For each step of the groove production, we have used new and sensitive techniques to determine the contribution of that step to the phase non-uniformity. Armed with an understanding of the errors and their origins, we could then implement process controls for each step. The plasma uniformity was improved for the silicon nitride mask etch process and the phase contribution of the plasma etch step was measured. We then used grayscale lithography, a technique in which the photoresist is deliberately underexposed, to measure large-scale nonuniformities in the UV exposure system to an accuracy of 3-5%, allowing us to make corrections to the optical alignment. Additionally, we used a new multiple-exposure technique combined with laser interferometry to measure the relationship between UV exposure dose and line edge shift. From these data we predict the contribution of the etching and photolithographic steps to phase error of the grating surface. These measurements indicate that the errors introduced during the exposure step dominate the contributions of all the other processing steps. This paper presents the techniques used to quantify individual process contributions to phase errors and steps that were taken to improve overall phase uniformity.
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Cynthia B. Brooks, Michael Gully-Santiago, Michelle Grigas, Daniel T. Jaffe, "New metrology techniques improve the production of silicon diffractive optics," Proc. SPIE 9151, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, 91511G (28 July 2014);