Fracture properties of LPCVD silicon nitride thin films from the load-deflection of long membranes

The bulge test is extended to the determination of the fracture properties of thin films. This was achieved by using long membranes and an accurate model describing their load-deflection response. The model includes the prestress and bending stiffness of the membrane material into the load-deflection response. The feasibility of the method was demonstrated with LPCVD silicon nitride films, which showed a maximum fracture stress between 10.8 and 11.7 GPa. Fracture reference stresses were computed according to the Weibull model for brittle fracture by integrating the membrane stress over the edge, surface, and volume of the samples, corresponding respectively to the assumption of dominant edge, surface, and volume flaws. The integration error from the contributions of the membrane corners and the short membrane sides was taken into account by a correction factor c. Finite element simulations showed that c approximate to 0 is a reasonable value. Independent of c, the assumption of edge failure gave slightly better agreement between experimental data and the Weibull model than the surface and volume flaw assumptions. For edge integration, a Weibull modulus m = 18.3 and mean fracture stress mu = 12.1 GPa were extracted from all sample, using a reference length L-E = 10(-3) m and c = 0. (C) 2002 Elsevier Science B.V. All rights reserved.