Mechanical cross-characterization of sputtered inconel thin films for MEMS applications

The characterization of Young's modulus, Poisson's ratio, linear coefficient of thermal expansion and residual stress of thin films is a critical issue for the design and development of MEMS. As inconel thin films have a relatively large thermal expansion coefficient together with a high electrical resistivity, they are good candidates for thermal actuators in MEMS. This work focuses on the determination of the mechanical properties of inconel thin films ranging from 0.4 up to 1.2 μm sputtered on a (100) silicon substrate. The atomic composition (6% Fe, 80% Ni, 14% Cr) of these films, close to that of inconel 600 alloys, has been determined by XRF analysis. A wafer curvature measurement technique is used to determine the residual stress and the linear thermal expansion coefficient. We measure residual stress between 152 and 342 MPa. The value of the linear thermal expansion coefficient is discussed but is found to be around 10 ppm/K−1. We perform bulge testing on micro machined square membranes to determine together the Young's modulus and residual stress of 0.4 μm films. Values of 166 GPa and 296 MPa are obtained, respectively. These results obtained by bulge testing are eventually compared with measurements made by nanoindentation. Good correspondence is found between the different methods.