While it might be argued that chemically adsorbed fluorine is not an ionic impurity, the Si-F bond has 70 % ionic character (7) and would be strongly polar-ized. Cunningham et al. (8) reported that a specimen of p-type silicon soaked in HF, rinsed and stored in a dry nitrogen ambient exhibited a strong n-type inver-sion layer. These data along with the demonstrated strong adsorption of fluoride point out the potential problems in device fabrication. While the results presented here are adequately ex-plained by chemisorption, subsequent work has shown that higher amounts of fluoride can be retained by different ypes of silica films. This is particularly true of heavily boron doped films where enhanced adsorp-tion has been observed. Further work will be required to completely understand the variation in fluoride re-tention by the different types of oxides. The use of fluorine-18 labeled etches as described in this work appears to offer the best method of studying this phe-nomenon

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