Low hydrogen content stoichiometric silicon nitride films deposited by plasma-enhanced chemical vapor deposition

Abstract

We have deposited silicon nitride films by plasma-enhanced chemical vapor deposition (PECVD) at 250 °C with properties similar to films prepared at 700 °C by low-pressure chemical vapor deposition (LPCVD). Films are prepared using silane and nitrogen source gases with helium dilution. The film properties, including N/Si ratio, hydrogen content and electrical quality are most sensitive to changes in the silane flow rate during deposition. For films deposited under optimized conditions at a substrate temperature of 250 °C, current versus voltage measurements in metal-insulator-semiconductor structures show the onset of carrier injection at 3-4 MV/cm, slightly lower than LPCVD films. When bias-stressed to 2 MV/cm, capacitance versus voltage measurements show some hysteretic behavior and evidence for positive fixed charge, similar to LPCVD films. For the optimized films: N/Si=1.33±.02; refractive index (λ=6328 Å)=1.980±0.01; dielectric constant (1 MHz) ∼7.5; density=2.7±0.1; and the etch rate in 10% buffered HF ranges from 32 to 70 Å/min. In addition, the hydrogen is distributed equally in Si-H and N-H groups, with a total hydrogen content <10 at.%. These films have a significantly lower hydrogen content than observed in other PECVD silicon nitride films deposited at this temperature. When the substrate temperature is increased to 350 °C, the films have the same Si/N ratio, and similar electrical properties; the hydrogen content is reduced to <6×10 21 cm-3, and the etch rate is 17 Å/s in 10% buffered HF solution.