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A Technique for Determining the Mechanical Behavior and Electrical Performance of Thin Films


 We describe a technique to determine the mechanical behavior and electrical performance of thin films. Thin films (2 μm) are deflected with a probe, and the displacement of the thin films and total electrical resistance are recorded. Nonlinear finite element models (ANSYS) are used to predict the corresponding force and stress. Three microstructures are built and tested: cantilever (80 μm long and 100 μm wide), bridge (290 μm long and 50 μm wide), and cross (320 μm long and 30 μm wide). No failures are observed at 15 μm deflection for all three structures, and a yield strength at least 1.34 GPa (4–20 times larger than the reported bulk value, but consistent with thin film theory) is inferred. The measured total resistance for every device ranges from open to 0.2 Ω. A direct correlation between the measured resistance and numerically predicted force (or contact pressure since the same probe tip is used in all tests) is noted, validating the numerical predictions. The bridge and cross designs appear feasible as a burn-in test socket, and we predict a mating force of 80–350 N for a 25 mm square chip with 10,000 solder balls on 250 μm spacing. This force will depend on the acceptable range of resistances as measured by our system.