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In this work, a simple structure of ultra-thin LiTaO 3 (LT) film on extremely high-resistance sapphire substrate (LTOS) was proposed. The PSC effect has little effect on the effective resistivity (peff) of the LT /sapphire interface in LTOS since the resistivity of sapphire is greater than 1014 Ω. cm (lack of sufficient free carriers), therefore the RF loss in LTOS substrate is limited. The 4-inch LTOS substrate was prepared by ion-cutting process, and high-Q resonators and a low insertion loss (IL) filter were demonstrated on the LTOS substrate. The resonator exhibits an extracted k2 of 6.62 % and a maximum Bode-Q (Qmax) of 2100, resulting in a FoM (k2×Qmax) of 139. The filter with a center frequency of 2231 MHz features a minimum IL of 0.61 dB and a 3-dB fractional bandwidth (FBW) of 3.6%. The SAW devices on LTOS substrate show a great potential for applications in RF wireless communications.

In this work, the phase velocity (V p ) and the electromechanical coupling factors (k t 2) of SH0 modes excited in X-cut LiNbO 3 (LN) films were studied by finite element analysis, and both of them show strong dispersion when LN films are bonded to foreign substrates. In particular, the V p of the SH0 mode in LiNbO 3 -on-SiC (LNOSiC) substrate can exceed 1.5 times the value in LN plate, so as the operating frequency. 4-inch LNOSiC substrate was fabricated by ion-cutting process. A group of SH0 mode resonators on the novel LNOSiC substrate were demonstrated, so as the ladder-type filter. Since part of the acoustic energy of the SH0 mode penetrates into the high-velocity SiC supporting substrate, the fabricated resonator exhibits a phase velocity of exceeding 5000 m/s. The ladder-type filter composed of the SH0 mode resonators with increased velocity shows a center frequency (f c ) of 3.93 GHz, an insertion loss (IL) of 2.05 dB and a 3-dB fractional bandwidth (FBW) of 14.2%. The acoustic devices on LNOSiC heterogeneous substrate are very promising for high frequency and wideband 5G front-ends.

In this work, a novel heterogeneous substrate of lithium tantalate thin film on silicon carbide (LiTaO 3 -on-SiC, LTOSiC) was proposed and prepared. Since the SiC features an extremely-high phase velocity, a low acoustic loss and a small thermal expansion coefficient (CTE), surface acoustic wave devices with high frequency, low loss and low TCF were achieved on the LTOSiC substrate. The fabricated shear horizontal surface acoustic wave (SH-SAW) resonator shows a maximum Bode-Q (Bode-Qmax) of 2235 and a low TCF of -9.4 ppm/K at 1.90 GHz, while the longitudinal leaky surface acoustic wave (LLSAW) resonator shows a high Bode-Qmax of 1180 and an improved TCF of -24.7 ppm/K at 4.40 GHz. The ladder-type filter composed of SH-SAW resonators was also demonstrated, showing a center frequency (fc) of 3.03 GHz, an insertion loss (IL) of 0.9 dB and a 3-dB bandwidth of 101 MHz. The TCFs at - 3 dB attenuation of the lower and higher frequency side of passband are -13.8 and -15.8 ppm/K, respectively. The acoustic devices on LTOSiC substrate possess the prospect of commercial applications in 5G era.

In this work, we introduce a novel architecture for monolithic integrated multiband filters based on lithium niobate on insulator (LNOI) platform. The advantages of the crystal cut of Y124° are introduced compared to Z-cut. Three typical acoustic modes are excited simultaneously in different in-plane directions of a 400 nm Y124°-cut LiNbO 3 thin film. Combining with the buried electrode structure, the multiband filters with a wide operating frequency range from L band to C band, and a wide fractional bandwidth (FBW) range from 3.5% to 26% are achieved. The proposed structure provides an attractive solution for monolithic integration of filters.