Design and implementation of high-performance and cost-effective white-light interferometric profile measurement systems

White light interferometry (WLI) has played an important role in nano- and micro-scale profile measurement technology. To meet the demand of high-accuracy, high-repeatability, and cost-effective measurement, the research activities on WLI and its applications are rapidly in progress. WLI is based on the superposition of waves with different but very close wavelengths to produce beat phenomena (or to generate detectable envelopes in the interferogram) and then to identify the locations of the zero-order interference fringes (or those of the maximum intensities of interference fringes without the optical path difference). The locations reflect the information of three dimensional (3D) surface profiles from the consecutively acquired images in a WLI system. The objective of this paper is to develop a high-accuracy and cost-effective WLI measurement system, especially for the surfaces of micro-mechatronic devices, micro-optical components, semiconductor devices, etc. In our approach, the feasibility of the use of spectral coherence properties to meet the system design requirement is first investigated. Specifically, proper spectral filters are employed to enhance the coherence length of the light source (i.e., that of the filtered light source) to an order of ten micrometers. Then, a Young's double-slit interference experiment with filtered and unfiltered white light sources is conducted to demonstrate the effectiveness of this technique. Also, we adopt a Michelson interferometric configuration as the optical module of the proposed WLI system, for the sake of its simplicity. Experimental results indicate that several inexpensive spectral filters, a lower-grade charge-couple-device (CCD) image sensor, and a PZT (piezoelectric transducer) with a lower movement resolution are merely needed to develop the WLI system, instead of the use of higher-grade optical and optomechanical components. It turns out that the proposed system with high-accuracy measurement performance is more cost-effective than others.