ELECTRON AND HOLE TRAPPING IN THE BURIED OXIDE OF UNIBOND WAFERS

Electron and hole traps have been studied in the buried oxide of Unibo nd wafers and are compared with thermal oxide samples of the same thic kness that were covered by polysilicon. The thermal oxide samples with and without a subsequent 1100 degrees C anneal have been examined. Th e electron and hole traps were filled by room temperature and cryogeni c temperature (50 or 60 K) x-ray irradiation. The characteristics of t he trapped carriers were studied by thermal excitation up to 350 K and field-induced tunneling up to 6 MV/cm. X-ray induced current through the oxides during and after irradiation was measured. Hole trapping in the Unibond and annealed thermal oxide is very similar and significan tly higher than the thermal oxide that was not annealed. The hole trap s are concentrated near the Si/SiO2 interfaces. The concentration of h ole traps is less than in SIMOX. The effects of electron traps are als o very similar in Unibond and the annealed oxide. Under negative bias, 1 MV/cm, at cryogenic temperatures and for doses above 1 Mrad(SiO2) t here is a net negative charge near the bottom Si/SiO2 interface. The t rapped electrons are released by warming to room temperature. The comb ination of electron and hole traps present near both Si/SiO2 interface s are also responsible for electron tunneling at the cathode interface during and after irradiation that is observed at all temperatures. Th e negative charge buildup and electron tunneling are not observed in t he unannealed thermal oxide.