Reports of the Academy of Sciences of the USSR

1. Vol. 162, No. 5

PHYSICAL CHEMISTRY

V. S. SOTNIKOV, A. S. BELYANOVSKII

ADSORPTION OF IONS OF CERTAIN METALS FROM ELECTROLYTES DURING THE ETCHING IN THEM OF GERMANIUM, SILICON, AND QUARTZ

(Presented by Academician A. P. Vinogradov, December 4, 1964)

This paper considers the adsorption of metal ions from electrolytes that have found wide application in the mass production of Ge, Si, and semiconductor devices made from them. Impurities adsorbed by the surface of the semiconductor material and by the electron–hole \(p\)–\(n\) junction during their etching and washing with water have an adverse effect on the yield of usable devices and on the stability of their parameters over time. The reagents supplied by the chemical industry would seem to have a sufficient degree of purity (impurity content \(10^{-7}\)—\(10^{-4}\%\)). However, adsorption from solutions also occurs at such impurity concentrations. In addition, during the etching of \(p\)–\(n\) junctions, ions of a number of elements that enter into the structure of the junction pass into the solution, for example In, Zn, Ga, As, Sb, Sn, and others. Moreover, depending on the structure of the junction and the etching time, the content of each of the listed impurities, as we have established, may reach \(10^{-2}\)—\(10^{-1}\%\). Therefore, the question of the adsorption of metal ions from etchants over a wide range of concentrations is very topical, especially since the literature contains very little information on adsorption on semiconductors from solutions in general and from special etchants in particular \((1–4)\).

The first work in this field appeared in 1956 \((^1)\). In it, using the radioactive isotope \(\mathrm{Na}^{24}\), the adsorption of Na cations from NaOH solutions on the surface of germanium diodes with an alloyed junction and of \(p\)–\(n\)–\(p\) transistors during their electrolytic etching was investigated. In the work of Kuleshova and Naumova \((^2)\), also with the aid of radioactive indicators, the adsorption of \(\mathrm{Na}^+\), \(\mathrm{Fe}^{3+}\), and \(\mathrm{Ca}^{2+}\) ions on germanium during its etching in 30% \(\mathrm{H_2O_2}\) was studied.

Fig. 1. Dependence of the magnitude of impurity adsorption: a—on silicon, on their concentration in KOH and CP; b—on germanium, on their concentration in KOH, CP, and \(\mathrm{H_2O_2}\); c—on quartz, on their concentration in KOH and CP.

Beginning in 1958, the authors carried out work on the study of the adsorption of ions of various metals (Fe, Cu, Ag, Au, Zn, In, Na, Rb, Sb) on germanium, silicon, and quartz, as well as on certain semiconductor devices from water and from various etchants, using radioactive indicators. In papers (6, 7) we presented the results of a study of the adsorption of ions of the indicated metals on the surface of Ge and Si from CP and H₂O₂ etchants. However, adsorption isotherms were not given there, nor were the kinetics and mechanism of ion adsorption from etchants considered.

In the present work we have studied the adsorption of Cu, Ag, Au, In, Sb, and Zn ions from chemical etchants: CP (a mixture of 49% HF and 65% HNO₃ in a ratio of 1 : 4) and 20% KOH on the surface of germanium, silicon, and quartz, and also from 30% H₂O₂ on the surface of Ge, as a function of the content of the indicated impurities in the solutions and of the contact time of the specimens with the solutions.

Etching in CP was carried out at room temperature (20°), and in alkali and peroxide at the boiling temperature (107° for KOH and 104° for H₂O₂). The procedure has been described by us earlier (3). The adsorption isotherms and kinetic curves are given in Figs. 1 and 2.

Discussion of the results

A characteristic feature of all the adsorption isotherms presented is their obedience to the Freundlich equation over wide concentration ranges. The Freundlich equation is violated only for Au, Sb, and In

Table 1

at relatively high concentrations. The constants of the Freundlich equations are given in Table 1.

In Figs. 2a, b are given the dependences of the adsorption values of the same elements on germanium and silicon on the contact time of the specimens with the solutions. A characteristic feature of all the curves presented is their rapid tendency toward saturation, which occurs especially rapidly during etching in CP (after 1–2 min). However, the form of the adsorption curves in the first minutes of etching is different for different etchants. Whereas during etching in KOH the adsorption value increases, during etching in CP and in H₂O₂ the adsorption value decreases and its equilibrium value is less than the initial one. For the case of adsorption on Ge from KOH, analysis of the curves made it possible to establish that they obey the equation

$$ N = N_\infty (1 - e^{-kt}), $$

where $N_\infty$ is the equilibrium value of the adsorption. Below we give the values of the coefficients $k$ for different elements:

Such a difference in the behavior of the adsorption curves is evidently explained by the different behavior of Ge and Si in the indicated solutions. The point

in that the onset of an equilibrium state during adsorption from etchants is determined not only by the usual condition of equality of the rates of adsorption and desorption, but also by the etching rate. In view of the fact that ever new layers with an already adsorbed impurity are etched away from the surface, the magnitude of adsorption at a high etching rate cannot increase with time. On the contrary, as a result of a decrease in the true surface area (especially in such a polishing etchant as CP), the total number of adsorbed atoms must decrease. An exception in this respect is the action of KOH on Ge. The etching rate in this case is negligibly small, and separation of layers with an already adsorbed impurity practically does not occur. Therefore the dependence of adsorption on time obeys the usual equation \(N = N_{\infty}(1 - e^{-kt})\). The values of the etching rates after mechanical grinding of the specimens on M-5 powder are given in Table 2. On single crystals of Ge and Si the etching rate was measured for the (111) plane.

Examination of Fig. 1 makes it possible to conclude that the adsorption values on one and the same adsorbent depend substantially on the solution in which etching takes place. Thus, for example, the magnitude of adsorption from KOH in all cases is 1–2 orders of magnitude greater than for the same elements from CP. There may be several reasons for this: 1) differences in the temperature regimes during etching in CP, KOH, and H₂O₂; 2) the electrochemical potentials of both Ge and Si, as well as of the adsorbates, have different values in different etchants; 3) the etching rate, as already indicated, is different in different solutions; 4) the magnitude of the true surface area of the specimens is, as we have shown (⁹), different when etching in different etchants; 5) metal ions in CP, KOH, and H₂O₂ solutions may be in different states.

Table 2

Values of the etching rates (μ/min)

Fig. 2. Dependence of the magnitude of impurity adsorption: a—on the time of etching of silicon in KOH and CP; b—on the time of etching of germanium in KOH, CP, and H₂O₂

As is known, the process of adsorption of ions from electrolytes will be possible in the case when the initial potential of the impurity–solution system is greater than the initial potential of the given electrode in the solution (although, in small quantities, those ions for which this condition is not satisfied may also be adsorbed).

It is known that real solids are in fact not homogeneous (deviations from stoichiometric composition, microcracks, etc.), and therefore ions from the solution will first of all be captured by those surface regions where the force field is most intense. Further—

the subsequent fate of the captured ions will differ depending on whether or not they can receive electrons from the surface of the solid. In the first case, metals are deposited on the surface in the elemental state; in the second, adsorption occurs in the ionic state. The sources of electrons can only be elemental Ge or Si, or their monoxides, but by no means the dioxides. When the adsorbate is deposited in the ionic state, the ions are first captured by the regions of the force field of greatest intensity; then, when these regions are already occupied, the ions are captured by neighboring regions, up to the filling of all free bonds. In the limit, adsorption of one monolayer occurs. Further adsorption, if it occurs, is already physical.

A different picture will obtain when the adsorbed elements are deposited in the elemental state. Uncharged metal atoms settle on certain regions of the surface, which leads to the formation of many galvanic microcouples, and the subsequent process is reduced to the electrolytic deposition of metal on these microcouples. The regions on which primary adsorption has occurred begin to grow, and the surface coating thus takes place not as an even layer, but in the form of numerous mounds that increase with time; this was observed by us when examining the surface of Ge and Si with an adsorbed impurity by means of an electron microscope.

On the basis of the foregoing, one may conclude that the amount of adsorbed substance when deposition occurs in the elemental state must be greater than when deposition occurs in the ionic state. In the latter case, there is a monolayer coating with an additional small amount of physically adsorbed ions; in the former case, deposition of the metal on the surface ends when all the mounds coalesce and the entire surface becomes covered with the deposited metal, so that the conditions for its further electrochemical deposition disappear. The amount of deposited metal in this case will considerably exceed a monolayer coating and will depend on the structure of the surface.

Comparison of Fig. 1a, b, c confirms this assumption. On quartz, in which Si is in the highest valence state, only ionic adsorption can occur. And indeed, the magnitude of adsorption on quartz for all the elements we investigated is significantly lower than on Ge and Si and does not exceed a monolayer coating \((\sim 10^{15}\ \text{atoms}/\text{cm}^2)\).

All the measurements mentioned above were carried out on samples of p-type Ge and Si with a resistivity of \(6.2—7.8\ \Omega\cdot\text{cm}\). To determine the dependence of adsorption on the type of conductivity, measurements were made of Cu and Ag adsorption on n-type Ge and Si with the same resistivity (from CP, KOH, and H₂O₂). It turned out that the conductivity type of Ge and Si, within the limits of experimental error, does not affect the adsorption magnitude of the indicated elements. A more detailed consideration of this question will be given in the following article.

Received
2 XII 1964

REFERENCES

¹ J. P. Wolsky, P. Rodriguez, W. Waring, J. Electrochem. Soc., 103, 606 (1956). ² И. М. Кушелов, А. Ф. Наумова, ЖФХ, 32, 62 (1958). ³ P. J. Holmes, R. C. Newman, Proc. Instr. Elect. Eng., B106, Suppl. 15, 287 (1959). ⁴ J. T. Law, Phys. Chem., 59, 543 (1955). ⁵ Electrochemistry of Semiconductors, Ed. by P. J. Holmes, 1962, p. 265. ⁶ В. С. Сотников, А. С. Белановский, ЖФХ, 34, 2110 (1961). ⁷ В. С. Сотников, А. С. Белановский, ДАН, 137, 1162 (1961). ⁸ В. С. Сотников, А. С. Белановский, Collection: Coprecipitation and Adsorption of Radioactive Elements, “Nauka,” 1965, p. 149.