Dresden 2011 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 43: Mechanical Properties II

MM 43.2: Talk

Thursday, March 17, 2011, 18:15–18:30, IFW D

The fracture toughness of silicon nitride thin films of different thicknesses as measured by bulge tests — •Benoit Merle and Mathias Göken — Department of Materials Science and Engineering, Institute I, University Erlangen-Nürnberg, Germany

A bulge test setup was used to determine the fracture toughness of amorphous low pressure chemical vapor deposited (LPCVD) silicon nitride films with various thicknesses in the range of 40 to 108 nm. The method used for this measurement relied on a special sample preparation with a Focused Ion Beam (FIB), in which a crack-like slit of a defined length was introduced in the center of a rectangular freestanding membrane. The membrane was then deformed in the bulge test until failure occurred, and the fracture toughness KIC of the film was calculated from the pre-crack length and the stress at failure. It was shown that the membranes were in a transition state between pure plane-stress and plane-strain, which however had a negligible influence on the measurement of the fracture toughness, because of the high brittleness of silicon nitride and its low Young's modulus over yield strength ratio. The fracture toughness KIC was measured to be constant at 6.3 +/- 0.4 MPa m1/2 over the whole studied thickness range, which compares well with bulk values. This means that the fracture toughness, just as the Young's modulus, is a size independent quantity for LPCVD silicon nitride. This presumably holds true for all amorphous brittle ceramic materials.