Radiation-Insensitive Silicon Oxynitride Films for Use in Silicon Devices

Silicon oxynitride (SiON) films (in MIS structures) with compositions near Si2ON2 are nearly insensitive to space-charge built-up for radiation doses up to ~10 Mrads. Bias during irradiation does not lead to instability unless certain threshold field values are exceeded. These values are 2.8 × 105 V/cm for negative bias and 1.5 × 106 V/cm for positive. Above the threshold for negative bias there is negative charge injection by the metal contact. The shift of the C-V curve, ΔVfb' is linearly related to the applied bias, VA: ΔVfb= VTH - VA (VTH is the threshold voltage). The composition range of interest is characterized by refractive indices from 1.74 to 1.82. The films are good barriers to sodium drift at 160°C, but are permeable to hydrogen gas and allow the usual annealing of interface states. Films deposited on bare Si surfaces show large negative Vfb. To obtain low surface charge densities, a thin SiO2 layer (40 to 180 Å) can be interposed between the SiON and Si or the Si surface can be pretreated chemically (~15 Å SiO2). Neither treatment affects radiation hardness. In preliminary bias-temperature tests (4 hours, 300°C, ±1.5 × 106V/cm) SiON films showed non-ionic -type drifts. For either bias polarity, films with interlayers showed drifts < 2V; those with pretreated Si surfaces showed drifts ~3V for negative bias and no drifts for positive. SiON films on bare Si surfaces are less stable (drifts ~5 to 10V) due to injection from the Si electrode.