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 (∼0.3 μm thick) were deposited onto (100) MgO single crystalline substrates by pulsed laser deposition at 750 °C and 200 mTorr O2 with a nominal 600-Å-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 δ>100) while retaining a useful dielectric tuning (=[C(0)–C(23 V/μm)]/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.