Once a surface relief pattern has been patterned in photoresist or etched into a substrate, how do you know if you have produced what you wanted? There are many different approaches to answering this question, but all of them fall into one of two categories: (1) metrological tests that measure the dimensions and geometry of the surface structures or (2) tests that measure the optical performance of the component. Both types of tests are necessary. Although dimensional measurements will provide information on the fidelity of the fabrication process, they cannot assure you that the element works properly. (Suppose someone goofed and the design is incorrect?) We now consider a number of techniques for both dimensional and optical testing of diffractive optical elements.
Dimensional measurements of diffractive optic structures are used both during and after fabrication of the element. Typical measurements include lateral feature sizes, locations of the transition points across the field of the DOE, grating depths, verticality of grating side walls, rounding of edges, surface roughness, and other geometrical factors. Although the performance of the DOE can usually be inferred from the measured geometry, metrological measurements are normally used to troubleshoot and quantify problems in the fabrication process. We now consider several common methods for the measurement of DOE geometries.
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