Rotated Interdigital Transducers With Matched Waveguide Boundaries for High-Performance Surface Acoustic Wave Devices

This work proposes a 2-D electrode structure consisting of rotated interdigital transducers (IDTs) and matched waveguide boundaries. By tilting the waveguide boundary so that the incident angle of the power flow is moderate, the energy leakage can be eliminated. Based on the Y42-cut LiTaO3/SiO2/sapphire (LTOS) (LiTaO3-on-sapphire, LTOS) substrate and 2-D electrode structure, high-Q shear horizontal surface acoustic wave (SH-SAW) resonators were realized. Besides, the suppression of transverse modes, the weakening of out-of-band (OoB) responses, and the regulation of electromechanical coupling coefficients (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$k_{\text {t}}^{2}$ </tex-math></inline-formula>) were also achieved. Among the fabricated SH-SAW resonators with different 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> and tilted angle <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>, resonators possess a high Q value when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert \gamma -\theta \vert ~\approx ~7^{\circ }$ </tex-math></inline-formula>, and the highest <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q} _{\text {p}}$ </tex-math></inline-formula> (Q value at antiresonance frequency) reaches 5231. When <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 resonator increases from 0° to 20°, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}_{\text {t}}^{2}$ </tex-math></inline-formula> of the corresponding resonator changes from 12.6% to 5.9%. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q} _{\text {p}}$ </tex-math></inline-formula> of the proposed resonators is larger than 3000, which exceeds four times that of the resonator with only rotating IDT. SH-SAW filters with different designs were fabricated, measured, and compared. The minimum insertion loss (ILmin) of the filter with matched <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> is 0.61 dB lower than that of the filter with only rotated IDT. Based on the monolithic LTOS substrate, the 3-dB bandwidths (BW3 dB) of the two SH-SAW filters with the proposed structure are 47 and 63 MHz, respectively, and the ILmin is 0.96 and 0.83 dB, respectively. Filters with the proposed structure also achieve steep skirts and over 50 dB of OoB rejection in adjacent bands. The proposed filters were also demonstrated on the mass-produced Y42-cut LiTaO3/SiO2/poly-Si/Si wafer. The proposed 2-D electrode structure shows great potential to realize multiple low-loss filters with different bandwidths on a single material platform and is suitable for mass production.