ANALYSIS OF THIN SiO₂ FILMS AND HYDROFLUORIC ACID FOR THE CONTENT OF TRACE IMPURITIES OF CERTAIN ELEMENTS BY THE APN METHOD

A. A. Kaplin, A. N. Pokrovskaya, A. G. Stromberg

(Presented by the scientific-methodological seminar of the Department of Physical Chemistry)

One of the main tasks of semiconductor technology is the manufacture of reliable devices capable of operating for a long time. The characteristics of semiconductor devices and the yield percentage of acceptable devices in their production are substantially affected by the degree of purity. The presence of impurities significantly worsens the electrical properties of semiconductors. Since SiO₂ films serve as a source of diffusion of microelements into the semiconductor material, the analysis of these films for impurity content, as well as of water, hydrofluoric acid, and hydrofluoric-acid-based etchants used for etching films, including SiO₂, and for cleaning the surfaces of solid circuits, is of great interest.

A small number of works have been devoted to the analysis of SiO₂ films for certain impurities; moreover, macrosamples of SiO₂ on the order of 1.0–2.0 g were taken for analysis. Weinshtein et al. [1, 2] determined, by amalgam-polarography, \(>3 \cdot 10^{-8}\%\) Cu and Pb in SiO₂. The sample taken for analysis was 2 g, the electrolysis time was 5 min, \(\varphi_e = -0.8\) V. The duration of the SiO₂ analysis was 12 hours. In the work of Vinogradova and Kameneva [3], a procedure is given for determining \((3.25—4.4)\,10^{-6}\%\) Bi against an HCl background, as well as a procedure for the simultaneous determination of Pb and Tl against a background of \(0.1\,N\) KCl in the presence of complexone III. According to this procedure, Pb is determined in an amount of \((2—3)\,10^{-8}\,M\), and Tl \(<1 \cdot 10^{-8}\,M\). We have not encountered works devoted to the analysis of microquantities of SiO₂ by the APN method.

Hydrofluoric acid and water are used to dissolve SiO₂ films. These same reagents are included in many other etchants and reagents used for cleaning surfaces in the production of integrated circuits. The APN method has previously been used to analyze HF for certain impurities; for example, \(>10^{-8}\%\) Zn, Pb and \(>5 \cdot 10^{-7}\) Cu were determined against a background of \(1\,N\) KOH and NaOH [4], as well as \(10^{-8}\,M\) Cu, Pb, Cd, Tl against a background of \(0.4\,N\) HCl [5].

The aim of the present work is the analysis of SiO₂ films for the content of Zn, Cd, Pb, Sn, In, Tl, Cu, Sb, Bi by the APN method, the analysis being carried out from a single sample with a weight of \(2 \cdot 10^{-4}\) g. Since H₂O and HF are used for etching SiO₂, they were analyzed for the content of the indicated 9 elements.

Preparation of SiO₂ for analysis is carried out according to the following scheme. To transfer the film into solution, diluted HF (1:3) is used, which removes the film in 1–2 min without dissolving the material.

substrate (in our case the substrate material is silicon). For evaporation of SiF$_4$ an optimum temperature was selected at which the fluorides of the elements being determined do not volatilize; the temperature should not exceed 100–110°C.

The next stage in preparation for amalgam-polarographic analysis of SiO$_2$ films is the choice of an indifferent electrolyte. Using the table of spectra [6], 0.25 N HCl was chosen as the background for determination of Cd, Pb, Cu, Sb, Bi; for determination of Sn—3 N HCl; for determination of Zn the most convenient background is 0.02 N NH$_4$F. The determination of In and Tl is carried out against a background of 2.5 M KOH; moreover, when determining Tl, sodium tartrate is added to the solution in order to bind Cd into a complex, in the presence of which Tl is determined and which interferes with the determination of Cd.

For the work a polarograph LP-60 (Czechoslovakia) is used, with a polarographic cell of Kuleshov design with inserted platinum cups. As the cathode we chose a mercury film electrode prepared by cementing a silver wire with epoxy resin into a glass tube; the anode is a saturated calomel electrode. Oxygen is purged from the solution, and the solution is stirred, with argon.

Determination procedure

The sample (Si + SiO$_2$) is weighed on an analytical balance with an accuracy of up to $10^{-5}$ g and transferred into a platinum cup previously checked for purity. To remove the SiO$_2$ film from the substrate, diluted hydrofluoric acid (1:3) is used in an amount of 2–3 ml. The HF acid is first purified by distillation in a palladium distillation apparatus and, before analysis, is checked for the content of all the elements being analyzed. To check HF for purity, we take 3 ml of it and evaporate it to dryness at $t = 100^\circ$C, then analyze it for impurity content by the procedure given below. The same applies to the water used in the analysis for preparing working solutions and etchants and for cleaning the surface. For analysis we take 10–15 ml.

After etching the film for 1–2 min, the substrate is carefully removed from the cup with fluoroplastic tweezers, washed with pure water over the cup containing the etched film, dried, and weighed with an accuracy of up to $10^{-5}$ g. From the difference in weights, the weight of the film being analyzed is determined. The excess hydrofluoric acid is removed by evaporation at $t^\circ$ — 100–110°C. Then 5 ml of the indifferent electrolyte 0.02 N NH$_4$F is added to the cup and, at $\varphi_\mathrm{e} = -1.7$ V (relative to the saturated calomel electrode), reduction of Zn ions is carried out for 10 min. We begin recording the anodic Zn peak from $\varphi = -1.2$ V; the Zn peak potential is equal to $-1.0$ V (relative to the saturated calomel electrode). The sensitivity of the instrument when recording the Zn peak is $1 \cdot 10^{-9}$ A/mm.

After evaporation of this background at $t^\circ$C, we add to the cup 5 ml of 0.25 N HCl and, at a potential of ($-1.2$ V), carry out electrolysis of Cd, Pb, Cu, Bi ions, which are recorded at peak potentials, respectively: $\varphi_{\mathrm{p}Cd} = -0.65$ V, $\varphi_{\mathrm{p}Pb} = -0.45$ V, $\varphi_{\mathrm{p}Cu} = -0.25$ V, $\varphi_{\mathrm{p}Bi} = -0.05$ V. The optimum accumulation potential for Sb is $-0.5$ V, and $\varphi_{\mathrm{p}Sb} = -0.15$ V.

Acidifying the background to an HCl concentration corresponding to 3 N, at $\varphi_\mathrm{p} = -0.45$ (i.e., at $\varphi_\mathrm{p}$ corresponding to $\varphi_{\mathrm{p}Pb}$), we obtain the total Sn + Pb peak, and judge the concentration of Sn ions from the difference between the (Sn + Pb) peak obtained against a 3 N HCl background and the Pb peak obtained against a 0.25 N HCl background or a neutral 0.02 N NH$_4$F background. We evaporate the hydrochloric—

Table 1

acidic background, add 5 ml of 2.5 N KOH and, at \(\varphi_{\mathrm{e}} = -1.9\) V, carry out reduction of In ions. Recording of the polarogram begins at \(\varphi = -1.1\) V; \(\varphi_{n\mathrm{In}} = -0.77\) V (relative to sat. cal. el.). After determining Zn, we add 0.1 g of dry sodium tartrate to the polarographic beaker and, at \(\varphi = -0.9\) V, carry out electrolysis of Tl ions, whose peak potential is \(-0.55\) V (relative to sat. cal. el.).

According to this procedure, analyses were carried out of water of various degrees of purification and HF used in integrated-circuit production technology, as well as of SiO\(_2\) films (Table 1). These results are not general, but characterize only the given technological process.

Conclusions

1. A procedure is proposed for determining a number of elements (Zn, Cd, Pb, Sn, In, Tl, Cu, Sb, Bi) in water of various degrees of purification and in HF used in IC production.

2. A procedure is presented for determining a number of elements in SiO\(_2\) films from a sample weighing \(2 \cdot 10^{-4}\) g. All 9 elements were determined from a single sample.

References

1. Yu. I. Vainshtein, K. Ya. Ginzburg. In the collection Methods for the Analysis of High-Purity Substances. “Nauka,” 64, 1965.

2. Yu. I. Vainshtein, K. Ya. Ginzburg. In the collection Methods for the Analysis of Chemical Reagents and Preparations. Moscow, IREA, issue 28, 1966, 125—133.

3. E. N. Vinogradova, A. I. Kamenev. Proceedings of the Commission on Analytical Chemistry of the USSR Academy of Sciences, 15, 1965, 175—178.

4. S. I. Sinyakova, N. V. Markova. In the collection Methods for the Analysis of Substances of Special Purity. Moscow, “Nauka,” 1965, 507—509.

5. P. Beran, J. Doležal, E. Hrabánková. Collect. Czechosl. Chem. Communs., 39, No. 1, 308—313, 1968.

6. R. F. Zarubina, N. A. Kolpakova, A. A. Kaplin. Zavod. laborator., 1971, No. 1, pp. 11—12.