Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 2012"

Characterization of Nontoxic Liquid-Metal Alloy Galinstan for Applications in Microdevices

Abstract: We have obtained interfacial properties of Galinstan, a nontoxic liquid-metal alloy, to help replace mercury in miniature devices. To prevent formation of an oxide skin that severely hinders the fluidic behavior of small Galinstan droplets and leads to inaccurate property data, we performed our experiments in a nitrogen-filled glove box. It was found that only if never exposed to oxygen levels above 1 part per million (ppm) would Galinstan droplets behave like a liquid. Two key properties were then investigated: contact angles and surface tension. Advancing and receding contact angles of Galinstan were measured from sessile droplets on various materials: for example, 146.8 and 121.5, respectively, on glass. Surface tension was measured by the pendant-drop method to be 534.6 10.7 mN/m. All the measurements were done in nitrogen at 28 with oxygen and moisture levels below 0.5 ppm. To help design droplet-based microfluidic devices, we tested the response of Galinstan to electrowetting-on-dielectric actuation.

An AlN MEMS Piezoelectric Microphone for Aeroacoustic Applications

Abstract: This paper describes the development of a micro- machined microphone for aircraft fuselage arrays that are utilized by aeroacousticians to help identify aircraft noise sources and/or assess the effectiveness of noise-reduction technologies. The developed microphone utilizes piezoelectric transduction via an integrated aluminum nitride layer in a thin-film composite diaphragm. A theoretical lumped element model and an associated noise model of the complete microphone system are developed and utilized in a formal design-optimization process. Optimal designs were fabricated using a variant of the film bulk acoustic resonator process at Avago Technologies. The experimental characterization of one design is presented here, and measured performance was in line with sponsor specifications, including a sensitivity of -39 μV/Pa, a minimum detectable pressure of 40.4 dB, a confirmed bandwidth up to 20 kHz, a 129.5-kHz resonant frequency, and a 3% distortion limit approaching 172 dB. With this performance-in addition to its small size-this microphone is shown to be a viable enabling technology for low-cost, high-resolution fuselage array measurements.

Quadrature-Error Compensation and Corresponding Effects on the Performance of Fully Decoupled MEMS Gyroscopes

Abstract: This paper presents experimental data about the sources of the quadrature error in a fully decoupled microelectromechanical systems gyroscope and demonstrates the extent of performance improvement by the cancellation of this error. Quadrature sources including mass, electrostatic-force, and mechanical-spring imbalances have been compared by FEM simulations, and spring imbalance has been found as the dominant source of the quadrature error. Gyroscopes have been designed with intentional spring imbalances and fabricated with a SOI-based silicon-on-glass fabrication process, the resulting quadrature outputs of the fabricated gyroscopes have been measured, and their agreement with FEM simulations has been verified. Next, it has been experimentally shown that the electrostatic nulling of the quadrature error with closed-loop control electronics improves the bias instability and angle random walk (ARW) of a fully decoupled gyroscope up to ten times. Moreover, the quadrature cancellation improves the scale-factor turn-on repeatability about four times and linearity about 20 times, reaching down to 119 and 86 ppm, respectively. Finally, the quadrature cancellation allows operating the gyroscope with higher drive-mode displacement amplitudes for an increased rate sensitivity. With this technique, outstanding bias instability and ARW performances of 0.39°/h and 0.014 °/√h, respectively, have been achieved.

Three-Axis Lorentz-Force Magnetic Sensor for Electronic Compass Applications

Abstract: A low-power microelectromechanical-systems (MEMS) three-axis Lorentz-force magnetic sensor is presented. The sensor detects magnetic field in two axes with a single MEMS structure. Three-axis sensing is performed using two perpendicular structures on the same die. The MEMS device is a micromechanical resonator, and sensing is conducted using excitation currents at the device's in-plane and out-of-plane mechanical resonant frequencies which are 20.55 and 46.96 kHz, respectively. A die-level vacuum seal results in in-plane and out-of-plane mechanical quality factors of 1400 and 10000, current, the sensor's noise is equivalent to 137 nT/√Hz for the respectively. With 0.58 mW used to provide the two-axis excitation z-axis magnetic field inputs and 444 nT/√Hz for the x-and y-axis fields. For the z-axis field measurements, Brownian noise is the dominant noise component, while the xand y-axis field measurements are limited by the electronic noise in the discrete capacitive-sensing electronics. The major source of offset error is residual motion induced by electrostatic force. The offset is reduced to 14 μT using a dc compensation voltage applied to the MEMS structure to null the electrostatic force. After compensation, the offset stability is 400 nT with a 0.7-s averaging time.

A Piezoelectric Parametric Frequency Increased Generator for Harvesting Low-Frequency Vibrations

Abstract: This paper presents the design, fabrication, and testing of a piezoelectric parametric frequency increased generator for harvesting low-frequency non-periodic vibrations. The generator incorporates a bulk piezoelectric ceramic machined using ultrafast laser ablation. The electromechanical transducer is designed as a clamped-clamped spiral beam in order to decrease the stiffness within a limited footprint. An internal mechanism up-converts the ambient vibration frequency to a higher internal operation frequency in order to achieve better electromechanical coupling and efficiency. To gain maximum power output, the optimum width and thickness values of a spiral up-conversion unit are computed via multi-physics finite-element analysis simulations. The fabricated device generated a peak power of 100 μW and an average power of 3.25 μW from an input acceleration of 9.8 m/s2 at 10 Hz. The device operates over a frequency range of 24 Hz. The internal volume of the generator is 1.2 cm3.

Laser-Driven Micro Transfer Placement of Prefabricated Microstructures

Abstract: Microassembly of prefabricated structures and devices is emerging as key process technology for realizing heterogeneous integration and high-performance flexible and stretchable electronics. Here, we report on a laser-driven micro transfer placement process that exploits, instead of ablation, the mismatch in thermomechanical response at the interface of a transferable microstructure and a transfer tool to a laser pulse to drive the release of the microstructure from the transfer tool and its travel to a receiving substrate. The resulting facile pick-and-place process is demonstrated with the assembling of 3-D microstructures and the placement of GaN light-emitting diodes onto silicon and glass substrates. High-speed photography is used to provide experimental evidence of thermomechanically driven release. Experiments are used to measure the laser flux incident on the interface. These, when used in numerical and analytical models, suggest that temperatures reached during the process are enough to produce strain energy release rates to drive delamination of the microstructure from the transfer tool.

Resonant PZT MEMS Scanner for High-Resolution Displays

Abstract: A resonant piezoelectric scanner is developed for high-resolution laser-scanning displays. A novel actuation scheme combines the principle of mechanical amplification with lead zirconate titanate (PZT) thin-film actuation. Sinusoidal actuation with 24 V at the mechanical resonance frequency of 40 kHz provides an optical scan angle of 38.5° for the 1.4-mm-wide mirror. This scanner is a significant step toward achieving full-high-definition resolution (1920 × 1080 pixels) in mobile laser projectors without the use of vacuum packaging. The reported piezoscanner requires no bulky components and consumes <; 30-mW power at maximum deflection, thus providing significant power and size advantages, compared with reported electromagnetic and electrostatic scanners. Interferometry measurements show that the dynamic deformation is at acceptable levels for a large fraction of the mirror and can be improved further for diffraction-limited performance at full resolution. A design variation with a segmented electrode pair illustrated that reliable angle sensing can be achieved with PZT for closed-loop control of the scanner.

A SU-8-Based Microfabricated Implantable Inductively Coupled Passive RF Wireless Intraocular Pressure Sensor

Abstract: With the growing demand in noncontact detection of human diseases, this paper presents an implantable passive wireless pressure sensor using an inductively coupled wireless sensing technique, particularly designed to monitor the intraocular pressure (IOP) of glaucoma patients. The microfabricated IOP sensor consists of a planar spiral gold coil inductor, a two-parallel-gold-plate (metal-insulator-metal) capacitor, and a SU-8 pressure-sensitive diaphragm. The IOP sensor is fully encapsulated inside biocompatible SU-8 stacking layers to isolate the IOP sensor from the biological tissue medium environment. By measuring the impedance phase dip frequency shift from the external coil, the IOP signal can be obtained through the implanted IOP sensor. The optimized size of the manually wound external coil was investigated. The readout distance is up to 6 mm from the implanted sensor. Characterization results show that the microfabricated IOP sensor has relatively high sensitivities-7035 ppm/mmHg in air and 3770 ppm/mmHg in saline medium-with pressure resolution lower than 1 mmHg, which is adequate for IOP monitoring application.

High-Range Angular Rate Sensor Based on Mechanical Frequency Modulation

Abstract: We report, for the first time, an angular rate sensor based on mechanical frequency modulation (FM) of the input rotation rate. This approach tracks the resonant frequency split between two X - Y symmetric high-Q mechanical modes of vibration in a microelectromechanical systems Coriolis vibratory gyroscope to produce a frequency-based measurement of the input angular rate. The system is enabled by a combination of a MEMS vibratory high-Q gyroscope and a new signal processing scheme which takes advantage of a previously ignored gyroscope dynamic effect. A real-time implementation of the quasi-digital angular rate sensor was realized using two digital phase-locked loops and experimentally verified using a silicon MEMS quadruple mass gyroscope (QMG). Structural characterization of a vacuum- packaged QMG showed Q factors on the order of one million over a wide temperature range from -40 °C to +100°C with a relative x/y mismatch of Q of 1 %. Temperature characterization of the FM rate sensor exhibited less than 0.2% variation of the angular rate response between 25°C and 70 °C environments, enabled by the self-calibrating differential frequency detection. High-speed rate table characterization of the FM angular rate sensor demonstrated a linear range of 18 000 deg/s (50 r/s, limited by the setup) with a dynamic range of 128 dB. Interchangeable operation of the QMG transducer in conventional amplitude- modulated and new FM regimes provides a 156-dB dynamic range.

Measurement of the Anisotropy of Young's Modulus in Single-Crystal Silicon

Abstract: In (100) silicon wafers, the most commonly used in microelectromechanical systems (MEMS) fabrication, the value of Young's modulus of a MEMS structure can vary by over 20%, depending on the structure's orientation on the wafer surface This anisotropy originates from the crystal structure of silicon We have directly measured the anisotropy of Young's modulus in the (100) plane of silicon from the measured resonance frequencies of a “wagon-wheel” test structure comprising an arc of identical microcantilevers fabricated in the structural layer of a (100) silicon-on-insulator wafer The direction of the principal axis of the cantilevers increased from 0° to 180 ° in 10° steps with respect to the [110] direction, allowing the angular dependence of Young's modulus to be experimentally mapped out The Young's modulus was measured to have a value of 170 GPa ± 3 GPa at 0° and 90 ° to the [110] direction and a value of 131 GPa ± 3 GPa at ±40° and ±50° to the [110] direction The measured values of Young's modulus and their angular dependence agree very well with the theoretical values that were recently reported, thereby experimentally verifying the theoretical calculations

Body-Biased Complementary Logic Implemented Using AlN Piezoelectric MEMS Switches

Abstract: This paper reports on the implementation of low-voltage complementary mechanical logic achieved by using body-biased aluminum nitride (AlN) piezoelectric microelectro-mechanical systems (MEMS) switches. By biasing the equivalent body of a four terminal mechanical switch with a fixed voltage, the threshold voltage of the mechanical transistor has been precisely tuned and the voltage swing used for implementing digital functionalities reduced to very low values (≤ ±2 V). Thanks to the use of a mechanical switching mechanism, the AlN MEMS switches have exhibited an extremely low subthreshold slope (0.065 mV/dec), which sets the promise for even further reduction of the voltage swing to less than 100 mV. By using opposite body biases, the same mechanical switch has been made to operate as an equivalent n-like or p-like (complementary) device. Two basic AlN mechanical switch elements have then been used to form a body-biased inverter operating at 100 Hz with a ±1.5-V voltage swing. Furthermore, low voltage and functionally complete logic elements (NAND and NOR) implemented by using body-biased complementary and thin-film (250 nm thick) AlN-based piezoelectric mechanical switches have been synthesized. Finally, scaling rules for these devices are derived, and the key challenges that will need to be addressed to achieve further miniaturization are presented.

Thin-Film Piezoelectric Materials For a Better Energy Harvesting MEMS

Abstract: Ferroelectric PZT-based perovskite thin films are widely studied for fabrication of compact piezoelectric energy harvesting (EH) power microelectromechanical systems (MEMS) due to their large piezoelectric coefficients. Output energy of the piezoelectric EH power MEMS is chiefly governed by their energy conversion rate, κ2 and/or (e2/e), where e and e denote their piezoelectric coefficient and dielectric constant. The values of (e2/e) are considered as figures of merit (FOM) for the piezoelectric EH power MEMS. At present nonferroelectric AlN thin films are considered as a candidate for a better piezoelectric EH power MEMS due to their high FOM values. These PZT-based thin films are mostly polycrystalline thin films of binary perovskite compounds, Pb(Zr, Ti)O3 (PZT). We have proposed single crystal thin films of PZT-based ternary perovskite compounds, Pb(Mn,Nb)O3-PZT (PMnN-PZT), instead of the binary perovskite PZT. The single crystal PMnN-PZT thin films have been successively fabricated by rf-magnetron sputtering. It is found that the FOM values of single c-domain/single crystal PMnN-PZT thin films are one order of magnitude higher than those of AlN thin films. This suggests output powers of the PMnN-PZT thin-film EH power MEMS are one order of magnitude higher than those of the AlN thin-film EH power MEMS.

Modeling and Characterization of Cantilever-Based MEMS Piezoelectric Sensors and Actuators

Abstract: Piezoelectric materials are used in a number of applications including those in microelectromechanical systems. These materials offer characteristics that provide unique advantages for both sensing and actuating. Common implementations of piezoelectric transduction involve the use of a cantilever with several layers, some of which are piezoelectric. Although most analyses of such a cantilever assume small piezoelectric coupling (SPC), the validity of this assumption has not been fully investigated. This paper presents closed-form expressions for the voltage developed across a piezoelectric layer in an N-layer cantilever used as a sensor (e.g., as a microphone) and for the displacement profile of an N-layer cantilever used as an actuator. This represents the first time these closed-form expressions have been presented without making the SPC assumption and are used to determine the validity of the this assumption. Furthermore, a new, more robust experimental technique for identifying the piezoelectric coefficient is demonstrated using an aluminum nitride (AlN) cantilever beam. The developed expressions are also used to predict the voltage across a piezoelectric layer in a beam containing AlN layers in response to a pressure excitation and are shown to be in close agreement with experimental results.

Investigation on Mechanically Bistable MEMS Devices for Energy Harvesting From Vibrations

Abstract: In this paper, mechanically bistable microelectromechanical systems devices are investigated for energy harvesting from mechanical vibrations. This approach is particularly suitable when random, weak, and broad-band vibrations in the low-frequency range are considered. These working conditions are, in fact, quite challenging and are often approached via arrays of linear resonant microdevices. Our approach allows, with a single device, to efficiently collect kinetic energy in the whole spectrum of frequencies of the incoming signal. Bistable behaviors are achieved through purely mechanical and fully compliant micromechanisms. Different structures have been analytically and numerically investigated, both in static and dynamic working conditions, and optimized results are proposed. The advantages of the proposed device, which exploit bistable dynamic behaviors, over linear and monostable strategies are presented in this paper: In the case of the incoming kinetic energy spread over a large bandwidth and confined at low frequencies, a larger fraction of the input mechanical energy is transferred to the mechanical-to-electrical conversion section of the harvester and, therefore, to the final user. A complete device design is proposed in this paper by taking into account a dedicated fabrication process which allows to obtain large inertial masses; electrostatic conversion has been considered and embedded into the device to evaluate the device performances in terms of the electric energy scavenged.

Design and Demonstration of an In-Plane Silicon-on-Insulator Optical MEMS Fabry–Pérot-Based Accelerometer Integrated With Channel Waveguides

Abstract: In this paper, we present a novel optical microelectromechanical systems (MEMS) accelerometer sensor dedicated to space applications. An in-plane Fabry-Perot (FP) microcavity (FPM) with two distributed Bragg reflectors (DBRs) is used to detect the acceleration. One of the DBR mirrors is attached to two suspended proof masses, allowing the FP gap to change while proof masses experience acceleration. Acceleration is then detected by measuring the spectral shift of the FPM. The optical accelerometer presented here uses silicon strip waveguides integrated with MEMS on a single silicon-on-insulator wafer, making it compact and robust. All of the device components are fabricated using one single fabrication step. Immunity to electromagnetic interference, high sensitivity and resolution capability, integrability, reliability, low cross-sensitivity, simple fabrication, and possibility of having two- and three-axis sensitivities are numerous advantages of our sensor compared to the conventional ones. The sensor performance demonstrated a 90-nm/g sensitivity and 111-μg resolution and better than 250-mg dynamic range.

A Physics-Based Predictive Modeling Framework for Dielectric Charging and Creep in RF MEMS Capacitive Switches and Varactors

Abstract: In this paper, we develop a physics-based theoretical modeling framework to predict the device lifetime defined by the dominant degradation mechanisms of RF microelectromechanical systems (MEMS) capacitive switches (i.e., dielectric charging) and varactors (i.e., creep), respectively. Our model predicts the parametric degradation of performance metrics of RF MEMS capacitive switches and varactors, such as pull-in/pull-out voltages, pull-in time, impact velocity, and capacitance both for dc and ac bias. Specifically, for dielectric charging, the framework couples an experimentally validated theoretical model of time-dependent charge injection into the bulk traps with the Euler-Bernoulli equation for beam mechanics to predict the effect of dynamic charge injection on the performance of a capacitive switch. For creep, we generalize the Euler-Bernoulli equation to include a spring-dashpot model of viscoelasticity to predict the time-dependent capacitance change of a varactor due to creep. The new model will contribute to the reliability aware design and optimization of the capacitive MEMS switches and varactors.

Resonant MEMS Mass Sensors for Measurement of Microdroplet Evaporation

Abstract: Microelectromechanical systems (MEMS)-based resonant mass sensors have been extensively studied due to their high sensitivity and small size, making them very suitable for detecting micro- or nanosized particles, as well as monitoring microscaled physical processes. In a range of physical and biological applications, accurate estimation and precise control of the evaporation process of microdroplets are very important. However, due to the lack of appropriate measurement tools, the evaporation process of microdroplets has not been well characterized. Here, we introduce a self-oscillating MEMS mass sensor with a uniform mass sensitivity to directly measure the mass changes of evaporating microdroplets. The mass sensor has a unique spring structure to provide spatially uniform mass sensitivity. The sensor's velocity is fed back to the actuation signal to induce self-oscillation, enabling rapid determination of the resonant frequency. The evaporation rates of single microdroplets of dimethyl sulfoxide and water at various temperatures are obtained. With the measured evaporation rates and the simulated surface area of the microdroplet, the enthalpies of vaporization of both liquids are extracted and found to be in agreement with those in the literature. The method developed in this work can be a valuable tool to enhance our understanding of microscaled physical processes involving rapid mass change, such as evaporation, deposition, self-assembly, cryopreservation, and other biological applications.

A Three-Axis CMOS-MEMS Accelerometer Structure With Vertically Integrated Fully Differential Sensing Electrodes

Abstract: This study presents a novel CMOS-microelectromechanical systems (MEMS) three-axis accelerometer design using Taiwan Semiconductor Manufacturing Company 0.18-μm one-poly-Si six-metal/dielectric CMOS process. The multilayer metal and dielectric stacking features of the CMOS process were exploited to vertically integrate the in-plane and out-of-plane capacitive sensing electrodes. Thus, the three-axis sensing electrodes can be integrated on a single proof mass to reduce the footprint of the accelerometer. Moreover, the fully differential gap-closing sensing electrodes among all three axes are implemented to increase the sensitivities and decrease the noise. The in-plane and out-of-plane sensing gaps are respectively defined by the minimum metal line width and the thickness of one metal layer by means of the metal wet-etching post-CMOS process. Thus, the capacitive sensitivities are further improved. The fully differential gap-closing sensing electrodes also bring the advantage of reduced cross talks between all three axes. As a result, the footprint of the presented three-axis accelerometer structure is only 400 × 400 μm2. Compared with existing commercial or CMOS-MEMS studies, the size is significantly reduced. The measured sensitivities (nonlinearities) are 14.7 mV/G (3.2%) for the X-axis, 15.4 mV/G (1.4%) for the Y-axis, and 14.6 mV/G (2.8%) for the Z-axis.

Fabrication of a Micromachined Two-Dimensional Wind Sensor by Au–Au Wafer Bonding Technology

Abstract: The design, fabrication, and performance of a micromachined 2-D wind sensor are presented. The sensor operates based on the detection of temperature and flow-dependent heat distribution on a hot sensing surface. It consists of a silicon sensing chip and a ceramic packaging substrate, in which the sensing chip is bonded to the front side of the ceramic packaging substrate through wafer-level gold bumps. The backside of the ceramic substrate provides a smooth surface for the sensor exposed to the wind flow. A silicon diaphragm was fabricated by wet etching to minimize its heat capacity, resulting in the improvement of the power consumption, response time, and resolutions. Experimental results show that the measurement of wind flow speed is demonstrated in the range from 0.5 to 40 m/s with the sensitivity more than 2.73 mW/ms-1. The sensor requires only 2 mW initial heating power, and in constant-temperature difference mode, the response time less than 1.4 s is obtained. By measuring temperature difference in two directions perpendicular to each other, the detection of direction in a full range of 360 has been achieved. The errors in the measured wind speed and direction after calibration are ±4% and ±2°, respectively.

The First Launch of an Autonomous Thrust-Driven Microrobot Using Nanoporous Energetic Silicon

Abstract: As the capability and complexity of robotic platforms continue to evolve from the macro to the micron scale, the challenge of achieving autonomy requires the development of robust, lightweight architectures. These architectures must provide a platform upon which actuators, control, sensing, power, and communication modules are integrated for optimal performance. In this paper, the first autonomous jumping microrobotic platform is demonstrated using a hybrid integration approach to assemble on-board control, sensing, power, and actuation directly onto a polymer chassis. For the purposes of this paper, jumping is defined as brief parabolic motion achieved via an actuation pulse at takeoff. In this paper, the actuation pulse comes from the rapid release of chemical energy to create propulsion. The actuation pulse lasts several microseconds and is achieved using a novel high-force/low-power thrust actuator, nanoporous energetic silicon, resulting in 250 μJ of kinetic energy delivered to the robot and a vertical height of approximately 8 cm.

Characterization of Contact Resistance Stability in MEM Relays With Tungsten Electrodes

Abstract: The impact of device operating parameters on the ON-state resistance (RON) of microelectromechanical relays with tungsten (W) electrodes is reported. Due to the susceptibility of W to oxidation, RON increases undesirably over the device operating cycles. This issue is aggravated by Joule heating when the relay is in the on state. The experimental results confirm that shorter ON time, as well as shorter off time, provides for more stable RON with respect to the number of ON/OFF switching cycles.

A Refreshable Braille Cell Based on Pneumatic Microbubble Actuators

Abstract: A refreshable Braille cell as a tactile display prototype has been developed based on a 2 x 3 pneumatic microbubble actuator array and an array of commercial valves. The microbubble actuator acting as a Braille dot consists of a parylene corrugated diaphragm and an overcoated elastomer layer. Polyurethane (PU) and polydimethylsiloxane (PDMS) elastomer-based Braille cells were fabricated and compared in terms of their displacement-pressure characteristics, force-displacement characteristics, dynamic response, and hysteresis of elastic deformation. Both PU- and PDMS-based pneumatic Braille cells demonstrate satisfactory properties against the static and vibrational tactile display requirements. The displacement of the PU-based Braille dot is 0.56 mm at a 0.2-Hz operating frequency, and the generated force is 66 mN at a 100-kPa applied pressure. The strong hysteresis observed in microbubble actuators made with PU elastomer is resolved by the use of PDMS elastomer. The refreshable Braille cell was also designed to meet the criteria of lightness and compactness to permit portable operation. The design is scalable with respect to the number of tactile actuators while maintaining fabrication simplicity.

Integrated Optofluidic Iris

Abstract: An optofluidic iris, which uses the absorption of an opaque liquid in combination with a second transparent liquid for defining optical apertures of variable diameters, tuned by directly deforming a ring-shaped liquid-liquid interface using electrowetting on dielectrics, is presented. The novelty lies not only in the absence of any moving parts or membranes but also in the design of the liquid ring, which allows planar microfabrication processes to be used exclusively for defining all functional parts of the iris. When compared to other optofluidic approaches, this concept combines ultracompactness, full integration, moderate drive voltage (75 V), low power consumption ( ; 86%), and an optical quality suitable for imaging applications, with a fabrication process which will allow mass fabrication.

Parametric Study of Zigzag Microstructure for Vibrational Energy Harvesting

Abstract: A comprehensive parametric study is presented on the vibration and the energy harvesting performance of a low-frequency zigzag energy harvester. The zigzag microelectromechanical systems (MEMS) vibrational energy harvesters have low natural frequencies which match the low-frequency range of ambient vibrations. The harvesters can, therefore, be designed to resonate with ambient excitation. The power produced by energy harvesters at resonance is orders of magnitude larger than off resonance power. The paper aims at providing an easy-to-use, comprehensive tool for designing the harvesters for different applications. The two key characteristics of the vibrational energy harvesters are their resonance frequency and their power transfer function. We formulate both vibrations and power production of the zigzag MEMS harvesters in nondimensional equations. The paper advances the state of the art in MEMS energy harvesting research area by identifying the dimensionless parameters governing mechanical vibrations and energy generation. We also investigate how the resonant frequency and the maximum power vary with each of the corresponding dimensionless parameters. The graphs summarize the parametric studies and provide sufficient tools for design of zigzag harvesters. The natural frequencies are related to six dimensionless variables, and the power transfer functions depend on 12 dimensionless parameters.

Real-Time Monitoring of Whole Blood Coagulation Using a Microfabricated Contour-Mode Film Bulk Acoustic Resonator

Abstract: We report a microelectromechanical systems contour-mode film bulk acoustic resonator (C-FBAR) to monitor in-vitro whole blood coagulation in real time. The C-FBAR has a suspended ring made of piezoelectric aluminum nitride excited in the radial-extensional mode. It operates at its resonant frequency of 150 MHz and possesses a quality factor of 77 in citrated human blood. The C-FBAR is characterized using aqueous glycerine solutions showing that it accurately measures the viscosity in the range of 1 to 10 centipoise. The C-FBAR, then, is used to monitor in-vitro blood coagulation processes in real time. Results show that its resonant frequency decreases as viscosity of the blood increases, during the fibrin generation process after the coagulation cascade. The coagulation time and the start/end of the fibrin generation are quantitatively determined. The C-FBAR has the potential to become a low-cost, portable, yet reliable tool for hemostasis diagnostics.

CMOS-Based High-Density Silicon Microprobe Arrays for Electronic Depth Control in Intracortical Neural Recording–Characterization and Application

Abstract: This paper reports on the characterization and intracortical recording performance of high-density complementary-metal-oxide-semiconductor (CMOS)-based silicon microprobe arrays. They comprise multiplexing units integrated on the probe shafts being part of the signal transmission path. Their electrical characterization reveals a negligible contribution on the electrode impedances of 139 ±11 kΩ and 1.2 ±0.1 MΩ and on the crosstalks of 0.12% and 0.98% for iridium oxide ( IrOx) and platinum (Pt) electrodes, respectively. The power consumption of the single-shaft probe was found to be 57.5 μW during electrode selection. The noise voltage of the switches was determined to be 5.6 nV/√Hz; it does not measurably affect the probe performance. The recording selectivity of the electrode array is demonstrated by electrical potential measurements in saline solution while injecting a stimulating current using an external probe. In-vivo recordings in anesthetized rats using all 188 electrodes with a pitch of 40.7 μm are presented and analyzed in terms of single neural activity and signal-to-noise ratio. The concept of electronic depth control is proven by performing mechanical translation of the probe shaft while electronically switching to adjacent electrodes to compensate the mechanical shift.

Mode-Localized Displacement Sensing

Abstract: We report the construction of a new class of micromachined displacement sensors that employ the phenomenon of vibration-mode localization for monitoring minute inertial displacements. It is demonstrated both theoretically and experimentally that the eigenstate-shifted output signal of such mode-localized displacement sensors may be as high as 1000 times greater than corresponding resonant-frequency variations that serve as the output in the more traditional vibratory resonant micromechanical displacement/motion sensors. The high parametric sensitivities attainable in such mode-localized displacement sensors, together with their inherent advantages of improved environmental robustness and electrical tunability, suggest an alternative approach in achieving improved sensitivity and stability in high-resolution displacement transduction.

Monolithic Integration of Pressure Plus Acceleration Composite TPMS Sensors With a Single-Sided Micromachining Technology

Abstract: This paper concerns the development of a single-side micromachined tire-pressure monitoring system (TPMS) sensor for automobiles, which is with a piezoresistive pressure sensor and a cantilever-mass piezoresistive accelerometer monolithically integrated in a 1.6 mm × 1.5 mm sized (111)-silicon chip. Single-wafer-based front-side silicon micromachining and metal electroplating technologies are employed to fabricate the device. Specially designed releasing trenches along (111) orientation are constructed to form the hexagonal pressure-sensitive diaphragm and the postsealed vacuum reference cavity. The fabrication of the accelerometer is also based on a hexagonal diaphragm that is latterly cut into suspended cantilevers and seismic mass. To achieve a high sensitivity, a high-density copper thick film is selectively electroplated to significantly increase the mass. The performance of the 115-g-ranged accelerometer is measured, exhibiting a sensitivity of 99.9 μV/g (under 3.3-V supply), nonlinearity of ±0.45% FS, and the noise floor of better than 0.2 g. The 750-kPa-ranged pressure-sensor sensitivity is measured as 0.108 mV/kPa (under 3.3-V supply), with the nonlinearity error smaller than ±0.1% FS and the temperature coefficient of sensitivity as -0.19%/°C FS before compensation. The noise floor of the pressure-sensor out- put signal is 0.15 kPa. The zero-point temperature coefficient is tested as -0.11%/°C FS and -0.024%/°C FS for the accelerometer and the pressure sensor, respectively. Fabricated with the low-cost front-side micromachining technique, the small-sized TPMS sensors are promising in practical applications and volume production.

Characterization of a Mechanical Motion Amplifier Applied to a MEMS Accelerometer

Abstract: In this paper, a mechanical amplification concept for microelectromechanical systems (MEMS) physical sensors is proposed with the aim to improve their sensitivity. The scheme is implemented using a system of micromachined levers (microlevers) as a deflection amplifying mechanism. The effectiveness of the mechanism is demonstrated for a capacitive accelerometer. A proof-of-concept single-axis mechanically amplified accelerometer with an amplification factor of 40 has been designed, simulated, and fabricated, and results from its evaluation are presented in this paper. The sensor's amplified output has a sensitivity of 2.39 V/g using an open-loop capacitive pick-off circuit based on charge amplifiers. Experimental results show that the addition of the mechanical amplifier does not alter the noise floor of the sensor. The measured natural frequency of the first mode of the sensor is at 734 Hz, and the full-scale measurement range is up to 7 g with a maximum nonlinearity of 2%. It is shown, through comparison with a conventional design, that the mechanically amplified accelerometer provides higher deflection without sacrificing bandwidth.

Hollow Out-of-Plane Polymer Microneedles Made by Solvent Casting for Transdermal Drug Delivery

Abstract: Although hollow microneedles have been proposed as an effective and convenient method for transdermal drug delivery, their expensive fabrication techniques to date have prevented their mass fabrication as a viable option. A novel method, based on solvent casting, is presented for inexpensive fabrication of hollow out-of-plane polymer microneedles. Microneedles are formed during a solvent evaporation process, which leaves a polymer layer around pillars in a prefabricated mold. The mold is fabricated using photolithography and can be used for consecutive solvent casting of microneedles. Arrays of microneedles with lengths up to 250 μm have been fabricated from clay-reinforced polyimide. Several mechanical tests were performed on solvent cast solid structures to find the optimum clay percentage in the polyimide that would lead to the highest compressive strength. The fabricated needles were tested for robustness, and it was observed that the needles were capable of withstanding on average compressive loads of up to 0.32 N. The suitability of the microneedles for skin penetration and drug delivery was demonstrated by injection of fluorescent beads into a skin sample.