Impedance-matched HTBARs with 3 Micron Film and 0.1 Scalability Enable Quantum Signal Processing

Researchers are continually seeking to improve the performance of acoustic resonators, vital components in signal processing and emerging quantum technologies, and a team led by Zi-Dong Zhang from Nanjing University of Aeronautics and Astronautics and Zhen-Hui Qin from Nanjing University now presents a significant advance in this field. Their work introduces a novel design for high overtone bulk acoustic resonators, overcoming limitations in impedance matching, unwanted interference, and scalability that plague conventional devices. The team, which also includes Yi-Han He, Yun-Fei Cheng, Hao Yan, and Si-Yuan Yu, demonstrates a laterally excited resonator that achieves exceptionally efficient energy transfer, exceeding ninety nine percent, and exhibits a stable and predictable frequency response. This innovative design, utilising a specific lithium niobate structure and a gridded electrode configuration, not only suppresses spurious modes and allows precise control over the resonator’s size, but also establishes a pathway towards highly integrated, robust multimode phonon sources for future quantum networks and microwave photonic circuits.

Lateral Excitation HBAR on Lithium Niobate

This research details the development and characterization of a novel high-Overtone Bulk Acoustic Resonator, termed a Laterally-Excited HBAR, or X-HBAR. This innovative design utilizes a unique excitation scheme with Lithium Niobate on a High Resistivity Silicon substrate, aiming to achieve high-frequency operation, high quality factor, and improved temperature stability. Unlike traditional HBARs, this design employs lateral excitation, simplifying fabrication and potentially reducing energy losses. The device consists of a metal layer deposited on Lithium Niobate, placed on a High Resistivity Silicon substrate.

Acoustic waves are excited laterally using a top-top electrode configuration, simplifying manufacturing by eliminating complex stacking and etching procedures. The High Resistivity Silicon substrate minimizes acoustic losses and creates a suspended structure, allowing the resonator to operate in thickness-shear mode. Experiments demonstrate that these resonators achieve frequencies in the GHz range, with high quality factors reaching up to 3350 at room temperature. The resonators exhibit a high frequency-quality product, indicating a good balance between frequency and quality factor, alongside excellent temperature stability.

Investigations into electrode gap spacing revealed its influence on insertion loss and quality factor, while low-temperature testing showed improved quality factors. The X-HBARs demonstrate competitive performance compared to other existing HBAR configurations in terms of frequency, quality factor, and frequency-quality product. They also offer potential advantages in fabrication simplicity and cost, establishing the laterally-excited X-HBAR design as a promising approach for achieving high-frequency, high-quality resonators. These resonators have potential applications in high-performance filters for wireless communication systems, high-sensitivity sensors, precise timing circuits, and as building blocks for advanced microelectromechanical systems.

Fabricating a High Overtone Shear Acoustic Resonator

Researchers engineered a laterally excited high overtone thickness shear bulk acoustic resonator, termed X HTBAR, to overcome limitations found in conventional designs. The fabrication process began with a custom-ordered lithium niobate-on-silicon substrate, polished down to a 3 micron thick lithium niobate layer. Metal electrodes, consisting of a chromium and gold bilayer, were formed using e-beam evaporation and a lift-off process, creating a fully planar excitation scheme. Scientists employed a calibrated vector network analyzer to characterize the resonator’s performance, collecting two-port scattering parameters under standard ambient conditions.

Electrical contact was established using specialized probes, and further measurements were conducted under vacuum and cryogenic temperatures. Complementing the experimental work, the team performed finite element analysis to simulate the resonator’s spectral response, verifying the X HTBAR’s operating mechanism. These combined experimental and computational approaches demonstrate high quality factors, frequency quality products exceeding ten to the power of thirteen at room temperature, and a stable free spectral range, establishing the X HTBAR as a promising component for quantum interconnects and microwave photonic integrated circuits.

Efficient Acoustic Excitation in Lithium Niobate Resonators

Researchers have developed a new type of high overtone bulk acoustic resonator, termed an X HTBAR, that overcomes limitations found in conventional designs. This device utilizes a 3 micron thick film of lithium niobate on a 500 micron silicon substrate, enabling highly efficient excitation of thickness shear modes through lateral electrodes, eliminating the need for bottom electrodes and confining acoustic energy. Experiments demonstrate that this configuration achieves energy transfer efficiency exceeding 99 percent, alongside a stable free spectral range. The X HTBAR exhibits comb-like phonon spectra spanning from 0.

1 to 1. 8GHz, and achieves high quality factors ranging from ten to the power of three to ten to the power of five. Measurements confirm frequency quality products exceeding ten to the power of thirteen at room temperature, alongside a low temperature coefficient of frequency, indicating exceptional stability. The device’s gridded electrode design suppresses spurious modes and allows for tunable mode volumes. This innovative design delivers superior acoustic impedance matching and reduced acoustic energy dissipation, resulting in enhanced performance. Finite element simulations and experimental data align, confirming a free spectral range consistent with theoretical calculations. The researchers demonstrate that the acoustic impedance between the lithium niobate and silicon layers is effectively matched, maximizing energy transmission and minimizing reflection losses, positioning the X HTBAR as a promising component for large-scale quantum interconnects and microwave photonic integrated circuits.

High Q Lithium Niobate Acoustic Resonator

This research demonstrates a novel bulk acoustic resonator, termed X HTBAR, fabricated using a lithium niobate-on-silicon platform. By combining the piezoelectric properties of lithium niobate with the acoustic impedance of silicon, the team achieved highly efficient excitation and coupling of acoustic modes with minimal energy loss. A key innovation lies in the lateral excitation scheme, which enables scalable resonant mode volumes while maintaining high spectral purity. Experimental characterization confirms the realization of stable, high-quality factor resonances, with frequency-quality factor products exceeding ten to the power of thirteen, and consistent free spectral ranges that align with theoretical predictions. The X HTBAR exhibits exceptional performance, even surpassing that of mechanical bound states achieved with more complex phononic crystals, and facilitates cooperative interactions between multiple phonon modes. The device’s lateral excitation scheme and use of lithium niobate contribute to a scalable resonant mode volume, enhanced energy storage, and efficient coupling, resulting in excellent integration compatibility and robustness against electrode-related perturbations.