Reliability Physics of Contact-Type MEMS Switches
Professor Nick McGruer
Department of Electrical and Computer Engineering
Electrostatically actuated microswitches and relays have been developed at Northeastern University and Analog Devices.1,2,3 These devices are approximately 100 x 100 microns in size. Packaged, eight-contact versions of the device have been tested up to 1010 cycles with a current of 10 mA and have shown a switch resistance variation of less than 0.2 Ohm over the test. Finite element analyses using ANSYS were performed to study the temperature distribution in the microswitch. The geometry of the model is shown in Figure 2. The modeling results show that in a microswitch with a typical geometry, the highest temperature is located away from the contact, in the thin film trace, and that the hottest spot moves further away from the contact as the contact radius increases. Experimental results show that in gold-gold contacts, lower contact resistances correspond to higher contact adhesion, and that for very low resistance contacts, the resistance decreases and the adhesion increases as the switch is cycled. In refractory metal contacts, contact adhesion is typically less, and the contact resistance can be very stable with appropriate packaging, but the contact resistance for these long-life contacts is higher than predicted by clean metal contact theory. Both hot switching and operation at higher currents degrade the contact lifetime, even when clean-metal contact theory does not predict significant heating at the contact. Failure tests show that for Au-Au contacts, the contact trace fails at a switch current of 0.35 A per contact and a voltage of 0.45 V and that the damaged region is 3-5 mm away from the center of the contact, in reasonable agreement with models of switches with enlarged contacts.
Professor McGruer is conducting research in the areas of microelectromechanical systems (MEMS) and nanotechnology. At Northeastern he has conducted a variety of microfabrication-related research projects including plasma-source ion implantation, fabrication of 0.1 to 2 micron-scale gated field emission devices, fabrication and characterization of microrelays, microspectrometers, micromirrors and other MEM sensors, and fabrication of 3-D microelectronic circuits. To carry out this work, Professor McGruer directs the Microfabrication Laboratory and the Scanning Electron Microscopy Facility. Before coming to Northeastern, Dr. McGruer was responsible for ion implant/diffusion process development for 0.8 and 1.25 micron CMOS technologies and investigated rapid thermal oxidation of silicon for MOS gate dielectrics at Sperry Semiconductor Operations in Egan Minnesota.