Elevated Microresonators for Investigating the Effect of Air Damping

It is known that the dissipative damping force due to the air film trapped between the bottom of surface micromachined resonators and the substrate on which they are fabricated decreases in magnitude as the separation between the two increases. In order to further investigate this effect and compare experimental findings with theory, a new test device that enables modulation of the damping interaction between a surface micromachined resonator and the substrate has been fabricated. The device, fabricated by the PolyMUMPs process, consists of a surface micromachined polysilicon microresonator, which is elevated out of the plane of the substrate with the assistance of a bimorph beam. The resonator-to-substrate separation of the elevated resonator can be varied by changing the temperature of the bimorph beam. Q factors for different separations have been measured. Experimental results show that the elevated microresonators have Q values which are 65% higher than similar microresonators fabricated close to the substrates. These experimental findings show good agreement with the theoretical model of damping used.