Total Dose Effects of Ionizing Radiation on MOS Structures at 90°K

The effects of ionizing dose on MOS device parameters at 90°K are much greater than at 0°C. This behavior is well predicted by the model and analysis developed by Mitchell - provided that the mobility-lifetime product, for electrons near the oxide-silicon interface, is chosen to give a best fit between experiment and theory. From the data on zero-biased and positively-biased devices, this product is Tu 8 ±2 x 10 16i 2/V at 90°K. This value is an order of magnitude greater than that reported by Mitchell for a room-temperature thermal oxide. No direct determination of the AT product near the metal-oxide interface can be made, but it is estimated that it is about a factor of ten larger than that indicated above. The data also show a bias dependence for the form of space charge distributions at both interfaces. At the oxidesilicon interface, the bias dependence of the charge distribution can be explained by assuming that some charge trapped very near the interface is neutralized by electrons tunneling from the silicon under the influence of the high field in that area. At the metal-oxide interface, the charge is compacted near the plate for increasingly negative voltages. Preliminary isochronal annealing data from 90°K to 300°K show that most of the damage anneals out at -2000K. The residual damage is less than that present after irradiation of equivalent devices to the same dose [1 Mrad(Si)] at 0°C.