The high selectivity in etching of dielectric films over crystalline silicon (c-Si) by hydrofluoric acid (HF) is routinely exploited in the fabrication of various silicon based devices such as microelectromechanical systems (MEMS) and silicon solar cells. In this paper it is shown that the high selectivity does not apply to highly doped n-type c-Si. We confirm experimentally that the etch rate of highly doped rc-type c-Si is about 0.8 nm/minute in diluted HF, which is more than 10 times higher than previous results reported in the literature and measured at lower carrier concentrations. We propose that a combination of factors is contributing to the observed fast etching rate of heavily doped ra-type c-Si. The proper understanding and exploitation of the etching mechanism of n~+ c-Si by HF can be very beneficial for the fabrication of silicon MEMS and solar cell devices.
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