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Talanta

Volume 71, Issue 5, 30 March 2007, Pages 1901-1905
Talanta

Determination of total fluoride in HF/HNO3/H2SiF6 etch solutions by new potentiometric titration methods

https://doi.org/10.1016/j.talanta.2006.08.022Get rights and content

Abstract

In the photovoltaic industry the etching of silicon in HF/HNO3 solutions is a decisive process for cleaning wafer surfaces or to produce certain surface morphologies like polishing or texturization. With regard to cost efficiency, a maximal utilisation of etch baths in combination with highest quality and accuracy is strived. To provide an etch bath control realised by a replenishment with concentrated acids the main constituents of these HF/HNO3 etch solutions including the reaction product H2SiF6 have to be analysed. Two new methods for the determination of the total fluoride content in an acidic etch solution based on the precipitation titration with La(NO3)3 are presented within this paper. The first method bases on the proper choice of the reaction conditions, since free fluoride ions have to be liberated from HF and H2SiF6 at the same time to be detected by a fluoride ion-selective electrode (F-ISE). Therefore, the sample is adjusted to a pH of 8 for total cleavage of the SiF62− anion and titrated in absence of buffers. In a second method, the titration with La(NO3)3 is followed by a change of the pH-value using a HF resistant glass-electrode. Both methods provide consistent values, whereas the analysis is fast and accurate, and thus, applicable for industrial process control.

Introduction

The determination of the fluoride concentration using an ion-selective electrode (F-ISE) either by direct potentiometry or by potentiometric titration with La(NO3)3 is known for numerous applications, e.g. pharmaceutical products [1] or potable water [2]. Detailed studies about interferences have shown no influence of any of the common anions such as chloride, nitrate, sulphate or phosphate [3]. The only significant interaction has to be referred to the hydroxide ion if its concentration is in the range of the fluoride content or higher [3], [4].

The direct potentiometry with a F-ISE is carried out after calibration and addition of TISAB-buffer. A robust and more exact method, especially, for the determination of the fluoride concentration is the use of the F-ISE as end-point indication during the titration of fluoride with lanthanum nitrate solution leading to precipitated lanthanum fluoride (Eq. (1)).

3F + La3+  LaF3The dependence of the titration on the pH-value was studied thoroughly using different systems of buffered and unbuffered solutions. The largest potential change was determined for neutral solutions without any addition of buffer. A lower pH-value adjusted by various buffer systems leads to a decrease of the potential change due to the formation of protonated fluoride species [5] that cannot be detected by the F-ISE. Although, the titration of neutral and unbuffered solution provides the best results these conditions are hardly to stabilize, mainly, because La(NO3)3 undergoes an acid hydrolysis. Buffering with acetate–acetic acid is necessary to set a solution permanently to a neutral pH. A problem going along with the use of acetate buffer is the shift of the equivalence point (EP) according to the amount of buffer that forms soluble lanthanum–acetate complexes. In contrast, there is no direct interference of the acetate-ion with the LaF3 crystal of the F-ISE [4], [5], [6], [7].

An increase of the sensitivity can be achieved by working in organic solvents. This affects the solubility of the precipitated LaF3 due to the altered polarity of the solvent. Mainly, dipolar aprotic solvents, a variety of alcohols and their mixtures with slight amounts of water [5], [8] contribute to an extension of the detection limit to nanomolar quantities of fluoride [9].

Recently, the F-ISE was applied for the determination of the total fluoride content in acidic etch solutions of silicon consisting of nitric acid (HNO3), hydrofluoric acid (HF) and the reaction product hexafluorosilicic acid (H2SiF6) [10]. The more exact and efficient titration with La(NO3)3 has not yet been performed for such strong acidic solutions. Hence, two different new titration methods for the determination of fluoride on the basis of La(NO3)3 are described in this paper. The methods are applicable for acidic solutions containing bounded fluoride species. One titration method uses the F-ISE for end-point detection and the other one is performed with a HF resistant glass-electrode. These methods combine several important advantages like excellent recovery, low relative standard deviation (R.S.D.), as well as the adherence of essential pH-values and the decomplexation of fluoride both without the use of buffer solution. Additionally, the determination of fluoride can be realised after the analysis of other important constituents of silicon etch solutions [10] right in the same aliquot and this approach leads to a minimal R.S.D.

Section snippets

Experimental

For the preparation of the synthetic etch solutions analytic grade acids were used. Acidic etch solutions for silicon contain the remaining educts nitric acid (HNO3, 65 wt.%, Merck) and hydrofluoric acid (HF, 40 wt.%, Merck) and the reaction product hexafluorosilicic acid (H2SiF6, 35 wt.%, Fluorchemie Dohna). The exact concentrations of the used acids were additionally verified by titration. All used volumes of the acids in the synthetic sample solutions were weighed at first (AT 200,

Titration of fluoride with La(NO3)3 and F-ISE for end-point detection

Etch solutions of silicon consist of HF and HNO3 plus the reaction product H2SiF6. At the typical pH-values < 1 of such mixtures fluoride is existent as H2F2 and as protonated solution species of SiF62− (the speciation of H2SiF6 is not yet fully resolved [11]) in the etch solution. For the precipitation with La(NO3)3 and for the detection with the F-ISE free fluoride is mandatory, so that the performed analysing methods require special preparation approaches. To obtain deprotonated fluoride

Conclusions

Two new potentiometric titration methods for the determination of fluoride in acidic solutions are presented. Based on the titration with La(NO3)3 and the F-ISE common approaches have been adapted to strong acidic etch solutions. Furthermore, the use of a glass-electrode for the detection of the EP in the fluoride titration has been established. This new method makes it possible to determine all important components of acidic etch solutions of silicon with only one analysing method, the

Acknowledgments

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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