Studies on the Determination of Ultramicroimpurities in Complexing Agents
Abstract Generated abstract
This study addresses the need for highly sensitive control of heavy metal ultramicroimpurities in complexing agents used to clean semiconductor surfaces. The authors investigate amalgam polarographic determination of Cu, Pb, Cd, In, and Zn in morin and quinalizarin solutions using a mercury film cathode and graphite anode, with particular attention to selecting suitable supporting electrolytes and electrolysis conditions. They find that quinalizarin analysis is most sensitive in 0.4 N NaOH, while morin analysis is conducted in neutral KCl medium to avoid mutual interference of cadmium and indium. The work develops procedures intended to determine these impurities at sensitivities of about 10^-5 to 10^-6 percent.
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STUDIES ON THE DETERMINATION OF ULTRAMICROIMPURITIES IN COMPLEXING AGENTS
L. M. Poskrebysheva, A. A. Kaplin, B. A. Kubrak
(Presented by the scientific seminar of the Department of Physical Chemistry)
The electrical parameters of semiconductor devices and their stability depend on the state of the semiconductor surface, which changes as a result of physical and chemical processes occurring on it [1].
The method of removing (desorbing) an impurity from the surface of a device depends both on its chemical composition and on the state in which the impurity was deposited on the surface. The best desorbents for most impurities are complexing agents.
As complexing agents, morin and quinalizarin are used. Morin, or 3,5,7,2,4-pentaoxyflavone, is an oxy-derivative of flavone [2], a yellow crystalline substance with a melting point of 290°C. It is soluble in water, in alcohol, in dilute acids and alkalis, and has the formula (see left).
Quinalizarin, or 1,2,5,8-tetraoxyanthraquinone (alizarin bordeaux) [2], has the following structural constitution:
Chinalizarin crystallizes in red rhombic needles (melting point 275°C) and sublimes readily. Chinalizarin is insoluble in water and slightly soluble in alcohol; as a phenol it dissolves readily in caustic alkalis, giving intensely colored red-violet solutions.
With atoms of heavy metals, chinalizarin forms complex compounds in which the metal atom enters into the structure of five- or six-membered rings, for example:
Since at present the semiconductor industry has no highly sensitive methods for monitoring the purity of the above-mentioned complexing agents, the aim of our work was to conduct studies for developing procedures for determining a number of heavy-metal impurities (Cu, Pb, Cd, In, Zn) in solutions of complexing agents by the APN method with a sensitivity of \(10^{-5}\)—\(10^{-6}\%\).
The studies were carried out in an ordinary polarographic cell; a mercury film electrode was used as the cathode, and graphite as the anode. In developing amalgam-polarographic procedures, one of the most important stages is the choice of the optimum supporting electrolyte. First of all, the possibility of determining impurities by carrying out polarography directly in solutions of morin and chinalizarin was investigated.
Chinalizarin is prepared by dissolving a weighed portion (0.1 g) of the crystalline substance in a liter of distilled water in an alkaline medium. On passing to a neutral medium, chinalizarin acquires a brownish color and precipitates in the form of red flakes. In an acidic medium chinalizarin is in the dissolved state and has an orange color. However, owing to the fact that in an acidic medium it is impossible to determine all five elements simultaneously (the electrooxidation of zinc occurs in the region of hydrogen-ion discharge), the dependence of the anodic current of the elements studied on the concentration of NaOH alkali in the solution was investigated. It was found that the greatest sensitivity of the elements studied is observed at an alkalinity of the solution of 0.4 N.
A morin solution is obtained by dissolving a 0.1 g weighed portion in 100 ml of isopropyl alcohol and adding 900 ml of hot water (60–70°C) to the resulting solution with gentle stirring until complete dissolution. Since cadmium and indium interfere with each other’s determination in an alkaline medium, all studies were carried out in a neutral medium in the presence of KCl.
On these backgrounds, the following dependences were obtained for choosing the conditions for carrying out the analysis:
- Height of the anodic peak of the element—electrolysis potential.
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Height of the anodic peak of the element — electrolysis time.
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Height of the anodic peak of the element — concentration of metal ions in solution.
On the basis of the investigations carried out and the dependences obtained, amalgam-polarographic methods were developed for determining a number of impurities in solutions of complexing agents used in the technology of semiconductor devices for cleaning semiconductor surfaces.
REFERENCES
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G. P. Peka. Physics of Semiconductor Surfaces. Publishing House of Kiev University, 1967.
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A. E. Chichibabin. Fundamentals of Organic Chemistry. Goskhimizdat, 1958.