The importance of thermoelastic damping as a fundamental dissipation mechanism for small-scale mechanical resonators is evaluated in light of recent efforts to design high-Q micrometer- and nanometer-scale electromechanical systems. The equations of linear thermoelasticity are used to give a simple derivation for thermoelastic damping of small flexural vibrations in thin beams. It is shown that Zener’s well-known approximation by a Lorentzian with a single thermal relaxation time slightly deviates from the exact expression.

Thermoelastic Damping in Micro- and Nano-Mechanical Systems.

Over the last couple of decades, the advancement in Microelectromechanical System (MEMS) devices is highly demanded for integrating the economically miniaturized sensors with fabricating technology. A sensor is a system that detects and responds to multiple physical inputs and converting them into analogue or digital forms. The sensor transforms these variations into a form which can be utilized as a marker to monitor the device variable. MEMS exhibits excellent feasibility in miniaturization sensors due to its small dimension, low power consumption, superior performance, and, batch-fabrication. This article presents the recent developments in standard actuation and sensing mechanisms that can serve MEMS-based devices, which is expected to revolutionize almost many product categories in the current era. The featured principles of actuating, sensing mechanisms and real-life applications have also been discussed. Proper understanding of the actuating and sensing mechanisms for the MEMS-based devices can play a vital role in effective selection for novel and complex application design.

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A Review of Actuation and Sensing Mechanisms in MEMS-Based Sensor Devices

MEMS switch is a movable device manufactured by means of semiconductor technology, possessing many incomparable advantages such as a small volume, low power consumption, high integration, etc. This paper reviews recent research of MEMS switches, pointing out the important performance indexes and systematically summarizing the classification according to driving principles. Then, a comparative study of current MEMS switches stressing their strengths and drawbacks is presented, based on performance requirements such as driven voltage, power consumption, and reliability. The efforts of teams to optimize MEMS switches are introduced and the applications of switches with different driving principles are also briefly reviewed. Furthermore, the development trend of MEMS switch and the research gaps are discussed. Finally, a summary and forecast about MEMS switches is given with the aim of providing a reference for future research in this domain.

Research Status and Development Trend of MEMS Switches: A Review.

Commercial success of microelectromechanical systems (MEMS) switches is limited by several issues. A high actuation voltage requires special circuitry solutions that increase size and cost of the switch. Another problem is the lack of reliability due to the stiction phenomenon. This paper presents a single-pole double-throw MEMS switch with electrostatic actuation and resistive contact. The device is based on an aluminum beam suspended by the torsion springs over the driving and signal electrodes. The design provides the pull-in voltage as low as 4.9 V. At the same time, the switch is equipped with the mechanism that protects it from stiction. The device is able to operate in the passive and active opening regimes. Recovery of the device after stiction in the hot switching conditions is demonstrated. In the cold mode, stiction is not observed at the transmitted DC power up to 25 mW. The resonant properties and response time of the switch are investigated. The on-resistance and the lifecycle are discussed. The proposed design is characterized by the high mechanical reliability. The main reason of failure is an increase of the on-resistance because of carbon accumulation on the platinum contacts.

Stiction-protected MEMS switch with low actuation voltage

The research at hand deals with the mechanical behavior of beam-like nanostructures. Nanobeams are assembled of multiple layers of different materials and geometry giving a layered nanobeam. To properly address experimentally noticed size effects in structures of this type, an adequate nonlocal elasticity formulation must be applied. The present model relies on the stress-driven integral methodology that effectively circumvents known deficiencies of other approaches. As a main contribution, a set of differential equations and boundary conditions governing the underlaying mechanics is proposed and applied to two benchmark examples. The obtained results show the expected stiffening nonlocal behavior exhibiting most of smaller and smaller structures and modern devices.

https://www.mdpi.com/2073-8994/12/5/717/pdf

Nonlocal Mechanical Behavior of Layered Nanobeams

Electrostatically actuated MEMS switch with the resistive contact is presented. Design of the switch includes the active contact breaking mechanism, which allows to detach the beam from the signal electrode in case of sticking. The mechanism is realized by the presence of two driving electrodes under the beam. The switch is fabricated by the surface micromachining. Finite element simulation and experimental investigation of the switch in a cold regime are performed.

A low actuation voltage bistable MEMS switch: design, fabrication and preliminary testing

MEMS switches have a wide range of potential applications in radio frequency and microwave systems due to promising characteristics. However, these devices are still not widespread because of reliability issues. The weak point of the resistive MEMS switch is the contact area that degrades during operation. This paper presents a single-pole double-throw switch having an extended lifecycle due to multiple contact points. The device is based on an aluminum beam suspended on torsion springs. It contains four beams connected to each other and actuated by a common driving electrode. Each beam provides the contact of a platinum bump with a signal electrode at the same contact force. Testing of the four-beam switch and the basic single-beam device is performed in the cold switching conditions. Contact resistance and lifecycle of both devices are analyzed and compared. Degradation of the contact bumps of the four-beam switch and the main failure mechanisms are discussed.

Contact resistance and lifecycle of a single- and multiple-contact MEMS switch

Microfluidic systems require a compact, energy-efficient and microtechnology-compatible actuator that pushes the liquid through the channels. Electrochemical devices are promising candidates, but they suffer from a long response time due to slow gas recombination. An actuator with a millisecond response time was demonstrated recently. A micron-sized chamber of the device with two titanium electrodes is sealed by a polydimethylsiloxane membrane. A series of microsecond voltage pulses of alternating polarity is applied to the electrodes. Nanobubbles generated in the chamber push the membrane up, but disappear quickly due to spontaneous combustion of hydrogen and oxygen. In this work, operation of the device is investigated in detail. The pulses with a frequency from 100 to 500 kHz are used for actuation. It is demonstrated that higher frequency and higher amplitude of the pulses provide larger deflection of the membrane, but finally the deflection is saturated. The stroke of 8-9 mu m can be achieved. In a cyclic operation regime the actuator is driven by series of pulses. If the time interval between the series is too short, the gas accumulates in the chamber. The membrane lifts during several cycles and then oscillates in the lifted position. In this regime the operating frequency as high as several hundred hertz can be achieved. The higher the frequency, the higher is the lift. The stroke also increases with the frequency, making a higher value more beneficial. Destruction of the electrodes is not observed, but the oxidation of titanium with time suppresses the gas production and decreases the membrane deflection. At a high frequency of the pulses the oxidation goes slower, but still significantly affects the performance. The oxidation of the electrodes is recognized as the main problem of the device. Methods to solve the problem are proposed.

A fast electrochemical actuator in the non-explosive regime

Resonant properties of the three-layer metallic cantilevers with 40 nm thickness are investigated. Two types of the nanocantilevers were fabricated: Cr-Al-Cr and Ti-Al-Ti. Resonant frequencies of the nanocantilevers were determined from the experimentally obtained resonant curves. Cantilever oscillations were excited by the electric force, the registration of the cantilever motion was performed by the optical lever method. Dependencies of the first and the second resonant frequencies on the cantilever length and width were experimentally obtained. The experimental data analysis and the comparison with the theoretical predictions were performed. Relations between the cantilever resonance properties and its dimensions and material are discussed.

Resonance properties of multilayer metallic nanocantilevers

An important step in the MEMS switch fabrication is the formation of a suspended micron-sized beam. Typically, the beam is made of gold due to its high electrical conductivity and chemical inertness. However, the fabrication process is complicated by poor suitability of Au for chemical etching and deformation of the beam under residual mechanical stress. An additional disadvantage is a high price of gold. Aluminium and its alloys are considered as a promising alternative. In this work, the magnetron-sputtered Al 99.99%, Al–1% Si and Al–1.5% Ti are tested as structural materials for a MEMS switch. We fabricate 1 μm thick beams and investigate surface morphology, electrical resistivity and mechanical properties. Single-layer films of Al 99.99% and Al–1% Si have coarse-grained microstructure with the root-mean-square roughness higher than 50 nm. The multi-step deposition reduces this value to 15 nm without significant deterioration of the resistivity and Young’s modulus. However, multilayer films of Al 99.99% have interlayer voids, which may degrade the switch reliability. Al–1.5% Ti provides much smoother and fine-grained film with plain sidewalls, which results in a higher quality factor of the beams. But this alloy has three times higher electrical resistivity than pure Al. Therefore, fabrication of the beam and transmission line from Al–1.5% Ti will increase insertion loss of the switch. A four-layer film of Al–1% Si is more preferable, since it also has fine microstructure and does not contain interlayer voids, but is close to pure aluminium in terms of resistivity.

Victor V. Naumov

Papers

A low actuation voltage bistable MEMS switch: design, fabrication and preliminary testing

Contact resistance and lifecycle of a single- and multiple-contact MEMS switch

A fast electrochemical actuator in the non-explosive regime

Resonance properties of multilayer metallic nanocantilevers

Testing of aluminium and its alloys as structural materials for a MEMS switch