Reduction of diffraction effect of UV exposure on SU-8 negative thick photoresist by air gap elimination

This paper reports a novel way to compensate the air gap between photomask and photoresist for eliminating UV light diffraction on photoresist, which greatly increases the sidewall straightness of high-aspect-ratio resist structures. In this research, SU-8 negative tone photoresist was used for experiments, and glycerol was employed as an index match material for bridging air gap between photomask and photoresist during exposure. Results showed that a high aspect ratio wall structure of 156 mum thick and 25 mum wide had a 45% pattern width error when exposed under 100 pm air gap, while glycerol compensated process accomplished a straight resist wall without appreciable error. This method is simple and cheap to employ, compared to the usage of costly thick-photoresist-film spinner for resist planarization. Numerical simulation on the diffraction effect upon the structure wall has also been conducted. The calculated and experiment wall profiles showed similarity in trend.