High-Steepness and Low-Loss SAW Filters With Fractional Bandwidth From 3.7% to 12.4% on a Monolithic X-Cut LiNbO3/SiO2/SiC Substrate

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 (
$k_{t}^{2}$
) 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 (
$\theta $
) of the SAW resonators are adjusted to obtain the desired
$k_{t}^{2}$
, 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 (
$\theta $
) from 0° to 42°, the range of
$k_{t}^{2}$
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
$k_{t}^{2}$
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
$k_{t}^{2}$
of 7.5%. While the rest of the resonators have a uniform in-plane orientation of 27° with
$k_{t}^{2}$
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.