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Phosphosilicate glass is known to stabilize silicon surface in planar structures against undesirable drift. It has been shown that the glass acts as a getter for sodium. The purpose of the experiments reported here is to clarity further the mechanism by which this glass improves silicon planar circuit reliability. Combined electrical (MOS) and radiochemical analysis of impurity and radiochemical measurement of phosphorus is done. Na24 Br82 impurity is diffused into samples before and after glassing all at 900Â°C. The most significant result is that, with glass overlay in place on 5000Ã thermal oxide, no appreciable silicon surface potential shift occurs, even if Na concentration throughout SiO2 is around 1018 cm-3. In addition, the Na does not appreciably drift in tge SiO2 during 50 min. at 200Â°C with fields of 106 v/cm. It is concluded that, in addition to gettering of Na by the glass, improved stability results because the glassing process renders Na in the oxide neutral. Detailed profiles show Na and Br trapping occurs at the glass/oxide interface as well as within the glass.