Bulge Testing of Silicon Nitride Thin Films at the Wafer Level

Inhalt:
This paper reports on the mechanical characterization of thin film long membranes made of silicon nitride (SiNx), grown by low pressure chemical vapor deposition (LPCVD), supported by silicon (Si) frames using the bulge test. A uniform pressure is applied to the diaphragms and the resulting deflection is measured as a function of the membrane dimensions and thickness. A mechanical model is used to analyze the load-deflection response, from which a plane strain modulus Eps of 279 GPa and prestress σ 0 of 1254 MPa are extracted for LPCVD SiNx. The pressure is increased until fracture occurs and the brittle material strength data are analyzed using pooled Weibull distributions from the stress profiles in the membrane at the fracture load. A Weibull modulus m of 40.1 and scale parameter σ 0,W of 10.3 GPa·(µm)3/m are obtained for 94-nm-thick samples, assuming that the fracturecausing flaws are uniformly distributed throughout the specimen volume. The possibility of the material strength being dictated by line, surface or volume flaws is investigated and size effects are discussed.