Defect dynamics and wear-out in thin silicon oxides

Abstract

The authors describe the results of a study of discrete, low-frequency fluctuations in the current through very thin insulator ( approximately 20 AA), small-area (1-2500 mu m²) metal-oxide-semiconductor tunnel diodes. Findings suggest that the discrete nature of the fluctuations arises from the activated transitions of oxide defects, either localised or extended, between a number of accessible metastable configurations, each of which has associated with it a different net trapped charge. Inelastic interactions between the fluctuating defects and the diode tunnel current can result in random energy input to the defects, which can effectively heat them above the bulk oxide temperature. At sufficiently high device biases, these interactions, coupled with interactions between the defects themselves, are strong enough that they can result in the creation of additional oxide defects, and thus lead to the gradual wear-out or deterioration of the oxide. A study of the complex fluctuations which arise during thin oxide wear-out and their effects on diode characteristics reveals that after an initial period of Si-SiO₂ interface defect creation, the oxide wear-out proceeds by the diffusion of defects into the oxide. These moving defects appear to be closely associated with hole traps in the oxide. A number of the characteristics of very thin oxide wear-out can also be seen or inferred in measurements on thicker oxides, suggesting that the studies of very thin oxide diodes are providing a particularly detailed picture of the early stages of trap state formation, wear-out and breakdown in tunnel oxides in general.