Study on high speed measurement of three-dimensional surface profiles using phase shifting interferometry

Abstract

A high speed measurement of surface profiles by using interferometry is presented. It is realized by using a high speed phase shifting interferometer which utilizes the self-injection locking of a frequency tunable laser diode. By using a confocal Fabry-Perot cavity and a linear modulation of the laser diode current, the laser frequency can be locked to the resonant modes of the Fabry-Perot cavity consecutively, which provides equal phase steps. The phase step error was evaluated to be about 3 MHz which corresponds to 0.2 nm in length measurement. To increase the measurement height, a high speed two-wavelength phase shifting interferometry is presented. With our test setup, the measuring range is extended to 10 μm by using two injection-locked frequency scanning lasers which offer fast and equidistant phase shifting of interference fringes. A technique to determine the unwrapped phase map in a frequency scanning PSI without the ordinary phase unwrapping process is proposed. This technique was realized in a microscope system. Wide frequency tunable range enabled one to have enough phase step interval despite the shortened difference between the lengths of reference and test arm caused by shortening the coherence length. Developed method to measure the three-dimensional surface shape is applicable to high speed in-line inspection of spacers in liquid crystal display panels or wafer bumps where measuring range is well determined and high speed measurements are essential.