Etch-stop behaviour of depletion layers

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Abstract
The authors demonstrated that the thickness of a diaphragm using the electrochemical etch-stop technique can be monitored with the applied reverse bias voltage on individual pn-junctions. It is thus possible to produce diaphragms of differing thickness on one wafer using different reverse bias voltage values. This effect is attributed to an etch-rate reduction of approximately a factor of ten when the etch front reaches the depletion layer of the pn-junction. Since the depletion-layer thickness is voltage dependent different diaphragm thicknesses can be obtained. For a metallurgical junction depth of 4.4 mu m diaphragm thicknesses of 5.5 mu m, 6.5 mu m and 9.5 mu m were obtained for respective reverse bias values of 2 V, 8 V and 20 V. Since depletion layers can also be obtained with metal oxide silicon semiconductor (MOS) structures, MOS-capacitors with n+ source diffusions were biased with 10 V gate to substrate and 3 V source to substrate voltages. Electrochemical etching yielded membranes of typically 3 mu m thickness. They are lightly p-doped in this case (3*1015 cm-3) and completely stress free. For MOS-structures no final etch-stop can be obtained, however, etch-through monitors are used as etch end-point detectors. Very thin, single-crystal diaphragms with thicknesses below 2 mu m are of importance in several applications. Such thin diaphragms were made with shallow implanted pn-junctions of 1.2 mu m junction depth and an electrical activation anneal. The resulting diaphragm thickness obtained was 2 mu m by sufficient overetching.

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