Experimental Studies on the Mechanism of Wet Chemical Etching of Silicon in HF∕HNO[sub 3] Mixtures

Abstract

A detailed kinetic study was performed to elucidate the mechanism of wet chemical etching of silicon in a HF-rich HF∕HNnormalO3 mixture. In contrast to earlier studies, the etch rates were determined by dissolution of only a few milligrams of silicon in carefully thermostatted acid mixtures in order to avoid a change in composition during the experiments and an uncontrolled warming of the etchant. All etch experiments were followed by chemical analytics. The etch rates were studied as a function of temperature, silicon content of the etchant (utilization), and stirring speeds. By choosing proper reaction conditions, intermediates of the reduction process of HNnormalO3 , such as normalN2normalO3 , were stabilized and spectroscopically identified. Furthermore, it was found that the nitrite ion concentration, measured in diluted etchant solution by ion chromatography, acts as a parameter for the reactive N(III) species in the concentrated etchant. Two different etch regimes were identified. In the region of high nitrite concentrations, the etch rate is apparently independent on the nitrite concentration. At lower nitrite concentrations, the etch rate decreases linearly with the nitrite concentration. Kinetic examinations showed that the reaction mechanism remains unchanged in both regimes. Furthermore, the kinetic parameters of nitrite decays were determined. The obtained results provide the first explanation of why an etch mixture of constant HF–HNnormalO3 ratio at given Si content can exhibit different etch behavior. A mechanistic model on the role of N(III) species and dissolved gases in the etching process including a suggestion of the rate-limiting step is presented. Consequences for technical applications are discussed.

Chemical etching of isolation grooves in high-power silicon devices

“…In HF-HNO 3 mixtures the electrochemical origin of etching process is confirmed by the formation of porous Si layers [9]. On the other hand, the presence of several combined equilibria between different nitrogen oxides was proposed [5] to lead to the formation of nitrous acid as a reactive species in the etching process (3), (4). Indeed, the best morphology of Si surface was obtained in HNO 3 -rich HF / HNO 3 / CH 3 COOH solutions, in an apparent agreement with this mechanism [5].…”

“…On the other hand, the presence of several combined equilibria between different nitrogen oxides was proposed [5] to lead to the formation of nitrous acid as a reactive species in the etching process (3), (4). Indeed, the best morphology of Si surface was obtained in HNO 3 -rich HF / HNO 3 / CH 3 COOH solutions, in an apparent agreement with this mechanism [5]. However, this mechanism does not explain how nitric acid is reduced on Si (111) surface passivated by hydrogen [10].…”

“…As a result, a lower dissociation and a higher oxidation degree of HNO 3 are achieved during the etching process. In addition, a lower polarity of acetic acid, as compared to water, leads to a better wetting of a partially hydrophobic Si surface [5]. It should be noted that the formation of deep isolation grooves in Si wafer with p-n junctions by wet etching differs from the etching process of a homogeneous substrate.…”

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.)…”