Mechanics of a curved electrode actuator operating in viscous dielectric media: simulation and experiment

Abstract

Curved electrode electrostatic actuators have been shown to have the potential to manipulate microparticles at high speeds in viscous dielectric media. However, there are no multiphysics models that fully describe the mechanics of these actuators in viscous dielectric media. This work is the first step in the direction of describing the actuator mechanics through Finite Element Method (FEM) simulations and validate it with the experiments. Simulations are performed using the commercial software Coventor MEMS+^® and the results are found to be in good agreement with experiments. Two observations are reported in this study. One, for a given media and low actuator displacements, the cutoff frequency of the actuator is independent of the actuation voltage and/or actuator displacement. Two, an actuator immersed in a high viscous media will have higher squeeze film damping and a lower cutoff frequency. Through a simple lumped-parameter model of the actuator, we report that the viscous damping effect by the substrate on the actuator submerged in methanol or water media becomes significant for actuation frequencies >100 Hz.

Appendix

Actuator Fabrication Recipe On the SOG wafer, a composite metal film comprising of 30 nm Cr and 200 nm Au is deposited on the silicon side by e-beam evaporation and patterned by photolithography to make electrical contact pads (a, b). Next, an etch mask to be used for bulk micromachining of actuator components is defined by depositing a 1000 nm thick SiO₂ layer using a plasma enhanced chemical vapor deposition (PECVD) process (processed at 250 °C using SiH₄/N₂O gases), patterned using a photoresist mask, and then etched using inductively coupled plasma assisted reactive ion etching process (RIE-ICP; using CHF₃/Ar gases) (c, d). The high-aspect ratio actuator features (curved electrodes, beam electrode, reinforcing beam) are machined by DRIE using a standard Bosch process in 5s:2s step cycles; the glass substrate acts as an etch stop (e). After DRIE, the movable structures of the actuator are released by timed HF (49% HF:H₂O::1:1 volume ratio) wet etch process for approximately 2 minutes at standard room temperature, triple rinsed in water, then in methanol, and then dried at room temperature (f). Next, the actuator is coated with 10 nm thick Al₂O₃ insulator film using atomic layer deposition (ALD) process (processed at 120 °C using TMA/O₂ precursors) (g). Finally, the 10 nm thick Al₂O₃ insulator film is anisotropically etched using inductively coupled plasma assisted reactive ion etching process (RIE-ICP; using BCl₃/Cl₂ gases) to unmask the Au/Cr electrical contact pads that were coated with Al₂O₃ during the ALD process (h) (Figs. 9 and 10).

Fig. 9

Fabrication scheme of the actuator. [Figure adapted from Ref. [8] with permission from Springer Nature Microsyst. Technol. 24, 3479 (2018). Copyright 2018 Springer-Verlag GmbH Germany, part of Springer Nature]

Fig. 10

A convergence plot for the nonzipped operating mode of the actuator for $\bar V$= 4 V in deionized water media. The convergence test shows that convergence begin to occur for Bernoulli beam elements ≥ 120; hence 120 beam elements are chosen for constructing the 3-dimensional FEM model of the actuator