chapter 9, Testing Diffractive Optical Elements

Excerpt

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.

9.1 Metrology

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.

9.1.1 Optical microscopy

Microscopes are perhaps the most commonly used tools for inspection of diffractive optical elements. Although these tools may not provide highly precise measurements, they are relatively inexpensive and easily available. Microscopes are typically used to find gross defects in pattern geometries and for rapid relative measurements of lateral feature dimensions. In many cases, the eyepiece of a microscope will contain a reticle with regularly spaced tick marks.