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Photoresist Aerosol Spray Can for Fabricating Microfluidic Devices
Keywords: Aerosol, Spray , Photoresist
Photolithography has been shown to be a viable technique for the rapid prototyping of microfluidic devices. The same materials, processes and equipment sets once used for wafer-scale fabrication of semiconductor devices and micro-electro mechanical systems (MEMS) have now been adapted for microfluidics. However, the tool sets derived from these industries are capital intensive and thereby create a barrier to rapid fabrication. These equipment costs can be overcome by replacing the common photolithographic method of spin-coating photoresist with a portable, self-contained aerosol spray can of photoresist. Spin-coating photoresist on a flat, circular substrate produces a very uniform coating for a variety of film thickness and with a film thickness variation of less than 1% across the entire surface. However, the efficiency of spin-coating method is extremely low and most of the dispensed resist used in the spin coating process is discarded and wasted. For non-circular, irregular shaped substrates, special techniques and/or equipment must be employed to avoid a buildup of the photoresist around the corners of the substrate. When the substrate is not flat and contains significant topography, either in the form of deep wells or high aspect ratio microscopic features, serious coating irregularities, known as striations or “comet tails” can result and significantly degrade the coating uniformity. Likewise, when the substrate has voids, through hole vias or perforations, vacuum cannot be used to affix the substrate to the spin-coating chuck thereby eliminating the ability to spin-coat the substrate altogether. There are, of course, several methods (other than spin-coating) which can be used for applying photoresist coatings, such as: spray, immersion and electro-deposit. However only spray coatings have received much attention. Air-brush coating techniques have also been examined, but this technique tends to produce many coating bubbles and is difficult to control the film thickness. Recently, photoresist materials have been formulated and spray-coating tools have been designed to accommodate substrates with deep topography. However, these tools are quite expensive and do not lend themselves to the low-cost production of prototype microfluidic devices. To enable the manufacture of low-cost, rapid prototyped, microfluidic devices without a capital investment or a coating track, a self contained, pressurized aerosol spray-can has been formulated complete with photoresist and propellant. In this photoresist spray can formulation, the photoresist, solvent and propellant produce a completely miscible, pressurized mixture, which when released through the built-in spray nozzle eliminates bubble formation in the resist coating. This pressurized, ballistic photoresist mixture has been produced for a wide variety of photoresist types, including: positive, negative and SU-8 chemically amplified negative photoresist. PDMS molds can now be prepared using SU-8 masters, which have simply been prepared by spray-coating SU-8 from an aerosol spray can. Pattern metallization of completed SU-8 or PMDS devices can also be accomplished by spray-coating an aqueous developable negative photoresist also from an aerosol spray can.
Harris R. Miller, Product Development Manager
MicroChem Corp.
Newton, MA

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