Microwave properties of BST thin film interdigital capacitors on low cost alumina substrates

Barium Strontium Titanate (BST) thin films have a permittivity that is dependent on applied electric-field. As electronically tunable lumped capacitors they can be used as microwave varactors in phase shifters, tunable filters, and delay lines. Generally BST films are deposited on single crystalline substrates. This paper reports on the microwave properties of BST films deposited on low cost alumina substrates by means of metal organic chemical vapour deposition using inexpensive copper metallization. Interdigital capacitors were fabricated yielding capacitors in the sub-picofarad range and tunable by voltages up to 120 V. Low frequency intrinsic Q's of over 100 were obtained. This reduced to external Q's of 17 at 26GHz for 0.5 mum electrodes. The maximum dielectric tunability was found to be about 1.64:1 (40%) for an applied field of 300 kV/cm at a bias of 120 V.