Hydrogen-free silicon oxynitride layers with a broad refractive index range produced at room temperatures through magnetron sputtering

Silicon oxynitride is a versatile material used in many nanofabrication processes, including the fabrication of waveguiding devices, solar panels, and semiconductors. Existing silicon oxynitride sputtering methods require high deposition temperatures or the use of hydrogen-containing precursors, creating issues. High temperature processes damage substrates; hydrogen-containing precursors increase the extinction coefficient of the deposited layer and negatively impact optical performance. Presented here is an optimized process for the ambient-temperature deposition of hydrogen-free silicon oxynitride and silicon nitride films using magnetron sputtering. A Box–Behnken experiment design was used to investigate the effect of the relative gas flow rates of argon, nitrogen, and oxygen (4:3:0 to 4:3:5). Target sputter power was varied from 2 to 6 kW. Deposition rate, refractive index, and extinction coefficient at 632 nm were measured as outputs and via response surface mapping a process was optimized for a low-loss tuneable refractive index SiON process. Target power, oxygen:nitrogen, and oxygen:argon flow ratio were all observed to impart a nonlinear effect on refractive index, resulting in an extrapolated refractive index range from 1.49 to 2.03 at 632 nm, exceeding the typical refractive index range of 1.5–2.0 achievable using plasma enhanced chemical vapor deposition. The process window suggested by the design of experiments yields silicon oxynitride and silicon nitride layers with near-zero extinction coefficients and sputter deposition rates from 0.11 to 0.31 nm/s. This enables the fabrication of SiON films with near-zero extinction coefficients and tuneable refractive indices beyond the standard, while ensuring strong adhesion at ambient temperatures. Validation runs were performed to confirm the validity of the process space.