Bulk oxide traps and border traps in metal–oxide–semiconductor capacitors

Abstract

Thermally stimulated current (TSC) and capacitance–voltage measurements are combined via a newly developed analysis technique to estimate positive and negative oxide-trap charge densities for metal–oxide–semiconductor (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 prominent TSC peaks are observed in these BSG films. A high-temperature peak near 250 °C is attributed to the Eγ′ defect, which is a trivalent Si center in SiO2 associated with an O vacancy. A lower temperature positive charge center near 100 °C in these films is likely to be impurity related. The higher temperature Eγ′ peak is also observed in 10, 17, and 98 nm thermal oxides. A much weaker secondary peak is observed near ∼60 °C in some devices, 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. Ratios of electron to hole trap densities in the thermal oxides range from ∼30% for radiation exposure to greater than 80% for high-field stress. It is suggested that the large densities of border traps associated with trapped holes in these devices may be due to high space-charge induced electric fields near the Si/SiO2 interface. In some instances, border traps can reduce near-interfacial electric fields by local compensation of trapped positive charge. This may provide a natural explanation for the large densities of border traps often observed in irradiated or electrically stressed MOS capacitors.