Strain-relieved Ba0.6Sr0.4TiO3 thin films for tunable microwave applications

This article discusses tunable dielectric thin films, with particularly emphasis on strain-relieved and defect-reduced tunable dielectric thin films that significantly reduce dielectric loss at microwave frequencies. Ba0.6Sr0.4TiO3 (BST) thin films (similar to0.3 mum thick) were deposited onto (100) MgO single crystalline substrates by pulsed laser deposition at 750 degreesC and 200 mTorr O-2 with a nominal 600-A-thick BST buffer layer. These films were observed to be strain relieved and show better dielectric properties by exhibiting a significantly high dielectric Q(=1/tan delta>100) while retaining a useful dielectric tuning (=[C(0)-C(23 V/mum)]/C(0)>20%, where C is the film capacitance) at 8 GHz compared to strained BST thin films. The BST buffer layer could be composed of any porous BST phase with randomly oriented grains between a nearly amorphous phase and a fully crystalline phase. A further increase in the dielectric Q was observed in strain-relieved BST thin films mixed with MgO and strain-relieved BST thin films doped with W. (C) 2002 American Institute of Physics.