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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 μm and the maximum step from 0.43 to 0.2 μm compared to a single pixelated exposure using the same mask design. [ABSTRACT FROM AUTHOR]

Abstract: We report a photoresist offset method using gray-scale technology to counteract the effect of aspect ratio dependent etching (ARDE), achieving controlled etch depths across a range of aspect ratios. Previously, we have reported the first fabrication of a deep phase Fresnel lens (PFL) in silicon through the use of gray-scale technology. As each PFL ridge becomes thinner at larger radii of the PFL, ARDE causes a significant reduction in profile accuracy. Thus, in this paper, a compensated aspect ratio dependent etching (CARDE) process is proposed and demonstrated to enable controlled ridge heights/depths on large diameter PFLs containing a variety of ridge widths. A compensation function is used during optical mask design to incorporate a photoresist offset, defining wide ridges with higher gray levels to locally modulate the time of etching, effectively giving thin ridges a head start during the etch. Multiple PFL profiles, both compensated and uncompensated, are designed and fabricated. Using a phasor-based profile evaluation method, PFLs fabricated using the CARDE process are shown to exhibit an increase in profile accuracy and calculated lens efficiency. [Copyright &y& Elsevier]