Investigation of silicon dioxide films by neutron activation analysis and autoradiography

Activation analysis and surface autoradiography were used to determine the concentration distribution and precipitation of contaminants in the internal part of the films and on the surfaces. The origin of the contaminants was studied by sampling each technological product following oxidation doped oxide deposition, heating and metallizing. Samples were irradiated at a thermal neutron flux of 3.2 . 1013 n . cm-2 . s-1. Irradiation times of 30 hrs and cooling times of about 24 hrs were used. After irradiation, samples were treated by 3N hydrochloric acid at 60 deg C to remove the surface contamination. The removal of the successive regions of the oxide films was carried out by chemical etching, using an NH4F-HF-H2O solution (90 cm3 conc. HF, 300 g NH4F, 600 cm3 H2O, NH4OH to ph=4.5), which has an etching rate of 200 A/min at 0 deg C. By this technique picein coating is used to protect parts other than the analyzed oxide film. For NAA measurements, slices were etched in 25 ml polyethene beakers containing 10 cm3 of solution in which the etchant was to be gamma-counted. For the autoradiography, Kodak AR 10 Striping film and ORWO RD 3-4 dosimeter film were used. The exposure times varied in the range of 24-72 hrs according to the activity of the samples. The distribution of the density of the autoradiography was determined by microdensitometry. Under the given conditions, the following detection limits of impurities could be obtained in silicon dioxide films: Na=80 ppb, Cu=20 ppb and Au=5 ppb. (T.G.)