Characterization of Thin-Film Decoupling and High-Frequency (Ba,Sr)TiO 3 Capacitors on Al 2 O 3 Ceramic Substrates

In this paper we present the results of the characterization of parallel-plate thin-film (Ba 1- x ,Sr x )TiO 3 (BST) capacitors, to demonstrate their suitability for use as decoupling capacitors (a capacitance as high as 0.34 µF and a capacitance density of up to 70 fF/µm 2 ) and as tunable RF components (a small capacitance from 0.5 pF to 16 pF, a high tunability of 4.22:1 at 10 V and a capacitance density of up to 34 fF/µm 2 ). 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 on Pt/TiO x /SiO 2 /Al 2 O 3 ceramic substrates. For large capacitors (2.25 mm 2 ), capacitance and tan δ were measured at low frequencies (1 kHz) using an LCR meter. Smaller capacitors (16 µm 2 to 961 µm 2 ) were characterized in the frequency range of 0.01–20 GHz. Capacitance, tan δ and equivalent series resistance (ESR) were extracted from two port scattering parameters obtained using a vector network analyzer (VNA). The relationships between dielectric loss, tunability and commutation quality factor (CQF) vs BST composition and deposition conditions were outlined.