Investigating composition-dependent fracture toughness in silicon-rich silicon nitride layers

This study systematically investigates the mechanical properties, particularly fracture toughness, of plasma enhanced chemical vapor deposited SiNx films by exploring variations in their chemical composition. Three films with sub-micrometer thickness (600 nm) and different stoichiometry, i.e., increasing silicon content, were deposited and characterized by X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopy, and transmission electron microscopy. Mechanical properties were determined using nanoindentation, applying the Oliver–Pharr method for hardness and elastic modulus and a procedure suitable for thin films, developed by Xia et.al., for fracture toughness measurement. Results revealed an enhancement in fracture toughness for the highest silicon-content film, while the intermediate sample showed no significant variations, suggesting the existence of a silicon-content threshold above which some toughening effect is initiated. The observed toughening mechanism is hypothesized to be related to the substantial dispersion of amorphous silicon heterogeneities, affecting intrinsic stress evolution and mechanical compliance of the films. These findings highlight the critical role of chemical composition and microstructural features in controlling mechanical performance and provide valuable guidelines for the customization of SiNx passivation layers in the semiconductor industry.