We investigated the technical possibility of exploiting a femtosecond pulse laser as the light source of low-coherence
interferometry for topographical inspection of silicon wafers. The intention was to measure both the front- and rear-surface
profiles of a silicon wafer simultaneously by illuminating from one side of the wafer only. To the end, the
spectrum of the femtosecond laser was widened using a photonic crystal fiber to yield wavelengths over the particular
range of 1000 to 1200 nm, which is not only transmittable through silicon but also detectable by an ordinary CCD
photodetector array. This tomographic scheme enables complete measurement of thickness profile and also detection of
internal voids such as cracks residing inside the wafer with high lateral and depth resolutions, which could be useful for
nondestructive testing of multi-layered packages of silicon wafers.
We present a new type of point-diffraction interferometer specially designed for industrial use with high immunity to external vibration encountered in the course of measurement process. The proposed interferometer uses thermally-expanded fibers instead of conventional pinholes as the point-diffraction source to obtain a high quality reference wave with an additional advantage of relatively easy alignment of optical components. Vibration desensitization is realized through a common-path configuration that allows the influence of vibration to affect both the reference and measurement waves identically so that it is subsequently cancelled out during the interference of the two waves. A spatial phase shifter is added to capture four phase-shifted interferograms simultaneously without time delay using a single camera to avoid vibration effects. Experimental results demonstrate that the proposed interferometer is capable of providing stable measurements with a level of fringe stabilization of less than 1 nanometer in a typical workshop environment equipped with no excessive ground isolation for anti-vibration.