David Schreiber

The pull-in behavior of electrostatically actuated bistable microstructures

The results of theoretical and experimental investigation of an initially curved clamped–clamped microbeam actuated by a distributed electrostatic force are presented. Reduced-order Galerkin and consistently constructed lumped models of the shallow Euler–Bernoulli arch were built and verified by numerical analysis, and the influence of various parameters on the stability was investigated. Due to the unique combination of generic mechanical and electrostatic nonlinearities, the voltage–deflection characteristic of the device may have two maxima implying the existence of sequential snap-through buckling and pull-in instability and of bistability of the beam. The first critical voltage can be higher or lower than the second one, while the stable deflections are significantly larger than in a straight beam. The minimal initial elevation required for the appearance of the snap-through in the electrostatically actuated beam is smaller than in the case of uniform deflection-independent loading; a closed-form approximation of this elevation was evaluated. The devices were fabricated from silicon on insulator (SOI) wafer using deep reactive ion etching and in-plane responses were characterized by means of optical and scanning electron microscopy. Model results obtained for the actual dimensions of the device were in good agreement with the experimental data.

Pull-in behavior and multistability of a curved microbeam actuated by a distributed electrostatic force

In this work we report on theoretical and experimental investigation of a multistability phenomenon in initially curved flexible clamped-clamped microbeams actuated in-plane by a distributed electrostatic force and fabricated from silicon on insulator (SOI) wafer using a deep reactive ion etching (DRIE) process. Theoretical results provided by a reduced order (RO) model of the shallow Euler-Bernoulli arch and backed by experiments indicate that in the device with non-monotonous stiffness-deflection dependence the voltage-deflection characteristic may have two maxima implying the existence of multiple stable configurations at the same voltage while the range of stable deflections is significantly larger and pull-in voltage is lower than in a straight beam.

Meso scale MEMS inertial switch fabricated using an electroplated metal-on-insulator process

In this work, we report on a novel simple yet robust two-mask metal-on-insulator (MOI) process and illustrate its implementation for the fabrication of a meso scale MEMS inertial switch. The devices were fabricated of a ?40??m thick layer of nickel electrodeposited on top of a 4??m thick thermal field oxide (TOX) covering a single crystal silicon wafer. A 40??m thick layer of KMPR??resist was used as a mold and allowed the formation of high-aspect-ratio (1:5) metal structures. The devices were released by the sacrificial etching of the TOX layer in hydrofluoric acid. The fabricated devices were mounted in a ceramic enclosure and were characterized using both an electromagnet shaker and a drop tester. The functionality of the switch, aimed to trigger an electrical circuit when subjected to an acceleration pulse with amplitude of 300 g and duration of 200??s, was demonstrated experimentally and the performance targets were achieved. The experimental results were consistent with the model predictions obtained through finite element simulations.

Large displacement low voltage multistable micro actuator

This paper presents the modeling, design, fabrication and characterization of electrostatic large displacement multistable micro actuators. The device incorporates multiple serially connected bistable elements realized as shallow curved beams of slightly varying length. Loaded by an increasing force provided by an electrostatic comb drive transducer, the device undergoes a sequence of snap- through events and exhibits multiple stable equilibrium configurations at the same voltage. A reduced order (RO) model built using the Rayleigh-Ritz procedure as well as a nonlinear finite element (FE) analysis were used in order to predict the actuator behavior and evaluate design parameters. Devices of four different configurations were fabricated by a deep reactive ion etching (DRIE) based process using silicon on insulator (SOI) wafers. Experimental results demonstrate that the multistable devices exhibit stable displacement of 90mum while four snap-through and snap-back events take place during loading and unloading respectively. Experimental results are found to be in good agreement with the theoretical predictions.

Pull-In Behavior of Electrostatically Actuated Multistable Microstructures

We present an analysis of the electromechanical behavior and stability of a capacitive-based Micro Electro Mechanical Systems (MEMS) device with non-monotonous stiffness-deflection dependence. As an example, we consider a flexible initially curved double clamped micro beam actuated by a distributed electrostatic force. Since the system exhibits both mechanical snap-through buckling and electrostatic pull-in instability, the equilibrium curve has two bifurcation points implying the existence of multiple equilibrium configurations. The multistability phenomenon described in the present work is a result of interaction between mechanical and electrostatic nonlinearities of the system and differs from the electrostatic pull-in based bistability and mechanical bistability associated with the snap-through buckling. The governing equations of the geometrically nonlinear curved Euler-Bernoulli beam are formulated in the framework of the shallow arch approximation. Actuating force is calculated using second order perturbation solution of the Laplace equation for an electric potential. A coupled electro mechanical model is built by the Rayleigh-Ritz method with linear undamped eigenmodes of a straight beam as base functions. After verification of the model results, we analyze the influence of initial geometry of the beam on the location (in terms of actuation voltage and deflections) of the critical points on the bifurcation diagram. It was found that for snap-through to take place, the initial elevation of the beam should be larger than a certain value whereas the existence of electrical pull-in instability is unconditional. In addition, the stable relative deflection of a curved beam is larger than of initially straight beam. Based on the model results, we present an example of a multistable actuator design. Devices of various configurations were fabricated of single crystal silicon using deep reactive ion etching and the existence of multiple stable states and multiple instability points was demonstrated experimentally.© 2007 ASME

Bistability under tension and its use in a threshold force sensor

We report on a first experimental demonstration of a bistability phenomenon in a contactless device suspended using initially curved flexible beams and pulled by electrostatic forces directed along the beams. Devices of several configurations were fabricated from a silicon on insulator (SOI) substrate using a deep reactive ion etching (DRIE) based process and were actuated in-plane in ambient air conditions. We introduce a novel, less sensitive to the fabrication tolerances, operational scenario involving actuation by two parallel-plate electrodes and demonstrate that the bistable device can be used as a force sensor based on a pull-in voltage monitoring.

Surface-modified reusable gold electrode for detection of dissolved oxygen

The behavior of gold electrodes for the detection of dissolved oxygen was studied by the method of cyclic voltammetry in a phosphate-buffered solution with physiological pH. Surface modification with electropolymerized poly (o-phenylenediamine) film was performed to improve electrode antifouling properties. The voltammetric signature of oxygen was considered in terms of film electropolymerization conditions and post-deposition conditioning of the electrodes. The changes in the chemical structure of the poly (o-phenylenediamine) films as a result of these factors were confirmed by X-ray photoelectron spectroscopy analysis. Following long post-deposition conditioning in a phosphate-buffered solution, the modified electrodes exhibited stable voltammetric signatures in repeated tests and during storage as well as in the presence of a dense population of Escherichia coli (characterized by negligible metabolic activity) in the buffer. The results are indicative of the improved electrode antifouling properties.

Electromagnetic Microactuators With On-Chip Resin-Bonded Permanent Magnets

In this work we present modeling, fabrication and characterization of novel electromagnetic microactuators with integrated resin-bonded hard magnets embedded in the handle of silicon-on-insulator (SOI) wafers. Trenches etched through the handle of the SOI wafers are filled with the resin-bonded magnet material and allowed to cure at ambient temperature. Clamped-clamped beams fabricated from the single crystal silicon device layer of the SOI wafer are fabricated above the resin-bonded magnet filled trenches. Applying alternating current through the beam produces steady out-of-plane displacements due to resistive Joule heating and excites in-plane resonant vibrations due to Lorentz force coupling. The 8 mm long, 32 μm wide beams produced a maximum in-plane amplitude of 4.2 μm under an applied 2.4 mA current while the resonant frequency was tuned by changing the current amplitude. The results provided by the coupled thermo-electro-mechanical model of the beam and backed by experiments suggest that the integrated resin bonded magnets can be efficiently used for the actuation of micro structures.Copyright