Process optimisation for compact, high aspect ratio SU-8 microstructures using x-ray lithography

摘要

X-ray lithographic conditions for high aspect ratio SU-8 resist structures are characterized for potential application in x-ray optics and bioMEMS. The effects of the main process parameters such as exposure dose, post exposure bake, development time and the packing density of the microfabricated features on the development depth and increase in feature size at the top portion of the resist (as compared to that in mask) were investigated. As test samples, we fabricated 1mm high, densely-packed SU-8 structures comprising of 30μm square pillars with spacings of 36μm, 12μm and 6μm. Dissolution rates are found to be longer for densely packed structures than predicted by simple physical models based on isolated structures. We examine the effect on dissolution rates of the density of features in our structures. We also optimised our process with respect to the parameters described above using the Taguchi method. We find that optimisation of the development time with exposure dose and post-exposure bake time can reduce the dimensional error to ～3% for certain densely-packed structures.