Novel switchable and dc bias controllable high quality factor high overtone bulk acoustic resonators HBAR

Abstract

The primary motivation of this thesis is to study a novel microwave acoustic resonator, the newlinehigh-overtone bulk acoustic wave resonator (HBAR), based on ferroelectric Ba0.5Sr0.5TiO3 newline(BST) thin films which is in the paraelectric phase at room temperature. newlineTraditional HBARs based on piezoelectric transduction mechanism are often chosen for a wide newlinevariety of applications such as sensing, microwave sources, quantum acoustodynamics (QAD) newlineand acoustic spectroscopy. The novel HBARs presented in this thesis utilize electrostriction newlinemechanism which in turn induces piezoelectric effect in the thin ferroelectric BST film under newlinean applied external DC field. This gives the HBARs an added advantage of having the newlinecapability of switching from a simple capacitor to a resonator. Even though the individual newlinetuning of each resonant mode is insignificant when compared to other tunable film bulk newlineacoustic wave resonators (FBAR) and solidly mounted resonators (SMR) based on ferroelectric newlinethin films because of the heavy mass loading the HBARs suffer from its thick substrate, newlineimportant parameters of the resonators like the coupling coefficient (and#119896;and#119905; newline2) and the quality factor newline(Q) of the HBAR are affected by the applied DC field, making the parameters controllable or newlinedependent on the external force i.e., DC field. newlineThe thesis introduces the HBAR technology by laying down a state-of-the-art literature review. newlineThe experimental methodologies and techniques used in designing, fabrication and newlinecharacterization of the HBAR is discussed in detail. Modeling using 1-D numerical simulation, newline2-D FEM and 2-D axisymmetric FEM are performed and correlated with experimental results. newlineHBARs are fabricated with various substrates (YAG, sapphire, silicon and fused silica). Using newlineHBARs made on YAG and sapphire substrates, the acoustic velocity, the mass density, the newlineelastic constant and the coupling coefficient of the BST thin film are extracted from its spectral newlinedata. The DC field dependent nature of the and#119896;and#119905; newline2 and the Q factor of both the HBARs are newlineestablished.