Silicon dissolution regimes from chemical vapour etching: from porous structures to silicon grooving

A new method of silicon grooving is developed based on chemical vapour etching (CVE) of silicon substrates. The CVE consists of exposing silicon wafers to acid vapours issued from a mixture of HF and HNO3. It was found that CVE of silicon results in the formation of Porous Silicon (PS). However, we noticed that, depending on the volume ratio of HF/HNO3, PS may transform in a white powder essentially composed of (NH4)2SiF6. It was shown that PS is a major phase above a volume ratio of 9:1, while the (NH4)2SiF6 phase becomes major at HF/HNO3 ranging between 2:1 and 4:1. The structural differences between PS and the (NH4)2SiF6 white powder was investigated by Fourier transformation infrared (FTIR) spectroscopy. The formation kinetics of the (NH4)2SiF6 white powder was found to depend on both acid mixture and silicon substrate temperatures. The high solubility of the (NH4)2SiF6 white powder enables us to groove silicon at different depth, with an accuracy of 0.2 mm. This grooving technique could be introduced in microelectronic engineering as well as for buried metal contact in solar cells processing.