Abstract A wideband high-selectivity bandpass filter based on a hybrid acoustic-electromagnetic architecture is proposed. The gold bonding-wire inductance is intentionally employed as a designable series element of the bulk acoustic wave (BAW) resonator to construct a dual band-stop notch network, enabling a single acoustic resonator branch to generate two controllable transmission zeros. The notch frequencies can be tuned by the BAW resonator area and bonding-wire geometry. The distributed microstrip/coupled-line circuit forms the wide passband, while three embedded dual band-stop notch networks sharpen the passband skirts and suppress out-of-band spurious responses. For verification, the hybrid filter is fabricated and measured. The fabricated filter achieves a center frequency of 3.29 GHz, a fractional bandwidth of 25.9%, a minimum insertion loss of 1.19 dB, lower-side and upper-side 3-to-20-dB roll-off bandwidths of approximately 235 MHz and 145 MHz, respectively, and out-of-band suppression exceeding 22 dB. The results validate the proposed parasitic-aware hybrid design for wideband, high-selectivity RF front-end applications.

This work demonstrates a dual-layer AlN bulk acoustic wave resonator (DBAR) that excites a higher order longitudinal mode without requiring periodic polarization inversion. Two same-polarization AlN piezoelectric layers are driven with opposite electric-field directions, enabling excitation of the second mode and suppressing the fundamental mode. For comparison, conventional FBAR and DBAR devices were fabricated. The DBAR operates at 7.3 GHz with k ${}_{\mathrm {eff}}^{2}$ of 5.4% and Qmax of 1020, whereas the FBAR resonates at 6.4 GHz with k ${}_{\mathrm {eff}}^{2}$ of 6.0% and Qmax of 442. The DBAR also exhibits an f x $Q$ value of 7446 and a temperature coefficient of frequency (TCF) of −20 ppm/°C. The results indicate that the DBAR structure can achieve a high $Q$ value in high-frequency operation above 6 GHz. Meanwhile, the thicker film stack improves the mechanical strength and process stability of the device, while enabling high-order-mode resonance without relying on polarization inversion.

This work presents an ultra-wideband bulk acoustic wave (BAW) filter enabled by the co-integration of Al0.7Sc0.3N resonators and a cap-wafer port-matching inductor that is fully compatible with wafer-level packaging (WLP). Four mass-loading configurations are employed to stagger the resonance frequencies and expand the passband, while the integrated matching inductors further improve impedance matching over the wide operating range. The cap-wafer inductors are realized with 2-µm-thick Au. The fabricated filter exhibits a center frequency of 3.1 GHz, a 355 MHz -3 dB bandwidth (11.5% FBW), and VSWR ≤ 1.5 in passband, with >36 dB out-of-band rejection and a minimum insertion loss of -1.53 dB.

Monitoring of bolt connection status is a crucial part of the health assessment of infrastructure such as buildings and railways. The traditional ultrasonic flight time measurement methods are mostly used for stress monitoring of bolts, at present, there is no effective method for detecting the bending deformation of bolts. This study proposes a method for measuring the bending degree of bolts based on the analysis of ultrasonic echo characteristics. The dynamic evolution mechanism of ultrasonic propagation characteristics within bent bolts is deeply explored via COMSOL numerical simulation. In this study, an ultrasonic signal transmission and reception system is designed and constructed to observe variations in acoustic wave propagation paths, extract the variation patterns of echo-related characteristic parameters, and achieve the evaluation of bolt bending degrees. A series of tests were carried out on bolts with bending angles in the range of 1 °~10 °, validating that the echo time of the measurement approach presented herein is insensitive to deviations in the bending angle. Moreover, the amplitudes of the first and second echoes decrease monotonically with the increase of the angle. With the gradual increase in the bolt bending angle, the position of the global maximum peak exhibits a significant backward shift, which further validates the effectiveness and feasibility of the measurement method presented in this work.

Conventional A1-mode Lamb-wave resonators exhibit high electromechanical coupling coefficients (K2) but suffer from the presence of numerous spurious modes. In this study, we propose a circular A1-mode Lamb-wave resonator that effectively addresses this issue. The design transforms the interdigital transducer (IDT) configuration from a traditional rectangular layout to a circular geometry. This approach achieves the cancellation of spurious modes utilizing their anisotropic in-plane acoustic velocities on Z-cut lithium niobate, while maintaining an effective excitation of the A1 mode due to its isotropic acoustic velocity on the surface. In addition, the variation of the IDT pitch and metallization ratio along the radial direction results in the excitation of independent spurious modes that conflict with each other, further enhancing the suppression effects. Experimental results demonstrate that the proposed resonator achieves a K2 as high as 32.14% and a quality factor (Q) of 524. These findings highlight a promising design strategy for the development of high-performance, spurious-mode-free A1-mode Lamb-wave resonators.

The real-time monitoring of bolt preload is vital for the predictive maintenance of critical infrastructures. Conventional approaches often rely on wired or battery-powered sensors, making them unsuitable for large-scale distributed deployments. This article presents a novel passive wireless bolt preload monitoring system based on the ultrasonic time of flight (ToF). The system includes a sensor and a reader coupled through magnetic resonance coils. The sensor, consisting of a piezoelectric transducer and a resonant coil, is installed directly on the bolt head. The reader includes a 4.5-MHz radio frequency (RF) signal transceiver and a ToF measurement circuit. RF energy is wirelessly delivered to the sensor through the resonant coupling network to excite longitudinal ultrasonic waves within the bolt. Echoes from internal interfaces are coupled back through the same path for ToF information extraction. Experimental results show a strong linear correlation ( R-2>0.999 ) between ToF and preload across various bolt sizes (M16, M18, and M20), with measurement errors below 2 % compared with commercial sensors. The system demonstrates robustness against spatial misalignment, fixture variability, temperature shifts, and humidity variations. This passive wireless, noninvasive, and high-precision system holds strong potential for long-term structural health monitoring (SHM) with minimal maintenance requirements.

In this paper, we present a fast-response thermal flow sensor with low power consumption and high sensitivity, which was fabricated using complementary metal-oxide-semiconductor micro-electromechanical systems technology. To enhance sensitivity and reduce power consumption, we propose a lithography-free, self-aligned post-complementary metal-oxide-semiconductor fabrication process. Characterization results demonstrate that the fabricated flow sensor could achieve high thermal efficiency, realizing an overheat temperature rise (Delta Th) of 58.76 K while maintaining the power consumption below 1 mW. The sensor also exhibits a high normalized sensitivity of 2131 mV m-1 s W-1, a low minimum detection limit of 0.88 mm s-1, and a rapid response time of 1.7 ms @ 1.67 m s-1 . The sensor was subsequently incorporated into a portable pulmonary function monitoring system, which includes a by-pass channel to enable a broad measurement flow range (+/- 1000 SLM) suitable for medical applications. Leveraging the highperformance sensor and an optimized flow channel design, the portable pulmonary function monitoring setup achieves an enhanced resolution of 28.5 mL s-1 and a significantly improved volume error of 1.6%. Experimental results validate the efficacy of the proposed thermal flow sensor and portable pulmonary function monitoring system, demonstrating its potential as a promising sensing technology for pulmonary disease diagnostics and medical applications, particularly in remote or resource-limited settings.

Humidity sensors are essential components in environmental monitoring and have widespread applications in healthcare, smart agriculture, and industrial manufacturing. Nevertheless, simultaneously achieving ultrafast response, high sensitivity, and long-term operational stability remains a significant challenge. In this work, we report a high-performance humidity sensor realized by integrating a highly oxidized graphene oxide (HOGO) film as the sensitive layer with a shear-horizontal (SH) plate wave resonator. The HOGO film is synthesized via a modified Hummers method, yielding a high density of oxygen-containing functional groups and structural defects. These features provide abundant water adsorption sites, strong hydrophilicity, and efficient mass transport pathways, which synergistically enhance the humidity-sensing performance. Experimental results demonstrate that the proposed sensor achieves high sensitivities of 14.56 kHz/% RH from 10% to 70% RH and 37.07 kHz/% RH from 70% to 90% RH with a 60 nm-thick HOGO film, along with ultrafast response and recovery times of 60 ms and 0.6 s under open air condition. Using standard evaluation in a closed chamber, the speed of response is 0.4 s and recovery 1.8 s. The sensor also shows good repeatability and long-term stability, characterized by an amplitude variation of only 0.1%. Owing to its outstanding performance, the proposed sensor shows strong potential for applications in health monitoring, non-contact humidity sensing, and human-computer interaction.

To address the high-dimensional, nonlinear optimization challenges in BAW filter design, an intelligent collaborative design framework based on Graph Neural Networks (GNN) and hybrid multi-objective optimization is proposed. This method models the filter topology as a graph structure and constructs a surrogate model using a Graph Attention Network (GAT). A continuous multi-level reward function is designed to quantify performance, and hybrid optimization is achieved by combining NSGA-II and PSO. Experimental results demonstrate that this approach can complete the optimization in approximately 14 minutes. The designed filter achieves an average insertion loss of -1.5 dB in the 3.2–3.5 GHz passband, with stopband suppression better than -40 dB. Compared to traditional methods, this approach achieves an order-of-magnitude speed improvement, providing a new pathway for the efficient and automated design of BAW filters.

This article proposes a fitting method based on the harmonic bulk acoustic resonator (HBAR) for precisely extracting the effective electromechanical coupling coefficient ( ${k}_{\textit {eff}}{}^{{2}}\text {)}$ and quality factor ( $Q$ value) of a film bulk acoustic resonator (FBAR). This method successfully simulated the impedance of the FBAR based on an Al ${}_{{0}.{7}}$ Sc ${}_{{0}.{3}}$ N film after release by precisely fitting the HBAR impedance and obtained its ${f}_{p},{k}_{\textit {eff}}{}^{{2}}$ , and $Q$ value. The experimental results show that compared with the measured results of FBAR, the relative error of ${k}_{\textit {eff}}{}^{{2}}$ predicted by the HBAR is less than 5%, and the relative error of the ${Q}_{p}$ value is less than 25%, the relative error of the ${Q}_{s}$ value is less than 26%. This work provides an efficient and reliable characterization tool for the process optimization and performance prediction of the FBAR based on HBAR.

Humidity sensors are essential components in environmental monitoring and have widespread applications in healthcare, smart agriculture, and industrial manufacturing. Nevertheless, simultaneously achieving ultrafast response, high sensitivity, and long-term operational stability remains a significant challenge. In this work, we report a high-performance humidity sensor realized by integrating a highly oxidized graphene oxide (HOGO) film as the sensitive layer with a shear-horizontal (SH) plate wave resonator. The HOGO film is synthesized via a modified Hummers method, yielding a high density of oxygen-containing functional groups and structural defects. These features provide abundant water adsorption sites, strong hydrophilicity, and efficient mass transport pathways, which synergistically enhance the humidity-sensing performance. Experimental results demonstrate that the proposed sensor achieves high sensitivities of 14.56 kHz/% RH from 10% to 70% RH and 37.07 kHz/% RH from 70% to 90% RH with a 60 nm-thick HOGO film, along with ultrafast response and recovery times of 60 ms and 0.6 s under open air condition. The sensor also shows good repeatability and long-term stability, characterized by an amplitude variation of only 0.1%. Owing to its outstanding performance, the proposed sensor shows strong potential for applications in health monitoring, non-contact humidity sensing, and human–computer interaction.

This paper presents a compact, low-loss narrowband bulk acoustic wave (BAW) ladder filter for the BeiDou-3 B1C band. An enhanced Mason equivalent-circuit model is employed, enabling concurrent optimization of resonator characteristics and filter response. To meet the stringent 32.736 MHz bandwidth, the effective electromechanical coupling coefficient (keff2 ) is intentionally reduced through engineering of the electrode-to-piezoelectric thickness ratio (te/tp approximate to 2.3-2.5). A ladder topology with individually tailored resonator areas is co-optimized to balance insertion loss, out-of-band rejection, chip area, and power handling. The fabricated AlN BAW resonator exhibits an effective keff2 of 3.9 %, a maximum quality factor (Qmax) of 3346 at 1.578 GHz, and a temperature coefficient of frequency (TCF) of approximately -31.75 ppm/degrees C. The resulting filter achieves a minimum in-band insertion loss of 1.16 dB, out-of-band suppression exceeding 36.6 dB, and an effective filter-level TCF of -30.5 ppm/degrees C. With a compact area of 0.82 x 0.62 mm2, the filter demonstrates a miniaturized, high-performance solution for BeiDou-3 B1C application.

This article proposes a fitting method based on the harmonic bulk acoustic resonator (HBAR) for precisely extracting the effective electromechanical coupling coefficient (k(eff) 2 ) and quality factor (Q value) of a film bulk acoustic resonator (FBAR). This method successfully simulated the impedance of the FBAR based on an Al0.7Sc0.3N film after release by precisely fitting the HBAR impedance and obtained its f(p), k(eff) (2) , and Q value. The experimental results show that compared with the measured results of FBAR, the relative error of k(eff) (2) predicted by the HBAR is less than 5%, and the relative error of the Q(p) value is less than 25%, the relative error of the Q(s) value is less than 26%. This work provides an efficient and reliable characterization tool for the process optimization and performance prediction of the FBAR based on HBAR

This work reports on a dual optimization of process and device to extend the bandwidth of bulk acoustic wave (BAW) filters based on Al0.7Sc0.3N piezoelectric film. The thicknesses of the AlN seed layer and Al0.7Sc0.3N film were adjusted to obtain high-quality Al0.7Sc0.3N film while maintain high effective electromechanical coupling coefficient (keff2) of the BAW resonators. X-ray diffraction of Al0.7Sc0.3N film show the full-width at half-maximum of the (002) orientation as 1.67 degrees with 0.91 nm surface roughness (Rq). The BAW resonator structure was also optimized based on above piezoelectric film, and a modified Mason model considering the area effect of frequency is proposed. The results show that the BAW resonator has a keff2 up to 22% at the operating frequency 3.5 GHz and a Bode quality factor of 992. The filter with a 3 dB bandwidth of 371 MHz was designed and fabricated on 8 inch wafers. The keff2 difference of the resonators across the entire 8 inch wafer is no more than 1.6%. The center frequency of the developed BAW filter is 3.35 GHz and the insertion loss is only 1.28 dB with an out-of-band rejection more than -38 dB.

This work reports an ultrawideband bulk acoustic wave (BAW) filter based on the codesign of a high-effective coupling coefficient ( ${k}_{\textit {eff}}^{{2}}\text {)}$ Al0.7Sc0.3N resonator and wafer-level-packaging (WLP) compatible cap-wafer integrated passive device (IPD) matching inductors. The fabricated BAW resonators exhibit a ${k}_{\textit {eff}}^{{2}}$ of 21.1% and a maximum Bode Q of 690. The 2- $\mu $ m-thick Au cap-wafer inductors provide an inductance of approximately 0.95 nH and a measured Q value of about 16 at 3 GHz. The cap-wafer inductors are introduced at the filter ports to reshape the port impedance and maintain a low voltage standing wave ratio (VSWR) across the enlarged passband without requiring PCB-level or postpackaging matching components. The resulting filter achieves a center frequency of 3.1 GHz, a 355-MHz 3-dB bandwidth (11.5% fractional bandwidth), an in-band VSWR $\le 1.5$ , out-of-band rejection > 36 dB, and a minimum insertion loss of −1.53 dB. The packaged filter also exhibits a temperature coefficient of frequency (TCF) of −27 ppm/°C, a measured ${\mathrm{P}}_{\text {1$\,$ {dB}}}$ of 35dBm, and a compact WLP size of $1.1\times 0.9\times 0.3$ mm. Failure analysis further indicates that the power-induced degradation is mainly associated with the BAW resonators rather than the cap-wafer inductors. These results demonstrate that cap-wafer IPD inductors provide an effective device–package–matching codesign route for wideband, miniaturized, and manufacturable BAW filters.

Film bulk acoustic resonators (FBARs) are widely used in radio frequency (RF) filters for wireless communication because of their high operating frequency and high quality factor. With the increase of high-power applications, ensuring device robustness has become a critical challenge. This study presents an investigation into the high-power failure behaviors and mechanisms of FBARs, specifically examining the role of active area, film thickness, and geometry. Experimental results demonstrate that small-area FBARs exhibit distinct failure characteristics compared to large-area devices. Small-area devices are governed by progressive spallation at electrode edges, which is induced by high-temperature oxidation and stress concentration, whereas large-area FBARs are prone to sudden structural fracture or short-circuiting caused by excessive thermal stress. Crucially, the study reveals a thickness-dependent transition in large-area devices, where short-circuiting and structural fracture correspond to distinct stress-severity regimes. Furthermore, dynamic evaluations demonstrate that these FBARs preserve strict electrical linearity right up to the point of catastrophic collapse. Based on these phenomenological findings, a thermo-mechanical coupling mechanism is proposed that goes beyond the conventional thermal-only model. Finally, we propose new design guidelines to enhance the power handling capability of FBAR devices.

Conventional wireless passive surface acoustic wave (SAW) strain sensors often employ rigid materials, limiting their application on curved surfaces. This letter presents a fully flexible wireless passive strain sensor, implemented by the flip-chip integration of an ultra-thin SAW device with a flexible antenna. The SAW sensor, fabricated on a 50- mu m ST-cut quartz substrate with 200-nm aluminum electrodes, is coupled with a flexible antenna made by patterning copper electrodes on a polyimide substrate. Experimental evaluations include both wired and wireless tests, demonstrating the system's sensitivity of 121.63 Hz/mu epsilon within a dynamic range of 5000 mu epsilon and a wireless transmission range exceeding one meter. Besides, the system also exhibits excellent linearity and repeatability. The fully flexible sensor design enhances the system's versatility, enabling deployment in challenging environments, such as confined or curved surfaces, particularly for structural health monitoring applications.

This article presents a novel resonant current sensor designed for contactless, miniaturized, high-sensitivity, and high-precision static dc current measurement. The sensor features a cantilever structure integrated with a thin-film lithium niobate lamb wave resonator (LWR) and a micromagnet. The working principle of the sensor is based on the torque and force induced by the magnetic field of the measured current, which causes a resonant frequency shift of the LWR. To integrate a 30 mu m thick cantilever with a 400 nm thick LWR, a dual-mask deep silicon etching process is employed. This process ensures that the structure can support the magnet while also maintaining the resonant region as a thin film that is favorable for LWR operation. Furthermore, to ensure precise magnet placement, a self-aligning process based on the attraction between the nickel layer and the magnet is introduced. Experimental results demonstrate a sensitivity of 2.2 kHz/A, a minimum current resolution of 0.6 A in open-loop testing, and a linear measurement range of up to 70 A for dc wire measurements. These results suggest that the proposed sensor has strong potential for applications in smart meters, battery management systems (BMS), and electric vehicle charging stations.

A super high-frequency (SHF) bandpass filter was designed and fabricated using low-temperature co-fired ceramic (LTCC) technology. To achieve miniaturization while suppressing undesired coupling effects that can generate spurious resonances and parasitic responses, a multilayer configuration was implemented, in which capacitive and inductive elements were allocated to specific layers and regions. Through utilizing the parasitic interactions between components, the number of discrete elements was reduced. Transmission zeros were introduced to enhance out-of-band (OoB) suppression through tailored routing of interconnecting lines, and mutual inductive coupling between inductors. The design was optimized using full-wave three-dimensional electromagnetic simulations. The fabricated LTCC filter exhibits a passband of 3.0-3.4 GHz with an OoB rejection exceeding 50 dB. Measured results are in good agreement with simulations, confirming the suitability of the proposed approach for SHF-band applications.