Low Temperature and DC Bias Dependence Study of Barium Strontium Titanate Based High Overtone Bulk Acoustic Resonator (HBAR)

This paper reports the study and characterization of a high overtone bulk acoustic resonator (HBAR) with varying dc bias voltages applied and with measurements performed at various temperature points. Important parameters of the resonators such as the effective coupling coefficient and quality factors are studied for the multiple resonant modes appearing in the frequency spectra with relation to effects from both temperature and dc bias applied. It is observed that the resonant modes intensity (S11, dB) and its distribution varies with different bias voltages as well as upon the polarity of the bias applied. Both the effective coupling coefficient and the quality factor (Q) of the modes are heavily influenced by the temperature at which the resonator operated. The transition temperature of the ferroelectric thin film Ba0.5Sr0.5TiO3 (BST) used as the transducer is also observed from the distribution of the coupling coefficient and quality factor with temperature. From the study of three different resonant modes (0.94 GHz, 1.52 GHz and 2.56 GHz), it is observed that the frequency quality factor product (fQ) has an inverse proportionality with temperature wherein with decrease in temperature, the recorded fQ increases. The dependence of fQ on temperature also increases with increase in frequency of the mode considered.