Towards A High Quality Factor DC Electric Field Switchable Barium Strontium Titanate Solidly Mounted Resonator
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Towards A High Quality Factor DC Electric Field Switchable Barium Strontium Titanate Solidly Mounted Resonator George N. Saddik1, Junwoo Son2, Susanne Stemmer2 and Robert A. York1 1 University of California Santa Barbara, Department of Electrical and Computer Engineering, Santa Barbara, CA 93106, U.S.A. 2 University of California Santa Barbara, Materials Department, Santa Barbara, CA 93106, U.S.A. ABSTRACT Barium strontium titanate solidly mounted resonators (SMR) were fabricated with three different acoustic Bragg reflectors (ABR) on a sapphire substrate. The three devices had ABR structures consisting of W/SiO2/W/SiO2, Mo/SiO2/Mo/SiO2, and Pt/SiO2/Pt/SiO2 respectively. The s-parameters of all three devices were measured. The results showed that the quality factor increased as a function of the material in the ABR structure. The quality factor for the devices with tungsten, molybdenum and platinum in the ABR structures are 101, 88, and 31, respectively. This investigation showed how the material in the ABR structure can contribute to the acoustic loss in the device. INTRODUCTION Voltage-tunable ferroelectric materials such as barium strontium titanate (BST) and strontium titanate (STO) have been investigated by several research groups for varactor applications such as tunable matching networks, phase shifters and tunable filters [1-3]. Recently it has been shown that BST and STO become piezoelectric under an applied dc bias [4, 5]. Several research groups have demonstrated ferroelectric bulk acoustic wave resonators using the thin film bulk acoustic wave resonator (TFBAR) and solidly mounted resonator (SMR) approaches [5,6]. Although it is believed that BST thin films can achieve quality factors in excess of 500, typical quality factors for BST SMR devices are in the 40 to 50 range. The loss mechanisms that can contribute to low quality factors include acoustic losses in the SMR layers, surface and interface roughness associated with the top and bottom interfaces between the BST layer and the electrodes, and excitation of lateral modes. The goal of this contribution is to investigate the acoustic losses in BST SMR due to the materials used in the acoustic Bragg reflector (ABR), which is one of the factors contributing to the low quality factor of our BST SMR devices. EXPERIMENT An acoustic Bragg reflector consists of multiple quarter-wavelength alternating high and low acoustic impedance layers. In this work, three BST SMRs with three different acoustic Bragg reflectors were designed and tested. The acoustic impedance and the thickness of each layer in the ABR are determined by [7]. All three devices consist of a 4-layer ABR, platinum bottom electrode, BST thin film and a platinum top electrode. The three devices had ABR structures consisting of W/SiO2/W/SiO2, Mo/SiO2/Mo/SiO2, and Pt/SiO2/Pt/SiO2 respectively. The Pt/SiO2/Pt/SiO2 ABR structure has been consistently used in all our previous BST SMR devices.
TE Pt BST BE Pt SiO2 W or Mo or Pt SiO2 W or Mo or Pt Sapphire Ti
Figure 1. Schematic diagram
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