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Unlike electromagnetic waves, acoustic vibrations waves can be used to transfer power directly through metal structures without being shielded. In this article, a novel design of a self-detachable acoustic wireless power transfer system that can be used to transfer power through the thickness of a steel plate is presented, which does not require the use of any couplant. Electro-permanent-magnets (EPMs) were used to provide magnetic clamping force along the perimeter of the receiver transducer disk to enhance coupling to the steel plate, while the transmitter transducer was bonded to the other side of the plate. The EPM clamping force can be switched ON/ OFF electronically with low power consumption. Unlike past work reliant on additional bonding materials or liquid/gel couplant, this approach enables the receiver to be attached and detached at will, opening up the possibility of a simple charging pad for unmanned aerial vehicles (UAVs) or other consumer devices for harsh environment applications. Power transfer efficiency up to 63% was achieved, and the effect of varying steel plate thickness and clamping force was also investigated. A finite element model was also constructed to understand the vibration mode shape.

This letter reports on the passive voltage amplification provided by X-cut Lithium Niobate (LN) S0 mode resonators when implemented as Matching Networks (MN) in the front-end of ultra-low-power, Internet of Things (IoT) receivers. In the first section, a simple gain analytical model is described. In the second part, devices design rules and geometries are disclosed. In the third and last section, measurements methodology and results are presented. Measured gains of 38.9 dB and 32.5 dB are reported at 110 and 570 MHz, respectively, for a capacitive load of ~100 fF. These performance represent, to the knowledge of the authors, the largest passive voltage gains ever recorded at the given frequencies for MEMS-based matching networks.

This paper presents a 1-port thickness-shear (TS) mode quartz resonator with very high voltage gain, suitable for use in “near-zero-power” radios. These devices offer voltage gains >100x (referenced to $50~\Omega $ ) at ~50 MHz and >300x at ~75 MHz with ~60 fF load, which is the highest voltage gain directly measured to date in a piezoelectric micromechanical resonator. Six resonators with resonance ${f}_{{\text {r}}}\approx 50$ MHz and identical electrode dimensions are characterized by S11 measurements and by directly measuring the voltage gain using a high-impedance active probe. Quality factor ( ${Q}$ ) up to 140,000 at ~50 MHz and electromechanical coupling ( ${k}_{{\text {eff}}}^{2}$ ) up to 0.445% are demonstrated. In several devices, voltage gain around or above 100 V/V is observed for load ${C}~\approx ~60$ fF, and the gain changes as expected with decreasing load impedance. Additionally, a similar 1-port quartz resonator design at ${f}_{{\text {s}}}~\approx ~75$ MHz is demonstrated to have voltage gain over 300 V/V (referenced to $50\Omega $ ) and ${Q}$ around 335,000. These quartz resonators are fabricated in a mature, commercially available process, promoting widespread integration into radio-frequency (RF) wake-up receivers (WURs). [2020-0196]

Ultrasonic guided plate waves (Lamb waves) can be used to transfer power along the length of metal plates, achieving longer distance wireless power transfer (WPT), while not being impeded by electromagnetic shielding from the metal plate. In this article, a fundamental study on the performance of Lamb wave WPT is presented, including modeling, simulations, and experimental verification. By using Macro-Fiber Composite (MFC), ${d}_{33}$ -mode, piezoelectric transducers bonded to a $64\,\,\text {mm} \times 406\,\,\text {mm} \times 1.6$ mm aluminum plate using an epoxy, power transfer of 0.47 W with 56% overall power transfer efficiency was achieved at a 204-mm distance. The measured frequency response of the power transfer efficiency matches well with the simulated results, and the effects of complex load impedance matching and transducer sizing were investigated. It is shown that the location where the efficiency is maximized roughly corresponds to the zero-order symmetrical mode (S0) standing wave patterns due to reflections from the plate edges. For practical implementation, the effect of using different methods to temporarily or permanently bond the MFC transducers to the metal plate was also investigated, as well as the effect of electrically grounding the metal plate.

The purpose of this work is to assess the performance of an AT-cut quartz device as a two-port resonant transformer for near-zero power-consuming wake-up receiver applications. The device consists of two, coupled thickness-shear mode trapped-energy ~50 MHz resonators on a single quartz substrate. Passive voltage gain up to 84 V/V (referenced to $50\Omega $ ) is observed when the device is driven by a low-power signal (≤ −30 dBm) at resonance. This transformer offers high voltage gain suitable for use in the front-end of a near-zero-power wake-up receiver. Additionally, this work shows that connecting two coupled-resonator quartz devices in a parallel-input, series-output (PISOT) configuration provides improvements in voltage gain. A 17% increase in gain is demonstrated for PISOT over that of a single coupled resonant transformer for various load impedances. This work is the first use of quartz piezoelectric transformers (PTs) for passive voltage gain, and the highest open-circuit gain demonstrated in a two-port micromechanical PT.

Origami manufacturing, the use of folding to create 3-D shapes using 2-D fabrication tools and techniques, is a promising avenue for creating low cost parts. In this letter, we demonstrate the hands-free cutting and folding of 3-D inductor geometries from blank metal foil using only a 2-D laser cutter. Laser forming, localized laser heating to create thermal stresses to deform a workpiece, is used to fold traces out of plane without manual handling. The laser forming of thin copper foil is first characterized, followed by cutting and folding of two 3-D inductors: a three-turn planar coil with folded overpass and a twelve-turn toroid. The planar coil was found to have inductance 199 nH and peak quality factor of 99.7 at 54.7 MHz, while the toroid had inductance 159 nH with peak quality factor 64.9 at 86.7 MHz. With laser cutters, one of the most widely available machine tools, this technology allows custom, high performance electrical passives to be rapidly fabricated without specialized equipment.

Thick metal barriers prevent the use of conventional electromagnetic wireless power transfer due to Faraday shielding effects. Here, for the first time, we demonstrate power transfer to and communication with a compact wireless sensor transponder fully embedded within a solid piece of metal through the use of ultrasonic waves. This technique is an important innovation for applications, such as structural health monitoring of solid metal structural components, which would be weakened by the presence of holes for wiring. The embedded sensor system consists of a single piezoelectric transducer used for both power and data communication, along with an energy storage unit and associated charging circuitry, as well as a representative sensor and sensor data acquisition electronics. The entire sensor transponder was packaged within a compact volume of 11.9 cm3 (0.73 in3) and can be further miniaturized to millimeter-scale sizes with the use of widely available system-on-chip technologies. The measured power transfer and sensor data results were found to closely match modeling results, achieving a power transfer efficiency of 33% at 440 kHz, showcasing the feasibility and potential of this approach.

Direct-write laser grayscale lithography has been used to facilitate a single-step patterning technique for multilayer lead zirconate titanate (PZT) thin films. A 2.55- $\mu \text{m}$ -thick photoresist was patterned with a direct-write laser. The intensity of the laser was varied to create both tiered and sloped structures that are subsequently transferred into multilayer PZT(52/48) stacks using a single Ar ion-mill etch. Traditional processing requires a separate photolithography step and an ion mill etch for each layer of the substrate, which can be costly and time consuming. The novel process allows access to buried electrode layers in the multilayer stack in a single photolithography step. The grayscale process was demonstrated on three 150-mm diameter Si substrates configured with a 0.5- $\mu \text{m}$ -thick SiO2 elastic layer, a base electrode of Pt/TiO2, and a stack of four PZT(52/48) thin films of either 0.25- $\mu \text{m}$ thickness per layer or 0.50- $\mu \text{m}$ thickness per layer, and using either Pt or IrO2 electrodes above and below each layer. Stacked capacitor structures were patterned and results will be reported on the ferroelectric and electromechanical properties using various wiring configurations and compared to comparable single layer PZT configurations.

This letter documents the design and demonstration of microelectromechanical systems (MEMS) piezoelectric transformers with a measured boost in effective figure of merit as high as 4.2X. This boost is enabled by an architecture approach, which takes transformers with figures of merit limited by the inherent effective electromechanical coupling factor, ${k}_{\textsf {eff}}^{\textsf {2}}$ , and device quality factor, and provides enhanced effective transformer figures of merit through architecture optimization. This is enabled by a parallel input series out transducer architecture inspired by prior approaches with multi-layer bulk piezoelectric transformers but first demonstrated here in MEMS with discrete transformers fabricated with ~45-MHz lead-zirconate-titanate (PZT)-on-silicon electromechanical resonators.

Wireless power transfer (WPT) through acoustic waves can achieve higher efficiencies than inductive coupling when the distance is above several times the transducer size. This paper demonstrates the use of ultrasonic phased arrays to focus power to receivers at arbitrary locations to increase the power transfer efficiency. Using a phased array consisting of 37 elements at a distance nearly 5 times the receiver transducer diameter, a factor of 2.6 increase in efficiency was achieved when compared to a case equivalent to a single large transducer with the same peak efficiency distance. The array has a total diameter of 7 cm, and transmits through air at 40 kHz to a 1.1-cm diameter receiver, achieving a peak overall efficiency of 4% at a distance of 5 cm. By adjusting the focal distance, the efficiency can also be maintained relatively constant at distances up to 9 cm. Numerical models were developed and shown to closely match the experimental energy transfer behavior; modeling results indicate that the efficiency can be further doubled by increasing the number of elements. For comparison, an inductive WPT system was also built with the diameters of the transmitting and receiving coils equivalent to the dimensions of the transmitting ultrasonic phased array and receiver transducer, and the acoustic WPT system achieved higher efficiencies than the inductive WPT system when the transmit-to-receive distance is above 5 cm. In addition, beam angle steering was demonstrated by using a simplified seven-element 1-D array, achieving power transfer less dependent on receiver placement.

Wireless power transmission is of paramount importance in stretchable electronics due to the challenges of fabricating effective electrical contacts between stretchable systems and the outside world. Poor stretchable inductor performance has been a major factor limiting the range of power transfer to stretchable systems. Here, through the development of stretchable inductors with the highest quality factors on record (as high as 174, more than double the highest previously demonstrated), we are able to increase the range of effective power transfer to stretchable electronics. Our stretchable inductors were fabricated using liquid-metal-filled channels in a soft silicone and can withstand strains up to 100%. Using this approach combined with magnetic resonance techniques, we show systems able to transfer power effectively at distances of one and two coil diameters, significantly beyond the half-coil diameter distance that was the previous state of the art. Using optimized designs, we demonstrate the efficiency of 64% at one coil diameter (40 mm) and a second system with the efficiency of 15.5% at two coil diameters (84 mm).

The fabrication and demonstration of on-chip thermite devices using evaporation driven assembly (EDA) of micro- and nano-particles into on-chip trenches is demonstrated here. EDA is a simple low-cost method for fabricating particle-based devices at atmospheric pressure and at or near room temperature. EDA of thermites features the controlled delivery of material into on-chip trenches, minimal waste and hazard while making thick devices, and flexible substrate choice. On-chip thermite devices are envisioned as single-use high-energy-density heat sources with low initiation energies for low power systems. These thermite devices may be used for on-chip thermal processing, soldering, fuzing, microelectromechanical or microfluidic system actuation, and so on. EDA is demonstrated to be a suitable method for making thermite devices 10’s of ${\mu }\text{m}$ thick, 10’s of ${\mu }\text{m}$ wide, and mm’s long. Burn rates up to 225 m/s and heating of devices to over 100 °C have been achieved, as has the actuation of shape memory alloy cantilevers having a transition temperature of 70 °C. These initial results show that significant localized heating can be achieved using these EDA fabricated thermite devices, which have the potential to be used for a wide range of applications. [2017-0163]

Currently available materials for III–V multijunction solar cells lattice matched to GaAs covering the spectral range from 1.65 to 1.82 eV are composed of either immiscible quaternary alloys or contain aluminum. We report the fabrication of a novel aluminum-free In $_x$ Ga $_{1-x}$ As $_{1-z}$ P $_z$ /Ga $_{1-y}$ In $_y$ P ( x > y ) strain-balanced multiple quantum-well (SBMQW) p-i-n solar cell structure lattice matched to GaAs, grown by metal–organic chemical vapor deposition. SBMQWs consist of alternating layers of In $_x$ Ga $_{1-x}$ As $_{1-z}$ P $_z$ wells and Ga $_{1-y}$ In $_y$ P barriers ( x > y ) under compressive and tensile strain, respectively. When compared with standard GaInP devices, SBMQW structures exhibit longer photoluminescence wavelength (680–780 nm) emission and enhanced light absorption with improved short-circuit current density. In this study, the SBMQW emission and absorption wavelength is controlled by adjusting the layer thickness of InGaAsP wells, while the arsenic and indium compositions are fixed. We show that carriers generated in QWs are extracted via thermionic emission. The proposed SBMQWs allow more flexibility in the design of current multijunction solar cells and future cells with more than four junctions. InGaAsP/GaInP SBMQWs may also be used in applications other than solar cells, such as light-emitting diodes (LEDs) and lasers, with the advantages of tuning the emission and absorption processes.

A completely integrated 3.3 V to 10 Vpk-pk bidirectional switched-capacitor ladder converter is presented for driving the piezo actuators of a 2 mm wing micro-robotic flying insect. The 0.1–10 MHz converter, fabricated in 1.2 V/3.3 V triple-well 0.13 $\upmu $ m CMOS, operates as a switched-mode amplifier with 0–3.3 V waveforms at the input and a constant 3x step-up while exhibiting symmetric drive strength for positive and negative load currents. A feedforward switching scheme that effectively bootstraps gate-drive signals for the power switches of the ladder converter has been implemented to enable output waveforms above both the supply and device voltage rating, between 0 V and ${\sim} $ 10 V. Non-overlapping clock signals float with the dc–500 Hz arbitrary input waveform, or a 2x scaled replica, while keeping all transistors within safe operating conditions. A voltage compliant sub-circuit, the Nested-Bootstrapped Switch (NBS) cell, is introduced as a building block for the creation of the gate-drive signals. On-chip MIM capacitors and 3.3 V I/O MOS switches for the ladder converter were sized based on a charge-multiplier representation of the converter and the output impedance concept. Experiments show actuation of a 2 mm wing prototype, an effective 5 nF peak load, and efficiency of 77% at 800 $\upmu $ W and 80% at 32 $\upmu $ W.

High-quality inductors are difficult to realize in stretchable electronics, since the thick parallel traces needed to minimize resistance also result in a highly rigid structure. Adding periodic waves or kinks to the traces of an inductor coil has been found to result in a more compliant structure that will not permanently deform or break after stretching by tens of percent. In this paper, the effects on electrical performance of creating inductor coils from stretchable wavy traces are investigated. Eight different tortuous trace designs were modeled and experimentally tested in one- and three-turn inductors, as well as an electrical transformer design. Incorporating waves into an inductor was found to result in a negative mutual coupling along the traces, degrading the inductor performance. The incremental self-inductance per resistance of the added length due to the waviness, $\sim 1.2$ nH/ $\Omega $ , was similar for all interconnect types tested. This value is less than a third that of using a longer straight conductor of similar cross section (4.4 nH/ $\Omega )$ , resulting in a drop in ratio of inductance to resistance and resulting peak quality factor in the measured inductors by as much as a factor of two.

This paper introduces a heterogeneous die integration process using electroplated copper to mount a bare die into a silicon handling wafer while simultaneously forming vertical, through-wafer vias. Deep reactive-ion etching is used to form openings in the handling wafer allowing the die to be flush-mounted for minimal device thickness. The backsides of the support wafer and die are coated by copper sputtering and electroplating, which physically secures the die in place. Electrical isolation is achieved through passivation of the silicon handle wafer sidewalls before copper electroplating. Wet thermal oxide growth was chosen over plasma-enhanced chemical vapor deposition as the sidewall passivation technique, as wet thermal oxide was found to yield superior coverage and uniformity. Topside interconnects were realized using a previously established thick-film copper metallization process. A $3\times 3$ die array was successfully produced and tested for die-to-die connectivity. Thermal modeling of the fabricated devices showed that power densities up to 1 W/cm $^{2}$ could be accommodated while keeping the maximum temperature below 85 °C.

This paper presents a multilayer wiring board that integrates a copper air-core microinductor to enable a highly compact, chip-scale power converter module. The wiring board is wafer-level fabricated with three 30-μm-thick electroplated copper layers and is subsequently detached from the fabrication wafer to yield a board that is only 90 μm thick for minimum overall module volume. Within this platform, a stacked-spiral air-core microinductor is designed for high-switching-frequency power conversion and yields high inductance density of 128 nH/mm 2 (100 nH/mm 3 by volume, including 600 μm clearances both above and below the inductor to minimize coupling with external conductors). Although this technology is anticipated to be more appropriate for emerging, experimental converters with switching frequencies >30 MHz, a proof-of-concept, ultraminiature (9 mm 2 footprint, 0.7 mm thick) power converter module is presented that utilizes the microinductor wiring board in conjunction with a commercially available surface-mount boost regulator (~4 MHz switching frequency). The converter module yielded a maximum output power of 153 mW at 60% efficiency for a volumetric power density of 24 mW/mm 3 based on the physical volume occupied by the module (13 mW/mm 3 based on the volume needing to be kept clear from external conductors).

Lattice-matched multiple quantum wells (MQWs) consisting of InxGa1-xAs wells with very thin GaAs0.2P0.8 barriers have been incorporated into a GaInP/GaAs tandem solar cell. InGaAs/GaAsP MQWs increase the short-circuit current of the GaAs cell by extending the absorption range, with minimal impact on an open-circuit voltage, thus alleviating current matching restrictions placed by the GaAs cell on multijunction solar cells. MQWs with very thin, tensile strained, high phosphorus content GaAsP barriers allow tunneling to dominate carrier transport across the MQWs and balance the compressive strain of the InGaAs wells such that material quality remains high for subsequent top cell growth. We show that the addition of the QW layers enhances the GaAs cell, does not degrade the performance of the GaInP top cell, and leads to potential efficiency enhancements.

This letter reports the design, fabrication, and performance of millimeter-wave transmission lines and devices in a tri-layer thick-copper (Cu) process. Beneath the Cu layers is a low-voltage lead zirconium titanate (PZT) MEMS stackup atop a nitride membrane, on high-resistivity silicon. The 10 μm thick Cu layers enable several transmission-line topologies, and the PZT-MEMS functionality enables tunability. We designed transmission lines and un-optimized noncontact distributed MEMS transmission line (DMTL) phase shifters for 75–110 GHz, and present static measurements here. Simulation and measurements are in close agreement, with measured CPW line loss 0.15 dB greater than simulated (0.27 dB/mm at 95 GHz). Excess loss is attributed to Cu roughness, conductivity, and membrane/strap supports. Measured noncontact DMTL phase shifters exhibit a mean of 40 $^{\circ}$ /dB when intrinsic line loss is subtracted.

This paper reports the performance of lead-zirconate-titanate (PZT)-on-silicon electromechanical resonators as thin-film piezoelectric transformers. The PZT-on-silicon resonators rely on the odd harmonics of the contour length-extensional modes to provide a convenient means for voltage and current transformations with a single layer of piezoelectric PZT. The resistive-load-dependent voltage gains and efficiencies are derived along with the peak efficiencies and open circuit voltage gains. The models of efficiency and voltage gain are compared to the experimental measurements of fabricated PZT-on-silicon piezoelectric transformers with 2, 4, and 10 μm of device silicon. The resonant frequencies of the devices range between 14 and 19 MHz. Peak efficiencies as high as 62% are measured and open circuit voltage gains as high as 5.7 are extracted from the measurements. The measured efficiencies with 50- Ω loads compare within 14% of the models on average (25% peak error). The extracted open-circuit voltage gains and their models compare within 22% on average (67% peak error). The trade between the load-dependent efficiencies and voltage gains are also presented for the n=3, 5, 9, and 13 harmonics.

Minority carrier transport across InGaAs/GaAsP multiple quantum wells is studied by measuring the response of p-i-n and n-i-p GaAs solar cell structures. It is observed that the spectral response depends critically upon the width of the GaAsP barriers and the device polarity. Electron tunneling is not as efficient as hole tunneling due to a higher conduction band barrier. The spectral response depends on the relative magnitude of the carrier lifetime as compared with the tunneling lifetime. This paper deduces an estimated electron lifetime of 110 ns in In0.14Ga0.86As wells and 25 ns in In0.17Ga0.83As wells, which agree with published results.

Multiple quantum wells (MQW) lattice matched to GaAs consisting of In0.14Ga0.76As wells balanced with GaAs0.24P0.76 barriers have been used to extend the absorption of GaAs subcells to longer wavelengths for use in an InGaP/GaAs/Ge triple-junction photovoltaic cell. Thin barriers with high-phosphorus composition are capable of balancing the strain from the InGaAs wells; thus, creating conditions to allow for thicker wells and for carrier tunneling to dominate transport across the structure. As a result, a larger percentage of the depletion region is occupied by InGaAs quantum wells that absorb wavelengths beyond 875 nm and the indium composition is not limited by thermionic emission requirements. Measurements at elevated temperatures and reverse bias suggest that a thermally assisted tunneling mechanism is responsible for transport through the barriers.

This paper presents the fabrication and characterization of air-core power inductors that leverage vertically-stacked, thick copper spiral windings to deliver high inductance densities >; 100 nH/mm2 and quality factors >; 10 on silicon substrates. The inductors are designed for on-chip or in-package integration with high-frequency power converter circuits to enable chip-scale power management in size-constrained applications. A process for realizing the inductors through multilevel, three-dimensional molding of electroplated copper structures has been enhanced with capabilities for four independent copper layers in thicknesses of up to 30 μm each. The impact of increasing the thickness of the stacked copper traces is specifically explored and compared to similar inductors with 10-μm-thick stacked windings. Impedance characterization revealed a drastic decrease in the series resistance in the thicker inductors as desired but also revealed undesired eddy current and magnetic coupling effects.

This letter documents the design and characterization of integrated and continuously tuned, radio frequency multilayer metal noncontact varactors with low-power, large-displacement, thin-film lead zirconate titanate (PZT) actuators. The devices, with capacitances in the 0.019-0.23-pF range, achieved high quality factors at the maximum capacitance state (Q = 100 at 8.7 GHz) and tuning ratios as high as 11.6:1 at 20 V actuation with nW-power consumption. The devices utilize three electroplated copper (Cu) layers ( ~10 μm per layer) directly fabricated on PZT microelectromechanical system actuators.

The high-electric field piezoelectric properties of a PZT-transduced, two-port, acoustic resonator are utilized to create a binary phase shift modulator. The modulator operates at frequencies below 100 MHz in a length extensional mode. Typical peak modulation voltage levels are 7 V. The design is configured as a subsystem with separate RF and data ports for use in low data rate, battery-powered networks.

The authors discovered a magnetic semiconductor material that can store spin orientations at room temperature. This property could be used to build a spintronic memory.

This paper presents the microfabrication and measurement of high-inductance-density, moderate-Q, air-core inductors, and transformers intended for switch-mode power supplies operating in the 100-500 MHz frequency range. The inductors and transformers were fabricated on Pyrex substrates with four layers of electrodeposited copper with each layer up to 10 ?m thick. Stacked winding layers allowed for mutual coupling between layers to increase areal inductance density. Inductors of various designs exhibited inductance densities of up to 100 nH/mm 2 and quality factors approaching 20 in the frequency range of interest. Transformers were formed by interleaving primary and secondary coils and were designed with increased inductance in the secondary coil for voltage gain. A fabricated transformer, 1.5 mm × 1.5 mm in area, yielded 46 nH primary inductance and 500 nH secondary inductance with a coupling coefficient of 0.63. Measurements indicated that a maximum transformer efficiency of 78% at 125 MHz would be possible.

This paper describes a low cost method to deliver magnetic nanoparticle materials using a liquid suspension deposition into ?m-sized structures that would comprise an inductor core. The delivery structure consists of a well and capillary where the well is the target for liquid delivery. Prior theory describing the dosing of MEMS capillaries with polymers is extended to NiFe2O4 nanoparticles in suspension for dosing capillaries constituting inductor type passives with ~200 times larger volumes (2 mm × 15 ?m × 10 ?m). Well and capillary structures were dosed with the magnetic nanoparticles suspension with and without the presence of a permanent magnetic field. Magnetization measurements show a difference in relative permeabilities of 4.2 and 3.3 with and without the presence of the magnetic field, respectively.

This letter documents the design, microfabrication, and testing of integrated and continuously tuned radiofrequency copper (Cu) inductors with low-power large-throw lead-zirconate-titanate (PZT) actuators. The devices, with inductances in the 3-10-nil range, achieved high-quality factors, Q >; 10 (1-6 Gilz range), and tuning ratios as high as 2.6:1 at 20-V actuation with nanowatt-power consumption. This was enabled by the monolithic integration of three electroplated Cu layers (~10 μm per layer) and PZT microelectromechanical system actuators. Two tunable inductor design types, positively and negatively coupled designs, and the tradeoff between tuning ratio and inductor Q are considered.