Radiation-induced trapping centers in thin silicon dioxide films

Abstract

In this paper the technological and scientific aspects of radiation-related charge trapping in thin SiO2 films are reviewed. These films are amorphous in nature and are thermally grown on single crystal silicon substrates serving as the insulating layer in metal-oxide-semiconductor (MOS) capacitors and transistors. The structure and operation of these devices are reviewed with special emphasis on the effect of charges trapped in the oxide. The technical importance of understanding the interaction of ionizing radiation with thin SiO2 films is illustrated with two practical examples. The first involves the operation of MOS transistors in environments where ionizing radiation is present, leading to an accumulation of positive space charge in the oxide. The second deals with process-induced defects generated by radiation encountered during the fabrication of devices by processes such as electron beam lithography or electron gun metallization. Unannealed traps of this type capture hot electrons producedin the substrate during the operation of the MOS transistor. In both these examples, the charging of the oxide results in instabilities which degrade operation. Its sensitivity to charge trapped in the insulator makes the MOS system an ideal vehicle for scientific study of these phenomena. The basic techniques for characterizing the density, capture cross-sections, and location are briefly discussed and applied to the problem of radiation-induced defects in thin SiO2 films. Ionizing radiation is shown to interact with the SiO2 in two modes. In the first it supplies carriers to fill pre-existing hole traps at the interfaces. In the second it creates electron and hole traps in the bulk of the thin film. These latter defects are in a neutral state after irradiation and are detectable only when either electrons or holes are subsequently injected into the oxide. The capture cross-sections, trap densities and location of these centers in the film are presented. The annealing treatments required to remove these traps from aluminium and polysilicon gate devices are also discussed. The number traps produced by an incident 25 KV electron beam is found to depend weakly on the dosage. A dipolar defect, produced by the ionizing radiation, seems to explain the behavior of the neutral centers. © 1980.