Electron and hole trapping in the buried oxide of Unibond wafers

Electron and hole traps have been studied in the buried oxide of Unibond wafers and are compared with thermal oxide samples of the same thickness that were covered by polysilicon. The thermal oxide samples with and without a subsequent 1100°C anneal have been examined. The electron and hole traps were filled by room temperature and cryogenic temperature (50 or 60 K) X-ray irradiation. The characteristics of the 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 significantly higher than the thermal oxide that was not annealed. The hole-traps are concentrated near the Si/SiO₂ interfaces. The concentration of hole traps is less than in SIMOX. The effects of electron traps are also very similar in Unibond and the annealed oxide. Under negative bias, 1 MV/cm, at cryogenic temperatures and for doses above 1 Mrad(SiO ₂) there is a net negative charge near the bottom Si/SiO₂ interface. The trapped electrons are released by warming to room temperature. The combination of electron and hole traps present near both Si/SiO₂ interfaces are also responsible for electron tunneling at the cathode interface during and after irradiation that is observed at all temperatures. The negative charge buildup and electron tunneling are not observed in the unannealed thermal oxide