Fast and scalable interfacial convective assembly of ... · Fast and Scalable Interfacial Convective Assembly of Nanoparticles For Interconnects Applications A Dissertation Presented

Fast and Scalable Interfacial Convective Assembly of Nanoparticles

For Interconnects Applications

A Dissertation Presented

by

Adnan Korkmaz

to

The Department of Mechanical Engineering

in partial fulfillment of the requirements

for the degree of

Doctor of Philosophy

in

Mechanical Engineering

Northeastern University

Boston, Massachusetts

April 2017

Dedicated to my parents, family and friends

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Contents

List of Figures v

List of Tables ix

List of Acronyms x

Acknowledgments xiii

1 Introduction 11.1 Nanoelements and their applications . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Nanofabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Assembly techniques of nanoelements . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3.1 Directed assembly of nanoelements . . . . . . . . . . . . . . . . . . . . . 21.4 Objective and significance of research . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Surface interactions and forces acting on particles and fluids 122.1 Forces on particles and fluids in colloidal systems . . . . . . . . . . . . . . . . . . 122.2 Electrostatic double layer forces and zeta potential . . . . . . . . . . . . . . . . . 142.3 Brownian force and velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.4 Van der Waals force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.5 Drag force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.6 Gravitational force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.7 Convective force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.8 Capillary force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.9 Marangoni force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.9.1 Marangoni instability in a shallow liquid pool . . . . . . . . . . . . . . . . 242.9.2 Marangoni effects on mass transfer . . . . . . . . . . . . . . . . . . . . . 25

2.10 Hydrophobic-hydrophilic surface interactions . . . . . . . . . . . . . . . . . . . . 262.11 Dielectrophoresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.12 Electrophoretic assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3 Experimental approach 303.1 Experimental facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.2 Interfacial convective assembly and characterization tools . . . . . . . . . . . . . . 31

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3.2.1 Template design and fabrication . . . . . . . . . . . . . . . . . . . . . . . 313.2.2 Particle suspension preparation and stability . . . . . . . . . . . . . . . . . 393.2.3 Interfacial convective assembly setup . . . . . . . . . . . . . . . . . . . . 413.2.4 Post characterization after the interfacial convective assembly . . . . . . . 42

3.3 Dielectrophoretic assembly and characterization tools . . . . . . . . . . . . . . . . 453.3.1 Template design and fabrication . . . . . . . . . . . . . . . . . . . . . . . 453.3.2 Nanoparticle suspension preparation . . . . . . . . . . . . . . . . . . . . . 473.3.3 Assembly setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.3.4 Post characterization after dielectrophoretic assembly . . . . . . . . . . . . 50

3.4 Electroplating and characterization tools . . . . . . . . . . . . . . . . . . . . . . . 513.4.1 Template design, fabrication and assembly setup . . . . . . . . . . . . . . 513.4.2 Post characterization after the electroplating . . . . . . . . . . . . . . . . . 51

4 Hypothesis 534.1 Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5 Results and discussion 555.1 Interfacial convective assembly of particles . . . . . . . . . . . . . . . . . . . . . 55

5.1.1 Assembly process and mechanism . . . . . . . . . . . . . . . . . . . . . . 555.1.2 In situ experiment results . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.2 Control of assembly process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665.2.1 Effect of evaporation of solvents on the interfacial convective assembly . . 705.2.2 Effect of miscibility of solvents on the interfacial convective assembly . . . 725.2.3 Fabrication of various types of nanostructures . . . . . . . . . . . . . . . . 735.2.4 Fabrication of nanostructures in hydrophobic surfaces . . . . . . . . . . . 76

5.3 Theoretical calculations-interfacial convective assembly . . . . . . . . . . . . . . . 765.4 Seed layer deposition using nanoparticles on ceramic surfaces for the printed elec-

tronics application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805.5 Fabrication of three-dimensional (3-D) nanostructures by interfacial convective as-

sembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.5.1 Interconnects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

5.6 Fabrication of three-dimensional (3-D) nanostructures by dielectrophoretic assem-bly of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.6.2 Experimental setup for the assembly . . . . . . . . . . . . . . . . . . . . . 885.6.3 Fabrication process of 3-D structures . . . . . . . . . . . . . . . . . . . . 895.6.4 Fabrication of various types of metallic 3-D nanostructures . . . . . . . . . 905.6.5 Fabrication of ZnSe nanostructures . . . . . . . . . . . . . . . . . . . . . 935.6.6 Fabrication of composite 3-D nanostructures . . . . . . . . . . . . . . . . 935.6.7 Fabrication of 3-D nanostructures on flexible substrates . . . . . . . . . . . 955.6.8 Electrical characterization of fabricated 3-D nanostructures . . . . . . . . . 97

6 Conclusion and Future Work 1026.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1026.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

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Bibliography 106

iv

List of Figures

1.1 Various assembly mechanisms based on particle confinement . . . . . . . . . . . . 41.2 Illustration of the influence of the evaporation-induced convective flow on the as-

sembly of particles on an oxygen-plasma-treated surface . . . . . . . . . . . . . . 51.3 Sketch of the particle and water fluxes . . . . . . . . . . . . . . . . . . . . . . . . 61.4 Location of micrometer-size particles in wetting films . . . . . . . . . . . . . . . . 71.5 Langmuir-Blodgett method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.6 Langmuir-Schaefer method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.7 Electrophoretic assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.1 Diagram of the relative magnitudes of displacement in one second due to threephysical mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 Schematic of zeta potential and the double layer . . . . . . . . . . . . . . . . . . . 152.3 Surface tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.4 Flow induced by unbalanced tangential forces at a fluid interface: the Marangoni

effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.5 Macro-Marangoni convection generated during the transfer of a solute across a

curved meniscus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.6 Flow generated by self-amplification of small disturbances . . . . . . . . . . . . . 212.7 Marangoni convection during mass transfer as a drop emerges from a 3-mm diame-

ter nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.8 Marangoni convection generated around an air bubble attached to a heated surface

in subcooled nucleate boiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.9 Surface tension variation and local film thinning for surface tension positive and

negative systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262.10 Surface energy differences between hydrophilic and hydrophobic surfaces . . . . . 262.11 Schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.12 Numerically calculated electric field lines . . . . . . . . . . . . . . . . . . . . . . 28

3.1 Wet etch bench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2 Laurell spinner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.3 Nanospec spectrophotometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.4 Zeiss Supra 25 scanning electron microscopy . . . . . . . . . . . . . . . . . . . . 323.5 Schematic of template fabrication process with nanopatterns . . . . . . . . . . . . 33

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3.6 Brewer spinner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.7 Quintel 4000 mask aligner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343.8 Schematic of template fabrication process for the in situ experiments . . . . . . . . 343.9 Schematic of the Si etching process with a hard mask . . . . . . . . . . . . . . . . 363.10 Si etching process with a hard mask . . . . . . . . . . . . . . . . . . . . . . . . . 373.11 Anatech SP-100 plasma system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.12 Unaxis plasma-therm 790 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.13 Phoenix contact angle measurement . . . . . . . . . . . . . . . . . . . . . . . . . 383.14 Schematic of patterned ceramic template fabrication process . . . . . . . . . . . . 393.15 Rotary evaporator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.16 Particle size and zeta potential analyzer . . . . . . . . . . . . . . . . . . . . . . . 403.17 Assembly setup for the interfacial convective assembly . . . . . . . . . . . . . . . 423.18 Park systems NX10 AFM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.19 Carl Zeiss 1540 cross beam system . . . . . . . . . . . . . . . . . . . . . . . . . . 433.20 Optiphot 200 fluorescence microscope . . . . . . . . . . . . . . . . . . . . . . . . 443.21 Electrical characterization using the probe station . . . . . . . . . . . . . . . . . . 443.22 Four point probe resistivity measurement . . . . . . . . . . . . . . . . . . . . . . 443.23 Nikon eclipse TE2000-U inverted microscope . . . . . . . . . . . . . . . . . . . . 453.24 Structure for the in situ experiment design . . . . . . . . . . . . . . . . . . . . . . 463.25 Bruce anneal furnace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.26 MRC 8667 sputtering systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.27 Micro automation 1006 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.28 Preparation of nanoscale patterns and a schematic that illustrates the dielectrophoretic

assembly of nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.29 Bath type and probe type sonicators . . . . . . . . . . . . . . . . . . . . . . . . . 493.30 Dielectrophoretic assembly setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.31 Nanopillar fabrication process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.32 Preparation of templates and electroplating setup . . . . . . . . . . . . . . . . . . 52

4.1 Schematic difference between the local evaporation-driven and the proposed assem-bly process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.1 Schematic difference between the local evaporation-driven and interfacial convec-tive assembly process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.2 Fluorescent microscope images of assembled 3-µm diameter of silica particles . . . 575.3 Assembly mechanisms-in situ experiments using an inverted microscope . . . . . . 585.4 Frame from the video showing the landing time of 0.01 wt%, 3-µm fluorescent silica

particle inside the 20-µm width lines . . . . . . . . . . . . . . . . . . . . . . . . . 605.5 Frame from the video showing the landing time of 0.01 wt%, 1-µm fluorescent silica

particle inside the 20-µm width lines . . . . . . . . . . . . . . . . . . . . . . . . . 615.6 Frame from the video showing the landing time of 0.01 wt%, 0.5-µm fluorescent

silica particle inside the features . . . . . . . . . . . . . . . . . . . . . . . . . . . 625.7 Frame from the video showing the landing time of 0.01 wt%, 3-µm fluorescent silica

particle outside the features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

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5.8 Frame from the video showing the landing time of 0.01 wt%, 1-µm fluorescent silicaparticle outside the features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

5.9 Frame from the video showing the landing time of 0.01 wt%, 0.5-µm fluorescentsilica particle outside the features . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

5.10 Effects of parameters on the assembly . . . . . . . . . . . . . . . . . . . . . . . . 675.11 Effect of assembly time on the interfacial convective assembly . . . . . . . . . . . 685.12 Arrangement of particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 695.13 Comparison of IPA and acetone to study the effect of evaporation on the assembly . 715.14 Comparison of IPA, water, and acetic acid to study the effect of evaporation on the

assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 725.15 Comparison of IPA, chloroform, and toluene to study the effect of miscibility on the

assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 745.16 SEM and AFM micrographs of assembled 30-nm fluorescent silica particles . . . . 755.17 Fluorescent and SEM micrographs of assembled 100-nm fluorescent silica particles 755.18 Assembly results of 51-nm fluorescent PSL particles . . . . . . . . . . . . . . . . 765.19 Assembly result of 30-nm silica particles on hydrophobic surface . . . . . . . . . . 775.20 Velocity measurement of different size of colloidal fluorescent silica particles inside

Deionized water (DI) \ Isopropyl alcohol (IPA) mixture at room temperature . . . 785.21 Velocity measurement of different size of colloidal fluorescent silica particles inside

DI \ IPA mixture at room temperature . . . . . . . . . . . . . . . . . . . . . . . . 795.22 Governing forces for different silica particle sizes in water and IPA . . . . . . . . . 795.23 Capillary force for different silica particle sizes in water . . . . . . . . . . . . . . . 805.24 Contact angle measurements on the ceramics . . . . . . . . . . . . . . . . . . . . 815.25 Contact angle measurements on the ceramics after cleaning process using solvents . 825.26 Surface characterization on the Al2O3 and AlN surfaces . . . . . . . . . . . . . . . 825.27 Interfacial convective assembly process on rough ceramic surfaces . . . . . . . . . 835.28 Top view SEM images of 40-nm copper particles assembled on Al2O3 . . . . . . . 835.29 Tilted-view SEM images of 40-nm copper nanoparticles assembled on Al2O3 . . . 845.30 Assembled copper nanoparticles a) before and b) after the thermal annealing . . . . 855.31 Electrical measurements on the annealed copper nanoparticles . . . . . . . . . . . 855.32 Surface and electrical characterization of the nanopillars . . . . . . . . . . . . . . 875.33 A schematic of the template and directed assembly process . . . . . . . . . . . . . 895.34 Fabricating 3-D nanostructures through electric field-directed assembly of nanopar-

ticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 915.35 SEM images of fabricated 3-D nanopillars . . . . . . . . . . . . . . . . . . . . . . 925.36 AFM image of fabricated 3-D silver nanopillars . . . . . . . . . . . . . . . . . . . 935.37 Effect of assembly paramters on silver nanostructure fabrication by the electric field

directed assembly method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945.38 SEM images of interconnects made of (a) tungsten and (b) silver . . . . . . . . . . 945.39 Fabrication of semiconducting and composite ZnSe nanopillars on Indium tin oxide

(ITO) on glass surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965.40 Fluorescent images of assembled a) gold, b) gold-quantum dots particles and c) their

SEM image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975.41 Assembly of Au nanoparticles on the flexible substrate . . . . . . . . . . . . . . . 98

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5.42 Electrical current plotted as a function of the applied voltage for the probe/PMMAconfiguration as shown in the SEM inset for (a) tungsten and (b) silver interconnect 99

5.43 Electrical characterization setup and results from the composite ZnSe nanostructure. 100

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List of Tables

1.1 Top-down and bottom-up fabrication techniques . . . . . . . . . . . . . . . . . . . 2

5.1 Properties of solvents studied for the effect of evaporation . . . . . . . . . . . . . . 715.2 Properties of solvents studied for the effect of miscibility . . . . . . . . . . . . . . 735.3 Assembly condition for tungsten and silver interconnects . . . . . . . . . . . . . . 905.4 Electrical properties of tungsten nanostructures . . . . . . . . . . . . . . . . . . . 995.5 Electrical properties of silver nanostructures . . . . . . . . . . . . . . . . . . . . . 101

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List of Acronyms

AC Alternating current

Al Aluminum

AlN Aluminum nitride

Al2O3 Aluminum oxide

AFM Atomic force microscopy

AHB Advanced high-performance bus

Ar Argon

ATLM Arbitrated transaction level model

Au Gold

CCD Charge-coupled device

CMP Chemical mechanical polishing

CNT Carbon nanotube

CVD Chemical vapor deposition

Cr Chromium

DEP Dielectrophoresis

DI Deionized water

DCM Dicholoromethane

DC Direct current

DRAM Dynamic random access memory

EHD Electrohydrodynamics

EP Electrophoresis

x

EPD Electrophoretic deposition

FB Brownian force

Fcap Capillary force

Fconv Convective force

Fdrag Drag force

Fgrav Gravitational force

FOTS Fluoro-octyl-trichloro-silane

HHF Hogg-healy-fuerstenau

HMDS Hexamethyldisilzane

IC Integrated circuit

ICP Inductively coupled plasma

ITRS International technology roadmap for semiconductors

ITO Indium tin oxide

IPA Isopropyl alcohol

MIBK Methyl isobutyl ketone

Ma Marangoni number

MRC Materials research corporation

NH4OH Ammonium hydroxide

NMP N-methyl-pyrrolidone

NPGS Nanometer pattern generation system

NPs Nanoparticles

O2 Oxygen plasma

PETG Polyethylene terephthalate glycol-modified

PSL Polystyrene latex

pH Potential of hydrogen

PMMA Poly (methyl methacrylate)

PVP Polyvinylpyrrolidone

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PEB Post expose bake

QD Quantum dots

Ra Rayleigh number

SEM Scanning electron microscopy

SERS Surface enhanced raman spectroscopy

Si Silicon

SDS Sodium dodecyl sulfate

SF6 Sulfur hexafluoride

SLPAD Solid-liquid phase arc discharge method

T Temperature

t Time

TLM Transaction level model

3D Three-dimensional

2D Two-dimensional

UV Ultraviolet

W Tungsten

wt Weight

ZnSe Zinc selenide

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Acknowledgments

I would like to express my sincere thanks and appreciation to my advisor, Prof. AhmedBusnaina for his support, mentorship and patience over the past five years. His academic vision andconstant encouragement helped me to achieve this extensive research.

I would like to thank my dissertation committee members Prof. Yung Joon Jung, andProf. Yongmin Liu. Their knowledge and insight truly enriched this study. Special thanks to Prof.Randall Erb, whose guidance was critical in my understanding of fundamental concepts for mythesis.

I would like to thank Dr. Sivasubramanian Somu for challenging me throughout the lastsix years and for his support and creativity. I would also like to thank Dr. Aditi Halder for herenthusiastic vision and collaboration. She has been a very influential mentor and it was pleasureworking with her. Special thanks to Dr. Cihan Yilmaz for his ongoing guidance, help, and friendshipover the years.

I would like to thank the Kostas staff David McKee, Scott McNamara, and Rich DeVito.I would not have been able to finish this work without their help. I also want to thank Matt Botti,Jess Viator, Eric Howard, Matt Rogers, and Anton Janulis.

Thank you to my lab mates Hobin Jeong, Eric Penchansky, Salman Abbasi, AlolikaMukhopadhyay, Burak Sancaktar, Dr. Sharon Kotz, Dr. Juk-Yung Lee, Dr. Asli Sirman, Dr.Hanchul Cho, Dr. Ankita Faulkner, Dr. Jungho Seo, Dr. Cem Apaydin, Dr. Zhimin Chai, Dr.Bingbing Wang, Dr. Jin Young Lee, Dr. June Huang, and Dr. Asanterabi Malima for creating ahappy work environment and for providing me with friendship over the years.

In the meantime, this is an opportunity to thank people who have shaped my academicpersonality prior to my arrival to Northeastern University. Special thanks go to my undergrad-uate advisor, Professor Nilufer Egrican, for her invaluable support and incessant encouragementthroughout my studies at Yeditepe University. She always helped me greatly, not just in shapingmy professional career path in the United States, but in every aspect of life as a caring and anencouraging mentor.

I feel very fortunate to meet with great friends during my graduate studies. Special thanksgo to my friends Mert Korkalı, Cem Bila, Umut Orhan, Seyhmus Guler, Cagrı Dikilitas, MahmutBurak Tarakcıoglu, Canberk Kalaycı, and Ismail Bilgin for taking part in the enjoyable moments ofmy doctoral years in Boston.

I would like to dedicate my dissertation work to Professor Yaman Yener, Senior AssociateDean of Engineering for Faculty Affairs at Northeastern University, who passes away on June 14,2013. He was a true inspiration for and a father figure of Turkish students not only at NortheasternUniversity, but also in the Greater Boston area. He will always remain a role model in the many liveshe touched (like mine). His priceless effort that allowed me to pursue a worthwhile academic career

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will always be remembered as one of the cornerstones of my lifetime. I feel very fortunate to haveknown such a great scholar during my years and one of his last students, prior to his early departurefrom this world. He will be remembered by me and many others with bottomless affection.

Finally, my deepest appreciation and love is reserved for my parents Ayhan and Durdane,for their endless support and love, and for making me who I am. Their love embraces me everywheredespite the long geographic distance between us.

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Abstract

Fast and Scalable Interfacial Convective Assembly of Nanoparticles For

Interconnects Applications

by

Adnan Korkmaz

Doctor of Philosophy in Mechanical Engineering

Northeastern University, April 2017

Dr. Ahmed Busnaina, Adviser

Directed assembly of nanoelements has been used to fabricate 1, 2, and 3-dimensional orhybrid nanostructures with unique properties to be used in many applications including electron-ics, optics, and energy for enhanced performances. However, current directed assembly techniqueshave challenges in developing highly scalable and high-rate assembly for precisely placing nanoele-ments. Template-directed fluidic assembly and electric field induced assembly approaches are themost common assembly techniques. In the template-directed fluidic assembly, nanoelements areassembled either vertically or horizontally at the air-liquid interface with the help of a capillaryforce, which guides nanoelements to the desired surfaces. This process is slow since there is noexternal force applied on the nanoelements, hence it requires hours to assemble nanoelements lessthan an inch square. In addition, the resolution of nanoelements assembly is limited because of thewettability of solvent, which is water. On the other hand, electric field induced assembly techniquesare fast and robust since the applied electric field guides the nanoelements to the desired surfaces.This process requires a conductive layer to attract the nanoelements.

Here, we present an entirely new evaporation-driven assembly technique called interfacialconvective assembly, in which colloidal particles are selectively and simultaneously integrated onthe patterned surfaces up to two minutes over several square inches area. We have assembled organicand inorganic particles such as Polystyrene Latex (PSL), silica, gold, and silver into deep channels,holes, wells, and trenches, with sizes ranging from 25 nm to 20 µm on either conductive or insu-lating surfaces such as silicon, glass, gold, and ceramic. We demonstrate with in situ experimentsthat there is a turnover process because of the density, surface tension, and volatility difference be-tween water and isopropyl alcohol (IPA). Particles migrate toward the surface, and once the solventinside the feature starts evaporating, particles are driven together and change their position like alake turnover. We have tested different solvents such as water, acetic acid, chloroform, and tolueneto investigate the effects of evaporation, miscibility, and wettability on the interfacial convectiveassembly. Experimental results showed that the solvent needs to have low surface tension, be misci-ble, and volatile to perform successful assembly. We also examine the effect of changing parameters

xv

(including assembly time, temperature, and concentration) to control the assembly process. Adjust-ing these parameters precisely led to the assembly of colloidal nanoparticles in complex shapesover large areas such as a micron-scale world map. We successfully fabricated copper thin film onrough ceramic surfaces such as aluminum nitride (AlN) and aluminum oxide (Al2O3). After the an-nealing process, the copper film was conductive and its conductivity was less than the bulk copperbecause of the grain boundaries. We have fabricated silver interconnects successfully in nano- andmicro-scale in one minute and performed an electrical characterization on the fabricated structures.

The overall significance of our results is threefold: first, the interfacial convective assem-bly developed here shortens the processing time at least by a factor of ten compared to the convectiveself-assembly approach. Secondly, there is no need to have a conductive layer at all for the assem-bly process. Lastly and more importantly, the assembly process works on hydrophobic surfaces aswell as hydrophilic ones because of the wettability of low surface tension solvent. These resultsindicate that our approach will facilitate the fabrication of novel nanostructures and lead to variousnanoscale device applications.

xvi

Chapter 1

Introduction

1.1 Nanoelements and their applications

The prefix nano, derived from the Greek nanos, meaning dwarf, is becoming widespread

in science and technology. Nanotechnology is the technology that deals with nanoscale materials[1].

The nanometer is a metric unit of length, and denotes one-billionth of a meter. Nanomaterials

are materials that are nanoscale at least in one dimension. Nanoparticles are studied because of

their size-dependent electrical[2], photonic[3], and chemical properties[4]. They’ve been com-

mercialized recently[[5],[6]]. Nanoparticles are used in many applications such as biology and

medicine[[7],[8],[9]].

A Carbon nanotube (CNT) is a cylindrical nanomaterial. They exhibit high strength and

unique electrical and thermal properties[10]. Their electrical conductivity is either metallic or semi-

conducting depending on their atomic structure[11]. They are used in applications such as nano-

electronics[12], sensors[13], and micro-batteries[14].

1.2 Nanofabrication

Nanofabrication is the design and manufacture of structures and devices with dimensions

measured in nanometers. It combines the techniques such as patterning, growing, forming, and

removing material with nanometer precision, repeatability, and control.

Nanofabrication methods can be listed in two different categories. There are top-down and

bottom-up fabrication techniques[[15],[16]] as shown in Table 1.1. The semiconducting industry

1

CHAPTER 1. INTRODUCTION

uses top-down techniques to pattern nanoscale structures with various lithography methods. The

bottom-up technique uses interactions from the atomic, molecular and supra-molecular levels.

Top-down fabrication Bottom-up fabricationE-beam lithography Vapor phase depositionOptical lithography Atomic layer depositionNanoimprint lithography Sol-gel nanofabricationWet/dry etching Molecular self assemblyLift-off processScanning probe lithography

Table 1.1: Top-down and bottom-up fabrication techniques

1.3 Assembly techniques of nanoelements

Directed assembly of nanoelements has been long used to create various types of patterns

with unique properties for applications in sensors[[17], [18], [19]], electronics[19], optics[[20],

[21]], biomedicine[22], high-density data-storage[[23], [24], [25]].

1.3.1 Directed assembly of nanoelements

1.3.1.1 Convective and capillary assembly of nanoparticles

Among the many directed assembly techniques, flow-driven assembly[26], such as con-

vective [27], and capillary-based assembly[[28], [29]], is one of the most effective methods to place

micro and nanoparticles into different arrangements on surfaces.

In convective assembly, the assembly is driven by the convective flow of the solvent in-

duced by evaporation on the contact line of the droplet, which leads to the formation of mono

and multilayer particles on patterned and non-patterned surfaces. Similarly, in capillary assem-

bly, the assembly is driven by the capillary forces[30] because of the deformation of the liquid-

fluid interface[31]. Using these assembly techniques, various types of nanoelements such as car-

bon nanotubes[[32] ,[33] ,[34] ,[35] ,[36]], nanowires[37], nanoparticles made from silica[38],

polystyrene [39], gold[40], virus-like particles[41] and cells[42] are assembled into lines and wells

on surfaces. Many experimental and numerical studies have been performed to investigate the effect

of various assembly parameters such as withdrawal rate[27], temperature[28], evaporation speed

[40] and pattern geometry[42]. The control of processes led to the creation of nanopatterns for

2


high-resolution printing[43], thin film coating[44] and SERS[40]. However, in these assembly tech-

niques, the processes are largely governed by the evaporation of water at three-phase contact line so

that the assembly takes place only at localized regions on surfaces. Therefore, scalability of these

approaches over large areas in a short time becomes a major challenge. In addition, these meth-

ods strictly depend on the properties such as surface functionalization and energy, which typically

requires hydrophobic/hydrophilic surface treatment processes[[45], [46]]. For example, convec-

tive assembly is generally performed on hydrophilic substrates with typical contact angle values

below 20o because the thickness of the solvent layer needs to be equal to or less than the particle

diameter[31]. Likewise, in capillary assembly, receding contact angles between 10o and 30o are pre-

ferred to overcome thermal fluctuations on small size particles for a highly efficient assembly[47].

The surface modification not only increases the complexity and time involved in the substrate prepa-

ration, but it also limits the utilization of these methods in a wide range of applications. Therefore,

there is a need for a versatile assembly technique that accommodates a wide range of micro and

nanoparticles, works quickly over a large area, upon either a hydrophobic or hydrophilic surface.

Convective assembly is one of the commonly used self-assembly methods for the ar-

rangement of ordered particle structures. The process can be used to produce ordered coatings over

a large area. It is obtained on wetted substrates with contact angle values below ∼ 20 . When the

contact angle of the aqueous solution increases, the confinement effect induced by the meniscus

decreases. Above a critical contact angle value, the horizontal force exerted by the liquid meniscus

becomes high enough to prevent particles from depositing onto a flat substrate. On patterned sur-

faces, the combined effects of capillary forces resulting from the local distortion of the meniscus

when the contact line is dragged over the structures and the geometrical confinement induced by the

structures lead to a selective immobilization of the particles in the recessed areas of the substrate,

whereas no particles are deposited in the surrounding areas [48]. The assembly mechanism is based

on the convective flow of a solvent-induced by evaporation of the droplet that drags the particles

toward the contact line and lateral capillary forces between particles as shown in Figure 1.1. In

a hydrophilic area, the assembly starts when the thickness of the solvent layer becomes equal to

the particle diameter[29], and the lateral capillary force helps to drive the particles to make close

and dense patterns. The combined effects of convective flow and attractive capillary forces that arise

when the top of the particles protrude from the solvent layer lead to the formation of extended layers

or multilayers of closely packed particles. In a hydrophobic area, on the other hand, the thickness

of the solvent does not approach the particle diameter and the horizontal force produced by the

liquid meniscus prevents particles from depositing and forming patterns[[48],[49],[50],[51]]. The

3


effect of temperature of oxygen-plasma-treated surface on the convective assembly was illustrated

in Figure 1.2.

Figure 1.1: Various assembly mechanisms based on particle confinement at the contact line of adroplet can be distinguished depending on the wetting properties and topographical patterning ofthe substrate. Convective assembly is obtained on wetting substrates for contact angle values below20 . The assembly mechanism is driven by the convective flow of solvent-induced by evapora-tion on the contact line of the droplet, which leads, on flat surfaces, to the formation of continuous2D layers of packed particles (a) or to 2D discontinuous arrangements on patterned surfaces (c).Capillary assembly takes place for receding contact angles θrec of the colloidal suspension greaterthan 20 . While no deposition occurs on flat surfaces (b), the combined effect of geometrical con-finement and capillary forces created when the meniscus is pinned on the structures of a patternedsubstrate can be used to deposit only one or a few particles (d). (Source [48])

In this technique, a pressure gradient, from the suspension toward the wetting film that

arises due to the water evaporation to produce a suspension influx from the bulk suspension toward

the wetting suspension film Figure 1.3. This influx consists of a water component, jw, and of a par-

ticle flux component, jp. The water flux compensates for the water evaporated from the film je, and

4


Figure 1.2: Illustration of the influence of the evaporation-induced convective flow on the assemblyof particles on an oxygen-plasma-treated surface. a) For Ts > Tdew, the hydrodynamic force createdby the flow drags particles to the contact line and leads to a monolayer formation. b) For Ts ≈ Tdew,the evaporation of the solvent in the drying region is nearly zero. c) For Ts < Tdew, condensationtakes place on the already assembled layer and creates a reverse flow of solvent that disassemblesthe monolayer. (Source [48])

the particle flux causes particles to accumulate in the film, thus forming dense structures. The thick-

ness of vertical wetting films increases from the plate-suspension-air contact line downward toward

the bulk suspension due to the hydrostatic pressure. Then, successive monolayers, bilayers, trilay-

ers, etc. are expected to be formed by the continuous particle flux, jp and fill up the space between

the substrate and the film surface. The capillary forces between the particles can gather particles

into layered arrays[[46],[51],[52]]. For steady-state assembly, a simple equation for balancing the

volumetric fluxes of the liquid and assembly of particles was proposed[27].

νc =β × l × je × ϕ

h× (1− ε)× (1− ϕ)(1.1)

In the Equation 1.1 where Vc is the growth velocity of the layer, je is the local evaporation

rate, l is the evaporation length, ϕ is the particle volume fraction, h is the thickness of the particle

array and ε, and the porosity of the array, where (0 < β < 1) is the coefficient of proportionality.

By controlling both the film thickness and the surface electric potential, latex particles

were nucleated in wetting films on mercury, glass and mica[46]. Sodium dodecyl sulfate (SDS) was

used to improve the wettability of mercury. The film was made thinner by re-treating with a solution

from the cell as shown in Figure 1.4.

Over the last decades, convective assembly technique has been used to assemble different

types of nanoparticles, as well as carbon nanotubes[[38],[53],[54]]. Recently, Kraus et al.[43] and

Malaquin et al.[48] successfully achieved monolayer assembly of sub-100 nm gold (Au) particles

on a template using the convective assembly technique. The process required physical patterning

as well as chemical modification on the template. Cha et al.[32], by a similar assembly technique,

assembled 5-nm gold particles utilizing highly hydrophobic and highly hydrophilic regions on the

5


Figure 1.3: Sketch of the particle and water fluxes in the vicinity of monolayer particle arraysgrowing on a substrate plate that is being withdrawn from a suspension. The inset shows the meniscishape between neighboring particles. Here, Vw is the substrate withdrawal rate, Vc is the arraygrowth rate, jw is the water influx, jp is the respective particle influx, je is the water evaporation flux,and h is the thickness of the array. (Source [27])

6


Figure 1.4: Location of micrometer-size particles in wetting films on mercury: (A) particles pushedtoward the film surface by the electric field; (B) particles sandwiched between the upper film surfaceand the substrate. (Source [46])

substrate without requiring three-dimensional physical patterning. It is important to note that the

assembly process in these methods usually takes hours and therefore may not be suitable for mass

production.

1.3.1.2 Langmuir-Blodgett and Langmuir-Schaefer techniques

Convective assembly method looks similar to the Langmuir-Blodgett technique however

the assembly mechanism is different [[44],[55],[56],[57],[58],[59]]. In this method, arrays are

formed from particles that are completely immersed in solvent, and they should not adsorb onto

the surface and the substrate as well. It is a powerful technique that can be used to assemble a

large-area monolayer of anisotropic building blocks. Although it is possible to form ordered arrays

of nanoparticles into 2D and Three-dimensional (3D) structures, the structures are not permanently

bonded to each other. The assemblies hold together in the liquid-air interface, they fall apart when

taken out from the medium.

The process is carried out in a water-filled through equipped with a mobile barrier and

a pressure sensor. Nanoparticles are dispersed in a volatile solvent that is immiscible in water,

typically chloroform or hexane. The solution is spread dropwise onto the water surface where it

7


spreads to an equilibrium surface pressure and evaporates, leaving behind a water-supported film of

particles. The high surface tension of water allows the interfacial region to easily support nanos-

tructures with dense material compositions[58]. The mobile barrier is used to laterally compress

the monolayer at a controlled speed. The resulting 2D superlattices can then be transferred onto

solid substrates by vertical dip-coating as shown in Figure 1.5. Deposition was performed on both

sides of the sample after the transfer. This method is only suitable for the film layer fabrication and

needs delicate control of the surface pressure. Otherwise, changes in orientation and the breaking

of packed structures were observed[55].

Similarly, nanoelements are transferred onto a sample using the Langmuir-Schaefer method

(Figure 1.6). Deposition was performed on one side of the sample after the transfer. This method

is only suitable for the film layer fabrication and needs delicate control of the surface pressure.

Otherwise, cracks and voids were observed[44].

Figure 1.5: Langmuir-Blodgett through: a) schematic of a water-filled Langmuir-Blodgett throughbefore compression, b) schematic of a water-filled Langmuir-Blodgett through after compression,c) image of a substrate being pulled vertically through a Langmuir monolayer of nanoelements. Thespeed of both the dip-coater and the mobile barrier are mechanically controlled.

Figure 1.6: Langmuir-Schaefer: a) evaporation of solvent on supernatant part, b) transfer onto ahydrophilic surface.

8


1.3.1.3 Electric field directed assembly

Charged or uncharged particles suspended in liquids are moved to a conductive surface us-

ing an externally applied electric field within a very short time (less than 1 minute) over a large area.

The most commonly used electric field directed assembly techniques are electrophoretic (Direct

current (DC) electric field) and dielectrophoretic (Alternating current (AC) electric field) assem-

bly. Metallic[60], semiconducting[61] and polymer[62] psrticles suspended in aqueous as well as

in non-aqueous solutions[[63],[64],[65],[66]].

The induced charge on the NPs is given in Equation 1.2 [67]

q = 4πεrε0(1 + κR)ζ (1.2)

where R is the radius of a colloidal particle, εr is permittivity of suspension, ε0 is per-

mittivity of free space, κ is inverse Debye length, and ζ is the zeta potential on the particles. The

inverse Debye length is

κ =

√2NAe2I

ε0εrkBT(1.3)

whereNA is Avagadro’s number, e is the elementary charge, T is the absolute temperature,

I is the ionic strength of the electrolyte.

I =1

2ΣnD=1cDz

2D (1.4)

where cD is the molar concentration of ion D and zD is the charge number of the ion.

In Electrophoresis (EP) the electric force exerted on charged particles can be tuned by

changing the electric properties of the liquid medium. This force causes the charged particle to

accelerate toward the oppositely charged electrode as shown in Figure 1.7. The electric field force

is given in Equation 1.5. In this equation, Fe is the electric field force; q is the carried charge and

E is the electric field.

Fe = Eq (1.5)

For a spherical particle, the frictional coefficient is given by the Stokes law ( Equation 1.6)

f = 3πµa (1.6)

9


Figure 1.7: A Schematic of electrophoretic deposition of particles onto the anode substrate.

where a is the diameter of the particle, and µ is the viscosity of the medium.

A charged particle in an electric field accelerates toward the oppositely charged electrode

[[60],[64]].An opposing force, Fd due to viscous resistance of the medium increases as the particle

velocity increases ( Equation 1.7).

Fd = υf (1.7)

where υ is the particle velocity and given by the Equation 1.8

υ = qE/(3πµa) (1.8)

Electrophoretic deposition (EPD) is a high-throughput process that is used to coat a flat

conductive substrate with micro- or nanoscale components[[68],[69]]. EPD is also used to assemble

colloidal gold nanoparticles into micro patterned photoresist trenches prepared via micro-transfer

molding[60].

1.4 Objective and significance of research

In a typical fluidic or convective assembly process, a convective flow on the particles is

induced by the evaporation of water at the three-phase contact line of a solution. The assembly

mechanism is based on the mass transport by the flux of both particle and water towards the assem-

bly region stimulated by the water evaporation from the menisci between neighboring particles and

the wetting film. Therefore, the assembly occurs only at the contact line so-called the accumulation

zone, which is only a small fraction of the substrate. The assembly process becomes very slow

10


since the assembly speed strongly depends on the evaporation rate of water along the contact line.

In addition, the assembly only initiates when the thickness of the thin liquid film becomes equal to

the particle diameter, which typically happens on a hydrophilic surface. In this study, the objective

is to develop a rapid process which could work on completely hydrophobic, completely hydrophilic

or partially functionalized surfaces (both hydrophobic and hydrophilic surfaces).

In this work, we have developed a new convective and interfacial assembly technique:

the so-called interfacial convective assembly, which does not depend on the confinement of parti-

cles induced at the three-phase contact line and does not require surface treatment on the substrate.

Therefore, this method provides the following advantages over previously developed evaporation

driven assembly techniques; i) particle assembly simultaneously start everywhere on the surface,

and the nanoelements are rapidly assembled over a large area. For example, using the convec-

tive assembly[43] technique, it could typically take several hours to assemble particles over several

square inch area. The interfacial convective assembly developed here can shorten this time at least

by a factor of 10, enabling the assembly of particles in a few minutes over several square inch

scale. ii) This assembly process does not require the use of hydrophilic surfaces. This eliminates

excessive chemical functionalization and time involved in the template fabrication while increasing

the applicability to various types of surfaces. Using this method, we assembled various types of

nanoparticles such as Polystyrene latex (PSL), silica, gold, silver, and copper on topographical pat-

terns. We demonstrated that the assembly can be scaled up to several square inch area in a very short

time. We investigated the assembly mechanism and studied the assembly parameters such as tem-

perature, concentration, and assembly time. The understanding of governing forces led to controlled

assembly of various sizes of particles into arrays of lines and vias including complex arrangements

such as world map down to 25-nm scale, which is more than 2 times smaller scale compared to pre-

viously reported convective driven nanoparticle assembly/printing[43]. The results indicate that the

presented approach opens remarkable opportunities for the automated high throughput and large-

scale assembly applications such as high-resolution printing[43] for printed electronics, optical and

medical devices[[20], [21], [22]].

11

Chapter 2

Surface interactions and forces acting on

particles and fluids

This chapter outlines the basic forces acting on the particles and surface interactions. Elec-

tric fields often cause fluid motion and in order to describe the consequence of exposing conducting

electrolytes to high-strength AC fields, the science of Electrohydrodynamics (EHD) is introduced.

In order to understand how fluids and colloidal particles move within micro- and nanosystems, it

is important to be able to determine the scale and range of forces that govern the behaviour under

different experimental conditions.

2.1 Forces on particles and fluids in colloidal systems

The particle and the fluid medium can only move through the action of an external force,

which broadly speaking can be divided into two categories. The first is a random or stochastic force

over which we have little control, while the second type of force is a deterministic force, which is

almost totally under our control. The random force originates in the thermal energy or temperature

of the system and is due to molecules continuously bumping into each other, or into the suspended

particles, causing them to move about in a random manner. This is Brownian motion and does not

lead to a net unidirectional particle movement. We have little control over this force, other than

through changing the viscosity of the suspending medium or the temperature.

Deterministic forces, on the other hand, are almost completely under our control and

can be exploited to move particles in well-defined ways. One obvious example of a deterministic

force is gravity, which causes both particle and fluid movement. Depending on the time scale

12

CHAPTER 2. SURFACE INTERACTIONS AND FORCES ACTING ON PARTICLES AND FLUIDS

of an experiment, this force can be ignored, or indeed utilized to produce, for example, selective

sedimentation of particles suspended in a fluid. For particles denser than the surrounding medium,

gravity pulls the particle downwards, but for sub-micrometre or nanoparticles this force is usually

small and has little or no effect during the time course of a typical experiment. Small particles

have low inertia. A particle will always move at a constant velocity in a constant force field (when

suspended in a viscous fluid). If a temperature gradient is present then the AC field will also cause

fluid motion due to local variations in the permittivity and/or conductivity of the fluid.

Electrical forces can act both on particles and on the suspending fluid. The major electrical

forces acting on small particles suspended in a fluid such as water, are EP and/or Dielectrophoresis

(DEP)[70]. EP occurs due to the action of the electric field on the fixed, net charge of the particle,

while DEP only occurs when there are induced charges, and only results in motion in a non-uniform

field (this can be a DC or an AC field). If a temperature gradient is present then the AC field will

also cause fluid motion due to local variations in the permittivity and/or conductivity of the fluid.

Figure 2.1: Diagram of the relative magnitudes of displacement in one second due to three physicalmechanisms: Brownian motion, gravity and dielectrophoresis as a function of particle radius from1 cm down to 1 nm (Source [70])

The magnitudes of the Brownian motion, gravity and dielectrophoresis, depend on several

variables, principally the dimensions of the system, the size of the particle, and for the electrokinetic

13


forces, with voltage and conductivity. Figure 2.1 shows the displacement of a particle over a time

interval of 1 second as a function of particle size. The first point to note is the effect of Brownian

motion. It can be ignored even for particles up to 1 µm in radius. Gravitational force scales linearly

with volume so that the bigger the particles the further they move. In a time frame of a few minutes,

this force can lead to relatively large displacements of particles such as cells, but its effect is almost

insignificant for sub-micrometre particles. For particles larger than 1 µm, the figure shows that it is

a relatively trivial matter to ensure that the DEP force dominates over both the gravitational force

and Brownian motion, i.e. DEP is the sole deterministic force over this period of time. However,

the situation is not as simple for smaller particles. Take for example a 50-nm diameter particle.

The lower DEP force line shows that, although gravity can be ignored, over short time intervals we

see that Brownian motion dominates particle displacement. A stronger DEP force leads to a larger

deterministic force so that movement becomes possible. The DEP force scales with the square of the

voltage and inversely with the cube of the distance, so that decreasing the characteristic dimensions

of the electrode by one order of magnitude can lead to a three orders of magnitude increase in the

DEP force.

2.2 Electrostatic double layer forces and zeta potential

When an uncharged surface is immersed into a liquid, it will attain a surface charge due

to preferential adsorption of ions present in the liquid or due to dissociation of surface groups. The

final surface charge has to be balanced by an equal but oppositely charged region of counter-ions,

some of which are bound to the surface within the so-called Stern layer, while others form the

diffuse electric double layer. Both layers are shown in Figure 2.2. Surfaces of nanoelements carry a

net charge either from the chemical groups exists on the surface or the absorption of the ions from

solution[71].

According to the Hogg-healy-fuerstenau (HHF) model, the electrical double layer force

interacting between a sphere and a plate with constant potential can be expressed[72] as shown in

Equation 2.1

FΨel = µεrε0R(Ψ2

01 + Ψ202)

κe−κH

1− e−2κH

[2Ψ01Ψ02

Ψ201 + Ψ2

02

− e−κH]

(2.1)

where Ψ01 is the zeta potential of the particle of radius R, Ψ02 is the zeta potential of the

substrate, εr is the dielectric constant of the medium, ε0 is the dielectric permitivity of a vacuum,

and κ the Debye - Huckel parameter of the electrolyte solution.

14


Figure 2.2: Schematic of zeta potential and the double layer. (Source [73])

According to Debye - Huckel approximation to Derjaguin - Landau - Verwey - Overbeek

theory [[74]], zeta potential ζ can be assumed to be equal to the surface potential φ0.

ζ = φ(R) =q/R

4πεrε0(1 + κR)−1 (2.2)

where q is the surface charge, R is the particle radius, εr is the relative permittivity of

the material, ε0 is the vacuum permittivity, κ is inverse Debye length, which is related to ionic

strength (I) of the solution, valance of the ionic species (z), Boltzmann constant (kB) and solution

temperature (T) with Equation 2.3

κ2 =2Iz2

εkBT(2.3)

The effect of ionic concentration and pH on the zeta potential for different types of par-

ticles has been investigated [[75],[76],[77]]. By increasing salt concentration, the absolute value of

zeta potential on the latex particles increases at higher Potential of hydrogen (pH) values and de-

creases as the salt concentration decreases[78]. At higher ionic concentrations the electrical double

layer thickness on the particles reduces, which decreases the zeta potential[79].

If all particles in suspension have a large negative or positive zeta potential then they will

tend to repel each other and there will be no tendency for particles to come together[80]. However,

15


if the particles have low zeta potential values then, there will be no force to prevent the particles

coming together.

2.3 Brownian force and velocity

Particles in solution possess a random force due to the thermal energy of the system,

causing them to move in a random manner[81]. The magnitude of the Brownian force (FB) was

modeled as a Gaussian white noise[82] process by Equation 2.4

FB = Gi

√12πaµkBT

∆t(2.4)

where µ is the dynamic viscosity of the medium, kB is the Boltzmann constant (1.38 x

10−23 J/K), T is the absolute temperature. Gi is zero - mean, unit variance independent Gaussian

random numbers and ∆t is the time used in the calculations.

The root - mean - square velocity (VN ) of a Brownian particle can be calculated[83] as

VN =

√3kBT

m=

1

a

√18kBT

πρa(2.5)

A Brownian-Reynolds number (Re) based on the Brownian velocity leads to

Re =1

υ

√18kBT

πρa(2.6)

where υ is the kinematic viscosity of the liquid.

2.4 Van der Waals force

There is a long-range attractive force between any molecules at a distance (≈ 1 nm)[84].

This is known as the van der Waals or Hamaker force. This force have an important role to play

in controlling the stability of colloidal particles. It has three different components which are the

orientation, Debye interaction and London dispersion forces. Orientation force arises due to an

interaction between two dipoles (Keesom). The other component is the induction (Debye interac-

tion), which is weaker than the other force, and last one is the London dispersion force which is

an attractive force[[85],[86]]. The van der Waals force exists between all atoms and molecules and

arises due to fluctuation of the electric charge around a molecule or atom. Interaction free energy

(Equation 2.7) is proportional to the inverse of the sixth power of the inter-molecular distances.

16


WV DW = −CV DWr6

= −(Cind + Corient + Cdisp)

r6(2.7)

The interaction force between a sphere of radius R and a flat plate at a separation distance

z0 is given by Equation 2.8[84]

FV DW =AR

6z0(2.8)

where A is the conventional Hamaker constant and given by

A = π2ρ1ρ2CV DW (2.9)

where ρ is the number density of molecules in the solid.

2.5 Drag force

The drag force is also effective on the particle. The fluid exerts a drag force on the particle

that affects the velocity of the particle. If the fluid is in motion, then the drag force pulls the particle

along. When a particle is moving relative to the fluid, it experiences a viscous drag force due to

the action of the fluid on the particle. Stokes solved the equations of motion of a rigid sphere in a

fluid in the laminar regime. The formula calculates the drag force on a sphere of diameter a moving

steadily in a fluid with velocity; V. Using the boundary condition, the magnitude of the drag force

is given by Equation 2.10

Fd = 3πµaV (2.10)

where V is the relative velocity between the particle and the fluid. This equation is valid

for Re < 1. Drag coefficient is

Cd =Fd

0.5ρV 2A(2.11)

where is the fluid density and A is cross sectional area of the spherical particle.

A = πa2/4 (2.12)

17


2.6 Gravitational force

In terms of micro-scale, gravity is dominant as a deterministic force, and the magnitude

of this force is given by Equation 2.13

Fgrav =1

6πa3(ρ2 − ρ1)g (2.13)

where ρ1 and ρ2 refer to the densities of the medium and the particle, respectively. g is

the gravitational acceleration constant and a is the particle diameter.

2.7 Convective force

If velocity is known then pressure and forces can be determined. According to first law of

Newton

Fconv = ma = mV

t(2.14)

where the mass of the particle is the density times the volume. a is the acceleration of the

particle. V is the velocity of the particle.

2.8 Capillary force

Deformation of the liquid surface creates the Capillary force (Fcap) as shown in Figure 2.3.

Capillary interaction increases with increasing interfacial deformation created by the particles. Two

similar particles floating on a liquid interface attract each other because the liquid meniscus deforms

to decrease the gravitational potential energy of the particles when they close each other. The origin

of this force is the particle weight[87].

There is a direct surface tension component in the Fcap, as surface tension pulls the contact

line of meniscus and the particle towards the contact line of meniscus and the surface[[87],[88]].

This force is called the surface tension force. The Fcap can be given by Equation 2.15[89]

Fcap = 4πRγL cos θ (2.15)

where R is radius of the sphere and γL is the surface tension of the liquid and θ is the

contact angle.

18


Figure 2.3: Movement of particles forced by the capillary force applied between two particles.

Fcap also depends on humidity. Humidity independence - Fcap is viable for spherical

particles above 1 µm radius, but below that there is a strong humidity dependence[90].

According to Vassileva’s work [91]], the net capillary forces between spherical particles

floating at a liquid-liquid interface can be calculated using the equation

Fcap = (F (γ)x + F (p)

x )ex + (F (γ)y + F (p)

y )ey (2.16)

where ex and ey are unit vectors. F(γ)x and F(γ)

y are because of the contribution of the

interfacial tension [91]]. F(p)x and F(p)

y are because of the contribution of the pressure distribution

over the particle surface [91]].

2.9 Marangoni force

When surface or interfacial tension changes from point to point in the interface, a tangen-

tial force equal to the gradient of the tension is developed, as shown in Figure 2.4. It is directed

from the low tension to high tension, and the magnitude of the resulting surface stress is given by

FtA

= 5IIσ (2.17)

where 5IIσ is the interfacial gradient of the boundary tension (the gradient taken with

respect to orthogonal coordinates tangential to the interface. Variations in tension frequently arise

during the transfer of heat or chemical species across the interface, owing to variations in interfacial

temperature or composition as occur in the multiphase separation processes. Movement along the

patch of interface is transmitted to the adjacent bulk phases via the ”no slip” condition, and the

19


significant bulk convection may result. The marangoni effect[92] is defined as bulk flows generated

by a spatial variation in boundary tension.

Figure 2.4: Flow induced by unbalanced tangential forces at a fluid interface: the Marangoni effect.

One example of Marangoni convection occurs when there is mass transfer of a solute

across a curved meniscus as shown in Figure 2.5. If the solute is transferring upward, it will be

depleted from the region beneath corner of the meniscus due to the geometric asymmetry of the

system. If the solute decreases interfacial tension, its depletion from the meniscus region will locally

increase interfacial tension here, leading to a force directed toward the wall. Tears of wine[93] is

an example of macro Marangoni convection. A film of wine, an ethanol/water solution, forms a

meniscus around the glass. The alcohol evaporates from the meniscus because it is more volatile

than water, leaving behind a water-rich film of relatively high surface tension. More liquid is then

drawn up the side of the glass where it accumulates in a ring that breaks into droplets that flow back

down into the wine.

Another type of Marangoni convection may occur as a result of a system’s inherent in-

stability with respect to small disturbances. For example, desorption of ethanol from a shallow

aqueous pool infinite in lateral extent (free of menisci) as shown in Figure 2.6. During the evapora-

tion, the surface region becomes enriched in water and will therefore have a higher surface tension

than that corresponding to the bulk solution beneath. Such a system, as all systems, is subject to

small disturbances. One component of such a disturbance may be a local dilation, bringing an eddy

of ethanol-rich liquid up to the surface. The surface tension will thus be locally reduced, and a

dilational force established. This will further dilate the surface, bringing still larger amounts of

ethanol-rich liquid to the surface until the disturbance has amplified itself to macroscopic propor-

tions, usually within a fraction of a second (Because this bulk flow results from the amplification of

20


Figure 2.5: Macro-Marangoni convection generated during the transfer of a solute across a curvedmeniscus (on the left). Convection generated during transfer of acetic acid across the interfacebetween benzene/chlorobenzene (on the bottom) and water. Source [94]

microscopic disturbances.). This convection would not occur if the transfer direction were reversed,

i.e., if the ethanol vapor were being absorbed into the water. Under these circumstances, a dilational

eddy would bring water-rich liquid to the surface, locally increasing the tension and generating a

converging flow that destroys the disturbance. When it does occur, the self-amplification of the

disturbance is opposed by the eroding influence of diffusion and viscosity, and only when a certain

critical concentration gradient exists will the disturbance grow. A similar phenomenon occurs when

a liquid pool is heated from below (or cooled from above, as by evaporation). Since surface tension

decreases with temperature, an eddy bringing warmer liquid to the surface from the interior may be

self-amplified[95].

Figure 2.6: Flow generated by self-amplification of small disturbances (micro-Marangoni effectresulting from hydrodynamic instability)

21


Usually a system unstable with respect to Marangoni convection for transfer of a solute

in one direction is stable for transfer in the other direction. In the stable case, however, macro

Marangoni convection may occur. It was shown in Figure 2.7 for the transfer of acetic acid across

an interface between a drop of ethyl acetate at the tip of a capillary tube and ethylene glycol. The

system is unstable with respect to micro Marangoni convection, manifest as interfacial turbulence,

when the solute is initially in the ethyl acetate phase, but not for the reverse transfer direction. In

the latter case, a single toroidal eddy is formed in the drop, as the interfacial tension at the tip of the

drop is lower than at the base.

Figure 2.7: Marangoni convection during mass transfer as a drop emerges from a 3 mm diameternozzle. Micro Marangoni convection during inward transfer, but a single macro Marangoni vortexduring outward transfer in the system ([[96]]

The Marangoni equation is

τ′′z − τ

′z +5IIσ + τ s = 0 (2.18)

where τ′′z and τ

′z are the viscous tractions of the adjacent bulk phases,

τ′′z = exτ

′′zx + eyτ

′′zy (2.19)

where ex and ey are unit vectors in the x and y directions and assuming Newtonian fluids

τ′′zx = µ

′′(∂v

′′z

∂x+∂v

′′x

∂z

)(2.20)

τ′′zy = µ

′′(∂v

′′z

∂y+∂v

′′y

∂z

)(2.21)

22


τ s represents the contribution of any intrinsic rheology. Equation 2.18 is a vector equation

yielding two scalar equations, for the x and y components, respectively. These are extracted by

taking the surface divergence (Equation 2.22) and the normal component (Equation 2.23) of the

surface curl of Equation 2.20 and Equation 2.21[97]. For Newtonian incompressible fluids;

−µ′′[(

∂2v′′z

∂z2

)+52

IIv′′z

]+ µ

′[(

∂2v′z

∂z2

)+52

IIv′z

]+52

IIσ +5II .τ s = 0 (2.22)

−µ′′ez .5II x

(∂v

′′II

∂z

)+ µ

′ez .5II x

(∂v

′′II

∂z

)+ ez .5II xτ s = 0 (2.23)

The variation in the boundary tension of the Marangoni effect caused by variations in

interfacial temperature, composition and electrical potential;

5IIσ =

(∂σ

∂T

)o

5II T +

(∂σ

∂C

)o

5II C +

(∂σ

∂E

)o

5II E (2.24)

where E is the local electrical potential. The boundary conditions for the thermal energy

equation at the interface are;

T′

= T′′and q

′+ q

′′+ ST = 0 (2.25)

where q′

and q′′

are heat fluxes from the adjacent bulk phases. From Fourier’s Law;

q′

= −k′ ∂T′

∂z(2.26)

where k is the thermal conductivity. ST is the generation (or consumption) of heat (per

unit area) because of any chemical reaction at the interface.

The boundary conditions for the convective diffusion equation for a species i;

C′i = mC

′′i and j

′i + j

′′i +Ri = 0 (2.27)

where m is the distribution equilibrium constant for species i, and j′i and j

′′i are fluxes of

species i from the adjacent bulk phases. From the Fick’s law;

j′i = −D′

i

∂C′i

∂z(2.28)

where Di is the diffusivity of species i and Ci its concentration. Ri is the generation (or

consumption) of species i (per unit area) because of the chemical reaction at the interface.

23


As the temperature of a solid surface is raised to the point where vapor bubbles are nu-

cleated, the efficiency of heat transfer generally increases sharply before nucleate boiling begins.

Although increased levels of natural convection may be part of the reason for this, it cannot be the

total explanation because the phenomenon is observed even in zero-gravity experiments and even

when the heated surface is facing downward. It is believed that the enhancement is traceable to the

presence of bubbles formed by dissolved air and attached to the heated surface[98]. Because such

bubbles have a higher temperature near their base, where they are attached to the solid surface, sur-

face tension gradients develop, and the resulting surface motion induces convection in the adjacent

liquid as shown in Figure 2.8. Significant enhancement in heat transfer were obtained in comparison

to the case in which the bubbles were not present.

Figure 2.8: Marangoni convection generated around an air bubble attached to a heated surface insubcooled nucleate boiling

2.9.1 Marangoni instability in a shallow liquid pool

Temperature or concentration gradients during heat and mass transfer normal to the pool

are considered adverse if, when sufficiently steep, they lead to instability and spontaneous flow such

as Benard cells[99]. The gradient supports a layer of liquid at the surface whose tension is higher

than that corresponding to the liquid in the interior.

The gradients may produce either adverse or stabilizing density stratification, which may

lead to natural convection. Density-driven natural convection in a liquid is the result of Rayleigh

instability[100] and coexists with surface tension driven flow that is Marangoni instability.

Surface-tension-caused instability in a shallow pool heated from below was first studied

by Pearson[101]. Instability during the transfer of a solute through a liquid-liquid interface was

studies by Sternling and Scriven[102].

24


The Marangoni number (Ma):

Ma =

(dσ

dT

)o

| dTodz| h

2

µα=

(dσ

dC

)o

| dCodz| h

2

µD(2.29)

where D is the solute diffusivity, α is the thermal diffusivity, µ is the dynamic viscosity,

T is the temperature, h is the enthalpy, and D is the solute diffusivity.

Adverse temperature (or concentration) profiles are subject to buoyancy-driven as well as

Marangoni instability. Rayleigh number (Ra):

Ra =gξκh4

αν(2.30)

where g is the gravitational acceleration, ξ is the coefficient of volumetric expansion, and

κ is the temperature gradient, h the depth of the pool, α the thermal diffusivity and ν the kinematic

viscosity. It is possible to make a direct comparison between the critical depth of a pool of a given

liquid with a known temperature drop as predicted by the Marangoni vs. the Rayleigh mechanism.

For most liquids, including high molecular weight alkanes, with a 1o temperature drop across the

layer, one finds a critical depth for Rayleigh instability of the order 3-10 mm, whereas for Marangoni

instability, it is less than 1 mm[103].

2.9.2 Marangoni effects on mass transfer

The Marangoni effects are manifested in two general ways. First, the presence of flow

in the immediate vicinity of fluid interfaces, as either macro or micro Marangoni convection (the

latter termed interfacial turbulence) may increase the overall mass transfer efficiency by factors up

to ten or more. Second, in some cases such flows may result in large changes in the interfacial

area available for transfer, either increasing or decreasing it from the case when boundary tension

gradients are absent[104].

Surface tension positive (σpos) systems were defined as those for which the more volatile

component has the lower surface tension, while surface tension negative (σneg) systems were those

for which the more volatile component has the higher surface tension. When the thickness of a

liquid film varies, the thinner region will equilibrate faster than the thicker region, and therefore

be enriched in the less volatile component as shown in Figure 2.9. In a σpos system, this leads to

higher surface tension in the thin area. 1-2 mN/m is required for sufficient surface tension difference

between the components.

25


Figure 2.9: Surface tension variation and local film thinning for a surface tension positive andnegative systems.

2.10 Hydrophobic-hydrophilic surface interactions

The surface energy is the total inter-molecular forces on material surface and the degree

of attraction or repulsion forces exerted among material[105]. The contact angle (wetting angle) is a

measure of the surface energy of a solid by a liquid. There are hydrophobic and hydrophilic surfaces

and they are defined by the angle between an edge of droplet and the surface underneath as shown

in detail Figure 2.10. If droplet spreads and wets a large area over the surface, then the contact

angle will be low and that surface is considered as hydrophilic. But if the droplet forms a sphere

that barely touches the surface, that surface is considered hydrophobic. Positive ions attached on

the surface cause water to spread out over wide areas[106].

Figure 2.10: Surface energy differences between hydrophilic and hydrophobic surfaces for thecases such as a) complete wetting, b) partial wetting, and c) non-wetting.

26


2.11 Dielectrophoresis

Conductivity is a measure of the ease with which charge can move through a material,

while permittivity is a measure of the energy storage or charge accumulation (at interfaces) in a

system. In the presence of an applied electric field, charge moves and piles up at either side of the

interface between the particle and the electrolyte as shown in Figure 2.11. Changing the pH will

alter the net charge on the surface. Most cells possess more acid than basic groups and so have

a characteristic net negative surface charge density. In order to maintain charge neutrality, ions

of opposite charge are attracted to the surface forming a thin layer of counter charge, called the

double layer. If the particle is now subjected to an external electric field, the double layer charges

experience a force and move.

Figure 2.11: Schematic diagram of how a dielectric particle suspended in a dielectric fluid polarisesin a uniform applied electric field E

Figure 2.12 (a) shows a particle with a polarisability greater than the suspending medium.

The electric field lines bend towards the particle, meeting the surface at right angles as if it were a

metal sphere, and the field inside the particle is nearly zero. The converse is shown in Figure 2.12(b),

where the particle polarisability is less than the electrolyte. The field lines now bend around the

particle as if it were an insulator. The field inside is similar to that outside. The field lines in a

non-uniform electric field are shown in figures Figure 2.12(c) and Figure 2.12(d). There are mainly

two characteristics of dielectrophoresis:

• the intrinsic electrical properties of a particle manifest themselves as a force which varies with

the applied frequency of the AC field. Therefore, particles can be moved in a non-uniform

27


Figure 2.12: Numerically calculated electric field lines for four different cases, defined by theparticle more polarisable or less polarisable than the suspending medium, in a uniform or a non-uniform electric field. For the more polarisable particle (a) and (c), the field lines are drawn to thesurface of the particle, becoming increasingly perpendicular as the polarisability increases, and thefield strength inside the particle is low. For the less polarisable particle (b) and (d), the field linesare bent around the particle and the field strength inside the particle is high. The arrows show thedirection of the force and movement in each case. (Source [70])

28


AC field.

• two different particles can exhibit quite different force-frequency spectra. The force is not

only different in magnitude but also in direction, so that the particles can move in opposite

directions in the same field with the same frequency. Dynamic separation of particles is

therefore possible.

2.12 Electrophoretic assembly

Electrophoretic deposition is the migration of charged particles/elements dispersed in a

suspension towards oppositely charged electrode under the influence of an applied uniform electric

field.

The Coulomb force on a particle is given by the product of the electric field and the charge

on the particle

FEP = QE =

∫Sσq dSE (2.31)

where Q is the total charge on the particle which, if the particle has a surface charge

density σq, is given by the integral of this charge density over the closed surface S of the particle.

29

Chapter 3

Experimental approach

This chapter describes the facilities and procedures used in the experiments. Most of the

experiments were performed at the George J. Kostas Research Center at Northeastern University. In

situ experiments were only performed in the lab of mechanical engineering department of Boston

University. The experiments were performed in the class 100 and 1000 clean rooms at NSF Center

for High-rate Nanomanufacturing in Northeastern University.

3.1 Experimental facilities

Experiments have been performed in the George J. Kostas Nanoscale Technology and

Manufacturing Research Center at Northeastern University. The 7,000 sq. ft. user facility includes

cleanroom space down to Class 10 and houses instruments for micro- and nanofabrication processes

and characterization studies, including metrology. The George J. Kostas Nanoscale Technology and

Manufacturing Research Center, established in 2005. This research facility has the capabilities of

the nano and microfabrication areas in the Egan Research Center. The Kostas facility houses a com-

plete 6 inch wafer fabrication line and consists of three areas. The first area comprising class 10 and

class 100 clean room spaces has the capability for optical and nanolithography processes, wet etch,

ion mill, Inductively coupled plasma (ICP) dry etch processes, plasma vapor deposition processes,

electron beam evaporation processes, oxidation and nitride processes, electrical and mechanical

property testing and characterization equipments.

The second area, which is class 10,000, is used for testing, imaging and design pro-

cesses. Equipment additions acquired since inception of the grant include: Field Emission Scanning

electron microscopy (SEM) (Carl Zeiss SUPRA25) with Nanometer pattern generation system

30

CHAPTER 3. EXPERIMENTAL APPROACH

(NPGS) e-beam nanolithography, PGT energy dispersive spectrum analyzer and Zyvex S100 nano-

manipulator capability and a PSIA XE7 Atomic force microscopy (AFM). A surface energy analysis

system is incorporated into this imaging room. Other tools in the chemistry laboratory are raman

spectrometer (Jobin Yvon Labram), Janis ST-500 electrical probe station, chemical hoods, thermal

incubator and centrifuge.

3.2 Interfacial convective assembly and characterization tools

3.2.1 Template design and fabrication

To fabricate templates for the nanoparticle assembly; Si wafers were cleaned in piranha

solution (H2SO4: H2O2 2:1) for 5 min at 110-115oC. Wafers were cleaned in the wet etch bench

as shown in Figure 3.1. Poly (methyl methacrylate) (PMMA) was coated on the wafer by Laurell

spinner (Figure 3.2) at 5000 rpm for 1 min. PMMA thicknesses (measured by Nano-Spec 200

optical profilometer, Figure 3.3) were 450 and 50 nm to pattern different width and aspect ratio of

features. Following spinning, the wafer was placed in a conventional oven at 100oC for 30 min.

E-beam lithography was performed by Zeiss Supra 25 SEM (Figure 3.4) to obtain various patterns.

Measured beam current was 41 pA at an acceleration voltage of 30 kV using an aperture with a

diameter of 10 µm. After exposure, photo-resist was developed in Methyl isobutyl ketone (MIBK)-

IPA 1:3 solution for 80 s, in IPA for 30 s, and DI for 5 min, respectively to remove all the residual

polymer (Figure 3.5).

Figure 3.1: Wet etch bench.

For the in situ experiments, glass chips were fabricated. A 500 µm thickness of borosil-

icate glass wafer was diced into 15 mm x 15 mm chips. Glass chips were cleaned in piranha for 3

min, rinsed in DI for 5 min and dried with nitrogen. The chips were baked at 200oC for 5 min on a

contact hot plate to dehydrate the surface. The SU8-2010 resist was spin-coated using the Brewer

31


Figure 3.2: Laurell spinner.

Figure 3.3: Nanospec spectrophotometer.

Figure 3.4: Zeiss Supra 25 scanning electron microscopy.

32


Figure 3.5: Schematic of template fabrication process with nanopatterns.

spinner (Figure 3.6) onto the organic-free glass chips. The spin conditions: the sample was ramped

to 500 rpm at 100 rpm/s acceleration. Then, the sample was ramped to 3000 rpm at an acceleration

of 300 rpm/s and hold for a total of 30 s. After the spinning, it was pre-baked at 65oC for 1 minute

and soft baked at 95oC for 2 minute on a hot plate to evaporate the solvent and densify the film. SU-

8 was exposed near Ultraviolet (UV) (350-400nm) with Quintel 4000 mask aligner (Figure 3.7) for

10 s. Following exposure, a Post expose bake (PEB) was performed at 65oC for 1 minute and 95oC

for 2 min on a hot plate to selectively cross-link the exposed portions of the film. The slow ramp

was performed to minimize stress, wafer bowing, and to resist cracking. Following PEB, the SU-8

resist was developed in the SU-8 developer (1-methoxy-2-proponal acetate) for 2 minute, rinsed in

IPA and dried by spinning (Figure 3.8).

Figure 3.6: Brewer spinner.

In order to test strong solvents for the interfacial convective assembly method, PMMA

patterned silicon samples were not be able to used because PMMA dissolves easily inside strong

solvents. Because of this, patterned and etched Si samples were fabricated. The fabrication process

33


Figure 3.7: Quintel 4000 mask aligner.

Figure 3.8: Schematic of template fabrication process for the in situ experiments.

34


started with the O2 plasma (for 1 min) by the Anatech system. Adhesion promoter (Hexamethyldisilzane

(HMDS)) has been coated on the cleaned silicon wafers using a spinner. HMDS promotes photore-

sist adhesion by creating a bond between the resist and silicon. The silizanes bond to the silicon in

the wafer while the methyls bond with the photoresist. Vapor priming allows for HMDS application

in a monolayer and reduces the chance of contamination. The adhesion promoter has been baked for

30 min at 100oC in an oven. Afterwards, LOR 3A resist has been coated to be used for the lift off

process and it was baked at 190oC for 6 min on a hot plate. S1813 (1.3 µm of thickness) photo-resist

was spin-coated on the adhesion layer and baked at 100oC for 1 min on a hot plate (Figure 3.9a).

The sample was exposed to UV by using a negative mask for 3.5 s. Following the exposing, the

sample was developed in AZ 726 (MIF) developer for 30 s (Figure 3.9b and Figure 3.10a). In or-

der to remove the residuals of any resists outside the patterns, the sample was exposed to the O2

plasma for 1 min. 50 nm of Cr film has been deposited on the sample using the e-beam evaporator

(Figure 3.9c and , Figure 3.10b). After the metal deposition, the sample was immersed into 1165

(microposit remover), acetone, and IPA for 5 min each (Figure 3.9d and , Figure 3.10c). Following

the lift off process, silicon on the patterned regions was etched by ICP (RF1: 250 W, RF2: 300 W,

Sulfur hexafluoride (SF6): 6 sccm, Ar: 4 sccm) (Figure 3.9e). Si etch rate was 10000 A/min. Cr

was wet-etched at 40oC until seeing the color difference on the sample surface (Figure 3.9f). The

sample was coated by S1827 (2.7 µm of thickness) photoresist as a protective layer. Then, it was

diced 15 mm x 15 mm. Following the dicing, the protective layer was removed by solvents and

the sample was exposed to the O2 plasma for 1 min. Fluoro-octyl-trichloro-silane (FOTS) has been

coated at 150oC for 30 min in a vacuum oven.

This method is also investigated on ceramic surfaces such as Aluminum nitride (AlN)

and Aluminum oxide (Al2O3). Ceramic samples were cleaned of organics using O2 plasma (for 1

minute) by Anatech SP-100 plasma system (Figure 3.11) or the uniaxis inductively coupled plasma

790 system (Figure 3.12). Contact angle of the ceramic substrates are measured before and after the

surface treatment using the Phoenix contact angle measurement (Figure 3.13). AZ 2020 negative

photoresist is spin-coated at 5000 rpm for 1 minute. Samples were baked at 100oC for 1 minute

and after exposure (using the Quintel aligner for 10 s) samples were post-baked at 100oC for 1

minute. Following the post-baking, AZ 2020 samples were developed in AZ MIF 300 for 20 s. The

fabrication process is simply shown in Figure 3.14

35


Figure 3.9: Schematic of the Si etching process with a hard mask (Cr). a) Photoresist coating on theSi surface. b) After exposing to UV and developing, an undercut was achieved. c) A thin layer ofCr was deposited on the top surface. d) After the lift off process, the photoresist has been removedcompletely. e) Si was etched anisotropically. f) Excess Cr layer has been removed by wet-etching.

36


Figure 3.10: Si etching process with a hard mask (Cr). a) S1813 and LOR was coated on the Sisurface. The sample was exposed to UV and developed. 0.5 µm of undercut was achieved. b) 50nm of Cr was deposited on the top surface. c) After the lift off process, the photo-resist has beenremoved completely.

37


Figure 3.11: Anatech SP-100 plasma system.

Figure 3.12: Unaxis plasma-therm 790.

Figure 3.13: Phoenix contact angle measurement.

38


Figure 3.14: Schematic of patterned ceramic template fabrication process.

3.2.2 Particle suspension preparation and stability

The aqueous, green fluorescent, different size of silica particles (silicon oxide, 25 and 50

mg/ml) suspension was purchased from Kisker Biotech GmbH & Co, Steinfurt; Germany (nominal

diameters: 30, 100, 500, 1000, 1500, 3000 nm). Particles were directly used from the commercial

suspensions. The zeta potential of fluorescent silica particles is -43 ± 5 mV.

The aqueous, red fluorescent PSL nanoparticles suspension was purchased from Thermo

scientific (Fluoro-Max Red, nominal diameter: 51 nm, 1wt%). The zeta potential of PSL particles

is -43 ± 5 mV. These particles have a density of 1.05 g/cm3 and refractive index of 1.59, are

used to study the assembly parameters due their good dispersion stabilities. We have performed

assembles with higher PSL concentrations and the excess amount of water was removed using the

rotary evaporator, which increases the concentration of the particle solution (Figure 3.15).

The aqueous gold nanoparticle suspension was purchased from British Biocell Interna-

tional (nominal diameter: 50 nm). The gold colloid is composed of chloroauric acid (< 0.01 %),

sodium citrate (< 0.00001 %), tannic acid (< 0.0000001 %) and potassium carbonate (< 0.0000001

%) and have citrate stabilization with a net negative surface charge. The zeta potential of the gold

nanoparticles is -41 ± 7 mV.

The aqueous silver nanoparticle suspension was purchased from nanocomposix (nominal

diameter: 25 ± 5 nm) and were capped with Polyvinylpyrrolidone (PVP). The zeta potential of

these particles is -24 mV.

39


Figure 3.15: Rotary evaporator is used to evaporate the excess amount of water in the solution andto increase the nanoparticle concentration.

In order to measure the stability of particle solutions, the zeta (ζ) potential is measured

using Malvern ZetaSizer ZS90 (Figure 3.16). It is capable of measuring both the zeta potential

and size. It uses light for the measurements. The frequency of the light changes with respect to

the reference frequency when the light is scattered due to the particle motion. The direction and

magnitude of the shift are directly related to the suspended colloidal particle mobility. Thus, the

particles only need to move a small fraction of their diameters for a change in frequency to be

detected, which is accomplished in a low electric field. The instrument can determine the average

electrophoretic mobility of the particles in solution[107].

Figure 3.16: Particle size and zeta potential analyzer.

40


Velocity is measured but mobility and ζ potential are calculated. Mobility is not a direct

measure of repulsive force; the ζ potential is. The relationship between ζ potential and mobility

depends on the theoretical model chosen. There are two classic models that result in two classic

limits: the Smoluchowski and the Huckel equations. They apply in opposite limits. These limits

have a common root: the magnitude of the dimensionless product κa, where a is the radius of

the kinetic unit. The parameter κ−1, called the double layer thickness (Debye length), plays an

important role in determining distance from the particle surface over various electrical potentials

are significant. Simple ions are very small: as a result, κa 1. The relationship between mobility

and zeta potential is[108]:

Huckel Limit;

µe = (2eζ)/(3µ) where µ is the viscosity of the suspending liquid for κa 1.

In the opposite limit, κa is very large, and the limiting equation is:

Smoluchowski Limit;

µe = eζ/µ for κa 1.

In general, mobility is related to the ζ potential by the following equation:

µe = (2eζ)/(3µ)f(κa,ζ) where f(κa,ζ) is a model-dependent function.

f(κa,ζ) = f(κa) = 3/2-4.5/(κa)+37.5/(κa)2-330/(κa)3 for κa > 1.

3.2.3 Interfacial convective assembly setup

Figure 3.17a shows the patterned Si substrate with PMMA. Patterns were wetted in IPA

for 2 min in a bath type sonicator (Crest ultrasonic cleaner, model no 1200T: 45 kHz, 200 W) to

remove the trapped air inside the features. They were filled using IPA as shown in Figure 3.17b. 100

µl of the aqueous suspension of colloidal particles was added on top of IPA as shown in Figure 3.17c.

Using of solvent can shed light on broader aspects of the interface. For example, the contact angle

of IPA is 13o on a Si surface, in contrast; it is 33o using deionized water. The volume ratio between

IPA and DI is 1:3 over 15 mm x 15 mm when 100 µl of DI was used. Similarly, the weight ratio

between IPA and DI is 1:4, therefore, the cause of the density.

A hydrophobic glass cover, FOTS coated by vapor deposition method, was used as a

top cover material to achieve uniform evaporation and eliminate the watermarks as shown in Fig-

ure 3.17d. To coat 0.15 mm thickness of glass with FOTS, glass was placed in a glass petri dish. 200

µl of FOTS was placed next to the microscope glass and covered by a petri dish. FOTS was evap-

orated physically to cover glass slide in a vacuum oven for 30 min at 150oC. The gap between the

41


glass cover and substrate was controlled using glass spacers positioned at the edge of the top-glass

cover.

Figure 3.17: Assembly setup was shown; a) patterned chip, b) wetting process using IPA, c) addingparticle solution, d) FOTS coated glass was placed to prevent watermarks and control evaporationrate of solvent.

3.2.4 Post characterization after the interfacial convective assembly

After the assembly process, the samples have been characterized using SEM, Carl Zeiss

1540 (Figure 3.20) and Park systems NX10 AFM (Figure 3.18).

Fluorescent particle imaging:

A Nikon Optiphot 200 fluorescence microscope (Figure 3.20) with a Micropublisher 5.0

cooled RTV camera was utilized to acquire optical images. Two different filters such as B2-A and

G2-A (Nikon Inc.) were used for the fluorescent particles.

Electrical measurements on the patterns were performed for using the Agilent analyzer

and Micromanipulator Co. Inc. probe station (Figure 3.21). Electrical measurements on thin metal

films were performed using the Four point probe tool. Four points of contact must be made between

the probes and sample. A current goes through the outer probes, and the difference in voltage is

measured between the two inner probes as shown in Figure 3.22.

42


Figure 3.18: Park systems NX10 AFM .

Figure 3.19: Carl Zeiss 1540 cross beam system.

43


Figure 3.20: Optiphot 200 fluorescence microscope.

Figure 3.21: Electrical characterization is performed using a) the Agilent station and b) the probestation. The probes are located and I-V measurements are performed using a software.

Figure 3.22: Four point probe resistivity measurement.

44


In order to understand the assembly mechanism, in situ experiments were performed by

using an inverted optical microscope (Nikon TE 2000-U, Figure 3.23) with a Hamamatsu ORCA-

Flash 4.0 camera. The sample was wetted by IPA and aqueous colloidal particle solution was added,

and the observation was shown in Figure 3.24.

Figure 3.23: Nikon eclipse TE2000-U inverted microscope.

3.3 Dielectrophoretic assembly and characterization tools

3.3.1 Template design and fabrication

An oxide layer was grown by wet oxidation on the silicon wafer using the Bruce anneal

furnace (Figure 3.25). The conductive layer for this assembly was formed by sputtering Cr/Au

(5 nm/100 nm) on SiO2 (500 nm)/Si wafer using the Materials research corporation (MRC) 8667

sputtering (Figure 3.26). The wafer was diced (into 15 mm x 15 mm chips) with a Micro automation

1006 dicing saw (Figure 3.27) and was pre-cleaned with piranha solution at 110oC for 5 min. A

positive photo-resist PMMA, (Poly-Methyl-Methacrylate, 950 PMMA A Resists) from Shipley Inc,

was then spun-coated using the Laurell spinner on the Cr/Au chips as an insulating layer. By

adjusting the spin speed, time and the anisol amount, the PMMA thickness is adjusted. The patterns

were designed in Design Cad drawing software and patterned on PMMA by using J.C. Nabity

system in the SEM. While generating the patterns, 10 µm aperture was used. The applied dose

amount has been varied between 0.5-2.0 to achieve perfectly exposed patterns for different pattern

dimensions. Following the e-beam writing, the templates have been developed in MIBK/ IPA (1:3)

45


Figure 3.24: Structure for the in situ experiment design to observe the assembly and the inset fromthe microscopy image.

46


solution for 80 s and IPA for 30 s. Then the templates were rinsed under running DI for 30 s. The

process is shown in Figure 3.28.

Figure 3.25: Bruce anneal furnace.

3.3.2 Nanoparticle suspension preparation

The aqueous gold nanoparticles were purchased from British biocell international (nom-

inal diameter: 5 nm and 20 nm). The pH of the suspension was adjusted by adding ammonium

hydroxide (Baker Co.). A portable pH meter, Sension2, purchased from Hach company, was used

to measure the pH of the suspension. The pH meter was calibrated with pH standards 7, 4 and 10

with a calibration slope -58. The pH of solutions is 10. The aqueous aluminum, tungsten particle

suspension were purchased from Meliorium technologies (nominal diameter: 10 nm). These par-

ticles are sonicated using a bath sonicator (Branson ultrasonic cleaner, model No B-22-4, 40 kHz,

125 W, Figure 3.29a) or probe sonicator (Q700 Sonicator, 20 kHz, 700 W, Figure 3.29b). The aque-

ous silver particle suspension was purchased from Nanocomposix (nominal diameter: 10 nm). This

suspension has citrate for stabilization and originally has a concentration of 1 wt%. DI is added to

the suspension prior to assembly to achieve a desired concentration of 0.06 wt%. The zeta potential

of these particles is -30.2 mV.

47


Figure 3.26: MRC 8667 sputtering systems.

Figure 3.27: Micro automation 1006, dicing tool.

48


Figure 3.28: Preparation of nanoscale patterns and a schematic that illustrates the dielectrophoreticassembly of nanoparticles.

Figure 3.29: a) Branson ultrasonic cleaner, b) Q700 probe sonicator.

49


3.3.3 Assembly setup

The template and a counter electrode (Cr/Au sputtered gold) were connected to a func-

tion/arbitrary waveform generator (Agilent 33220A) and dipped into the particle suspension (Fig-

ure 3.30). Following the application of a sinusoidal AC electric field with or without a DC offset,

the template and the counter electrode were pulled up from the suspension using a dip coater (KSV

NIMA) at a controlled speed (85 mm/min). Finally, the PMMA layer on the template was removed

using acetone for metallic nanopillars. The nanopillar fabrication process is simply shown in Fig-

ure 3.31.

Figure 3.30: Dielectrophoretic assembly setup.

3.3.4 Post characterization after dielectrophoretic assembly

Following the assembly process, top-view and tilted images are taken using Zeiss Supra

25 SEM.

Fluorescent particle imaging

A Nikon Optiphot 200 fluorescence microscope was used for the fluorescent particles.

50


Figure 3.31: Nanopillar fabrication process.

3.4 Electroplating and characterization tools

3.4.1 Template design, fabrication and assembly setup

An oxide layer was grown by wet oxidation on the silicon wafer using the Bruce anneal

furnace (Figure 3.25). The conductive layer for this assembly was formed by sputtering Cr/Au (5

nm/100 nm) on SiO2 (500 nm)/Si wafer using MRC 8667 sputtering (Figure 3.26). The wafer was

diced into 15 mm x 15 mm size of chips with Micro automation 1006 dicing saw (Figure 3.27) and

was pre-cleaned with piranha solution at 110oC for 5 min. A positive photoresist PMMA was then

spun-coated using the Laurell spinner on the Cr/Au chips as an insulating layer. By adjusting the

spin speed, time and the anisol amount, the PMMA thickness is adjusted. The patterns are designed

in Design Cad drawing software and patterned on PMMA by using J.C. Nabity system in the SEM.

While generating the patterns, a 10 µm aperture was used. The applied dose amount has been varied

between 0.5-2.0 to achieve perfectly exposed patterns for different pattern dimensions. Following

the e-beam writing, the templates have been developed in MIBK/ IPA (1/3) solution for 80 s and

IPA for 30 s. Then, the templates were rinsed under running DI for 30 s. The process is shown in

Figure 3.32.

3.4.2 Post characterization after the electroplating

Following the electroplating process, top-view and tilted images are taken using Zeiss

Supra 25 SEM.

51


Figure 3.32: Preparation of templates and electroplating set up.

52

Chapter 4

Hypothesis

In this chapter, we explain our hypothesis on the basis of limited evidence as a starting

point for further investigation.

4.1 Hypothesis

In a typical convective and fluidic assembly process, a convective flow on the particles is

induced by the evaporation of water at the three-phase contact line of a solution (Figure 4.1a). The

assembly mechanism is based on the mass transport by the flux of both particle and water towards

the assembly region stimulated by the water evaporating from the menisci between neighboring

particles and the wetting film. Therefore, the assembly occurs only at the contact line, the so-called

the accumulation zone, which is only a small fraction of the substrate. Assembly process becomes

slow since the assembly speed strongly depends on the evaporation rate of water along the contact

line. In addition, the assembly only initiates when the thickness of the thin liquid film becomes

equal to the particle diameter, which typically happens on a hydrophilic surface. The disadvantages

of conventional evaporation-driven assembly process:

• Depends on liquid-surface interface.

• Requires surface functionalization.

• Assembly process takes hours, not suitable for practical applications.

• Assembly over large areas in a short time is challenging.

• Assembly into high aspect ratio geometries is difficult due to the challenge in wetting.

53

CHAPTER 4. HYPOTHESIS

In our proposed work (Figure 4.1b), the hypotheses are:

• Investigating the dependency of evaporation on liquid-surface interface.

• Examining the dependency to surface functionalization.

• Inspecting the assembly process with use of volatile solvent.

Figure 4.1: Schematic difference between the local evaporation-driven and the proposed assemblyprocess. a, Only at the three-phase contact line of water, the particles are derived to the surfacesdue to the convective flow generated by the evaporation of water. Reproduced from ref. [43]. b, Inthe proposed method, low surface tension and miscible solvent layer was introduced to investigateeffect of wetting, miscibility, and evaporation.

54

Chapter 5

Results and discussion

5.1 Interfacial convective assembly of particles

5.1.1 Assembly process and mechanism

In a typical convective assembly process, a convective flow on the particles is induced

by the evaporation of water at the three-phase contact line[51] of a solution (Figure 5.1a). The

assembly mechanism is based on the mass transport by the flux of both particle and water towards

the assembly region stimulated by the water evaporation from the menisci between neighboring

particles and the wetting film. Therefore, the assembly occurs only at the contact line so-called the

accumulation zone[43], which is only a small fraction of the substrate. Assembly process becomes

slow since the assembly speed strongly depends on the evaporation rate of water along the contact

line. In addition, the assembly only initiates when the thickness of the thin liquid film becomes

equal to the particle diameter, which typically happens on a hydrophilic surface.

In the interfacial convective assembly process, a layer of low surface tension solvent such

as alcohol is first spread onto a surface to facilitate the template wettability regardless of hydropho-

bicity/hydrophilicity of the surface. Then, an aqueous dispersion of colloidal particles is directly

placed on top of the solvent film. Finally, the aqueous dispersion can be covered with a glass slide

to achieve a controlled evaporation rate of liquids (Figure 5.27b). Over time, the low surface tension

solvent is displaced with the colloidal particles due to difference in i) the surface tension[93], ii) the

evaporation rate, and iii) the specific gravities between the solvent and water. Mixing can also be

observed depending on the miscibility of the solvent with water. Due to the displacement between

the solvent and water, a directional convective flux of water is generated from the aqueous solution

55

CHAPTER 5. RESULTS AND DISCUSSION

Figure 5.1: Schematic difference between the local evaporation-driven and interfacial convectiveassembly process. a, Only at the three-phase contact line of water, the particles are derived to thesurfaces due to the convective flow generated by the evaporation of water. Reproduced from ref.[43]. b, The addition of solvent enables very fast and simultaneous assembly over large area. c,Sketch of the experimental apparatus for the interfacial convective assembly on patterned substratesand the optical image of assembled of 3-µm silica particles inside 20-µm with of lines over 2 cm x2 cm area.

56


to the solvent, which induces a hydrodynamic drag force on the dispersed particles and directs the

particles toward the surface to be assembled. The particle assembly simultaneously initiates every-

where on the surface and therefore the particles are rapidly assembled over large areas. Figure 5.1c

illustrates the successful assembly result using 3-µm diameter of silica particles inside wide micro

scale patterns over 2 cm x 2 cm area in 2 min. Fluorescent images of these particles are shown in

Figure 5.2.

Figure 5.2: Fluorescent microscope images of assembled 3-µm diameter of silica particles; a) zoomout, b) zoom in.

5.1.2 In situ experiment results

To establish a better understanding of the assembly mechanism, we performed in-situ

experiments using an inverted fluorescence microscope (For a detailed description of the experi-

mental set-up and construction of the pattern, see Figure 3.24). Since sub-micron size particles are

not clearly visible under the microscope due to the resolution limit, we examined the trajectory of

micron size particles during the assembly. The movements of 3-µm diameter silica particles were

traced from the bottom surface of a glass substrate as shown in Figure 5.3a.

In order to understand the effect of evaporation on the assembly, we identified the time that

takes for the first particle to reach inside/outside the patterns at room temperature. It is expected

that IPA outside the patterns will evaporate at a faster rate compared to IPA inside the patterns

due to the larger surface area. Therefore, the particles should assemble outside the patterns at a

faster rate. In situ experiments showed that it only took 13 s for the 3-µm particle to reach outside

57


Figure 5.3: Assembly mechanisms-in situ experiments using an inverted microscope. a, Three-dimensional schematic illustration of the three-step process for the assembly. Step 1: the turnoverprocess due to the density difference between IPA and water. Step 2: evaporation/mixing of IPAoutside the pattern. Step 3: the displacement process because of the Benard-Marangoni effectinduced by the evaporation from a two-component mixture, where the two miscible liquids differby both their volatility and surface tension gradient. b, Landing time of 0.01 wt%, 3-µm diametersilica particle inside the pattern. c, Landing time of 0.01 wt%, 3-µm diameter silica particle outsidethe pattern, highlighting that time increases. d, Landing time of silica particles as a function ofparticle diameter were identified inside and outside the patterns. All of the experiments above wereperformed at room temperature.

58


the patterns (on the photoresist) although the same particle assembled inside the pattern in 30 s

(Figure 5.3b and c). These results indicated that evaporation of the solvent has an effect on the

interfacial convective assembly process. The influence of evaporation on the assembly is further

discussed below. Based on these experiments, the assembly process can be described with the

following three-steps. In step 1, water and IPA mix like a lake turnover[109] (Figure 5.3)a) since

they are completely miscible, and IPA is less dense and more volatile than DI. At the end of the

turnover, IPA replaces water above the patterns, however, there is still IPA left in the patterns since

evaporation is not complete inside the patterns. In step 2, the evaporation of IPA outside the patterns

start and the particles are dragged and assembled only outside the patterns. In step 3, the IPA inside

the patterns evaporate enabling the assembly of particles inside the patterns. During the entire

assembly process, resulting evaporation from the IPA-water mixture causes the Benard-Marangoni

([110], [111], [112]) convection since the two miscible liquids differ in terms of their both volatility

and surface tension. At the end of the assembly process, the substrate is tilted perpendicular to the

ground to remove excess particles from the non-patterned hydrophobic surface (measured contact

angles for PMMA and SU8-2010 are 70o and 75o, respectively) via both the Stokes drag force and

gravity. Likewise, in the convective/capillary assembly, excess particles are cleared away by moving

the liquid meniscus during the assembly process[43]. Figure 5.3d shows the time takes for different

size of silica particles to reach either outside or inside the patterns as a function of particle diameter.

In situ experiments, glass microscope slide was used to carry the prefabricated samples.

Figure 5.4 shows that the 3-µm diameter of fluorescent silica particles reaches the bottom surface

of the features only in 30 s at room temperature (0.01 wt%, no cap). Experiments were lasted for

different particle sizes such as 1 and 0.5-µm at the same conditions. Figure 5.5 and Figure 5.6 show

the optical image of landing of 1 and 0.5-µm silica particles. Landing times were 75 and 203 s for

1 and 0.5-µm, respectively. It was observed that the Brownian motion becomes dominant, and the

landing time increases significantly when the particle size decreases. The relative humidity was 52

% during all the experiments.

Similarly, landing time of silica particles with different sizes outside the features (top

surface of the resist) was recorded. Figure 5.7 shows optical images (using the bright filter) showing

the landing time of 3-µm diameter of particles with 0.01 wt% concentration at room temperature.

Figure 5.8 and Figure 5.9 show the optical images (using the green filter) for the landing time of

1-µm and 0.5-µm diameter of particles, respectively. Landing times are 13 s, 40 s, 130 s for 3-µm,

1-µm and 0.5-µm, respectively. Landing time decreases with increasing particle size since the effect

of gravity increases.

59


Figure 5.4: Frame from the video showing the landing time of 0.01 wt%, 3-µm fluorescent silicaparticle inside the 20-µm width lines is 30 s.

60


Figure 5.5: Frame from the video showing the landing time of 0.01 wt%, 1-µm fluorescent silicaparticle inside the 20-µm width lines is 75 s.

61


Figure 5.6: Frame from the video showing the landing time of 0.01 wt%, 0.5-µm fluorescent silicaparticle inside the features is 203 s.

62


Figure 5.7: Frame from the video showing the landing time of 0.01 wt%, 3-µm fluorescent silicaparticle outside the features is 13 s.

The results demonstrated the assembly of microparticles inside the arrays of large micron

scales patterns. However, it would be desirable to assemble nanoscale size particles using the same

method since it can enable fabrication of nanostructures with unique photonic[3], electrical[2], and

chemical[113] properties. To create such nanopatterns, we have assembled 30-nm silica particles

into the 300-nm width of lines. Unlike micron size particles, we observed that the time to initiate

the assembly of nanoparticles in the nanoscale patterns is much longer compared to the assembly

in microscale. For example, when the assembly was performed using 100 µl, 25 mg/ml, 30-nm

silica particles at 40oC, no assembly was observed inside the patterns up to 10 min. After 10 min, a

sudden increase in the assembly was observed for the same conditions. On the other hand, the time

to assemble 3-µm silica particles with same other conditions was 2 min.

At the moment of the displacement process induced by evaporation, colloidal particles are

dragged into the feature by the Marangoni forces. We note that there is a significant difference in

the assembly time between nano and micro-scale particles using the same assembly conditions such

as concentration, temperature. As the particle size decreases (D3 > D2 > D1), particle drag velocity

increases (Vd3 > Vd2 > Vd1) and assembly time increases (t1 > t2 > t3) since the drag force (Fd1

> Fd2 > Fd3) due to the evaporation is directly related to the particle diameter.

63


Figure 5.8: Frame from the video showing the landing time of 0.01 wt%, 1-µm fluorescent silicaparticle outside the features is 40 s.

64


Figure 5.9: Frame from the video showing the landing time of 0.01 wt%, 0.5-µm fluorescent silicaparticle outside the features is 130 s.

65


5.2 Control of assembly process

To further investigate and understand the assembly kinetics, we studied the factors in-

cluding temperature, concentration as well as the assembly time on the same dimensional nanoscale

pre-patterned template. These patterns define the particle arrangement and govern the assembly

mechanism. Template consists of 16 of 20-µm spaced array of 200-nm lines on 15 mm x 15 mm Si

substrate.

We found that temperature played a crucial role in deciding the assembly time. Adjusting

the temperature of the solution is a convenient means to increase the evaporation rate of the solvent,

and thus for obtaining high assembly yields. The convection equilibrium-dynamic diffusion mod-

ulates significantly in terms of temperature since the evaporation rate, Stokes drag force as well as

the particle interaction force depends on temperature. To observe the trend experimentally, 30-nm

standard fluorescent silica particles (25 g l−1) were assembled at different temperatures. Interest-

ingly, assembly coverage increased exponentially with increasing temperature for a constant time; 2

min as shown in Figure 5.10a. The second remark concerns the effect of concentration in solution.

We observed that the concentration of the particle had to be sufficient to cover the features[33] in

addition to the particle flux (Figure 5.10b). It is crucial to highlight the concentration since the

leading force is the liquid evaporation during the assembly.

Observations during the study indicate that there is a threshold time to cover desired po-

sitions completely with nanoparticles at the same temperature as provided in Figure 5.10c. Other

complementary temperature studies may also be useful. For instance, when the assembly time for

30-nm fluorescent silica particles is 10 min at 40oC, the assembly coverage is less than 25%. Sur-

prisingly, the increase of temperature as a consequence of heating increased the assembly yield up

to 100% in 16 min. Overall, the results show that even if the dispersed particles are arranged com-

pletely inside the features, IPA inside the features needs to be evaporated to obtain a full coverage

(please see Figure 5.3-step3).

Assembly yield of nanoparticles was further validated as a function of temperature. As ev-

idenced by the multitude of examples, at room temperature; assembly of nanoparticles takes hours

to achieve full assembly coverage inside the patterns. Interestingly, at a higher temperature than

room temperature, same assembly coverage can be obtained within 2 min as shown in Figure 5.10d.

We stress that as a result of the increase in temperature, assembly time reduces exponentially. All

of the graphs in Figure 5.10 are obtained by subsequent image processing of 100x magnified fluo-

rescence microscope images using ImageJ program. Measured relative humidity is 52% during all

66


the experiments.

Figure 5.10: Effects of parameters on the assembly. Assembly of 30-nm fluorescent silica particlesin PMMA patterned silicon templates. Template consists of 20 of 20-µm spaced array of 200-nmlines on 15x15 mm Si substrate. a, Efficiency of the assembly was represented as a function oftemperature (T). Assembly time (t) is fixed at 2 min for each point while the temperature was variedbetween room temperature and 75oC. b, Corresponding efficiency of the assembly was shown asa function of the concentration (C). T was set to 75oC and t was set to 2 min for each point. Theresults show how the concentration is effective on the assembly. c, Efficiency of the assembly asa function of t. T is fixed at 40oC for each point, showing no significant change until 10 min. d,T-dependent t for a full coverage. C is fixed at 25 g l−1 at each point while the temperature wasvaried between room temperature and 75oC. Results are representative of at least three independentexperiments (each performed in duplicate) with same prefabricated patterns. Scale bar, 10-µm.

Effect of assembly time in the interfacial convective assembly is shown in Figure 5.11.

These SEM images demonstrate a threshold assembly time for the given conditions.

The forces leading to the formation of nanopatterns are general, and the approach should

be applicable to a large variety of imperfect nanoparticles. To demonstrate the versatility of the

approach in terms of size, and type of particle, we exhibited assembly of diverse nanoparticles such

67


Figure 5.11: Effect of assembly time on the interfacial convective assembly using 30-nm silicaparticles for a)10 min, b) 12 min, and c) 16 min.

68


Figure 5.12: Arrangement of particles. Assembly of gold, PSL, silica and silver nanoparticlesin PMMA patterned silicon templates. a, SEM micrographs of assembled 50-nm gold particlesinside 10-µm-spaced array of 300-nm lines. b, SEM micrographs of assembled 51-nm fluorescentPSL particles in 400-nm lines. c, SEM micrographs of assembled 30-nm fluorescent silica particlesinside 400-nm lines. d, SEM image of assembled 25-nm silver particles in 30-nm width trenches. e,Fluorescence micrograph of the world map consists of 1.2-µm spaced array of 100-nm fluorescentsilica particles, and the inset shows high magnification SEM image of particles in 600-nm diameterholes. Scale bars in the inset figures, 200-nm.

69


as gold, PSL, silica, and silver. Assembly was performed in simple as well as complex geometries.

Figure 5.12a shows SEM micrograph of assembled 50-nm gold particles in PMMA patterned 10-

µm-spaced array of 300-nm lines on Si substrate (see Supplementary Information for the template

fabrication). Figure 5.12b shows SEM micrograph of assembled 51-nm PSL particles in a patterned

10-µm-spaced array of 400-nm lines on Si substrate. Figure 5.12c presents corresponding SEM

micrograph of assembled 30-nm fluorescent silica particles in a patterned 10-µm-spaced array of

400-nm lines on Si substrate. Nanomaterials comprising silver nanoparticles are not only attractive

for novel optical and electrical conducting properties, but are also suitable for addressing funda-

mental structural issues such as enzyme electrode design in glucose biosensor[114] and collection

of closely spaced microelectrodes for the Surface enhanced raman spectroscopy (SERS)[115]. Sin-

gle particle assembly was performed using 25-nm silver particles in 30-nm lines on Si substrate as

shown in Figure 5.12d. In this process, 5 g l−1 PVP coated silver nanoparticles were centrifuged

for 30 min at 13200 rpm. The supernatant fluid was taken out at the end of the centrifuging process;

thereby increase the concentration from 5 to 15 g l−1. The rest of the fluid was mixed using a vortex

mixer and used for the assembly experiments. As might be expected, it is also possible to assemble

nanoparticles over a large area inside complicated geometries. Figure 5.12e exhibits fluorescence

micrograph of the world map consists of 1.2-µm spaced array of 100-nm silica particles, and the

inset shows high magnification SEM image of particles in 600-nm diameter holes.

5.2.1 Effect of evaporation of solvents on the interfacial convective assembly

In order to see the effect of evaporation on the interfacial convective assembly, acetone

(highly volatile and miscible solvent) has been tested and compared with IPA. Acetone is a good

candidate to test as a solvent because the surface tension of acetone is very close to the surface

tension of IPA as shown in Table 5.1 and it spreads over the surface like IPA. The surface gravity of

acetone is also smaller than water.

Silicon templates (please see Figure 3.9) were used for the assembly to test different

solvents since acetone dissolves photoresists including the SU8. The template with a pattern depth

of 3.5 µm were immersed into IPA and sonicated for several seconds to wet the surface (including

the patterns). 1 µm size of fluorescent silica (100 µl, 50 mg/ml, and aqueous solution from Kisker

Inc.) particle solution has been added on top of IPA and covered with a cover glass (cap) (300 µm

gap height between the surface of substrate and the cap). The sample was heated to 50oC over a

hot plate and kept for 5 min. The temperature was chosen below 56oC, since it is the boiling point

70


of acetone. Following the heating process, the cap was taken out and the sample was left to dry by

tilting at ambient conditions (Figure 5.13a). Similarly, acetone has been tested instead of IPA using

the same assembly conditions but partial assembly was observed (Figure 5.13b). Drag force was

calculated for 1 µm size of silica particle using the velocity of 18 x 10−6 m/s. This velocity was

measured using the IPA-water mixture during the in-situ experiments.

Table 5.1: Properties of solvents studied for the effect of evaporation.

Figure 5.13: Comparison of solvents a) IPA and b) acetone to study the effect of evaporation onthe assembly (Exposure time is 1 s for both).

In a different study, assembly result with IPA (Figure 5.14a) has been compared with the

assembly results of using DI and acetic acid instead of IPA. Silicon templates (please see Figure 3.9)

were used for the assembly to test these solvents. The template with a pattern depth of 3.5 µm were

immersed into DI and sonicated for several seconds to wet the surface (including the patterns). 1

µm size of fluorescent silica (100 µl, 50 mg/ml, and aqueous solution from Kisker Inc.) particle

solution has been added on top of DI and covered with a cover glass (cap) (300 µm gap height

between surface of substrate and the cap). The sample was heated to 40oC over a hot plate and kept

for 20 min. Following the heating process, the cap was taken out and the sample was left to dry

by tilting at ambient conditions. There was almost no assembly as seen in Figure 5.14b. Similarly,

71


acetic acid has been tested instead of DI using the same assembly conditions. Figure 5.14c shows

that the assembly rate of acetic acid is much higher than water and but lower than IPA.

Figure 5.14: Comparison of a) IPA, b) DI, and c) acetic acid to study the effect of evaporation onthe assembly (Exposure time is 1 s for all).

5.2.2 Effect of miscibility of solvents on the interfacial convective assembly

In order to see the effect of miscibility on the interfacial convective assembly, chloroform

(highly volatile and immiscible solvent) and toluene (partially volatile and immiscible solvent) have

been tested and compared with IPA (miscible solvent). Chloroform is a good candidate to test as a

solvent because the surface tension of chloroform is very close to the surface tension of IPA and it

is highly volatile as shown in Table 5.2. Besides, it spreads over the silicon surface like IPA. The

surface gravity of chloroform is slightly higher than water.

Silicon templates (please see Figure 3.9) were used for the assembly to test different

solvents since chloroform dissolves photoresists including the SU8. The template with a pattern

depth of 3.5 µm were immersed into IPA and sonicated for several seconds to wet the surface

72


(including the patterns). 1 µm size of fluorescent silica (100 µl, 50 mg/ml, and aqueous solution

from Kisker Inc.) particle solution has been added on top of IPA and covered with a cover glass

(cap) (300 µm gap height between the surface of substrate and the cap). The sample was heated to

50oC over a hot plate and kept for 10 min. The temperature was chosen below 61oC, since it is the

boiling point of chloroform. Following the heating process, the cap was taken out and the sample

was left to dry by tilting at ambient conditions (Figure 5.15a). Similarly, chloroform and toluene

have been tested instead of IPA using the same assembly conditions. However, no assembly was

observed (Figure 5.15b and Figure 5.15c). These experiments have showed that solvents need to

mix each other for the assembly. Drag force was calculated for 1 µm size of silica particle using

the velocity of 18 x 10−6 m/s. For the calculations, the particle velocity was measured using the

IPA-water mixture during the in-situ experiments.

Table 5.2: Properties of solvents studied for the effect of miscibility.

5.2.3 Fabrication of various types of nanostructures

The forces leading to the formation of nanopatterns are general, and the approach should

be applicable to various nanoparticles. To demonstrate the versatility of the approach in terms

of size, and type of particle, we exhibited assembly of diverse materials such as silica and PSL.

Figure 5.16 presents corresponding SEM and AFM micrographs of assembled 30-nm silica particles

in a patterned 10-µm spaced array of 400-nm lines on Si substrate. Figure 5.17 exhibits fluorescence

micrograph of the world map consists of 1.2-µm spaced array of 100-nm silica particles, and the

inset shows high magnification SEMimage of particles in 600-nm diameter vias.

The control of wetting enabled the assembly of nanoparticles into several types of prefab-

ricated PMMA nanopatterns, including trench, via, cross, s-structures with the diameter changing

from 50 to 600-nm. As shown in Figure 5.18, 51-nm PSL particles showed high coverage results

inside the features.

73


Figure 5.15: Comparison of a) IPA, b) chloroform, and c) toluene to study the effect of miscibilityon the assembly (The inset shows the aqueous particle solution on toluene with a high contact angle).

74


Figure 5.16: SEM (on the left) and AFM (on the right) micrographs of assembled 30-nm fluorescentsilica particles inside 400-nm lines.

Figure 5.17: Fluorescent (on the left) and SEM (on the right) micrographs of assembled 100-nmfluorescent silica particles inside 600-nm diameter vias.

75


Figure 5.18: Assembly results of 51-nm fluorescent PSL particles in a) 300-nm width trenches, b)500-nm width trenches, c) 100-nm width trenches, d) diagonal patterns, e) 200-nm diameter holes,f) curved trenches.

5.2.4 Fabrication of nanostructures in hydrophobic surfaces

In order to test the applicability of the interfacial convective assembly on completely

hydrophobic surfaces, PMMA patterned silicon template was coated with FOTS after patterning

(and development). After the coating, contact angle of PMMA increased from 66o to 100o. 30-nm

silica particles were assembled successfully inside the patterned areas as shown in Figure 5.19.

5.3 Theoretical calculations-interfacial convective assembly

In order to calculate the convective drag force, the velocity was measured by analyzing

digital video frames. Particle movement with respect to the fluid on top of the photoresist was

recorded using a Charge-coupled device (CCD) camera. Figure 5.20a shows the displacement of a 3-

µm silica particle in 1.47 s at room temperature. The velocity found was 73x10−6 m/s. Figure 5.20b

shows the displacement of a 1-µm silica particle in 2 s at room temperature. Its velocity was

18x10−6 m/s. Figure 5.20c shows the displacement of a 0.5-µm silica particle in 5 s at room

temperature. Its velocity was 8x10−6 m/s. Experimental results agreed well with the previous

theoretical studies[110].

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Figure 5.19: Assembly result of 30-nm silica particles on FOTS coated (hydrophobic surface),PMMA patterned silicon template.

In contrast to the microparticles, 30-nm fluorescent silica particles were incorporated into

20-µm spaced array of 200-nm lines in hours at room temperature since small particles have low

inertia. There is an ongoing challenge in the fluid-driven assembly of nano-scale particles due to the

nature of dominant forces corresponding to the particle size. To ascertain the assembly mechanism,

there is a need to calculate the forces acting on the particles. In terms of micro-scale, gravity is

dominant as a deterministic force, and the magnitude of this force is given by Fgrav = 1/6 Π a3

(ρ2-ρ1)g, where ρ1 and ρ2 refer to the densities of the medium and the particle, respectively. g

is the gravitational acceleration constant and a is the particle diameter. Considering the forces in

the nano-scale, the Brownian force becomes dominant (see Figure 5.22 for details) and the mag-

nitude of the Brownian force was modeled as a Gaussian white noise process by Li[82], FB = Gi√((12ΠaµkBT )/∆t), where µ is the dynamic viscosity of the medium, kB is the Boltzmann con-

stant (1.38x10−23 J/K), T is the absolute temperature. Gi is zero-mean, unit variance independent

Gaussian random numbers and ∆t is the time used in the calculations.

Besides to the gravitational force and the Brownian force, in practice, the drag force is also

effective on the particle due to the convective drag flow driven by IPA evaporation. The fluid exerts

a drag force on the particle that affects the velocity of the particle. If the fluid is in motion, then the

drag force pulls the particle along. When a particle is moving relative to the fluid, it experiences

a viscous drag force due to the action of the fluid on the particle. Stokes solved the equations of

motion of a rigid sphere in a fluid in the laminar regime. The formula calculates the drag force on a

sphere of diameter a moving steadily in a fluid with velocity; V. Using the boundary condition, the

magnitude of the drag force is given by FD = 3Π µaV, where V is the relative velocity between the

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Figure 5.20: Velocity measurement of different size of colloidal fluorescent silica particles insideDI \ IPA mixture at room temperature; a) 3-µm size of particle moves between t = tosec and t =(to+1.47)sec with Vparticle = 73x10−6 m/s, b) 1-µm size of particle moves between t = tosec and t= (to+2)sec with Vparticle = 18x10−6 m/s, c) 0.5-µm size of particle moves between t = tosec and t= (to+5)sec with Vparticle = 8x10−6 m/s. to is the initial time.

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Figure 5.21: Velocity measurement of different size of colloidal fluorescent silica particles insideDI \ IPA mixture at room temperature. Velocity of particle increases with increasing particle size.

Figure 5.22: Governing forces for different silica particle sizes in water and IPA. Velocity usedhere for drag forces are based on measured velocities shown in Figure 5.20.

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Figure 5.23: Capillary force for different silica particle sizes in water.

particle and the fluid. This equation is valid for Re < 1. Drag coefficient is CD = FD/(0.5 ρ V2 A)

where ρ is the fluid density and A = Π a2/4 is cross sectional area of the spherical particle. Velocities

used here for drag force calculations are based on measured velocities shown in Figure 5.20. For

particles less than 500 nm, velocity of 500 nm size of particles has been used for the calculations

because of the limitations of optical imaging during in situ experiments.

The root-mean-square velocity (VN ) of a Brownian particle can be calculated as[83] νN =√((3kbT )/m) = 1/a

√((18kbT )/Πρa) (see Supplementary Information for the force calculations

in Table 1). A Brownian-Reynolds number (Re) based on the Brownian velocity leads to Re = 1/ν√((18kbT )/Πρa) where ν is the kinematic viscosity of the liquid.

Another governing force on interfacial convective assembly is the capillary force which

is shown in Figure 5.23.

5.4 Seed layer deposition using nanoparticles on ceramic surfaces for

the printed electronics application

Printed electronics[116] are used in industrial applications such as radio frequency iden-

tification (RFID)[117]. Inkjet printing[118], electroplating[119] and atomic layer deposition[120]

are commonly used techniques for the printed electronics. Atomic layer deposition method uses gas

for a chemical process in a high vacuum chamber to form slowly a thin film. Inkjet printing method

uses electrically functional electronic or optical inks to print patterns on various surfaces. Resolution

is limited in this method. Electroplating method uses electrical current to reduce dissolved metal

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cations to form a metal coating on an electrode. Electroplating is typically performed on conducting

surfaces. To electroplate on non-conductive surfaces, a conductive seed layer is needed. The seed

layer should have good adhesion to the non-conductive surface and the electroplated film. Interfa-

cial convective assembly technique has been used to deposit the seed layer on ceramic surfaces in

this study.

Before starting the process, contact angle of ceramic substrates were measured as shown

in Figure 5.24. AlN is more hydrophobic than Al2O3. In order to increase the surface energy, the

samples were either cleaned in piranha solution or treated by O2 plasma. Different solvents such

as N-methyl-pyrrolidone (NMP) and Dicholoromethane (DCM) were also tested to clean AlN and

Al2O3. (Figure 5.25). Surface characterization has been performed on both AlN and Al2O3 using

SEM and AFM (Figure 5.26).

Suspension preparation: 40-nm diameter of metal basis copper nanowpowder (from US

research nanomaterials, Inc) was dispersed in DI using the probe sonicator and different concen-

trations such as 1 wt%, 2 wt% and 5 wt% were prepared. No other chemical was included in the

solution. Sonication time was 30 min.

Sample preparation: Ceramic samples were cleaned using piranha solution for 5 min.

Following the piranha cleaning process, samples were rinsed under running DI and dried with N2.

Assembly process: Ceramic samples were wetted using IPA (Figure 5.27). Copper nanopar-

ticle suspension (100 µl) was placed on top of the solvent and the assembly process has been initi-

ated over a hot plate at 75oC. Assembly process was performed in 7 min.

A uniform assembly was obtained successfully over all the sample surface as shown in

Figure 5.28. Samples were cut using liquid N2 to image their cross-section view. Tilted-view images

are shown in Figure 5.29.

Figure 5.24: Contact angle measurements on the AlN and Al2O3 surfaces.

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Figure 5.25: Contact angle measurements on the AlN and Al2O3 surfaces after the cleaning processusing solvents such as NMP and DCM.

Figure 5.26: Surface characterization on the a) Al2O3 and b) AlN surfaces using SEM and AFM.

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Figure 5.27: Interfacial convective assembly process on ceramic with a rough surface.

Figure 5.28: Top-view SEM images of 40-nm copper particles (5 wt%) assembled on Al2O3.

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Figure 5.29: Tilted-view SEM images (from different regions) of 40-nm copper particles (5 wt%)assembled on Al2O3.

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Oxidation of copper nanoparticles[121] is an important issue needs to be addressed for

the applications. In order to eliminate the effects of oxidation after the assembly process, copper

nanoparticles were thermally annealed in H2/N2 (5%/95%) medium at 500oC for 30 min using a

conventional furnace. Color of assembled copper nanoparticles has changed from black to reddish

brown as shown in Figure 5.30. Copper oxide is reduced to copper metal (Equation 5.1)

CuO +H2 → Cu+H2O (5.1)

Figure 5.30: Assembled copper nanoparticles on Al2O3 a) before and b) after the thermalannealing.

Electrical characterization: Following the thermal annealing process, four point probe

measurements were performed on the annealed copper nanoparticles as shown in Figure 5.31. The

bulk resistivity (ρ) is calculated using Equation 5.2 where Rs is the sheet resistance and t is the film

thickness. Obtained bulk resistivity was 15 times higher than the bulk copper resistivity.

Figure 5.31: a) Electrical measurements on the assembled 40-nm copper nanoparticles (after theannealing) using the four point probe. b) Sheet resistance of electroplated copper film on the seedlayer deposited on SiO2 surface [122].

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ρ = Rs × t (5.2)

5.5 Fabrication of three-dimensional (3-D) nanostructures by interfa-

cial convective assembly

5.5.1 Interconnects

The presented assembly technique also enables fabrication of nanostructures for inter-

connect applications. 5-nm of silver particles were succesfully assembled into PMMA patterned

Au surface in 3 min at 60oC and they were formed into nanostructures. To show that fabricated

nanostructures have promises in interconnects, we measured and demonstrated the electrical char-

acteristics of our fabricated nanopillars using an SEM-based in situ Zyvex S-100 nanomanipulator.

On the basis of our measurements, the lowest resistivity (calculated) for 50-nm in diameter and 150-

nm in height of silver NP-based pillar (Figure 5.32a) is 346 µΩ.cm (Figure 5.32b), which is an order

of magnitude 2 times higher thank the bulk resistivity of silver (1.59 µΩ.cm). This is adjunct to the

nanoscale dimensions of fabricated structures. The difference might be due to structural defects

and discontinuties in the pillar, variations in the pillar diameter and electron scattering from grain

boundaries. When the dimensions of metals are in the range of the mean free path (∼51 nm for

silver)[123], the resistivity of metals becomes higher than their bulk resistivity[124] due to electron

scattering from grain boundaries and interfaces[125].

5.6 Fabrication of three-dimensional (3-D) nanostructures by dielec-

trophoretic assembly of nanoparticles

5.6.1 Introduction

With the scaling down of devices and Integrated circuit (IC)s, it becomes increasingly

challenging to fabricate such interconnects using convectional techniques. For example, filling

high aspect ratio structures below 16 nm is challenging. As patterns shrink, etching, cleaning, and

filling high aspect ratio structures will be challenging, especially for low-k dual damascene metal

structures and Dynamic random access memory (DRAM) at nano-dimensions. Three-dimensional

control of interconnect structures will be required. To overcome these challenges, International

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Figure 5.32: Surface and electrical characterization of the nanopillars. a, SEM image ofNP-based silver nanopillars; inset: high resolution SEM image. b, Demonstration of the I-V char-acteristic of fabricated silver nanostructures.

technology roadmap for semiconductors (ITRS) outlines the necessity of developing alternative

novel fabrication techniques that enable to be scaled down. In practice, common materials used for

interconnects include tungsten ([126], [127]), copper ([128], [129]), and silver ([130], [131]).

Copper is used for local, intermediate and global interconnection and fabricated using

electroplating. In addition, tungsten is desirable in interconnects. Because it experiences low

electromigration[126] and has advantage on metallization process and air gap formation. Tungsten

interconnects are fabricated using Chemical vapor deposition (CVD) and used for contact plugs,

vias, and local metallization[127]. However, it is significantly challenging to create high aspect ra-

tio tungsten interconnects[132]. There have been newly developed technologies to make nanoscale

and high aspect ratio W interconnects. Common processes to fabricate tungsten nanorods and

nanowires include methods such as vapor phase synthesis[133], hydrothermal mechanisms[134],

thermal-evaporation[135], and various CVD ([136], [137], [138], [139], [140], [141], [142], [143]).

Common fabrication methods for producing silver nanorods and nanowires include seed-mediated

growth ([144], [145]), chemical reduction synthesis[146], Solid-liquid phase arc discharge method

(SLPAD)[147], UV photoreduction technique[148], and CVD[149]. While commonly used, most of

these methods require unique conditions such as high temperature, high vacuum and the addition of

chemicals. There are also challenges in fabrication of copper interconnects[150]. Line and via side-

wall roughness, intersection of porous low-k voids with sidewall, barrier roughness will adversely

affect electron scattering in copper lines and cause increases in resistivity. Line edge roughness,

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trench depth and profile, via shape, etch bias, thinning due to cleaning, Chemical mechanical pol-

ishing (CMP) effects. Combinations of materials and processes used to fabricate new structures

create integration complexity. 3D chip stacking circumvents of traditional interconnect scaling by

providing enhanced functional diversity. Engineering manufacturable solutions that meet cost tar-

gets for this technology is a key interconnect challenge according to the 2011 ITRS[151]. Therefore,

silver is one of the metal that is considered for replacing copper because silver has a high electrical

conductivity[130] which is useful for micro-interconnects as resistivity increases with decreasing

linewidth ([127], [140]). Due to issues related to fabrication introduction and added cost Ag will

have to show significant improvements vs. Cu at narrow line dimensions for industry adoption to

occur according to the 2011 ITRS.

We have recently demonstrated that a new manufacturing technique, based on directed as-

sembly of nanoparticles, which can produce metallic interconnects down to 25 nm and below[152].

This process is performed at ambient conditions without using the chemical additives therefore

making it highly suitable for cost-effective, green nano-manufacturing. In addition, we have shown

that produced gold nanopillars have polycrystalline nature, have an electrical resistivity that is lower

than or equivalent to electroplated gold. In this work, we demonstrated that tungsten and silver in-

terconnects can be fabricated using the previously reported technique. Aqueous silver and tungsten

nanoparticles were assembled into pre-patterned vias and fused into solid nanostructures down to

40 nm with high aspect ratio and smooth sidewalls. We acquired the resistivities of tungsten and

silver from the measurements. Resistivity values for nanorods and nanowires depend on the method

of fabrication. Tungsten nanorods have a reported electrical resistivity from between 1-4 orders of

magnitude above the bulk resistivity, varying highly with fabrication method ([137], [138], [140],

[141], [153], [154]). Silver nanorods have reported electrical conductivity between 1-2 orders of

magnitude above the bulk resistivity ([145], [149]). This method of fabricating tungsten and silver

nanorods without the use of additional chemicals and at ambient conditions. The method used sim-

ply includes e-beam lithography and directed electrical assembly technique. This process has been

shown to have prior success with the fabrication of nanostructures with other materials[152].

5.6.2 Experimental setup for the assembly

Conventional micro/nano fabrication and e-beam lithography techniques were performed

to fabricate the templates with vias of various dimensions and arrays. Figure 5.33 shows schematics

of the template and directed assembly process. A 5 nm of chrome (Cr) and 100 nm of gold (Au)

88


were sputtered onto a SiO2/Si (500 nm/380 µm) wafer, which was then diced into 15 mm x 15 mm

chips. Subsequently, 150 nm thick of PMMA was spin-coated at 5000 rpm for 1 min onto the pre-

cleaned (H2SO4/H2O2, 2:1) Cr/Au surfaces. E-beam lithography was conducted on the template

using a Supra 25 SEM with a beam blanker and controlled by a J.C. Nabity e-beam generation

system.

Figure 5.33: A schematic of the template and directed assembly process.

After the exposed PMMA was developed in MIBK and IPA (1:3), the patterned template

was placed parallel to the counter electrode so that the metal sides were facing each other and con-

nected to a function waveform generator. These chips were submerged into the aqueous nanoparticle

solution, and an electric field was applied to it[152]. The aqueous tungsten Nanoparticles (NPs) sus-

pension was purchased from Meliorum Technologies (nominal diameter: 10 nm, the zeta potential:

-53.3 mV). The aqueous silver NPs suspension was purchased from nanoComposix, Inc. (nominal

diameter: 10 nm, the zeta potential: -45.2 mV). The assembly conditions of tungsten and silver NPs

were given in Table 5.3. The counter and template were removed from the suspension at a con-

trolled speed of 85 mm/min using a KSV NIMA dip coater. Electrical characterization was carried

out inside the SEM with the help of a ZYVEX (S100) Nanomanipulator. Tungsten probes with a

tip radius of 20 nm were used to contact tungsten and silver nanostructures to the gold electrode

separately.

5.6.3 Fabrication process of 3-D structures

The fabrication process of 3-D nanostructures involves directing colloidal nanoparticles,

using DEP[70], toward a template. The template is a substrate comprising a conductive film coated

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Parameter Tungsten SilverConcentration (wt%) 1 0.06AC voltage (V) 12 12DC voltage (V) 2 0Frequency (kHz) 50 50Pulling speed (mm/min) 85 85pH 10 6.7Time (s) 450 270-450

Table 5.3: Assembly condition for tungsten and silver interconnects

by an insulator such as PMMA that has nanoscale patterns such as vias, as shown in Figure 5.34a.

An AC electric field is applied between the template and a counter electrode positioned 5 mm away

from the template in the particle suspension. The electric field creates a dielectrophoretic force on

the particles, moving them toward the vias where the magnitude of electric field is highest[152]. The

surface charge of the particles would create an induced dipole moment in the ionic atmosphere[155].

As the particles assemble, the applied electric field induces their fusion, forming arrays of solid

nanostructures (Figure 5.34b). The fusion of small colloidal chains into structures such as wires

under an applied electric field arises from localized joule heating, induced by the applied AC voltage

at the nanoparticle junctions[156]. Following the assembly and fusion process, the insulator layer

is removed (if necessary) to obtain freely standing 3D nanostructure such as nanopillars as shown

in Figure 5.34c.

5.6.4 Fabrication of various types of metallic 3-D nanostructures

In addition to gold, other metal nanopillars can also be fabricated on a gold or any con-

ductive surface using similar assembly parameters used for fabrication of the gold nanopillars. 5 nm

gold nanoparticles with 1.5×1011 of concentration were used to fabricate gold nanopillars with 80

nm diameter and 1 µm space (Figure 5.35a). Ammonium hydroxide (NH4OH) was added to adjust

the pH and zeta potential. Measured pH of the solution was 10. 12 Vpp was applied at 50 kHz for 1

min to create gold nanostructures.

Aluminum (Al) nanoparticles (nominal diameter: 5 nm, 30 mg/50 ml) were used to fab-

ricate Al nanostructures with 80 nm diameter and 5 µm space (Figure 5.35b). Assembly was per-

formed without the addition of NH4OH in 90 s. Zeta potential, conductivity and of the solution are

-65.6 mV, 0.353 mS/cm and 9.96 respectively.

Silver (Ag) nanoparticles (nominal diameter: 5 nm, 1 wt%) were used to fabricate Ag

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Figure 5.34: Fabricating 3D nanostructures through electric field-directed assembly of nanopar-ticles. (a,b) Nanoparticles suspended in aqueous solution are (a) assembled and (b) fused in thepatterned via geometries under an applied AC electric field. (c) Removal of the patterned insulatorfilm after the assembly process produces arrays of 3D nanostructures on the surface. Source ([152]).

nanostructures with 40 nm diameter and 1 µm space (Figure 5.35c). Assembly was performed

without the addition of NH4OH in 3 min (power was turned off for 10 s in each 90 s). Zeta potential,

conductivity, and pH of the solution are -45.2 mV, 0.615 mS/cm and 10 respectively.

Tungsten (W) nanoparticles (nominal diameter: 5 nm, 1 wt%) were used to fabricate W

nanostructures with 40 nm diameter and 1 µm space (Figure 5.35d). 12 Vpp with 2 VDC offset was

applied at 50 kHz for 7.5 min (power was turned off for 10 s in each 90 s) to create 3D tungsten

nanostructures.

Desired pillar geometry on a template can be achieved, with the same pillar dimensions,

by adjusting the spacing between the vias. The fabricated nanopillars have smooth side walls for

all dimensions fabricated as shown in AFM image (Figure 5.36). The high-magnification in SEM

images Figure 5.35c) and Figure 5.35d) show that the aspect ratios of the pillars could be adjusted

by controlling the diameter and depth of the vias.

The results show that the frequency and amplitude of the applied voltage can be adjusted

to fabricate a uniform arrays of nanopillar for different via geometries. To achieve a successful

particle assembly and chaining in the vias, the DEP force needs to be higher than a certain threshold.

When a lower voltage (6 Vpp, 50 kHz) is applied, the DEP and chaining forces decrease, resulting

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Figure 5.35: SEM images of fabricated 3D nanopillars made of a) gold, b) aluminum, c) silver, andd) tungsten nanoparticles.

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Figure 5.36: AFM image of fabricated 3D silver nanopillars.

in a partially assembled and fused particles in the vias (Figure 5.37a). On the other hand, the

application of a higher voltage ( 20 Vpp, 50 kHz) increased the assembly rate but resulted in over

deposition on the PMMA surface (Figure 5.37b). The frequency of the applied voltage can be

controlled to achieve a successful nanopillar formation (Figure 5.37c and Figure 5.37d). Assembly

process can be controlled by changing the assembly time (Figure 5.37e). The ability to adjust the

nanopillar height by changing the assembly time might be important for applications requiring high

aspect ratios[157].

Using the presented directed assembly method, nanopillars made of various materials such

as tungsten and silver can be fabricated on any conductive surface as shown in the SEM images

(Figure 5.38). Control of the governing parameters enabled successful fabrication of nanopillar

arrays down to 25 nm with controlled dimensions, high aspect ratio, and good uniformity.

5.6.5 Fabrication of ZnSe nanostructures

As a side work to show the capability of the method, semiconducting materials such

as Zinc selenide (ZnSe) with either n or p dope were studied for the assembly on ITO on glass

substrates. The resistance of ITO film on glass surface is ∼15 Ω. After the optimization of the

assembly parameters such as voltage, frequency, concentration, optimized assembly results were

showed in Figure 5.39 for n-, p-, and both respectively.

5.6.6 Fabrication of composite 3-D nanostructures

To demonstrate the capability of nanoscale composite structures, we fabricated gold-

fluorescent Quantum dots (QD) (Figure 5.40). Fabrication of the structures requires precise control

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Figure 5.37: a, b, Assembly processes performed at different applied voltages. Frequency: 50 kHz;assembly duration: 90 s. (a) Applied voltage: 6 Vpp. (b) Applied voltage: 20 Vpp. Inset: High-angleSEM image of an assembly area indicating high assembly rate in the vias. c, d, Assembly processperformed at different applied frequencies. Applied voltage: 12 Vpp; assembly duration: 90 s. (c)Applied frequency: 100 kHz. (d) Applied frequency: 10 kHz. Inset: Higher-magnification SEMimage of the silver assembly area after PMMA removal, revealing particle over-deposition on thegold surface. The scale bars in the insets are 20 nm.

Figure 5.38: SEM images of interconnects made of (a) tungsten and (b) silver.

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of nanopillar height within the via. To achieve a desired nanopillar height, we estimated the assem-

bly rate of gold nanoparticles by simply altering the assembly time (other parameters such as 12

Vpp and 50 kHz were kept constant).

The ability of controlling the pillar height enabled us to obtain any desired height for the

gold part of nanopillar by adjusting the assembly time. Following the formation of gold segment,

we assembled fluorescent quantum dots into the remaining part of the via. Assembly conditions for

the quantum dots part; 12 Vpp with 2.5 VDC offset was applied at 10 kHz for 10 s. The resulting

composite nanostructure is clearly shown by the fluorescent microscopy images in Figure 5.40b.

Joule heating is calculated using the electrical resistance and the current passing through

the contact.

Q = I2pRct (5.3)

where I is the magnitude of the electric field and t is the time period of the current passing

through the particle-surface contact. Joule heating causes an increase in nanoparticle temperature

that is calculated by[152]

4T =3I2pρcz0t

4π2cv,pa2r3(5.4)

where4T is the temperature difference and the surface, cv,p is the specific heat capacity

of bulk particle material and z0 is the separation distance between the particle. z0 is typically 0.4

nm[158].

The contact radius, a between the particle and the surface due to the adhesion-induced

deformation is needed to calculate the Joule heating at nanoparticle junctions. The contact radius

can be calculated using the Maugis and Pollock (MP) model[159].

5.6.7 Fabrication of 3-D nanostructures on flexible substrates

Electric field directed assembly method is applicable to both hard and soft substrates.

A Polyethylene terephthalate glycol-modified (PETG) film (0.5 mm of thickness) coated with a

150-200 nm of ITO (conductive, 19 Ω) has been used as a substrate. 150 nm of PMMA has been

coated following the oxygen plasma treatment (to increase the adhesion between the surface and

PMMA) on the substrate and to clean the surface from the organics). E-beam lithography has been

performed on the sample. 5 nm of gold nanoparticles were assembled and fused using the electric

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Figure 5.39: (a) Fabricaton of n-type ZnSe nanopillars on ITO on glass surfaces. Applied voltage:12 VPP and 0 VDC ; frequency: 20 kHz. Assembly duration: 5 times 90 s with 5 s off in between.(b) Fabricaton of p-type ZnSe nanopillars. Applied voltage: 12 VPP and 0 VDC ; frequency: 10kHz. Assembly duration: 5 times 90 s with 5 s off in between. (c) Fabrication of composite ZnSenanopillars.

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Figure 5.40: Fluorescent images of assembled a) gold, b) gold-fluorescent QD particles, and c)their top view SEM image.

field directed assembly method on the patterned regions to fabricate 3D gold nanostructures as

shown in Figure 5.41.

5.6.8 Electrical characterization of fabricated 3-D nanostructures

To demonstrate that fabricated nanostructures have promises in the interconnects, the elec-

trical characteristics of our manufactured nanopillars were performed using an SEM-based in situ

Zyvex S-100 nanomanipulator. We compared the electrical characteristics of our fabricated tungsten

and silver nanopillars with other reported nanostructures fabricated by conventional methods such

as CVD[136] and seed-mediated growth[144]. CVD is one of the most widely used method for the

fabrication of tungsten nanowires and for uniformly filling vias in integrated circuits. Put simply,

CVD is the technique of releasing a gas precursor onto a heated substrate. In the process, a solid

deposit forms. Some processes induce the reaction by shooting a focused ion beam at the substrate,

but this fabricates wires that are not pure tungsten[136] and, according to Li, et al., this impurity is

the root cause of the nanowires having a resistivity higher than tungstens bulk resistivity[138]. In

addition, when the dimensions of metals are in the range of the mean free path, the resistivity of met-

als becomes higher than their bulk resistivity[124] due to electron scattering from grain boundaries

and interfaces[125]. Thus, the bulk resistivity is not suitable to characterize the electrical properties

of these nanostructures.

We performed these measurements using two tungsten probes having a tip diameter of 20

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Figure 5.41: Assembly of Au nanoparticles on a PETG substrate.

nm; one of the probes contacted the nanopillars while the other contacted the thin gold layer under

the PMMA. The pillars either broke or bended readily when the measurements were performed after

the PMMA layer had been removed. Therefore, we did not remove the PMMA layer during the I -

V measurements; that is, we monitored the encapsulated nanopillars as shown in Figure 5.42. The

PMMA layer also served as a dielectric barrier during the measurements, preventing any possible

current leakage between the probe and the underlying gold surface.

The contact between the probe and the nanopillar effects the reliability of the measure-

ments. In our experiments, achieving good contact (slight penetration into pillars) between the

probe and a nanopillar was difficult due to the small size of pillar diameter. Indeed, we obtained

large variations in resistance (from tens of Ω to hundreds of kΩ) for small size nanopillars depending

on the quality of the contact.

The observed I-V characteristic of W nanopillars was plotted in Figure 5.42a besides the

SEM inset image. The result was compared with the other conventional processes in Table 5.4

to demonstrate that these fabricated nanostructures are promising in nanoelectronics. These pro-

cesses require the use of chemicals, high temperature, high vacuum and the process is slow. The

tungsten interconnects reported in this study measured a resistivity value of 2900 µΩ.cm, an or-

der of magnitude 2 times the bulk resistivity (5.28 µΩ.cm)[160]. Tungsten interconnects showed

higher resistance than the bulk value because of the oxidation([161], [162]). As mentioned earlier,

when the dimensions of metals are in the range of the mean free path (∼41 nm for tungsten)[163],

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the resistivity of metals becomes higher than their bulk resistivity. The resistivity of each metal is

calculated by using the equation, Equation 5.5:

Table 5.4: Electrical properties of tungsten nanostructures

Fabrication method Diameter (nm) Length (µm) Resistivity (µΩ.cm)FIB CVD[138] 19 10 200FIB CVD[141] 900 30 3000FIB CVD[164] 215 80x10−3 55

Damascene CVD[140] 20 132 13EBI CVD[154] 200 110x10−3 19000

Electrodeposition[165] 40 10 50Directed assembly 150 0.15 2900

ρ = RA

l(5.5)

where ρ is the resistivity, R is the resistance, A is the surface area, l is the length of the

nanopillar.

Figure 5.42: Electrical current plotted as a function of the applied voltage for the probe/PMMAconfiguration as shown in the SEM inset for (a) tungsten and (b) silver interconnect.

Similarly, the measured I-V characteristic of Ag nanopillars was plotted in Figure 5.42b

beside the SEM image. The measured resistivity of these silver interconnects reported a value of

87.9 µΩ.cm, an order of magnitude times larger than the bulk resistivity (1.59 µΩ.cm) (20). The dif-

ference in resistivity might be because of the structural imperfections and discontinuties in the pillar,

and electron scattering from grain boundaries. As mentioned earlier, when the dimensions of metals

are in the range of the mean free path (∼52 nm for silver) (43), the resistivity of metals becomes

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Figure 5.43: Electrical characterization set-up and results from the composite ZnSe nanostructure.a) Agilent station, Zyvex nano manipulator, and SEM were all connected to each other for theelectrical measurements, b) Probes with nanotips were placed inside the SEM, c) The probe wasin contact with the fabricated ZnSe nanostructure, d) I-V measurements of ZnSe nanostructures forvarious voltages.

100


higher than their bulk resistivity. The result was compared with other conventional processes in Ta-

ble 5.5 to demonstrate that these fabricated nanostructures are promising in nanoelectronics as the

measured values are coherent with the other methods such as CVD and seed-mediated growth. The

other electrical measurements were performed using either the two terminal electrode method[166]

or four-point probe method[145].

Table 5.5: Electrical properties of silver nanostructures

Fabrication method Diameter (nm) Length (µm) Resistivity (µΩ.cm)EBI CVD[166] 45-110 60 33

Seed-mediated growth[145] 40 20 12.5Directed assembly 115 0.15 87.9

101

Chapter 6

Conclusion and Future Work

In this chapter, we conclude the presented work and present suggestions for future re-

search:

• 5.1 Conclusions

• 5.2 Future Work

6.1 Conclusions

Here, a novel assembly technique called interfacial convective assembly, in which col-

loidal particles are selectively and simultaneously assembled inside the prefabricated surfaces, was

presented. Using this method, we assembled various types of micro and nanoparticles such as silica,

PSL, gold and silver on topographical structures. We demonstrated that this assembly technique is

scalable up to several square inch area in a few minutes and is capable of fabricating nanostructures

with line widths down to 25-nm. This scale is more than two times smaller than previously reported

convective driven nanoparticle assembly/printing studies. We performed in situ studies using mi-

croparticles to establish a better understanding of the process. In situ experiment results show that

colloidal particles reach the bottom surface of the inner and outer structured region in different time.

During the process, particles migrate toward the substrate surface, and once the solvent inside the

structures starts evaporating, particles are driven together because of the difference in the surface

tension, evaporation rate, and specific gravity between the solvent and water. Tuning the parameters

precisely led to the well-organized nanoparticles in remarkably complex shapes such as the world

map. The interfacial convective assembly developed here shortens the processing time at least by a

102

CHAPTER 6. CONCLUSION AND FUTURE WORK

factor of ten compared to the convective self-assembly approach. This assembly approach will facil-

itate fabrication of a large family of nanostructures, including a wide flexibility in the composition,

size and shape of the constituent nanoparticles.

• Particles from 5 nm to 3 µm size in diameter were assembled successfully by interfacial

convective assembly method.

• Assembly of spherical shaped particles in stable and aqueous solution result in a shape closer

to the shape of pattern than the random shaped particles in unstable solution for the interfacial

convective assembly.

• Interfacial convective assembly has been performed on silicon, glass, gold, and ceramic sur-

faces.

• In situ experiments have showed that time for a particle to reach the bottom of pattern takes

much longer than the time for a particle to reach top layer of the pattern.

• Efficiency of interfacial convective assembly increases exponentially with increasing temper-

ature.

• Efficiency of interfacial convective assembly increases exponentially with increasing concen-

tration.

• Miscible and highly (or non-) volatile solvents are not suitable for the interfacial convective

assembly.

• Immiscible solvents are not suitable for the interfacial convective assembly.

• Interfacial convective assembly is applicable to different geometry of patterns and does not

depend on orientation of patterns.

• Interfacial convective assembly could work on hydrophobic and hydrophilic surfaces.

• DCM is a good solvent to clean ceramic surfaces.

• Uniform thin film deposition was obtained successfully by interfacial convective assembly on

rough surfaces such as AlN and Al2O3.

103


• Assembled copper nanoparticles (by interfacial convective assembly method) as a thin film

could be conductive after annealing process in inert environment. Obtained bulk resistivity

was 15 times higher than bulk copper resistivity.

• Assembled tungsten nanoparticles (by interfacial convective assembly method) could be used

as an interconnect. The tungsten nanopillars reported in this study measured a resistivity

value of 2900 µΩ.cm, an order of magnitude 2 times the bulk resistivity (5.28 µΩ.cm).

• Assembled silver nanoparticles (by interfacial convective assembly method) could be used as

an interconnect. On the basis of our measurement, calculated resistivity of assembled silver

was an order of magnitude 2 times higher than bulk resistivity. It is mainly because of the

grain boundaries. It could be improved by annealing assembled nanoparticles.

By using the electric field directed assembly method, the control of the deriving forces

led to a precise and repeatable assembly of various types of nanoparticles onto the desired loca-

tions of surfaces. In contrast to conventional methods such as electroplating or thin film deposition,

this method can fabricate solid nanostructures from nanoparticles made of conducting and semicon-

ducting materials at room temperature and pressure without the need for a seed layer and chemical

additives. The method is fast, precise, and can be used to fabricate a variety of 3D nanostructures

with feature sizes down to 40-nm (depends on the resolution of patterning) in less than a minute over

a large area. The pillars were fabricated uniformly from gold, tungsten, aluminum, silver, doped

ZnSe, and QD.

• Control of the governing parameters enabled successful fabrication of nanopillar arrays down

to 25 nm with controlled dimensions, high aspect ratio, and good uniformity.

• Fabrication of 3D nanostructures was performed by the electric field directed assembly method

on both the flexible and hard substrates.

• Hybrid 3D nanostructures were fabricated using the materials such as gold and QD; p type

ZnSe and n type ZnSe.

• The tungsten interconnects reported in this study measured a resistivity of 2900 µΩ.cm, an

order of magnitude 2 times the bulk resistivity.

• The measured resistivity of silver interconnects reported a value of 87.9 µΩ.cm, an order of

magnitude times larger than the bulk resistivity.

104


• I-V characterization of the composite ZnSe nanostructure shows semiconducting behavior

above 1 V. On the other hand, it has metallic behavior below 1 V.

6.2 Future Work

The ability to perform rapid assembly of various nanoparticles can pave the way for fab-

ricating nanoparticle-based interconnects for the IC. The assembly and electrical characterization

results, presented in this work, show the potential for further development of this nanoparticle-based

chips. Some suggestions for the future research are as follows:

• Annealing of assembled silver nanoparticles.

• Performing electrical characterization on the annealed silver nanostructures.

• Electroplating of silver to fabricate nanopillars.

• Performing electrical characterization on the electroplated silver nanostructures.

• Comparing the electrical measurement results of assembled and electroplated silver nanos-

tructures.

• Analyzing numerically the interfacial convective assembly technique.

• Comparing the experimental results of interfacial convective assembly technique with numer-

ical analysis.

• Applying electric field during interfacial convective assembly to see effect of electric field on

assembly process.

105

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