Power MEMS Research Articles

Fabrication of Microhotplates Based on Laser Micromachining of Zirconium Oxide

We present a novel approach to the fabrication of MEMS devices, which can be used for gas sensors operating in harsh environment in wireless and autonomous information systems. MEMS platforms based on ZrO2/Y2O3 (YSZ) are applied in these devices. The methods of ceramic MEMS devices fabrication with laser micromachining are considered. It is shown that the application of YSZ membranes permits a decrease in MEMS power consumption at 4500C down to ∼75 mW at continuous heating and down to ∼ 1 mW at pulse heating mode. The application of the platforms is not restricted by gas sensors: they can be used for fast thermometers, bolometric matrices, flowmeteres and other MEMS devices working under harsh environmental conditions.

Terrain Classification for Track-driven Agricultural Robots

A long-term goal of agricultural automation is to deploy intelligentrobots to facilitate labor-intensive tasks such as crop care or selective harvesting with minimum human supervision. To achieve this goal, the agricultural robots must be able to adapt themselves in response to various terrain conditions. The reason is that the terrain characteristics can jeopardize the performance of a robotin carrying out a taskor even causing it being trapped in the field. The aim of this work is to evaluate the effectiveness of using an intelligent algorithm, i.e. support vector machine (SVM) in recognizing various terrain conditions in an agricultural field. For this purpose, asmall tracked-driven mobile robot together witha terrain test bed has been developed. The terrain test bed emulates three types of terrain conditions, i.e. sand, gravel and vegetation. The tracked-driven robot is embedded with a low power MEMS accelerometer for measuring vibration signals resulted from the track-terrain interaction. An experimental study was conducted usinga SVMtrained with three different kernel functions, i.e. linear function, polynomial function and radial basis function (RBF). The results showed that the SVM can recognize different terrain conditions effectively. This work contributes to devising a self-adaptive agricultural robot in coping with changing terrain conditions.

A Tunable Palladium-based Capacitive MEMS Hydrogen Sensor Performing High Dynamics, High Selectivity and Ultra-low Power Sensing

We developed an ultra-low power MEMS hydrogen (H2) sensor performing high dynamics together with high sensitivity up to the Lower Explosive Limit (LEL), i.e. 4% vol. H2 in dry air. The architecture consists of Al clamped-clamped beams (700 nm- thick, 800μm-total width, 140 or 240μm-length) with both ends made of a Pd/Al bimorph (200/700 nm-thick) on a quarter of its length, acting as actuators, released above a split-bottom electrode. The initial stress defines the initial deflection of the structure while the Pd-hydriding induces compressive stress variation in the Pd actuator layer and therefore the membrane deflection. A capacitive transduction is used to continuously measure the gap variation due to the H2 absorption and hydriding kinetic. Results show a fast response time of less than 5 s for an effective concentration of about 0.2% vol. H2 in dry air or N2 mixture, with no cross sensitivity.

Ultra-low Power MEMS Activity Sensor for Wireless Health Monitoring System

Ultra-low Power MEMS Activity Sensor for Wireless Health Monitoring System

Full ceramic micro solid oxide fuel cells: towards more reliable MEMS power generators operating at high temperatures

A full ceramic μSOFC integrated in silicon is presented here for the first time, able to operate up to 750 °C.

Porous La0.6Sr0.4CoO3−δ thin film cathodes for large area micro solid oxide fuel cell power generators

Porous La0.6Sr0.4CoO3−δ thin films were fabricated by pulsed laser deposition for being used as a cathode for micro solid oxide fuel cell applications as MEMS power generators. Symmetrical La0.6Sr0.4CoO3−δ/yttria-stabilized zirconia/La0.6Sr0.4CoO3−δ free-standing membranes were fabricated using silicon as a substrate. A novel large-area membrane design based on grids of doped-silicon slabs was used. Thermo-mechanical stability of the tri-layer membranes was ensured in the intermediate range of temperatures up to 700 °C. In-plane conductivity of ca. 300 S cm−1 was measured for the cathode within the whole range of application temperatures. Finally, area specific resistance values below 0.3 Ω cm2 were measured for the cathode/electrolyte bi-layer at 700 °C in the exact final micro solid oxide fuel cell device configuration, thus presenting La0.6Sr0.4CoO3−δ as a good alternative for fabricating reliable micro solid oxide fuel cells for intermediate temperature applications.

Modeling and Analysis of Micro Hybrid Gas Spiral-Grooved Thrust Bearing for Microengine

The micro hybrid spiral-grooved thrust bearing is a promising candidate to support the rotating elements in power MEMS devices such as micro gas turbine engines. However, the realization of hybrid thrust bearings has encountered a number of technical challenges due to the very high rotating speed and DN number (the product of the inner diameter and the rotational speed of the bearing, mm · rpm) to achieve high power density, the super thin gas film between rotors and thrust pad, and the relative large fabrication uncertainties according to the imperfection of the fabrication technology. In this paper, the configuration of a micro hybrid spiral-grooved thrust bearing for power MEMS is designed, and the steady and dynamic characteristics of this kind of bearing are then analyzed comprehensively, with the consideration of both the rarefaction effects and the influence of potential microfabrication defects. The nonlinear equations of molecular gas-film lubrication describing the gas rarefaction effects in a micro hybrid bearing are discretized by the finite volume method and solved by the Newton–Raphson techniques. The small perturbation technique is employed to study the dynamic behavior of a micro hybrid bearing. The results show that the micro hybrid thrust bearing exhibits better steady-state and dynamic performance than the existing micro hydrodynamic and hydrostatic bearings and that the hybrid bearings are likelier to be stable than their hydrodynamic counterparts, especially when the frequency number is high. The load capacity of the micro hybrid bearing increases slightly with the number of orifices and gradually with the diameter ratio of the orifice. The microfabrication defects of clogged orifices could lessen the load capacity and the dynamic coefficients of the hybrid thrust bearing. The model developed in this paper can serve as a useful tool to provide insight into micro hybrid gas thrust bearing-rotor systems.

Influence of Carbonization Conditions in Hydrogen Poor Ambient Conditions on the Growth of 3C-SiC Thin Films by Chemical Vapor Deposition with a Single-Source Precursor of Hexamethyldisilane

This paper describes the characteristics of cubic silicon carbide (3C-SiC) films grown on a carbonized Si(100) substrate, using hexamethyldisilane (HMDS, Si2(CH3)6) as a safe organosilane single precursor in a nonflammable H2/Ar (H2 in Ar) mixture carrier gas by atmospheric pressure chemical vapor deposition (APCVD) at 1280°C. The growth process was performed under various conditions to determine the optimized growth and carbonization condition. Under the optimized condition, grown film has a single crystalline 3C-SiC with well crystallinity, small voids, low residual stress, low carrier concentration, and low RMS. Therefore, the 3C-SiC film on the carbonized Si (100) substrate is suitable to power device and MEMS fields. This paper describes the characteristics of cubic silicon carbide (3C-SiC) films grown on a carbonized Si(100) substrate, using hexamethyldisilane (HMDS, Si2(CH3)6) as a safe organosilane single precursor in a nonflammable H2/Ar (H2 in Ar) mixture carrier gas by atmospheric pressure chemical vapor deposition (APCVD) at 1280°C. The growth process was performed under various conditions to determine the optimized growth and carbonization condition. Under the optimized condition, grown film has a single crystalline 3C-SiC with well crystallinity, small voids, low residual stress, low carrier concentration, and low RMS. Therefore, the 3C-SiC film on the carbonized Si (100) substrate is suitable to power device and MEMS fields.

Non-linear dynamic analysis of the ultra-short micro gas journal bearing-rotor systems considering viscous friction effects

Due to the micro-fabrication limitations and the low thickness of the silicon wafer, the length-to-diameter ratio (L/D) of the gas journal bearings in Power MEMS is about one order lower than that of the conventional bearings, which suggests that the viscous friction force in the micro-bearing is comparable to the load capacity. The effects of viscous friction force on non-linear dynamic characteristics of the ultra-short micro-bearing-rotor system are studied in this paper. The molecular gas-film lubrication model, which valid for arbitrary Knudsen numbers, is systematically coupled with the rotor kinetic equations and solved simultaneously to investigate the non-linear dynamic behavior of the system. The center orbits, phase portraits, Poincare maps, and FFT spectra of the system response at different L/D ratio, rotor mass, and bearing number, and the corresponding bifurcation diagrams for cases of ignoring and considering viscous friction force are inspected and compared. The results indicate that, if the viscous friction force is not taken into account in the case of low L/D ratio, the low-frequency large-amplitude self-excited whirl motion will be predicted as the increase of the rotor mass and the bearing number. However, when the viscous friction force is included in the non-linear dynamic model, the rotor motion becomes more stable under the same conditions, as the synchronous motion with smaller amplitude prevails.

A lumped model with phase analysis for inline RF MEMS power sensor applications

This paper proposes a microwave lumped model with attenuation and phase mechanisms for GaAs monolithic microwave integrated circuits (MMIC)-compatible inline RF MEMS power sensors, in order to evaluate reflection and insertion losses and phase characteristics of the sensors. The purpose of the power sensors is measuring a certain percentage of the incident RF power coupled by a MEMS membrane, and their model is achieved based on the microwave port network theory. During the inline power detection, an undesired phase shift is generated by the MEMS membrane. Using this model, the additional phase shift is analyzed to show the phase change of the detected RF signal, with the effects of the membrane height and width on the phase. The three power sensors with the MEMS membrane 50μm, 100μm and 147μm in width are measured to validate the lumped model up to 13.5GHz, and the sources of small errors are discussed. Furthermore, experiments show that the inline RF MEMS power sensors with the membrane 100μm and 147μm in width have resulted in average phase changes of about 14% and 35% over the entire frequency compared with that 50μm above the same coplanar waveguide (CPW) lines, respectively.

Development of low frequency, insulating thick diaphragms for power MEMS applications

Major challenges of micro thermal machines are the thermal insulation and mechanical tolerance in the case of sliding piston. Replacing piston by membrane in microengines can alleviate the latter and lead to planar architectures. However, the thermal isolation would call for very thick structures which are associated to too high resonant frequencies which are detrimental to the engine performances. A thermal and mechanical compromise is to be made. On the contrary, based on fluid structure interaction, using an incompressible fluid contained in a cavity sealed by deformable diaphragm it would be possible to design a thick, low frequency insulating diaphragm. The design involves a simple planar geometry that is easy to manufacture with standard microelectronics methods. An analytical fluid-structure model is proposed and theoretically validated. Experimental structures are realized and tested. The model is in agreement with the experimental results. A dimensionless model is proposed to design hybrid fluid structures for micromachines.

A comparative study of misalignment detection using a novel Wireless Sensor with conventional Wired Sensors

The advancement in low cost and low power MEMS sensors makes it possible to develop a cost-effective wireless accelerometer for condition monitoring. Especially, the MEMS accelerometer can be mounted directly on a rotating shaft, which has the potential to capture the dynamics of the shaft more accurately and hence to achieve high monitoring performance. In this paper a systematic comparison of shaft misalignment detection is conducted, based on a bearing test rig, between the wireless sensor measurement scheme and other three common sensors: a laser vibrometer, an accelerometer and a shaft encoder. These four sensors are used to measure simultaneously the dynamic responses: Instantaneous Angular Speed (IAS) from the encoder, bearing house acceleration from the accelerometer, shaft displacements from the laser vibrometer and angular acceleration from the wireless sensor. These responses are then compared in both the time and frequency domains in detecting and diagnosing different levels of shaft misalignment. Results show the effectiveness of wireless accelerometer in detecting the faults.

A three-channel thermoelectric RF MEMS power sensor for GaAs MMIC applications

This paper presents an X-band three-channel thermoelectric radio frequency microelectromechanical systems (RF MEMS) power sensor with three inputs and one output, in order to measure the power with up to three RF signals. This power sensor is based on the principle of power–heat–voltage conversion and accomplished with the GaAs monolithic microwave integrated circuit (MMIC) technology. The three inputs are used to transmit RF signals over coplanar waveguide (CPW) lines and the one output collects the thermovoltage. To show good electromagnetic and thermal isolations, signal crosstalks for the three-input and one-output power sensor are quantified by the S-parameters simulation and the power handling measurement, respectively. In addition, the measured power sensor has resulted in three input reflection coefficients of less than −21.5dB for frequencies up to 12GHz. Experiments demonstrate the sensor has the good linearity of the power with respect to the thermovoltage at X-band, with sensitivities of about 53.1μVmW−1, 56.3μVmW−1, 58.9μVmW−1 for the three inputs at 12GHz, respectively.

Fabrication and analysis of high-performance piezoelectric MEMS generators

In this paper, we have designed a fabrication process for microgenerators by bonding a piezoelectric ceramic Pb(Zr,Ti)O3 (PZT) plate to a silicon on insulator (SOI) wafer. The key techniques of the process include the low-temperature bonding technique using conductive epoxy resin, thinning of the bulk PZT using mechanical lapping and wet-etching combined method, and the micromachining of bulk ceramics by dicing. Through the development and optimization of the process, a piezoelectric MEMS power generator array was successfully fabricated. The typical device is selected to characterize the output performance of the microgenerators, while the composite beam dimension of PZT and silicon layer is about 3080 µm × 800 µm × 31 µm and the dimension of Ni proof mass is about 900 µm × 800 µm × 450 µm. The experimental results show that the output voltage, output power and power density of this device are 2.72 VP-P, 11.56 µW and 28 856.7 µW cm−3 at the resonant frequency of 514.1 Hz when it matches an optimal resistive load of 70 kΩ under the excitation of 1g acceleration. The output performance of this device is higher, compared with that of other reported MEMS power generators, which demonstrates that this novel technique has great potential to fabricate high-performance piezoelectric MEMS energy harvester.

An 8–12 GHz microwave frequency detector based on MEMS power sensors

An 8–12 GHz microwave frequency detector is presented, which is based on MEMS power sensors, by sensing the phase delay of two separated signals in two different tracks. The phase delay, proportional to frequency of the input microwave signal, leads to variation of the output power. Two different kinds of MEMS power sensors, a thermoelectric microwave power sensor and a MEMS capacitive power sensor, are adopted to measure the variation. The former is used for measuring the lower power while the latter for the higher one. The presented detector has been designed, optimized and fabricated successfully using the GaAs MMIC process. For the microwave signals of 10, 15 and 20 dBm, the measured result shows that the output thermovoltage decreases with the frequency based on the thermoelectric microwave power sensor and sensitivities of the detector better than 0.033, 0.110 and 0.343 µV MHz−1 are measured, respectively. Furthermore, the sensitivity of detector is directly proportional to the power of the input microwave signal. For the microwave signals of 24 dBm, the measured result shows that the capacitance decreases with the frequency based on the MEMS capacitive power sensor and the sensitivity of 0.013 fF MHz−1 is measured. The frequency measurement demonstrates the validity of the presented design.

GaAs MMIC fabrication for the RF MEMS power sensor with both detection and non-detection states

This paper presents the GaAs MMIC fabrication for the coupled RF MEMS power sensor with an impedance matching structure for improving the microwave performance, a capacitance compensating structure for obtaining the wideband response, and two shunt capacitive MEMS switches for achieving the conversion of the detection and non-detection state. In the fabrication process, we focus on discussing three key techniques of the fabrication such as the release of the MEMS membrane and switches, the protection of thermopiles and the process of the backside cavity, and propose the effects of these techniques on the power sensor. The validity of the fabrication is analyzed by the experiments, with the reflection and insertion losses of less than −17dB and 0.8dB in the detection state as well as that of less than −19dB and 0.6dB in the non-detection state, and the good power handing capability. Furthermore, the measured resistance of the thermopiles of the power sensor is about 172kΩ, which corresponds to the calculated signal-to-noise ratio of 2.5×106W−1.

Integrated micro fuel cell with on-demand hydrogen production and passive control MEMS

An ever increasing demand for packaging more energy on-board to meet the needs of power hungry microsystems is driving the miniaturization of power generators. We report a fully integrated Power MEMS, in the 10-μL size, designed to deliver high energy and power densities. On-board hydrogen production and an efficient control scheme that facilitates integration with a fuel cell membrane electrode assembly are key elements for micro energy conversion. A millimeter-scale reactor produces hydrogen by hydrolysis of CaH2 and LiAlH4, to yield energy densities of the order of 200 Whr/L. A passive microfluidic control scheme, incorporating surface tension to pump water in a microchannel for hydrolysis and microvalve control using hydrogen backpressure, facilitates delivery and regulation and eliminates bulky auxiliaries that consume parasitic power. We tested the ability of this control scheme to improve uniformity of power delivery during long periods of lower demand, with fast switching to mass transport regime on the order of seconds, and realized peak power density of up to 391.85 W/L. Prototypes have been tested for duty periods from 2–48 h, with multiple switching of power demand in order to establish performance across multiple regimes. Critical to the realization of the integrated power MEMS, and its energy and power density, are effects of water transport and byproduct hydrate swelling on hydrogen production in the microreactor. While CaH2 showed superior hydrogen release kinetics that enhances power density, LiAlH4 provided greater energy density due to reduced byproduct expansion that permitted increased hydrogen production.

Piezoelectric MEMS generator based on the bulk PZT/silicon wafer bonding technique

AbstractA MEMS‐based piezoelectric energy harvester was fabricated by bonding a piezoelectric ceramic Pb (Zr,Ti)O3 (PZT) plate to silicon wafer. The generator structure of a trapezoidal shape composite cantilever beam with nickel metal mass was designed. The novel fabrication techniques to obtain PZT thick film with excellent piezoelectric properties, including a low‐temperature PZT—Si bonding using epoxy resin as the intermediate adhesive layer and thinning of the bulk PZT ceramics by mechanical lapping and polishing method, were carried out. A piezoelectric MEMS power generator array was successfully fabricated and the measurement results for a microgenerator with typical dimension were also presented. The test result shows that this novel technique has great potential to fabricate high‐performance piezoelectric MEMS energy harvester.

A microdevice with large deflection for variable-ratio RF MEMS power divider applications

In this paper, we present the design and fabrication of a large deflection MEMS actuator directly integrated as a variable-ratio radio frequency (RF) power divider in a waveguide. The device is based on a tilting microplate suspended by SU-8 springs and driven by a vertical comb drive actuator. The fabricated device is characterized by laser Doppler vibrometer and white light interferometer measurements. The dynamic measurement confirms the resonance frequency of the torsional mode being around 370 Hz, from which the spring stiffness has been extracted and used for static modeling of the device. The fabricated microplate devices achieved deflection angles of 5.9° with a dc actuation voltage of 30 V. First RF transmission measurements show good agreement with results from electromagnetic field simulations. A variable power split ranging from equal division up to a ratio of more than 1:2 is measured at 82.5 GHz whilst keeping the amount of dissipated power below 25%. This is the first reported actuated RF MEMS device integrated in a metal waveguide operating at such high frequency.

Study on the Performance of Gas Journal Bearings for Power MEMS

Gas journal bearings, which are used to support radial loads in a rotating machine, have somewhat unusual requirements in Power MEMS deriving from the extremely shallow structures. With the reference Knudsen number being included, the modified Reynolds equation for gas journal bearings based on Burgdorfer’s first order slip boundary condition is put forward. The boundary condition for modified Reynolds equation is given. The numerical method is employed to solve the modified Reynolds equation to obtain the pressure profiles, load capacities and attitude angles of gas journal bearings for Power MEMS under different reference Knudsen numbers and eccentricity ratios. Numerical analysis shows that the pressure profiles and non-dimensional load capacities decrease obviously with the reference Knudsen number increasing, and the attitude angle changes conversely. Moreover, when the eccentricity ratio is smaller, the effect of gas rarefaction on the attitude angle is less.