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This paper presents a new method to improve the performance of SAW devices based on transferred lithium niobate thin films. Amorphous silicon replaces the single-crystal silicon to eliminate a conductive layer generated in the thin-film transfer processes. As a result, the electromechanical coupling of the surface-guided acoustic wave is maximized, and the substrate loss is reduced in the amorphous-silicon-based hybrid wafer. The fabricated resonator shows an extracted electromechanical coupling coefficient of 22% with over 60 dB impedance radio, which is 30 dB higher than the resonator with parasitic surface conductive effect. The fabricated delay lines show excellent propagation properties of the excited surface-guided acoustic wave with a minimum propagation loss of 18.1 dB/mm at 2.35 GHz, which further demonstrate the elimination of the conductive layer and outstanding acoustic energy confinement between lithium niobate and amorphous silicon. This work has shown the strong potential of the silicon-SAW platform for front-end signal processing. © 2022 IEEE.

This work presents an acoustic platform using solidly mounted thin-oriented lithium niobate (LiNbO3) film on silicon (Si). A thin layer of amorphous Si eliminates a conductive layer generated in the thin-film bonding processes and contributes to acoustic energy confinement and thermal frequency stability. The gigahertz operating frequency is achieved by resorting to the guided acoustic wave shear-horizontal wave (SH0) in a 400-nm-thick X-cut LiNbO3 thin film. Due to the elimination of the parasitic surface conduction (PSC) effect, the electromechanical coupling of the guide acoustic wave is maximized in the Si-based heterogeneous wafer. Due to the minimized acoustic impedance mismatch between surface material and substrate, longitudinal and higher order spurious modes are suppressed. Due to the stiff substrate with a small temperature expansion coefficient (TEC), the solidly mounted structure features a reduced temperature coefficient of frequency (TCF) and improved power handling. The fabricated resonator shows an extracted electromechanical coupling coefficient of 22.8%, a high loaded Q of 1208 at 1.6 GHz, and a TCF of -36 p/min/K. The compressed filter is demonstrated with a minimum insertion loss (IL) of 0.6 dB, a fractional bandwidth (FBW) of 8%, an out-of-hand rejection of 30 dB, a TCF of -38.5 p/min/K at roll-off, and a miniaturized footprint of 0.4 mm(2). The performance has shown the strong potential of the LiNbO3-Si platform for front-end applications in 5G.

This work studies the influence of substrate loss on the performance of acoustic resonators and characterizes the effective substrate resistivity of LiNbO 3 thin-film on a high-resistivity Si (HR-Si) wafer. Coplanar waveguides (CPWs) are utilized to extract the effective substrate resistivity of LiNbO 3 film on Si over a frequency range from 1 to 40 GHz with two different film thicknesses: 400 nm and 1.6 µm. The effective resistivities of released and unreleased CPW structures are experimentally extracted and analyzed to show that a suspended structure on a thinner LiNbO 3 film has less substrate loss due to a smaller electric field filling factor in the film. As a result, the removal of Si wafer by a release process brings more substrate loss reduction for a thin film than that on a thick film.

This paper presents a short review of the microwave acoustics area, where exciting material innovations and performance advancements have been made in the past decade. The ever-growing demand for more sophisticated passive signal processing functions on-chip has fueled these developments. As a result, microwave acoustic devices have maintained performance leadership in mobile applications. By evaluating three fundamental parameters, namely electromechanical coupling (k 2 ), quality factors, and frequency scalability, of microwave acoustics, this paper aims to, extensively but not exhaustively, capture the rationales behind approaches achieving higher performance microsystems. Outlooks for different material systems and addressing their underlying challenges are also offered in hopes of establishing a balanced roadmap for future microwave acoustics development.

This work presents a new design of an acoustic-only dual-band bandpass filter operating beyond 6 GHz. The operating frequencies are achieved by resorting to the third-order antisymmetric Lamb wave mode (A3) in an 800 nm thick Z-cut lithium niobate thin film. The dual-band operation utilizes a fine-controlled regional thinning technique on lithium niobate thin film. Due to the spurious-free and high-Q performance of the A3 dual-resonance lithium niobate resonators, the fabricated dual-band bandpass filter has been demonstrated with small in-band ripples and sharp roll-offs. The two transmission bands are contiguous and located at 6.75 GHz and 7.35 GHz, respectively, with a fractional bandwidth of 2.5%. These two passbands share the same static capacitance leading to a miniaturized footprint of 0.6 mm 2 , which shows the strong potential of this dual-band design for front-end applications.

This work presents a new class of microelectromechanical system (MEMS) resonator toward 60 GHz for the fifth-generation (5G) wireless communications. The wide range of the operating frequencies is achieved by resorting to different orders of the antisymmetric Lamb wave modes in a 400-nm-thick Z-cut lithium niobate thin film. The resonance of 55 GHz demonstrated in this work marks the highest operating frequency for piezoelectric electromechanical devices. The fabricated device shows an extracted mechanical Q of 340 and an f x Q product of 1.87 × 10 13 in a footprint of 2 × 10 -3 mm 2 . The performance has shown the strong potential of LiNbO 3 antisymmetric mode devices for front-end applications in 5G high-band.

This work presents an improved design that exploits dispersion matching to suppress the lateral spurious modes in the thin-film lithium niobate first-order antisymmetric Lamb wave mode (A1) resonators. The dispersion matching in this work is achieved by micro-machining the lithium niobate thin film to balance the electrical and mechanical loading of electrodes. The fabricated devices based on the proposed method exhibit spurious-free responses with a maximum Q{3dB} of 692 and k{t}{}-{2} of 28%. The demonstrated method herein could overcome a significant hurdle that is currently impeding the commercialization of A1 devices. © 2020 IEEE.

This work presents the first demonstration of a wideband hybrid monolithic acoustic filter in the K u -band that overcomes the electromechanical coupling (k t 2) limit on the fractional bandwidth (FBW) of conventional ladder topology acoustic filters. This work employs the bandwidth-widening technique of combining a shunt inductor to a resonator to compensate for low k t 2. The filter combines electromagnetic and acoustic structures to take the advantages of both: wideband, high Q, and compact sizes. The fabricated filter based on the fifth-order antisymmetric Lamb wave mode (A5) with 1.1% k t 2has a 3-dB FBW of 2.93% at 14.7 GHz, stopband insertion loss of 19.5 dB, with a small footprint of 2.25 mm 2 .

This paper demonstrates the first nonreciprocal network based on switched low-loss acoustic delay lines. The four-port circulator is built upon a recently reported frequency-independent, programmable, nonreciprocal framework based on switched delay lines. The design space for such a system, including the origins of the insertion loss (IL) and harmonic responses, is theoretically investigated, illustrating that the key to better performance and low-cost modulation signal synthesis lies in a large delay. To implement a large delay, we resort to in-house fabricated low-loss, wideband lithium niobate (LiNbO 3 )SH0 mode acoustic delay lines employing single-phase unidirectional transducers. The four-port circulator, consisting of two switch modules and one delay line module, has been modularly designed, assembled, and tested. The design process employs time-domain full circuit simulation, and the results match well with measurements. An 18.8-dB nonreciprocal contrast between IL (6.6 dB) and isolation (25.4 dB) has been achieved over a fractional bandwidth of 8.8% at a center frequency 155 MHz, using a record low switching frequency of 877.19 kHz. The circulator also shows 25.9-dB suppression for the intramodulated tone and 30 dBm for IIP3. Upon further development, such a system can potentially lead to future wideband, low-loss chip-scale nonreciprocal radio frequency systems with unprecedented programmability.

This paper presents a new class of micro-electromechanical system (MEMS) C-band filters for 5G front-ends. The filter is comprised of resonators based on the first-order asymmetric Lamb wave (A1) mode in thin-film lithium niobate. The fabricated filter has been demonstrated at 4.5 GHz with sharp roll-off, flat in-band group delay, and spurious-free response over a wide frequency range. Specifically, the filter shows a 3-dB fractional bandwidth (FBW) of 10%, an insertion loss (IL) of 1.7 dB, an out-of-band (OoB) rejection of -13 dB, and a compact footprint of 0.36 mm 2 . The demonstrations herein mark the largest fractional bandwidth (FBW) achieved for acousticonly filters at 5G frequencies.

This paper demonstrates the first nonreciprocal network based on switched low-loss acoustic delay lines. The four-port circulator is built upon a recently reported frequency-independent, programmable, nonreciprocal framework based on switched delay lines. The design space for such a system, including the origins of the insertion loss (IL) and harmonic responses, is theoretically investigated, illustrating that the key to better performance and low-cost modulation signal synthesis lies in a large delay. To implement a large delay, we resort to in-house fabricated low-loss, wideband lithium niobate (LiNbO 3 )SH0 mode acoustic delay lines employing single-phase unidirectional transducers. The four-port circulator, consisting of two switch modules and one delay line module, has been modularly designed, assembled, and tested. The design process employs time-domain full circuit simulation, and the results match well with measurements. An 18.8-dB nonreciprocal contrast between IL (6.6 dB) and isolation (25.4 dB) has been achieved over a fractional bandwidth of 8.8% at a center frequency 155 MHz, using a record low switching frequency of 877.19 kHz. The circulator also shows 25.9-dB suppression for the intramodulated tone and 30 dBm for IIP3. Upon further development, such a system can potentially lead to future wideband, low-loss chip-scale nonreciprocal radio frequency systems with unprecedented programmability.

This paper reports a 2.48 GHz tri-coil and rectifier design implemented in a system that demonstrates simultaneous wireless power transfer and communication to a 0.1 mm by 0.1 mm coil. The tri-coil link and rectifier successfully rectified and demodulated a 20 dBm amplitude-shift keyed (ASK) RF signal modulated at a rate of 1 Mb/s. Additionally, a 5.7 GHz tri-coil link was fabricated to validate the frequency scalability of this technology platform for other unlicensed bands and was measured in a customized experimental testbed to account for the effects of lateral misalignment between coils. The 5.7 GHz tri-coil design had a measured peak RF power transfer efficiency of -29 dB with a vertical separation of 1 mm, which is ten times the load coil diameter. © 2017 IEEE.

This paper reports a design methodology for a tri-coil system that can wirelessly transfer power from a macro-scale probe to a chipscale apparatus over a long distance and with a high efficacy. Such systems can be employed for enabling the wireless charging and communication between implanted body sensors and smart phones as well as between hardware roots of trust and their interrogation probes. A design example has been offered and subsequently validated by an experimental testbed consisting of micro-fabricated coils on both sides of a Silicon substrate. As predicted by the analytical models, the measured power transfer efficacy (PTEF) of the designed system is as high as -27 dB at the resonance, confirming an orders-of-magnitude higher PTEF than that of prior WPT systems over a distance greater than 5 times the coil diameter.