Film bulk acoustic wave resonators (FBARs) with relatively high Q-factor are considered good candidates to be used in the RF module of chip-scale atomic clocks (CSACs). In order to simulate and analyze the resonant properties, the Mason equivalent circuit of the FBAR device is introduced, which consists of five parts including top electrode layer, low temperature silicon oxide layer, piezoelectric layer, bottom electrode layer and a composite support structure layer. With the practical processing conditions considered, the piezoelectric layer with a reasonable thickness of 1.30um is selected to achieve a FBAR device with resonant frequency 4.60GHz and Q-factor 278 by the simulation and analysis. In order to further improve the Q-factor, SiO2 thin films with thicknesses from 0.10um to 0.50um placed between the top electrode and piezoelectric layer are introduced. However, as the SiO2 thin film is introduced, the resonant frequency of the FBAR device will drop. In order to keep the resonant frequency fixed to 4.60GHz, the thickness of the piezoelectric layer is adjusted. Finally, the FBAR device resonating at 4.60GHz with Q-factor 627 is achieved, the thicknesses of the SiO2 film and piezoelectric layer of which are 0.20um and 0.69um respectively. The Q-factor of the FBAR device improves about 350, and the FBAR device is expected to be used in CSACs.
Observation of the fundamental Nyquist noise limit in an ultra-high Q-factor cryogenic bulk acoustic wave cavity
