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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 [micro]m and the maximum step from 0.43 to 0.2 [micro]m compared to a single pixelated exposure using the same mask design. [2008-0164] Index Terms--Grayscale lithography, microelectromechanical systems (MEMS), micromachining, three-dimensional (3-D) lithography.

We report the first fabrication and development of a deep phase Fresnel lens (PFL) in silicon through the use of gray-scale lithography and deep-reactive ion etching (DRIE). A Gaussian tail approximation is introduced as a method of predicting the height of photoresist gray levels given the relative amount of transmitted light through a gray-scale optical mask. Device mask design is accomplished through command-line scripting in a CAD tool to precisely define the millions of pixels required to generate the appropriate profile in photoresist. Etch selectivity during DRIE pattern transfer is accurately controlled to produce the desired scaling factor between the photoreist and silicon profiles. As a demonstration of this technology, a 1.6-mm diameter PFL is etched 43 [micro]m into silicon with each grating profile designed to focus 8.4 keV photons a distance of 118 m. [1128] Index Terms--Deep-reactive ion etching (DRIE), gray-scale lithography, phase Fresnel lens (PFL), three-dimensional (3-D) structures.