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CHARACTERIZATION OF STRAIN RATE DEPENDENCE IN THE MECHANICAL BEHAVIOR OF GOLD THIN FILMS

The effect of strain rate on the mechanical behavior of gold thin-films for RF-MEMS applications was examined. Microscale, free-standing, dog bone shaped specimens were subjected to uniaxial tension tests at strain rates ranging between 10 s and 10 s via a custom built testing apparatus. The specimens, fabricated using ebeam evaporation, had gage sections 1000 μm long and 200 μm wide, with thickness varying between 500 and 650 nm. The specimens were fabricated as 200 nm multi-layers with 100 nm average grain size. Force and displacement data were collected and used to construct stress-strain curves. The specimens exhibited elastic-nearly perfectly plastic behavior at all strain rates excluding the slowest (10 s) where local stress peaks were observed. Material yield strength and peak stress, ranging from 120-300 MPa and 140-365 MPa respectively, both increased as the strain rate increased. For high strain rates the ductility was measured to be about 1.9%, increasing to 4.5% for the lowest strain rate. These values were found to be consistent with those published in literature. In general, the material exhibited qualitative trends as expected for the variation in strain rate except, as mentioned before, at the slowest strain rates. INTRODUCTION Microelectromechanical Systems (MEMS) have the potential to revolutionize the space industry. Some of the space specific advantages of MEMS include the drastic reduction in mass, volume, and power requirements. This will directly impact spacecraft, test facilities, and most important, launch costs. However, the utilization of MEMS in space applications has been limited due to concerns about their reliability and survivability in the harsh space environment. Only thorough testing of MEMS devices and materials will ever abate these concerns. Gold has been identified as an ideal candidate material for radio frequency (RF) MEMS due to its chemical inertness, low processing temperature, and outstanding electrical conductivity. A literature survey showed that testing of gold thin films is a recent development and much about the mechanical behavior at the microscale is still uncertain. Gudlavalletti et al. studied the stress-strain response of gold thin films as early as 2002, reporting values for ductility, yield and maximum stress. Espinosa and Prorok have done studies on gold thin films since 2003 investigating sizeeffects on mechanical behavior using a Membrane Deflection Experiment (MDE). More recently in 2003, Li and Cima conducted bulge tests on gold thin films reporting values for bulk modulus and yield strength, the later of which were slightly higher than those values measured by Espinosa and Prorok. This research project was designed to investigate the effects of strain rate on the mechanical behavior of gold thin films focusing specifically on the changes in ductility, yield stress, and strength as a function of strain rate. MATERIALS Specimen Fabrication The specimens used for this research project were freestanding evaporated gold thin films fabricated in Univ. of Virginia’s Microfabrication Laboratory using modern photolithographic techniques. First, a silicon wafer was covered with AZ4110 photoresist and spun up to 7000 RPM for 30 seconds. This resulted in a layer of photoresist 0.7 um thick. The wafer was then allowed to sit for a few minutes before it was softbaked