BULK OXIDE TRAPS AND BORDER TRAPS IN METAL-OXIDE-SEMICONDUCTOR CAPACITORS

Thermally stimulated current (TSC) and capacitance-voltage measurement s are combined via a newly developed analysis technique to estimate po sitive and negative oxide-trap charge densities for metal-oxide-semico nductor (MOS) capacitors exposed to ionizing radiation or subjected to high-field stress. Significantly greater hole trapping than electron trapping is observed in 3% borosilicate glass (BSG) insulators. Two pr ominent TSC peaks are observed in these BSG films. A ;high-temperature peak near 250 degrees C is attributed to the E'(gamma) defect, which is a trivalent Si center in SiO2 associated with an O vacancy. A lower temperature positive charge center near 100 degrees C in these films is likely to be impurity related. The higher temperature E'(gamma) pea k is also observed in 10, 17, and 98 nm thermal oxides. A much weaker secondary peak is observed near similar to 60 degrees C in some device s, which likely is due to metastably trapped holes in the bulk of the SiO2. Negative charge densities in these thermal oxides are primarily associated with electrons in border traps, which do not contribute to TSC, as opposed to bulk electron traps, which can contribute to TSC. R atios of electron to hole trap densities in the thermal oxides range f rom similar to 30% for radiation exposure to greater than 80% for high -field stress. It is suggested that the large densities of border trap s associated with trapped holes in these devices may be due to high sp ace-charge induced electric fields near the Si/SiO2 interface. In some instances, border traps can reduce near-interfacial electric fields b y local compensation of trapped positive charge. This may provide a na tural explanation for the large densities of border traps often observ ed in irradiated or electrically stressed MOS capacitors. (C) 1998 Ame rican Institute of Physics. [S0021-8979(98)03423-9].