Stress relaxation of free-standing aluminum beams for microelectromechanical systems applications

We investigated the uniaxial tension and stress relaxation properties of micron-scale Al beams for microelectromechanical systems applications in a piezoactuator-driven test apparatus. Pure aluminum and Al-1.5 at.% titanium free-standing beams were fabricated using micromachining procedures. In the tensile tests, we found the yield strength of the Al beams to be approximately 95 MPa. We also observed a significant strengthening effect in the alloyed samples, which had a yield strength approximately 85% higher than the pure Al samples. In stress relaxation tests, we observed a substantial load-drop (about 56% after 10 min) in the Al films, and we propose here that grain boundary sliding is responsible for this relaxation. By comparison, the relaxation of the Ti-alloyed samples was only 15%. We believe that this difference results from the Al3Ti precipitates that form at the Al grain boundaries in the alloy samples. (C) 2000 American Institute of Physics. [S0003-6951(00)00923-2].