A DC voltage dependent switchable acoustically coupled BAW filter based on BST-on-silicon composite structure

Abstract

A DC voltage dependent switchable, bulk acoustic wave (BAW) filter is presented in this paper. The filter has a composite structure consisting of a barium strontium titanate (BST) layer sandwiched between top and bottom platinum (Pt) electrodes deposited on silicon (Si) and oxide (SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ) layers. The electrostrictive property of ferroelectric BST allows for the filter to be turned on and off by applying an electric field across the BST layer. The Si and SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> layers are used to increase the overall quality factor and mechanical strength of the structure. When the filter is turned on, the 3 dB bandwidth is 43 MHz at a center frequency of 1.08 GHz. After resonating out the input and output static capacitances of the filter using shunt inductors, the filter exhibits an insertion loss of 8.5 dB with the application of 25 V DC bias. The return loss and out-of-band rejection of the filter are 18 dB and 29 dB, respectively. In its off state, with no DC bias applied, the isolation between input and output ports is 37 dB.