SU-8 On Pmma - A New Technology For Microfluidics

Conventional SU-8 lithography process for fabricating microfluidic devices often uses silicon or glass as substrate materials. Since silicon and glass are very hard and brittle, drilling fluid access holes or dicing the substrates into individual devices are difficult. We investigated the use of polymethylmethacrylate (PMMA) as a new substrate material. PMMA, an amorphous thermoplastic, was chosen because it is easy to drill or cut, biocompatible, transparent, and hundreds of times cheaper than silicon or glass wafers. Moreover, its thermal expansion coefficient ideally matches that of SU8. PMMA poorly resists solvents, and has low glass transition temperature (105C). Thus, the conventional process was modified. The substrate was only cleaned with isopropyl alcohol and deionized water. The baking temperature was lowered to 90C. In addition, a “base layer” of SU-8, helping to achieve a high quality structural pattern, was coated before coating the actual structural SU-8 layer. A Tesla valve, a non-moving part microfluidic valve, was successfully fabricated in SU-8 using the presented process. However, the PMMA substrate bowed due to the thermal residual during baking steps. Despite the bowing problem, which can be solved by increasing substrate thickness, we conclude that PMMA is a promising material for SU-8 process.