Permanent Radiation Effects in Hardened Al2O3 MOS Integrated Circuits

Complementary-symmetry metal-oxide semiconductor (COS/MOS) inverter circuits have been fabricated with aluminum oxide as channel insulator. This oxide was formed either by plasma-anodization of Al or by vapor deposition at 450°C using Al-isopropoxide. The RCA CD-4007 circuit configuration was used for these studies. The inverters were subjected to ionizing radiation with the applied gate biases encountered under normal operation (0 and +10 volts). For all units tested, the shifts in the inverter characteristics were due to shifts in the threshold voltages of the individual transistors. No measurable deterioration in gain due to interface state generation was observed. These results confirm the expected radiation hardness measured previously with MOS capacitors. Based on the net voltage shifts in the transfer characteristics of the complementary inverters, plasma-grown A12O3 shows an increase of more than an order of magnitude in hardness over units with SiO2 channel oxide. The hardness of units with deposited A12O3 was found to be intermediate to these. The increased hardness of A12O3 over SiO2 is attributed, at least in part, to the presence of trapping centers for negative charge. This serves to balance the positive trapped oxide charge so that a considerable reduction in the net oxide charge is attained.