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Failure of micron scale Single Crystal Silicon bars due to torsion developed by MEMS micro instruments

  • Taher Saif (a1) and N. C. MacDonald (a2)

Abstract

We present an experimental study on a single crystal silicon (SCS) bar subjected to pure torsion using MEMS micro instruments. The bar is in the form of a pillar, anchored at one end to the silicon substrate. It is attached to a lever arm at the other end. The pillar has a minimum cross sectional area at its mid height. The cross section coincides with the (100) plane of SCS. Torsion is generated by applying two equal forces on the lever arm on either side of the pillar. Two micro instruments apply the forces. Each consists of an electrostatic actuator and a component that calibrates it. The actuator generates high force (≈ 200 µN at 50 V) and is capable of developing large displacements (≈ 10 μm). Calibration involves determination of the force generated by the actuator at an applied voltage, as well as the linear and higher order spring constants of its springs. Each microinstrument is thus calibrated independently.

With the application of forces by the two micro instruments, a torque is generated which twists the pillar. The angle of twist at different applied voltages are recorded using an angular scale. The corresponding torques are determined from the calibration parameters of the actuators. Torque is applied until the pillar fractures. Two such sample pillars, samples 1 and 2, are tested. There cross sectional areas are 1 and 2.25 µm2. We find that both the pillars behave linearly until failure. The stresses prior to fracture are evaluated based on anisotropic theory of elasticity. Samples 1 and 2 fail at shear stresses of 5.6 and 2.6 GPa respectively. The fracture surfaces seem to coincide with the (111) plane of SCS.

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References

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[1] Tang, W. C., Nguyen, T. H., and Howe, R. T., Laterally driven polysilicon resonant microstructures, Sensors and Actuators, Vol. 20, pp 2532, 1889.
[2] Saif, M. T. A., and MacDonald, N. C., “Measurement of forces and spring constants of micro instruments”, Review of Scientific Instruments, vol. 69, no. 3, pp 14101422, March 1998.
[3] Zhang, Z. L. and MacDonald, N. C., Fabrication of Submicron High-Aspect-Ratio GaAs Actuators, J - MEMS, Vol. 2, No. 2, pp. 6672 (1993).
[4] Shaw, K. A., Zhang, Z. L. and MacDonald, N. C., SCREAM I: A Single Mask, Single-Crystal Silicon, Reactive Etching Process for MicroElectroMechanical Structures, Sensors and Actuators A, Vol. 40, pp. 6370 (1994).
[5] Lekhnitskii, S. G., Theory of Elasticity of an Anisotropic Body, Mir Publishers, Moscow, 1981.
[6] Saif, M. T. A. and MacDonald, N. C., Micro Mechanical Single Crystal Silicon Fracture Studies - Torsion and Bending, Proceedings of the Ninth Annual International Workshop on Micro Electro Mechanical Systems (MEMS 96), San Diego, California, Feb 11-15, 1996, pp 105109.

Failure of micron scale Single Crystal Silicon bars due to torsion developed by MEMS micro instruments

  • Taher Saif (a1) and N. C. MacDonald (a2)

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