RADIATION EFFECTS AT LOW ELECTRIC-FIELDS IN THERMAL, SIMOX, AND BIPOLAR-BASE OXIDES

We have performed thermally-stimulated-current (TSC) and capacitance-v oltage measurements on 370-1080 nm thermal, SIMOX, and bipolar-base ox ides as functions of bias, dose rate, and temperature during irradiati on. Base oxides built in a development version of Analog Devices' RF25 process show much more interface-trap buildup than XFCB oxides. Both net-oxide-trap and interface-trap charge densities for RF25 capacitors are enhanced significantly during low-dose-rate or high-temperature i rradiation at 0 V over high-rate, 25 degrees C exposures. TSC measurem ents show the increase in net-oxide-trap charge density is due to a de crease in trapped electron density with decreasing dose rate or increa sing irradiation temperature (at least to 125 degrees C), and not by i ncreased trapped hole density. Similar enhancement of net-oxide-trap a nd interface-trap charge density with decreasing dose rate is found fo r soft thermal oxides irradiated at 0 V, but not 5 V. These results st rongly suggest that space charge effects associated with holes metasta bly trapped in the bulk of the oxide can cause the enhanced bipolar ga in degradation seen at low dose rates and/or high temperatures in many technologies. No enhanced radiation-induced charge trapping is observ ed for low-dose-rate or high-temperature, 0 V irradiation of SIMOX cap acitors. Implications for hardness assurance tests are discussed.