RAPID TOOLING USING SU-8 FOR INJECTION MOLDING MICROFLUIDIC COMPONENTS

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1 Header for SPIE use RAPID TOOLING USING SU-8 FOR INJECTION MOLDING MICROFLUIDIC COMPONENTS Thayne L. Edwards *1, Swomitra K. Mohanty 1, Russell K. Edwards 2, Charles Thomas 2, A. Bruno Frazier 1 1 Georgia Institute of Technology, MiRC, 791 Atlantic Drive, Atlanta, GA University of Utah, 50 South Central Campus Drive, Room 2202, Salt Lake City, UT 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 SU- 8 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 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 materials 1. 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: gte639t@prism.gatech.edu; Telephone: (404) ; Fax: (404) ; WWW:

2 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-embossing 5 and injection molding 6 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 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 Fabrication of the SU-8 Micromold 2. MATERIALS AND METHODS 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 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 Polycarbonate Polycarbonate (PC), trade name Lexan, is an amorphous material with a glass transition (T g ) 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 mm/mm. Low shrinkage helps to achieve high dimensional tolerances. However, unlike other

3 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/cm 3. 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 Polypropylene Polypropylene is a semicrystalline plastic with a melt temperature (T m ) of 177 C and a T g of -15 C. Because of its low T g, 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 mm/mm. PP has a high MFI of 23 g/10 min and a density of 1.06 g/cm 3. Because of its low T g, 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.

4 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 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 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

5 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 SU-8 Micromold 3. RESULTS AND DISCUSSION 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 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.

6 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 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 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.

7 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.

8 height (um) Polypropylene Polycarbonate SU-8 Mold 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. 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 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 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 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 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 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