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ISBN 978-952-60-4640-2 ISBN 978-952-60-4641-9 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 ISSN 1799-4942 (pdf) Aalto University School of Electrical Engineering Department of Automation and Systems Technology www.aalto.fi
BUSINESS + ECONOMY ART + DESIGN + ARCHITECTURE SCIENCE + TECHNOLOGY CROSSOVER DOCTORAL DISSERTATIONS
Aalto-D
D 6
9/2
012
Veikko Sariola
Droplet Self-A
lignment: H
igh-Precision R
obotic Microassem
bly and Self-Assem
bly A
alto U
nive
rsity
Department of Automation and Systems Technology
Droplet Self-Alignment: High-Precision Robotic Microassembly and Self-Assembly
Veikko Sariola
DOCTORAL DISSERTATIONS
Aalto University publication series DOCTORAL DISSERTATIONS 69/2012
Droplet Self-Alignment: High-Precision Robotic Microassembly and Self-Assembly
Veikko Sariola
Doctoral dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the School of Electrical Engineering for public examination and debate in Auditorium AS2 at the Aalto University School of Electrical Engineering (Espoo, Finland) on the 29th of June 2012 at 12 noon (at 12 o’clock).
Aalto University School of Electrical Engineering Department of Automation and Systems Technology
Supervisor Professor Heikki Koivo Instructor Adjunct Professor Quan Zhou Preliminary examiners Associate Professor Takafumi Fukushima, Tohoku University, Japan Assistant Professor Pierre Lambert, Université libre de Bruxelles, Belgium Opponents Research Professor Aarne Oja, VTT, Finland Assistant Professor Pierre Lambert, Université libre de Bruxelles, Belgium
Aalto University publication series DOCTORAL DISSERTATIONS 69/2012 © Veikko Sariola ISBN 978-952-60-4640-2 (printed) ISBN 978-952-60-4641-9 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 (printed) ISSN 1799-4942 (pdf) Unigrafia Oy Helsinki 2012 Finland The dissertation can be read at http://lib.tkk.fi/Diss/
Abstract Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi
Author Veikko Sariola Name of the doctoral dissertation Droplet Self-Alignment: High-Precision Robotic Microassembly and Self-Assembly Publisher School of Electrical Engineering Unit Department of Automation and Systems Technology
Series Aalto University publication series DOCTORAL DISSERTATIONS 69/2012
Field of research Control Engineering
Manuscript submitted 20 October 2011 Manuscript revised 19 April 2012
Date of the defence 29 June 2012 Language English
Monograph Article dissertation (summary + original articles)
Abstract Droplet self-alignment is a microassembly process where the surface tension of liquid aligns microparts to a substrate. Traditionally, solder has been used, but using unconventional liquids, such as water or adhesives in air, has several attractive properties. Water is compatible with most materials and processes, it is easy to achieve good droplet confinement and it evaporates quickly. Adhesives have the ability to make irreversible bonding. Nevertheless, achieving adhesive droplet self-alignment is difficult, because adhesives generally have a small surface tension. Both liquids can be adapted to low temperature processes.
This thesis describes water droplet self-alignment in high detail, by measuring yield, accuracy, capabilities to build complex structures, and speed of the process. Experiments have been done using an environment-controlled microassembly station and recorded using high-speed microscopy. The results show that droplet self-alignment can achieve industrially relevant performance, and the results may be used as a basis of future process design rules.
A new, microfabricated silicon capillary gripper has been developed, which picks microparts using the surface tension of water. Pick-and-place experiments showed that microparts are self-aligned to the tool by droplet self-alignment. The developed gripper enables handling microparts accurately and delicately.
Finally, new patterned oleophilic / oleophobic surfaces have been developed that enable self-alignment using oil-like liquids, including low-temperature curable adhesives. Self-alignment using an industrial adhesive on the patterns was demonstrated. While this surface was used for droplet self-alignment, a micropatterned oleophilic / oleophobic surface may well have other applications outside droplet self-alignment.
Keywords droplet self-alignment, microassembly, self-assembly, oleophobic, wetting, surface tension
ISBN (printed) 978-952-60-4640-2 ISBN (pdf) 978-952-60-4641-9
ISSN-L 1799-4934 ISSN (printed) 1799-4934 ISSN (pdf) 1799-4942
Location of publisher Espoo Location of printing Helsinki Year 2012
Pages 153 The dissertation can be read at http://lib.tkk.fi/Diss/
Tiivistelmä Aalto-yliopisto, PL 11000, 00076 Aalto www.aalto.fi
Tekijä Veikko Sariola Väitöskirjan nimi Pisaran itsekohdistus: Tarkkuusmikrokokoonpano hyödyntäen robotiikkaa ja itsekokoonpanoa Julkaisija Sähkötekniikan korkeakoulu Yksikkö Automaatio- ja systeemitekniikan laitos
Sarja Aalto University publication series DOCTORAL DISSERTATIONS 69/2012
Tutkimusala Systeemitekniikka
Käsikirjoituksen pvm 20.10.2011 Korjatun käsikirjoituksen pvm 19.04.2012
Väitöspäivä 29.06.2012 Kieli Englanti
Monografia Yhdistelmäväitöskirja (yhteenveto-osa + erillisartikkelit)
Tiivistelmä Pisaran itsekohdistus on mikrokokoonpanoprosessi, jossa nesteen pintajännitys kohdistaa mikrokokoisia kappaleita alustalle. Perinteisesti nesteenä on käytetty juotetta. Epätavanomaisten nesteiden, kuten veden tai liiman, käyttö ilmassa on useilla tavoilla houkuttelevaa. Vesi on yhteensopiva useimpien materiaalien ja prosessien kanssa. On helppo valmistaa pintoja, jotka rajaavat veden kostumista. Lisäksi vesi haihtuu nopeasti. Liimat taas pystyvät muodostamaan pysyvän liitoksen. Liimapisaraan perustuva itsekohdistus on kuitenkin hankalaa, koska liimoilla on pieni pintajännitys. Molempia nesteitä voidaan käyttää matalissa lämpötiloissa.
Tässä väitöskirjatyössä tutkittiin vesipisaraan perustuvaa itsekohdistusta mittaamalla sen saantoa, tarkkuutta, kykyä rakentaa monimutkaisia rakenteita ja nopeutta. Kokeita on tehty mikrokokoonpanoasemalla, joka sijaitsee olosuhteiltaan säädellyssä kammiossa. Kokeet nauhoitettiin suurnopeusmikroskoopilla. Tulokset näyttävät, että pisaraan perustuvalla itsekohdistuksella voidaan saavuttaa teollisesti merkittävä tehokkuus, ja tuloksia voidaan hyödyntää tulevaisuuden prosessisuunnittelun pohjana.
Lisäksi väitöskirjatyössä on kehitetty uudentyyppinen, piipohjainen kapillaaritarttuja, joka poimii mikrokappaleita veden pintajännitystä käyttäen. Poimintakokeissa osoitettiin, että kappaleet itsekohdistuvat työkaluun pisaran avulla. Tarttuja mahdollistaa mikrokappaleiden käsittelyn tarkasti ja niitä vahingoittamatta.
Väitöskirjassa näytetään myös, että kuvioituja oleofobisia / oleofiilisiä pintoja voidaan käyttää liimapisaraan perustuvassa itsekohdistuksessa. Itsekohdistus näytettiin kokeellisesti käyttäen kaupallista liimaa ja kehitettyä pintaa. Vaikka kyseistä pintaa käytettiin itsekohdistukseen, kuvioidulla oleofobisella / oleofiilisellä pinnalla voi hyvin olla muitakin sovelluksia.
Avainsanat pisaran itsekohdistus, mikrokokoonpano, itsekokoonpano, oleofobinen, kostuminen, pintajännitys
ISBN (painettu) 978-952-60-4640-2 ISBN (pdf) 978-952-60-4641-9
ISSN-L 1799-4934 ISSN (painettu) 1799-4934 ISSN (pdf) 1799-4942
Julkaisupaikka Espoo Painopaikka Helsinki Vuosi 2012
Sivumäärä 153 Luettavissa verkossa osoitteessa http://lib.tkk.fi/Diss/
6
List of publications
7
List of publications
This thesis consists of an overview and of the following seven publications
which are referred to in the text by their Roman numerals.
I. Veikko Sariola, Quan Zhou, and Heikki N. Koivo, “Hybrid
microhandling: A unified view of robotic handling and self-
assembly,” J. Micro-Nano Mechatronics, vol. 4, no. 1, pp. 5-16,
2008.
II. Veikko Sariola, Mirva Jääskeläinen, and Quan Zhou, “Hybrid
microassembly combining robotics and water droplet self-
alignment,” IEEE Transactions on Robotics, vol. 26, no.6, pp. 965-
977, 2010.
III. Veikko Sariola, Quan Zhou, and Heikki N. Koivo “Three
Dimensional Hybrid Microassembly Combining Robotic
Microhandling and Self-Assembly,” in Proc. IEEE Int. Conf. Robotics and Automation, ICRA’09, pp.2605-2610, 12-17 May
2009.
IV. Mirva Jääskeläinen, Veikko Sariola, and Quan Zhou,
“Environmental effects on droplet self-alignment assisted hybrid
microassembly,” in Proc. IEEE Int. Symp. Assembly and Manufacturing, ISAM’09, pp. 177-182, 2009.
V. Bo Chang, Veikko Sariola, Mirva Jääskeläinen, and Quan Zhou,
“Self-alignment in the stacking of microchips with mist-induced
water droplets,” J. Micromechanics and Microengineering, vol. 21,
no. 1, 2011.
VI. Veikko Sariola, Ville Liimatainen, Tatu Tolonen, Reidar Udd, and
Quan Zhou, “Silicon Capillary Gripper With Self-alignment
Capability,” in Proc. IEEE Int. Conf. Robotics and Automation,
ICRA’11, pp. 4098-4103, May 2011.
VII. Bo Chang, Veikko Sariola, Susanna Aura, Robin H. A. Ras, Maria
Klonner, Harri Lipsanen, and Quan Zhou, “Adhesive Self-assembly
of Microchips on Oleophilic/Oleophobic Patterned Surfaces in
Ambient Air,” Applied Physics Lett., vol. 99, no. 3, 2011.
8
Contributions of the Author
The author was the primary designer and conductor of the experiments in
[I - IV, VI], including setting up and automating the experimental platform.
The author was solely responsible for the mathematical analysis in [II] and
[VI], and for the machine vision algorithms in [V]. The author did all
microfabrication in [II - VII].
Adj. Prof. Quan Zhou was the instructor of all work. In [II] and [IV],
Mirva Jääskeläinen conducted part of the experiments (Section IVA, test
Sets 2 and 3 and Section IIIC, respectively). In [V] and [VII], Bo Chang
conducted the self-alignment experiments. In [VI], Ville Liimatainen, Tatu
Tolonen and Reidar Udd contributed to the mechanical setup of the
platform and pick-and-place experiments. Susanna Aura contributed the
porous ORMOCER® material in [VII]; the patterning was developed by the
author. Ph. D. Robin Ras developed concept of the surface functionalization
method used in [VII].
Publications [I - III, VI] were written primarily by the author. The author
contributed significantly to the writing of [IV], [V] and [VII].
Symbols
9
Symbols
Symbol Unit Definition
A m2 Surface area of the liquid interface
dtdx / m/s First derivative of x w.r.t. time t . The speed of a
part. 22 / dtxd m/s2 Second derivative of x w.r.t. time t . The
acceleration of a part.
E J Surface energy
f Fraction of solid surface area wet by the liquid,
10 �� f
F N Forceg m/s2 Gravitational acceleration
h m Liquid droplet film thickness
ph m Part thickness
l m Part length
CL m Capillary length of liquid
m kg Part mass
Cp Pa Characteristic pressure of liquid
r Surface roughness, ratio of the actual surface area
to the geometric surface area, 1�r
2,1R m Principal radii of curvature
*2,1R Non-dimensionalized principal radii of curvature
T Nm Torque
V m3 Droplet volume
w m Part widthx m x-bias, the difference between the initial position
and the equilibrium position of a part during self-
alignment
z m Height
10
Symbol Unit Definition
*z Non-dimensionalized height
Greek symbols� J/m2 Surface energy. For liquids equal to surface
tension.
SL� , LG� ,
SG�
J/m2 Surface energies of the solid-liquid, liquid-gas
and solid-gas interfaces, respectively
xE �� / N Partial derivative of E w.r.t. x��� /E Nm Partial derivative of E w.r.t. �
p� Pa Overpressure inside a meniscus
*p� Non-dimensionalized overpressure
� º Contact angle
0� º Contact angle of a perfectly smooth surface
1� , 2� º Contact angles of surface patterns 1 and 2,
respectively Pa·s Liquid viscosity kg/m3 Liquid density
p kg/m3 Part density
� º Pad edge angle
� º Part rotation around z-axis
Abbreviations
11
Abbreviations
ECA Electrically conductive adhesive
FSA Fluidic self-assembly
I/O Input / output
ICP-RIE Inductively coupled plasma reactive-ion etching
LMA Low melting point alloy
PCB Printed circuit board
PECVD Plasma-enhanced chemical vapor deposition
RFID Radio-frequency identification
RIE Reactive-ion etching
RMS Root mean square
SAM Self-assembled monolayer
SEM Scanning electron microscope
SU-8 A negative, high aspect-ratio photoresist
12
Contents
13
Contents
LIST OF PUBLICATIONS 7
Contributions of the Author 8
SYMBOLS 9
ABBREVIATIONS 11
CONTENTS 13
1. INTRODUCTION 15
1.1 Droplet self-alignment 161.1.1 Robotic microassembly and self-assembly 171.1.2 Terminology 18
1.2 Objectives of the research 19
1.3 Methods and tools 20
1.4 Thesis contributions 21
1.5 Related research 22
2. SURFACES WITH PATTERNED WETTING PROPERTIES 25
2.1 Surface energy 25
2.2 Contact angle 25
2.3 Manipulating contact angles 272.3.1 Surface chemistry 272.3.2 Surface roughness 28
2.4 Patterned wetting: surface patterns and protrusions 29
2.5 On the validity of the wetting models 30
3. DROPLET SELF-ALIGNMENT 33
3.1 Modeling of droplet self-alignment 343.1.1 Quasi-static modeling 34
14
3.1.2 Numerical modelling using Surface Evolver 363.1.3 Dynamic modeling and liquid properties 36
3.2 Handling strategies 383.2.1 Droplet self-alignment assisted robotic microhandling 383.2.2 Droplet self-alignment of mismatching shapes 393.2.3 Flipping parts 403.2.4 Capillary gripping 41
4. EXPERIMENTAL WORK 43
4.1 Microrobotic test platforms 434.1.1 Pick-and-place robots 434.1.2 Liquid droplet dispensing 444.1.3 High speed microscopy and machine vision 44
4.2 Microfabricated surfaces with patterned wetting properties 454.2.1 Hydrophilic surfaces using silicon dioxide 454.2.2 Hydrophobic surfaces using CHF3 plasma 454.2.3 Oleophilic / oleophobic patterns using porous ORMOCER® with perfluorinated trichlorosilanes and gold 464.2.4 Sharp edges using SU-8 464.2.5 Fabrication of a capillary gripper using inductively coupled plasma reactive-ion etching 47
5. RESULTS AND DISCUSSION 49
5.1 Water droplet assisted release in robotic microhandling [I – V] 495.1.1 Yield and statistical modelling [II] 505.1.2 Accuracy, speed and trajectories [II] 515.1.3 Mismatching and 3D structures [I] - [III] 515.1.4 Effects of environmental conditions on self-alignment [IV] 535.1.5 Parallel delivery of liquid using mist [V] 54
5.2 Silicon capillary gripper with self-alignment capability [VI] 55
5.3 Adhesive droplet self-alignment on oleophilic / oleophobic surfaces [VII] 56
6. CONCLUSIONS 59
ACKNOWLEDGEMENTS 63
REFERENCES 65
APPENDIX A: NEGLECTING GRAVITY IN SELF-ALIGNMENT 75
APPENDIX B: ENERGY OF A TWISTED MENISCUS 77
APPENDIX C: PUBLICATIONS 79
Introduction
15
1. Introduction
There are two prevailing trends in the semiconductor industry: “more
Moore”, transistors being cramped up in smaller and smaller space, and
“more than Moore”, the heterogeneous integration of different types of
component, such as logic, memory, radio frequency circuits, signal
conditioning, sensors, actuators, mechanics, fluidics, optoelectronics and
optics [1–3]. The fabrication processes and the material prerequisites for
the components may be incompatible, so that it is not possible to fabricate
them all on a common substrate [4]. One solution is to manufacture the
wafers separately, cut the wafers into dies, and assemble those dies into a
single package. Increasingly, the dies are stacked vertically in 3D, which
allows smaller packaging of the devices [3]. Currently, this is done using
high-performance robotic microassembly.
In the manufacturing of relatively simple semiconductor devices,
assembly has long been the major cost factor. For example with high
performance flip chip assembly of radio frequency identification devices
(RFID), assembly makes up 65% of the costs [5]. Due to the serial nature of
the robotic assembly, any throughput increase is transferred to the price
when manufacturing large volumes. Lowered manufacturing costs would
not only bring more profit, but open up new application fields by e.g.
bringing RFID tags to consumer products. Relatively high cost of the tags
has still hindered their acceptance [6].
Increasing the complexity of the components translates to an increasing
number of input / output (I/O) pads. Also, the total size of the packaged
device is still decreasing; consumers expect the same functionality in an
ever decreasing package size. Together these factors decrease the I/O pitch
(distance between the I/O pads), so the pads need to be aligned more
accurately. Especially optical interconnects need to be aligned very
accurately for good optical coupling [7]. Active alignment methods can
achieve this, but with a significant throughput penalty. High accuracy
passive alignment and high yield assembly methods would enable more
complex products [8][9].
16
1.1 Droplet self-alignment
When a droplet of liquid is placed between a part and a receptor site, the
droplet forms a meniscus and aligns the part to the substrate. This is called
droplet self-alignment (Fig. 1). The phenomenon is a consequence of the
surface energy of the droplet: the energy is minimized when the surface
area of the droplet is minimized, i.e. when the part is aligned to the receptor
site.
Figure 1 Principle of droplet self-alignment. a) A droplet of liquid forms a meniscus between a part and a receptor site. b) The surface tension of the droplet aligns the part to the receptor site.
The key to droplet self-alignment is to confine the wetting of the droplet
between the part and the receptor site. The wetting properties of the
surfaces depend on the surface materials, the droplet liquid, addition of
surfactants, medium (air, water, vacuum etc.), temperature, surface charge
etc. Two general approaches for inhibiting liquid spreading can be
identified: surface patterns [10], as done in the receptor site of Fig. 1,
and sharp edges [11], as done with the part in Fig. 1.
Solder self-alignment (where the droplet is solder) has been
extensively studied and is being used in the industry, for example in flip-
chip or surface-mount reflow soldering [9], [12–20]. In recent
developments, shaking has been used to help the self-alignment [21] and
solder self-alignment has been demonstrated in the presence of a viscous
fluid [19], which is important for flip chip assemblies using “no flow”
underfill. For solder, it is relatively easy to achieve high wetting contrast
using metallic pads. Nevertheless, it has its drawbacks; melting solder
requires relatively high temperatures, so that heat-sensitive materials and
parts cannot be used. The use of Electrically Conductive Adhesives (ECAs)
together with Low Melting point Alloys (LMAs) has been proposed [22],
[23]. The self-alignment was possible by the surface tension of the LMA,
but not with adhesives alone.
The use of unconventional liquids in air has several attractive properties.
These liquids include water [24] and adhesives (or resins, as they are
sometimes called) [25]. Adhesives can be adapted to low temperature
processes, and have the ability to make irreversible bonding. Nevertheless,
achieving adhesive droplet confinement for self-alignment on planar
Introduction
17
surfaces has not been demonstrated so far, partly because adhesives have
small surface tension and behave therefore like oils, not being easily
confined inside the patterns. Comparison of the typical viscosities and
surface tensions of adhesives, water and solders is shown in Fig. 2.
One of the differences between solder self-alignment and water droplet
self-alignment is that the solder balls are also used as electrical connectors.
Therefore, typically multiple solder joints are used, laid out in a matrix
pattern. In contrast, water droplet self-alignment usually uses a single
droplet, which can have a size comparable to the size of the part.
Water alone is not capable of making permanent bonding, but several
methods for combining water droplet self-alignment and bonding have
been proposed, e.g. using silicon oxide pads and a small amount of
hydrogen fluoride (HF) in the water [26], or using metallic bonding after
the water droplet self-alignment [27]. Water is an attractive alternative
because it is compatible with many materials and processes, it has relatively
high surface tension and therefore it is relatively easy to achieve good
droplet confinement. Furthermore, water evaporates rather quickly, leaving
the part in close contact with the substrate.
Figure 2 Comparison of surface tensions and viscosities of adhesives, water and solders in their typical operating temperatures. [25], [28–32]
1.1.1 Robotic microassembly and self-assembly
Microrobotic assembly methods can achieve either high throughput or
small part size, but not both at the same time. There is a tradeoff: the
smaller the part size, the slower the operation is [33]. Furthermore, there is
a fundamental limit in accuracy that can be achieved using optical
microscopes. Also, robotic assembly methods have long been known to
suffer from “sticking effects”, adhesive forces between the tool and the
object [34].
102
103
10-1
100
101
102
103
104
105
106
Surface tension [mN / m]
Surface tensions and viscosities of liquids
Adhesives
SoldersWater
18
Using droplet self-alignment for the final, fine alignment is an attractive
option. The accuracy of sensors or actuators does not affect the alignment
accuracy. Rather, the fabrication accuracy of the part and the substrate sets
the accuracy, so it can be very high [35], even when the robot is very fast.
Also, if the part is brought into contact with a liquid droplet (e.g. water), the
surface tension helps overcoming the adhesive force between the tool and
the object [36].
Since the beginning of the 1990s, there has been an increased interest in
self-assembly of mesoscopic and microscopic parts, in which the parts are
designed in such a way that they are automatically attracted to their
assembly sites. Self-assembly of microparts is typically based on surface
interactions [37]. The parts are typically delivered randomly to the
assembly sites, e.g. in fluid suspension [38].
Many self-assembly methods include droplet self-alignment as a part of
the process [37], [39], [40]. Self-assembly is inherently parallel, and droplet
self-alignment defines the accuracy. Therefore, such methods could yield a
higher assembly throughput rate and a higher alignment precision than
robotic placement [41]. In general, however, self-assembly methods have
not been used to create very complex devices. Furthermore, since the
assembly steps need to be considered during fabrication, reconfiguration of
the assembly is difficult compared to traditional methods.
Self-assembly has been used to build a range of interesting structures, for
example macroscopic crystalline structures [42], a microsized replica
keyboard [43] and a DNA smiley [44]. While such structures are impressive
scientific feats, their assembly methods are not directly applicable in the
assembly of microchips.
Surface tension is not the only force that has been used for self-alignment.
Self-alignment has been studied using several other physical forces,
including 3D shape matching [45], magnetic [46] and
electrophoretic forces [47]. Mastrangeli et al. [4] and the author [I]
have reviewed alternative methods in more detail.
Many of the ideas and techniques used in self-assembly can be adapted
and combined with traditional robotics. The author discusses this concept
of hybrid assembly in [I], but in this summary only one combination, the
combination of droplet self-alignment with robotics, is discussed.
1.1.2 Terminology
Droplet self-alignment has been called by several other names, including
solder self-alignment (when the liquid is solder) [48], resin self-alignment (when the liquid is resin, actually a type of adhesive) [25], surface tension
Introduction
19
driven self-assembly [49], capillary self-assembly [50] and fluidic self-assembly [51]. The terminology is yet to be fully established across research
fields, and the author himself has been unable to adapt a single standard
when publishing in journals of different fields. To avoid confusion, this
summary reserves the term self-assembly only for stochastic, parallel
assembly methods. Self-alignment refers to a process where forces bring
the part accurately to its desired position and orientation. A self-assembly
process may or may not include a self-alignment step.
Sometimes it makes sense to distinguish between self-centering and self-alignment. In this thesis, self-centering refers to position correction, while
self-alignment includes orientation correction. Achieving self-centering, but
not self-alignment, is typical for axially symmetric droplet joints [52]. In
Section 3.2.1 and [VI], a special case, which achieves self-alignment, but not
self-centering, is detailed. However, in general self-alignment includes self-
centering, which is assumed hereon unless otherwise noted.
The word droplet underlines that the liquid used is not necessarily
solder. Fluidic and liquid self-alignment could be easily confused with
fluidic self-assembly methods, where the whole process is done in a liquid
medium. Capillary self-alignment is confusing, as there is no capillary
(small narrow tube), or capillary action (liquid rising spontaneously in a
narrow tube). Therefore, this summary uses the term droplet self-alignment, but the reader should be aware of the possibly synonymous
terms in the literature.
The word microassembly highlights that distinct microparts combined,
as opposed to monolithic microfabrication methods where all the required
functionalities are built on the device by sequential material deposition,
removal and modification steps. Microparts should have at least one
dimension less than 1 mm. In this summary, assembly is understood as the
placement of only a single part on a substrate or another part. The assembly
of multiple parts can often be broken down into several single-part
assembly operations, which is why multipart assembly is not considered
separately. Subsequent packaging operations, such as sealing or wire
bonding, are out of the scope of this thesis and are therefore excluded from
the definition.
1.2 Objectives of the research
The objective of this research is to increase the accuracy of the robotic
microassembly process by combining it with droplet self-alignment using
unconventional liquids, such as water and adhesives. The goal is to have
20
benefits of both technologies, without the drawbacks. The specific research
objectives include:
� Characterizing the water droplet self-alignment process in high
detail, including yield, accuracy, its capabilities to build complex structures, and studying the speed of the process
� Developing a new miniaturized capillary gripper which picks up
a micropart using the surface tension of water. The droplet self-aligns the micropart to the tool
� Developing new patterned surfaces that enable droplet self-
alignment using water and oil-like liquids, including low-
temperature curable adhesives
1.3 Methods and tools
This research is multidisciplinary, combining elements of microrobotics,
microfabrication, surface chemistry and automation, which are
cross-scientific in their own right. The approach taken is predominantly
experimental.
The research builds upon previous microrobotic research conducted by
the research group during the last 20 years [53]. For the experiments [I -
V], [VII], a microhandling station was setup. The station comprises of
microrobotic pick-and-place tweezers that can handle parts down to about
50 μm, several microscopes, positioning stages and liquid dispensing
systems.
In most experiments, the robot picked a micropart, liquid was delivered to
the receptor site and the part was placed in contact with the droplet. The
part was released, and it self-aligned to the receptor site.
The self-alignment is a relatively fast process. To observe the process in
more detail, high-speed optical microscopes were used [II], [III], [V].
Furthermore, the ambient environment (temperature and humidity) affect
microhandling and droplet self-alignment, so the ambient environment was
carefully controlled using an environmental control system [54], [IV].
Accurate dispensing of the water was achieved using non-contact [I - IV]
microfluidic dispensing systems. As an alternative, a parallel dispensing
method based on water mist condensation was studied [V]. Adhesives were
dispensed using contact dispensing methods [VII].
For making the test parts and surfaces with patterned wetting properties,
several microfabrication methods were used, including standard
lithography, etching, plasma processing and surface chemistry. Some
special techniques that were used include making dummy parts and
patterns using SU-8 [55], [56], [I - V], [VII]. SU-8 is a spin-coated, thick
Introduction
21
photoresist which can be used to make structures as thick as 450 μm with
aspect ratios around 1:10 or even 1:20. Also, Inductively Coupled Plasma
Reactive Ion Etching (ICP-RIE) was used for making protruded wetting
patterns on silicon and through-silicon microchannels [VI]. Commercial
chips were used in [IV], [VI].
For hydrophilic surfaces, silicon dioxide was grown using thermal growth
process [VI] or deposited using plasma-enhanced chemical vapor
deposition (PECVD) from silane (SiH4) precursor [IV], [V]. Hydrophobic
fluorocarbon surfaces were deposited using trifluoromethane (CHF3)
precursor in a plasma process [IV], [V]. For oleophobic surfaces, porous
ORMOCER® was functionalized using fluorinated trichlorosilanes [VII].
ORMOCER® is an inorganic-organic polymer with a silicon skeleton.
1.4 Thesis contributions
This thesis brings new knowledge in using unconventional liquids in
droplet self-alignment, including water and adhesives, instead of the more
traditionally used solder.
This thesis brings new, experimental data from the droplet self-alignment
process itself, by studying the phenomenon using high-speed microscopy
and an environment-controlled microassembly station. Yields, accuracies,
capabilities and the speed of the process have been measured. This
information can be used as a basis of future process design rules.
A new, miniaturized microgripper that integrates water droplet self-
alignment in a capillary microgripper has been developed. The tool may be
used in future microassembly platforms which need to handle parts
accurately and delicately. Current flip-chip machines actively measure the
misalignment between the picked part and the tool; the newly developed
tool may remove the need for this and simplify the picking operations
considerably. The developed tool is suitable for smaller parts than the ones
previously used in capillary gripping.
Finally, a new fabrication method has been developed for patterned
oleophobic / oleophilic surface for adhesive droplet self-alignment. This is,
as far as the author knows, the first time adhesive droplet self-alignment
has been achieved using oleophobic surfaces for the droplet confinement.
While this surface was used for droplet self-alignment, producing a
micropatterned oleophilic / oleophobic surface itself is no trivial task, and
may well have other applications outside droplet self-alignment.
22
1.5 Related research
Several authors have reported water droplet self-alignment [24], [27], [35],
[57–60]. Parts with lateral dimensions of several millimetres [27], [35],
[57], [60] and around one millimetre [24], [59] have been used. Compared
to those works, the parts used in this thesis work were significantly smaller
(from 50 μm to around 300 μm). Consequently, also smaller amount of
water was used.
Fukushima et al. [61] reports water droplet self-alignment assisted release
from tweezers. Compared to the work done in this thesis, the initial
misalignment was not accurately controlled and the chips were larger than
the parts used in this thesis. Refs. [61] and [35] also report an assembly
approach using water droplet self-alignment for positioning chips on an
intermediate carrier wafer. The self-alignment accuracy was 0.43 μm.
Furthermore, hydrofluoric acid solution together with silicon dioxide
surfaces has been used for achieving final bonding [57]. The bonding
strength was over 10 MPa.
Sato et al. [24] reports water droplet self-alignment using surface
patterns. The results show that the alignment accuracy is decreased by the
friction force between the microparts, the existence of areas which are not
wet on the high wettability area and the overflow to the low wettability area.
Theoretical analysis showed that hexagonal pattern created the largest
restoring force of the studied patterns.
Noda et al. [58] reports droplet self-alignment using a significantly larger
water droplet and a smaller chip (75 μm × 75 μm × 5 μm) than the ones
used in this thesis work. The droplet was either water or methanol solution.
The chip floats on an excessively large (diameter around 1 mm) droplet,
which slowly evaporates. After the droplet evaporation, the chip is located
within the hydrophilic region of the surface, but not positioned in a definite
location or orientation inside the region.
Tsai et al. [59] reports self-alignment using a solid edge for inhibiting
water droplet spreading. The study shows that large droplets decrease self-
alignment accuracy. Furthermore, the chips stay aligned when transporting
the substrate.
Kaneda et al. [60] reports modelling and experimental study on
oscillations of a tilting circular pad on a glycerine or a water droplet. While
those results are important for understanding the dynamics of the tilt
oscillations, it is hard to predict lateral dynamics or random variations in
the self-alignment process based on that work. The tilting motion was
captured on video using high speed microscopy. In this thesis work, high
speed microscopy was also used.
Introduction
23
Chapuis et al. [27] reports a combination of a water droplet and a solder
bump self-alignment. First a large water droplet self-aligns the microchip to
the substrate, after which smaller solder bumps are reflowed and perform
the final self-alignment. That study focused on fabrication methods of the
chips and substrate, but the self-alignment process itself was only shortly
studied. The alignment error was estimated to be below 20 �m on both
axes.
Bark et al. [62] reports the use of surface tension as a gripping force. The
gripper in that work was significantly larger (4 mm × 4 mm square) than
the gripper presented in this work. Both grippers had a similar rectangular
geometry. The self-alignment effect was reported [62], but the phenomenon
was not modelled.
Aoyama et al. [63], Lambert and Delchambre [64], Lambert [65] and
Biganzoli et al. [66] report different capillary gripper designs. In all of the
aforementioned work, the grippers were axially symmetric and could not
achieve self-alignment; only self-centering. Furthermore, the parts handled
were significantly larger (> 1 mm) than the ones used this thesis. One
important difference between the previous works and this work is that in
this work, the capillary gripper was microfabricated from silicon, allowing
greater control of the gripper head shape and definition. Vasudev and Zhe
[67] report manipulating the gripping force using electrowetting. The
electrowetting gripper did not have a liquid inlet, so that the
microfabrication of the electrowetting pads would still have to be integrated
into the fabrication process of the liquid inlet.
Moon et al. [22] and Yasuda et al. [23] report self-alignment using LMA
filled ECAs. Adhesive droplet self-alignment was reported in Kim et al. [25].
The wetting contrast was achieved with sharp edges, instead of surface
patterns, and the chips were larger than the ones used in this thesis.
Ethylene glycol droplet self-alignment using only sharp edges and similar
parts as the ones in this thesis has been reported by Corral et al. [68], who
was a member of the research group of the author.
24
Surfaces with patterned wetting properties
25
2. Surfaces with patterned wetting properties
2.1 Surface energy
When bulk material is cut, work must be done to break the intermolecular
bonds of the material. The work done is proportional to the area of the cut.
The work is stored as energy in the interface, and one can associate a
material interface with energy defined as
AE � (1)
where E is the energy stored in the interface, � is the surface energy
(constant, units 2/ mJ ), and A is the area of the interface. The surface
energy depends on several factors including materials, chemical
composition, surface roughness, charge and temperature at the interface.
For liquids, surface energy is equal to the surface tension of the liquid.
Surface tension makes liquid interfaces act like a rubber sheet and is the
reason that liquid droplets tend towards spherical shape. With the energy
definition it is fairly easy to see why airborne liquid droplets assume a
spherical shape: spheres have the minimal surface area for a fixed volume,
minimizing the surface energy.
2.2 Contact angle
The Young equation establishes the contact angle at which a liquid droplet
comes at rest when placed on surface. Young equation states that
SGLGSL ���� � cos (2)
where SL� , LG� and SG� are the surface energies of the solid-liquid, liquid-
gas and solid-gas interfaces, respectively (See Fig. 3 for definition) and � is
the contact angle of the liquid.
26
Figure 3 When a droplet placed on a surface, the contact line settles at the contact angle, which is a characteristic of the solid-liquid-gas combination used.
The Young equation (2) is a consequence of the force balance at the
contact line. Cohesive forces try to curl the liquid droplet into a sphere,
while adhesive forces try to spread the liquid on the surface.
In practice, the Young equation is not always useful for predicting the
contact angle of a new material, partly because neither SL� or SG� is known
accurately. Instead, one just resorts to measuring the contact angle, since it
can be done fairly easily.
For a particular liquid, surfaces can be divided into two categories:
� High wettability surfaces, which have contact angles less than 90º.
When the liquid used is water, such surfaces are called hydrophilic, in
the case of oil they are called oleophilic (or more generally: lyophilic)
and when both types of liquids wet a surface it is called omniphilic.
� Low wettability surfaces, which have contact angles over 90º. Water
repellent surfaces are called hydrophobic, oil repellent oleophobic or
lyophobic and surfaces repelling all types of liquids omniphobic.
Water is a special case. Being an extremely polar solvent, there are
comparatively large cohesive forces associated with it and thus it is
relatively easy to find surfaces that are hydrophobic. For less polar liquids,
such as oils, this is not the case.
The Young equation (2) does not cover several nonidealities observed in
real surfaces. When a liquid contact line moves along the surface, the
contact angle can assume a range of angles between advancing and
receding contact angle denoted as A� and R� , respectively, depending on
the direction of the movement of the contact line [69]. This is called contact angle hysteresis and is a friction phenomenon.
When a droplet is dispensed on a surface, it is possible that the contact
line is advancing, i.e. the observed contact angle is A� on every side of the
droplet. On the other hand, when for example a droplet is evaporating, it is
possible that the contact line is receding everywhere, i.e. the observed
contact angle is R� on every side of the droplet. The observed contact angle
can also be anything in between A� and R� , if no care is taken to ensure the
direction of the movement of the contact line [70]. Thus, without specific
knowledge on how a contact angle measurement was done, a single contact
Surfaces with patterned wetting properties
27
angle measurement should only be understood as an upper bound for R�
and as a lower bound for A� . For some materials, the angle hysteresis is
very low ( RA �� � ) so a single contact angle fully describes the wetting on
the surface.
Surface chemistry and surface roughness also affect the observed contact
angle, which is discussed in the following Subsection. For simplicity, it is
assumed that the contact angle hysteresis is negligible, but the reader
should be aware that this is not necessarily the case. Both roughness,
especially local surface curvature [71], and chemistry [72] also affect the
contact angle hysteresis.
2.3 Manipulating contact angles
For the purposes of this thesis, it is of special interest to explore how the
wetting properties of a surface can be manipulated. Since it is the outermost
layer on the surface that defines the wetting properties, chemical methods
can be used to alter the adhesive properties of a surface. Surface roughness
also affects the contact angle and can have radical effects on the contact
angle, even when the materials stay the same.
In the following two subsections, these two methods are detailed.
2.3.1 Surface chemistry
Silicon dioxide is a naturally hydrophilic material (Fig. 4a). This is due to
having a lot of hydroxyl (–OH) groups at the surface [73], which are created
when the oxide comes into contact with water. Those hydroxyl groups have
strong interactions with water molecules, resulting in large adhesive forces.
Fluorocarbons (Fig. 4b) have been shown to belong to ultra-low
wettability materials [74]. This phenomenon likely arises from the low
polarizability and high ionization potential of the carbon-fluorine bond
[75]. Consequently, fluorocarbon coatings exhibit excellent repellency
towards water and in some cases even oil.
A breakthrough in tailoring surface properties was the development of
self-assembled monolayers (SAMs) [76]. SAMs are formed by amphiphilic
molecules that have two ends, head and tail. The head shows specific
affinity to the substrate, while the tail group can be chosen for its interfacial
properties, in effect allowing greater freedom in the choice of surface
material and the choice of the interfacial energy (Fig. 4c).
Common SAMs include thiols (head group –SH) that can bond with gold
and trichlorosilanes (head group –SiCl3) that can bond with hydroxyl
28
groups. For achieving low interfacial energies, SAMs with fluoroalkane tail
groups are used.
Figure 4 Three different ways to use surface chemistry and materials to manipulate interfacial energy: a) Silicon oxide is a naturally hydrophilic material due to the large number of hydroxyl groups on its surface; b) Fluorocarbons are naturally hydrophobic materials; and c) SAMs can be used to tailor interfacial energies, by the choice of the tail group R.
2.3.2 Surface roughness
The Wenzel model [77] of surface roughness is based on the following
observation: when the roughness of a surface increases, the actual surface area of the interface is increased (Fig. 5a). Wenzel model
states that
0coscos �� r (3)
where � is the apparent contact angle, 0� is the contact angle for a perfectly
smooth surface and 1�r is the roughness factor (ratio of the surface area
to the projected surface area). Two cases can be identified:
a) When º900 �� , 0�� � i.e. the contact angle is decreased or stays
equal
b) Conversely, when º900 �� , 0�� � i.e. the contact angle is
increased or stays equal
The important observation is that the surface cannot go from high
wettability to low wettability, in spite of the roughness, if the assumptions
of the Wenzel model are valid.
However, in porous surfaces, pockets of gas may be trapped in pores of
the solid (Fig. 5b). This reduces the contact area between the liquid and the
solid, leading to Cassie-Baxter model [69]:
Surfaces with patterned wetting properties
29
1coscos 0 �� ffr �� (4)
where 1,0�f is the fraction of the droplet area that is in contact with the
surface.
When 1 f , (4) is reduced to (3). When 0�f , º180�� regardless of
the original contact angle. This explains how roughness can make highly
wettable materials non-wettable.
In practice, neither of the models is very good for predicting contact
angles: factors r and f are often unknown for a new material beforehand.
Again, it is often easiest to measure contact angles experimentally. The
models serve as an explanation on what kind of mechanisms contribute to
the observed contact angle and how to manipulate it. The Cassie-Baxter
model suggests that, in order to make surfaces with extremely low
wettability, one should have significant porosity, with pockets of gas trapped under the liquid.
Figure 5 Effects of surface roughness on contact angle: a) in Wenzel model, the wetting of the rough surface is complete; and b) in the Cassie-Baxter model, pockets of gas are trapped under the droplet, reducing the area in contact with the droplet.
2.4 Patterned wetting: surface patterns and protrusions
When a droplet is placed on an area with high wettability, framed by edges
with low wettability, the liquid spreading stops at the edges of the area and
the droplet assumes the shape of the area (Fig. 6a). In this summary, any
such shape inhibiting liquid spreading is called a pad.
To completely wet the pad, there is a minimum volume required for the
droplet. Below this volume, the contact line does not reach all the way to
the pad edge and the position of the droplet inside the pad is ambiguous1.
At the edge, the droplet can have any contact angle between the contact
angle of the pad and the contact angle of the background (Fig. 6b);
consequently, there is a range of volumes that can fully wet the pattern.
1 Methods for spontaneously positioning droplets into the center of a pad have been developed [100]; they are based on surface energy gradient. Such methods might be helpful for droplet self-alignment, but remain yet to be tested.
30
When the droplet volume goes past the maximum volume, the liquid
overflows the pad edge and wets also the background.
Figure 6 a) When a droplet is placed on a pad with high wettability (�1) on a low wettability background (�2), the droplet assumes the shape of the pad. b) When there is too little liquid, the pad is not fully wetted. If there is too much liquid, the liquid overflows and wets the background. c) Liquid spreading can also be inhibited by a sharp edge.
Pads with sharp edges can also inhibit liquid spreading [11]. A geometric
condition for the liquid confinement was already derived by J. W. Gibbs:
00 )º180( ���� ���� (5)
where � is the angle of the pad corner (see Fig. 6c).
The ability of a sharp angle to inhibit liquid spreading is similar to having
high wettability surface patterns on low wettability background, with a
theoretical difference in contact angles.
One of the attractive properties of using sharp edges instead of surface
patterns is that sharp edges can naturally confine even low surface tension
liquids [25]. However, in many cases it is undesirable or even impossible to
introduce a significant profile to the pads. Therefore, whichever approach is
appropriate depends on the particular task at hand.
Throughout this summary, many figures are drawn so that sharp edges
inhibit liquid spreading. This is for the reason that large part of the research
in this thesis concentrated especially on confining wetting using sharp
angles. In most cases the wetting confinement could have also been
possible using surface patterns.
2.5 On the validity of the wetting models
There has been considerable discussion on the validity of the Wenzel and
Cassie-Baxter models [78–80]. The discussion stems from an
experimentally proven observation: The contact angle behaviour is only determined by interactions of the liquid and the solid at the contact line alone [80]; not the average properties of the contact area.
��º180
Surfaces with patterned wetting properties
31
Any surface properties (roughness, surface energy) inside the contact area
of the droplet have no contribution to the observed contact angle.
The author fully agrees with the contact line view; and, in fact, takes
identical stance with [79] in Fig. 6: After overflowing the pattern in Fig. 6b,
the contact angle is exactly 2� , the surface energy of the pad has no
contribution to the observed contact angle.
It seems that the controversy rises partly from misinterpretation of the
Cassie-Baxter model. The author believes that Cassie and Baxter fully
understood that it is the surface properties at the contact line that
determine the contact angle, and that their analysis of the surface energy
should be only applied to the infinitesimal area covered by the moving
contact line. They observed contact angle instability when a water droplet
moved on a coarse wire mesh, when the contact line moved discontinuously
from wire to wire [69]. Had they thought that the contact angle is set by the
average properties of the whole contact area such instability would be
unexpected.
Cassie and Baxter considered homogenous surfaces that have roughly
similar wetting properties throughout and with microscopic surface
roughness. It was for this reason that the surface properties at the contact
line could be replaced by the average properties of the surface.
Regardless of the controversy, Wenzel and Cassie-Baxter modelswere originally formulated to explain why rough or porous surfaces have low wettability. The fact that they have is already enough for this
thesis.
32
Droplet self-alignment
33
3. Droplet self-alignment
When a droplet of liquid is placed between a part and a substrate with
matching wetting patterns, the droplet forms a meniscus between the
patterns and aligns the part to the substrate. This is called droplet self-
alignment (Fig. 7). The phenomenon is a consequence of the surface energy
of the droplet: the energy is minimized when the surface area of the droplet
is minimized i.e. when the patterns are perfectly centered and aligned to
each other. The confinement of the droplet inside the patterns can be
achieved by any of the methods discussed in Subsection 2.4.
Figure 7 Illustration of the droplet self-alignment principle. a) A droplet forms a meniscus between a part and a receptor site. b) The surface tension of the droplet self-aligns the part to the receptor site. c) Perspective view of the phenomenon. d) View from the top: droplet self-alignment can correct both position and orientation of the part.
In this summary, the freely moving object is called the part (sometimes also
called the chip), while the substrate is the supporting structure that the part
is assembled on. In practice, the substrate can be e.g. a PCB, a wafer or
another part; in this thesis, mostly dummy parts and substrates were used
in order to demonstrate the principle. A receptor site is the shape on the
substrate which inhibits the liquid spreading and to which the part self-
aligns.
34
The difference between the initial location of the part and the final
location of the part, after the successful self-alignment, is called the bias. In
some other contexts [24], this is called the initial error. Term error was not
adopted in this work, because the bias could be accurately controlled and
thus is considered a process input.
3.1 Modeling of droplet self-alignment
3.1.1 Quasi-static modeling
Quasi-static models of droplet self-alignment only consider the energies
associated with the liquid droplet interface [15], and possibly the
gravitational potential energy associated with the liquid or microscopic
parts. The scaling effect [81] explains why the gravity is negligible
compared to capillary forces when the dimensions are small. This holds
true when the characteristic dimensions are much smaller than the
capillary length of the liquid; in practice this is so when the dimensions
approach sub-millimeter sizes (the capillary length CL of water is 2.7 mm).
This is generally true for the liquid joints used in droplet self-alignment.
Also the parts used in this thesis are of submillimeter size, so their gravity
can be neglected. This can be shown using non-dimensionalization of the
Young-Laplace equation, full derivation given in Appendix A.
Using quasi-static models, one can calculate the energy associated with
the interface as a function of the part location and orientation. As a result,
one gets the shape of the energy well, which can be used to study
problematic self-alignment configurations e.g. local minima or positions
where the restoring force is small [82].
The energy well depends on the shape of the part and the receptor site. In
this thesis, only square-shaped or rectangular parts and receptor sites have
been studied. The reason for this is that many dies are rectangular, because
they are cut from a wafer along horizontal and vertical lines. A single
rectangular receptor site can achieve both self-centering and self-
alignment. Rectangular parts have 180º symmetry positions, while square
parts have 90º symmetry. This means that there are multiple
configurations with identical energy.
A simplistic geometric model of the surface energy is presented in Fig. 8.
The droplet shape is approximated with a right parallelepiped.
Droplet self-alignment
35
Figure 8 Approximating droplet shape with a right parallelepiped.
By increasing the bias x in the geometric model, the volume of the liquid
stays constant and the surface areas of the front, back, top and bottom faces
stay constant. Only the surface areas of left and right faces (indicated in Fig.
8) increase, and their total energy is given by
2222 hxwAE � �� (6)
where w is the width of part, h is the height of the liquid film and x is the
bias in x-direction (see Fig. 8). It is clear that the energy minimum with
respect to x is found when 0 x i.e. the part achieves self-centering.
Even if such a geometric model is grossly simplified, it already brings
useful information about self-centering. By taking the derivative of (6) w.r.t
x-bias, the force F is
22/2/ hxwxxEF �� ��� � (7)
The force starts to drop as the part approaches the equilibrium, being
exactly zero when the part is in equilibrium. When x is much smaller than
h , (7) can be approximated with
hwxFhx
/2�����
(8)
In practice, due to necking effects, contact angle hysteresis and other
wetting related phenomena, the droplet height cannot be controlled
accurately. However, it is more feasible to control the droplet volume Vand h is a monotonic function of V .
Decreasing h , by decreasing V , increases the force near the equilibrium
point, i.e. smaller droplets create larger forces. This suggests that smaller
droplets could be beneficial to the droplet self-alignment. On the other
hand, intuitively, using too small droplets, no droplet self-alignment can be
36
possible. Therefore, the droplet volume was selected as one of the key
parameter of the study, and its effects on the process yield was measured in
multiple experiments. This will be discussed in Subsection 5.1.1.
To show self-alignment, one should consider the energy of a meniscus as a
function of part rotation. This derivation is left to Appendix B.
3.1.2 Numerical modelling using Surface Evolver
The simplistic geometric model already captures the basic features of self-
alignment, but it is still a very crude approximation. Real surface shape is
smooth, more like shown in Fig. 9a and 9b.
The Surface Evolver software [83] can be used to numerically study
minimal surfaces more accurately. The software breaks the surface into
smaller elements, and tries to minimize the energy, by optimizing the
location of each vertex. During the optimization constraints are respected,
such as liquid volume being constant and that the droplet wets only inside
the pads. The actual optimization is based on well-known methods, such as
the gradient, Hessian or conjugate gradient method. As a result, the shape
of the energy well can be calculated more accurately.
The difference between the Surface Evolver simulation and the geometric
approximation of (6) can be very small so that (6) may be a good enough
approximation for some applications. Equation (6) only allows calculation
of the force in lateral direction. When moving the part in vertically with a
constant droplet volume, the liquid will bulge, neck or retract from edge of
the pad and the situation is more complex. More general cases are usually
handled with Surface Evolver, when analytical and geometrical
approximations would be complicated.
3.1.3 Dynamic modeling and liquid properties
The static models do not explain damping, which is a result of
hydrodynamic forces of the liquid. Assuming Newtonian fluid and linear
velocity profile inside the liquid [15], the viscous force can be approximated
with
dtdx
hwlF
� (9)
Droplet self-alignment
37
Figure 9 Example of droplet self-alignment simulation using Surface Evolver. Contact angles on the receptor site and the part are assumed to be 30º. The boundary conditions of the pads were assumed to be one-sided i.e. the contact line is forced to be inside the edges of the pads, but not necessarily at the pad edge. The liquid is assumed to be water (� = 72.8 mJ / m2), the size of the receptor site and the part is 300 μm × 300 μm and the liquid amount is 3 nL. a) Shape of the surface with bias between the receptor site and the part; b) Shape of the surface with perfect alignment. Notice the small necking of the liquid; and c) comparison of the energies calculated using (6) and Surface Evolver. In Surface Evolver, the x-bias was initially 0 m and the liquid thickness h was optimized so that the meniscus was in equilibrium. The h was then fixed for the rest of the simulation. In eq. (6), the liquid
thickness was approximated with wlVh .
where is the viscosity of the liquid, l is the length of the part (see Fig. 8)
and dtdx / is the velocity of the part. This approximation is valid only for
high viscosity liquids or small h . Eq. (9) underestimates the viscous force
for low viscosity liquids or large h ; in such cases, the liquid flows have to be
taken into account by solving Navier-Stokes equations [84] or some
simplification of thereof [85], [86].
Quasi-static modeling assumes that at each instant the surface is at
equilibrium and the force can be calculated by taking the derivative of the
energy. The force calculation can be done numerically using either Surface
Evolver or by some analytical approximation. For this approximation to be
valid, the dynamics of the liquid surface should be much faster than the
dynamics of the part. This is generally true for droplet self-alignment
applications [41].
When the bias is small, one can use (8), (9) and Newton’s law to get a
linear second-order dynamic equation of the part:
022
2
��hwx
dtdx
hwl
dtxdm �
(10)
38
where m is the mass of the part and 22 / dtxd is the acceleration of the
part. This explains some of the basic dynamic properties of the self-
alignment. The dynamics can be over- or under-damped, depending on the
balance between the mass, surface tension, viscosity and dimensions. When
the system is under-damped, the part overshoots during the self-alignment
and oscillates. It is clear that the choice of liquid has an effect on self-
alignment.
3.2 Handling strategies
3.2.1 Droplet self-alignment assisted robotic microhandling
As discussed in Subsection 1.1.1, droplet self-alignment can be used in many
applications in robotic microassembly and self-assembly. One particular
use is to help positioning parts after coarse positioning by a robot.
In this thesis work, the following handling strategy was extensively
studied: first, a microgripper picks a micropart from a surface and brings it
close to a receptor site. A droplet is dispensed on the receptor site, and the
gripper brings the part in contact with the droplet. When the part is
released, the surface tension of the droplet self-aligns the part to the
receptor site. Finally, at least in the case of water, the droplet evaporates;
leaving the part adhered to the receptor site. This handling strategy is
illustrated in Fig. 10.
In the simplest case, the shape of the receptor site and the part match
exactly. But the following Subsection discusses that this does not need to be
the case and good alignment can still be achieved.
Figure 10 Droplet self-alignment experiments using a tweezer-type robot. a) A pad with sharp edges is the receptor site. Receptor site could also be surface patterns; b) Liquid (in most cases water) is dispensed on the receptor site; c) Microgripper approaches the liquid droplet with a part in its grasp. In this thesis, the whole backside of the part is used and the liquid spreading is inhibited by the sharp edges. Surface patterns could also have been used; d) The part is placed in contact with the droplet and released; e) The part is self-aligned to the receptor site. In the case of water, the water evaporates and leaves the part aligned and adhered to the receptor site.
Droplet self-alignment
39
3.2.2 Droplet self-alignment of mismatching shapes
If the shapes of the part and the receptor site do not match exactly, there
exist multiple energy minima for the part (Fig. 11a). If, however, the part is
released outside the receptor site, the part can still align to the receptor site,
as shown in Fig. 11b. This happens when the part is sufficiently damped.
With too little damping (small liquid viscosity or large h ), the part can
overshoot past the edge and pull the contact line with it, away from the left
pad edge. In such a case, the part could end up to one of the positions in
Fig. 11a.
Figure 11 Self-alignment of mismatching shapes. a) There exist multiple energy minima, so the final position after the self-alignment may be ambiguous; b) If, however, the self-alignment is started outside the border of the receptor site and the system is sufficiently damped, the part aligns to the edge of the receptor site.
It is interesting to note that in all cases in Fig. 11, the rotation is still
corrected by the self-alignment. So, in this particular case, it is possible to
have self-alignment, without self-centering.
When only one edge confines the wetting changes, the shape of the energy
well is not the same as one side of the energy well of a perfectly matching
part and a receptor site. This happens because the liquid is able to wet
further on the receptor site. As a consequence, the alignment is not perfect,
but the part may align a bit inside the edge of the receptor site. When the
liquid film is thin, this misalignment is negligible.
When the liquid film is very thick, the ambiguity in the energy well
disappears completely. This is illustrated in Fig. 12. There is enough liquid
to wet the receptor site completely, and a small part can center on a larger
receptor site.
40
Figure 12 Alignment of a small part on a larger receptor site with a large amount of liquid. The wetting of the receptor site is complete, and the part aligns to the middle.
When there is only a small mismatch in the size of the part and the receptor
site, small droplet is already enough for making the position unique and
align the part to the middle of the receptor site. Thus droplet self-alignment
is robust: the part aligns in the middle of the receptor site even if there are
small tolerances in the part or receptor site dimensions, due to
manufacturing.
Finally, the alignment is even possible in two dimensions (Fig. 13). By
starting the alignment over both edges of a corner, a part self-aligns to the
corner of a large receptor site.
Figure 13 Alignment of a small part on a corner of a large receptor site. If the alignment is started with edges of the part outside both edges of the large receptor, the part aligns to the corner of the receptor site.
3.2.3 Flipping parts
In addition to realizing lateral part movement and part rotation around z-
axis, the surface tension of a droplet can be used to power rotations around
x- or y-axis. The principle is illustrated in Fig. 14. This can be used to flip
parts in place, which would otherwise require either dexterous
manipulations [87], complicated kinematics capable of object-centric
rotations [88] or rotating the part by pushing it against an external object
[89].
The phenomenon is known in solder assembly as part tomb-stoning [90],
where a passive component spontaneously rises up due to the surface
tension of the droplet. In electronics assembly, it is usually considered a
manufacturing defect. However, surface tension powered rotations have
also been used to create self-assembled out-of-plane structures in a
controlled manner [91].
When using surface tension powered rotations with a microrobotic
handling platform and a water droplet dispenser, interesting handling
possibility emerges: since the robot can approach the receptor from a
controlled direction, the rotation direction and final orientation of the part
Droplet self-alignment
41
can be chosen during manipulations. The water droplet dispenser allows
dispensing droplets on the fly and performing multiple manipulations with
the same part and receptor site, with the liquid evaporating after each self-
alignment. Combining multiple such manipulations, the part can rotated
with almost any desired face facing the receptor site.
Figure 14 Using surface tension of a droplet to flip parts. A side of the part wets and when the part is released, the part rotates so that the side faces the receptor site.
3.2.4 Capillary gripping
So far, droplet self-alignment has been discussed in the context of parts
self-aligning on a receptor site after the part is placed on the receptor site.
In a capillary gripper, the surface tension of a droplet is used to pick a part
from a surface. The surface tension lifts the part from the surface and self-
aligns the part to the tool. The principle is shown in Fig. 15.
The working principle of a capillary gripper is as follows: first, a water
droplet is formed in the head of the gripper. For this purpose, the gripper
usually contains a small water inlet (capillary) through which the water is
pumped (Fig. 15a). The gripper approaches the part on the surface, and the
droplet forms a meniscus between the gripper and the part (Fig. 15b). If the
surface tension is able overcome the adhesion of the part to the surface, the
part is picked (Fig. 15c) and self-aligns to the gripper. The gripper is usually
axially symmetric, so that only self-centering is possible. If, however, the
gripper is e.g. square, also self-alignment is possible (Fig. 15d-e).
Capillary gripping has several attractive features. First, due to the scaling
of the capillary force, the force becomes comparatively larger than inertia
when the parts are minimized. Second, the self-alignment mechanism may
simplify manipulations considerably due to eliminating the need for
measuring the misalignment between the tool and the part after picking.
Finally, the liquid joint provides compliance during manipulations and
applies force evenly on the surface, protecting the parts from damage.
One problem with capillary gripping is the release of the parts: if the
picking force was large enough to overcome the part-surface adhesion, the
part-surface adhesion is not large enough to release the part from the tools
42
grasp. Several solutions to this problem have been proposed in [92],
including pumping excessive amount of liquid through the gripper head.
Figure 15 Picking parts using a capillary gripper. a) A square-shaped gripper head approaches the part. Water droplet is inserted through the capillary. b) The gripper touches the part. The water droplet forms a meniscus between the gripper and the part. c) The gripper retracts and picks the micropart using the surface tension of the water. d-e) The part self-aligns to the gripper head, using surface tension.
Experimental work
43
4. Experimental work
4.1 Microrobotic test platforms
To capture the phenomenon of droplet self-alignment in detail, test
platforms were needed, which could a) position the microparts near the
receptor sites with known accuracy; b) dispense accurate amounts of liquid;
and c) record the phenomenon in real time. To these ends, different kinds
of custom microrobotic test platforms were developed, which included
accurate positioning stages, water dispensers and high speed microscopes.
An example of a test platform is shown in Fig. 16.
Figure 16 A microrobotic test platform used to study droplet self-alignment in [I – V].
4.1.1 Pick-and-place robots
In [I – V, VII], a tweezer microgripper was used to pick-and-place the parts
for self-alignment. The tweezer microgripper could handle relatively small
parts, down to 50 μm.
The gripper also suffers from the “sticky finger” phenomenon when
handling very small parts. Nevertheless, the surface tension of the liquid
can overcome the adhesion of the part to the tip and releases the part from
the tip.
To achieve accurate timing, positioning and repeatability in the
experiments, high level of automation was required. The test platforms
44
were computer controlled, and the experiments were automated using the
LUA scripting language [93].
4.1.2 Liquid droplet dispensing
Several different concepts for delivering the liquid to the assembly site were
tested. For water, a non-contact dispenser (GeSIM / PicPIP), capable of
shooting a water droplet to the assembly site, was used in [I – IV]. Each
droplet could be tuned to have a volume of about 60 to 250 pL. Larger
droplets could be formed by dispensing multiple droplets.
As an alternative, the parallel delivery of water droplets using
condensation from humid air was tested in [V]. Water mist was created
using an air humidifier (Bionaire / Ultrasonic Compact BU1300W-I), and
guided near the assembly site using tubing. The humidifier could be turned
on and off by the computer through an I/O card and a relay.
In [VI], water was delivered to a capillary microgripper using through-
silicon microchannel. On the backside of the silicon, 1/16” tubing was
connected using microfluidic connectors (Upchurch / Nanoport™). The
liquid amount was controlled using a volume controlled pump (Cavro /
XCalibur, Syringe size 25�l).
In [VII], adhesive liquid was used instead of water. The adhesive had a
much higher viscosity, so that a non-contact dispenser could not be used.
Instead, a contact dispenser (EFD Inc. / Mikros TM dispense pen) was used
for the liquid delivery. The contact dispenser makes a small droplet at its
head and touches the receptor site, delivering controlled amount of the
liquid on the receptor site.
4.1.3 High speed microscopy and machine vision
To observe the self-alignment phenomenon in detail, the process was
recorded using a high speed camera (Imperx / IPX-VGA210-G), capable of
recording up to 3000 frames per second. The camera was mounted on an
optical microscope. This enabled recording of the time it takes for the self-
alignment to complete [II], observing the trajectories of the part during
self-alignment [II] and recording videos of the self-alignment in high detail
[III].
In [V], machine vision algorithms were used to estimate the amount of
condensed water on the patterns. The algorithms were able to locate and
measure the size of each droplet from individual images. Using the
algorithms, the water condensation process was shown to be linear with
respect to time, and the amount of liquid could be easily controlled by
controlling the time of the condensation.
Experimental work
45
4.2 Microfabricated surfaces with patterned wetting properties
In this thesis, confined wetting has been achieved using both sharp edges
and surface patterns (see Subsection 2.4). All studies used sharp edges in
one way or another, while in part of the studies, surface patterns were also
used [IV, V, VII]. Table 1 summarizes the materials and liquids used in this
thesis, and the corresponding contact angles.
Table 1 Contact angles of different materials and liquids used in this thesis.
Material Liquid Contact angle (º)
Silicon dioxide Water < 10 [94]
SU-8 Water 65 [VII], 81 [V]
Delo 18507 adhesive 47 [VII]
Gold Delo 18507 adhesive 53 [VII]
CHF3-plasma
deposited fluorocarbon
Water 105 [95]
Functionalized porous
ORMOCER®
Olive oil 133 [VII]
Delo 18507 adhesive 119 [VII]
4.2.1 Hydrophilic surfaces using silicon dioxide
Hydrophilic surfaces were made from silicon dioxide, and two methods for
creating silicon dioxide surfaces were used: thermal oxidization [VI] and
depositing silicon dioxide in PECVD process from silane (SiH4) source gas
[IV], [V].
Silicon wafers were wet oxidized in oxidation furnaces. Patterning of the
oxide was achieved by using lithography and by dry etching the oxide in
RIE. A practical problem with the wet oxidation furnaces used is that any
metallized wafers are not allowed. Therefore, only new and clean wafers can
be thermally oxidized.
For wafers that had already patterns on them, silicon dioxide was
deposited using the PECVD process. Silicon dioxide patterns alone were
nearly invisible under an optical microscope. To see the edges of the
patterns clearly, a thin layer of aluminum was sputtered under the patterns
before SiO2 deposition.
4.2.2 Hydrophobic surfaces using CHF3 plasma
In [IV], [V], deposition of fluorocarbon surfaces was achieved using
trifluoromethane (CHF3) plasma [95]. Five minutes in pure CHF3 plasma
was enough to make the surfaces hydrophobic. The surface treatment was
stable enough that a photoresist could be spincoated, exposed, developed
46
and removed from top of it without significantly altering the hydrophobic
qualities of the surface.
4.2.3 Oleophilic / oleophobic patterns using porous ORMOCER® with perfluorinated trichlorosilanes and gold
In [VII], patterned oleophilic / oleophobic surfaces were developed for
testing the self-alignment using industrial adhesives. The oleophobic areas
were made out of ORMOCER® [96]. ORMOCER® is an inorganic-organic
polymer with a silicon skeleton. When exposed to oxygen plasma, the
organic material is removed, leaving only porous skeleton behind [97].
Furthermore, the silicon skeleton is oxidized.
After the oxygen exposure, gold patterns were defined on the
ORMOCER®, using lift-off lithography. Finally, perfluorinated
trichlorosilanes were used to functionalize the surface. The trichlorosilanes
are able to bind hydroxyl (–OH) groups of the porous ORMOCER®, but
not on gold. As a result, the gold remains oleophilic but the porous
ORMOCER® becomes oleophobic. The full fabrication sequence in shown
in Fig. 17.
Figure 17 Fabrication process of oleophobic/oleophilic patterned surface. Not drawn to scale. (a) ORMOCER® resist is spin coated on a silicon wafer; (b) oxygen plasma treatment to make porous ORMOCER®; (c) lithography; (d) gold evaporation; (e) lifting-off gold; (f) functionalization with trichlorosilane.
4.2.4 Sharp edges using SU-8
A large part of the self-alignment experiments were done using rectangular
microparts, made out of SU-8 (Fig. 18). SU-8 is a spin-coated, thick
photoresist which can be used to make structures as thick as 450 μm with
aspect ratios around 1:10 or even 1:20. SU-8 was very suitable for making
thick parts with small (around 100 μm) lateral dimension in various shapes
and sizes very quickly. For the self-alignment experiments, SU-8 has the
nice property of being naturally relatively omniphilic, and the edge of the
patterns can be used as a sharp edge for confined wetting. Furthermore,
Experimental work
47
SU-8 is optically transparent, allowing the observation of the self-alignment
process from above.
Figure 18 A scanning electron microscope (SEM) micrograph of an SU-8 part. The part size is approximately 100 μm × 100 μm × 40 μm.
The SU-8 parts used did not have any kind of electrical functionality. For
real world applications, silicon chips would have been more practical. Some
experiments were done using silicon chips [IV, VI], but SU-8 allowed much
faster prototyping. A silicon surface can be easily oxidised, which makes it
omniphilic so many of the techniques tried with SU-8 are expected to carry
over to silicon chips also. For very fine control of the wetting, any of the
methods discussed in Subsection 2.3 could have been used.
4.2.5 Fabrication of a capillary gripper using inductively coupled plasma reactive-ion etching
In [VI], inductively coupled plasma reactive-ion etching (ICP-RIE) was
used to create near vertical walls and through-silicon microchannels for
fabricating a silicon capillary gripper. Since polymer based photoresists are
not selective enough for ICP-RIE, the photoresist pattern was first
transferred to a hard mask from thermally grown silicon dioxide and
sputtered aluminum.
Two different etch depths (through-silicon channels and gripper head
shape) were achieved using nested masks: outer mask was made of
aluminum and masked the through-silicon etching, and inner mask was
made of silicon dioxide and masked the gripper head etching. The
fabrication method was adapted from [98] and is illustrated in Fig. 19.
48
Figure 19 Etching two different depths using nested masks. The figure is not drawn to scale. a) Silicon oxide patterns are defined on the silicon wafer. On top of the oxide, aluminium patterns are defined; b) Through silicon channels are etched, using aluminium as the mask; c) The aluminium is removed; d) Second silicon etching uses the oxide as a mask. e) Perspective view of the final structure.
Liquid was inserted through the through-wafer channel. One of the nice
side benefits of using silicon dioxide as a mask material was that it could be
left on the surface. The naturally omniphilic silicon dioxide, bordered by
sharp edges, was good for achieving confined wetting of water [VI].
Results and discussion
49
5. Results and discussion
Papers [I] – [V] show how water droplet self-alignment can be used to help
part placement in robotic microassembly. Specifically, a tweezer
microgripper was used (see Section 4.1.1) to pick-and-place microparts on
water droplets, which self-aligned the microparts to the assembly sites after
releasing. In those studies, a microdispenser was used for local water
droplet delivery.
Paper [I] surveys the merits of robotic microassembly and self-assembly.
Self-assembly has high efficiency, as it can have very high throughput. On
the other hand, robotic handling has good capabilities, in the sense that it
can build complex structures and can be adapted by reprogramming to the
assembly task. Based on the analysis, the paper proposes the combination
of droplet self-alignment with robotics, demonstrates the handling strategy
in experiments and analyses the proposed handling method using the
results of the survey. In paper [II], the yield, accuracy and speed of the
handling method are experimentally measured. Paper [III] reports the
building of 3D structures using the aforementioned handling method.
Paper [IV] reports the effects of the ambient environment conditions
(temperature, humidity) on the handling method. In paper [V], parallel
delivery of water by condensation from humidified air was reported.
Paper [VI] shows how water droplet self-alignment can be used to help
part picking in robotic microassembly. Specifically, a new microfabricated
capillary microgripper is presented, where the picked micropart is self-
aligned to the tool.
Finally, paper [VII] reports how oleophilic / oleophobic patterns can be
fabricated and used for adhesive droplet self-alignment.
The results are now explained more thoroughly.
5.1 Water droplet assisted release in robotic microhandling [I – V]
In paper [I], the merits of the droplet self-alignment assisted robotic
microhandling were qualitatively evaluated, by performing the following
50
experiments: a) aligning two, identically shaped 300 μm × 300 μm square-
shaped SU-8 parts on each other, and b) aligning a smaller part on the
corner of a larger part (see Subsection 3.2.1). The following major points
were identified for further study:
1) The handling strategy is robust, in the sense that the self-alignment
can be successful, even when the robot has large positioning errors.
Good alignment accuracy can still be achieved. This was quantitatively
measured in paper [II], by determining the yield as a function of the
bias and droplet volume.
2) The handling strategy is accurate, since it is determined by the
manufacturing precision of the parts and the receptor sites. The
droplet self-alignment assisted release overcomes the sticking effects
[34], where the part is adhered to the tool. This is because the
capillary force from surface tension is larger than the adhesion of the
part to the releasing tool [61]. The alignment accuracy is mostly
limited by the fabrication accuracy of the microparts. The alignment
accuracy was quantitatively measured in paper [II].
3) Self-alignment can happen even when the size of the parts do not
match exactly. This allows great flexibility in the handling, and
different possibilities were explored in papers [II] and [III].
4) The ambient environment has an effect on the handling strategy, due
to affecting the evaporation rate of the droplets. This was
quantitatively measured in paper [IV]. Furthermore, if the humidity of
the environment can be made high, no water dispenser is necessary, as
the water can be delivered to the receptor site through condensation.
This possibility was further discussed in paper [V].
5.1.1 Yield and statistical modelling [II]
In paper [II], the yield of the handling strategy, presented in [I], was
reported as a function of the process parameters. A large number (> 500) of
experiments with varying x-, y-, z-bias and water amount were conducted,
and the success of each self-alignment was recorded.
Using logistic regression [99], the yield was statistically modeled. The
model was used to calculate the optimal droplet volume and z-bias. For
300 μm × 300 μm parts, these were 1.8 nL and 47 μm, respectively. For the
range of acceptable parameters, the model was used to show that 98% yield
could be achieved when a) x-bias was inside ±82 �m; b) y-bias was inside
±88 �m; c) z-bias was inside 25 and 69 μm; or d) droplet volume was
between 0.97 and 3.07 nL; while other parameters were kept optimal. Of
the positioning dimensions, z-bias had the tightest bounds.
Results and discussion
51
Furthermore, the model predicts correctly the intuitive result that the
yield is maximum when there is no x- or y-bias.
5.1.2 Accuracy, speed and trajectories [II]
In paper [II], the final alignment accuracy of the self-alignment process for
matching and mismatching shapes was measured using a scanning electron
microscope. Water droplets and SU-8 parts were used. In all cases, the
root-mean-square accuracy of the self-alignment was close to 2 μm and
0.4º.
SU-8 side walls have a slight taper, which is a result of the lithography
process. This alone can create a size difference of ~ 1-2 μm between the top
and the bottom side of the part, and was not taken into account when
conducting the experiments. As discussed in Section 3.2.2, the self-
alignment process is expected to be robust, in the sense that the part should
center to the receptor site even when there are small size differences.
Nevertheless, it is possible that these size differences made the alignment
accuracy worse, and parts with better manufacturing precision should be
used to measure the ultimate limits of droplet self-alignment. The
alignment accuracy was still reasonably high.
The alignment accuracy was measured using high-vacuum SEM. First, the
water droplet self-aligned SU-8 parts on top of SU-8 parts were left to air
dry. This drying process created a fairly strong bonding between the two
SU-8 parts, as discussed in [III]. While the bonding strength was not
measured quantitatively, qualitatively it was so strong, that detaching these
two parts using tweezers created significant damage to the parts before the
bonding failed. The chemical origin of this bond is not known. Therefore, it
was expected that a combination friction and adhesion would keep the
parts firmly in place, even when the samples were introduced to high
vacuum.
Using high-speed microscopy, the duration of the water droplet self-
alignment was measured to be around 100 ms, and to have large variations
from one test to another. In some cases, the standard deviation was as large
as 146 ms. Nevertheless, the alignment was successful in all cases,
regardless of the variations in the trajectories.
5.1.3 Mismatching and 3D structures [I] - [III]
As discussed in Section 3.2.1, self-alignment is even possible when the part
sizes are not matching exactly. In paper [I], first results of aligning parts to
corners of larger parts were shown (Fig. 20). The liquid amount was
identified as a critical parameter for the success of this handling strategy.
52
In paper [II], further testing of the alignment of mismatching shapes was
carried out. In the tests, 95% yield was achieved when aligning a part on the
corner of a larger part. In edge alignment experiments, the RMS-accuracy
in y-direction was measured as 3.4 μm and 0.3º. Fig. 21 shows a SEM
micrograph after the alignment of a square SU-8 part on the end of a
rectangular receptor site.
In paper [III], the edge alignment was shown in a reversed context: the
top part was large and rectangular, while the receptor site was significantly
smaller (Fig. 22). Using this method, overhanging structures could be
created.
Finally, in paper [III], flipping microparts 90º by surface tension was
shown (Fig. 23), as discussed in Section 3.2.3. By approaching the receptor
site from opposite direction, the direction of the flip could be reversed. The
handling strategy is an easy way to realize object-centric rotations, which
usually would otherwise require complicated kinematic structure.
Figure 20 Aligning a small part on the corner of a larger part. a) The gripper approaches with the part in its grasp. b) A water droplet is dispensed on the corner of the receptor site. c) The part contacts with the droplet, and the droplet forms a meniscus between the part and the corner of the receptor site. d-f) The part self-aligns to the corner of the receptor site. Originally published in [I]. With kind permission from Springer Science and Business Media.
Figure 21 Aligning a small, square-shaped, SU-8 part onto the end of a larger, rectangular part. The square part is approximately 300 μm × 300 μm × 40 μm. The picture is from one of the self-alignment experiments from [II]. Image was taken using a SEM.
Results and discussion
53
Figure 22 Aligning a larger part on a smaller part, creating an overhanging structure. The part is a 600 �m × 300 �m × 40 �m and the receptor site is 150 �m × 300 �m × 40 �m. Originally published in [III]. © 2009 IEEE
Figure 23 Flipping parts using droplet self-alignment. This particular manoeuvre is very difficult for conventional microrobotics. a) The gripper approaches the receptor site with a part in its grasp. b) A water droplet is dispensed on the receptor site. c) The droplet wets the side of the part, forming a meniscus between the receptor site and the side of the part. d) The part is released and rotates 90º, the side now facing the receptor site. e-f) Water evaporates, leaving the part aligned and adhered to the receptor site. Originally published in [III]. © 2009 IEEE
5.1.4 Effects of environmental conditions on self-alignment [IV]
The water droplet self-alignment is affected by the ambient environment,
namely its temperature and humidity. This is partly explained by the fact
that the water droplet evaporation rate is affected by the liquid volume,
temperature and the humidity of the environment.
In paper [IV], firstly the droplet evaporation time was measured as a
function of temperature, humidity and liquid volume (Fig. 24). Naturally,
smaller droplets evaporated faster in lower humidity and higher
temperature. In self-alignment experiments, decreased relative humidity
was observed to increase self-alignment duration when aligning silicon
parts on hydrophilic patterns. Also, in very low humidity (5%), small water
54
droplets evaporated too quickly, before the self-alignment was completed,
resulting in failed alignment.
The results suggest that very low humidity and small droplet is
detrimental to water droplet self-alignment, due to droplet evaporating too
quickly. However, this can be compensated by increasing the water droplet
size.
Figure 24 Droplet evaporation time as a function of humidity and liquid volume in a) 25°C and b) 40°C. Smaller droplets evaporated faster in lower humidity and higher temperature. Size of each droplet was approximately 300 pL. Data from [IV].
5.1.5 Parallel delivery of liquid using mist [V]
In paper [V], a study on a method for delivering water to the receptor sites
by condensation from water mist was reported. An ultrasonic humidifier
created water mist, which was delivered to the vicinity of the receptor sites
(Fig. 25). The mist condenses on the surface, forming the droplet for self-
alignment.
In the paper, it was shown that the water condensation process is
approximately linear with respect to time and thus the amount of water can
be easily controlled by adjusting the duration of the humidifying. Part of the
difficulty in developing the model was to estimate the amount of water on
the receptor sites. A machine vision technique was developed that identified
droplets and estimated their volume from the microscope images.
The fact that there are also some droplets on the background, outside the
receptor site, complicates the wetting behaviour. The results showed that
with too large a bias, the yield of the self-alignment drops, partly because
the droplet is not fully confined inside the receptor site. Nevertheless, with
a suitably small bias (< 60% of part size in one direction), a 100% yield was
achieved.
The paper shows that using a humidifier is a viable strategy for the water
delivery. This eliminates the need for the water droplet dispenser, which is
Results and discussion
55
a very delicate component in the system. The parallel nature of the water
mist condensation makes this strategy very appealing, as compared to the
serial nature of the single droplet dispenser.
Figure 25 Water mist is created using a humidifier, and directed near the parts. The mist condenses on the parts, forming the droplet for self-alignment. [V]
5.2 Silicon capillary gripper with self-alignment capability [VI]
In paper [VI], a novel capillary microgripper was reported. The basic
working principle of capillary gripping was outlined in Subsection 3.2.4.
The gripper was fabricated out of silicon using the method detailed in
Subsection 4.2.5. The final, fabricated gripper is shown in Fig. 26.
Figure 26 a) Two different gripper head shapes on a wafer, before singulation. The through-silicon holes are visible in the middle of the gripper heads. The die singulation trenches are also etched during the through-silicon etching. The singulation is done by cutting the small silicon bridges. b) Singulated gripper. A microfluidic connector is attached on the backside of the gripper. [VI]
Using Surface Evolver and geometric models, the gripper was shown to
have self-alignment capability, even when the parts do not have exactly the
same size as the gripper (Fig. 27).
Furthermore, using Surface Evolver models, the gripping force was
estimated for various gripper / part configurations. The gripping force was
estimated to be in the range of 100 μN to 1 mN for parts with lateral sizes
ranging from 250 μm to 500 μm.
The self-alignment capability of the gripper was demonstrated
experimentally in pick-and-place experiments. An image sequence of a part
aligning to the tool is shown in Fig. 28.
56
Figure 27 Shape of the energy well of the gripper. a) All models predict that the gripper is able to self-center the part when both the gripper and the part are matching squares; b) If, however, the part is rectangular, but the gripper is square, there are multiple minima for small amount of liquid, as predicted by the Surface Evolver model. With large enough amount of liquid, the minimum becomes unique and the part centers to the gripper. Data from [VI]
Figure 28 a-d) An image sequence of an experiment showing the self-alignment of a silicon chip to the capillary microgripper. Originally published in [VI]. © 2011 IEEE
5.3 Adhesive droplet self-alignment on oleophilic / oleophobic surfaces [VII]
In paper [VII], adhesive (i.e. glue) was used as the self-alignment droplet.
The components were made out of SU-8 and the alignment patterns were
oleophilic / oleophobic patterns made on ORMOCER®, fabricated using
the method described in Subsection 4.2.3. Electron micrograph of porous
ORMOCER® surface is shown in Fig. 29.
Results and discussion
57
Figure 29 ORMOCER® surface after exposure to oxygen plasma. The porous silicon skeleton is left behind and it is highly oxidized.
Fig. 30 shows the patterns and the contact angle of the adhesive on the
patterns. The achieved contrast was large enough to confine the liquid on
the patterns without the adhesive wetting on the background.
Fig. 31 shows a self-alignment experiment using a droplet of adhesive on
the patterns. The successful self-alignment was shown with bias up to 90
μm for 200 μm × 200 μm microparts. The test was repeated 8 times with
100% yield when the amount of adhesive was 0.5 nL – 1.5 nL. The self-
alignment failed for various reasons when the amount of adhesive was
significantly outside this range.
The accuracy was estimated to be less than 1 μm in x-axis and 3.5 μm in y-
axis, where the measurement of this accuracy was limited by the accuracy of
the optical microscopes.
The duration of the self-alignment was about 500 ms, which is about 10
times longer than with water droplets of similar size and parts of similar
dimensions. The reason for this difference is the increased viscosity of the
adhesive.
58
Figure 30 Contact angle contrast on oleophilic / oleophobic patterns. a) Oleophilic gold patterns on fluorinated porous ORMOCER® background, fabricated using the method described in Section 4.2.3. b) Contact angle of an adhesive droplet on the oleophobic surface: 119º. c) Contact angle of adhesive droplet on the oleophilic patterns: 53º. Reprinted with permission from [VII]. Copyright 2011, American Institute of Physics.
Figure 31 a-c) An image sequence of an adhesive droplet self-alignment. The part is made of SU-8. The self-alignment takes about 500 ms. x- and y-bias were 35μm and 90μm, respectively. Reprinted with permission from [VII]. Copyright 2011, American Institute of Physics.
Conclusions
59
6. Conclusions
Droplet self-alignment using unconventional liquids, such as water and
adhesives, was studied experimentally. Several experiments were
conducted to measure properties that are relevant for practical applications
of the technique: yield, accuracy, duration and adaptability to handle parts
of varying size.
The alignment accuracy that was achieved is comparable to the
manufacturing precision of the microparts and patterns used in the
experiments. The accuracy is close or even better than the accuracies of the
best currently available industrial robots. Parts with more accurate
definition should be used to see the limits of the droplet self-alignment
process itself. The main benefit of using droplet self-alignment is that it can
achieve high accuracy without using high precision robotics.
In [VII], the accuracy of the adhesive droplet self-alignment (~ 3.5 μm)
was measured using optical microscopes, which had a comparable
resolution to the accuracies measured. Therefore, it is possible that the real
alignment accuracy is better than reported. As a future work, the alignment
accuracy should be validated using SEM or some other method e.g. vernier
patterns [35].
To achieve high yield, z-bias and droplet volume should be optimized and
controlled more accurately than x- or y-bias. When other parameters were
optimal, water droplet self-alignment could correct positioning errors as
large as half the lateral dimension of the part, but the positioning tolerances
in z-bias were much tighter. In some extreme cases, water droplet self-
alignment was observed to be successful even when the lateral bias was
almost as large as the size of the part i.e. when only a small edge of the part
was overlapping with the receptor site.
The estimated yield of the alignment process (around 99%) using water
droplet self-alignment is comparable or better than the yield of many self-
assembly processes [4]. However, even higher yields can be expected from
industrial chip assembly processes and to even measure accurately yields
that high, a full-scale industrial test plant would be required. Nevertheless,
for many applications, the achieved yield could already be high enough.
60
Droplet self-alignment is a very fast process, occurring in the time range
of 100 ms in the assembly cases presented in this summary. Whether
droplet self-alignment time should be counted into the cycle time of the
whole assembly process of a device depends on the process: if the
subsequent assembly steps cannot be started before the self-alignment step
is complete, the droplet self-alignment time has to be included in the cycle
time. In cases where the part can be left to self-align spontaneously, the
self-alignment time is not critical.
As a future work, droplet self-alignment methods could be integrated with
electrical connections and bonding steps. Bonding could already be
achieved using adhesives as discussed in Section 5.3, suggestions for
integrating droplet self-alignment with electrical connections were given in
Subsection 1.1.
There is ongoing work on the commercialization of the droplet self-
alignment process. Existing pick-and-place machines can be adapted for
the process, as long as they are able to house the liquid dispensing systems.
The required accuracy of the pick-and-place machine is very low, because
the droplet self-alignment can correct even large placement errors.
Some limitations of the results obtained in this thesis should be explained.
First of all, in [I – VI], water was used for droplet self-alignment, and the
results can be, to some extent, liquid specific. Some of the problems were
identified in [VII] and solutions were offered for adhesives and other oil-
like liquids. However, the work done in this thesis cannot be used to infer
what liquid properties are ideal for droplet self-alignment, because different
liquids were not exhaustively tested. On theoretical considerations alone,
high surface tension is expected to help liquid confinement, and low
viscosity and high surface tension speed up the self-alignment process.
These considerations cannot be used to conclude that high surface tension
and/or low viscosity are always beneficial for droplet self-alignment. In
future research, the experiments done in this work should be expanded by
using various liquids and the pros and cons of each liquid should be
carefully evaluated. Once the critical liquid properties are identified,
methods should be developed to tailor and enhance the desired liquid
properties. To close the circle, the handling method should be
benchmarked against solder self-alignment in comparable experiments.
Droplet self-alignment has been modeled, but the models often include
several simplifications, such as perfect contact line pinning to the pad edge,
no contact angle hysteresis, quasi-static meniscus etc. The typical output of
the models is the shape of the meniscus in equilibrium and self-alignment
forces. However, for wider adoption of droplet self-alignment, the models
should ultimately be able to answer a simple question: using a particular
Conclusions
61
combination of materials, liquids, and process parameters, will droplet self-
alignment work (as judged by practical criteria, such as yield, speed or
alignment accuracy)? While such model may still be quite far away, one
prerequisite for it is the prediction of wetting behavior (contact angle) of a
liquid on a new material. Development of such models in near future seems
more likely.
Furthermore, the microrobotic platform and the microgripper in
particular can have some effect on the results of the droplet self-alignment
experiments in [I – V]. When releasing the microparts, the part can adhere
unevenly to the tips of the microgripper. With a different type of a gripper,
e.g. a vacuum gripper, the adhesion force would be towards a different
direction and would have a different magnitude, which could affect the
success of the droplet self-alignment. As a future research topic, different
grippers should be tested.
Finally, the results, as grouped under the research objectives (Subsection
1.2), can be summarized as
� Several experiments were done to study water droplet self-alignment
assisted robotic placement. Yield, accuracy, capabilities to build
complex structures, and speed of the process were evaluated.
� A new capillary gripper was fabricated from silicon and experiments
showed that the gripper can pick and self-align parts
� Oleophilic / oleophobic patterns were developed for adhesive droplet
self-alignment. Self-alignment of parts on the patterns was shown
experimentally.
62
Acknowledgements
63
Acknowledgements
I am grateful to Adj. Prof. Quan Zhou for his active guidance throughout my
research. Indeed, he hired me as a young student and introduced me to the
whole field of micro- and nanotechnology. Also, this work would have not
been possible without the encouraging leadership of Prof. Heikki Koivo. He
helped in the preparation of this summary, but most importantly, visits to
his office were crucial for keeping the spirits high.
It has been an honor to work with the current and past members of the
Micro- and Nanorobotics research group, including Ville Liimatainen, Bo
Chang, Iiris Routa, Antti Virta, Reidar Udd, Mirva Jääskeläinen, Petri
Hänninen and Petteri Korhonen. They managed creating a comfortable and
a casual atmosphere, which made work days much more enjoyable. I thank
Prof. Yrjö Konttinen for introducing me to the field of cell biology in the
past year, during which I completed writing the summary of this thesis. I
am indebted to Dr. Tech. Antti Pohjoranta, Matias Lahti and my father
Prof. Hannu Sariola for proofreading and invaluable comments on the draft
of the summary. This work was made possible by the financial support I
have received from the Graduate School in Electronics,
Telecommunications and Automation, the Academy of Finland, the
European Commission and the Emil Aaltonen foundation. Assoc. Prof.
Takafumi Fukushima and Asst. Prof. Pierre Lambert did outstanding work
pre-examining this thesis.
I am grateful for the enthusiastic support from my family, mother Anna-
Paula, father Hannu and sisters, who primed me with a deep interest in the
natural world. Finally, I thank my wife Laura, for her loving support and
crazy projects which keep the life worth living for. Had she not wanted to
make Carmen dance with robots, we would not have met.
Any factual errors and omissions remain my sole responsibility.
Espoo, April 2012
Veikko Sariola
64
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74
Appendix A: Neglecting gravity in self-alignment
75
Appendix A: Neglecting gravity in self-alignment
The shape of a liquid meniscus is governed by the Young-Laplace equation
under gravity:
21 /1/1 RRgzp �� � � (11)
where p� is the overpressure inside the meniscus, is the density of the
liquid, z is the height, g is the gravitational acceleration and 2,1R are the
principal radii of curvature. Eq. 11 can be non-dimensionalized by using the
capillary length g
LC �
and the characteristic pressure CC Lp /� , and
setting CLzz /* , Cppp /* � � , CLRR /2,1*
2,1 to arrive at
*2
*1
** /1/1 RRpz � �� (12)
Hydrostatic pressure (gravity acting on the droplet) can be neglected when
CLzz ���� 0* .
For a rectangular part where the liquid fills the bottom of the part fully (as
in Fig. 8), the overpressure under the part due to the part weight is
ghwl
gwlhwlmgp pp
pp
� (13)
where m is the mass of the part, p is the density of the part and ph is the
thickness of the part. Thus
� /*Cpp gLhp � (14)
76
Assuming that �p (for example, the density of SU-8 is
3kg/m1,190 p ), 2/1/ Cp Lg �� and we get Cp Lhp /* �� . The gravity of
the part can be neglected when Cp Lhp ����� 0* .
To summarize: Assuming that a) the part bottom is fully wetted; b) the
part density is close to the density of the liquid; and c) the height of the
liquid film and the height of the chip are much smaller than the capillary
length of the liquid, the gravity can be neglected. Similar conclusions
cannot be drawn for parts with large lateral dimensions, where the droplet
wets only a small portion of the part bottom, since the overpressure due to
chip weight can be much larger.
Appendix B: Energy of a twisted meniscus
77
Appendix B: Energy of a twisted meniscus
To show self-alignment, the energy of the meniscus should be calculated as
a function of part angle � . In Appendix B of ref. [41], equation for the
surface area of a rectangle upon a twist was developed. When a rectangle
with the length l and height h is twisted by angle � along the axis of h ,
the surface area A is given by [41]
22
2 1ln12
aahahlA ���� �
(15)
whereh
la2�
. This can be rewritten as
)(arcsinh1 22
aaahA �� �
(16)
If the chip is assumed to have same length and width ( wl in Fig. 8), the
total energy from twisting all the four faces is given by
)(arcsinh144 22
aaahAE �� �
�� (17)
The torque T is then given by
)(arcsinh144 22
2
aaahAET ��� ��
� ��
� �
��
��
(18)
In ref. [41], the equation for torque was given in a slightly different form,
but it can be checked that the both forms are equivalent. The direction of
torque was chosen as negative, to highlight that the torque is self-aligning.
Fig. 32 shows energy and torque plotted as a function of part angle.
78
Figure 32 Calculating surface energy and torque of a twisted meniscus using equations 17 and 18. Following values were used: 2mJ/m8.72 � , �m10 h and �m300 l . a)
Energy as a function of chip angle. The energy minimum is exactly when º0 � . b) Torque as a function of chip angle. Nm0 T exactly when º0 � .
The method can be extended to rectangular chips ( wl � ), by considering
the energy and torque of front and back faces separately from the energy
and torque of left and right faces.
Appendix C: Publications
79
Appendix C: Publications
�
9HSTFMG*aegeac+
ISBN 978-952-60-4640-2 ISBN 978-952-60-4641-9 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 ISSN 1799-4942 (pdf) Aalto University School of Electrical Engineering Department of Automation and Systems Technology www.aalto.fi
BUSINESS + ECONOMY ART + DESIGN + ARCHITECTURE SCIENCE + TECHNOLOGY CROSSOVER DOCTORAL DISSERTATIONS
Aalto-D
D 6
9/2
012
Veikko Sariola
Droplet Self-A
lignment: H
igh-Precision R
obotic Microassem
bly and Self-Assem
bly A
alto U
nive
rsity
Department of Automation and Systems Technology
Droplet Self-Alignment: High-Precision Robotic Microassembly and Self-Assembly
Veikko Sariola
DOCTORAL DISSERTATIONS
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