Stress Relaxation of Gold/Polysilicon Layered MEMS Microstructures Subjected to Thermal Loading

We study both experimentally and theoretically the stress relaxation behavior of a series of gold/polysilicon MEMS microstructures fabricated by the MUMPs surface micromachining process and subjected to thermal loading. We measured, using an interferometric microscope with a custom-built temperature chamber, full field deformed shapes (and from these determined the average curvatures in x and y directions) of gold (0.5 μm thick)/polysilicon (1.5 or 3.5 μm thick) beam and plate microstructures. The microstructures are initially thermal cycled between room temperature and 190°C to stabilize the gold microstructure. After the initial thermal cycle, the microstructures are cooled from 190 °C to 120 °C and held at 120°C for about six weeks. During the isothermal hold, stress relaxation is observed in all of the microstructures. Both material and structural issues contributed to the observed deformation response. The coupling of the two was apparent in plate microstructures where the initial cooling caused them to buckle, but the stress relaxation then caused them to substantially unbuckle. We attempted to model the stress relaxation process by assuming simple power-law creep in the gold $ε˙=Aσn$⁠, and assuming that the polysilicon did not relax at the modest temperature of 120 °C. We found that with such a simple model we could not obtain self-consistent results among all measurements. However, with a stress component of n = 5, good agreement was obtained among beam and plate microstructures using a value of A that depended on the microstructure thickness. The reasons for this are unclear presently, but these results resonate with similar studies of thin films on thick substrates in microelectronics contexts.