Heterogeneous Integration of 42°YX LiNbO₃/SiO₂/Quartz for Wideband and Spurious-Free SAW Resonators

As a promising material, lithium niobate (LiNbO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>, LN) thin films on heterogeneous substrates have received widespread attention in the field of radio frequency (RF) for broadband filters in recent years. This article proposes a wafer-scale 42°YX LN/SiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>/36°YX quartz (LNOQ) substrate for spurious-free shear horizontal surface acoustic wave (SH-SAW) resonators. Through 3-D finite element simulation, the structures of the LNOQ and interdigital electrodes are optimized to get a large 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">${k} ^{{2}}$ </tex-math></inline-formula>) and good confinement of the acoustic energy, as well as to suppress the spurious modes. The LNOQ wafer fabricated by the ion-slicing process exhibits a high film uniformity of 608.16 nm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm ~1.4$ </tex-math></inline-formula>%. The full-width at half-maximum (FWHM) of the high-resolution X-ray diffraction rocking curve of the LN is only 39.6 arcsec, which has the characteristics of a single crystal. The prepared SAW resonator features a large <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k} ^{{2}}$ </tex-math></inline-formula> of 31.56%, high quality factor of 1048, and a clear response over a wide frequency range. The demonstrated LNOQ platform shows great potential in broadband RF wireless communications.