Influence of Material Properties of PECVD Silicon Nitride Films Prepared at 150 °C from Highly Diluted SiH_4 in N_2

摘要

We report on the electrical properties of silicon nitride (SiN_x) films deposited at 150°C from highly diluted SiH_4 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 NH_3 to obtain good dielectric quality in the low-temperature films. An amount of NH_3 equal to the net SiH_4 in the gas mixture was found to be adequate, and further addition of NH_3 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 10~9 Ωcm and 4 MV/cm to 10~(13) Ωcm 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 10~5, a threshold voltage of 1.1 V, a subthreshold slope of 1.2 V/dec, and a field effect mobility of 0.04 cm~2/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.