Fracture properties of thermal silicon oxide thin films from the load-deflection of long SiNX/SiO2 membranes

The bulge test was successfully extended to the determination of the fracture properties of silicon oxide/silicon nitride thin film sandwiches. This was achieved by using long membranes and applying a comprehensive mechanical model describing the mechanical response of these membranes deflected into a plane-strain deflection profile under a differential pressure. The model is able to cope with thin films with arbitrary z-dependent prestress and plane-strain modulus, where z denotes the coordinate perpendicular to the membrane. It consistently takes into account the bending rigidity and the stretching stiffness of the layered material and enables accurate calculations of the load-deflection response and of the stress distribution throughout the composite membrane as a function of the load, and in particular at the critical pressure leading to the fracture of the membranes. The method was applied to six different types of membranes made of of 300-nm-thick LPCVD silicon nitride and 450-nm-thick to 980-nm-thick thermal silicon oxide films produced at 950 degrees C and at 1000 degrees C. Extracted prestress levels, plane-strain moduli, and fracture stresses are 0.97 +/- 0.05 GPa, 301.7 +/- 36.1 GPa, and 9.7 +/- 0.3 GPa, respectively, for the LPCVD silicon nitride. For the silicon oxide -236.5 +/- 49.0 MPa, 69.8 +/- 46.1 GPa, and 0.8 GPa to 1.48 GPa were found for the prestress, plane-strain modulus, and fracture stress, respectively.