Rapid Prototyping and Development of Microfluidic and BioMEMS Devices J. Sasserath and D. Fries Intelligent Micro Patterning System Solutions, LLC St. Petersburg, Florida (T) 727-522-0334 (F) 727-522-3896 www.intelligentmp.com Introduction In order to reduce the cost of manufacturing and provide the flexibility needed to respond to cyclical economic conditions, outsourcing of manufacturing functions is commonly used. This model has proven successful in many high technology industries. For example, the rapid growth of the Taiwanese semiconductor manufacturing industry has been based on this outsourcing model. Additionally, there are over fifty (50) commercially available fabrication facilities for silicon-based MicroElectroMechanical systems (MEMS) in the United States alone 1. A similar concept is presented for outsourcing the design, prototyping, and testing of BioMEMS and microfluidic devices. Although conceptually similar to the silicon device centers, many fundamental differences exist and will be discussed below. Additionally, an overview of a BioMEMS/microfluidic device outsource fabrication facility will be provided. Lastly, success stories using this model will be provided. Key Factors that Drive the BioMEMS/Microfluidics Outsource Model In order to respond to the needs of device designers in these areas, a number of significant differences will exist between this outsource and traditional silicon manufacturers. These are all a result of the differences in device needs, as well as the economic considerations that are frequently present with these devices. Substrates: Whereas silicon fabs are set up to deal with standard, well defined silicon wafers, the BioMEMS and microfluidic communities are often forced to work with non-silicon materials. Considerations that drive these include the need to work with large devices (which can often exceed a few square centimeters in size), incompatibility of silicon with many fluids, and the need to start with low cost materials. Examples of substrates often used in these applications include glass slides, PMMA squares, PVDF & other polymeric materials, and rigid plastics that have topography. Critical Features: While silicon processes often strive for higher device densities and small, sub-micron geometries, BioMEMS/Microfluidics often require thicker materials and larger features. The result is that many of the high priced pieces of process equipment utilized in silicon fabs are not needed for BioMEMS/Microfluidics fabrication. This expense elimination is critical for many cost sensitive applications. Sasserath and Fries, Page 1 of 11
Rapid Prototyping: CMOS and other silicon based devices are often well understood and accurate models exist for designers. These allow for rapid design of new devices, while fabrication of new products may take up to a few weeks. BioMEMS and microfluidics designers often do not have the wide range of modeling, simulation and design tools available and, hence, rely on empirical results to finalize device design and fabrication. Hence, multi-week fabrication cycles are expected and could slow device development to unacceptable levels. The result is that an outsource fabrication facility for these devices must overcompensate and be able to rapidly prototype new designs in only a few days to provide speedy time to market for new devices. Device Cost Sensitivity: BioMEMS and Microfluidics are often more cost sensitive than silicon devices. This cost requirement must be achieved, even though volumes are typically smaller than comparable silicon runs. Whereas silicon device orders are often in lots of many thousand devices, BioMEMS and microfluidic devices may only be ordered in lots of 50-100 pieces. This presents a difficult economic challenge to fabricators who need to produce low cost devices in small quantities. The Solution-A Successful Fab Model A solution to the cited problems is available. A commercial design and fabrication facility has been developed that addresses the above issues. In addition to providing a production outsource for new devices, the presented solution will allow companies to do prototyping and test new ideas without the need to establish a new fabrication line. This ultimately offers advantages to the new user such as reduced capital costs, faster time to market, and allows the customer to focus on their core competencies. In addition to providing specific solutions to the critical areas discussed earlier, the outsource is set up to provide a single point where design, fabrication, and testing of new devices can occur. This is needed to ensure complete communication during the device development process so that all of the details needed to successfully prototype new designs will not be lost. Additionally, even though users may choose to only use a portion of these services, having a core group of individuals who are experienced in each of these individual steps will further reduce the risk associated with new product design. Maskless Photolithography Exceeds the User s Needs The key process for transferring images to an electronic or microfluidic device is termed Photolithography. It is used in semiconductor and other device manufacturing (e.g. Lab on a chip, microfluidic devices, optoelectronics). Standard photolithography processes utilize photomasks (masks) as a critical part of the image transfer process. A standard photolithography process involves four major steps which are described below: Standard Photolithography Steps: Step 1: Photoresist Coating A substrate, an object onto which the image is transferred or projected, is coated with photoresist, a liquid polymeric material. The photoresist is the material that the image will be transferred to during the photolithography process. The coating process is performed by spinning the substrates at speeds between 1000 and 5000 rpm. Photoresist is deposited onto Sasserath and Fries, Page 2 of 11
the substrate surface during this dynamic movement to ensure even coating over the entire substrate surface. Another alternative is to employ dry film photoresists which can be laminated into place to create the photopatternable surface. Step 2: Exposure Once the substrate has been coated with photoresist, the substrate is then exposed on an exposure tool. In standard processes, the system shines light through a glass plate which is partially coated with chrome. This plate, termed a photomask or mask, has the master image of the device on it. By shining light through it and onto the substrate, individual areas of the photoresist are selectively exposed to light. This exposure causes a chemical change in the photoresist. Step 3: Development Once exposed, the substrate is then immersed in a developer solution. Developer solutions are typically aqueous and will dissolve away areas of the photoresist that were exposed to light. Therefore, after successful development, the photoresist is patterned with the master image that was provided by the photomask. Step 4: Hardbake After development, the substrate is baked in an oven or hot plate at temperatures between 100-120 o C. This is needed to drive off liquids that may have been absorbed on the substrate and to crosslink the remaining photoresist. Crosslinking the polymer increases mechanical and chemical stability of the material, allowing it to be used in further substrate processing. In the above standard process, the cost and time for device processing are heavily influenced by the availability and cost of the photomask used to impart the pattern. An ideal solution would be to employ a maskless technology that can yield a large number of possible, arbitrary designs depending on the device. An ideal maskless exposure system, Intelligent Micro Patterning s SF-100, exceeds the requirements of BioMEMS and microfluidic device fabricators. The SF-100 is a sophisticated photoimaging sytem which takes any full scale image created on a Windows -based computer, and reduces that image down to a size as small as 5 microns, maintaining all relative proportions and resolutions. The SF-100 is used in the exposure step of a typical photolithography process. A standard Windows -based personal computer is interfaced directly to the system, providing system control and image storage for the exposure process. A schematic of the system is shown in Figure 1 and a photo of the system is shown in Figure 2. The Maskless Processing Sequence (1) Any Windows -based software may be used to create the desired design. This might be a design for a microchip, a MEMS device, a microfluidic device, patterned surface chemistries, a circuit, etc. (2) The computer generated image is electronically transferred to the patented Smart Filter assembly. Sasserath and Fries, Page 3 of 11
(3) Light is introduced into the system using a polychromatic white light source. (4) A direct coupled optical delivery system ensures efficient transfer of this energy to our patented Smart Filter sub-assembly. The Smart Filter incorporates all of the optical and electronic components necessary to transfer an image onto the substrate. Through shaping and optimization of the light path, the projected image is free of distortion, and uniform throughout the exposure area. a. Light emanating from the Smart Filter is broadcast directly onto the surface of the substrate. b. Since the area of this image is typically only a few square centimeters, a step and repeat motion may be used to expose the entire surface of the substrate. To controllably move the substrate during these activities, a highly accurate xyz stage is incorporated into the base unit. Piezoelectronic motors provide a step increment of 0.25 micron, ensuring accurate and reliable registration between levels. c. Additionally, by using the high-resolution microscope above the substrate, the user may control image to substrate alignment. This provides the capability of fabricating multi-layer devices. Multiple layers are often required for more complex devices, where many functions are integrated together to provide greater device performance. d. A removable UV filter is placed between the light source and substrate during the alignment sequence so as to avoid substrate exposure during image to substrate alignment. This filter is necessary in order to prevent the photoresist from exposing while the substrate is being aligned to the image. If the filter were not present, the photoresist would be exposed during the alignment process, resulting in a thinned photoresist thickness. This thinning would cause punch through and repeatability problems at subsequant etch and deposition steps. Sasserath and Fries, Page 4 of 11
Advantages of Maskless Photolithography Over Standard Techniques Table 1 describes some of the advantages and disadvantages of maskless photolithography based on smart filter technology when compared to other standard photoresist exposure systems. Other Process Technologies Provide Complete Device Fabrication Capabilities With the photolithography capability defined, there are other processes which must be made available to customers in order to be able to fabricate a completed device. A number of these processes are described below. Note that in all cases, these processes can be run on standard silicon wafers, as well as with many other non-standard substrates. Sizes can range from pieces of wafers to large 300mm square wafers or linear tape materials in roll to roll processes. Additionally, substrate materials can vary. A partial list of acceptable materials includes glass, plastics, ceramics, metals, and many others. Deposition of Metals: In order to make electrical contact within the device and to the outside world or in creating electrodes for electronic detectors or making metal based components, metal deposition capabilities are required. Although a number of methods are commercially available for this, plating and electroless methods both offer the cost advantages needed when working with BioMEMS and microfluidic devices. Additionally, both thin and thick films can be deposited here, offering significant process flexibility to the user. Metals that can be deposited using these techniques include gold, nickel, platinum, paladium, and copper. Silkscreen Making and Film Deposition: For devices that can be produced with lessstringent imaging requirements, silk-screening is available. This process requires that a silkscreen master image first be produced and then, using this master image, a number of thick paste films can be deposited. Typical thicknesses for the deposited films are 10-20 um and curing is often performed between 75-150C. Wet Chemical Milling: Using wet chemical etching techniques, patterning of the above films can be accomplished. Processes are available for a number of metals, including aluminum, stainless steel, and copper. Processing temperatures vary depending on the specific user requirements, but are generally less than 60C. These processes are fully compatible with the photoresist imaging processes described earlier. Casting & Molding: Hard casts and molds can be fabricated using the photoresist imaging techniques described above. If many low cost copies of this product need to be manufactured or 3 dimensional copies of other hard objects need to be produced, casting and molding techniques can be used with these masters.. In either case, copies are made of plastics, such as PDMS (polydimethyl siloxiane), and the work is performed between 25-100C. Features as small as 10 um and as large as 10 mm have been successfully reproduced using casting and molding processes. For a large number of devices, micromolding combined with injection molding techniques may be used to produce testing and diagnostic components Other Photoimageable Materials: Using dry film diazo-photoresist systems, laminate films between 15-150 um thick can be successfully produced. These are produced using standard photresist imaging techniques. Additionally, ceramics can also be directly patterned. This clearly eliminates the damage, poor selectivity and material Sasserath and Fries, Page 5 of 11
redeposition issues associated with etching ceramic films. Single layers are deposited in 10 um intervals and, through the use of multiple coats, can be deposited to much higher thicknesses. Once patterned, the ceramic material is cured at 850C. Ceramic materials are excellent insulators and packages for applications that are used at either high temperature or in harsh environments, such as implantables in the body. Finally, polyimide materials can be patterned, which can be used as interlevel dielectric, passivation top coat layers, or as polymeric diagnostic components and implantable devices. Results Prove Concept: Using the above model, a number of advanced devices have been designed, fabricated and tested. A partial listing of these is given below. A thin film glucose monitor is a good example of how the aforementioned technology can be applied. This sensor is shown in Figure 3. Although this is not a complete device, the ability to process plastic materials is critical for many BioMEMS and microfluidic devices. The lithography process described earlier has been used in many such applications. Figure 4 shows an example of a 50 mm x 50 mm square that was patterned with standard g-line (426nm) photoresist materials. If further processing was required, other steps, such as plating or etching could be performed to fabricate a more complicated structure. This device has the desired materials sought in a disposable diagnostic device. The substrate is made of polycarbonate which can be injection molded to high tolerance and the patterned layer can be laminate coated followed by photo patterning to create the analytical sequences. The polymeric matrix in Figure 5 is actually a 3 dimensional lattice that was produced by exposing multiple layers of photosensitive materials sequentially. Through the repeated exposure of individual layers, a structure with significant height was produced. This structure has utility as a cell capturing matrix or as a scaffolding structure for biomaterials. Using this technique, combined with the inherent flexibility of the maskless photolithography process, biomaterials can be grown in specific shapes and sizes, which maybe useful in future tissue transplant or regeneration procedures. Finally, Figure 6 shows advanced capabilities that are available using Smart Filter technology. This figure shows highly curved surfaces that have been patterned with photoresist materials. Figure 8 clearly demonstrates a 750um diameter cylinder that has photoresist patterns around its entire circumference. This material is important since they provide for more applications that can take advantage of thin film processing techniques available in the fab. The ability to fabricate highly precise mechanical devices and integrate electronic components onto mechanical devices offers the medical diagnostic device designer significant flexibility and capabilities that are not available with standard silicon processes. Summary An external BioMEMS and microfluidic fabrication capability has been discussed. Although similar in principle to silicon-based commercial fabrication facilities, a number of critical changes have been implemented to ensure success. These include: The ability to handle non-standard, non-silicon substrates, Large critical feature sizes, Sasserath and Fries, Page 6 of 11
Rapid prototyping capabilities, and Low cost operation. The key to success for this commercial fabrication capability is the ability to do maskless photolithography. Combining the flexibility of this process with other standard thin film processing techniques, many varied devices have been successfully fabricated. These include microfluidic and BioMEMS devices made from thin films, plastics, glasses, and other curved substrates. This novel microfabrication technology permits new designs and new devices and will ensure that customer s next generation devices can be developed quickly and at low cost. Acknowledgements The authors would like to thank Addys Gonzalez Sasserath for her contributions in providing simple, easy to read descriptions of the maskless photolithography process, the SF-100 maskless exposure system operation, and the comparison table of the different photolithographic processes. Sasserath and Fries, Page 7 of 11
Figure 1 - Schematic of the SF-100 Maskless Exposure System Figure 2 - Photo of the SF-100 Maskless Exposure System Sasserath and Fries, Page 8 of 11
Figure 3 Thin Film Glucose Monitor Figure 4 Photoresist Patterned Polycarbonate Plastic Material Figure 5 Polymer Lattice Used for Biomaterials Growth Figure 6 750 um Diameter Stainless Steel Rod Patterned Around Circumference with Photoresist Sasserath and Fries, Page 9 of 11
Table 1:Comparison of Smart Filter Technology to Optical Exposure Technologies Requiring Photomasks Exposure Technology Require Photomasks to Generate Pattern? Substrate Size Requirements Defect Levels Smart Filter Technology Contact Printing Proximity Printing Mono- Chromatic Stepper No Yes Yes Yes Accommodates substrates of various shapes, materials, and sizes. Silicon wafers and other substrates ranging from 15mm to over 300mm long have been processed. Low, since substrates are only handled by backside and have no mechanical contact during process. Manual System set for single substrate size. Most applications support only standard silicon wafer sizes, 75mm, 100mm, 125mm, or150mm High, since wafers come in contact with photomasks during exposure. System set for single substrate size. Most applications support only standard silicon wafer sizes, 75mm, 100mm, 125mm,150mm or 200mm Low, since wafers are only handled by backside and have no mechanical contact during process. Cassette to Cassette System set for single substrate size. Most applications support only standard silicon wafer sizes, 75mm, 100mm, 125mm,150mm or 200mm. 300mm wafer processing possible. Very Low Wafer Handling Cassette to Cassette Cassette to Cassette System Size Small Medium Medium Large Minimum 5 um <1.0 um <1.0 um <0.5 um Feature Size Time from completion of device design to start of first exposure <10 minutes is needed per revision to transfer design file to SF-100 system computer for exposure Exposure Field.63 cm x.63 cm 24 hours or more per revision are needed for fabrication and inspection of each photomask. Entire Wafer Surface 24 hours or more per revision are needed for fabrication and inspection of each photomask. Entire Wafer Surface 24 hours or more per revision are needed for fabrication and inspection of each photomask. 2.0 cm x 1.0 cm Sasserath and Fries, Page 10 of 11
1 R. Grace, Overview of the MEMS Industry, Presented at Tampa Bay and MEMS Conference, University of South Florida, St. Petersburg, Florida, February 28, 2002. Sasserath and Fries, Page 11 of 11