Microelectromechanical Systems Actuators Research Articles

Structural in situ studies of shape memory alloy (SMA) Ni–Ti thin films

Ni–Ti SMA thin films formed by sputtering have been attracting great interest as powerful actuators in micro-electromechanical systems (MEMS) such as micro-valves, micro-fluidic pumps and micro-manipulators. Successful implementation of Ni–Ti micro-actuators requires a good understanding of the relationship among processing, microstructure and properties of Ni–Ti thin films. At the ROssendorf BeamLine (ROBL-CRG) at ESRF, we carried out a series of experiments that clearly illustrate the benefit of in situ studies, not only during annealing, but also during sputtering. The in situ sputtering experiments during film growth were performed using a magnetron sputter deposition chamber installed into the six-circle diffractometer of the materials research station. This facility allowed us to follow, almost in “real time”, the structural evolution of the deposited thin films as a consequence of changing deposition parameters.

Serpentine Spring Corner Designs for Micro-Electro-Mechanical Systems Optical Switches with Large Mirror Mass

In this paper, we describe the design principles of serpentine springs with high reliability for the micro-electromechanical systems (MEMS) optical switches with large mirror mass. The most often seen failure mode of the MEMS optical switches under reliability tests is the breaking of these springs, which provide the restoring force for the MEMS actuators. The breaking points are usually at the turning corner of the serpentine springs when the MEMS optical switches are under a high G shock test or a vibration test. In order to overcome the difficulties, we redesigned the corner shapes of the springs with careful consideration. We will discuss the theoretical analysis and simulation modeling for the corner shapes of serpentine springs. MEMS optical switches with redesigned serpentine springs are fabricated and tested to prove the proposed design. The results show that the MEMS optical switches with new serpentine springs can pass rigorous reliability tests.

MEMS actuators and sensors: observations on their performance and selection for purpose

This paper presents an exercise in comparing the performance of microelectromechanical systems (MEMS) actuators and sensors as a function of operating principle. Data have been obtained from the literature for the mechanical performance characteristics of actuators, force sensors and displacement sensors. On-chip and off-chip actuators and sensors are each sub-grouped into families, classes and members according to their principle of operation. The performance of MEMS sharing common operating principles is compared with each other and with equivalent macroscopic devices. The data are used to construct performance maps showing the capability of existing actuators and sensors in terms of maximum force and displacement capability, resolution and frequency. These can also be used as a preliminary design tool, as shown in a case study on the design of an on-chip tensile test machine for materials in thin-film form.

Sliding mode-based microstructure torque and force estimations using MEMS optical monitoring

The determination of microstructure state is becoming of increasing importance for microelectromechanical system (MEMS) sensors and actuators. Sliding mode-based microtorque estimation for a rotary micromotor, assuming the availability of a MEMS optical monitoring data stream, is presented. The dynamic model of the rotating MEMS system and the electrostatic torque are identified. The technique uses the estimated position signal to approach the actual measured position signal, obtainable through optical monitoring, to simultaneously estimate the microtorque and load torque. The estimated microtorque and load torque are low-pass filtered to eliminate the switching behavior inherent in the sliding-mode estimator. The experimental setup of a lateral comb resonator with optical monitoring is presented and discussed. The simulation results of both electrostatic and load torque estimations are presented, as well as the experimental force estimation of a lateral comb resonator device.

Numerical simulation of boundary-layer disturbance evolution

The use of numerical simulations to study the development of boundary-layer disturbances is illustrated for a number of different incompressible flow configurations. These include cases where the disturbances are generated by, or interact with, flow-control devices in the form of compliant panels, suction slots and microelectromechanical systems actuators. The velocity-vorticity system of governing equations used for the simulations is reviewed, along with the numerical discretization.

Fatigue Characteristics of the Si Moveable Comb Inserted into MEMS Optical Devices

This paper focuses on the fatigue characteristics of the single crystal silicon (SC-Si) cantilever in relation with the critical design of micro electro-mechanical systems (MEMS). Development of MEMS actuators for optical communication usage is carried out successfully, for example, in optical switches and variable optical attenuators (VOA). In those devices, fatigue characteristics of the MEMS structure are crucial to its practical application. However, fatigue tests using real structures have not been carried out well. In this research, the fatigue life has been inspected at the actual device, under actual usage conditions for the first time. We obtained fracture rate λ from experimental results, and the value of Failure in Time (FIT) A was about 0.3 FIT. This result indicates that these MEMS devices having enough reliability for practical usage.

Micro-mechanical switch array for meso-scale actuation

Traditional micro-electro mechanical systems (MEMS) actuators have limited stroke and force characteristics. This paper describes the development of a hybrid actuation solution, which utilizes a micro-machined actuator array to provide switching of the mechanical motion of a larger meso-scale piezo-electric actuator. One motivating application of this technology is the development of a tactile display, where discrete mechanical actuators apply vibratory excitation at discrete locations on the skin. Specifically, this paper describes the development, fabrication, and characterization of a 4 × 5 micro-actuator array of individual vibrating pixels for fingertip tactile communication. The individual pixels are turned on and off by pairs of microscopic thermal actuators, while the main vibration is generated by a vibrating piezo-electric plate. The fabrication sequence and actuation performance of the array are also presented.

Mechanically tunable photonic crystal structure

We report a tunable nanophotonic device concept based on flexible photonic crystal, which is comprised of a periodic array of high-index dielectric material and a low-index flexible polymer. Tunability is achieved by applying mechanical force with nano-/microelectromechanical system actuators. The mechanical stress induces changes in the periodicity of the photonic crystal and consequently modifies the photonic band structure. To demonstrate the concept, we theoretically investigated the effect of mechanical stress on the anomalous refraction behavior and observed a very wide tunability in the beam propagation direction. This concept provides a means to achieve real-time, dynamic control of photonic band structure and will thus expand the utility of photonic crystal structures in advanced nanophotonic systems.

Electrochemical Codeposition of Magnetic Particle-Ferromagnetic Matrix Composites for Magnetic MEMS Actuator Applications

This paper reports on a new electrochemical codeposition technique for microelectromechanical systems (MEMS) actuator application. Various magnetic composite materials can be obtained by incorporating hard magnetic particles with electrodeposited magnetic (either soft or hard ferromagnetic materials) metal matrices. The incorporated magnetic particle investigated is barium ferrite and the metal matrices are electroplated nickel, CoNiMnP, and an electroless plated nickel-phosphorus alloy. Scanning electron microscopy results show that particles can be successfully incorporated into all the investigated metal matrices. The resulting magnetic properties show that materials produced by the electrochemical codeposition technique have a high coercivity of up to 2120 Oe and a maximum magnetic energy density, of up to This indicates that the reported technique has the potential of becoming an important technique for the deposition of hard magnetic materials for magnetic MEMS actuators. © 2004 The Electrochemical Society. All rights reserved.

Electrostatic micromembrane actuator arrays as motion generator

A rigid-body motion generator based on an array of micromembrane actuators is described. Unlike previous microelectromechanical systems (MEMS) techniques, the architecture employs a large number (typically greater than 1000) of micron-sized (10–200 μm) membrane actuators to simultaneously generate the displacement of a large rigid body, such as a conventional optical mirror. For optical applications, the approach provides optical design freedom of MEMS mirrors by enabling large-aperture mirrors to be driven electrostatically by MEMS actuators. The micromembrane actuator arrays have been built using a stacked architecture similar to that employed in the Multiuser MEMS Process (MUMPS), and the motion transfer from the arrayed micron-sized actuators to macro-sized components was demonstrated.

Forcing a planar jet flow using MEMS

We present the results of an experimental study in which a planar laminar jet of air was forced by an array of micro-electromechanical systems (MEMS) micro-actuators. In the absence of forcing, the velocity profile of the experimental jet matched the classic analytic solution. Driving actuators on either side of the jet in-phase or anti-phase, respectively, excited the symmetric or anti-symmetric mode of instability of the jet. Asymmetric forcing, using MEMS actuators on only one side of the jet, was also investigated.

Thin Film Piezoelectrics for MEMS

Thin film piezoelectric materials offer a number of advantages in microelectromechanical systems (MEMS), due to the large motions that can be generated, often with low hysteresis, the high available energy densities, as well as high sensitivity sensors with wide dynamic ranges, and low power requirements. This paper reviews the literature in this field, with an emphasis on the factors that impact the magnitude of the available piezoelectric response. For non-ferroelectric piezoelectrics such as ZnO and AlN, the importance of film orientation is discussed. The high available electrical resistivity in AlN, its compatibility with CMOS processing, and its high frequency constant make it especially attractive in resonator applications. The higher piezoelectric response available in ferroelectric films enables lower voltage operation of actuators, as well as high sensitivity sensors. Among ferroelectric films, the majority of the MEMS sensors and actuators developed have utilized lead zirconate titanate (PZT) films as the transducer. Randomly oriented PZT films show piezoelectric e31,f coefficients of about −7 C/m2 at the morphotropic phase boundary. In PZT films, orientation, composition, grain size, defect chemistry, and mechanical boundary conditions all impact the observed piezoelectric coefficients. The highest achievable piezoelectric responses can be observed in {001} oriented rhombohedrally-distorted perovskites. For a variety of such films, e31,f coefficients of −12 to −27 C/m2 have been reported.

Fatigue damage evolution in silicon films for micromechanical applications

In this paper we examine the conditions for surface topography evolution and crack growth/fracture during the cyclic actuation of polysilicon microelectromechanical systems (MEMS) structures. The surface topography evolution that occurs during cyclic fatigue is shown to be stressassisted and may be predicted by linear perturbation analyses. The conditions for crack growth (due to pre-existing or nucleated cracks) are also examined within the framework of linear elastic fracture mechanics. Within this framework, we consider pre-existing cracks in the topical SiO2 layer that forms on the Si substrate in the absence of passivation. The thickening of the SiO2 that is normally observed during cyclic actuation of Si MEMS structures is shown to increase the possibility of stable crack growth by stress corrosion cracking prior to the onset of unstable crack growth in the SiO2 and Si layers. Finally, the implications of the results are discussed for the prediction of fatigue damage in silicon MEMS structures.

Modeling and experimental characterization of the chevron-type bi-stable microactuator

A compliant bi-stable micromechanism allows two stable states within its operation range to remain at one of the local minimum states of potential energy. Bi-stable energy characteristics offer two distinct and repeatable stable states that require no power input to maintain. In this paper we suggest a new theoretical model of the chevron-type bi-stable microactuator using equivalent stiffness in the rectilinear and rotational directions. From this model, the range of the spring stiffness in which the bi-stable mechanism can be operated is analyzed and compared with the results of finite element analysis (FEA) for buckling analysis. The analysis of the equivalent stiffness model shows that the forces necessary for the forward and backward actuation are almost linearly proportional to the equivalent stiffness, also in agreement with that of FEA. Based on the analysis, a novel chevron-type bi-stable microelectromechanical systems (MEMS) actuator with hinges and coupling bars is proposed for the improvement of a stable latch-up operation. The thickness and orientation of the hinge is determined through FEA in the light of reliable operation, stroke requirement, mechanical stress, and process constraint. The change in the cross-sectional area during the fabrication process is also considered to take into account its effects on the reduction of equivalent stiffness of the bi-stable MEMS actuator. The fabricated chevron-type microactuator showed a reliable bi-stable operation with a 60 µm stroke at 36 V input voltage, in agreement with the results of the equivalent stiffness model. Therefore, these results confirm that the chevron-type bi-stable MEMS actuator using hinges with coupling bars is applicable to optical switches.

A Supervisory Wafer-Level 3D Microassembly System for Hybrid MEMS Fabrication

Hybrid MEMS (microelectromechanical systems) integrate solid-state ICs with MEMS sensors and actuators. It is widely believed that such systems will bring fundamental technological impacts and significant social benefits. Hybrid MEMS manufacturing requires the development of new fabrication, packaging and interconnection technologies in which microassembly plays a critical role. Microassembly is the assembly of objects with microscale and/or mesoscale features under microscale tolerances. It integrates techniques from many different areas such as robotics, computer vision, microfabrication and surface science. This paper studies the design and implementation of microassembly systems through the introduction of a supervisory microassembly workcell. This workcell is developed for 3D assembly of large numbers of micromachined thin metal parts into DRIE (deep reactive ion etching) etched holes in silicon wafers. It overcomes a major limitation of current MEMS fabrication techniques by allowing the use of incompatiable materials and fabrication processes to build complex-shaped 3D MEMS structures. The system is able to perform reliable and efficient wafer-level microassembly operations within a supervisory framework. Microassembly brings new and unique issues to robotics research. The major components of microassembly systems are analyzed. Results on micromanipulator design, illumination modeling and control, and microgripper design are presented.

Optical scanner using a MEMS actuator

Abstract Optical scanners have numerous applications from bar code readers and laser printers in industry to corneal resurfacing and optical coherence tomography in medicine. We have developed an optical scanner fabricated using photolithography on a polyimide substrate. This scanner uses an electrostatic microelectromechanical system (MEMS) actuator to tilt a gold-coated mirror resting on 3 μm thick polyimide torsion hinges to steer an optical beam. The linear actuator used is the integrated force array (IFA), a network of hundreds of thousands of deformable capacitors, which electrostatically contract with an applied differential voltage. IFAs are 2.2 μm thick patterned, metallized polyimide films 1 cm long and either 1 or 3 mm wide depending on the application. The mirror support structures were modeled using one-dimensional beam theory and ANSYS finite element analysis and then fabricated using a three-layer process with polyimide and gold on silicon wafers. Side scanning structures have been fabricated with tables 1.125 or 2.25 mm wide. The completed devices were coated with a 500 A thick conformal coating of parylene for protection from the environment. These devices have demonstrated optical scan angles up to 146° for applied voltages up to ±50 V. These devices were also used to steer a laser beam in a prototype bar code reader to demonstrate functionality.

Failure mechanisms of a MEMS actuator in very high vacuum

The tribochemical and mechanical origins of wear of a microelectromechanical systems (MEMS) actuator (electrostatic lateral output motor) operated in very high vacuum (10 −7 torr) are reported in this study. Failure mechanisms in vacuum were determined and then compared to those in dry air, which is one of the harshest environments for causing early failure. Durability in vacuum was poor, even worse than that in dry air. Poor durability in vacuum is related to the kinetics of wear and reformation of the native oxide at asperity contacts. Devices failed due to catastrophic wear in vacuum, and more wear debris was generated than in dry air. There was a fundamental difference in wear debris morphology for devices run in vacuum and dry air. In vacuum, wear debris took the form of pulled-out polysilicon grains. In dry air, wear debris was an agglomerate of smaller particles, which were largely comprised of SiO 2. An oxide layer reformed quickly enough in the air mediated wear process to provide some protection, but resulted in oxygen rich wear debris. In vacuum, the passivating native oxide layer was removed exposing reactive areas on the surface, which led to junction formation at Si–Si asperity contacts. It is proposed that interfacial bonds formed at asperity contacts were stronger than the cohesive bonds within the polysilicon, which resulted in grain pull-out in an adhesive wear process.

MEMS-enabled thermal management of high-heat-flux devices EDIFICE: embedded droplet impingement for integrated cooling of electronics

Abstract This paper reports the development of embedded droplet impingement for integrated cooling of electronics (EDIFICE). The EDIFICE project seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes in the range 70–100 W/cm2, employing latent heat of vaporization of dielectric fluids (50–100 μm droplets) to achieve these high heat removal rates. Micro-manufacturing and micro electro-mechanical systems (MEMS) will be discussed as enabling technologies for innovative cooling schemes recently proposed. A novel feature to enable adaptive on-demand cooling is MEMS sensing (on-chip temperature, remote IR temperature and ultrasonic dielectric film thickness) and MEMS actuation. EDIFICE will be integrated within the electronics package and fabricated using advanced micro-manufacturing technology (e.g., deep reactive ion etching (DRIE) and complementary metal-oxide-semiconductor (CMOS) CMU-MEMS). The development of EDIFICE involves modeling, CFD simulations, and physical experimentation on test beds. In this study, numerical simulations are performed to investigate EDIFICE jet impingement cooling with a dielectric coolant and the influence of several parameters such as jet diameter, jet velocity, and latent heat effects. This paper also presents flow visualization of micro-jet break-up, induced by MEMS micro-nozzles of irregular shapes and flow swirling to generate droplets with desirable dispersion. To enhance liquid spreading on the impingement surface and to create a thin film for effective evaporation, MEMS micro-structured surfaces are fabricated. All of these components are made from silicon and enabled by integrated-MEMS process technologies.

Quantum mechanical actuation of microelectromechanical systems by the Casimir force.

The Casimir force is the attraction between uncharged metallic surfaces as a result of quantum mechanical vacuum fluctuations of the electromagnetic field. We demonstrate the Casimir effect in microelectromechanical systems using a micromachined torsional device. Attraction between a polysilicon plate and a spherical metallic surface results in a torque that rotates the plate about two thin torsional rods. The dependence of the rotation angle on the separation between the surfaces is in agreement with calculations of the Casimir force. Our results show that quantum electrodynamical effects play a significant role in such microelectromechanical systems when the separation between components is in the nanometer range.

Micromachining technology for lateral field emission devices

We demonstrate a range of novel applications of micromachining and microelectromechanical systems (MEMS) for achieving efficient and tunable field emission devices (FEDs). Arrays of lateral field emission tips are fabricated with submicron spacing utilizing deep reactive ion etch (DRIE). Current densities above 150 A/cm/sup 2/ are achieved with over 150/spl middot/10/sup 6/ tips/cm/sup 2/. With sacrificial sidewall spacing, electrodes can be placed at arbitrarily close distances to reduce turn-on voltages. We further utilize MEMS actuators to laterally adjust electrode distances. To improve the integration capability of FEDs, we demonstrate batch bump-transfer of working lateral FEDs onto a quartz target substrate.