Mass and Electron Balance for the Oxidation of Silicon during the Wet Chemical Etching in HF/HNO₃ Mixtures

Abstract

The stoichiometry of the wet-chemical etching of silicon in concentrated HF/HNO 3 mixtures has been studied. By quantifying the major reaction products in solution, the established model that 3 mol of Si are oxidized by 4 mol of HNO 3 to yield 4 mol of NO could not be confirmed. In HNO 3 -rich HF/HNO 3 mixtures, approximately 1.1 mol of HNO 3 are required to oxidize 1 mol of Si. Excess HNO 3 leads to massive accumulation of N(III) species in the etchants and massive formation of nitrous oxides due to incomplete reduction of the HNO 3 . An excess of HNO 3 leads to higher consumption and poorer utilization indicated by the massive accumulation of N(III) species in the etchants. In HF-rich mixtures, only 0.9 mol of HNO 3 are needed to oxidize 1 mol of Si yielding a lower accumulation of N(III) species and a higher utilization of the HNO 3 . Two parallel pathways contribute to the oxidation of silicon in such solutions: (i) via the oxidation by HNO 3 and reactive intermediates generated by the reduction of HNO 3 and (ii) via the formation of hydrogen. A comprehensive treatment covering alkaline etching, electrochemical etching in HF media, and etching in concentrated HF/HNO 3 mixtures is proposed based on the reactivity of the hydrogen terminated silicon surface against the applied oxidizing agent.

Stain Etching of Silicon With and Without the Aid of Metal Catalysts

“…The reductions of VO 2 + and Fe 3+ do not evolve gas, unlike the much more complicated reduction of NO 3 - (15,16); therefore, only H 2 is generated in the etching reaction. Silicon is a very poor catalyst for H 2 production.…”

The Stoichiometry of Electroless Silicon Etching in Solutions of V2O5 and HF

“…[1] The wet-chemical etching of Si in HNO 3 /HF, an electroless electrochemical process, is an essential step in the manufacture of solar cells. [2] When performed under conditions for the formation of porous silicon (por-Si), the process is known as stain etching. It can be initiated by a variety of more suitable oxidizing agents [3] and can be used for the low-cost production of nanocrystalline por-Si powder [4] or Si nanocrystals.…”

“…The interpretation of reaction data is far from straightforward, although detailed experiments have greatly improved our understanding of etching in the electropolishing regime. [2] The discovery of stain etching with V 2 O 5 as the oxidant [8] provides a unique opportunity to study the stoichiometry of stain etching. The half-reaction that injects a hole into (equivalently, removes an electron from) the Si valence band is:…”

Porous Silicon Formation by Stain Etching

“…Acker and co-workers (Acker et al 2012;Hoffmann et al 2011;Steinert et al 2005Steinert et al , 2006Steinert et al , 2007Steinert et al , 2008Henssge and Acker 2007;Acker and Henssge 2007;Weinreich et al 2007;Jadzinsky et al 2007) The rate of stain etching is enhanced by the presence of defects. This characteristic has been used to create a hybrid amorphous porous/defect-followed mesoporous structure (Woo et al 2012a, b).…”

“…Unfortunately, the reduction of NO 3 À and the myriad other nitrogen-containing species that are formed as by-products are extremely complex. Only recently have Acker and co-workers been able to establish the role of these various species in the electropolishing regime (Acker et al 2012;Hoffmann et al 2011;Steinert et al 2007Steinert et al , 2008Henssge and Acker 2007;Acker and Henssge 2007;Steinert et al 2005Steinert et al , 2006. This complexity (i.e., high sensitivity to composition, temperature, extent of reaction, various dissolved gases, etc.)…”