Double-Exposure Grayscale Photolithography

A double-exposure grayscale photolithography technique is developed and demonstrated to produce three-dimensional (3-D) structures with a high vertical resolution. Pixelated grayscale masks often suffer from limited vertical resolution due to restrictions on the mask fabrication. The double-exposure technique uses two pixelated grayscale mask exposures before development and dramatically increases the vertical resolution without altering the mask fabrication process. An empirical calibration technique was employed for mask design and was also applied to study the effects of exposure time and mask misalignment on the photoresist profile. This technology has been demonstrated to improve the average step between photoresist levels from 0.19 to 0.02 mum and the maximum step from 0.43 to 0.2 mum compared to a single pixelated exposure using the same mask design.