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RAPID TOOLING USING SU-8 FOR INJECTION MOLDING
MICROFLUIDIC COMPONENTS
Thayne L. Edwards*1, Swomitra K. Mohanty1, Russell K. Edwards2, Charles Thomas2, A. Bruno Frazier1
1
Georgia Institute of Technology, MiRC, 791 Atlantic Drive, Atlanta, GA 30332
University of Utah, 50 South Central Campus Drive, Room 2202, Salt Lake City, UT 84112
2
ABSTRACT
In this work we present an injection molding tool fabricated using current micromachining techniques. The process was used to
fabricate micro fluidic channels in a plastic substrate with depths of approximately 27 µm. The process can easily be altered to
form channels of varying depths ranging from a few microns to approximately 100 µm. The tool was made using a
photosensitive epoxy (SU-8) on silicon. Complex two dimensional micro channel / chamber shapes and intersections are also
achievable because the process for defining SU-8 is lithography. Two polymers were successfully used for injection molding
the channels, clear rigid polycarbonate and opaque flexible polypropylene. Plastic replications of the inverse pattern of the SU8 tool were made with the described process. Fabrication time of the tool was approximately 30 minutes and survived without
failure, 22 shots for polypropylene and eight shots for polycarbonate (Lexan). Some deformities of the channels were observed
and were more pronounced in the PP channels. Channel height was increased by 2-3 µm due to a ridge that was formed due to
shear forces generated during the release stage of the process. The channel width shrunk slightly (approximately 1.7%
maximum for polypropylene) after release from the mold.
Keywords: SU-8, injection molding, microfluidics, surface replication, micromolding
1. INTRODUCTION
The need for faster and less expensive techniques to manufacture microfluidic components and systems is always present as
these devices move from research, to prototyping, and eventually to large-scale production. One common production method
that has proven itself in the past for creating parts faster and cheaper is injection molding. This technique has also been applied
to micromachined parts and will most likely play a large role in the future of micromachining. This paper explores the use of
the use of SU-8 epoxy in the rapid fabrication of a micromold tool to be used in an injection molding process.
1.1. Micromolding Techniques
Several molding techniques have been developed over the years using materials such as silicon, metals, and polymers. One
popular method for micro molding is the cast-molding technique. Yuh-Min Chiang et al. have characterized cast molding of
various polymers using different types of materials1. Their work uses structures created on silicon, glass, nitride, and SU-8.
Using ICP etching, structures can be well defined within a silicon wafer, which can then in turn be used as a mold master for
molding polymers. However, this type of molding can be time consuming and tedious to do. Wafers defined using ICP etching
can take a long time to define depending on the structure one is making. In addition, cast-molding requires a slow curing
polymer to be used as its substrate so that the resulting mold has all the features desired. One other issue associated with this
type of molding is the release of the mold from the master. At times, removing a mold can be difficult resulting in a damaged
product. The time and effort used in these processes can be long and tedious. Another technique that is used for cast molding
is to use to epoxy-based polymer SU-8 as a surface mold structure. Using photolithography, a design can be easily defined on a
silicon wafer creating a mold quickly and easily.
As mentioned above metals can be used in micromolding masters. One popular metal used in various electro-formed
applications is nickel.3,4 Micro electro-formed nickel can be built on the surface of a silicon wafer, and that wafer can then be
*
Correspondence: Email: gte639t@prism.gatech.edu; Telephone: (404) 894-2030; Fax: (404) 894-4700;
WWW: http://users.ece.gatech.edu/frazier/mil.html
used to mold a part. One method for micro electroforming involves depositing metal seed layers on a silicon wafer, and then
using thick photoresist to act as a mold for the metal as it electroplates in place. Another method of is done using deep X-ray
lithography (LIGA process) to define high aspect ratio structures where metal is plated forming micro mold master. These
molds are often used in hot-embossing5 and injection molding6 because of their increased durability over a polymer structure.
However, these methods of making molds are time consuming, taking several hours to make. Results can also vary a little with
micro electroforming metal, which can be problem if the structures created have a tight tolerance. But once a mold is created
using these processes, several devices can be created quickly using a hot embosser or injection molder. The application of
MEMS to making plastic technologies has great potential.
1.2. SU-8 as a Material for Micromolding
Using SU-8 as a micro master-mold for injection molding is an attractive method because of the speed one can create a mold
master, and then create several parts. This method is novel way for rapid-prototyping, making it possible to literally create
hundreds of parts in the same time it would take to make one part using a cast-molding technique. This method also has clear
advantages over micro-electroforming metals for mold masters in that creating an SU-8 master can take less than 2 hours
(depending on the size of the structures built) as opposed to a metal mold, which can take several hours. Injection molding also
gives very high repeatability in parts, especially when using the increasingly popular electric injection molding machines now
available on the market.
2. MATERIALS AND METHODS
2.1. Fabrication of the SU-8 Micromold
A three-inch, single-side polished, silicon wafer is coated with a 27-micron thick layer of SU-8 using spin coating. The wafer
is then exposed to UV light using a dark field mask creating a negative mold-master of the part desired. The dimensions for the
channels in this case are 7 mm wide and 3 cm long. If desired, the SU-8 master mold can be baked at 200 ºC for 30 minutes to
ensure all solvents are driven out of the resist and to make the mold master harder and durable. Once the mold is patterned, a
thin layer of titanium is sputtered followed by 1000 angstroms of aluminum on the surface of the SU-8. This aluminum serves
to reduce the sticking of the polymer to the mold master, see Figure 1.
A hydraulic injection-molding machine was used for this experiment. An aluminum insert was machined for this purpose. A
circular well with dimensions slightly larger than 3 inches wide and 1 mm deep was machined in to the surface of the first
aluminum insert. This insert is where the SU-8 master mold is positioned prior to injection molding, see Figure 2. On the
second aluminum insert, another well was machined out directly opposite of the first well. The dimensions of the second well
are slightly larger than 3 inches and 1 cm deep. This well allows plenty of room for the mold to be imprinted into the polymer
and established the thickness of the plastic part. Once the wafer was in place, polyimide tape was used along the edges to
fasten the mold to the aluminum inserts. The tape also had the effect of keeping the wafer from breaking as the plastic flowed
across the edges creating large shear forces.
2.2. Plastics Used and Their Properties
Two polymer materials were selected to explore the manufacturing issues of the microfluidic channels over a range of varying
polymer properties. Issues of interest are the replication characteristics of the polymer and the survivability of the silicon SU-8
tool. The two polymers used were Lexan (polycarbonate, PC) and polypropylene (PP). The two polymers represent extremes
in polymer properties and processing requirements. Polycarbonate is a tough, rigid material. It is relatively viscous in the melt
requiring high temperatures and pressures to properly fill and pack the mold. On the other had, PP is flexible and compliant. It
flows much easier in the melt resulting in lower processing temperature and pressures. The individual characteristics of each
polymer are explained below.
2.2.1. Polycarbonate
Polycarbonate (PC), trade name Lexan, is an amorphous material with a glass transition (Tg) around 150 °C. Because it is
amorphous, it is clear and rigid. Like all amorphous plastics, its shrinkage is low over a wide temperature range. PC has a
typical shrinkage of 0.005 mm/mm. Low shrinkage helps to achieve high dimensional tolerances. However, unlike other
Figure 1. Fabrication of SU-8 master mold. SU-8 is patterned and then approximately 500 angstroms of
titanium is sputtered (not shown) followed by 1000 angstroms of aluminum over the SU-8, The mold is
then placed in the injection molder where plastic is molded over the SU-8 and then released.
amorphous plastics such as polystyrene, PC is comparatively tough. It has a notched IZOD impact test of 17 ft-lb/in compared
to 0.65 ft-lb-in for PP. These characteristics make PC an ideal choice for optical devices such as compact discs and eye lenses.
PC is relatively viscous in the melt having a melt flow index (MFI) of 7 g/10 min and a density of 1.20 g/cm3. Because of its
high melt viscosity, PC requires higher temperatures, faster injection speeds and greater pressures to properly fill and pack the
mold. These properties can pose processing challenges in thin molds.
2.2.2. Polypropylene
Polypropylene is a semicrystalline plastic with a melt temperature (Tm) of 177 °C and a Tg of -15 °C. Because of its low Tg, the
amorphous region remains in a soft flexible state at room temperatures. The soft amorphous regions tie the rigid crystal
structures together acting as spring. This allows PP to be flexible and relatively soft at room temperatures. PP percent
crystallinity varies depending on the processing conditions. Because of the uniform nature of the crystals, the crystals are
denser than the amorphous regions. The formations of the crystal structures cause the part to shrink much more than an
identical amorphous part. A factor of the overall shrinkage is therefore a function of the percent crystallinity of the part.
Typical shrinkage for PP is 0.011 mm/mm.
PP has a high MFI of 23 g/10 min and a density of 1.06 g/cm3. Because of its low Tg, the polymer flows well when heated
above the melt temperature. For this reason, PP can be injected at relatively low processing temperatures and pressures. This
makes it easier to fill and pack the mold. Because of its flexibility, the part can be removed from the tool easier reducing the
possibility of breaking or cracking.
Figure 2. Illustration of setup for Injection molding machines. Specially
designed aluminum insert is placed within the injection mold clamps. The
SU-8 master mold is then placed within the aluminum insert.
PC has a water absorption of 0.35 percent. It is therefore necessary to dry the polymer before it is injection molded. Prior to
molding, a volume of PC was dried for 12 hours at a temperature of 121 °C. The PP did not require drying.
2.3. Injection Molding Machine and Controller
The parts were molded on a Boy 50 injection molding machine that had been modified by the installation of a closed loop
servo-valve hydraulic system produced by MOOG. The MOOG controller allows the injection phase to operate under closed
loop velocity control and the hold phase to operate under closed loop hold pressure control.
The machine has a standard mold base installed that was modified to hold one aluminum insert on both sides of the mold. On
the movable side, an aluminum insert was created to house the silicon wafer. On the opposite stationary side, the insert had a
thin disk cavity machined slightly smaller and directly opposite to the silicon wafer.
2.4. Process Parameters
The MOOG controller installed on the injection molding machine operates under a sequential three-phase cycle. The name and
order of each phase is the injection, hold and cooling phases respectively. The injection phase forces the plastic into the mold
constrained to a velocity profile. The MOOG controller allows the operator to modify the profile. In this set of experiments,
the velocity profile was constant. Both PC and PP were injected at a constant velocity of approximately 3.8 cm/s. This velocity
was determined by running experiments on PC with a dummy wafer installed. The experiment was conducted by molding a set
of sequential parts. The first part was molded under a fast injection velocity. On each successive part, the magnitude of the
velocity profile was decreased while each part was observed for defects. A range of parts molded under fast injection velocities
had surface irregularities. The speed was decreased until the surface defects were no longer apparent. The injection velocity
used for PC was also used for PP. The PP parts did not show the surface irregularities over the operating range of injection
velocities.
The MOOG controller was programmed to switch from the velocity to hold phase by a predetermined screw position. Once the
screw position passed the set point, the controller entered the holding phase. The holding phase constrains the hydraulic
pressure to a pressure profile. Again, for this experiment, the holding pressure profile was set at a constant through the entire
holding phase. The holding phase packs the mold and keeps the plastic from running back out of the gate until gate freeze. If
the gate is not frozen when the holding pressure is released, the part will deflate causing sink marks. As with the injection
velocity, the holding pressure and time were also determined by experimentation. The holding pressure was set to a low value
and parts were molded. Because of the low holding pressure, short shots occurred. The pressure set point was increased for
each successive part until the part was properly packed without flash. The holding time was determined by setting the time for
a relatively long period. The time was decreased until deflated parts were molded. The hold time was then increased. The
holding pressure and time for PC was approximately 6.2 kPa and 7seconds while the PP was 1.7 kPa and 6 seconds
respectively.
After the holding time expires, the controller changes from the holding phase to the cooling phase where the part continues.
Once the part has solidified, the part is ejected and the cycle can be repeated. The cooling times for both plastics were
approximately 15 seconds.
Other parameters set in the injection molding machine include barrel and mold temperatures and backpressure. The barrel
temperature for PC was 282 °C and 218 °C for PP. The backpressure for both plastics was set to 0.7 kPa with no
decompression. The mold had no cooling or heating. The mold relied on convection to cool the part.
After the proper set points were determined, addition parts were molded and discarded until stable conditions were reached;
indicated by stable performance of the barrel heaters at their set points. The dummy wafer was removed and the SU-8 silicon
wafer was inserted. A series of samples were molded in consecutive order until the silicon tool failed. Each part was marked
and stored. As in the case with PP, more than 22 parts were molded with no indication of failure.
3. RESULTS AND DISCUSSION
3.1. SU-8 Micromold
The first SU-8 micromold used for surface replication of the microfluidic channel was imaged with the Tencor, model P-10
profilometer and is shown in Figure 3. The dimensions of this end of the channel are 27 µm × 0.7 cm. The surface of the SU-8
mold appears very smooth and flat with the exception of a small ridge that appears around the top edge of the mold. The height
of this ridge is insignificant when viewed in a two-dimensional profile, as will be shown later. The sides of the SU-8 structure
appear to be sloped but are actually straight. The sloped wall effect in Figure 3 is due to the slope of the profilometer stylus.
The walls are shown to be straight with an SEM image of the same SU-8 micromold, shown in Figure 4. The SEM also shows
the smoothness and flatness of the SU-8 mold. No noticeable defects are seen in this figure. The structure is the inverse pattern
of the channel to be formed from the polypropylene and polycarbonate plastics.
3.2. Polycarbonate Channels
A three-dimensional profile of a channel from one of the PC parts, duplicated from the mold shown in Figure 3, is shown in
Figure 5. The bottom surface of the channel appears very flat and smooth. On the top of the channel walls there are defects
present in the form of bumps and pits. These defects are typically caused by dust particles, as the injection molding machine
was not located in a clean room environment. These defects are not crucial because of their location on this part; however,
defects such as these may be significant in some other application. Again, the walls appear sloped because of the angle of the
profilometer stylus. The walls are actually quite straight as can be seen in an SEM image of the corner of the channel as shown
in Figure 6. However, unlike the walls in Figure 3 on the SU-8 mold, these walls are rounded at the top. This rounded edge
may be because the part did not fill completely with PC at the base corner of the micromold. The round edge may also be from
releasing the part from the mold before it had solidified completely. In addition to the rounded edge, there is also a small ridge
at the top edge of the channel. The ridge formed from the shrinkage of the plastic around and the mold and also the shear force
between the plastic and SU-8 during mold release. However, the ridge is quite small and does not extend very far into the
plastic wall. The plastic parts were rigid and mostly transparent.
3.3. Polypropylene Channels
One end of a channel from a PP part was also imaged with the three-dimensional profilometer and is shown in Figure 7. This
part is nearly identical to the PC part with a few noticeable differences. Like the PC part the surface of the channel and channel
walls are very smooth and flat. However, defects are again noticeable in the surface, possibly due to dust particles present
during the mold process. The walls are rounded at the top as in the PC part; however, the rounding is not as significant in the
PP channel as in the PC channel. The PP plastic was more fluid and easier to inject than PC, which allowed the base corner of
the mold to be more completely filled. There is a ridge around the edge of the channel, which appears taller and extends further
away from the channel cavity than in the case of the PC part. This effect is also noticeable in the SEM image of the corner of
the channel, Figure 8. Polypropylene is much softer and easier to deform than PC. These properties allowed a larger ridge
formation from the shear force during the release step of the process. These parts are soft, flexible and completely opaque.
3.4. Tool Failure
Failure of the tool occurred either when the silicon wafer broke or when a portion of the SU-8 micromold was pulled from the
silicon substrate. The first mode of failure usually occurred when the wafer was lifted slightly from the aluminum cavity in
which it was seated, and plastic then flowed behind the wafer and broke it. This problem was solved by placing thin, Kapton
tape around the edge of the wafer. Breakage still occurred in some cases, however. For this case, the wafer would typically
break at the entrance region for the plastic. The reason for this is probably due to the high shear forces on the wafer due to the
very high plastic velocity at that point.
The second mode of failure was when the SU-8 mold was pulled from the substrate. The reason for this mode of failure is like
due to the plastic shrinking around the micromold during the cooling phase and not releasing easily from the mold during the
ejection phase. Evidence of this is found in the SEM images of the PC and PP channels, Figure 4 and Figure 6. The ridge
around the edge of the channel is due to the high shear force between the plastic and SU-8 walls when the part is released from
the mold.
3.5. Comparison of PC and PP
The replicate parts of the SU-8 mold using PC were more difficult to accomplish than with PP due to the material properties as
explained earlier. Tools lasted anywhere from one to eight shots when PC was used to replicate the mold. The tool used to
replicate the mold with PP showed no signs of failure after 22 shots, at which point the experiment was terminated.
Two-dimensional profiles of the SU-8 micromold, PC channel, and PP channel are overlaid in Figure 7 to show a comparison
between the replicate channels and the original mold. In this figure, the ridges that were observed earlier are more easily
noticed. Although only a small ridge is apparent for the PC channel, the overall channel height is increase by 2-3 µm. The
ridge is much more pronounced in the PP channel with the walls 2-3 µm higher than the mold and then actually dipping below
the original micromold channel height further away from the channel. The channel width shrunk in the case of the PP channel,
but did not change much for the PC channel. Also noticeable is the rounded edge at the top of the walls. The slope seen in the
walls is again due to the slope of the profilometer stylus.
The injection molded components were bonded with a planar, plastic cover plate to achieve enclosed leak-free micro channels.
The bonding was achieved using both solvent bonding and UV curable adhesives. In both cases, the bonding liquids were
routed around the perimeter of the microchannels using wicking channels designed into the injection molded component. The
two halves are aligned and mechanically clamped prior to application of the bonding solvent and / or adhesive.
4. CONCLUSION
The rapid and inexpensive production of plastic micro fluidic channels has been accomplished using injection molding of two
different types of plastics, polycarbonate (rigid) and polypropylene (soft). The injection mold tool used was fabricated in a
rapid process of patterning SU-8 on a silicon wafer using common lithographic techniques. The SU-8 in some cases was
covered with a thin bi-metal layer, but was observed to have no effect on the performance or lifetime of the mold in replicating
the channels. The SU-8 mold was approximately 27 µm in height and was shown to be very smooth and flat, with little or no
defects. The process time without the metal coating was approximately 30 minutes. Previous methods demonstrated used mold
materials such as nickel and polycrystalline silicon. Common process times for both these methods are on the order of several
hours.
Figure 3. Profile of SU-8 mold on a silicon wafer. The dimensions are
27µm x 0.7mm. The walls appear to be sloped only because of the
angle of the profilometer stylus.
Figure 4. SEM image of one end of the
SU-8 mold. The walls are straight and
the surface smooth without defects.
Figure 5. Profile of PC channel made from the mold in Figure 3. The
walls are slightly curved at the top.
Figure 6. SEM image of the corner of
the PC channel in Figure 5.
Figure 7. Profile of polypropylene (PP) channel made from the mold
in Figure 3. The walls are curved similar to the PC channel in Figure 3.
The surface defects are from dust particles.
Figure 8. SEM image of the corner
of the PP channel in Figure 5. The
ridge is wider than in the PC channel,
Figure 7.
5
0
height (um)
-5
-10
-15
Polypropylene
-20
Polycarbonate
-25
SU-8 Mold
-30
0
0.2
0.4
0.6
0.8
1
1.2
1.4
width (mm)
Figure 9. Two-dimensional overlay profile of the two polymer channels and the SU-8
mold.
The plastic channels showed deformities for both the PC and PP materials. A small ridge around the edge of the channel raised
the height of both channels by about 2-3 µm. The ridge was more pronounced in the softer PP plastic channels. The PP
channel width also shrunk more so than the rigid the PC plastic channels. Also in both plastic channels, a rounded edge was
noticed at the top of the channel wall. This rounded edge may be due to an incomplete filling of the channel or not enough
cooling time before releasing the part from the mold.
The two modes of failure were breaking the silicon wafer and pulling the SU-8 from silicon substrate. When using PC for
replicating channels, failure was usually seen after 1-8 shots. However, when using the softer PP plastic, no failure was seen in
the mold up to 22 shots, at which time the experiment was terminated.
ACKNOWLEDGEMENTS
Support for this project was provided by the University of Utah Technology Innovation Grant, the Whitaker Foundation, and
MicroChem Corporation.
REFERENCES
1.
2.
3.
4.
5.
6.
Yuh-Min Chiang, Mark Bachman, Charles Chu, G.P LI., “Characterizing the process of cast molding microfluidic
systems”, SPIE Conference on Microfluidic Devices and Systems II, Santa Clara, California, September 1999, Vol 3877,
pp. 303-311.
Byung HoJo, and David J., Beebe “Fabrication of Three Dimensional Microfluidic Systems by Stacking Molded
Polydimethylsiloxane (PDMS) Layers, ” SPIE Conference on Microfluidic Devices and Systems II, Santa Clara,
California, September 1999, Vol 3877, pp. 222-229.
I. Papautsky, S. Mohanty, R. Weiss, A.B. Frazier, “High Lane Density Slab Gel Electrophoresis by Micromachined
Instrumentation,” BMES and EMBS, Atlanta, GA, USA Oct 13-16, 1999, Session 9.5.2.
J. Brazzle, I. Papautsky, A. Bruno Frazier, “Micromachined Needles Arrays for Drug Delivery or Fluid Extraction”, IEEE
Engineering in Medicine and Biology, November/December 1999, pp. 53-58.
H. Becker, U. Heim, O. Rotting, “The fabrication of polymer high aspect ratio structures with hot embossing for
Microfluidic applications”, SPIE Conference on Microfluidic Devices and Systems II, Santa Clara, California, September
1999, Vol 3877, pp. 74-79.
Boone, T.D., Hooper, H.H., Soane, D.S., Integrated chemical analysis on plastic microfluidic devices, 1998 Solid State
Sensor and Actuator Workshop, Hilton Head Island, SC, June 8-11, pp. 87-92.