Titrimetric determination of silicon dissolved in concentrated HF–HNO3-etching solutions
Introduction
Isotropic wet chemical etching of silicon by HF–HNO3 acid mixtures is an essential step in the processing of semiconductor and solar grade silicon. The dissolution of silicon is described by the following net equation (according to ref. [1]):3Si + 18HF + 4HNO3 → 3H2SiF6 + 4NO + 8H2O.
Main applications are the removal of surface roughness of wafers after slicing from silicon ingots (saw damage) and the texturing of silicon wafer surfaces for solar cells.
In practice, etching solutions of varying acid contents are used. In order to obtain a certain etch rate or texturing effect either freshly prepared mixtures from concentrated acids are diluted with water or already used etching solutions are refreshed several times by the addition of concentrated acids. The challenge is to find the optimum compromise between maximum utilization of the etching mixtures and economically reasonable etching rates. Among other parameters like the acid concentrations, acid mixing ratio, and temperature, the content of dissolved silicon affects mainly the etching rate. Therefore, fast and precise determination of the silicon in the etching solution independently on the acids contents is required for technical application.
From the analytical point of view, the acids act as a heavy matrix that prevents or complicates the analysis as in the case of the photometric determination of H2SiF6 as silicomolybdic acid [2], which is applicable only for low silicon contents up to 0.3% H2SiF6 in HF. A silicon determination by AAS [3], [4] requires a time consuming calibration and several dilution steps of the concentrated samples. The methods based on precipitation of K2SiF6 and subsequent titration of the acid and later of K2SiF6 are time consuming as well and affected by the efficiency of K2SiF6 precipitation (e.g. in organic solvents or in the cold) [5], [6], [7].
The present paper describes a robust determination of silicon by potentiometric titration in synthetic HF–HNO3-etching solutions regardless of the concentrations of HF and HNO3 and their mixing ratio. The H2SiF6 content can be obtained for any etching solution, which is unsaturated with respect to silicon according to Eq. (1).
The titration of complex acid mixtures is characterized by an overlap of the equivalent points. As a general rule, pKs values for two acids should differ more than four units to separate their equivalence points by titration [8]. Potentiometric titration of HF–HNO3-mixtures yields only to a single equivalence point due to the close pKs values of 3.17 at 20 °C for HF [9] and −1.34 at 30 °C for HNO3 [9]. In HF–HNO3-mixtures containing dissolved silicon after etching, silicon is present as dissociated species of H2SiF6. H2SiF6 has a complex and widely unresolved speciation [10]. H2SiF6 is considered as a strong acid comparable to sulphuric acid. The second dissociation constant pKs,2 is estimated as 0.22 [10].
The titration of all the mentioned acids according to the net simplified Eqs. (2), (3), (4) in one mixture should result in a common, irresolvable equivalence point.H2SiF6 + 2NaOH → Na2SiF6 + 2H2OHNO3 + NaOH → NaNO3 + H2OHF + NaOH → NaF + H2O.
In the second step, the hexafluorosilicic ion reacts with NaOH under formation of silicid acid at a pH around 8 [11].Na2SiF6 + 4NaOH → Si(OH)4 + 6NaF.
Section snippets
Experimental
HF–HNO3-acid mixtures were prepared from analytical grade acids (HF 40% (w/w), HNO3 65% (w/w), Merck, Darmstadt, Germany). Pieces of p-type silicon wafers (4 in. diameter, polished, 675 μm thickness; Silchem GmbH, Freiberg, Germany) were slowly dissolved in 50 mL of a mixture of 70% (v/v) HF and 30% (v/v) HNO3 at 1 °C. During dissolution the reaction vessels were covered with a Teflon cap. Immediately after dissolution of the silicon, the etching solution was warmed within 20 min to a temperature of
Titration of HF–HNO3–(NH4)2SiF6 solutions
The complex behavior of HF–HNO3 mixtures after dissolution of Si can be approached by titration of HF–HNO3 mixtures of known acid concentrations with defined volumes of a (NH4)2SiF6 stock solution added to. Using synthetic HF–HNO3–(NH4)2SiF6 mixtures the titration of the HF and HNO3 protons is clearly separated from the titration of the hexafluorosilicate anion in the titration curve.
The titration curves show two equivalence points (Fig. 1). The titrant volume of the first equivalence point is
Conclusions
A potentiometric titration method for the determination of total silicon content in HF–HNO3-etching solutions is presented. After establishing a calibration curve, the silicon content can be determined independently on the HF–HNO3 mixing ratio, i.e. independently on the density, and on the state of aging of the mixture. The method is applicable only at silicon contents equal or below the theoretical maximum solubility according to Eq. (1). This method provides a fast analysis of unknown etching
Acknowledgements
The authors gratefully acknowledge the European Regional Development Fund 2000–2006 and the Free State of Saxony for funding within the project “SILCYCLE” under contract number 8323/1293 at the Sächsische Aufbaubank (SAB).
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