Study on soft hot embossing process for making microstructures in a cyclo-olefin polymeric (COP) film

Abstract Thermoplastic polymers are the primary materials for fabricating commercial microfluidic devices. Despite many excellent properties, the low thermal conductivity is a common limiting factor in speeding up temperature-dependent biological processes, particularly for polymerase chain reactions. There is a need to develop a fabrication process to create thin-film microfluidic devices that can have a small thermal mass and a short microchannel-to-surface distance. This type of device requires the depth of micropatterns to be very close to the film thickness, which can encounter serious fractures during the demolding process. To overcome this challenge, we develop a soft hot embossing process to create micropatterns in a 188 µ m thick cyclo-olefin polymeric (COP) film with a high embossing-depth to film-thickness ratio. The advantage of using a soft master is it can easily be peeled off from the molded film without causing a fracture from micropatterns. Polydimethylsiloxane (PDMS) is used as the soft silicone master, and four different 110 µ m high micropatterns are studied, including ribs, grooves, and circular columns and cavities. PDMS masters for creating a 110 µ m deep microchannel with different arrays of 70 µ m deep microwells are also investigated. The heights of these one-layer and two-layer PDMS masters are 58.8% and 95.7% of the film thickness. Experimental findings show that less than 3% height variation can be achieved using a single-layer PDMS master with a low aspect ratio. For the two-layer micropatterns, it was found that a dense array with a smaller gap between microwells can have a better pattern transfer. In summary, this study demonstrates the feasibility of using a soft master to create deep or tall micropatterns in a COP film. The possibility of using a soft hot embossing process to create micropatterns for thin-film microfluidic devices is verified.