2212-Tl-Ba-Ca-Cu-O films on 3" sapphire wafers for high frequency filters

Due to their higher critical temperature of above 100 K compared to Y₁Ba₂Cu₃O₇ thin films, high temperature superconducting Tl₂Ba₂Ca₁ Cu₂O₈ films are well suited for applications with regard to cryocooler operation. In the case of high frequency devices there exists not only the demand for high quality films but also to prepare them on suitable substrate materials such as sapphire. We report the preparation of high quality Tl₂Ba₂Ca ₁Cu₂O₈ films on both sides of cerium oxide buffered 2 inch and for the first time on single side buffered 3 inch sapphire wafers. The films were prepared by means of the well known two-step process. This technology allows us to prepare crack-free films with a thickness of up to 400 nm. The best 2 inch films reach a surface resistance of about 50 μΩ at 5.6 GHz and 77 K as well as a critical current density as high as 4 MA/cm² at 77 K and critical temperatures distinctly above 100 K, whereas the 3 inch films show values of 340 μΩ at 3.9 GHz and 77 K, 1 MA/cm², and 105 K. We prepared 4-pole band pass filters from the 2 inch double-sided films which were able to operate up to 88 K within their designed performance