Space Charge Analysis in Thin SiO2 Films: Local Vs. Uniform Degradation Models

Abstract

ABSTRACTThe behavior of small MOS structures (area 3 μm2), subjected to stresses caused by electron injection, demonstrates an obvious non-parallel shift in the tunneling characteristics (NPS effect), measured in the wide current range (3·10-16 -3·10-8 A) from either tunneling interface. This paper describes and compares two different approaches for the NPS effect interpretation: uniform and local degradation models (UDM and LDM). The UDM implies a modification of the oxide tunneling barrier due to area-uniform electron trapping inside the tunneling distance which could lead to the NPS effect [14]. It is shown that any spatial distribution of negative oxide charge is inapplicable to interpret the NPS effect within the framework of the UDM. The second approach uses the assumption of small (area ∼ 10-3 μm2) structure-stable regions (SSR) formed in noncrystalline SiO2 films during thermal growth. Due to the high resistance of SSRs to the degradation - negative charge generation - electron injection is gradually localized in the SSRs, avoiding a screening effect of electron trapping. It is shown that the influence of non-uniformity in the electron injection appears to be more significant, when degraded MOS structure is tested at low voltages (NPS effect).