Studies on the Development of Methods for Analyzing Macro-Scale Samples and Films of Germanium Monoxide for Cadmium, Lead, and Copper Content by the APN Method
Abstract Generated abstract
This study develops procedures for determining cadmium, lead, and copper impurities in crystalline germanium monoxide and very small germanium monoxide films used in microelectronic applications by anodic polarographic analysis on a mercury-film electrode. The authors examine dissolution conditions, identify a heated diluted mixture of water, ammonium hydroxide, and hydrogen peroxide as effective for dissolving samples, and show that germanium interferes with direct polarographic determination, requiring matrix removal by repeated evaporation with hydrochloric acid to distill germanium as tetrachloride. Optimized conditions enabled simultaneous determination of the three metals by the standard-addition method after matrix separation. Analyses indicated that deposited films contained substantially higher impurity levels than the initial crystalline material, with increases attributed to deposition conditions and possibly substrate effects.
Full Text
STUDIES ON THE DEVELOPMENT OF METHODS FOR ANALYZING MACRO-WEIGHT SAMPLES AND FILMS OF GERMANIUM MONOXIDE FOR CADMIUM, LEAD, AND COPPER CONTENT BY THE APN METHOD
A. A. Kaplin, N. P. Pikula, Z. S. Mikhailova
(Submitted by the scientific-methodological seminar of the Department of Physical Chemistry)
Recently, the analysis of macro-weight samples (films weighing \(0.001\ \text{g}\)), which are used for manufacturing microcircuits employed in radio electronics, has become increasingly important.
Our task was to develop a method for analyzing GeO films on a glass substrate and the initial crystalline germanium monoxide for the content of Cd, Pb, and Cu impurities. It was also necessary to determine how the impurity composition in the films changes with changes in the deposition conditions (\(t^\circ\), \(P\), substrate material).
Selection of a Solvent for GeO
According to the literature data [1], it is known that GeO is sparingly soluble in cold water, but dissolves in chlorine water and in a mixture of \(\mathrm{NH_4OH}\) and \(\mathrm{H_2O_2}\). We carried out a series of studies to select a solvent for the initial germanium monoxide (gray-black in color).
On the basis of the data presented in Table 1, the following conclusion can be drawn: the initial germanium monoxide dissolves best in a mixture of \(\mathrm{H_2O_2}\) and \(\mathrm{NH_4OH}\) upon heating. Further studies showed that, with dilution of the mixture, the solubility of GeO increases. Therefore, the best solvent for GeO (\(0.05\ \text{g}\)) is a mixture of \(8\ \text{ml}\ \mathrm{H_2O} + 1\ \text{ml}\) conc. \(\mathrm{NH_4OH} + 1\ \text{ml}\ \mathrm{H_2O_2}\) (30%) at \(t^\circ = 80\text{–}90^\circ\mathrm{C}\). A \(0.05\ \text{g}\) sample of GeO dissolves in this solvent in 1–1.5 hours.
In studying the solubility of GeO films on a glass substrate, it was established that the films (\(\sim 0.001\ \text{g}\)) dissolve in 4–5 drops of a warm diluted mixture of \(\mathrm{H_2O_2} + \mathrm{NH_4OH}\). In addition, GeO films dissolve in concentrated HCl.
Selection of Optimal Conditions for the Simultaneous Determination of Cd, Pb, Cu
From the literature data [3] it may be concluded that Cd, Pb, and Cu can be determined polarographically simultaneously in supporting electrolytes of NaCl, KCl, HCl, \(\mathrm{NH_4Cl}\), \(\mathrm{NH_4F}\), \(\mathrm{K_2CO_3}\), and others.
Table 1
Results of studies on the solubility of GeO samples
| No. | GeO sample weight, g | Solvent | Dissolution conditions | Dissolution results |
|---|---|---|---|---|
| 1 | 0.10 | 6 ml cold H₂O | \(t^\circ = 22^\circ\mathrm{C}\) | Dissolution is not noticeable; a light-brown turbidity formed at the bottom of the beaker, which does not disappear after 24 hours |
| 2 | 0.10 | 6 ml hot H₂O | \(t^\circ = 60\text{--}70^\circ\mathrm{C}\) | Red-brown turbidity |
| 3 | 0.10 | 10 ml H₂O + Cl₂* | \(t^\circ = 22^\circ\mathrm{C}\) | Dissolution is not noticeable. After 24 hours there is turbidity at the bottom |
| 4 | 0.10 | 10 ml H₂O + Cl₂* | \(t^\circ = 90^\circ\mathrm{C}\) | No dissolution is visible |
| 5 | 0.10 | 6 ml 9 N HCl + Cl₂* | \(t^\circ = 80\text{--}90^\circ\mathrm{C}\) | Partial dissolution (due to the formation and volatilization of GeCl₄) |
| 6 | 0.10 | 6 ml 9 N HCl + Cl₂* | \(t^\circ = 21^\circ\mathrm{C}\) | No dissolution |
| 7 | 0.10 | 3 ml H₂O + + 2 ml NH₄OH + + 2 ml H₂O₂ |
\(t^\circ = 21^\circ\mathrm{C}\) | Weak evolution of gas bubbles on the surface |
| 8 | 0.10 | 6 ml H₂O + + 2 ml H₂O₂ + + 2 ml NH₄OH |
\(t^\circ = 90^\circ\mathrm{C}\) | Dissolution proceeds with evolution of gas bubbles. Dissolved in 4 hours |
| 9 | 0.05 | 8 ml H₂O + + 1 ml H₂O₂ + + 1 ml NH₄OH |
\(t^\circ = 70\text{--}80^\circ\mathrm{C}\) | Dissolution in 1–1.5 hours |
* Cl₂ is passed into the beaker for 1.5–2 hours from a reaction flask, where Cl₂ is obtained by the reaction:
\[ \mathrm{MnO_2 + 4HCl = MnCl_2 + Cl_2 + 2H_2O} \ [2] \]
when concentrated HCl is added to MnO₂.
We studied the dependences of the heights of the anodic peaks of Cd, Pb, and Cu on the electrodeposition potential, on the electrolysis time, and on concentration against backgrounds of 0.1 N NH₄F, 0.1 N HCl, and 1.0 N HCl.
From the linear character of the dependence \(I — \tau_{\mathrm{el}}\) for these elements it follows that, over 20 min of electrolysis, depletion of the solution is practically absent.
From the dependence \(I — \varphi_{\mathrm{el}}\) it is clear that the optimum potential for simultaneous electrodeposition of Pb, Cd, and Cu is \(\varphi_{\mathrm{el}} = -1.2\ \mathrm{V}\), at which the limiting value of the currents is reached for all elements (Pb, Cd, Cu).
The rectilinear dependence \(I—C\) makes it possible to estimate the concentrations of these elements in solution by the standard-addition method.
Effect of Germanium on the Determination of Pb, Cu, Cd
The effect of germanium on the determination of Cd, Pb, and Cu was studied as follows: a GeO film (0.001 g) is dissolved in 3 drops of 9 N HCl, and water is added to a volume of 5 ml (final composition of the resulting background: GeCl₂(GeCl₄) + HCl). At an electroaccumulation potential ($\varphi_{\mathrm{el}} = -1.6$ V), a dark-colored film of reduced Ge forms on the mercury-film electrode. Owing to the formation of a germanium film on the electrode, the hydrogen overvoltage on mercury decreases, the residual current increases, diffusion of metals from the solution into the electrode is hindered, and semiconductor Ge films may form with the metals being deposited on the electrode. When the potential is decreased from −1.6 to 0.0 V, the germanium film does not dissolve from the electrode. Consequently, Ge interferes with the determination of Cd, Pb, and Cu.
We arrived at the same conclusion when analyzing macro-weighed portions of GeO after dissolving them in NH₄OH + H₂O₂.
Thus, for the determination of Cd, Pb, and Cu in germanium monoxide, preliminary removal of the matrix from the solution is necessary.
Removal of Ge from Solution
In the analysis of germanium, germanium dioxide, and germanium tetrachloride by various methods, the matrix is usually removed by distillation in the form of germanium tetrachloride [4], germanium fluoride [5], by distillation of the products of the chemical interaction of Ge with iodine and sulfur [6], or by extraction of Ge with CCl₄ from hydrochloric acid solutions [6].
Kamenev et al. [7–9], in the polarographic analysis of Ge and GeO₂ for the content of a number of impurities (Cd, Pb, Tl, Bi, Sb, Cu), recommend converting germanium and germanium dioxide to germanium tetrachloride by dissolution in chlorine-saturated hydrochloric acid. Concentration is carried out by removing the bulk of the germanium tetrachloride by evaporation or by distillation in a distillation apparatus, followed by electrolytic deposition of the impurities from the solution (after addition of the background) on a stationary mercury electrode with a silver contact.
We have shown that Ge is removed from solution if concentrated HCl is added to GeO dissolved in NH₄OH + H₂O₂, containing an excess of H₂O₂, and the contents of the beaker are heated almost to moist salts at $t^\circ = 90$–100°C (H₂O₂ in an acidic medium is an oxidizing agent, i.e., it converts Ge²⁺ to Ge⁴⁺, which promotes more complete removal of Ge in the form of GeCl₄). More complete removal of Ge occurs upon a second evaporation of the salts with concentrated HCl.
The residue after the second evaporation is dissolved in the background and polarographed.
Experimentally, by the “added–found” method, we have shown that, with twofold evaporation of the solution with HCl at $t^\circ = 90$–100°C, losses of Cd, Pb, and Cu practically do not occur.
On the basis of the studies carried out, we proposed procedures for analyzing crystalline germanium monoxide and GeO films for the content of Cd, Pb, and Cu.
The procedures are based on dissolving macro-weighed portions ($g = 0.050$ g) and films ($g \simeq 0.001$ g) of germanium monoxide in a mixture of H₂O : NH₄OH : H₂O₂ (8 : 1 : 1); subsequent twofold evaporation of the solution, with addition of concentrated HCl (to distill off GeCl₄), to moist salts; dissolution of the moist residue in the background; and polarographing of the resulting solution.
The content of the impurities Cd, Pb, and Cu was estimated by the method of additions. A series of analyses of macro-samples and GeO films was carried out (Table 2).
Table 2
Results of analyses of macro-samples and films of germanium monoxide
for the content of Cd, Pb, Cu by the APN method on a mercury-film electrode
after separation of the matrix by distillation in the form of GeCl\(_4\)
Analysis conditions:
weight of macro-samples — 0.050 g, weight of films — 0.001 g;
background 0.1 N NH\(_4\)F;
\(\varphi_{\mathrm{el}} = -1.8\) V (relative to the saturated calomel electrode);
\(\tau_{\mathrm{el}} = 5\) min;
\(S = 1 \cdot 10^{-8}\) A/mm.
The impurity content was estimated by the method of additions
| Samples | Cd | Pb | Cu |
|---|---|---|---|
| Macro-samples GeO | \((1.63 \pm 0.28)\cdot 10^{-5}\) | \((2.70 \pm 0.52)\cdot 10^{-5}\) | \((2.30 \pm 0.47)\cdot 10^{-5}\) |
| Films No. 1 | \((5.62 \pm 0.08)\cdot 10^{-4}\) | \((3.48 \pm 0.60)\cdot 10^{-3}\) | \((1.30 \pm 0.18)\cdot 10^{-2}\) |
| Films No. 2 | \((2.00 \pm 0.08)\cdot 10^{-4}\) | \((3.60 \pm 0.62)\cdot 10^{-3}\) | \((1.30 \pm 0.20)\cdot 10^{-2}\) |
| Films No. 3 | \((3.00 \pm 0.06)\cdot 10^{-4}\) | \((1.10 \pm 0.20)\cdot 10^{-3}\) | \((0.55 \pm 0.01)\cdot 10^{-2}\) |
Films 1, 2, and 3 were obtained by vacuum deposition of the initial GeO onto glass substrates at various temperatures and pressures.
From the analyses presented in Table 2 it may be concluded that the impurity content in GeO depends on the deposition conditions and increases by a factor of 10–100 in comparison with the initial crystalline GeO. The increase in the impurity content in the films may also depend on the substrate material (we were unable to check the purity of the surface of the glass substrate before depositing the GeO film onto it).
Conclusions
-
The solubility of macro-samples and films of GeO was studied; it was shown that Ge interferes with the polarographic determination of Cd, Pb, and Cu.
-
Optimal conditions were found for the simultaneous polarographic determination of Cd, Pb, and Cu in GeO after removal of the matrix by distillation of GeCl\(_4\). Procedures were developed for analyzing macro-samples and films of GeO for the content of Cd, Pb, and Cu.
-
It was shown that after deposition onto substrates the content of Cd, Pb, and Cu impurities in GeO increases by a factor of 10–100 in comparison with the initial GeO.
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