Paper MN-WeM4
Methodology for Electromechanical Characterization of Resonant Micro Structures Actuated by Acoustic and Fringing Electrostatic Fields

Wednesday, November 9, 2016, 9:00 am, Room 102B

In resonant micro and nanoelectromechanical sensors (MEMS/NEMS), the excitation of vibratory motion is usually achieved through implementation of electrostatic, piezoelectric, magnetic, thermal or optical transduction. Most of these methods require additional conductive layers, which broaden the vibrational characteristics of the device and induce residual stresses. In addition, often nonlinear actuation forces may alter the spectral characteristics of the structure. Inertial excitation by an external shaker is widely used for mechanical dynamic characterization. However, simultaneous electrical excitation and detection requires wire bonding and packaging, which is not always suitable at the initial stages of characterization or for wafer level testing. In this work we report on a methodology for efficient electromechanical characterization of resonant micro structures using a combination of acoustic and fringing electrostatic fields. We show that omnidirectional acoustic excitation of unpackaged, electrically connected using micromanipulators, devices is a convenient alternative to inertial excitations.

Using deep reactive ion etching (DRIE), 500 μm long and 16 μm wide cantilevers were fabricated from silicon on insulator (SOI) wafers with 5 μm and 2 μm thick device and buried thermal silicon dioxide layers, respectively. A cavity was etched within the handle of the wafer to allow for high unobscured out-of-plane vibrational amplitudes. An actuating electrode was fabricated from the device layer and was located symmetrically at the two sides of the beam. First, in order to investigate the mechanical response, flexural out-of-plane vibrations of microcantilevers were excited by a miniature acoustic speaker. The transducer frequency band was chosen near the fundamental mode of the cantilevers. Resonant frequencies and quality factors of the devices were measured optically with a laser Doppler vibrometer (LDV). An acoustic field was simultaneously monitored by a microphone located in a proximity of the cantilever. The microphone output, after re-normalization, was subtracted from the LDV output in order to eliminate the influence of the spectral characteristics of the transducer itself. Next the devices were operated simultaneously by the acoustic and electrostatic fields. The distributed electrostatic force acts in the direction opposite to the deflection and serves as an effective elastic foundation. As a result, the stiffness of the cantilever increases with increasing voltage. By applying a steady DC voltage of 100 V we demonstrate that the fundamental frequency of the beam harmonically excited by an acoustic field can be tuned upward by 2.5%.