Electrostatic MEMS Research Articles

Analysis and design of an electrostatic MEMS microphone using the PolyMUMPs process

This paper presents the analysis and design of an electrostatic MEMS microphone using the PolyMUMPs process with an additional back-etch processing step. Circular and square (simply supported and c...

J2230203 静電型MEMSスピーカーの検討

Recently the realization of the electrostatic speaker is expected with the popularization of smartphones. In this paper, the structure of the electrostatic MEMS speaker is suggested and the characteristic is measured. The electrostatic MEMS speaker has a comb-drive actuator which vibrate in a longitudinal direction. The speaker can be drive with zero bias by introducing the Potassium-ion electret into the comb-erectrodes. The design concept of this device is making of the MEMS speaker which realize a sound pressure level 70〜80 dB in a rsonance frequency at 2 V with the size of the vibration board which is equal to an earphone. The sound from the fabricated speaker was heard at applied voltage 30 V and charged voltage 250 V. when the relation between the vibration velocity and the frequency was measured by the laser Doppler vibrometer, the value roughly match up to the simulation. However, the problem is that various resonance mode come about and the drive voltage is higher than the design value. In the future, the resonance mode will be confirmed and low voltage driving will realized by reviewing the design.

Contact damping in microelectromechanical actuators

We examine the significance of the energy loss mechanisms active in electrostatic MEMS actuators. We find that the dominant loss mechanism changes depending on the actuator mode of operation. We find that the active mechanisms in the order of their significance are: fluid-structure interactions dominant for actuators operating in air, actuator-substrate interactions dominant for actuators in contact with a substrate under vacuum, and intrinsic loss mechanisms dominant for actuators in-flight under vacuum. Further, experimental results show that the quality factor of an electrostatic MEMS actuator drops drastically as the actuator first comes into line contact with a substrate. As the contact area expands along the actuator length, the quality factor increases. Measurements under 1 Torr vacuum show a three-fold increase in the quality factor as the contact area expands from a line to 30% of the actuator area. This increase in the quality factor is attributed to the drop in the contribution of friction forces into energy losses as contact expands and adhesion forces increase.

Dynamical solutions of singular parabolic equations modeling electrostatic MEMS

We study the electrostatic MEMS-device parabolic equation, $${u_{t} - \Delta u = \frac{\lambda_{\rho}(x)}{(1 - u)^{2}}}$$ with Dirichlet boundary condition and a bounded domain $${\Omega}$$ of $${\mathbb{R}^{N}}$$ . Here $${\lambda}$$ is positive parameter and $${\rho}$$ is a nonnegative continuous function. In this paper, we investigate the behavior of solutions for this problem. In particular, we show small initial value yields quenching behavior of the solutions. While large initial data leads global existence of the solutions.

Effect of Dielectric Charging on Capacitance Change of an SOI Based CMUT

Effect of dielectric charging on the performance of SOI based Capacitive Micromachined Ultrasonic Transducers (CMUT) has been investigated. Measurements on an SOI based CMUT show that that the capacitance change as a function of DC bias is considerably higher than analytically calculated values. Investigation shows that this deviation in capacitance from analytically calculated values is due to the combined effects of different dielectric charging phenomena due to a strong electric field, trap charges in the SOI oxide layer, the charge motion associated with the leakage current through the buried oxide layer, and the air in the CMUT cavity. Additionally, this charging effect degrades the transduction efficiency as the induced polarization reduced the effective bias across the CMUT. It is concluded that the buried oxide (BOX) layers in SOI wafers are not suitable for use as dielectric spacers in electrostatic MEMS devices. Keywords: Capacitance, CMUT, dielectric charging, leakage current, MEMS, microfabrication, SOI.

Real-time DC-dynamic biasing method for switching time improvement in severely underdamped fringing-field electrostatic MEMS actuators.

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input bias voltage. However, the tradeoff for the improved switching performance is a relatively long settling time to reach each gap height in response to various applied voltages. Transient applied bias waveforms are employed to facilitate reduced switching times for electrostatic fringing-field MEMS actuators with high mechanical quality factors. Removing the underlying substrate of the fringing-field actuator creates the low mechanical damping environment necessary to effectively test the concept. The removal of the underlying substrate also a has substantial improvement on the reliability performance of the device in regards to failure due to stiction. Although DC-dynamic biasing is useful in improving settling time, the required slew rates for typical MEMS devices may place aggressive requirements on the charge pumps for fully-integrated on-chip designs. Additionally, there may be challenges integrating the substrate removal step into the back-end-of-line commercial CMOS processing steps. Experimental validation of fabricated actuators demonstrates an improvement of 50x in switching time when compared to conventional step biasing results. Compared to theoretical calculations, the experimental results are in good agreement.

Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input bias voltage. However, the tradeoff for the improved switching performance is a relatively long settling time to reach each gap height in response to various applied voltages. Transient applied bias waveforms are employed to facilitate reduced switching times for electrostatic fringing-field MEMS actuators with high mechanical quality factors. Removing the underlying substrate of the fringing-field actuator creates the low mechanical damping environment necessary to effectively test the concept. The removal of the underlying substrate also a has substantial improvement on the reliability performance of the device in regards to failure due to stiction. Although DC-dynamic biasing is useful in improving settling time, the required slew rates for typical MEMS devices may place aggressive requirements on the charge pumps for fully-integrated on-chip designs. Additionally, there may be challenges integrating the substrate removal step into the back-end-of-line commercial CMOS processing steps. Experimental validation of fabricated actuators demonstrates an improvement of 50x in switching time when compared to conventional step biasing results. Compared to theoretical calculations, the experimental results are in good agreement.

Innovative strategy for designing rotating actuators – motors case study

In presented work the Finite Element Methods combined with MATLAB was introduced. This strategy enables changing of all parameters of a few types of magnetic motors and electrostatic micromotors MEMS. Furthermore, this strategy allowed for taking advantage of FEMM, namely possibility of calculations of complicated geometries without additional operations.

Binary MEMS gas sensors

A novel sensing mechanism for electrostatic MEMS that employs static bifurcation-based sensing and binary detection is demonstrated. It is implemented as an ethanol vapour sensor that exploits the static pull-in bifurcation. Sensor detection of 5 ppm of ethanol vapour in dry nitrogen, equivalent to a detectable mass of 165 pg, is experimentally demonstrated. Sensor robustness to external disturbances is also demonstrated. A closed-form expression for the sensitivity of statically detected electrostatic MEMS sensors is derived. It is shown that the sensitivity of static bifurcation-based binary electrostatic MEMS sensors represents an upper bound on the sensitivity of static detection for given sensor dimensions and material properties.

A free boundary problem modeling electrostatic MEMS: I. Linear bending effects

The dynamical and stationary behaviors of a fourth-order evolution equation with clamped boundary conditions and a singular nonlocal reaction term, which is coupled to an elliptic free boundary problem in a non-smooth domain, are investigated. The equation arises in the modeling of microelectromechanical systems and includes two positive parameters $$\lambda $$ and $$\varepsilon $$ related to the applied voltage and the aspect ratio of the device, respectively. Local and global well-posedness results are obtained for the corresponding hyperbolic and parabolic evolution problems as well as a criterion for global existence excluding the occurrence of finite time singularities which are not physically relevant. Existence of a stable steady state is shown for sufficiently small $$\lambda $$ . Non-existence of steady states is also established when $$\varepsilon $$ is small enough and $$\lambda $$ is large enough (depending on $$\varepsilon $$ ).

On the Partial Differential Equations of Electrostatic MEMS Devices with Effects of Casimir Force

We analyze pull-in instability of electrostatically actuated microelectromechanical systems, and we find that as the device size is reduced, the effect of the Casimir force becomes more important. In the miniaturization process there is a minimum size for the device below which the system spontaneously collapses with zero applied voltage. According to the mathematical analysis, we obtain a set U in the plane, such that elements of U correspond to minimal stable solutions of a two-parameter mathematical model. For points on the boundary \({\Upsilon}\) of U, there exists weak solutions to this model, which are called extremal solutions. More refined properties of stable solutions—such as regularity, stability, uniqueness—are also established.

Design and simulation of a novel electro-thermally actuated lateral RF MEMS latching switch for low power applications

A new lateral electro-thermally actuated latching RF MEMS switch is presented in this paper. In contrast to conventional electrostatic or electro-thermal MEMS switches which require an actuation voltage to hold the switch in on or off state, this switch has a true mechanical latching design in such a way that there is no power required to hold the switch on or off. The switch structure only consumes power while transition between states. In order to satisfy low voltage operation, electro-thermal actuators are chose as the drive and latching actuators of the switch. The required actuation voltage for drive and latching actuators is 6 V. The switch consumes a power of <99 mW while transition from off to on state and also consumes a power of 46 mW for 0.3 ms in transition from off to on states. FEM simulations show that the return loss of the switch is below ?10 dB up to 140 GHz and is below ?20 dB up to 40 GHz. The insertion loss of the switch is less than ?1 dB up to 150 GHz. The switch isolation when it is off is below ?20 dB up to 160 GHz. The switch has potential applications in low voltage, low power and high performance RF tuning and switching applications.

Characterization of MEMS Resonators via Feedthrough De-embedding of Pulsed-mode Response

In this paper, we analyze and give experimental evidence of the efficiency of pulsed-mode actuation for the characterization of capacitive MEMS resonators. In particular, we show how to process the output signal in pulsed mode to eliminate parasitic feedthrough effects and characterize the resonator. We test our approach on a pressure sensor, developed by THALES Avionics, used for avionics applications. Contrary to the existing state-of-the-art techniques, the method described in this paper can be used to characterize an electrostatic MEMS without a spectrum analyzer or high-performance ADC.

Output-Feedback Control of a Chaotic MEMS Resonator for Oscillation Amplitude Enhancement

The present work addresses the problem of chaos control in an electrostatic MEMS resonator by using an output-feedback control scheme. One of the unstable orbits immersed in the chaotic attractor is stabilized in order to produce a sustained oscillation of the movable plate composing the microstructure. The orbit is carefully chosen so as to produce a high amplitude oscillation. This approach allows the enhancement of oscillation amplitude of the resonator at a reduced control effort, since the unstable orbit already exists in the system and it is not necessary to spend energy to create it. Realistic operational conditions of the MEMS are considered including parametric uncertainties in the model and constraints due to the difficulty in measuring the speed of the plates of the microstructure. A control law is constructed recursively by using the technique of backstepping. Finally, numerical simulations are carried out to confirm the validity of the developed control scheme and to demonstrate the effect of controlling orbits immersed in the chaotic attractor.

Stability and Dynamic Analysis of the Electrostatic MEMS Actuators

The nonlinear dynamic behavior of the micro actuators is analyzed in this paper. In general, analysis of the electrostatic device is quite difficult and complicated due to the electrostatic coupling effect and the nonlinear electrostatic force. In this study, a hybrid method for the micro-structure system, which combines the differential transformation and finite difference approximation techniques, is used to overcome the nonlinear electrostatic coupling phenomenon. The analysis takes account of the electrostatic coupling effect, the fringing field effect, the residual stress, the nonlinear electrostatic force and squeeze-film damping effect. Finally, the actuating conditions which ensure the stability of the micro actuators are identified by reference to phase portraits.

Surface Micromachined MEMS Capacitors With Dual Cavity for Energy Harvesting

A novel electrostatic MEMS capacitors array with dual capacitive cavities for energy harvesting from environmental vibration has been developed. This paper presents the new design, fabrication, modeling, and complete set of characterization of a MEMS capacitors array energy harvester with dual cavities, which achieves considerably higher energy generation than similar devices with a single capacitor. Rocking instability, that is, perturbations that may result in movable plates not in parallel to fixed plates, of the movable plate has been studied based upon comprehensive dynamic simulations using MATLAB. Resultant symmetric breaking causes drastic reduction on power generation. The devices have been successfully fabricated using surface micromachining technology and experimentally characterized. The resonance frequency is observed at 500 Hz. A maximum power of 2.5 μW across the load resistors is achieved under a high DC bias of 15 V and extremely intense excitation above 5 g. Detailed dynamic performance of the MEMS capacitors array energy harvester are presented in the paper. Experimental comparison between device with single capacitor and capacitors array are addressed.

The Response of Electrostatic MEMS Structure under Mechanical Shock and Electrostatic Forces

For MEMS devices actuated by electrostatic force, unexpected failure modes can be hardly predicted when the electrostatic force coupled with the shock. A response model is established when a micro cantilever subjected to electrostatic force and mechanical shock. First, based on the theory of transverse forced vibration in vibration mechanics, the equation of motion under shock and electrostatic fore is presented. Then the reduced order model is gained after simplifying by mode superposition method. The computing results indicate that: the shock amplitude and duration are the key factors to affect the reliability of the device; the shock load and electrostatic forces make the threshold voltage much lower than the anticipated value. The micro cantilever may collapse to the substrate even at a voltage far lower than the pull-in voltage. This early dynamic pull-in instability may cause some failures such as short circuit, adhesion or collision damage.

A new model for electrostatic MEMS with two free boundaries

A moving boundary problem with two free boundaries modeling a two-dimensional idealized MEMS device with pull-in instability is discussed. We use a fixed point argument to show that the model possesses stationary solutions for small source voltages. We also give a rigorous evidence that solutions of the model converge towards solutions of the associated small aspect ratio equation in the vanishing aspect ratio limit.

Two-axis MEMS scanner with transfer-printed high-reflectivity, broadband monolithic silicon photonic crystal mirrors

We present a two-axis electrostatic MEMS scanner with high-reflectivity monolithic single-crystal-silicon photonic crystal (PC) mirrors suitable for applications in harsh environments. The reflective surfaces of the MEMS scanner are transfer-printed PC mirrors with low polarization dependence, low angular dependence, and reflectivity over 85% in the wavelength range of 1490nm~1505nm and above 90% over the wavelength band of 1550~1570nm. In static mode, the scanner has total scan range of 10.2° on one rotation axis and 7.8° on the other. Dynamic operation on resonance increase the scan range to 21° at 608Hz around the outer rotation axis and 9.5° at 1.73kHz about the inner rotation axis.

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.