X-ray micromachining SU-8 resist for a terahertz photonic filter

Abstract

Deep x-ray lithography (DXL) was used to fabricate two-dimensional photonic crystals (PCs) for a terahertz high-pass filter. Instead of the conventional poly(methyl methacrylate) resist, high-sensitivity SU-8 resist was used to pattern the ultra-deep (1 mm), and high aspect ratio (>30) PC structures. In this study, an x-ray mask with thick Si membrane was used in the DXL process owing to the high-sensitivity nature of SU-8. The robust mask structure can significantly improve the fabrication yield of the conventional x-ray mask with a very thin (< 2 µm) membrane. Preliminary results demonstrate that SU-8 has poor pattern definition after DXL process, probably due to its high sensitivity. This phenomenon can be eliminated by dissolving oxygen into the resist to ‘quench’ the excessive photochemical reaction. Thanks to the high adhesion property of SU-8, the sticking problem of the high aspect ratio crystals is easily solved with natural drying by reducing the unbalanced capillary force. After optimizing all the processes, the proposed DXL SU-8 technique successively fabricated the terahertz crystals with high efficiency, high precision (< 1 µm) and high surface quality (Ra ∼12 nm). The resist pillars were then deposited with a 300 nm thick gold layer for subsequent terahertz measurement. The measurement results show that the PC structure acts as a high-pass filter in the terahertz range, agreeing with the simulation results.