Sodium Ion Drift through Phosphosilicate Glass ‐ SiO₂ Films

The field‐assisted transport of sodium ions through phosphosilicate composite films on silicon substrates has been investigated. The effects of concentration (in the 0 to 8 mole per cent [m/o] range), sodium contamination level (between 10¹¹ and 10¹⁵ Na⁺ ions/cm²), field, temperature, and time have been quantitatively determined. For a limited quantity of transported sodium, this quantity is approximately proportional to the square root of biasing time; exponentially dependent on temperature and field, increases linearly with the initial sodium contamination level; and decreases drastically with increasing concentration. For example, seven more orders of magnitude of time are required to drift a given number of Na⁺ ions through a composite layer than through a 2150Aå thick layer of pure, thermally grown . The results are consistent with a model based on the emission of Na⁺ ions from traps in the phosphosilicate layer. It was concluded that phosphosilicate glass films, having negligible intrinsic polarizability, can be used to effectively stabilize IGFET threshold voltages against changes (0.1V in 10 yr at 80°C under a field of ) due to the presence of large amounts of sodium.