Low temperature studies of SIMOX charge trapping

The charge trapping of the buried layer is studied by irradiating SIMOX (separation by implanted oxygen) capacitors at cryogenic temperatures and comparing the result to thermal oxide control samples. The thermal and SIMOX oxide thicknesses are 4000 Å and 3700 Å, respectively. The buildup of positive charge during an X-ray irradiation at 60 K with the gates grounded is shown. Data are shown for doses ranging from 7 to 870 krad(SiO₂). Over this range, both oxides have similar net positive charge buildup. In addition, the results of isochronal annealing are shown for a thermal oxide capacitor and two SIMOX capacitors from the same wafer. The effects of field stressing after irradiation but while still at 60 K are also shown. Above 30 V (about 0.8 MV/cm), there is a large increase in the net positive charge that continues to increase to at least 90 V