Microwave Properties of Parallel Plate Capacitors based on (Ba,Sr)TiO_3 Thin Films Grown on SiO_2.Al_2O_3 Substrates

摘要

Ba_(0.7)Sr_(0.3)TiO_3 (BST) single and quadruple layer capacitors with Pt electrodes were fabricated together on polycrystalline alumina substrates with a SiO_2-based multicomponent amorphous buffer layer (SiO_2/Al_2O_3). 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, (Ba_(0.7)Sr_(0.3))TiO_3 and (Ba_(0.5)Sr_(0.5))TiO_3, were grown by metal-organic decomposition (MOD) and RF magnetron reactive sputtering. The capacitance density of 90-140 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 mm~2 were fabricated using photolithography and ion milling techniques. For large capacitors (0.125 to 2.25 mm~2), capacitance and tan5 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 mm~2 capacitors fabricated on SiO_2/Al_2O_3 was 7.3x10~(-9) A/cm~2 at 300 kV/cm (65 fF/μm~2), about 2 times lower than for (Ba_(0.7)Sr_(0.3))TiO_3 films deposited by MOD (1.4x10~(-8)A/cm~2 at 300 kV/cm, 34.5 fF/μm~2). Furthermore, the tunability of (Ba_(0.7)Sr_(0.3))TiO_3 deposited by both methods on SiO_2/Al_2O_3 was ～60% at 350 kV/cm.