AVS 50th International Symposium
    Microelectromechanical Systems (MEMS) Tuesday Sessions
       Session MM-TuP

Paper MM-TuP1
Electrostatic Actuation in BioMEMS

Tuesday, November 4, 2003, 5:30 pm, Room Hall A-C

Session: Poster Session
Presenter: T.L. Sounart, Sandia National Laboratories
Authors: T.L. Sounart, Sandia National Laboratories
T.A. Michalske, Sandia National Laboratories
Correspondent: Click to Email
Electrostatic MEMS actuators exhibit fast response times and are easily integrated into microsystems because they can be fabricated with standard silicon IC micromachining processes. Although electrostatic actuators have been used extensively in "dry" MEMS, they have received little attention in microfluidic bioMEMS, despite the added advantage of 80 times the energy density in water relative to that in air. This is probably because electrostatic actuation in most liquid media presents new challenges such as electrolytic gas generation, anodic oxidation, and electrode polarization. Electrolysis is avoided completely at O(1) V electrode potentials, and although such potentials are also too low for actuation in air, they are sufficient to actuate many devices in water. Unfortunately, at equilibrium ionic solutes in conducting fluids screen the electrode potential (electrode polarization) and disable the actuator. We are currently investigating electrostatically-driven biological sensors and other bioMEMS devices by employing ac drive signals to prevent charge screening, which enables electrostatic actuation in many liquids, at potentials low enough to avoid electrochemistry. Here we measure the frequency response of an interdigitated silicon comb drive actuator in liquids spanning a decade of dielectric permittivities and four decades of conductivity, and present a simple theory that predicts the characteristic actuation frequency. The analysis demonstrates the importance of the native oxide on silicon actuator response, and suggests that the actuation frequency can be shifted by controlling the thickness of the oxide. For native silicon devices, actuation is initiated at frequencies less than 10 MHz, in electrolytes of ionic strength up to 100 mmol/L, and thus electrostatic actuation is feasible in many bioMEMS and other microfluidic applications.