Toward crack-free thick silicon nitride through thermal stress compensation

The deposition of thick, crack-free stoichiometric silicon nitride (Si 3 N 4 ) by low-pressure chemical vapour deposition (LPCVD) is fundamentally limited by high tensile residual stress, which restricts film thickness to a few hundred nanometers. Here, we show that substrate selection provides an effective way to overcome this limitation without modifying any other deposition conditions. Under identical LPCVD growth at 820 °C, SiN x films exhibit different mechanical behaviour depending on the substrate, with early cracking on SiO 2 /Si and fused quartz, delayed cracking on Si, and crack-free growth to at least 990 nm on sapphire with near-zero residual stress. Combining experiments, molecular dynamics simulations, and thermoelastic analysis, we find that the intrinsic tensile stress generated during growth is comparable under the LPCVD-relevant simulated condition, whereas thermal mismatch provides the dominant substrate-dependent contribution to the final residual stress state. This thermo-elastic compensation reduces the crack-driving force and leads to a strong, nonlinear increase in the critical thickness at which cracking occurs. These results establish substrate engineering as a simple and scalable strategy to control stress and extend the accessible thickness range of LPCVD SiN x , providing a robust pathway toward mechanically stable thick films for integrated photonics and high-power optical applications.