Radiation tolerance of double layer field oxides

A radiation-hard field oxide has been fabricated for integrated circuits, eliminating the radiation-induced field inversion observed with pyrogenic field oxides. The radiation-hardened oxide was fabricated in conventional silicon integrated circuit process techniques and does not degrade under subsequent thermal stresses (up to 1000°C). The field oxide is built up of a double layer consisting of a thick, chemically deposited and doped SiO₂ layer on top of a thin, thermally grown SiO₂ film. The influence of the dopants of the CVD (chemical vapor deposited´) oxide as well as the annealing temperature on the radiation hardness of the field oxide is examined in order to find optimized process parameters. Compared to thermal oxides, a 2.5-order-of-magnitude improvement in radiation tolerance is observed with regard to the threshold voltage shift. The double layer field oxide can replace the thermal oxide in VLSI bipolar and MOS technologies. The elimination of guard bands, closed cell design, and gated insulator structures in MOS circuits is rendered possible. Significant improvement in speed and circuit density can be achieved by eliminating additional stray capacitance and space for guard structures