Radiation induced charge in SIMOX buried oxides: Lack of thickness dependence at low applied fields

Commercially prepared Separation-by-IMplantation-of-OXygen (SIMOX) wafers having buried-oxide (BOX) thicknesses ranging from 80 to 400 nm, and thermal oxides of similar thicknesses, were exposed to various doses of 10 keV x-rays. The net positive charge trapped in the buried-oxides during the radiation was determined by dual capacitance-voltage (C-V) and point-contact current voltage (I-V) measurements. For the thermal oxides, using metal-oxide-semiconductor (MOS) CV techniques, the amount of trapped charge was found to have the expected linear dependence on oxide thickness. For the SIMOX buried oxides, however, the amount of net trapped charge was found to be independent of BOX thickness when the oxides were biased at 0.05 MV/cm (a typical operating field). The SIMOX results are explained in terms of bulk-oxide hole and electron trapping.