The Effect of Patterned vs. Unpatterned Acoustic Bragg Reflector on Barium Strontium Titanate Solidly Mounted Resonator
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The Effect of Patterned vs. Unpatterned Acoustic Bragg Reflector on Barium Strontium Titanate Solidly Mounted Resonator George N. Saddik and Robert A. York University of California Santa Barbara, Department of Electrical and Computer Engineering, Santa Barbara, CA 93106, U.S.A. ABSTRACT Barium strontium titanate solidly mounted resonators were fabricated with patterned and unpatterned acoustic Bragg reflectors on a sapphire substrate. The patterned and unpatterned solidly mounted resonator devices had acoustic Bragg reflectors consisting of Pt/SiO2/Pt/SiO2. The s-parameters of both devices were measured. The results showed that the quality factor increased for the device with the patterned acoustic Bragg reflector structure. The quality factors for the devices with patterned and unpatterned acoustic Bragg reflector structures were 54 and 115 and 28 and 86, respectively at the resonant and antiresonant frequencies. This investigation shows how an unpatterened acoustic Bragg reflector can contribute to the degradation of the overall quality factor of the device. INTRODUCTION The electrostrictive property of thin-film strontium titanate (STO) and barium strontium titanate, Ba0.5Sr0.5TiO3, (BST) is manifested as a voltage-induced piezoelectricity [1]. During the past few years several research groups have demonstrated a new-type of voltage-activated bulk acoustic wave resonators that exploit this effect using the thin film acoustic wave resonator (TFBAR) and solidly mounted resonator (SMR) approaches [2-3]. The SMR structure employs a distributed Acoustic Bragg reflector (ABR) or acoustic “mirror” under the device to isolate it from the substrate. There are several mechanisms that can contribute to the degradation of the device performance such as surface and interface roughness associated with the top and bottom interfaces between the BST layer and the electrodes, acoustic losses in the SMR layers, excitation of lateral modes and the use of an unpatterned “blanket” acoustic Bragg reflector. Several publications mention that a blanket ABR will degrade the SMR performance and advice to pattern the ABR. However, they do not quantify this degradation. The intention of this contribution is to investigate the performance degradation of the BST SMR due to the use of a blanket ABR. EXPERIMENT An acoustic Bragg reflector consists of multiple quarter-wavelength alternating high and low acoustic impedance layers. In this work, two BST SMRs were fabricated, with a blanket and patterned ABR, respectively. The ABR in both cases consisted of a 4-layer Pt/SiO2/Pt/SiO2 structure, which has been used in all our pervious BST SMR devices. The BST SMR was fabricated on a c-plane sapphire substrate. The platinum and silicon dioxide layers were deposited by electron-beam evaporation and ICP plasma enhanced chemical vapor deposition, respectively. Following each silicon dioxide layer deposition a high temperature densification
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(a) SMR Device
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(b) Figure 1. Photograph
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