Thickness-extensional second order mode bulk wave resonator using face-bonded X-cut quartz thin plates

Abstract The demand for high-frequency oscillators has been increasing due to high-speed, large-capacity communications at base stations and data centers. Therefore, a Quartz (Qtz) resonator with a high resonance frequency is also highly desired, so we investigated a Qtz bulk acoustic wave (BAW) resonator using the thickness-extension (TE) mode, which has a higher velocity than the thickness-shear (TS) mode. This study presents the simulation and fabrication results of a TE mode resonator using an X-Qtz plate bonded with geometrically similar polarity planes (face-bonded Qtz plate). In the simulation, we investigated the influences of the third Euler angle when bonding and the difference in Qtz thickness. As a result of the fabrication using 10 μm or 20 μm thickness Qtz, the face-bonded Qtz samples excited a TE second mode with less spurious and ripples. In a face-bonded sample where each Qtz layer is 20 μm thick, the resonance frequency is about 155 MHz, the impedance ratio was 42.5 dB, Q mr was 1,720, and Q ma was 3,870. Taking 25 °C as the reference, the temperature coefficient of frequency (TCF) between 25 °C and 85 °C is −7.3 ppm °C for f r and −7.5 ppm °C for f a .