From angstroms to microns: Extending the measurement range of optical profilers

Optical profilers are very good for looking at the microstructure of a surface; however, they do not provide a very large dynamic range. They are limited to slopes which don't change the optical path difference between adjacent pixels by more than a half of the measurement wavelength (this corresponds to height changes of one-quarter wave). Many possible applications of optical profilers call for measuring the height of a step which is greater than a quarter of a wavelength, or for looking at structures of rough surfaces. Using the techniques of two-wavelength phase-shifting interferometry,1-4 the dynamic range of an optical profiler can be extended without sacrificing its high measurement precision.