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This paper reports a bimorph pinned piezoelectric micromachined ultrasonic transducer (PMUT) for enhanced sound pressure output and electromechanical coupling factor for space imaging applications. An equivalent circuit model has been built to analyze the dynamic behavior of PMUTs and prototype devices have been designed, fabricated. Experimental results show a 58% measured increase in the electromechanical coupling factor and pulse-echo measurements show a 4-meter traveling distance in air under a 133 kHz driving frequency. As a demonstration example, a relatively long-distance (>1.5 meters) ultrasound imaging test in air is conducted using a $4\times 4$ bimorph, pinned dual-electrode PMUTs array based on the transmission beamforming scheme. As such, this work enhances and extends the PMUT performances and capabilities for potential broad applications. [2021-0102]

This letter describes a blood flowmeter by the pulsed wave Doppler (PWD) ultrasound detection procedure utilizing piezoelectric micromachined ultrasonic transducers (pMUTs) with the aluminum nitride (AlN) film as the piezoelectric material. The transceiver electronics for the PWD scheme are constructed for system control and signal processing, and a blood-mimicking fluid (BMF) is synthesized for flow velocity tests. Both the average Doppler frequency-shift and Doppler power spectrum density (PSD) shift with respect to the flow velocity have been characterized and validated, and the flow direction can also be recognized. Finally, a real-time pulsatile flow measurement is demonstrated to emulate the feasibility of monitoring blood flows in practice. [2021-0077]

Miniaturized self-powering systems that integrate both energy harvesters and energy storage units as the power sources are essential to realize maintenance-free wireless sensor networks, implantable medical devices, and active radio frequency identification systems. On-chip micro supercapacitors (MSCs) are promising candidates for energy storage in such systems by providing high power densities, fast charge/discharge rates, and long cycle life. Researchers have been improving the performances, especially energy and power densities, of MSCs in recent years. This paper reviews the fundamental working mechanisms and design considerations of on-chip MSCs with special emphasis on the advantages of 3-D configurations. Typical fabrication methods are summarized, and their effects on the device performance and system integration are analyzed. In particular, the power generation of micro energy harvesters and the power consumption of typical wireless micro systems are surveyed, providing the basic and targeting performance requirements of future MSCs that can be integrated with them. [2017-0069]

This paper presents a passive wireless sensor for simultaneously and remotely measuring pressure and temperature under harsh environments. The sensor consists of a dual $LC$ (inductor and capacitor) resonant circuit, one without a cavity and the other with a cavity capacitor for temperature and pressure sensing, respectively. The low-temperature co-fired ceramic technology is used to fabricate the sensor, making it suitable for high-temperature harsh environment operations. Experimental results show the prototype sensor has temperature sensitivity of 8.15 kHz/°C and pressure sensitivity of 1.96 MHz/Bar up to 400°C. [2016-0157]

This paper presents the concept, basic theory, fabrication, and testing results of dual-electrode bimorph piezoelectric micromachined ultrasonic transducers (pMUTs) for both air- and liquid-coupled applications. Both the theoretical analyses and experimental verifications under the proposed differential drive scheme display high drive sensitivity and an electromechanical coupling energy efficiency that is as high as $4\times $ of the state-of-the-art pMUT with a similar geometry and frequency. The prototype transducers are fabricated in a CMOS-compatible process with the radii of 100–230 $\mu \text{m}$ using aluminum nitride as the piezoelectric layers with the thicknesses varying from 715 to 950 nm and molybdenum (Mo) as the electrodes with a thickness of 130 nm. The tested operation frequencies of the prototype transducers are 200–970 kHz in air for possible ranging and motion detection applications, and from 250 kHz to 1 MHz in water for medical ultrasound applications such as fracture healing, tumor ablation, and transcranial sonothrombolysis. A $12\times 12$ array structure is measured to have the highest intensity per voltage squared, per number of pMUTs squared, and per piezoelectric constant squared ( $I_{n}= I/(VNd_{31})^{2})$ among all reported pMUT arrays. The generated acoustic intensity is in the range of 30–70 mW/cm2 up to 2.5 mm from the transducer surface in mineral oil with a driving voltage of 5 $\text{V}_{{\mathrm{ac}}}$ , which is suitable for battery-powered therapeutic ultrasound devices. [2015-0305]

This paper reports an investigation into the various dissipation mechanisms that can affect polycrystalline diamond micromechanical resonators. Double-ended tuning fork and cantilever resonators were fabricated from 1–5- $\mu \text{m}$ thick microcrystalline diamond films. It is shown that the quality factor of the low frequency ( N 2 for various times up to 660 min is used to modify the resonator surface and is shown to result in a threefold increase in quality factor up to 365 000 at 26.6 kHz. [2015-0094]

A rapid and manufacturable technique to bond silicon to steel for microelectromechanical system (MEMS) sensor applications is presented. Using an induction heating system, the process is simple, clean, inexpensive, and fast, with bonding occurring in 3-5 s. Heat generation is isolated to within several micrometers below the steel surface, allowing for a controllable thermal budget that does not damage the surface treatment of steel. The process requires minimal surface conditioning and may be used with a variety of different metal systems. Bonds formed with adhesion layers of gold, silver, and nickel showed excellent mechanical characteristics, surviving surface strains exceeding +/- 2000 μe. The transferred strain was quantified by ANSYS simulation and measured experimentally for silicon substrates of varying thicknesses, with the 100 μm-thick substrates demonstrating the greatest strain sensitivity. A double-ended tuning fork MEMS resonant strain sensor, bonded to a steel parallel, demonstrated a repeatable 5.4-fold improvement in strain sensitivity compared to epoxy bonding. As such, this robust bonding process is well-suited for bonding MEMS sensors to monitor components in the automotive, manufacturing, and other harsh environment industries.

Microrelays with liquid metal wetted contacts have been demonstrated using bidirectional electrothermal electromagnetic actuators. These relays were fabricated with the Metal MUMPs foundry process, which has a 20-mum-thick nickel structural layer. The operating voltage is under 0.5 V. The measured breakdown voltage and off-state resistance are greater than 200 V and 100 MOmega, respectively, and the gold-to-gold contact resistance is around 0.3 Omega. When the contacts are wetted with liquid gallium alloy (melting point at -20degC), the measured contact resistance can be as low as 0.015 Omega. As such, these bidirectional relays could have potential applications in high-power switching systems with low contact resistance using liquid metal wetted contacts.

This paper presents a device-level microelectromechanical systems (MEMS) packaging process with accelerated tests and reliability analysis. Surface-micromachined microresonators are sealed inside microcavities by a rapid thermal processing (RTP) aluminum-to-silicon nitride bonding and packaging technique. Chip-to-chip bonding is used to form packages both under atmospheric pressure and in vacuum. The hermeticity of the package seals are evaluated by IPA (isopropyl alcohol) leak tests. The vacuum seal is evaluated by measuring the Q-factor (quality factor) of the packaged microresonators. The measured Q-factor of a vacuum-packaged comb-resonator is 1800/spl plusmn/200, corresponding to a 200 motor vacuum inside the micro cavity, and has not degraded over 37 weeks of shelf-life. The reliability information is evaluated by combining accelerated testing of the packages in a harsh environment (an autoclave chamber, 130/spl deg/C, 2.7 atm and 100% RH) and statistical analysis. The mean-time-to-failure (MTTF) of the packaged device is estimated as 29.7 weeks in an autoclave chamber, and tests on vacuum-packaged devices have confirmed the estimation.

A microplastic lens array has been successfully constructed on top of a 500-/spl mu/m-thick PC (Polycarbonate film) by using a micro hot intrusion process. A single-layer LIGA process is used to fabricate the high-aspect-ratio nickel mold insert that has circular hole patterns of 80 /spl mu/m in diameter and 200 /spl mu/m in depth. Under the hot intrusion process, plastic material can be intruded into these circular-shape holes and stopped at desired depth under elevated temperature and pressure to fabricate microlenses. By adjusting the embossing load, temperature and time, the curvature and height of the lens are controllable when the same mold insert is used. The optical properties of these microlenses have been characterized and the average radius of curvature is found as 41.4 /spl mu/m with a standard deviation of 1.05 /spl mu/m. Experimental characterization and theoretical model are conducted and developed for the micro-intrusion process in terms of the radius of curvature and height of the lenses and they correspond well with experimental data within 5% of variations. The focusing capability of the lenses is demonstrated by comparing the images of laser light with and without using the lenses. When the projection screen is placed 200 /spl mu/m away from the lens, the full-width at half-maximum (FWHM) for the lens is 110 /spl mu/m while the original FWHM of the optical fiber is 300 /spl mu/m.

A plastic micro drug delivery system has been successfully demonstrated by utilizing the principle of osmosis without any electrical power consumption. The system has an osmotic microactuator (see Su, Lin, and Pisano, J. Microelectromech., vol. 11, pp. 736-7462, Dec. 2002) and a polydimethylsiloxane (PDMS) microfluidic cover compartment consisting of a reservoir, a microfluidic channel and a delivery port. The typical dimension of the microfluidic channel is 1 cm in length with a cross-sectional area of 30/spl times/100 /spl mu/m/sup 2/ to minimize the diffusive drug flow while pressure drop remains moderate. Using oxygen plasma to activate the surfaces of polymers for bonding, the osmotic actuator is bonded with the PDMS cover while liquid drug can be encapsulated during the bonding process. Employing the net water flow induced by osmosis, the prototype drug delivery system has a measured constant delivery rate at 0.2 /spl mu/L/h for 10 h with an accumulated delivery volume of 2 /spl mu/L. Both the delivery rate and volume could be altered by changing the design and process parameters for specific drug delivery applications up to a few years. Moreover, the induced osmotic pressure can be as high as 25 MPa to overcome possible blockages caused by cells or tissues during drug delivery operations.

This paper points out the small typographical author errors found in the above paper [1] to eliminate any confusion for the readers.

A micromachined battery based on liquid electrolyte and metal electrodes for on-demand and disposable usages has been successfully demonstrated. The microbattery uses gold as the positive electrode and zinc as the negative electrode and is fabricated by using the standard surface micromachining technology. Two kinds of electrolytes have been tested, including the combination of sulfuric acid/hydrogen peroxide and potassium hydroxide. The operation of the battery can be on-demand by putting a droplet of electrolyte to activate the operation. Theoretical voltage and capacity of the microbattery are formulated and compared with experimental results. The experimental study shows that a maximum voltage of 1.5 V and maximum capacity of 122.2 /spl mu/W-min have been achieved by using a single droplet of about 0.5 /spl mu/l of sulfuric acid/hydrogen peroxide.

Variations in micromachining processes cause submicron differences in the size of MEMS devices, which leads to frequency scatter in resonators. A new method of compensating for fabrication process variations is to add material to MEMS structures by the selective deposition of polysilicon. It is performed by electrically heating the MEMS in a 25/spl deg/C silane environment to activate the local decomposition of the gas. On a (1.0/spl times/1.5/spl times/100) /spl mu/m/sup 3/, clamped-clamped, polysilicon beam, at a power dissipation of 2.38 mW (peak temperature of 699/spl deg/C), a new layer of polysilicon (up to 1 /spl mu/m thick) was deposited in 10 min. The deposition rate was three times faster than conventional LPCVD rates for polysilicon. When selective polysilicon deposition (SPD) was applied to the frequency tuning of specially-designed, comb-drive resonators, a correlation was found between the change in resonant frequency and the length of the newly deposited material (the hotspot) on the resonator's suspension beams. A second correlation linked the length of the hotspot to the magnitude of the power fluctuation during the deposition trial. The mechanisms for changing resonant frequency by the SPD process include increasing mass and stiffness and altering residual stress. The effects of localized heating are presented. The experiments and simulations in this work yield guidelines for tuning resonators to a target frequency.

A thermal-bubble-actuated micropump by the principles of liquid/vapor phase transition and nozzle-diffuser flow regulation is successfully demonstrated. The micropump consists of a resistive heater, a pair of nozzle-diffuser flow controller and a 1 mm in diameter, 50 /spl mu/m in depth pumping chamber. The actuation mechanism comes from periodically nucleating and collapsing thermal bubbles. A net flow is generated from the nozzle to the diffuser by the nozzle-diffuser flow controller. Two heater designs, single-bubble and dual-bubble actuation mode, have been investigated. In the single-bubble pumping mode, a maximum flow rate of 5 /spl mu/l/min is measured when the driving pulse is 250 Hz at 10% duty cycle under an average power consumption of 1 W. A similar flow rate of 4.5 /spl mu/l/min is achieved in the dual-bubble pumping mode, at the driving pulse of 5% duty cycle at 400 Hz with lower average power consumption, 0.5 W. The static pumping pressure is measured at a maximum value of 377 Pascal when the net volume flow rate is zero. As an application example in a microfluidic device, this valve-less micropump is used in a microfluidic system to enhance the fluid mixing by agitating the flows.

This paper presents a microactuator that utilizes osmosis to produce mechanical actuation without consuming any electrical energy. The microactuator is made of cellulose acetate with cylindrical chamber of 500 to 2000 /spl mu/m in diameter and of 200 to 1000 /spl mu/m in depth. Sodium chloride is chosen as the osmotic driving agent to be placed inside the chamber. A semipermeable diaphragm made of cellulose acetate is processed at the bottom of the chamber to control the water flow. Either a cellulose acetate diaphragm or an impermeable diaphragm made of vinylidene chloride and acrylonitrile copolymer is spin-coated on top of the chamber as the actuation diaphragm. Using the principle of osmosis, this water-powered, osmotic microactuator can employ high osmotic pressure (a chemical potential) up to 35.6 MPa to provide hydrostatic pressure for mechanical actuation. Experimental measurements show that up to 800 /spl mu/m vertical diaphragm movement (diaphragm size of 800 /spl mu/m in diameter) and constant volume expansion rate of 4.5 to 11.5 nL/h can be achieved. When integrated with other microfluidic devices, this osmotic microactuator could serve as a clean, compact and inexpensive fluidic actuation source.

A glass vacuum package based on localized aluminum/silicon-to-glass bonding has been successfully demonstrated. A constant heat flux model shows that heating can be confined locally in the dielectric layer underneath a microheater as long as the width of the microheater and the thickness of silicon substrate are much smaller than the die size and a good heat sink is placed underneath the silicon substrate. With 3.4 W heating power, /spl sim/0.2 MPa applied contact pressure and 90 min wait time before bonding, vacuum encapsulation at 25 mtorr (/spl sim/3.33 Pa) can be achieved. Folded-beam comb drive /spl mu/-resonators are encapsulated and used as pressure monitors. Long-term testing of vacuum-packaged p-resonators with a Quality Factor (Q) of 2500 has demonstrated stable operation after 69 weeks. A /spl mu/-resonator with a Q factor of /spl sim/9600 has been vacuum encapsulated and shown to be stable after 56 weeks.

Miniaturization is introduced as a novel methodology to enhance mass transport in a chemical vapor deposition process. As a result, amorphous carbon formation during the synthesis of single-walled carbon nanotubes (SWNTs) can be deterred. Miniaturization also maintains a laminar flow pattern to ensure a stable growth condition. A system with micrometer-sized reaction chambers has been constructed to experimentally verify this concept. The results show that clean and small-diameter SWNTs with approximately millimeter length can be quickly synthesized using ethylene as the source gas. Similar experimental parameters in a conventional large-scale system failed to produce SWNTs with comparable quality due to catalyst poisoning.

A hermetic package based on localized aluminum/silicon-to-glass bonding has been successfully demonstrated. Less than 0.2 MPa contact pressure with 46 mA current input for two parallel 3.5-/spl mu/m-wide polysilicon on-chip microheaters can raise the temperature of the bonding region to 700/spl deg/C bonding temperature and achieve a strong and reliable bond in 7.5 min. The formation of aluminum oxide with silicon precipitate composite layer is believed to be the source of the strong bond. Accelerated testing in an autoclave shows some packages survive more than 450 h under 3 atm, 100% RH and 128/spl deg/C. Premature failure has been attributed to some unbonded regions on the failed samples. The bonding yield and reliability have been improved by increasing bonding time and applied pressure.

Self-buckling behavior of micromachined beams under resistive heating is described by an electromechanical model with experimental verifications. This model consists of both electrothermal and thermoelastic analyses for beam-shape polysilicon microstructures that are fabricated by a standard surface micromachining process. When an input electrical current is applied, Joule-heating effects trigger the thermal expansion of beam structures and cause mechanical buckling. The standard testing devices are clamped-clamped bridges, 2-/spl mu/m wide, 2-/spl mu/m thick, and 100-/spl mu/m long. It is found that a minimum current of 3.5 mA is required to cause beam buckling. Under an input current of 4.8 mA, a lateral deflection of 2.9/spl plusmn/0.2 /spl mu/m at the center of the bridge is measured with a computer image processing scheme. The experimental measurements are found to be consistent with analytical predictions. A discussion of modeling considerations and process variations is presented.

A combination of surface- and bulk-micromachining techniques is used to demonstrate the feasibility of fabricating microhypodermic needles. These microneedles, which may be built with on-board fluid pumps, have potential applications in the chemical and biomedical fields for localized chemical analysis, programmable drug-delivery systems, and very small, precise sampling of fluids. The microneedles are fabricated in 1, 3, and 6 mm lengths with fully enclosed channels formed of silicon nitride. The channels are 9 /spl mu/m in height and have one of two widths, 30 or 50 /spl mu/m. Access to the channels is provided at their shank and distal ends through 40-/spl mu/m square apertures in the overlying silicon nitride layer. The microneedles are found to be intact and undamaged following repetitive insertion into and removal from animal-muscle tissue (porterhouse steak).

A simulation program is developed which is capable of calculating the output responses of piezoresistive pressure sensors as a function of pressure and temperature. Analytical models based on small and large deflection theories have been applied to predict the sensitivity and linearity of pressure sensors. Surface-micromachined diaphragms with square or circular shapes, fabricated by a low pressure chemical vapor deposition sealing process, are designed and tested to verify the program. They are made of polysilicon and have a standard width (diameter) of 100 /spl mu/m and thickness from 1.5 to 2.2 /spl mu/m. Various parameters of the piezoresistive sensing resistors, including length, orientation, and dopant concentration, have been derived and constructed on top of the diaphragms. For a 100-/spl mu/m-wide 2-/spl mu/m-thick square-shape pressure sensor, calculated and experimental results show that int sensitivity of 0.24 mV/V/(Ibf/in/sup 2/) is achieved. Experimentally, non a maximum linearity error of /spl plusmn/0.1% full-scale span) is found out on a 100-/spl mu/m-wide 2.2-/spl mu/m-thick square-shape pressure sensor. Both sensitivity and linearity are characterized by the diaphragm thickness and the length of the sensing resistors.

A passive micro strain gauge with a mechanical amplifier has been designed, analyzed, and tested. The mechanical amplifier provides a high gain such that residual strain in thin films can be directly measured under an optical microscope. This strain gauge can be in situ fabricated with active micro sensors or actuators for monitoring residual strain effects, and both tensile and compressive residual strains can be measured via the strain gauge. It is shown that a very fine resolution of 0.001% strain readouts can be achieved for a micro strain gauge with a 500 /spl mu/m-long indicator beam. Beam theories have been used to analyze the strain gauge with a mechanical amplifier, and the results were verified by a finite-element analysis. Experimental measurements of both polysilicon and silicon-riched silicon-nitride thin films fabricated by surface micromachining processes are presented.