Measuring the Mechanical Properties of Thin Metal Films by Means of Bulge Testing of Micromachined Windows

Abstract

Free-standing films of gold and aluminum have been fabricated using standard micro-machining techniques. LPCVD silicon nitride films are deposited onto (100) silicon wafers. Square and rectangular silicon nitride membranes are made by anisotropic etching of the silicon substrates. Then, metal films are deposited onto the silicon nitride membranes by means of evaporation. Finally, the sacrificial silicon nitride film is etched away by means of reactive plasma etching, resulting in well-defined, square and rectangular metal membranes.

Bulge testing of square windows allows one to determine the biaxial modulus of the film as well as the residual stress in it. Testing rectangular windows yields the plane-strain elastic modulus and the residual stress. Since deformation in rectangular membranes approaches plane-strain deformation, this geometry is ideal for studying the plastic properties of the metal films. Stress-strain curves can be readily determined from the load-deflection curves of rectangular membranes. The gold films have a biaxial modulus of 161±3 GPa and a plane-strain modulus of 105±5 GPa, slightly lower than the literature values for a (111) textured film. The yield stress of these films is approximately 231±17 MPa at 10⁻⁴% plastic strain. The elastic moduli of the aluminum films are 105±3 GPa and 76.4±0.7 GPa, respectively; the yield stress of these films is 187±30MPa.