Optimized ultraviolet grayscale process for high vertical resolution applied to spectral imagers

NanoCarb is a miniature Fourier transform imaging spectrometer dedicated to the measurement of atmospheric CO₂ and CH₄. The key element of NanoCarb is an array of Fabry–Perot microinterferometers having a stepcase shape. Lateral dimensions and height of each step depend on the used material, the focusing lenses, and the targeted optical path difference to be measured. In this paper, we developed a grayscale lithography process for a large surface patterning with high vertical resolution. This process is combined with plasma etching to transfer the as-obtained resist patterns into the silicon substrate. This method is an efficient and quick way for the realization of such arrays into silicon. A low contrast resist (ma-P1225G) was used for better control of the step height, and we investigated the effect of two annealing processes on the contrast curve slope: the soft bake and the postexposure bake. Therefore, combining the two processes leads to a 20 nm step height resolution in resist and 50 nm in silicon.