Compensated aspect ratio dependent etching (CARDE) using gray-scale technology

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.