High Q Lateral-Field-Excited Bulk Resonator Based on Single-Crystal LiTaO₃ for 5G Wireless Communication

Yanmei Xue,Mansun Chan,Xiaodong Yang,Changjian Zhou,Xiu Yin Zhang

doi:10.1109/jeds.2021.3066369

Abstract

The paper presents the design and fabrication of lateral-field-excited (LFE) resonators based on 42° Y-cut single-crystal LiTaO 3 (LT) on silicon dioxide (SiO 2 ). A simple coplanar top electrode is defined to excite the bulk acoustic wave modes in the suspended LT/SiO 2 structure, and the fabrication process that only involves two lithography steps is more simplified compared to that of commercial film bulk acoustic wave resonators. For a model structure consisting of LT(670 nm)/SiO 2 (1500 nm) thin film, two types of acoustic modes are both piezoelectrically active in the LT film: the first one is the thickness-shear mode with a resonance frequency of 2.46 GHz, an electromechanical coupling ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{eff}^{2}$ </tex-math></inline-formula> ) of 1.4%, a high quality factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> ) of 1690, and the second one corresponds to longitudinal mode with a resonance frequency of 4.39 GHz, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k} _{eff}^{2}$ </tex-math></inline-formula> of 1.2%, a high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> of 1590, which is among the highest reported for piezoelectric MEMS resonators operating at this frequency range. The excellent performance would enable application scenarios including high-resolution sensors, low-phase-noise oscillators, and low-loss, high selectivity filters in the sub-6 GHz range for the fifth-generation (5G) wireless communication.

Highlights

With the development of large-volume and high-speed communication systems in the past decades, there is an everincreasing demand for high-frequency acoustic devices
For frequency above 3 GHz, the mainstream filters used in wireless communication systems are film bulk acoustic wave resonators (FBARs) [1], where the thickness-extention mode (TEM) is utilized to obtain a relatively wide bandwidth
The scanned frequency is firstly set at a large range to obtain the acoustic mode frequency, narrowed to achieve a 31 kHz frequency step size to obtain an accurate calculation of Q and ke2ff

Summary

INTRODUCTION

With the development of large-volume and high-speed communication systems in the past decades, there is an everincreasing demand for high-frequency acoustic devices. For frequency above 3 GHz, the mainstream filters used in wireless communication systems are film bulk acoustic wave resonators (FBARs) [1], where the thickness-extention mode (TEM) is utilized to obtain a relatively wide bandwidth. Shear mode bulk acoustic wave that can be readily excited with the lateral-field-excited (LFE) resonators has attracted great attention related to liquid-phase sensing applications [2]–[4]. XUE et al.: HIGH Q LATERAL-FIELD-EXCITED BULK RESONATOR BASED ON SINGLE-CRYSTAL LiTaO3 coplanar electrodes are patterned on top of the piezoelectric thin film, which promises a simpler fabrication process and a lower cost. Compared with FBARs with piezoelectric thin films sandwiched by electrodes, LFE resonator does not require a bottom electrode, enabling the use of single-crystal piezoelectric thin films. There have been few studies on single-crystal piezoelectric thin films for high-frequency bulk acoustic resonators. The LFE resonators with a high frequency and a high Q, enable them strong candidates for high-resolution sensors, low-phasenoise oscillators and low-loss narrowband filters for radio frequency wireless communication, such as LTE-Advanced contiguous carrier aggregation which requires up to 20 MHz bandwidth [13] and IEEE 802.11a wireless LAN system occupying 16.6 MHz at a center frequency of 5 GHz [14]

RESONATOR FABRICATION

RESULTS AND DISCUSSION

THE DEPENDENCY OF RESONANCE FREQUENCY ON TEMPERATURE

CONCLUSION