Estimating the Surface Adhesion Force Using Pull-in/-out Hysteresis in Comb-Drive Devices

Surface adhesion forces play a critical role in the operation of NEM relays, but characterization data for micro/nanoscale contact areas of relevant materials are scarce. A novel technique to estimate the adhesion force between two contacting surfaces is presented. This method uses the hysteresis in the pull-in/-out voltages measured experimentally in electrostatically actuated comb-drive and circular NEM devices with two contacting electrodes. In these geometries the electrostatic actuation force is independent of the device displacement and the surface adhesion forces can be analytically calculated from the force balance describing the pull-out event. Finite-element ANSYS simulations of the electromechanical behavior of the devices support the analytical approach. Adhesion forces of 0.081 µN and 0.167 µN were calculated for Au-Au and Ti-Ti contacts with areas of 750 nm × 300 nm and 100 nm × 300 nm, respectively.