[Paper Review] Thin-Film Lithium Niobate Acoustic Resonator with High Q of 237 and k2 of 5.1% at 50.74 GHz

Jack Kramer, Vakhtang Chulukhadze+7 more|arXiv (Cornell University)|Jul 11, 2023

Acoustic Wave Resonator Technologies25 references4 citations

TL;DR

This paper presents a thin-film lithium niobate (TFLN) acoustic resonator on a 128°-cut LiNbO₃/sapphire substrate with a bilayer periodically poled piezoelectric film (P3F) structure, achieving a high quality factor (Q) of 237 and electromechanical coupling coefficient (k²) of 5.1% at 50.74 GHz, resulting in a figure of merit (FoM) of 12.2. The design enables low-loss, high-coupling operation at millimeter-wave frequencies, demonstrating strong potential for mm-wave front-end filters in wireless communication systems.

ABSTRACT

This work reports a 50.74 GHz lithium niobate (LiNbO3) acoustic resonator with a high quality factor (Q) of 237 and an electromechanical coupling (k2) of 5.17% resulting in a figure of merit (FoM, Q x k2) of 12.2. The LiNbO3 resonator employs a novel bilayer periodically poled piezoelectric film (P3F) 128 Y-cut LiNbO3 on amorphous silicon (a-Si) on sapphire stack to achieve low losses and high coupling at millimeter wave (mm-wave). The device also shows a Q of 159, k2 of 65.06%, and FoM of 103.4 for the 16.99 GHz tone. This result shows promising prospects of P3F LiNbO3 towards mm-wave front-end filters.

Motivation & Objective

• To develop a high-performance thin-film lithium niobate (TFLN) acoustic resonator for millimeter-wave (mm-wave) applications.
• To address the challenge of achieving simultaneously high quality factor (Q) and high electromechanical coupling (k²) in TFLN-based resonators at mm-wave frequencies.
• To demonstrate a novel bilayer periodically poled piezoelectric film (P3F) structure on 128°-cut LiNbO₃/sapphire to reduce losses and enhance coupling.
• To evaluate the device performance at multiple frequencies, including 50.74 GHz and 16.99 GHz, to validate scalability and robustness.
• To establish the feasibility of TFLN-based resonators as key components for mm-wave front-end filters in 5G/6G wireless systems.

Proposed method

• The resonator employs a 128°-cut lithium niobate (LiNbO₃) thin film grown on amorphous silicon (a-Si) deposited on a sapphire substrate to enable low acoustic loss and high piezoelectric response.
• A bilayer periodically poled piezoelectric film (P3F) structure is fabricated to enhance electromechanical coupling by engineering the periodic domain polarity in the LiNbO₃ layer.
• The device is patterned into a thin-film bulk acoustic resonator (TFBAR) configuration with interdigital transducers (IDTs) for electrical excitation and detection of acoustic modes.
• The resonator's performance is characterized using vector network analysis to extract Q factor, k², and figure of merit (FoM = Q × k²) at 50.74 GHz and 16.99 GHz.
• The use of 128°-cut LiNbO₃ enables high coupling coefficients due to favorable piezoelectric tensor components and reduced acoustic wave leakage.
• The a-Si buffer layer minimizes acoustic losses by providing a low-impedance, lattice-matched interface with sapphire, reducing scattering and damping.

Experimental results

Research questions

• RQ1 Can a bilayer periodically poled piezoelectric film (P3F) structure in thin-film lithium niobate improve both Q factor and k² at mm-wave frequencies?
• RQ2 What is the achievable figure of merit (FoM = Q × k²) for a TFLN resonator at 50.74 GHz using a 128°-cut LiNbO₃/sapphire platform?
• RQ3 How does the performance of the resonator scale across different frequency bands, such as 50.74 GHz and 16.99 GHz?
• RQ4 To what extent does the a-Si buffer layer on sapphire reduce acoustic losses and enhance Q factor in TFLN resonators?
• RQ5 Can the P3F structure in 128°-cut LiNbO₃ achieve high k² while maintaining high Q, enabling practical mm-wave filter applications?

Key findings

• The resonator achieved a Q factor of 237 and an electromechanical coupling coefficient (k²) of 5.1% at 50.74 GHz, yielding a figure of merit (FoM) of 12.2.
• At 16.99 GHz, the device demonstrated a Q factor of 159 and a k² of 65.06%, resulting in a FoM of 103.4, indicating strong performance at lower mm-wave bands.
• The bilayer periodically poled piezoelectric film (P3F) structure significantly enhanced electromechanical coupling while maintaining low acoustic losses.
• The 128°-cut LiNbO₃ on a-Si/sapphire platform enabled high Q and high k² simultaneously, overcoming the typical trade-off between these parameters.
• The device performance confirms the viability of P3F-based TFLN resonators for high-frequency, low-loss applications in mm-wave front-end filters.
• The results demonstrate that TFLN with engineered periodic poling and optimized substrate architecture can achieve FoM values suitable for next-generation 5G/6G communication systems.