Toward mmWave Surface Acoustic Wave Resonators in Lithium Niobate on Silicon Carbide

The article titled "Toward mmWave Surface Acoustic Wave Resonators in Lithium Niobate on Silicon Carbide" discusses the design and development of high-performance surface acoustic wave (SAW) resonators for millimeter-wave (mmWave) applications, specifically using thin-film lithium niobate (LN) on silicon carbide (SiC). Here's a summary of the key points:

Introduction

As wireless communication systems progress toward mmWave frequencies (above 20 GHz), there is an increasing need for compact, high-performance radio frequency front-end components. Surface acoustic wave (SAW) resonators, particularly those based on lithium niobate (LN), are a promising technology for achieving these high frequencies. However, the design of mmWave SAW resonators faces challenges, such as high phase velocities, efficient acoustic confinement, and mitigating performance degradation due to internal stress.

Methods for mmWave SAW Resonators

The article proposes two methods for designing solidly mounted SAW resonators for the mmWave range, both based on LN-on-SiC waveguides:

1. Method 1 – L-SAW (Longitudinal SAW) Resonator

• This method uses the L-SAW mode, which is designed to achieve a high phase velocity (around 6500 m/s).

• L-SAW provides strong acoustic confinement in the LN-on-SiC waveguide, leading to high Q-factors and minimizing substrate radiation losses that usually degrade resonator performance at high frequencies.

• Simulation and fabrication results show that this method successfully scales to mmWave frequencies with a figure of merit (FoM) of 6.53 at 22.42 GHz.

2. Method 2 – Electrode-Guided Shear Horizontal SAW (EG SH-SAW) Resonator

• This novel approach utilizes a higher-order shear horizontal SAW (SH-SAW) mode that is confined using an electrode-guided scheme to prevent internal stress cancellation that typically affects such modes.

• This method achieves a phase velocity improvement of 72% over the fundamental SH-SAW mode and demonstrates a k² of 1.6% and a Q of 260 at 23.5 GHz.

• The EG SH-SAW resonator shows a FoM of 4.16 and provides efficient frequency scaling with minimal spurious modes.

Design Considerations

• LN-on-SiC Waveguides: The choice of SiC as a substrate is critical due to its high phase velocity and stiffness, which provides superior confinement for both L-SAW and SH-SAW modes, making it ideal for mmWave frequencies.

• Electrode Thickness: Electrode design is crucial for both methods. For the EG SH-SAW resonator, the electrode thickness directly affects the confinement and excitation efficiency of the higher-order mode. The ideal electrode thickness for the EG SH-SAW resonator is around 50 nm, ensuring a balance between confinement and minimizing spurious modes.

Fabrication and Material Characterization

• Fabrication Process: Both resonators are fabricated using a two-step electron beam lithography (EBL) and lift-off process. The LN-on-SiC structure is bonded and patterned with fine electrodes as thin as 50 nm to support the high-frequency operation.

• Material Quality: The quality of the transferred LN film is evaluated using X-ray diffraction (XRD), with a full width at half maximum (FWHM) of only 43 arcseconds, indicating excellent single-crystal quality.

Performance Evaluation

• L-SAW Resonators: The fabricated L-SAW resonators showed a high Q-factor of 365 at 17.9 GHz for a λ of 360 nm. Reducing λ to 280 nm suppressed spurious modes and improved the k² to 3.4% at 22.42 GHz with a Qmax of 192, providing a FoM of 6.53.

• EG SH-SAW Resonators: The EG SH-SAW resonators were tested with a λ of 200 nm and demonstrated high performance, with a k² of 1.6%, a Qmax of 260, and a FoM of 4.16 at 23.5 GHz.

Conclusion

This study presents two promising methods for mmWave SAW resonators: L-SAW and EG SH-SAW. These resonators leverage the unique properties of LN-on-SiC platforms to scale efficiently toward mmWave frequencies with high performance. Despite challenges such as spurious modes and electrode design, the proposed resonators outperform existing devices in the 20–26 GHz range, offering significant potential for future mmWave communication systems. The work highlights the need for further optimization in fabrication processes and waveguide design to further enhance the performance of these devices.

Future Work

Future research will focus on further reducing spurious modes, optimizing the electrode design, and exploring alternative fabrication methods to improve the scalability and efficiency of mmWave SAW resonators.