Direct inkjet printing of composite thin barium strontium titanate films
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L. Sengupta, X. Zhang, S. He, and L. Chiu Paratek Microwave, Columbia, Maryland 21045 (Received 7 May 2003; accepted 12 September 2003)
Composite Ba0.6Sr0.4TiO3/MgO thin films with 60% tuning and tan ␦ of 0.007 at 2 GHz were deposited using metal organic decomposition inks by spin coating on single crystal MgO substrates. The films with approximately 1 mol% MgO in Ba0.6Sr0.4TiO3 had a better tuning/loss ratio than either the 0 or the 10 mol% MgO substituted films. Crystalline Ba0.5Sr0.5TiO3 films were produced on both MgO and alumina substrates by inkjet printing of metalorganic precursors with subsequent thermal decomposition followed by annealing at 900 °C. Barium strontium titanate lines as narrow as 100 m were printed on the alumina substrates. The inkjet-printed films were predominantly (100) oriented on MgO and (110) oriented on alumina. The crystalline quality of the inkjet-printed films was improved by annealing at 1100 °C for 3 h in oxygen. Both the printed and the spin-coated films had smooth surfaces (300 Å root-mean-square roughness) as required for subsequent deposition of high-resolution metal electrodes. An inkjet-printed Ba0.5Sr0.5TiO3 film (3500 Å) on MgO annealed at 1100 °C had 20% tunability of the dielectric constant (⑀) at 9.1 V/m direct current bias and tan ␦ < 0.002 at 1 MHz.
I. INTRODUCTION
Barium strontium titanate (BST) and related materials are enabling for a whole generation of new highperformance, low-cost, tunable devices to be used for high-frequency communications applications.1–3 A current leading approach to tunable dielectric devices is based on screen-printed BST/MgO composites with the film thickness ranging between 4 and 20 m. Composite BST/MgO thick films have a high tuning/loss ratio at GHz frequencies, and tunable microwave devices such as filters, phase shifters, and varactors based on these materials demonstrate superior microwave performance.4 At present, these devices require tuning voltages 艌50 V. A reduction of operating voltage to 5 V or less is highly desirable so as to simplify control circuits, improve compatibility, and further reduce cost. One approach to lowering the control voltages is to reduce the distance between the tuning electrodes in either a lateral or
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J. Mater. Res., Vol. 18, No. 12, Dec 2003 Downloaded: 06 Dec 2014
parallel plate configuration. Using smooth thin BST films with submicron thickness should allow significantly reduced electrode spacing. We have developed a liquid-precursor-based metalorganic decomposition (MOD) process for producing thin films of composite tunable dielectrics.5 The MOD liquid precursors (inks) may be readily coupled with direct write deposition techniques such as inkjet printing. As is the case for screen printing, inkjet printing is an inexpensive ap
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