Phosphosilicate glass stabilization of FET devices

The threshold voltage of MOSFET devices can be effectively stabilized from changes due to field-assisted motion of Na
⁺
in the gate oxide by the addition of a phosphosilicate glass (PSG) layer. The effectiveness of the glass for this purpose is markedly enhanced by increasing the P
2
O
5
concentration of the PSG. However, polarization of the PSG layer can, in turn, cause an appreciable instability of the threshold voltage. It is shown that detailed knowledge of the behavior of PSG layers permits prediction of the threshold stability of P
2
O
5
-treated FET devices. Thus, threshold stability can be maintained to within 0.1 V/1000 Å under device operating conditions by making a proper compromise on PSG thickness and P
2
O
5
concentration. Such stabilizing films offer satisfactory protection against realistic Na
⁺
contamination levels. Quantitative data on these phenomena are presented, and a simple structural model is given to account for the polarization and the Na
⁺
trapping behavior of the films. The formation of PSG films by doping of SiO
2
with P
2
O
5
at elevated temperatures is discussed.