Characterization of mechanical properties of MEMS materials

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(1) We developed a new type of tensile testing method to characterize thin films utilized as structural materials for MEMS.In the proposed "on-chip tensile testing method", test is carried out on a silicon chip containing a loading mechanism and a tensile specimen on a same chip. The developed system allows both quasi-static and dynamic fatigue testing.(2) We designed and fabricated test chips for different film materials such as (a) single crystal silicon in three different orientations, (b) SiO2, (c) Si3N4, (d) poly-crystalline silicon. We obtained fracture stress and strain, and Young's modulus of above-mentioned materials.(3) Single crystal silicon showed 5% elastic strain until fracture. This is much higher than values obtained from bulk silicon by bending test. We also observed a size-effect of the specimen on fracture strain. Smaller specimen showed larger fracture strain though both are tested by on-chip method.(4) Cyclic loading influenced on the fracture of single crystal silicon. Number of cycles when silicon fractured decreased according to the increase in applied peak value of strain. The fracture mechanism is under discussion.(5) Crack growth of the silicon specimen was observed in-situ by using a carbon-nano-tube attached on an AFM probe tip. From a change in crack profile, it was suggested plastic deformation might be introduced to the specimen. This is against the common knowledge that silicon does not show any plasticity at room temperature. Further investigation is needed.(6) We succeeded in tensile test for Si3N4 film 0.1 micron thick. We further designed a test system allowing a tensile test for films 0.01 micron thick. We expect a surface effect appear on the film property by decreasing the thickness.