Abstract
The authors describe their design for a paddle-like cantilever beam sample to relieve non-uniform stress distribution in beam-bending tests of the mechanical properties of thin film applications to MEMS. We added the sample to a custom-designed system equipped with an electrostatic panel and optical interferometer. The system overcomes problems associated with using nano-indentation for testing, and reduces errors tied to the amount of contact force required to bend the beam. Accurate paddle cantilever beam deflection was obtained using a four-step phase-shifting process with a Michelson interferometer. Film strain was determined using a simple force equilibrium equation. Residual stresses were measured at −41.3 MPa for 150 nm silver film, −3.2 MPa for 150 nm gold film, and −16.8 MPa for 150 nm copper film. We observed residual stresses for copper films at different thicknesses. The results indicate high tensile stress forms during the early deposition stage for thin copper film due to grain coalescence, and a decrease in stress with an increase in film thickness. In copper films with thicknesses greater than 153 nm, lattice relaxation associated with the surface mobility of metallic atoms changed residual stress from tension to compression.
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Acknowledgments
The authors wish to thank the Taiwan National Center for High-Performance Computing for access to their facilities, and employees of the Center for Measurement Standards, Taiwan Industrial Technology Research Institute, for their technical assistance. We are also grateful to Prof. Walter Brown of Lehigh University for his valuable input. This work was supported by the Taiwan National Science Council (NSC97-2221-E-005-012). Partial support was provided by the Republic of China Ministry of Education.
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Tong, CJ., Cheng, YC., Lin, MT. et al. Optical micro-paddle beam deflection measurement for electrostatic mechanical testing of nano-scale thin film application to MEMS. Microsyst Technol 16, 1131–1137 (2010). https://doi.org/10.1007/s00542-009-0999-7
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DOI: https://doi.org/10.1007/s00542-009-0999-7