H-Line Transparency Analysis Of Microlithography For Millimeter Scale High-Aspect Ratio Su-8 Structures

This paper reports transparency characteristics of the h-line in the microlithography for the fabrication of ultra-tall SU-8 microstructures. Since the h-line (405 mn) in the lithography system has higher transparency to the SU-8 photoresist compared to that of the (365 mu), the h-line is more suitable for fabrication of ultra-tall SU-8 microstructures for the expanded design capability of the microdevices. The transparency of the h-line through the SU-8 was experimentally verified as a function of film thickness and exposure time. Approximately 62% to the initial intensity of the h-line was observed at the 2.4 mm thick SU-8 film. The transparency of the SU-8 was also decreased by the exposure time. The h-line exposure to the 2.4 mm thick SU-8 film for 3500 seconds decreased the SU-8 transparency to the h-line by, 17.7% due to the crosslinking of the SU-8. With the proposed exposure scheme, an array of 2-mm tall micropillar array, and 1.5-mm long inclined micropillar array were successfully fabricated. Also, 7-mm tall micropillar was fabricated as a state-of-art microstructure. As an application, an array, of 1000-mu m-tall micropillars were successfully fabricated to form a 100-turn 3-D toroid inductor. The fabricated inductor showed average inductance of 950 nH in the frequency range of 0.1 to 10 MHz.