A novel capacitance bulge apparatus and measurements of the mechanical properties of Al thin films using it

Thin metal films often play an important role as structural elements or reflective surfaces in MEMS applications. Mechanical properties of the films are important due to their influence on the performance of MEMS devices that involve bending or stretching metal parts. In order to gain a better understanding of the mechanical behavior of thin metal films, we have developed a novel bulge system and measured mechanical properties of aluminum thin films. The thin films were prepared by e-beam evaporation of high purity Al onto 2 or 3mm x 12 mm rectangular silicon nitride membrane windows in silicon frames. N-2 gas was used to pressurize and thus bulge the membranes. The bulge height was measured based on changes of capacitance between the membrane and a fixed, closely spaced electrode. This apparatus provides resolution of approximately 50 nm in bulge height at a data acquisition rate of 100/sec and provides strain rates in the membrane up to 10(-5)/sec. The stability of the apparatus allows stress relaxation measurements to be made to times of many hours. Time dependent elastic modulus changes of 1 mu m Al films were measured over periods of times under constant stress.