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We studied mechanical properties and alloy effects of free-standing aluminum microbeams (50 × 500 × 2 μm) in a piezo-actuator-driven test apparatus with a load resolution of ±0.2 mN and a displacement resolution of ±10 nm. Pure Al and Al-2%Ti microbeams were fabricated using micromachining techniques. In tensile tests, we found the yield strength to be approximately 120 MPa for the pure Al beams, and approximately 75% higher for the alloyed beams. We examined the results with respect to those of bulk materials and thin films adhered to substrates. In stress relaxation tests, we observed a load drop of 56% over 10 min for the pure Al beams. We attributed this to grain boundary sliding and the nature of a free-standing thin film, i.e., the absence of a substrate. For the alloyed beams, the load drop was only 16%. We believed the difference was due to Al3Ti precipitates formed at grain boundaries, which hindered dislocation movements. We used TEM to reveal the microstructural features of the microbeams.
micro electro mechanical systems (MEMS), micromachining, thin film, substrate, piezo-acruator, yield strength, alloy, stress relaxation, grain boundary
Ph.D. candidate, Stanford University, Stanford, CA
Member of technical staff, Lucent Technologies, Inc., Reading, PA
Professor, Stanford University, Stanford, CA