Fabrication of large SU-8 mold with high aspect ratio microchannels by UV exposure dose reduction

Patterned SU-8 can be used as a master mold for soft lithography. Fabrication of SU-8 molds with high aspect ratio microchannels for this purpose is non-trivial due to the contrary imperatives of mold hardness and faithful replication of the photomask pattern. Increased UV exposure time, which improves resist hardness, is found to result in overexposure of the shadowed resist and unresolved pattern structure in the wafer center. We have found that reduction of UV exposure dose from the resist manufacturer's recommendation of 500-600mJ/cm² to 350mJ/cm² permits the successful fabrication of dense SU-8 gratings with relatively wide (80μm) SU-8 bars separated by narrow (10μm) microchannels with aspect ratio of 10 over the entire 100mm-diameter wafer. The underexposed SU-8 was rather soft but could be sufficiently hardened to attain a useable hardness (Vickers hardness (VH) number of 25) by hard baking at a relatively low temperature (95°C). A hard baking time of 20min resulted in saturation of the hardness; further increase of hard baking time resulted in no significant increase in hardness. The hard-baked SU-8 gratings were successfully used for replication of soft silicone rubber. Without hard-baking, the silicone rubber broke cohesively within the SU-8 gratings during demolding. A method of fabricating 100mm-diameter SU-8 mold consisting of 80μm wide SU-8 bars separated by 10μm narrow channels with aspect ratio of 10 for soft lithography has been demonstrated.