High-Steepness and Low-Loss SAW Filters With Fractional Bandwidth From 3.7% to 12.4% on a Monolithic X-Cut LiNbO<sub>3</sub>/SiO<sub>2</sub>/SiC Substrate
Xiaoli Fang, Jinbo Wu, Shibin Zhang, Pengcheng Zheng, Juxing He, Xinjian Ke, Kai Huang, Jicong Zhao, Haiyan Sun, Min Zhou, Xiaomeng Zhao, Xin Ou
Abstract
To integrate the ladder-type surface acoustic wave (SAW) filters of different frequency bands on a monolithic substrate and improve skirt steepness, a new approach of selectively adjusting the electromechanical coupling coefficient (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{t}^{2}$ </tex-math></inline-formula>) of built-in resonators without reducing their Q values is investigated. Based on the X-cut LiNbO3/SiO2/SiC (LNOSiC) substrate, the in-plane orientations (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta $ </tex-math></inline-formula>) of the SAW resonators are adjusted to obtain the desired <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{t}^{2}$ </tex-math></inline-formula>, while long dummy electrodes are applied to suppress the transverse leakage of the acoustic field to maintain a high Q value. For resonators with in-plane orientations (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta $ </tex-math></inline-formula>) from 0° to 42°, the range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{t}^{2}$ </tex-math></inline-formula> is continuously adjustable from 31% to 5%, while the measured maximum Bode-Q values are maintained around 1800. By presetting the in-plane orientations of resonators, the n25, n66, n40, 2.4 GHz Wi-Fi, n41, and n78 filters with FBW from 3.7% to 12.4% and out-of-band rejection over 35 dB were demonstrated. On the other hand, to meet the high requirements of steep skirts, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{t}^{2}$ </tex-math></inline-formula> of specific resonators in n41 and 2.4 GHz Wi-Fi filters are selectively reduced. Among a set of n41 filters of different designs, the one with steeper skirts has a shunt resonator with an in-plane orientation of 39° and a corresponding <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{t}^{2}$ </tex-math></inline-formula> of 7.5%. While the rest of the resonators have a uniform in-plane orientation of 27° with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{t}^{2}$ </tex-math></inline-formula> of 18%. The filter demonstrates a 3-dB passband from 2496 to 2685 MHz, a minimal insertion loss (IL) of 1.29 dB, and reaches 16 dB rejection at 2483 MHz, which is the edge of the 2.4 GHz Wi-Fi band. The fabricated 2.4 GHz Wi-Fi filter demonstrates a 3-dB passband of 2388–2482 MHz, a minimum IL of 1.32 dB, and achieves 30 dB rejection at 2496 MHz on the edge of the n41.