Low-temperature silicon nitride for thin-film electronics on polyimide foil substrates

We optimized silicon nitride (SiNx) layers, deposited by 13.56 MHz plasma enhanced chemical vapor deposition (PECVD) at 150°C, to provide a high quality gate dielectric layer for the amorphous silicon thin film technology on polyimide foils. The layers were deposited from mixtures of silane, ammonia, and hydrogen. We varied the H2 flow rate from 55 to 220 sccm and the rf power from 5 to 50 W, while the pressure was kept at 500 mTorr and the ratio of ammonia to silane flow at 10:1. The best film was obtained from the gas composition of SiH4:NH3:H2=1:10:44 and the rf power of ∼20 W. This film grows at the rate of 1.5 A/s, has a refractive index n=1.80, a dielectric constant e=7.46, a dielectric breakdown field >3.4 MV/cm, a Si/N ratio of ∼0.67, and a hydrogen content of ∼2×1022 cm−3, and etches in 10:1 buffered HF at a rate of 61 A/s.