Journal of Undergraduate Research in Bioengineering
65
Optimization of Process Parameters for Micropart Capillary
Assembly with Precision Positioning
Andrew X. Zhou,1 Shaghayegh Abbasi,2 Rajashree Baskaran,2 Karl F. Böhringer2
1
2
Department of Bioengineering, University of Washington, Seattle, Washington 98195
Department of Electrical Engineering, University of Washington, Seattle, Washington 98195
Abstract: The assembly of microdevices is currently done mainly with the pick and place method. However, this
method is slow and expensive for assembly requiring high precision. Thus, the development of a new fast, yet highly
precise method of assembly is the focus of current research. Self-assembly in liquid medium, where microdevices
spontaneously arrange themselves into more complex systems using capillary forces, has been shown to be a potential
candidate for parallel batch assembly with precision positioning. However, studies of how the design and process
variables affect the precision of the method have yet to be investigated. The main barrier to achieving the desired
precision in the x-y direction is the tilting of parts during the assembly process. In this project, we investigated the
effect the volume of adhesive has on tilting and developed a theoretical model for the tilting. Our data show that the
magnitude of tilting increases as the volume of adhesive used increases, in accordance with the model. In addition, we
studied a method using vertical vibration with a speaker to correct the tilting. Our data show that this method is
practical under certain conditions of adhesive coverage on the binding sites. With further investigation, tilting should
be able to be corrected consistently.
1. INTRODUCTION
Much research has been done in the past few
decades on microelectromechanical systems
(MEMS). These functional microdevices include
electrical components such as sensors and
actuators.
Complex
systems
of
these
microdevices can be used in a wide variety of
areas such as chemical analysis, biomedical
instrumentation and telecommunications [9].
However, different microdevices cannot be
manufactured on the same substrate due to
incompatibility in fabrication steps and other
issues; hence an assembly method is needed to
put them together in a system after fabricating
separately.
Currently, most of these micro systems are
created using the pick and place method, also
known as a top-down approach. This method
requires the use of a robot to pick individual
devices up and place them in the desired location
for assembly. Assembly with the pick and place
method is effective and can be very precise on
the micro scale. However, there are several
problems with this technique. First, it can be very
expensive if high precision is needed. The
method is also very slow when large numbers of
devices need to be assembled since the speed is
limited by the number of robots or robot arms [5].
Finally, there is a problem known as sticking. On
very small devices (less than a millimeter),
electrostatic, van der Waals, and surface tension
forces become stronger than the force due to
gravity, causing devices to stick to the robots that
are moving them [9]. Thus, a new technique
needs to be developed that is cheaper and faster,
but still has the precision of the pick and place
method.
Self-assembly, also known as a bottom-up
approach, has been the focus of recent research.
Self-assembly refers to a process where
microdevices spontaneously arrange themselves
into a more complex system. This technique
borrows an idea from nature that has been used
for millions of years – the self-assembly of
biological molecules to create a living organism.
Since self-assembly is a parallel process where
multiple devices can be assembled at once, it can,
in principle, create huge systems of correctly
placed microdevices quickly and efficiently [7].
Recent research has also shown that selfassembly can realistically compete with the pick
and place method in terms of precision [5]. This
makes self-assembly a very promising technique
for mass fabrication of small devices. Various
forces have been studied that could possibly
drive the spontaneous assembly process,
including gravitational [10], electrostatic [2,8],
and capillary forces [5,6,7,9].
Capillary-based self-assembly shows promise
as an effective method because capillary forces
are dominant relative to other forces on small
scales, allowing better self-alignment [5]. The
technique involves the use of hydrophilic and
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Journal of Undergraduate Research in Bioengineering
hydrophobic surfaces to selectively apply a
lubricant (a hydrocarbon) to certain areas of the
wafer (substrate) to be assembled on. The
microdevices (parts) have the same hydrophilic
and hydrophobic pattern, and will selectively
bind to the substrate in the desired orientation.
The driving force for this self-assembly and
alignment is the minimization of surface energy,
which is achieved by minimizing the area of
hydrophobic surfaces exposed to the hydrophilic
surroundings. The lubricant can then be solidified
by a curing method, holding the assembled parts
in place [5]. However, problems with the
precision of this technique have yet to be solved.
The main barrier to obtaining the desired
precision is the tilting of microdevices during the
assembly process. Tilting can also cause
problems for sensitive optical applications [4].
To improve the effectiveness of this method, the
tilting has to first be understood and then
corrected. Introduced in this article is a model for
part tilting, which can be used to minimize tilt
angles by altering assembly process parameters.
In addition, a method for correcting tilt is
demonstrated for situations where tilt angles
cannot be minimized below desired levels.
2.2 Substrate and Part Fabrication
Figure 1 summarizes the fabrication process for
the parts and substrates. The substrates were
fabricated on a 4-inch silicon wafer. Due to the
need for high binding precision, the rectangular
binding sites were defined using lithography. In
this process, photoresist AZ4620 was first spin
coated and lithographically patterned on the
substrate. Cr and Au were then evaporated on to
the substrate with thicknesses of 100Ǻ and
1000Ǻ, respectively. Next, a lift-off procedure
was done by soaking the wafer in acetone
overnight to remove any Cr and Au that were
deposited off the binding sites, thus fully defining
the final shape of the binding sites. To make the
gold binding sites more hydrophobic, the wafers
were cleaned with O2 plasma and then soaked in
1mM dodecanethiol in ethanol overnight. Thiol
2. MATERIALS AND METHODS
2.1 Substrate and Part Design
The silicon parts had dimensions of 5 x 5 x
0.1mm and were nonfunctional (no electrical
connections). The large surface area to thickness
ratio was required in order for the capillary forces
to be strong enough to align the parts, and also
for the final purpose of the assembly, which is
3D stacking of high interconnect density
electronic chips on the substrate. The binding
sites on both the substrate and part were made
out of gold and had dimensions of 3.65 x 4.9mm.
Rectangular biding sites were chosen because
there are fewer possible assembly orientations (2)
in comparison to square binding sites (4). This
allows for more flexibility in the circuitry design
on the part due to the decrease in symmetry that
is required. Past simulations have shown that the
rectangular binding sites should have a length to
width ratio of greater than 1.3:1 to consistently
achieve correct assembly orientations [3].
Figure 1. Summary of part and substrate fabrication process. A
silicon wafer is lithographically patterned with photoresist and
evaporated with Cu and Au. After a lift-off procedure, the gold
binding sites are coated with a self-assembled monolayer of
dodecanethiol to make the binding sites more hydrophobic.
molecules selectively bind to gold, creating a
self-assembled
monolayer
(SAM)
of
dodecanethiol on the surface of the gold binding
sites. Parts were fabricated by lithography in the
same way as the substrates except that SAM
coating was not done for the parts. An additional
step was needed in part fabrication to create the
desired part dimensions of 5 x 5 x 0.1mm. The
wafer of parts was thinned to 0.1mm by a
grinding process from the back side of the wafer
and then diced into 5 x 5mm sections.
Journal of Undergraduate Research in Bioengineering
2.3 Self-Assembly Using Capillary Forces
Figure 2 summarizes the general process for part
self-assembly using capillary forces. The
fabricated substrate with hydrophobic binding
sites was first placed in water. Next, a heat
curable adhesive was placed onto the
hydrophobic binding sites using a micropipettor.
The heat curable adhesive was made of 97%
triethyleneglycol dimethacrylate and 3% benzoyl
peroxide by weight, and was capable of
solidifying after heating. Since the adhesive was
also hydrophobic, it selectively covered the
hydrophobic gold binding sites. Parts were then
introduced into the water and allowed to selfassemble by energy minimization as discussed
previously. Once the parts were self-assembled,
the water was heated to 70°C for 2 hr to solidify
the heat curable adhesive.
2.4 Tilt Correction
Tilt correction was achieved using vertical
vibration with a speaker. First, parts were
67
allowed to self-assemble in a container of water
as described previously. The container was then
secured to a stage of a speaker connected to a
waveform generator outputting a 15Hz, 9V peakto-peak sinusoidal wave. The vibration frequency
was chosen based on the resonant frequency of
the vibration system (speaker) since it gives
maximum energy to the part-substrate system.
The assembled parts were allowed to vibrate for
2 min. The tilt angles of the parts were measured
before and after vibration using an optical camera.
3. RESULTS
3.1 Tilt Model
The tilt angle of the part was modeled using force
balance analysis. In the model, it was assumed
that one edge of the tilted part was always in
contact with the substrate, which was shown to
be true over 90% of the time in experiments. As
shown in Figure 3A, the two main forces acting
on the part are gravity and the capillary force.
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Journal of Undergraduate Research in Bioengineering
The capillary force can be calculated using the
Young-Laplace equation:
consistently overestimates the actual values at all
volumes of adhesive.
⎛ 1
1 ⎞
Fc = ΔPA = γ AW ⎜ + ⎟ A
⎝ R1 R2 ⎠
(1)
where Fc is the capillary force, ΔP is the pressure
difference between the adhesive and the water,
γAW is the adhesive-water interfacial tension, A is
the binding site area, and R1 and R2 are the radii
of curvature defined in Figure 3A. The force of
gravity Fg in the opposite direction of Fc can be
calculated using the equation
Fg = mg cos(α )
(2)
where m is the mass of the part, g is the
acceleration due to gravity, and α is the tilt angle.
In this situation, the force due to gravity is
minimal or near zero because the part is so small.
For a tilted part to be in an equilibrium state, the
capillary force must be equal and opposite the
gravitational force. In order for the capillary
force to be near zero, ΔP must also be near zero,
corresponding to an R1 of infinity, or a straight
interfacial line between the water and adhesive,
as seen in Figure 3B. The relationship between
adhesive volume and tilt angle can then be
calculated using geometry, resulting in the
equation
Figure 3. Model for tilting. (A) The two main forces
acting on the part are gravity and the capillary
force. (B)With the assumption that the gravitational
force is near zero, it was concluded that the
interfacial surface between the adhesive and water
is close to a straight line.
3.3 Tilt Correction
Simulations using Surface Evolver showed that
the global energy minimum of an assembled
system is at a tilt angle of 0°. As discussed
previously, the driving force for self-assembly is
⎛α ⎞
Vα = WL2 sin ⎜ ⎟
⎝2⎠
(3)
where W and L are the width and length of the
binding site, and α is the tilt angle.
3.2 Model Validation
To validate the tilting model, controlled amounts
of the heat curable adhesive were placed on the
binding sites of the substrate using a
micropipettor, ranging from 3 μl to 17 μl. Part
assembly and heat curing were performed as
previously described. Tilt angles were then
measured using an optical camera. At least five
trials were done for each adhesive volume. A
comparison between the model (Eq. 3) and
experimental results is shown in Figure 4. As
demonstrated by the model, tilt angle increases as
the adhesive volume is increased. Experimental
results follow a similar trend, but the model
Figure 4. Model validation. The tilt model is
plotted with the part tilt angle as a function of
adhesive volume. Experimental results are also
shown. Error bars represent standard deviation.
energy minimization. Thus, assembled parts
should tend to an orientation of zero tilt.
However, due to friction and other attractive
forces between the part and substrate, there are
Journal of Undergraduate Research in Bioengineering
69
other local energy minima where the part can be
in equilibrium. As seen in Figure 5, parts
approaching the binding site from the side push
the adhesive so that it is no longer distributed
evenly across the binding site. This results in the
system reaching an equilibrium position in which
the part is tilted (a local energy minimum).
Figure 5. Part tilting. A part is self-assembled from
the right side and becomes tilted after assembly is
completed.
To correct tilting, external energy must be
supplied to push the system to its global energy
minimum. Vertical vibration with a speaker was
used to supply the external energy in this
experiment. Different volumes of adhesive were
placed on the binding sites using a micropipettor,
ranging from 3 μl to 11 μl, to test the
effectiveness of tilt correction at different
adhesive volumes. At least five trials were done
for each adhesive volume. Tilt correction can be
clearly seen in an example in Figure 6. Figure 7
shows part tilt angles before and after vibration at
multiple adhesive volumes. As seen in the plot,
tilt was corrected to below 2° at all tested
volumes.
4. DISCUSSION
4.1 Tilt Model
As seen in the results, the experimental values
were consistently lower than the values predicted
by the model. The likely explanation for this
trend is that the assumption of part-substrate
contact may not always be true. In addition, the
assumption of near zero gravitational force
produced a simplified model. Future models can
Figure 6. Example of tilt correction.
(A) A part is tilted after assembly. (B)
The same part is shown with tilt
corrected after vertical vibration with
a speaker.
incorporate a more accurate force analysis with
fewer simplifications. The current tilt model can
be used in applications where there is a
maximum acceptable tilt angle. The model
allows for the calculation of the maximum
volume of adhesive that can be used in order to
guarantee that the tilt angle is below a required
limit. As determined by the model and the
experimental results, the tilt angle can be
minimized by decreasing the adhesive volume.
However, there are limitations to how much the
adhesive volume can be reduced. The adhesive
must fully cover the binding site and the
minimum amount of adhesive that can be
deposited on a binding site will be determined by
the depositing method and adhesive contact angle.
In certain situations, larger than desired tilt
angles will be unavoidable and an effective tilt
correction method must be used.
4.2 Tilt Correction
The key to tilt correction is to supply external
energy to the system so that the part is released
from its local energy minimum equilibrium
position, giving the part an opportunity to fall
into its global energy minimum. Vertical
vibration with a speaker was able to accomplish
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Journal of Undergraduate Research in Bioengineering
Figure 7. Tilt correction with vertical vibration. The tilt angle of assembled parts before and after vibration
is plotted for various adhesive volumes. Error bars represent standard deviation.
this by supplying kinetic energy to the system.
The results confirmed that the global energy
minimum at all adhesive volumes is indeed the
flat state, as predicted by the Surface Evolver
simulation.
4.3 Future Work
The tilt angle model described here gives the
relation between tilt angle and adhesive volume.
In the next step, by taking into account the
gravity force, we would be able to find a more
complete relation between tilt angle and all
assembly parameters such as water-adhesive
interfacial tension, part density, and adhesive
volume. This relation can then be verified by
doing experiments where all assembly
parameters are fixed and only one parameter is
changing. Another goal is to be able to deposit
adhesive on the binding sites using dip coating
instead of using a micropipettor. Dip coating
allows adhesive to be deposited on multiple
binding sites at once, speeding up the overall
process. In addition, the volume of adhesive that
is deposited will be very consistent on all binding
sites and can be easily controlled by controlling
the velocity and direction of dipping. Finally,
more studies can be done on tilt correction to
optimize the amplitude and frequency of
vibration around the resonant frequency of the
adhesive, which will give the best correction
results.
ACKNOWLEDGEMENTS
This work was supported by research grants from
Intel Corporation and grant number NSF EEC
9529161. Special thanks also to the University of
Washington Engineered Biomaterials (UWEB)
Research Experience for Undergraduates
program. In addition, the authors would like to
thank the Microfabrication Laboratory at the
Washington Technology Center for helping with
the clean room microfabrication process.
REFERENCES
1. Abbasi S, Zhou AX, Baskaran R, Böhringer
KF. Part tilting in capillary-based selfassembly: modeling and correction methods.
IEEE International Conference on Micro
Electro Mechanical Systems, Tucson, USA,
2008;1060-1063.
2. Böhringer KF, Goldberg KY, Cohn MB,
Howe
RT,
Pisano
AP.
Parallel
microassembly using electrostatic force fields.
IEEE International Conference on Robotics
and
Automation,
Leuven,
Belgium,
1998;483-496.
3. Fang J, Wang K, Böhringer KF. Selfassembly of PZT actuators for micropumps
Journal of Undergraduate Research in Bioengineering
4.
5.
6.
7.
8.
9.
10.
with high process repeatability. Journal of
Microelectromech Systems 2006;15:871-878.
Scott KL, Howe RT, Radke CJ. Model for
micropart planarization in capillary-base
microassembly. The 12th International
Conference on Solid State Sensors, Actuators
and Microsystems, Boston, USA, 2003;13191322.
Srinivasan U, Liepmann D, Howe RT.
Microstructure to substrate self-assembly
using
capillary
forces.
Journal
of
Microelectromech Systems 2001;10:17-24.
Terfort A, Bowden N, Whitesides GM.
Three-dimensional
self-assembly
of
millimeter-scale
components.
Nature
1997;386:162-164.
Terfort A, Whitesides GM. Self-assembly of
an operating electrical circuit based on shape
complimentarily and the hydrophobic effect.
Advanced Materials 1998;10:470-473.
Tien J, Terfort A, Whitesides GM.
Microfabrication through electrostatic selfassembly. Langmuir 1997;13:5349-5355.
Xiong X, Hanein Y, Fang J, Wang Y, Wang
W, Schwartz DT, Böhringer KF. Controlled
multibatch self-assembly of microdevices.
Journal of Microelectromechanical Systems
2003;12:117-127.
Yeh HJ. Smith JS. Fluidic Assembly for the
integration of GaAs light-emitting diodes on
si substrates. IEEE Photonics Technology
Letters 1994;6:706-708.
71