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, ????Vfb' is linearly related to the applied bias, VA: ????Vfb= 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 ????) can be interposed between the SiON and Si or the Si surface can be pretreated chemically (~15 ???? 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.