Reactive Species Generated during Wet Chemical Etching of Silicon in HF/HNO3 Mixtures

The role of intermediate species generated during wet chemical etching of silicon in a HF-rich HF/HNO3 mixture was studied by spectroscopic and analytical methods at 1 °C. The intermediate N2O3 was identified by its cobalt blue color and the characteristic features in its UV−vis and Raman spectra. Furthermore, a complex N(III) species (3NO+·NO3 -) denoted as [N4O6 2+] is observed in these solutions. The time-dependent decay of the N(III) intermediates, mainly by their oxidation at the liquid−air interface, serves as a precondition for the study of the etch rate as function of the intermediate concentration measured by Raman spectroscopy. From a linear relationship between etch rate and [N4O6 2+] concentration, NO+ is considered to be a reactive species in the rate-limiting step. This step is attributed to the oxidation of permanent existing Si−H bonds at the silicon surface by the reactive NO+ species. N2O3 serves as a reservoir for the generation of NO+ leading to a complete coverage of the silicon surface with reactive species at high intermediate concentrations. As long as this condition is valid (plateau region), the etch rate is constant and yields a smooth silicon surface upon completion of the etching. If the N2O3 concentration is insufficient to ensure a coverage of the Si surface by NO+, the etch rate decreases linearly with the N2O3 concentration and results in a roughening of the etched silicon surface (slope region).