Table of contents

Welcome to this special section of the Journal of Micromechanics and Microengineering (JMM). This section, co-edited by myself and by Professor Jeffrey Lang of the Massachusetts Institute of Technology, contains expanded versions of selected papers presented at the Power MEMS meeting held in Atlanta, GA, USA, in December of 2012.

Professor Lang and I had the privilege of co-chairing Power MEMS 2012, the 12th International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications. The scope of the PowerMEMS series of workshops ranges from basic principles, to materials and fabrication, to devices and systems, to applications. The many applications of power MEMS (microelectromehcanical systems) range from MEMS-enabled energy harvesting, storage, conversion and conditioning, to integrated systems that manage these processes.

Why is the power MEMS field growing in importance? Smaller-scale power and power supplies (microwatts to tens of watts) are gaining in prominence due to many factors, including the ubiquity of low power portable electronic equipment and the proliferation of wireless sensor nodes that require extraction of energy from their embedding environment in order to function. MEMS manufacturing methods can be utilized to improve the performance of traditional power supply elements, such as allowing batteries to charge faster or shrinking the physical size of passive elements in small-scale power supplies. MEMS technologies can be used to fabricate energy harvesters that extract energy from an embedding environment to power wireless sensor nodes, in-body medical implants and other devices, in which the harvesters are on the small scales that are appropriately matched to the overall size of these microsystems. MEMS can enable the manufacturing of energy storage elements from nontraditional materials by bringing appropriate structure and surface morphology to these materials as well as fabricating the electrical interfaces required for their operation and interconnection. Clearly, the marriage of MEMS technologies and energy conversion is a vital application space; and we are pleased to bring you some of the latest results from that space in this special section.

Approximately 130 papers were presented at the Power MEMS 2012 conference. From these, the 20 papers you have before you were selected based on paper quality and topical balance. As you can see, papers representing many of the important areas of power MEMS are included: energy harvesters using multiple transduction schemes; MEMS-based fabrication of compact passive elements (inductors, supercapacitors, transformers); MEMS-enabled power diagnostics; MEMS-based batteries; and low power circuitry adapted to interfacing MEMS-based harvesters to overall systems. All of the papers you will read in this special section comprise substantial expansion from the proceedings articles and were reviewed through JMM's normal reviewing process.

Both Professor Lang and I hope that you will share our enthusiasm for the field of power MEMS and that you will find this special section of JMM exciting, interesting and useful.

 Sincerely,

 Mark G Allen

This paper focuses on improving the past performance of ear canal energy harvesting by developing a piezo-earpiece prototype and a multiphysics finite element method to integrate more complex and effective piezoelectric structures. The proposed handmade piezo-earpiece prototype is a custom-moulded soft earpiece wrapped in a layer of flexible polyvinylidene fluoride. A finite element model is developed in order to estimate the energy harvesting capability of the piezoelectric layer. A prototype of the energy harvester is fabricated and tested. The experimental results show that the proposed device is able to generate 44 μJ of energy corresponding to an average value of 70 μW per jaw opening and closing cycle. Simulation of energy harvesting based on the finite element method is validated by the experimental results, thus confirming it as a versatile tool to model, optimize and evaluate the performance of other types of flexible piezoelectric structures for in-ear energy harvesting applications.

This paper presents a compact power electronic device, called an 'electrodynamic transformer (ET)', that transfers electrical power between isolated circuits through electrodynamically coupled vibrations of a mechanical beam structure. Background motivating factors are discussed, and an equivalent circuit model of the ET is presented. A microscale (10 mm³) ET is designed, fabricated and characterized, achieving ∼40% maximum efficiency. Taking advantage of the unique circuit topology of the ET equivalent circuit model, a dc/ac power inverter is then implemented with only two external components, a MOSFET and a capacitor.

In this paper, a piezoelectric cantilever with a Helmholtz resonator (HR) is proposed as a sound pressure sensor that generates a sufficiently large output voltage at a specific frequency without a power supply to drive the sensing element. A Pb (Zr, Ti) O₃ (PZT) cantilever with dimensions of 1500 µm × 1000 µm × 2 µm is designed so that its mechanical resonance frequency agrees with the target frequency. When sound pressure is applied at the target frequency, a large piezoelectric voltage can be obtained due to a high amplification ratio. Additionally, the PZT cantilever is combined with a HR whose resonant frequency is designed to be equal to that of the cantilever. This multiplication of two resonant vibration systems can generate detectable signals by sound pressures of several Pascals. The fabricated sensor generated a piezoelectric voltage of 13.4 mV Pa⁻¹ at the resonant frequency of 2.6 kHz. Furthermore, the fabricated sensor performed as an electrical trigger switch when a sound pressure of 2 Pa was applied at the resonant frequency.

This paper demonstrates an impact-triggered thermoelectric generator that uses latent heat liberated from the crystallization of supersaturated sodium acetate trihydrate (SSAT). A volume of SSAT is encapsulated in a polydimethylsiloxane chamber and capped with a steel plate. The device triggers when an external impact is applied to the steel plate, leading to an exothermic crystallization process. The heat creates a thermal gradient across a thermoelectric module which in turn generates electrical output. A maximum hot-junction temperature of 58 °C can be achieved, resulting in an instantaneous output power of 2.08 mW across 185 Ω load using a 3.3×2.5×1 mm³ commercially available Bi₂Te₃ thermoelectric module with 290 junctions.

A high-output-voltage micro-thermoelectric generator (µTEG) has been developed by fabricating thermocouples having a high aspect ratio (HAR) with a high integration density. They have been made by a novel and simple fabrication method, in which thermoelectric nanopowders are filled in a photosensitive glass mold by using aerosol deposition. It is followed by hot isostatic pressing to improve the thermoelectric property. This method has the possibility of increasing the aspect ratio of thermocouples drastically while increasing their toughness. We have fabricated thermocouples with an aspect ratio of 3.5 and a high integration density of 620 TCs cm^–2. Their Seebeck coefficient and electrical resistivity are 290 µV K^–1 and 1.5 mΩ cm, respectively, which make them as good as the thermocouples fabricated by hot pressing. By using the method, we have fabricated a µTEG chip having an area of 25 mm² in which 56 thermocouples are arranged in an area of 9 mm². The µTEG reaches a thermal resistance of 17.1 K W^–1, output voltage efficiency of 0.16 V cm^–2 K^–1 and output power efficiency of 9.3 µW cm^–2 K^–2. These HAR thermocouples have an advantage for energy harvesting from a human body because they can result in a high temperature difference because of their high thermal resistance.

We report microfabricated toroidal inductors with nanolaminated ferromagnetic metallic cores for chip-scale, high-power switching converters. The fabrication process of the toroidal inductor is based on individual manufacturing of partial windings (i.e. bottom and vertical conductors) and nanolaminated magnetic core, and integrating them by means of a drop-in approach. The nanolaminated ferromagnetic metallic cores presented in this paper consist of many multilayers of electrodeposited CoNiFe films, each layer with sub-micron thickness, with a total core thickness exceeding tens of microns. The beneficial magnetic properties (i.e. high saturation flux density and low coercivity) of CoNiFe alloys are well suited for chip-scale inductors as they achieve both large energy storage capacity as well as minimized volumetric core losses at high operating frequencies due to their nanolaminated structure. A drop-in integration approach, introduced to combine the microfabricated toroidal inductor windings with the magnetic cores, allows ease of integration. An advantage of this hybrid approach over monolithic fabrication in this application is the potential use of a wide variety of core materials, both microfabricated and bulk-fabricated, and which may or may not ultimately be CMOS-compatible. Exploiting this drop-in approach, 30-turn- and 50-turn-toroidal inductors integrated with nanolaminated CoNiFe cores, having 10 mm outer diameter and 1 mm thickness, have been successfully developed. Both types of inductors exhibit inductances higher than 1 µH at frequencies up to tens of MHz, showing ten times the inductance of an air core device with the same nominal geometry. The peak quality factor of the 30-turn-toroidal inductor reaches 18 at 1 MHz.

Resonant-based vibration harvesters have conventionally relied upon accessing the fundamental mode of directly excited resonance to maximize the conversion efficiency of mechanical-to-electrical power transduction. This paper explores the use of parametric resonance, which unlike the former, the resonant-induced amplitude growth, is not limited by linear damping and wherein can potentially offer higher and broader nonlinear peaks. A numerical model has been constructed to demonstrate the potential improvements over the convention. Despite the promising potential, a damping-dependent initiation threshold amplitude has to be attained prior to accessing this alternative resonant phenomenon. Design approaches have been explored to passively reduce this initiation threshold. Furthermore, three representative MEMS designs were fabricated with both 25 and 10 μm thick device silicon. The devices include electrostatic cantilever-based harvesters, with and without the additional design modification to overcome initiation threshold amplitude. The optimum performance was recorded for the 25 μm thick threshold-aided MEMS prototype with device volume ∼0.147 mm³. When driven at 4.2 ms⁻², this prototype demonstrated a peak power output of 10.7 nW at the fundamental mode of resonance and 156 nW at the principal parametric resonance, as well as a 23-fold decrease in initiation threshold over the purely parametric prototype. An approximate doubling of the half-power bandwidth was also observed for the parametrically excited scenario.

High-surface area, three-dimensional (3D) microstructures are designed and fabricated by the sequential electroplating of sacrificial and structural layers in a photoresist mold. A conformal coating of electrochemically deposited nickel hydroxide (Ni(OH)₂) films on these MEMS-enabled multilayer structures enabled the formation of functional electrodes for electrochemical energy storage devices. The characterization of the electrodes is performed galvanostatically at various charge and discharge rates. Electrodes with a varying number of laminations are shown to yield areal capacities from 0.1 to 5.2 mAh cm^–2. Power characteristics of the electrodes are determined by applying ultra-high charge rates of up to 120 C. At this high charge rate, the electrode is able to deliver 90% of its capacity.

Solenoid configuration of micro inductor, which has advantages of high quality factor and low loss, is needed in micro energy and power electronics applications but it is difficult to prepare using conventional microfabrication processes. In this work, we present a new microelectromechanical systems-based technology of micro solenoid-type inductor by a newly developed cylindrical projection photolithography method. Direct electroplating process of copper film on coil patterns was also successfully developed for achieving thick windings so that thick photoresist-based electroplating molds are not needed. Micro solenoid-type inductor prototypes of the winding pitch of about 40 µm, the winding number of 20 and 50, and the winding thickness of about 14 µm, were successfully fabricated on a 1 mm diameter glass capillary. The prepared 20-turn and 50-turn micro inductors were of inductance of 69 and 205 nH at 30 MHz, respectively.

This paper presents a MEMS energy harvesting device which is able to generate power from two perpendicular ambient vibration directions. A CYTOP polymer is used both as the electret material for electrostatic transduction and as a bonding interface for low-temperature wafer bonding. The device consists of a four-wafer stack, and the fabrication process for each wafer layer is described in detail. All the processes are performed at wafer scale, so that overall 44 devices can be fabricated simultaneously on one 4-inch wafer. The effect of fabrication issues on the resonant frequency of the device is also discussed. With a final chip size of about 1 cm², a power output of 32.5 nW is successfully harvested with an external load of 17 MΩ, when a harmonic vibration source with an RMS acceleration amplitude of 0.03 g (∼0.3 m s⁻²) and a resonant frequency of 179 Hz is applied. These results can be improved in an optimized design.

Supercapacitors (SCs) as energy storage devices are advantageous in their rapid charge/discharge capabilities and their immense charge storage capacity. Two important components of a SC are the electrically conductive electrodes (anode and cathode) and an electrically non-conductive separator between the two electrodes. This paper details a fabrication process for nanofibrous carbon electrodes and a nanoporous polymer separator using all SU-8 based electrospinning and post electrospinning processes, such as lithographical patterning, conversion of the nanofibrous polymer to carbon structures using heat treatment (carbonization) and their assembly to complete a SC. The process produces immensely porous electrodes with good conductivity; it is scalable and economical compared with the carbon nanotube electrode approach. High throughput tube nozzle electrospinning for nanofiber (NF) production and its photolithographical patterning have been employed to facilitate manufacturability. The dependence of the NF morphology on the carbonization temperatures is studied. Also, SC testing and characterization are discussed.

This paper presents a complete, self-contained energy harvesting system composed of a magnetic energy harvester, an input-powered interface circuit and a rechargeable battery. The system converts motion from daily human activities such as walking, jogging, and cycling into usable electrical energy. By using an input-powered interface circuit, the system requires no external power supplies and features zero standby power when the input motion is too small for successful energy reclamation. When attached to a person's ankle during walking, the 100 cm³ system prototype is shown to charge a 3.7 V, 65 mAh lithium-ion polymer battery at an average power of 300 µW. The design and testing of the system under other operating conditions are presented herein.

This paper presents three-dimensional (3D) micro supercapacitors with thick interdigital electrodes supported and separated by SU-8. Nanoporous carbon materials including graphene and activated carbon (AC) are used as active materials in self-supporting composites to build the electrodes. The SU-8 separators provide mechanical support for thick electrodes and allow a considerable amount of material to be loaded in a limited footprint area. The prototypes have been accomplished by a simple microelectromechanical systems (MEMS) fabrication process and sealed by polydimethylsiloxane (PDMS) caps with ionic liquid electrolytes injected into the electrode area. Electrochemical tests demonstrate that the graphene-based prototype with 100 µm thick electrodes shows good power performance and provides a considerable specific capacitance of about 60 mF cm⁻². Two AC-based prototypes show larger capacitance of 160 mF cm⁻² and 311 mF cm⁻² with 100 µm and 200 µm thick electrodes respectively, because of higher volume density of the material. The results demonstrate that both thick 3D electrode structure and volume capacitance of the electrode material are key factors for high-performance micro supercapacitors, which can be potentially used in specific applications such as power suppliers and storage components for harvesters.

An all-solid electrochemical supercapacitor has been developed using a nanostructured nickel and titanium nitride template that is coated with ruthenium oxide by atomic layer deposition (ALD). The electrode morphology was based on a high surface area biotemplate of genetically modified Tobacco mosaic virus. The biotemplate automatically self-assembles at room temperature in aqueous solution. Nafion® perfluorosulfonate ionomer dispersion was cast on the electrodes and used as a solid proton-conducting electrolyte. A 5.8 F g⁻¹ gravimetric capacity (578 µF cm⁻² based on footprint) was achieved in Nafion electrolyte, and the device retained 80% of its capacity after 25 000 cycles. The technology presented here will enable thin, solid, flexible supercapacitors that are compatible with standard microfabrication techniques.

This paper reports on a novel non-contact thermal imaging method with high spatial, temporal and temperature resolution for microdevice evaluation. This method uses a temperature sensitive paint with europium (III) thenoyltrifluoroacetone (Eu(TTA)₃). The luminescence of Eu(TTA)₃ was excited by a short pulsed UV light; a temporal resolution as high as 0.2 ms was achieved under normal CCD camera observation. Also, a spatial resolution of about 39 µm and a temperature fluctuation of about ±0.2 °C were demonstrated.

The functionality of paper-based diagnostic devices can be significantly enhanced by their integration with an on-board energy source. Here, we demonstrate the fabrication of paper-based electrochemical cells on wax paper using CO₂ laser surface treatment and micromachining. A four cell zinc–copper battery shows a steady open-circuit voltage of ∼3 V and can provide 0.25 mA for at least 30 min when connected to a 10 kΩ load. Higher voltages and current values can be obtained by adjusting the number and size of electrochemical cells in the battery without changing the fabrication process.

This paper presents a comprehensive analysis of a MEMS voltage step-up converter for energy harvesting and other low-power applications. The step-up operation is based on isolating the charge of a mechanically variable capacitor and varying the gap between the electrodes by an appropriate method of providing an actuation force. A bi-stable device is discussed and was specifically designed for static energy harvesting. This device features a separate electrostatic actuator element to manipulate the variable capacitor electrodes. Prototypes were then fabricated using a dicing-free silicon-on-insulator process. The devices have been arbitrarily designed to produce an output voltage which is five times the magnitude of the input (M = 5). Due to leakage currents, it was necessary to cascade the MEMS capacitors in parallel to obtain a higher capacitance level. Parasitic fringing capacitances have a substantial impact on the overall capacitance value of the MEMS device and so the measured multiplication level of the devices is limited to M = 2.125. With four devices in parallel, a maximum output voltage of 35.4 V was obtained for a 24 V input was measured. However, a maximum output voltage of ≈60 V is achievable if the capacitance value was further increased by connecting more devices in parallel or if or the load resistance was increased beyond 1 GΩ.

We report the first successful demonstration of an optical microelectromechanical systems (MEMS) sensing platform for the in situ characterization of electrochemically induced reversible mechanical changes in lithium-ion battery (LIB) electrodes. The platform consists of an array of flexible membranes with a reflective surface on one side and a thin-film LIB electrode on the other side. The membranes deflect due to the active battery material volume change caused by lithium intercalation (expansion) and extraction (contraction). This deflection is monitored using the Fabry–Perot optical interferometry principle. The active material volume change causes high internal stresses and mechanical degradation of the electrodes. The stress evolution observed in a silicon thin-film electrode incorporated into this MEMS platform follows a 'first elastic, then plastic' deformation scheme. Understanding of the internal stresses in battery electrodes during discharge/charge is important for improving the reliability and cycle lifetime of LIBs. The developed MEMS platform presents a new method for in situ diagnostics of thin-film LIB electrodes to aid the development of new materials, optimization of electrode performance, and prevention of battery failure.

An infusion pump powered by body heat is investigated in this paper, with the goal of addressing the needs of dermal wound healing. The infusion pump incorporates a Knudsen gas pump, a type of thermally driven pump, to pneumatic push the pharmaceutical agent from a reservoir. Two designs are considered: an integrated pump and reservoir, and a design with cascaded pump and reservoir. Thermal models are developed for both pumps, and the simulations agree well with the experimental results. The integrated pump and reservoir design uses hydrophobic materials to prevent a flow from occurring unless the infusion pump is placed on a human body. Flow rates in the µL min⁻¹ range for the integrated pump and reservoir, and approximately 70 µL min⁻¹ for the cascaded pump were obtained. The dynamic behavior of the cascaded pump is described based on the thermal models. Multiple copies of the cascaded pump are easily made in series or parallel, to increase either the pressure or the flow rate. The flow rate of multiple pumps in series does not change, and the pressure of multiple pumps in parallel does not change.

We present a novel wafer-level fabrication method for 3D solenoidal microtransformers using an automatic wire bonder for chip-scale, very high frequency regime applications. Using standard microelectromechanical systems fabrication processes for the manufacturing of supporting structures, together with ultra-fast wire bonding for the fabrication of solenoids, enables the flexible and repeatable fabrication, at high throughput, of high performance air core microtransformers. The primary and secondary solenoids are wound one on top of the other in the lateral direction, using a 25 µm thick insulated wire. Besides commonly available gold wire, we also introduce insulated copper wire to our coil winding process. The influence of copper on the transformer properties is explored and compared to gold. A simulation model based on the solenoids' wire bonding trajectories has been defined using the FastHenry software to accurately predict and optimize the transformer's inductive properties. The transformer chips are encapsulated in polydimethylsiloxane in order to protect the coils from environmental influences and mechanical damage. Meanwhile, the effect of the increase in the internal capacitance of the chips as a result of the encapsulation is analyzed. A fabricated transformer with 20 windings in both the primary and the secondary coils, and a footprint of 1 mm², yields an inductance of 490 nH, a maximum efficiency of 68%, and a coupling factor of 94%. The repeatability of the coil winding process was investigated by comparing the data of 25 identically processed devices. Finally, the microtransformers are benchmarked to underline the potential of the technology in rendering air core transformers competitive.

In this study, the investigation of surface-treatment of chemically inert graphitic carbon microelectrodes (derived from pyrolyzed photoresist polymer) for improving their attachment chemistry with DNA molecular wires and ropes as part of a bionanoelectronics platform is reported. Polymer microelectrodes were fabricated on a silicon wafer using standard negative lithography procedures with negative-tone photoresist. These microelectrode structures were then pyrolyzed and converted to a form of conductive carbon that is referred to as PP (pyrolyzed polymer) carbon throughout this paper. Functionalization of the resulting pyrolyzed structures was done using nitric, sulfuric, 4-amino benzoic acids (4-ABA), and oxygen plasma etching and the surface modifications confirmed with Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and electron dispersion x-ray spectroscopy (EDS). Post surface-treatment analysis of microelectrodes with FTIR and Raman spectroscopy showed signature peaks characteristics of carboxyl functional groups while EDS showed an increase in oxygen content in the surface-treatment procedures (except 4-ABA) indicating an increase in carboxyl functional group. These functional groups form the basis for peptide bond with aminated oligonucleotides that in turn could be used as molecular wires and interconnects in a bionanoelectronics platform. Post-pyrolysis analysis using EDS showed relatively higher oxygen concentrations at the edges and location of defects compared to other locations on these microelectrodes. In addition, electrochemical impedance measurements showed metal-like behavior of PP carbon with high conductivity (|Z| <1 KΩ) and no detectable detrimental effect of oxygen plasma surface-treatment on electrical characteristic. In general, characterization results—taken together—indicated that oxygen plasma surface-treatment produced more reliable, less damaging, and consistently repeatable generation of carboxyl functional groups than diazonium salt and strong acid treatments.

We present the first tunable v-shaped mirror, also known as Fresnel mirror, that may be used to generate a quasi-nondiffracting line pattern, for example for applications in laser lithography or nanomachining with femtosecond lasers. The aperture of the device is 5 mm with a surface flatness of better than λ/10, and the range of the tilting angle is 1.3–38 mrad, resulting in a fringe spacing of 123 μm down to 4.2 μm for red light at 633 nm. In contrast to usual cantilever-comb setups, the mirrors are supported on a PDMS layer and tilted by a single piezoelectric actuator, providing a high resonance frequency of 5.1 kHz. The device is fabricated using laser rapid prototyping of silicon and a casting processes of soft polymers. We show the static and dynamic characterization of the mirror and the verification of the optical functionality.

We describe a novel on-chip microfabrication technique for the alkali-metal vapor cell of an optically pumped atomic magnetometer (OPAM), utilizing an alkali-metal source tablet (AMST). The newly proposed AMST is a millimeter-sized piece of porous alumina whose considerable surface area holds deposited alkali-metal chloride (KCl) and barium azide (BaN₆), source materials that effectively produce alkali-metal vapor at less than 400 °C. Our experiments indicated that the most effective pore size of the AMST is between 60 and 170 µm. The thickness of an insulating glass spacer holding the AMST was designed to confine generated alkali metal to the interior of the vapor cell during its production, and an integrated silicon heater was designed to seal the device using a glass frit, melted at an optimum temperature range of 460–490 °C that was determined by finite element method thermal simulation. The proposed design and AMST were used to successfully fabricate a K cell that was then operated as an OPAM with a measured sensitivity of 50 pT. These results demonstrate that the proposed concept for on-chip microfabrication of alkali-metal vapor cells may lead to effective replacement of conventional glassworking approaches.

In this work a silicon voltage controlled microelectromechanical tuning fork resonator with electrostatic actuation and separate frequency tuning electrodes is presented. The released device is fabricated using a silicon-on-insulator wafer by a two-step process involving only focused ion beam masking and cryogenic deep reactive ion etching. This process is ideal for rapid prototyping, as the time to turn a design into the final device is only a few hours. The design of the resonator is optimized to accommodate the restrictions of the fabrication process, to maximize the frequency tuning range and to minimize the biasing voltage. Separating tuning and driving electrodes enables the resonance frequency adjustment by over 70 000 ppm (f_center > 1.5 MHz, quality factor Q ≈ 2000) with a tuning voltage of 29 V in an open loop mode.

This study presents the theoretical derivation of the discriminant D as a structural and material criterion for determining whether bistability can occur in micromechanically bistable mechanisms. When D < 0, the mechanism displays bistable behavior if an appropriate force is applied to push the bistable mechanism, whereas when D > 0, bistable behavior cannot occur. The proposed V-beam bistable mechanism was successfully fabricated with various beam lengths and tilted angles. The experiments conducted in this study validated the theoretical study of bistability. A comparison of the theoretical solutions and experimental results shows good agreement. Results further show that to design a bistable V-beam mechanism, the tilted angle should be larger for the same beam length, whereas the beam length should be longer for the same tilted angle. The developed discriminant D can be used to predict if a bistable mechanism can achieve bistable behavior based on structural sizes and material properties. Consequently, researchers can reduce trial-and-error experiments when designing a bistable mechanism. A V-beam with a larger tilted angle of up to 5° was successfully fabricated to act as a bistable mechanism, compared to a 3.5° tilted angle in existing studies. Consequently, the proposed method has the advantages of shorter beam lengths and smaller device areas.

This paper presents the fabrication and characterization of a micro alkaline direct ethanol fuel cell. The device has been conceived as a feasibility demonstrator, using microtechnologies for the fabrication of the current collectors and traditional techniques for the membrane electrode assembly production. The fuel cell works in passive mode, as expected for the simplicity required for micro power systems. Non-noble catalysts have been used in order to implement the main advantage of alkaline systems, showing the feasibility of such a device as a potential very-low-cost power device at mini- and micro scales.

This paper presents the first ultra-low-power complementary metal–oxide–semiconductor (CMOS)-based measurement technique for monitoring the cold-switched dynamic behavior of ohmic radiofrequency microelectromechanical systems (RF MEMS) switches in real time. The circuit is capable of providing precise information about contact timing and ohmic contact events. Sampling of dynamic events at frequencies of 1 and 5 MHz shows contact timing accuracy of 99% when compared with real-time true-height information obtained from laser Doppler vibration data. The technique is validated for an ohmic RF MEMS switch with multiple bounces. The actuation voltage has also been designed to enhance bouncing behavior to more clearly study the performance and limits of the presented technique. More than 13 bounces are successfully captured by the electronic measurement technique. The weakest bounces exhibit vertical displacements of less than 20 nm as recorded by a laser Doppler vibrometer. This demonstrates the ability to capture precise timing information even for weak contacting events. A detailed discussion of how parasitics influence this technique is also presented for the first time.

Stroboscopic white light interferometry (SWLI) has been known as a useful measurement technique for vibrating samples such as micro-electro-mechanical systems (MEMS) or micro-opto-electro-mechanical systems (M(O)EMS) because it enables dynamic mode reconstruction and characterization of the tested system. An approximate model simulation without any experimental confirmation previously indicated that the duty cycle of the light could reduce the accuracy of the measurement. To provide a comprehensive insight into this important phenomenon, the study investigated theoretically and experimentally the effect of duty cycle of the light. An atomic force microscopy cantilever beam vibrating at its second resonant frequency was measured and the experimental measurements were analyzed and compared with the simulated results. In general, a reasonable correspondence between the mathematical model and the experimental measurements has been observed when the duty cycle is less than 15% and the average deviation is kept within 15.4% of the vibration amplitude. However, it is verified that the SWLI using white light LED has its physical detection limits when the cycle time of the strobed light or the light exposure time of the imaging device is more than 20%, in which the maximum measured error can significantly exceed 38.4% of the vibration amplitude.

A micromachining process for a conductive microtip electrode array has been developed using a combination process of reactive ion etching. A localized conical ultra-micro electrode (UME) with submicron width is realized on the tip end for electrochemical measurements. A height of several tens of microns of the silicon microtip is achieved, and the area beyond the UME is covered with dielectric material for electrical insulation. A high-aspect-ratio silicon microtip is fabricated using anisotropic and isotropic etching with varying the gap and diameter of the etch masks. Cyclic voltammetry is conducted to characterize the electrochemical properties of the UMEs. The results indicate that the electrical connection on the UME works successfully, and the measured peak currents were found to be in good agreement with the values estimated from analysis based on conical UME theory.

We present analytic expressions for three common measures of the dynamic response in gap-closing electrostatic resonators. We show that peak gain, peak sharpness and logarithmic decrement are distinctively different and are not equal to the quality-factor of the unloaded system. We experimentally validate our theoretical predictions by characterizing the dynamic response of test devices. The significance of this work is that it clarifies the correct way in which the performance of MEMS resonators should be reported to avoid ambiguity.

A nonlinear method is proposed to calculate the adhesion energy (strain energy release rate) of stiction-failed μcantilever beams with large deflections. The proposed method uses a nonlinear theory for the deflection of a beam and an energy method for calculating the beam's strain energy. It is shown that current models used to predict μcantilevers' profile breakdown when the beam deflection exceeds 27% of the thickness due to the onset of longitudinal stresses in the μcantilevers. Because the present model captures longitudinal stresses in the μcantilevers and consequently their contribution to the strain energy, mode I and mode II contributions to the adhesion energy can be discerned. A set of experiments are performed using the peel test scheme with poly-Si μcantilever stiction failed on a poly-Si substrate. Results processed using the present model indicate that the adhesion energy of the μcantilevers actually increases with increased height of the μcantilever's base. This increase in the adhesion energy is attributed to the manner of loading that the μcantilevers experience which leads to increased contact area and the concomitant increase of adhesion.

A novel method of fabricating titanium superhydrophobic surfaces by ultrafast laser irradiation is reported. The ultrafast laser irradiation creates self-organized microstructure superimposed with nano-scale roughness, after which a fluoropolymer coating is applied to lower the surface energy of the textured surface and achieve superhydrophobicity. The focus of this study is to investigate abrasion effects on this mechanically durable superhydrophobic surface. The mechanical durability is analyzed with linear abrasion testing and microscopy imaging. Linear abrasion tests indicate that these surfaces can resist complete microstructure failure up to 200 abrasion cycles and avoid droplet pinning up to ten abrasion cycles at 108.4 kPa applied pressure, which roughly corresponds to moderate to heavy sanding or rubbing in the presence of abrasive particles. The wear mechanisms are also investigated and the primary mechanism for this system is shown to be abrasive wear with fatigue by repeated plowing. Although these results demonstrate an advancement in mechanical durability over the majority of existing superhydrophobic surfaces, it exemplifies the challenge in creating superhydrophobic surfaces with suitable mechanical durability for harsh applications, even when using titanium.

In this work, we present an alternative, low cost method for the fabrication of a heat exchanger utilizing metal-based microchannels using the UV-LiGA technique. Lithography is used to pattern dry film negative photoresist (Ordyl P-50100) on the substrate. The resist is laminated over the substrate and exposed with a UV source. The use of dry film resist allows for simple and inexpensive microchannel patterns without requiring advanced cleanroom equipment. Following the lithography process, electrodeposition of metals is used to fill the recesses patterned in the resist. In this work, nickel has been electroplated into the bounding resist structure. After electroplating, the remaining resist is dissolved leaving free standing metal structures. The fabricated exchanger is then evaluated based on thermal absorption of simulated waste heat sources and capillary action of the metal channels themselves. Channels are fabricated to heights of 60, 70 and 90 μm respectively on copper substrate using these methods. Working fluid mass transfer rate from the heated microchannel heat exchanger (MHE) is utilized as a basic metric of operation. The mass transfer rate recorded from the nickel-based MHE is 2.19, 2.81 and 3.20 mg s⁻¹ respectively for the different channel heights. This implies an effective thermal power consumption rate of 1.66, 2.13 and 2.42 kW m⁻² respectively. By contrast, an MHE fabricated with 115 and 142 μm tall channels on silicon substrate is shown to evaporate up to 2.84 and 3.04 mg s⁻¹ respectively, giving an effective thermal power consumption of 2.15 and 2.31 kW m⁻² respectively. An investigation of working fluid contact angle with the electroplated nickel surface is also presented. The surface is found to be a porous structure stemming from the electroplating process.

In this study, we have demonstrated the fabrication of a microbeam array (MBA) with various thicknesses and investigated the suitability it for an acoustic sensor with wide-range frequency selectivity. For this, an MBA composed of 64 beams, with thicknesses varying from 2.99–142 µm, was fabricated by using single gray-scale lithography and a thick negative photoresist. The vibration of the beams in air was measured using a laser Doppler vibrometer; the resonant frequencies of the beams were measured to be from 11.5 to 290 kHz. Lastly, the frequency range of the MBA with non-uniform thickness was 10.9 times that of the MBA with uniform thickness.

This paper proposes an optimal design of the thermal-actuated, piezoresistive-sensed resonator fabricated by a foundry-provided CMOS-MEMS process. The optimal design is achieved both by quantitatively comparing the mechanical properties of different composite films as well as by deriving an analytical model for determining the device dimensions. The analytical model includes a stress model of an asymmetric mechanical structure and a piezoresistivity model of the heavily doped, n-type polysilicon film. The analytical model predicts that the optimal length of the displacement sensor is 200 μm when the thermal actuator is 200 μm in length and the absorption plate is 100 μm in length. Additionally, the model predicts the resistivity of the polysilicon film of (6.8 ± 2.2) mΩ cm and the gauge factor of (6.8 ± 2.9) when the grain size is (250 ± 100) nm. Experimental results agree well with simulation results. Experimental data show that the resonant frequency of the device is 80.06 kHz and shifts to 79.8 kHz when a brick of Pt mass is deposited on the resonator. The mass of the Pt estimated from the frequency shift is 4.5419 × 10⁻¹² kg, while estimated from the measured dimension is 4.4204 × 10⁻¹² kg. Sensitivity of the resonant sensor is calculated to be 1.8 × 10² Hz ng⁻¹. Experimental results further show that the polysilicon film used in the experiments has a grain size of (241 ± 105) nm, an average gauge factor of 5.56 and average resistivity of 5.5 mΩ cm.

The effect of Cr buffer layer thickness on the open-circuit voltage generated by thin-film thermoelectric modules of Bi_0.5Sb_1.5Te₃ (p-type) and Bi₂Te_2.7Se_0.3 (n-type) materials was investigated. A Cr buffer layer, whose thickness generally needs to be optimized to improve adhesion depending on the substrate surface condition, such as roughness, was deposited between thermoelectric thin films and glass substrates. When the Cr buffer layer was 1 nm thick, the Seebeck coefficients and electrical conductivity of 1 µm thermoelectric thin films with the buffer layers were approximately equal to those of the thermoelectric films without the buffer layers. When the thickness of the Cr buffer layer was 1 µm, the same as the thermoelectric films, the Seebeck coefficients of the bilayer films were reduced by an electrical current flowing inside the Cr buffer layer and the generation of Cr₂Te₃. The open-circuit voltage of the thin-film thermoelectric modules decreased with an increase in the thickness of the Cr buffer layer, which was primarily induced by the electrical current flow. The reduction caused by the Cr₂Te₃ generation was less than 10% of the total voltage generation of the modules without the Cr buffer layers. The voltage generation of thin-film thermoelectric modules could be controlled by the Cr buffer layer thickness.

In this experimental study, flow boiling in mini/microtubes was investigated with surface enhancements provided by polyhydroxyethylmethacrylate (pHEMA) coatings (of ∼30 nm thickness) on inner microtube walls. Flow boiling heat transfer experiments were conducted on microtubes (with inner diameters of 249, 507 and 998 µm) having inner surfaces of pHEMA coatings, which increase heat transfer surface area, enable liquid replenishment upon bubble departure, provide additional nucleation sites, and serve for extending critical heat flux (CHF) enhancing boiling heat transfer. The de-ionized water was utilized as the working fluid in this study. pHEMA nanofilms of thickness ∼30 nm on the microtube walls were coated through an initiated chemical vapor deposition technique. Experimental results obtained from the coated microtubes were compared to their plain surface counterparts at two mass flux values (10 000 and 13 000 kg m⁻² s⁻¹). In comparison to the plain surface microtubes, the coated surfaces demonstrated an increase up to 24% and 109% in CHF and heat transfer coefficients, respectively. These promising results support the use of pHEMA coated microtubes/channels as a surface enhancement technique for microscale cooling applications.

In this paper a self-oscillator based on a polysilicon free–free beam resonator monolithically integrated and packaged in a 0.35 µm complementary metal–oxide–semiconductor (CMOS) technology is presented. The oscillator is capable of providing a 350 mV_PP sinusoidal signal at 25.6 MHz, with a bias polarization voltage of 7 V. The microelectromechanical systems (MEMS) resonator is packaged using only the back-end-of-line metal layers of the CMOS technology, providing a complete low-cost CMOS–MEMS processing for on-chip frequency references.

We present a new plasma etch process optimized for etching piezoelectric aluminum nitride (AlN) film deposited on thin molybdenum (Mo) metal electrode. Such film stack finds application in the integration of AlN-based RF microelectromechanical systems devices. The process is based on Cl₂/BCl₃/Ar gas chemistry with added buffer gas in inductively coupled plasma reactive ion etching system. The new gas mixture overcomes a generic problem of etched surface roughness without significant drop in AlN etch rate. Using design of experiment, the process window is optimized for improving selectivity to Mo and reducing microtrenching while maintaining smooth etched surface. Finally, an etching rate of 280 nm min⁻¹ with reliable etch stop on Mo electrode and smooth bottom surface is reported. The integration suitability of the developed etch process is tested by etching 2.0 to 5.0 µm size square shaped via holes in 1.0 µm thick (0 0 2) oriented piezoelectric AlN on 0.2 µm thick Mo electrode while integrating contour mode resonators.

In this paper we investigate the effect of Ar concentration and shadow mask material on the etch rate, surface roughness, and micromask pit density and depth for reactive ion etching (RIE) of x-cut alpha quartz. The ratio of SF₆ to Ar is varied at constant power and pressure to find an optimum Ar concentration for thinning x-cut quartz while maintaining good surface quality. The recipe is used with an STS320 RIE system to produce smooth, through-hole-free, insulating quartz membranes of 7 µm thickness in selective locations on 100 µm thick x-cut quartz wafers. We demonstrate that surface quality is much improved by replacing at least 75% of the SF₆ by volume with Ar, and that for non-inductively coupled plasma RIE the standard mask material nickel is detrimental to surface quality for long etch times.

Current advances in single cell sequencing, gene expression and proteomics require the isolation of single cells, frequently from a very small source population. In this work we describe the design and characterization of a manually operated microfluidic cell sorter that (1) can accurately sort single or small groups of cells from very small cell populations with minimal losses, (2) that is easy to operate and that can be used in any laboratory that has a basic fluorescent microscope and syringe pump, (3) that can be assembled within minutes, (4) that can sort cells in very short time (minutes) with minimum cell stress, (5) that is cheap and reusable. This microfluidic sorter is made from hard plastic material (PMMA) into which microchannels are directly milled with hydraulic diameter of 70 µm. Inlet and outlet reservoirs are drilled through the chip. Sorting occurs through hydrodynamic switching ensuring low hydrodynamic shear stresses, which were modeled and experimentally confirmed to be below the cell damage threshold. Manually operated, the maximum sorting frequencies were approximately 10 cells min⁻¹. Experiments verified that cell sorting operations could be achieved in as little as 15 min, including the assembly and testing of the sorter. In only one out of ten sorting experiments the sorted cells were contaminated with another cell type. This microfluidic cell sorter represents an important capability for protocols requiring fast isolation of single cells from small number of rare cell populations.

Several smart active materials have been proposed and tested for the development of microactuators. Among these, conjugated polymers are of great interest because miniaturization improves their electrochemical properties, such as increasing the speed and stress output of microactuators, with respect to large-scale actuators. Recently we developed a novel fabrication process to obtain robust free-standing conductive ultra-thin films made of the conjugated polymer poly(3, 4-ethylenedioxythiophene) doped with the polyanion poly(styrenesulfonate) (PEDOT:PSS). These conductive free-standing nanofilms, with thicknesses ranging between a few tens to several hundreds of nm, allow the realisation of new all polymer microactuators using facile microfabrication methods. Here, we report a novel processing method for manufacturing all polymer electrochemical microactuators. We fabricated and patterned free-standing PEDOT:PSS/SU8 bilayer microactuators in the form of microfingers of a variety of lengths using adapted microfabrication procedures. By imposing electrochemical oxidation/reduction cycles on the PEDOT:PSS we were able to demonstrate reversible actuation of the microactuators resulting in bending of the microfingers. A number of possible applications can be envisaged for these small, soft actuators, such as microrobotics and cell manipulation.

Microporous membranes have a number of applications in microfluidic devices including biotechnology and displays (Hagedon et al 2012 Nature Commun.3 1173, De Jong et al 2006 Lab Chip6 1125–39). Though commercially available micro-porous membranes are widely available using the ion track-etch method, there are few economical techniques to make mechanically robust membranes with dual pore sizes and/or surface texture. We demonstrate fabrication of high strength polyamide (poly(p-phenylene-2,6-benzobisoxazole)) membranes using a hybrid microreplication and mask-less photolithographic method. The fabricated films are only ∼6–10 µm thick, yet adequately strong to be released and handled with diagonals of at least several inches. The films contain dual pore sizes (32 µm, 6 µm) with the smaller pores achieving 1:1 aspect ratio, and the films include a highly-engineered surface texture. These films are demonstrated in application for electronic-paper displays, and exhibit robust optical switching and diffuse (paper-like) reflectance.

We have studied the surface quality of millimetre-scale optical mirrors produced by etching CZ and FZ silicon wafers in potassium hydroxide to expose the {111} planes. We find that the FZ surfaces have four times lower noise power at spatial frequencies up to 500 mm⁻¹. We conclude that mirrors made using FZ wafers have higher optical quality.