Lamb wave resonators (LWRs) based on lithium niobate on insulator (LNOI) substrates operating in an A1 mode at frequencies above 5 GHz typically exhibit high electromechanical coupling coefficients (K-2) ranging from 20% to 30%. This makes them promising candidates to replace current bulk acoustic wave technology in next-generation broadband RF filters. However, the K-2 of conventional LWRs still struggles to meet application requirements for bandwidths exceeding 1 GHz, such as WiFi-6E. This paper presents an analytical optimization approach for rapid and comprehensive scanning of the full 3D Euler space to find the maximum effective K-2 of LWR devices. Such a full scan is difficult to achieve by the conventional time-consuming finite element method (FEM). The K-2 results for a selected subset of Euler angles obtained through this analytical approach align with FEM simulation results. The optimized results show that LWRs on LNOI can achieve the highest K-2 of 59.92% at Euler angles of (0, -151 degrees, -60 degrees), providing bandwidths exceeding 1 GHz at 5 GHz. To validate this optimization approach, LWRs with various orientations were fabricated on both 41 degrees YX-cut and Z-cut LNOI wafers, with measured K-2 values at different Euler angles agreeing well with both analytical and FEM results. The proposed K-2 optimization method serves as a valuable guideline for selecting substrate cuts and device orientations in the design of ultra-broadband filters for next-generation telecommunications.

BAW (Bulk Acoustic Wave) filters, commonly implemented in ladder or lattice topologies, have been widely adopted as RF filters for high frequencies. However, the bandwidth of these filter topologies is limited by the effective electromechanical coupling coefficient of the resonators, which is decided by the piezoelectric material and is difficult to increase. In this study, a new topology is proposed to construct a N77 full band filter with high steep skirts which can coexist with N79 band by hybrid integrating BAW resonators with IPD (Integrated Passive Device) devices. The passband of the filter utilizes the left part of the band-stop filter to form large bandwidth and steep skirt. BAW resonators based on Al 0.8 Sc 0.2 N are connected to IPD circuit through flip-chip bonding. An N77-band 3.3-4.2 GHz filter has been designed and fabricated, demonstrating an average insertion loss of -2 dB, an out-of-band rejection of -28 dB, an average roll-off of 250 MHz and a compact effective area of only 1 mm × 0.85 mm.

This paper demonstrates a 3.4 GHz micro-acoustic filter with excellent in-band insertion loss, rectangle coefficient, and out-of-band rejection. The proposed filter was based on a novel topology by integrating the bulk acoustic wave (BAW) resonators with an on-chip clamping inductor, the introduction of the lumped components is beneficial for improving the filters' stopband rejection while having minimal influence on the rectangle coefficient and in-band insertion loss. Al0.8Sc0.2N material was selected as the piezoelectric film of the BAW device and the inductor was fabricated on the same wafer with BAW devices. The manufactured filter has a minimum in-band insertion loss of similar to 1.5 dB, a 3 dB bandwidth of about 170 MHz, and a stopband rejection of nearly -30 dB. The experimental results verify the design and show great applications for 5G communication and beyond.

With the increase of operating frequency above 4 GHz, the required resonator size is reduced to hundreds of $\mu \text{m}^{{2}}$ , which is proportional to 1/ ${f}^{\,{2}}$ . In traditional, the size reduction of the resonator will induce its parallel resonant frequency decrease, thus resulting in the decrease of its effective coupling coefficient ( ${k}_{\,{t}}^{\,{2}}$ ). Besides the parallel resonant frequency, we found that the serial resonant frequency of resonators working above 3 GHz also varies with its area size. However, this effect is not taken into account in conventional circuit models, leading to the failure of high-performance filter design. To address this issue, a modified Mason model of bulk acoustic wave (BAW) resonators was proposed in this work. Using the modified model, a high-performance N79 band (4.8–4.96 GHz) filter based on Al0.904Sc $_{{0.096}}\text{N}$ piezoelectric film was designed and fabricated, which achieved a minimum insertion loss of −0.93 dB and a bandwidth of 240 MHz. The power capacity and wire-bond package of BAW filters were further studied. The whole process of the Mason model modification, filter design, and fabrication pave the road for high-frequency filter applications.

To meet the stringent requirements of 5G communication, we proposed a high-performance bulk acoustic wave (BAW) filter based on single crystal AlN piezoelectric films on a SiC substrate. The fabrication of the BAW filter is compatible with the GaN high electron mobility transistor (HEMT) process, enabling the implementation of the integration of the BAW device and high-performance monolithic microwave integrated circuit (MMIC). The single crystal AlN piezoelectric film with 650-nm thickness was epitaxially grown on the SiC substrate by Metal Organic Chemical Vapor Deposition (MOCVD). After wafer bonding and substrate removal, the single crystal AlN film with electrode layers was transferred to another SiC wafer to form an air gap type BAW. Testing results showed that the fabricated resonators have a maximum Q-factor up to 837 at 3.3 GHz resonant frequency and electromechanical coupling coefficient up to 7.2%. Ladder-type filters were developed to verify the capabilities of the BAW and process, which has a center frequency of 3.38 GHz with 160 MHz 3 dB bandwidth. The filter achieved a minimum 1.5 dB insertion loss and more than 31 dB out-of-band rejection. The high performance of the filters is attributed to the high crystallinity and low defects of epitaxial single crystal AlN films.

Langasite (LGS) surface acoustic wave (SAW) sensor is considered as an ideal wireless passive sensing technology application in complex environments. However, existing LGS-based SAW strain sensor has low strain range and its sensing accuracy is affected by temperature greatly as well as strain, which can not meet the application requirements. By exploiting ultra-thin substrate and a custom calibration algorithm, this paper proposes an ultra-thin 100 $\mu \text{m}$ LGS SAW with (0°, 138.5°, 72°) Euler angle to extend strain range to 1200 $\mu \varepsilon $ at $500^{\circ }\text{C}$ , which is twice higher than the state-of-the-art. The ultra-thin SAW sensor has lower temperature coefficient and hysteresis loop effect, compared with thick LGS SAW. Aiming to improve sensing accuracy, the mechanism of temperature effects on strain sensitivity is investigated in different temperature stages, including thermal expansion mismatch effect and high-temperature glue strain transfer ratio effect. The mechanism can explain strain sensitivity curve under various temperature nicely. Based on the mechanism, a new calibration method is also developed to eliminate temperature effects. The testing results show that this calibration method can improve the measured strain accuracy effectively.

This Letter reports a new bulk acoustic wave (BAW) filter based on single crystal AlN piezoelectric film which has the potential application in 5G wireless communication. The single crystal AlN is deposited on SiC substrate by MOCVD and the air-cavity structure BAW device is fabricated. Testing results show that the fabricated resonators have Q-factor up to 837 and electromechanical coupling coefficient up to 7.2% with resonant frequency 3.2 GHz. The ladder-type filters are also developed on the same wafer, which have a center frequency of 3.38 GHz and 3 dB bandwidth 160 MHz, minimum insertion loss of 1.5 dB, and out-of-band rejection above 31dB. High performance of the filters comes from low defects of the single crystal AlN film, which demonstrates promising potential of single crystal AlN filters in 5G and future 6G applications.

Surface acoustic wave sensors (SAW), as a wireless passive sensor, are susceptible to temperature disturbance for high-temperature strain measurements, which results in poor accuracy and repeatability of the strain sensing. This paper investigates temperature effects on SAW strain sensors. Results show that high temperature not just induces frequency drift, but also reduces strain transfer ratio due to glue softening. The resonant frequencies of the langasite (LGS) SAW sensor have a linear relationship with strain for fixed temperatures, while the strain sensitivity is greatly affected by temperature. The results also show that the difference of measured strain sensitivities between the two resonant frequencies for one device is much less affected by temperature because they suffer the same strain transfer ratio and temperature characteristics, but different strain responses. A method to eliminate temperature effects has been proposed and accurate high-temperature strain measurements with SAW sensor were conducted to extend strain range up to 380 mu epsilon with high repeatability under 500 & nbsp;C.