A low-loss BST thin film on initial nucleation layer for micro and millimeter wave tunable phase shifter

Abstract Barium strontium titanate, (Ba,Sr)TiO 3 (BST), thin films have been deposited on (1 0 0)-oriented MgO substrate by combining preparation of initial layer by Pulsed Laser Deposition and main layer by Metal-Organic-Decomposition method. Films with an initial layer of 20, 30 and 40 nm thickness and final thickness of 400, 650 and 800 nm have been obtained. Physical and dielectric properties of the BST thin films have been characterized from the viewpoint of frequency-agile micro and millimeter circuit applications. The results reveal that Ba 0.6 Sr 0.4 TiO 3 thin films have a good crystallinity with characteristic orientation that is affected by the deposition conditions of the initial layer. Interdigital capacitor with a gap of 10 μm has been characterized and the dielectric loss and tunability are as low as 0.002–0.004 and 12%, respectively, at frequency of 1 MHz for the applied voltage from ∓40 to ±40 V. At microwave frequencies, classic-shaped coplanar waveguide lines formed on BST/(1 0 0) MgO were investigated. A differential phase shift of 18° was obtained at 20 GHz with insertion loss of about −2 dB at 60 V for Au/Cr interconnection. Finally, a three-stage LC-ladder-type phase shifter with variable capacitors of BST thin film has been fabricated considering the experimental results obtained for the classic-shaped coplanar waveguide lines and a maximum phase shift of 40° is obtained at 20 GHz and 60 V.