New insights into acidic wet chemical silicon etching by HF/H 2O–NOHSO 4–H 2SO 4 solutions

Abstract

Acidic wet chemical etching of crystalline silicon has been examined by utilization of HF–NOHSO 4–H 2SO 4 mixtures. In light of our previous studies the effects of nitrosyl ion concentrations on etching rates were studied time- and temperature resolved. The reactivity of crystalline silicon surfaces in HF/H 2SO 4 solutions is determined by NO +-ion concentrations at the silicon/electrolyte interface, measured by ion chromatography. Quantitative solution analysis proofed accumulation of ammonium ions and indicated the conversion of NO + as limiting for the overall etching process. Direct participation in the rate-limiting step was confirmed by calculation of activation energies. Increasing NO +-ion contents cause transition from reaction ( E A=55 kJ mol −1) to diffusion controlled ( E A=10 kJ mol −1) etching procedures. In combination with time and concentration dependent studies of produced structures a convenient regime for selective texturing or polishing polycrystalline silicon surfaces is reported. Qualitative analysis by 19F-NMR and Raman spectroscopy identified SiF 5 −/HF 2 − complexes as well as elementary hydrogen (H 2) as hitherto unknown products of silicon dissolution reactions in HF–NOHSO 4–H 2SO 4 mixtures. Based on our findings a strategy for fundamental investigations of relevant reaction pathways is presented and discussed with regard to reported mechanistic concepts.