Week 3: Patterning nanostructures and transferring to … · 2020. 11. 18. · Considerations for Determining Tools for ... • General characteristics - Diffraction is not a limitation

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Mechanical & Industrial Engineering Department, Louisiana State University

NIH P41 Center for BioModular Multi-Scale Systems for Precision Medicine

Sunggook Park

Week 3: Patterning nanostructures and transferring to thermoplastics

November 16, 2020

Sunggook Park

L.R. Daniel ProfessorMechanical & Industrial Engineering Department

and Center for BioModular Multiscale System for Precision Medicine (CBM2),Louisiana State University

“Thermoplastic Microfluidics and Nanofluidics Webinar” Webinar Series



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Contents

• Patterning nanostructures for nanomolding molds

• Transfer nanostructures into plastics using nanoimprint lithography

- Fundamental limitations and practical guidance for successful molding

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• Introduction to nanomolding



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equipment for hot molding of waffles- use of thermoset "polymers"- parallel (small scale) production

hot embossing carousel with 4 stations

materials stamps

coating

process optimization

throughputyield

automation

rheology

tools

applications and demonstrators

Nanomolding - As Easy As Kitchen Technology3

Slide from Dr. H. Schift, PSI, Switzerland



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Nanoimprint ProcessPolymer coating

Embossing

Polymer filmSubstrate

Heating T >Tg

Stamp

Cooling T < Tg

Demolding

New materials for resists

Stamps fabrication

Resist rheology

Anti-adhesive layers

New applications

Processcontrol

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Contents




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milli micro nano

10-3 10-6 10-9

Typi

cal

obje

cts

mechanicalmachining optical lithography beam lithography

SPM lithographyself organisation

R e p l i c a t i o n

mresolution

LIGA gears chip transistorSHT

moleculeswatch parts

lighte- beam

Nanoimprint Lithography

Small Scale Fabrication Technologies20



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Considerations for Determining Tools for Nanomold Fabrication

• Direct vs. resist-based nanopatterning• Compatibility with fabrication tools for micro to macro structures• Materials

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- Si preferred due to the availability of high accuracy post-processing tools (i.e. RIE)- Glass-based materials and metals depending on fabrication protocols.

(1) Structure requirement

• 2D vs 3D nanopatterning • Patterning area (serial vs. parallel patterning)

(2) Process compatibility

• Availability of tools• Costs

(3) Other considerations



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Considerations for Determining Tools for Nanomold Fabrication

- EVU lithography is most expensive- FIB and EBL are available in many microfabs.

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E-beam lithography (EBL) / focused ion beam milling (FIB) / interference lithography (IL) / EUV lithography

• Availability of tools and costs

- Direct, 3D: FIB- Resist-based, 2D: EBL, IL, EUV sidewalls controlled by exposure dose and follow-up processes.- Also related to materials and processing sequence

• 2D vs 3D nanopatterning // direct vs resist-based nanopatterning // compatibility with micro/macroscale patterning tools

- Parallel: IL, EUV lithography- Serial: EBL, FIB- Negative vs. positive patterns

• Patterning area (serial vs. parallel patterning)



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Electron Beam Lithography - System

LEICA LION LV1

Substrate holder

Electron source in LION LV1 with energy 1 - 40 keV

Essectial components: electron source, electron optics, precision sample stage with laser-interferometer position control

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Electron Beam Lithography

• General characteristics

- Diffraction is not a limitation on resolution.- Resolution depends on beam size, can reach ~ 5nm.

- Direct writing method (No mask is required)

• Issues

- Low throughput research tool or low pattern density manufacturing

- Proximity effect reduce resolution with dense patterns.

- High vacuum slow and expensive

- Patterning on a resist layer.

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- Resolution limited by lateral scattering of secondary electrons

- Processing: high level of complexity, expense, andmaintenance intensive

Scattering of secondary electrons

Proximity effect



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Electron Beam Lithography Process

http://nanooptics.uni-graz.at/ol/work/m_ebl.html

Spin-coating a resist

Electron beam writing

Developing

Pattern transfer byetching or lift-off

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• This process applies to other resist-based nanopatterning methods.

• The ultimate resolution is limited by the follow-up processes including development, etching, and lift-off.

The finial pattern size and sidewall profiledepend on the development condition



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Sub-10 nm Features by EBL11

The state of the art of electron beam lithography with aberration-corrected STEM scanning transmission electron microscopy.

(A) Example of the capability of this system (2 nm isolated feature size and 5 nm half-pitch).

(B) Micrograph of a grid formed in ZnO demonstrating a sub-10 nm resolution. The facility is a Leica VB6 UHR electron beam lithography machine located at the Cavendish Laboratory.

From Fundamentals and Applications of Nano Silicon in Plasmonics and Fullerines, 2018

https://www.sciencedirect.com/book/9780323480574



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Hot Embossing Lithography

Lift-off

master: embossing stamp- fabrication with e-beam exposure and RIE- material: Si or SiO2 (not specified)- size: down to 10 nm- period: 40 nm- depth: 60 nm

metal dots- material: not specified- excellent uniformity over 1 square inch

molded structure- material: PMMA on Si

Source: S. Chou, NSL, 1998 Mold Fabrication by E-Beam Lithography

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Focused Ion Beam (FIB) – Dual Beam System

3. Ion beam rasters over surface to remove material.

1. Electron beam used for imaging or patterning (e-beam litho)2. Liquid Metal Ion Source (LMIS) usually uses gallium because it is a

liquid at close to room temperature

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Focused Ion Beam (FIB)

3. Ions can sputter the sample4. Ions also create secondary

electrons & secondary ions• Secondary electrons are

used to generate images.

1. Ions are much more massive than electrons. 2. Ions do not travel as deep into the sample, but they

have much more momentum.

Giannuzzi, L.A.; Stevie, F.A. (Ed.) Introduction to Focused Ion Beams Springer, New York, 2005.

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Advantages• Smaller feature sizes than photolithography are possible.

(Smallest feature size on the order of 10-15 nm.)• Maskless technique• No need for developing (can see results immediately in SEM)• 3D patterning by laterally varying milling conditions.

Disadvantages• Direct write process - slow throughput• Gallium implantation and sample damage• Re-deposition of sputtered Gallium

FIB Advantages & Disadvantages16



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FIB Milling Considerations

Beam size (beam current), beam shape, dwell time/ number of passes, pixel spacing

https://www.raith.com/products/elphy-multibeam.html

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SEM

AFM

• 3D nanofunnels seamlessly interfaced to a nanochannel

• FIB milled using a bmp image file

• Facilitate the loading of DNA molecules into a nanofluidic channel

• Fabrication and metrology by Jinsheng Zhou

5 µm

FIB Milled 3D Nanofunnels

Work done by Jinsheng Zhou in Mike Ramsey’s lab at UNC.

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Fabricate nanoscale structures via FIB

Schematic diagram of metal-assisted FIB milling

(Menard and Ramsey 2010) Though the ion beam spot diameter can be as small as 7 nm (for a FEI Quanta 3D, Ga source), it is still

challenging to fabricate sub-50 nm structures, especially in width. With metal-assisted FIB milling, sub-50 nm even sub-5 nm structures have been demonstrated.

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Interference Lithography – System• Interference patterns due to highly coherent beams combined with high power.• Patterning on a resist layer



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Laser Interference Lithography - Patterns



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EUV-Interference Lithography• Modern synchrotron radiation



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XIL pattern on PMMA NIL Stamp on Si Imprinted on PMMA

• EUV-IL can be a new option to fabricate large area stamps for NILwith patterns below 100nm periods.

• NIL is capable of patterning large area, dense patterns.

1mm

EUV-IL for Imprint Molds



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Contents




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viscous melt

amorphous semicrystalline

brittlebrittle

plasticable to flow

decomposition decomposition

thermo-elastic

(rubbery)

solid(glassy - hard)

viscous melt

visco-elastic

bendable

injection molding

welding

cold molding / shaping(grinding / machining)

„cold“ drawing

warm molding ofsemifinished

products

ability to mold depending on temperature

plasticable to flow

pressing, extrusionrolling of mold materials

Thermoplastic Polymers: States

Tg

Tm,amor

Tdecomp

Tg

Tm,crys

Tm,amor

Tdecomp

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Limit of Replication (Thermal Molding)

a α

degree of freedom of the chain (α=109.5°)

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2/12/122/12 NaRS ⋅

=><

=><

• Radius of gyration, <S2>1/2

<R2>1/2: RMS end-to-end distance

<S2>1/2

<R2>1/2

<S2>1/2: RMS distance of an endfrom the gravity center

• PMMA (25k and 950k)Monomer (C5H8O2), Mw=100 g/molC-C distance (a) ~ 0.15 nmN=2×250=500 for Mw=25 kg/mol

=2×9500= 19000 for Mw=950 kg/mol

25k 950k<R2>1/2 4.7nm 20.7nm<S2>1/2 1.9nm 8.4nm

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Beyond the Limit (UV NIL)Continuous step formation and soft lithogrpahy

Q. Xu and G.M. Whitesides, JACS 127 (2005) 854

• Used low Mw UV resin• The height of replicated could be resolved within ~ 0.4 nm No limitation in terms of fidelity of molding

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• As P increases, Tg increases.

Too low --- P --- Too highNo filling No filling

Too low – T – Too highLow mobility high thermal stress

Too short – t – Too highNo filling Crystallization

Molding T, P, tPVT for a-PS

Heating

Cooling

Pressurizing

Releasing

• Crystallization of polymers is significant for many cases!

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• Imprinting at 150°C at 50 MpaDemolding at 70 °C



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Need to Consider Polymer Rheology:Stamp Geometries

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Stamp

Plastic substrate



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Rheology vs. Stamp GeometriesAssumptions:

Momentum Eq.Navier-Stokes)

0=∂

∂+

∂∂

−zy

pp zyσ

Constituent Eq. z

yzy u

u∂

∂−= 0µσ

- Incompressible- Quasi-stationary flow- L >> s- Shear viscosity, µ = µ0

−= 2

02

30 112 hhFLst

ff

µ• Stamp sinking time

(or filling time)

Continuity Eq.dt

tdhLydzuLth

y)(2

2/)(

0⋅−=⋅ ∫

B.C.

vy = 0 for z = ±h(t)/2

p = 0 for y = ±s/2

σzy = 0 for z =0

Schift and Heydermann: “Nanorheology“ in “Alternative Lithography”, in Kluwer Academic/Plenum Publisher 2003.



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Need to Consider Polymer Rheology:Stamp Geometries

• Rule: keep the distances of lateral polymer flowas small as possible

1. Prevent large features surrounded by large unstructured area

2. Distribute patterns over whole area

3. Use auxiliary structures

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Stamp

Plastic substrate



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Inhomogeneous Structures

Filling time: tf1 = 25 tf2

Stamp structures Imprinting Imprinted PMMA

Stamp bends!! Inhomogeneous residual layer thickness

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• Rule: Minimize lateral non-uniformity Important when imprinted patterns need to be transferred in subsequent processes.



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Size Dependence Filling Behavior

σ=0σ = 29.7mN/m

Decrease inFeature size

• Simulation based on Finite Volume Method with surface tension effect

Jeong et al., Fibers and Polymers 3 (3) (2002) 113

• In microscale molding, polymer fills along the structural walls first.

• In nanoscale molding, polymer fills via plug-like flow.

Decrease inSurface energy



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Demolding PhysicsNIL Process

- Energy balance per debonding length

Wt, ∆U, Qf, Γd : energy per unit areaVf: volume fraction of structure

Externalwork

IncreaseIn elasticenergy

Frictionaldissipation

Debondingenergy



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How to Improve Demolding Practically?

2/13

d 23

=Γ KRπγ

• Adhesion (Γd) at solid/solid interface

Anti-adhesive coating Stamp geometryMaterial selection



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Anti-adhesion Coating on Si Stamps

39°

107°

BeforeAfter

Contact angle Vs Time plot

• Vacuum deposition is preferred for stamps with nanostructures.• Optimized coating time was determined to be about 5 mins.

ClSi

FH H F F FF F F

F F FF F FH HCl Cl

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Material Surface energy / mJ/m2

Glass 83.4

Si (111) 1240

Si with native oxide 39 - 57Polytetrafluoroethylen (PTFE) 19

F13-TCS 12Narayan et al., MRS Proceeding 2018.Park et al., Microelectron. Eng. 2014

2/13

d 23

=Γ KRπγ

F13-TCS

Improvement of adhesion from Si with native oxide is a factor of 2-3. Reduced surface energy also contributes to reducing friction.



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How to Improve Demolding Practically?

2/13

d 23

=Γ KRπγ

• Adhesion (Γd) at solid/solid interface

Anti-adhesive coating Stamp geometryMaterial selection



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Use Materials with Low Modulus

Significant reduction in adhesion at stamp/substrate interface due to low Young’s modulus.Reduce thermal stress due to similar thermal expansion coefficients of polymer molds Use of high-ends fabrication tools such as EBL and FIB is limited to fabrication of Si master mold

Use polymer (or resin) as NIL molds!!!

Need to consider trade-off between improved demolding and decreased structural robustness.

• Width variations: 8% from Si master to UV resin stamp.25% from UV resin to PMMA



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Durability of Resin Stamps

• No noticeable degradation was seen for multiple replications.

Elastic behavior of the resin stamp. Resin stamps are durable!!



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Stress distribution for different Td Simulated normalized stress

Optimization of Demolding TMeasured demolding force

20 40 60 80 1000

20

40

60

80

100

Dem

oldi

ng fo

rce

/ N

Demolding temperature / OC

• Trade-off between adhesion and thermal stress There is an optimal demolding temperature.• Keep the movement vertical to the stamp surface direction.

If it is not possible, determine the demolding direction based on the structure.



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Avoid…

Local stress can be reduced by modifying mold insert with draft angle.

Mizuhata (2008) Microelecton. Eng.

Undercut

• Cryo-RIE produces stamps with high aspect ratio structures and smooth sidewalls.

• Replicated via UV-NIL up to the aspect ratio of 17.

Rough Surface

Silvestre (2016) Microelecton. Eng.



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High Aspect Ratio NIL

Shear stress evolution during demolding

FEM Simulation for Stamp Structure with a Draft Angle

Local stress can be reduced by modifying mold insert with draft angle.

Cadarso et al., Microsys & NanoEng (2017)Si Stamp made via cryo-RIE

UV-Imprinted in OrmoComp resist

• Cryo-RIE produces stamps with high aspect ratio structures and smooth sidewalls.

• Replicated via UV-NIL up to the aspect ratio of 17.



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• Select appropriate nanomanufacturing methods for mold fabrication depending on constraints imposed by materials, process conditions, and applications.

• Successful molding requires time and labors for process optimization. However, think a bit more of material properties and their physics on molding and demolding. Then, you can save a lot of time in developing your processes.

Summary40

Documents

Week 3: Patterning nanostructures and transferring to … · 2020. 11. 18. · Considerations for Determining Tools for ... • General characteristics - Diffraction is not a limitation