Influence of material properties of PECVD silicon nitride films prepared at 150°C from highly diluted SiH₄ in N₂

We report on the electrical properties of silicon nitride (SiN_x) films deposited at 150°C from highly diluted SiH₄ in N ₂ by a plasma enhanced chemical vapor deposition (PECVD) method. The films were also prepared below 200°C for comparison. Although the N ₂ dilution gas acted as a source of nitrogen atoms inside the SiN_x film, it was necessary to introduce NH₃ to obtain good dielectric quality in the low-temperature films. An amount of NH ₃ equal to the net SiH₄ in the gas mixture was found to be adequate, and further addition of NH₃ resulted in little improvement. For SiN_x films deposited at 150°C, the NH ₃ addition decreased the C-V hysteresis (ΔV_th from 15 V to 3 V, and increased the resistivity and the breakdown field strength from 10⁹ Ωcm and 4 MV/cm to 10¹³ flcm and 7 MV/cm, respectively. When these films were applied as a gate dielectric layer, the resulting TFT prepared at 150°C showed an on/off current ratio higher than 10⁵, a threshold voltage of 1.1 V, a subthreshold slope of 1.2 V/dec, and a field effect mobility of 0.04 cm²/Vsec. Under a dc bias stress of V_D = V_G = 25 V, the on-current of this TFT was stable over a period of 5000 seconds.