Optical in situ monitoring of silicon diaphragm thickness during wet etching

We have developed a method for in situ monitoring silicon diaphragm thickness during wet chemical etching. The principle of the thickness measurement is multiple-beam interference spectroscopy. This interference is caused by the multiple reflection at both surfaces of the silicon diaphragm. The interference spectrum was observed in the near-infrared region of the spectrum, from 800 nm to 1000 nm. This range is determined by the absorption spectra of silicon and etchant, and by the spectral sensitivity of the photomultiplier. Two monitoring systems, that is, a transmission system and a reflection system, were developed. Using these systems, diaphragm thicknesses from 2 mu m to 20 mu m could be monitored. The reflection system was developed to avoid the effect of bubble generation. Thickness non-uniformity reduces the contrast of the interference spectrum. In order to solve this problem, the area illuminated by the focused light has to be reduced, and the sensitivity of the photodetector has to be high.