Many Microelectromechanical Systems Research Articles

Bistable Configurations of Compliant Mechanisms Modeled Using Four Links and Translational Joints

Bistable mechanical devices remain stable in two distinct positions without power input. They find application in valves, switches, closures, and clasps. Mechanically bistable behavior results from the storage and release of energy, typically in springs, with stable positions occurring at local minima of stored energy. Compliant mechanisms offer an elegant way to achieve this behavior by incorporating both motion and energy storage into the same flexible element. Interest in compliant bistable mechanisms has also recently increased because of the advantages of bistable behavior in many micro-electro-mechanical systems (MEMS). Design of compliant or rigid-body bistable mechanisms typically requires simultaneous consideration of both energy storage and motion requirements. This paper simplifies this process by developing theory that provides prior knowledge of mechanism configurations that guarantee bistable behavior. Configurations which include one or more translational, or slider, joints are studied in this work. Several different mechanism types are analyzed to determine compliant segment placement that will ensure bistable mechanism operation. Examples demonstrate the power of the theory in design.

Characterisation of piezoelectric thin films by areal laser scanning

Piezoelectric thin films have great potential as transducer materials for many micro electromechanical systems (MEMS). An efficient characterisation technology for the piezoelectric properties of the thin films is becoming particularly essential for both fundamental materials research and practical applications. However, the variation among the reported data in the literature for piezoelectric films is significantly large as no widely accepted standard measurement technique has been popularly used. In this paper, we report the characterization method and results of our ferroelectric lead zirconate titanate (PZT) thin film samples by areal laser scanning. It is demonstrated that the monitoring of the vibration of the surface area, rather than a single point as reported previously, could result in much more comprehensive and accurate information about the piezoelectric performance of the films. The longitudinal piezoelectric dilatation data of the ferroelectric films are acquired at different frequencies and voltages. A theoretical analysis is given to further clarify the relationship between the detected vibration magnitude and the piezoelectric coefficient. The advantages of using the laser scanning vibrometer for characterising piezoelectric thin films are discussed in comparison with other methods.

Infrared thermal velocimetry for nonintrusive flow measurement in silicon microfluidic devices

Measurement of velocity is essential for verification of the performance of many microelectromechanical system (MEMS) based fluidic devices. MEMS devices are usually made of silicon which is transparent at infrared wavelengths. Infrared thermal velocimetry has been developed to measure the velocity in silicon microchannels utilizing the infrared transparency of silicon. Applications are broad and include MEMS fluidic devices and bio-fluidic elements. Briefly, an infrared laser heats the flowing fluid in a silicon microchannel and the recording of the thermal image of the moving heated liquid permits determination of the average flow velocity. It has been demonstrated that this method can be used to measure a wide range of velocities (1 cm/s–1 m/s or higher) with an accuracy of ±10%. The velocity of the maximum radiative intensity point (VMRIP) along the center of the channel is chosen to characterize the velocity in the channel. Numerical results show good agreement with the experimental results.

Alkoxyl monolayers as anti-stiction coatings in Si-based MEMS devices

Reducing surface energy is key to the success of many microelectromechanical systems (MEMS). In this report we present a strategy for the efficient assembly of alkoxyl monolayers onto a silicon surface to control surface energy. This is achieved by an all-liquid process in which the hydrogen terminated silicon surface resulting from aqueous HF etching is coated with a close-packed alkoxyl monolayer. The adhesion to silicon surface is reduced by a factor of 40 by the monolayer coating and friction coefficient of the coated surface is only 10-2. These coatings are successfully implemented in a model MEMS structure: cantilever beam array (CBA). Release-stiction is eliminated for polycrystalline silicon beams with a thickness of only 2 μm but with lengths up to 2 mm. Electrostatic actuation of coated beams in a controlled environment shows that the monolayer coating prevents in-use stiction at relative humidity as high as 90%.

공리적 설계를 이용한 비대칭 내부 짐벌을 가진 진동형 자이로스코프의 강건설계

Recently, there has been considerable interest in micro gyroscopes made of silicon chips. It can be applied to many micro-electro-mechanical systems (MEMS): devices for stabilization, general rate control, directional pointing, autopilot systems, and missile control. This paper shows how the mechanical design of the gyroscope can be done using axiomatic design, followed by the application of the Taguchi robust design method to determine the dimensions of the parts so as to accommodate the dimensional variations introduced during manufacturing. Functional requirements are defined twofold. One is that the natural frequencies should have fixed values. and the other is that the system should be robust to large tolerances. According to the Independence Axiom, design parameters are classified into a few groups. Then, the detailed design process is performed following the sequence indicated by the design matrix. The dimensions of the structure are detemlined to have constant values for the difference of frequencies without consideration of the tolerances. It is noted that the Taguchi concept is utilized as a unit process of the entire axiomatic approach.

Tribological Challenges in Micromechanical Systems

Despite much progress in surface micromachining technology, adhesion, friction and wear remain key issues, severely limiting the realization and reliability of many microelectromechanical systems (MEMS) devices. In this article, we focus on the use of molecularly thin organic films as release and anti-stiction coatings for MEMS. The various classes of organic films explored for MEMS are reviewed here, followed by a discussion of the current limitations and areas for improvements for this coating technology.

AN INVESTIGATION OF COMPLIANT MICRO-HALF-PANTOGRAPHS USING THE PSEUDORIGID BODY MODEL1*

Many microelectromechanical systems (MEMS) require a mechanism that amplifies motion. This article investigates the use of the pseudorigid body model (PRBM) to design fully compliant micro-half-pantographs for this purpose. The results demonstrate the ability to amplify motion with a compliant mechanism with no assembly or pin joints. The PRBM was used to develop equations for input-output displacement, force-deflection, and maximum nominal stress-deflection predictions. Predictions compared favorably to finite-element and experimental results. This work emphasizes further that the traditional method of trial-and-error design can be substituted by the pseudorigid body model approach. *Communicated by B. Gilmore.

High-Aspect-Ratio Structures for MEMS

Many microelectromechanical systems (MEMS) use the changing capacitance of movable parallel plates to drive and sense motion. An increase in this capacitance improves the performance of these micromechanical structures by means of increased electromechanical coupling for lower driving voltages and increased sensitivity of the micromechanical motion.

Transferable silicon nitride microcavities

The ability to construct self-supporting or suspended structures is essential for many micro-electro-mechanical systems (MEMS). Well-established technologies exist for the fabrication of such structures using deposition of SiNx grown by low-pressure chemical vapour deposition (LPCVD). However, such techniques are not suitable for temperature-sensitive substrates. While lower process temperatures (<300°C) can be achieved through plasma-enhanced chemical vapour deposition (PECVD), restrictions may still be imposed on the order in which processing steps can occur, making difficult or even prohibiting the use of MEMS technology for semiconductor materials such as HgCdTe (MCT). A novel self-supporting SiNx membrane technology that can be applied to temperature-sensitive semiconductor devices has been developed. To demonstrate this technology, a microcavity structure was constructed on a reusable GaAs substrate. The structure was then removed and bonded via van der Waals forces to a new substrate.

Self-assembled monolayers as anti-stiction coatings for MEMS: characteristics and recent developments

Despite significant advances in surface micromachining technology, stiction remains a key problem, severely limiting the realization and reliability of many micro-electro-mechanical systems (MEMS) devices. In this article, we focus on self-assembled monolayers as release and anti-stiction coatings for MEMS. Their formation mechanism, the microstructure coating process, and the characteristics of the coated microstructures are described, followed by a discussion of the current limitations, areas for improvements and recent progress for this coating technology.