Diagnostics of Impurity Composition of High–Pure Monosilane by Results of the Analysis of a Test Silicon Single Crystal

Elements III and V of groups of the Periodic System and carbon are the most important impurities in silicon. The estimation technique of carbon, boron and phosphorus impurity content in high−purity monosilane has been proposed. The technique involves the preparation of polycrystalline silicon by silane decomposition, growing a test single crystal by the floating zone melting method and analysis of single crystal samples by IR spectroscopy. Calculation of impurity concentration in polycrystalline silicon were performed using results on their content in the test single crystal samples, data on impurity distribution of in the liquid−solid system and sample coordinates along the ingot length. Effective impurity distribution coefficient in the «solid−liquid» system for specific growing conditions have been calculated using the Burton−Prim−Slichter equation. 

Results for the test silicon samples with natural isotopic composition and the enriched 28Si isotope obtained from monosilane samples with different impurity contents have been reported. Results of IR spectroscopic research of impurity composition for the test silicon single crystal are in agreement with the concentration data obtained by chromatography. The concentration of III and V group impurities in monosilane were in the range 4 ⋅ 10−9—2 ⋅ 10−6 at. %, and for carbon 2 ⋅ 10−6—6 ⋅ 10−4at.%. The measurement uncertainty by IR spectroscopy method for carbon impurity does not exceed 15 %, for boron and phosphorus — 20%. We show that the upper limit of carbon content in monosilane detected using this method is determined by its solubility in silicon, while the bottom limit depends on the detection accuracy of the IR spectroscopy technique and possible background contamination.

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