A method for evaluating the temperature coefficient of frequency of piezoelectric thin film without removing substrate

Temperature stability is one of the important factors of RF filters in mobile phones. In general, the temperature coefficient of frequency (TCF) of bulk acoustic wave resonator is evaluated after the device fabrication is completed. It would be convenient if it were possible to evaluate the TCF before removing substrate. In this study, we propose a new method to evaluate the TCF of piezoelectric layer using the structure of piezoelectric thin film with supporting substrate (HBAR structure). In general, it is difficult to avoid the effect of substrate and to extract the TCF consisting solely of a piezoelectric layer. To eliminate the unwanted multiple acoustic resonance due to the substrate in the frequency domain of HBAR, the multiple acoustic reflection in the substrate was eliminated in the time domain response. Then, we can observe broad resonance and anti-resonance peak originating from the resonance of piezoelectric layer. Using Mason’s equivalent circuit model, we theoretically found that resonance frequency is not significantly affected by the TCF of substrate in the response after above signal processing. Based on these findings, the direct measurement of TCFs for ScAlN and MgZnO piezoelectric thin films with various concentrations of Sc and Mg was achieved without removing substrate.