Usage induced changes to surface topography and material properties in polysilicon MEMS electrothermal structures

This paper presents the results of an experimental study of electrothermal poly-Si MEMS structures wherein changes to the surface topography and material properties are observed due to use. The ex-situ AFM characterization reveals changes in the surface topography after cyclic actuation. The extent of topical SiO2 appears to increase with the number of actuation cycles and increasing stress levels on the polysilicon surfaces. The differences in the surface topography and oxide thickness are characterized as a function of fatigue cycling and in-situ annealing of the electrothermal actuators. FEA analyses were performed to evaluate the magnitude and distribution of stresses in the actuators to compare stress effects from oxide development on electrothermomechanical structures. With the observation of topographical changes, the intrinsic material property like resistivity was also affected. A change of 1.4% was seen for a 20% duty cycle, 3.1% for 50% duty cycle and 4.1% for 80% duty cycle. Similar experiments were performed for sealed devices in order to observe the changes in resistivity under inert conditions. A comparison of change in resistivity for sealed devices and nonsealed devices was done. Finally, force-distance curves were plotted to ascertain the adhesion forces for the actuator surfaces before and after actuation. The adhesion forces increases from ∼7nN (un-actuated chevron) to ∼40nN (10,000 cycles).