We demonstrate coupled bulk and surface acoustic wave (BAW/SAW) resonators with significantly improved coupling efficiency as compared to SAW only devices. Two sets of stacks are investigated: one ...uses a thin film piezoelectric material with a bottom electrode on GaN substrate (i.e., AlN/Mo/GaN) and another includes a piezoelectric thin film with bottom metal on a Si substrate (i.e., AlN/Mo/Si). A vibrating BAW transducer induces a SAW in the substrate and between two sets of interdigitated transducers improving the coupling efficiency by a factor of <inline-formula> <tex-math notation="LaTeX">{\geq }\, \mathsf {8}{x} </tex-math></inline-formula> as compared to the conventional SAW. The performance of the coupled BAW/SAW resonators using each of the Si and GaN substrates is compared using FEM analysis. By using FEM, coupling efficiency of the hybrid BAW/SAW with AlN/Mo/Si stack is 2.4%, whereas it is 3.2% for the SAW using AlN/Mo/GaN stack.
This letter reports the first results on a new generation of anti-symmetric lamb-wave devices using a simple solidly mounted platform of AlN/Mo/Si and operating at very-high-to-super-high frequency ...range. AlN/Mo/Si-based solidly mounted resonators (SMRs) with sub-micrometer interdigitated transducers were fabricated using the electron beam lithography and liftoff process. Plasma etching was used to etch vias in ground pads to get access to the bottom electrode for grounding. By comparing the floating, grounded, and no bottom electrode device performances, it has been observed that the presented SMR design with grounded bottom electrode results in enhancement of the coupling efficiency by 500% compared with conventional SAW devices. In addition, these devices are more mechanically robust and can be monolithically integrated with a CMOS platform without requiring complex packaging processes compared with conventional released lamb-wave resonators.
Microelectromechanical systems sensors have been intensively developed utilizing various physical concepts, such as piezoresistive, piezoelectric, and thermoresistive effects. Among these sensing ...concepts, the thermoresistive effect is of interest for a wide range of thermal sensors and devices, thanks to its simplicity in implementation and high sensitivity. The effect of temperature on the electrical resistance of some metals and semiconductors has been thoroughly investigated, leading to the significant growth and successful demonstration of thermal-based sensors, such as temperature sensors, convective accelerometers and gyroscopes, and thermal flow sensors. In this paper, we review the fundamentals of the thermoresistive effect in metals and semiconductors. We also discuss the influence of design and fabrication parameters on the thermoresistive sensitivity. This paper includes several desirable features of thermoresistive sensors and recent developments in these sensors are summarized. This review provides insights into how it is affected by various parameters, and useful guidance for industrial designers in terms of high sensitivity and linearity and fast response.
In this paper, we present ultra-high quality factor (<inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula>) SAW resonators fabricated on aluminum gallium nitride on gallium ...nitride on silicon (AlGaN/GaN/Si) heterostructures with <inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula> exceeding 6000 at room temperature. We characterize their temperature response in a broad range of temperature (−196°C to 500°C or 77 K to 773 K). The effect of doping on the <inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula> and temperature coefficient of frequency (TCF) of the GaN-based SAW resonators is analyzed, for the first time, using un-intentionally doped GaN (UID-GaN), carbon doped GaN (C-doped), and Si doped GaN. The <inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula> value for UID-GaN and C-doped GaN is similar and decreases from 2000 to 1000 as the temperature is increased from 77 K to 773 K. The <inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula> value of Si-doped GaN is higher than UID and C-doped GaN by a factor of 3 (6622 at 77 K) and decreases with increase of temperature. This value of <inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula> at 1.8 GHz is the highest reported amongst aluminum nitride (AlN) or GaN-based SAW resonators. For extreme high temperatures (≥573 K) the TCF value is half the TCF value of UID and C-doped GaN, which shows the possibility of engineering the TCF by tuning the doping concentration. For low temperatures (≤150 K) C-doped and UID-GaN show a turn over point in TCF curve which shows their promise for cold clocks. 2020-0163
The experimental demonstration of aluminum scandium nitride (AlScN)‐on‐cubic silicon carbide (SiC) heterostructure thin film micromachined resonant transducers operating in a high‐temperature ...environment up to 600 °C is reported. Macroscopic and microscopic vibrations are investigated through a combination of ultrasensitive laser interferometry techniques and Raman spectroscopy. An average linear temperature coefficient of resonance frequency (TCf) of <1 ppm °C−1 within the temperature range from room temperature to 200 °C, and an average linear TCf of −16 ppm °C−1 between 200 and 600 °C, from the fundamental‐mode resonance of AlScN/SiC circular diaphragm resonator with a thickness of 1.9 µm and diameter of 250 µm, is obtained. Higher‐order modes exhibit much larger TCf, which make them strong candidates as high‐temperature‐tolerant temperature sensors or ultraviolet detectors. Raman spectroscopy indicates that the turning points of the peak positions of the longitudinal optical phonon modes of both 3C‐SiC and AlScN occur in almost the same temperature region where the turnover point of TCf is observed, suggesting that the microscopic vibrations in the crystal lattice and the macroscopic oscillation of the diaphragm are naturally mediated by the residual strain inside the materials at varying temperature.
This work demonstrates aluminum scandium nitride on silicon carbide (AlScN/SiC) heterostructure thin film micromachined resonant transducers operating in a high‐temperature environment up to 600 °C. Macroscopic and microscopic vibrations are investigated through a combination of laser interferometry techniques and Raman spectroscopy. The results provide insight for understanding the resonant characteristics and intrinsic material properties of AlScN/SiC at both device and crystal structure levels.
Improving and optimizing the processes for transfer printing have the potential to further enhance capabilities in heterogeneous integration of various sensing materials on unconventional substrates ...for implantable and stretchable electronic devices in biosensing, diagnostics, and therapeutic applications. An advanced transfer printing method based on sacrificial layer engineering for silicon carbide materials in stretchable electronic devices is presented here. In contrast to the typical processes where defined anchor structures are required for the transfer step, the use of a sacrificial layer offers enhances versatility in releasing complex microstructures from rigid donor substrates to flexible receiver platforms. The sacrificial layer also minimizes twisting and wrinkling issues that may occur in free‐standing microstructures, thereby facilitating printing onto flat polymer surfaces (e.g., polydimethylsiloxane). The experimental results demonstrate that transferred SiC microstructures exhibit good stretchability, stable electrical properties, excellent biocompatibility, as well as promising sensing‐functions associated with a high level of structural perfection, without any cracks or tears. This transfer printing method can be applied to other classes of wide bandgap semiconductors, particularly group III‐nitrides and diamond films epitaxially grown on Si substrates, thereby serving as the foundation for the development and possible commercialization of implantable and stretchable bioelectronic devices that exploit wide bandgap materials.
Employing a dissolvable film as a supporting layer for the fabrication of free‐standing silicon carbide microstructures, the present work eliminates the wrinkling and twisting phenomena associated with nanomembranes grown at high temperatures. This technique enables transfer‐printing of diverse microstructures of wide band gap semiconductors onto a soft substrate, creating a new class of stretchable electronics for biosensing and implanting applications.
In this paper, we report on a low-cost, environment-friendly and wearable thermal flow sensor, which can be manufactured in-house using pencil graphite as a sensing hot film and biodegradable ...printing paper as a substrate, without using any toxic solvents or cleanroom facilities. The hot film flow sensor offers excellent performance such as high signal-to-noise ratio ( 40 for an air flow velocity of 1 m s−1), high sensitivity to airflow (53.7 mV(m s−1)−0.8) and outstanding long-term stability (almost no drift in 24 h). The sensor can be comfortably affixed to the philtrum of patients and measures human respiration in realtime. The results indicate that the wearable thermal flow sensors fabricated by this solvent-free and user-friendly method could be employed in human respiratory monitoring.
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