An Unstructured Finite-Volume Method for Structure–Electrostatics Interactions in MEMS

Radio-frequency microelectromechanical systems (RF MEMS) are widely used for contact actuators and capacitive switches, and involve metal–dielectric contact. In these devices, the structure is activated by an electrostatic force, whose magnitude changes as the gap closes. It is advantageous to model fluid and structural mechanics and electrostatics within a single comprehensive numerical framework to facilitate coupling between them. In this article, we extend a cell-based finite-volume approach popularly used to simulate fluid flow to characterize structure–electrostatics interactions. The method employs fully implicit second-order finite-volume discretization of the integral conservation equations governing elastic solid mechanics and electrostatics, and uses arbitrary convex polyhedral meshes. Results are presented in this article for a fixed-fixed beam under electrostatic actuation.     

TRANSIENT,INTERFEROMETRIC IMAGING OF POLYCRYSTALLINE SILICON MEMS DEVICES DURING LASER HEATING

To characterize microelectromechanical systems (MEMS) during laser processing, imaging and metrology techniques need to be developed that are capable of visualizing the dynamics of MEMS structures and that are compatible with high power lasers. This study describes a high-speed, interferometric imaging system for measuring out-of-plane deflections of polycrystalline silicon (polysilicon) MEMS structures. A HeNe laser with a wavelength of 632.8 nm illuminates the structures. Since polysilicon is semitransparent at 632.8 nm, interference occurs between light waves reflected from the top of the microstructure and light waves transmitting through the structure and reflecting off the substrate. The interference produces fringes along microstructures that correspond to changes in height. Using a high-speed CCD camera, transient interferometric images of the dynamic motion of MEMS devices are captured. The imaging technique is demonstrated by obtaining the first transient images of the repair of adhered polysilicon microcantilevers, due to Nd:YAG pulsed laser heating. The experimental results clearly show failed structures peeling from the substrate as they are repaired.     

The behavior of structures based on the characteristic strain model of creep

There has been much work over the past two decades to aid the design and assessment engineer in the selection of a suitable material model of creep for high temperature applications. The model needs to be simple to implement as well as being able to describe material response over long times. Familiar creep models, as implemented in the majority of nonlinear finite element analysis systems, are still widely used although not always accurate in modeling creep behavior at the end of the secondary phase. The Characteristic Strain Model (CSM) has been shown to be able to effectively model creep behavior at long times; it is simple to implement and requires a minimum of creep data. This paper examines the ability of the CSM to model the recognized behavior of the steady state creep of simple structures under multi-axial stress.     

Review on microfabricated micro-solid oxide fuel cell membranes

Micro-solid oxide fuel cells (μ-SOFC) are promising power sources for portable electronic devices. This review presents the current status of development of microfabricated micro-solid oxide fuel cell membranes for power delivery. The μ-SOFC membranes are developed using micro-electro-mechanical system (MEMS) fabrication and machining techniques. The different designs of free-standing μ-SOFC membranes and μ-SOFCs deposited on porous substrates are presented. The materials used in the μ-SOFC anode, electrolyte and cathode are discussed and compared along with their microstructures. The electrical performance data of the different μ-SOFC designs are compared and discussed. High μ-SOFC performances of 677 mW cm⁻² were demonstrated at temperatures as low as 400 °C.     

Monte Carlo simulations of gas flow and heat transfer in vacuum packaged MEMS devices

The flow and heat transfer in vacuum packaged MEMS devices is numerically studied by using the direct simulation Monte Carlo method. The problem is simplified as an enclosure with a hot surface at the bottom. If the bottom is considered at a completely uniform high temperature, flow will be induced by the thermal stress difference from discontinuous temperature distribution effect. The heat transfer is weakened by the rarefied gas effect when compared with the continuum-based solution. If the bottom temperature is partly high and continuously decreases besides the hot part, the gas flow near the bottom surface will be remarkably enhanced due to the temperature gradients. As a result, the heat transfer on the hot chip surface is also enhanced. The current research can improve the understanding of gas flow and heat transfer in vacuum packaged MEMS devices.     

Microsystems enabled photovoltaics: 14.9% efficient 14 μm thick crystalline silicon solar cell

Crystalline silicon solar cells 10-15 times thinner than traditional commercial c-Si cells with 14.9% efficiency are presented with modeling, fabrication, and testing details. These cells are 14 μm thick, 250 μm wide, and have achieved 14.9% solar conversion efficiency under AM 1.5 spectrum. First, modeling results illustrate the importance of high-quality passivation to achieve high efficiency in thin silicon, back contacted solar cells. Then, the methodology used to fabricate these ultra thin devices by means of established microsystems processing technologies is presented. Finally, the optimization procedure to achieve high efficiency as well as the results of the experiments carried out with alumina and nitride layers as passivation coatings are discussed.     

Pd/Ag alloy as an application for hydrogen sensing

A highly sensitive H₂ gas sensor was fabricated using a Micro Electromechanical Systems (MEMS) procedure having an embedded micro-heater. The palladium-silver (Pd/Ag having stoichiometric ratios 77:23) thin film was deposited by the RF/DC magnetron sputtering and used as the hydrogen sensing layer designed as a zig-zag pattern. Morphological and structural properties of the Pd/Ag thin film was studied by Field emission scanning electron microscope (FESEM), Atomic force microscopy (AFM) and Energy Dispersive Analysis of X-rays respectively. The working temperature of the micro heater showed a linear relation with variations of the heater voltage. The electro thermal properties of the H₂ sensor were studied by finite element method (FEM). The sensing properties of the fabricated H₂ sensor as the change of electrical resistance were studied with respect to hydrogen concentration and temperature. Experimental results showed high sensor response and response time after application of the heater voltage. The sensing properties of the alloyed Pd/Ag thin film were more improved than those of pure palladium. The maximum sensor response (R_s) of the fabricated H₂ sensor was 14.26% for 1000 ppm H₂. The sensor response of the fabricated H₂ sensor showed linear behavior with the heater voltage (operating temperature) and positively corresponded with the hydrogen concentration.     

ANALYSIS OF SUBSTRATE EFFECTS ON THE RESPONSE OF PYROELECTRIC THIN-FILM INFRARED SENSORS

An infrared (IR) sensor with a lead titanate (PbTiO₃) thin film using the technology of microelectromechanical systems (MEMS) has been designed, fabricated, and developed. Anisotropic wet etching of Si(111) orientation in ethylenediamine/pyrocatechol (EDP) solution is used to process a micromachined cantilever beam for better thermal isolation and the array form available. The major IR-sensing part on the cantilever beam consists of a 5-nm PbTiO₃ layer deposited by RF sputtering, and a gold (Au) black layer evaporated as an IR radiation absorber, with the active cantilever dimensions of 200 x 100 x 5 mu m³ formed by the etching processes. The substrate effect on the performance of the IR sensor has been studied in both experimental and theoretical analysis. The cantilever structure exhibits much superior performance in both theory and experiment to that of a traditional IR-sensing bulk structure under incident radiation at a wavelength of 970 nm.     

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300°C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous growth regime to the mixed-phase (amorphous+microcrystalline) growth regime versus accumulated film thickness [the a→(a+μc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic layers by RF plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using the maximum possible flow ratio of H₂ to SiH₄ that can be sustained while avoiding the a→(a+μc) transition. More recent studies have suggested that a similar strategy is appropriate for optimization of p-type Si:H thin films. The simple phase diagrams can be extended to include in addition the thickness at which a roughening transition is detected in the amorphous film growth regime. It is proposed that optimization of a-Si:H in higher rate RF PECVD processes further requires the maximum possible thickness onset for this roughening transition.     

Feasibility of a fully autonomous wireless monitoring system for a wind turbine blade

Condition monitoring (CM) of wind turbine blades has significant benefits for wind farm operators and insurers alike. Blades present a particular challenge in terms of operations and maintenance: the wide range of materials used in their construction makes it difficult to predict lifetimes; loading is stochastic and highly variable; and access can be problematic due to the remote locations where turbines are frequently located, particularly for offshore installations. Whilst previous works have indicated that Micro Electromechanical Systems (MEMS) accelerometers are viable devices for measuring the vibrations from which diagnostic information can be derived, thus far there has been no analysis of how such a system would be powered. This paper considers the power requirement of a self-powered blade-tip autonomous system and how those requirements can be met. The radio link budget is derived for the system and the average power requirement assessed. Following this, energy harvesting methods such as photovoltaics, vibration, thermal and radio frequency (RF) are explored. Energy storage techniques and energy regulation for the autonomous system are assessed along with their relative merits. It is concluded that vibration (piezoelectric) energy harvesting combined with lithium-ion batteries are suitable selections for such a system.     

Hydrogen production by methanol reforming in a non-thermal atmospheric pressure microplasma reactor

On board reforming of hydrocarbons for fuel cell feed has become an attractive research topic due to the low energy densities of batteries. The implementation of a microplasma as a means for reforming the liquid fuel methanol is explored in this work. Hydrocarbon reforming is commonly accomplished through catalysis, but catalysts have a number of limitations such as poisoning, coking, coarsening, long start-up times and excessive costs. Published studies have shown the viability of plasma reforming but none have succeeded in achieving suitable system efficiencies for portable applications. Non-thermal microplasmas are particularly attractive for reforming due to their extremely high electron and power densities and the scale of microplasma devices make them well suited for portable applications. This study describes experimental microplasma reactors reforming methanol. The reactors are based on the microhollow cathode discharge (MHCD) structure fabricated with microelectromechanical systems (MEMS) fabrication techniques. Through modeling the reaction for all five experiments, conversions within the microchannel were found to be nearly 100%. Despite the variations in the five experiments due to input electrical power, flow rate and concentration, the model was validated in each test. The experiments discussed in this work show the promise of a portable, non-thermal microplasma reformer that generates hydrogen for fuel cells for portable power.     

Microscale combustion research for microthermophotovoltaic systems

A novel power MEMS concept, a micro thermophotovoltaic (TPV) system, is described in this work, which would use hydrogen as fuel and would be capable of delivering 1–10 W electrical power in a package less than 1 cubic centimeter in volume. A micro combustor is one of the most important components of a micro TPV system. A high and uniform temperature distribution along the wall of a micro combustor is required to get a high electrical power output. However, sustaining combustion in a MEMS‐sized combustor will be largely affected by the increased heat losses due to the high surface‐to‐volume ratio, which tends to suppress ignition and quench the reaction. In order to test the feasibility of combustion in micro devices and determine relevant factors affecting micro combustion, numerical and experimental work was carried out. Results indicated that a high and uniform temperature could be achieved along the wall of a flame tube. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 369–379, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20078     

Effects of Thermoelastic Damping on The Operation of The Ring Shape Anchored Contour Mode Disk Resonators

Thermoelastic damping is one of the most important energy loss mechanisms in MEMS resonators especially when the resonator is miniaturized to achieve higher frequencies. Based on thermal energy produced as the result of the resonator expansion and contraction, by considering the thermoelastic coupled equation, this article presents a solution to thermoelastic damping for previously demonstrated ring shape anchored contour mode disk RF MEMS resonator. This research proves that the thermoelastic damping has a negligible effect on the quality factor of the resonator. In addition the results mention that this effect becomes stronger when the surface to volume ratio is decreased for achieving higher frequency resonators. Obtained results reveal that the resonator could be utilized for ultra-low far-from-carrier phase noise oscillators.     

Degradation mechanism study of PTFE/Nafion membrane in MEA utilizing an accelerated degradation technique

Nafion membranes are widely used for commercial membrane electrode assemblies (MEAs) in proton exchange fuel cells (PEMFCs). The polytetrafluoroethylene (PTFE)/Nafion (PN) composite membrane has the advantages of being low in cost, high in mechanical strength, and does not swell excessively. This study focuses on the properties of PTFE/Nafion membranes and PTFE/Nafion MEAs by comparing the durability and performance of the PN MEAs to commercial Nafion 211 MEAs. In an accelerated degradation test (ADT), the characterization of PTFE/Nafion and Nafion MEAs were analyzed using in-situ electrochemical methods such as polarization curves, AC impedance, cyclic voltammetry (CV), and linear sweep voltammetry (LSV). The results demonstrate an increase in the internal resistance on the PTFE/Nafion MEA only. The three mechanisms behind this unique result were proposed to be: (a) Separation of the catalyst layer from the membrane due to creep deformation; (b) Separation of the outer Nafion layer film from the core PTFE/Nafion membrane due to creep deformation; (c) Degradation of the Nafion plane (or Nafion dissolution) from the PTFE surface.     

Water management in membrane electrolysis and options for advanced plants

The development of polymer electrolyte membrane electrolysis (PEMEL) is driven by increasing performance to decrease the costs of electrolysis systems. One option for increasing power density is decreasing the Ohmic losses within the cell. This can be enabled by using thinner membranes, although the disadvantage of thin membranes is their lower diffusion resistivity for water, hydrogen and oxygen what influences the efficiency and the operating conditions. In this paper the water transport and the Ohmic resistance of catalyst coated membranes with different thickness are analyzed. The disadvantage of high water permeability in thin membranes can be used to change the feed configuration in stacks and systems. It is possible to feed the electrolysis only from the cathode, which simplifies the mass transport (single phase) in the anode's porous transport layer and reducing stack and system dimensions, as well as costs.     

Operation of a proton-exchange membrane fuel cell under non-humidified conditions using thin cast Nafion membranes with different gas-diffusion media

Thin membranes in polymer electrolyte membrane fuel cells (PEMFCs) enhance the back diffusion of water from cathode to anode and allow operation of the PEMFC under dry conditions. In this work, thin cast Nafion membranes are prepared to operate the PEMFC under non-humidified conditions at various temperatures. Also, the effect of gas-diffusion media (GDM) on cell performance is examined using two different GDM that have distinct physical properties. Single cells with thin cast membranes provide better performance than those with commercially available Nafion 112. This improvement is due to better back-diffusion of water and lower membrane resistance. The performance of cell using GDM with low porosity is superior to that of a cell using GDM with high porosity. The fuel cell can be operated successfully under non-humidified conditions with a thin cast membrane and low porosity GDM.     

Phosphorus-doped glass proton exchange membranes for low temperature direct methanol fuel cells

Phosphorus-doped silicon dioxide thin films were used as ion exchange membranes in low temperature proton exchange membrane fuel cells. Phosphorus-doped silicon dioxide glass (PSG) was deposited via plasma-enhanced chemical vapor deposition (PECVD). The plasma deposition of PSG films allows for low temperature fabrication that is compatible with current microelectronic industrial processing. SiH₄, PH₃ and N₂O were used as the reactant gases. The effect of plasma deposition parameters, substrate temperature, RF power, and chamber pressure, on the ionic conductivity of the PSG films is elucidated. PSG conductivities as high as 2.54 × 10⁻⁴ S cm⁻¹ were realized, which is 250 times higher than the conductivity of pure SiO₂ films (1 × 10⁻⁶ S cm⁻¹) under identical deposition conditions. The higher conductivity films were deposited at low temperature, moderate pressure, limited reactant gas flow rate, and high RF power.     

Micro thermoelectric cooler: Planar multistage

A suspended, planar multistage micro thermoelectric (TE) cooler is designed using thermal network model to cool MEMS devices. Though the planar (two-dimensional) design is compatible with MEMS fabrication, its cooling performance is reduced compared to that of a pyramid (three-dimensional) design, due to a mechanically indispensable thin dielectric substrate (SiO₂) and technical limit on TE film thickness. We optimize the planar, six-stage TE cooler for maximum cooling, and predict ΔT_max = 51 K with power consumption of 68 mW using undoped, patterned 4–10 μm thick co-evaporated Bi₂Te₃ and Sb₂Te₃ films. Improvement steps of the planar design for achieving cooling performance of the ideal pyramid design are discussed. The predicted performance of a fabricated prototype is compared with experimental results with good agreements.     

Energy harvesting: State-of-the-art

This paper presents a brief history of energy harvesting for low-power systems followed by a review of the state-of-the-art of energy harvesting techniques, power conversion, power management, and battery charging. The advances in energy harvesting from vibration, thermal, and RF sources are reviewed as well as power management techniques. Examples of discrete form implementation and integrated form implementation using microelectromechanical systems (MEMS) and CMOS microelectronic processes are also given. The comparison between the reviewed works concludes this paper.     

基于IEC61970标准的微电网能量管理系统

微电网将成为未来智能电网的有力补充和有效支撑,其能量管理系统MEMS具有广阔的应用前景.初步设计了基于IEC61970等国际标准的MEMS的系统平台,从而实现分布式电源能量的最优利用.详细介绍了微电网中CIM的扩展建模,并根据微电网的特点设计了MEMS高级应用软件的功能.