A New Class of High-Overtone Bulk Acoustic Resonators Using Lithium Niobate on Conductive Silicon Carbide

This letter presents a new class of High-overtone Bulk Acoustic Resonators (HBARs) with only top electrodes but excellent performance using lithium niobate (LN) thin films on conductive silicon carbide (SiC). The 4-inch single-crystalline lithium niobate thin film on conductive SiC (LNCSiC) substrate was prepared by ion-cutting process. Since the conductive SiC serves as both the resonant cavity and the floating electrode, the HBARs based on LNCSiC with only top electrodes were designed and fabricated. The devices show a wide frequency response span exceeding 2.0 GHz, a frequency spacing of 10 MHz, an excellent f ·Q
_LAKIN
product of 9.6×10
¹³
Hz (f ·Bode-Q of 1.1×10
¹⁴
Hz), and a low temperature coefficient of frequency (TCF) of -18.9 ppm/°C. The frequency response shift of the proposed HBAR can be achieved by just adjusting the in-plane orientation of the top electrodes. Additionally, a multi-band filter was numerically synthesized based on the proposed HBARs, showing a spectral spacing of 10 MHz and a passband span of more than 2.0 GHz. Upon further optimizations, the proposed HBARs with a wide frequency response, a high-f ·Q, and a low-TCF will be very promising for radio-frequency applications.