Optical in situ monitoring of silicon diaphragm thickness during wet etching

We developed a method for in-situ monitoring of 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, with a spectrometer and a photomultiplier. This range is determined by absorption spectra of silicon and etchant, and by the spectral sensitivity of the photomultiplier. Two monitoring systems, that is, transmission system and reflection system were developed. The reflection system proved to be better as it is not affected by bubble generation. Using this system, the diaphragm thickness from 2 μm to 20 μm could be monitored. The thickness nonuniformity reduces the contrast of interference spectrum. In order to solve this problem, the measurement part illuminated by the focused light has to be reduced, and the sensitivity of the photodetector also has to be high