Microwave Properties of Parallel Plate Capacitors based on (Ba,Sr)TiO 3 Thin Films Grown on SiO 2 /Al 2 O 3 Substrates
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Microwave Properties of Parallel Plate Capacitors based on (Ba,Sr)TiO3 Thin Films Grown on SiO2/Al2O3 Substrates I.P. Koutsaroff, T. Bernacki, M. Zelner, A. Cervin-Lawry, A. Kassam, P. Woo, L. Woodward, and A. Patel Gennum Corporation, 970 Fraser Drive, Burlington, Ontario L7L 5P5 Canada [email protected] ABSTRACT Ba0.7Sr0.3TiO3 (BST) single and quadruple layer capacitors with Pt electrodes were fabricated together on polycrystalline alumina substrates with a SiO2-based multicomponent amorphous buffer layer (SiO2/Al2O3). This paper presents the results of the characterization of these capacitors, to demonstrate their suitability for application as decoupling (high value) capacitors and as components in tunable RF applications (e.g., phase shifters and filters). BST films of different compositions, (Ba0.7Sr0.3)TiO3 and (Ba0.5Sr0.5)TiO3, were grown by metal-organic decomposition (MOD) and RF magnetron reactive sputtering. The capacitance density of 90140 nm thick BST films was in the range of 20 to 70 fF/µm 2. Parallel plate capacitors with areas from 16 µm 2 to 2.25 mm2 were fabricated using photolithography and ion milling techniques. For large capacitors (0.125 to 2.25 mm2), capacitance and tanδ were measured at low frequencies (1 KHz - 1 MHz) using an LCR meter. Smaller capacitors (16 µm 2 to 3600 µm 2) were additionally characterized in the frequency range of 50 MHz - 20 GHz. In such case, capacitance, tanδ and equivalent series resistance (ESR) were extracted from two port scattering parameters obtained using a vector network analyzer (VNA). The relationship between dielectric loss, tunability and calculated figure of merit vs. BST composition and deposition temperature was outlined. In addition, loss and ESR at high frequencies was investigated. The typical achieved leakage current density of sputtered BST films for 2.25 mm2 capacitors fabricated on SiO2/Al2O3 was 7.3x10-9 A/cm2 at 300 kV/cm (65 fF/µm2), about 2 times lower than for (Ba0.7Sr0.3)TiO3 films deposited by MOD (1.4x10-8 A/cm2 at 300 kV/cm, 34.5 fF/µm2). Furthermore, the tunability of (Ba0.7Sr0.3)TiO3 deposited by both methods on SiO2/Al2O3 was ~60% at 350 kV/cm. INTRODUCTION Barium Strontium Titanate (BST) is an important material used in integrated passive devices, decoupling capacitors [1], multi-chip modules (MCM), and chip-scale packaging. BST also has shown great potential for the fabrication of tunable RF devices [2, 3] such as voltage-controlled oscillators [4], tunable filters [5], and phase shifters [6]. BST forms a solid solution for all Ba/Sr ratios that makes it ideally suitable for the wide temperature range of applications. Based on the required thickness and microstructure, BST thin films might be deposited by a variety of deposition techniques, such as MOCVD [7], CSD(MOD) [8], RF magnetron reactive sputtering [9, 10], etc. However, the combination of critical properties such as high dielectric constant, low loss, low leakage current and high tunability have not been achieved simultaneously. The ability to reproducibly grow
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