Frequency Agile Microwave Applications Using (Ba,Sr)TiO 3 /Y 3 Fe 5 O 12 Multilayer Grown by Pulsed Laser Deposition
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ABSTRACT (Ba,Sr)TiO3 (BST) thin films have been deposited by pulsed laser deposition (PLD) onto single crystal Y3 Fe 5O1 2 (YIG) substrates with/without an MgO buffer layer. The structure and microwave properties of BST films have been investigated as function of substrate orientation and 02 deposition pressures (5-800mTorr). The orientation of BST film is varies with the deposition conditions. The dielectric constant, loss tangent, and change in dielectric constant with an applied electric field have been measured at room temperature using interdigitated capacitors at 0.1 - 20 GHz. Polycrystalline BST films have a high tunability (-40%) with a dc bias field of 67kV/cm and a dielectric Q (=l/tanS) between 30 and 40, while (001) oriented BST films have a lower tunability (-20%) but higher dielectric Q (-50). A coplanar waveguide transmission line was fabricated from a (001) oriented BST film on (11 1)YIG which exhibited a 170 differential phase shift with an applied dc bias of 21 kV/cm. An equivalent differential phase shift was achieved with a magnetic field of 160Guass.
INTRODUCTION Ferroelectric thin films are being used to develop a new class of tunable microwave devices.[1,2] The ferroelectric material (Bao.sSro.s)TiO 3 (BST) exhibits a large electric field dependant dielectric constant, which can be used to produce a resonant frequency shift in a tunable oscillator or a time delay (phase shift) in a transmission line. A concern in the development of tunable microwave circuits, such as a coplanar waveguide (CPW) transmission lines, is the large change in the characteristic impedance (Zo) of the device, which occurs when the dielectric constant of the ferroelectric is reduced by a factor of four or more. A novel approach to this problem is to fabricate devices from a ferroelectric/ferrite multilayer. In a coplanar waveguide (CPW) transmission line, the differential phase shift (Aq0) of CPW transmission line between two bias states can be expressed by following, C
1
J__-2~
-
p)()
wherefis microwave frequency, I is the length of transmission line, c is light velocity in vacuum, and eff and peff are effective dielectric constant (permittivity) and permeabilty of device, respectively, and superscript 1 and 2 are indicating zero bias and an applied bias states, respectively. The characteristic impedance Zo of the CPW transmission line is related as following equation, IF -- "
(2)
Using eqs. (1) and (2), estimated A0 and Z. are shown in Table 1 and 2, respectively. The values are calculated at the operating frequency of 15GHz for an 1cm long transmission line, with a 2:1 change in rff (/,iff) from zero to non-zero electric (magnetic) bias field. The differential phase 123 Mat. Res. Soc. Symp. Proc. Vol. 603 ©2000 Materials Research Society
shift increases more with two bias fields than either an electric field or a magnetic field. The characteristic impedance of the transmission line behaviors differently; it changes a factor of v/2 with either an electric field or a magnetic field by itself, while it remain
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