METHOD FOR FORMING RF-PROPERTIES-OPTIMIZED COMPOSITIONS OF (RE) Ba2Cu3O7-delta THIN FILM SUPERCONDUCTORS

Abstract

The films of this invention are high temperature superconducting (HTS) thin films specifically optimized for microwave and RF applications. In particular, this invention focuses on compositions with a significant deviation from the 1:2:3 stoichiometry in order to create the films optimized for microwave/RF applications. The RF/microwave HTS applications require the HTS thin films to have superior microwave properties, specifically low surface resistance, R s , and highly linear surface reactance, X s , i.e. high J IMD . As such, the invention is characterized in terms of its physical composition, surface morphology, superconducting properties, and performance characteristics of microwave circuits made from these films.

We claim: 
     
         1 . A method for forming a superconducting article comprising the steps of:
 providing a substrate,   providing at least one buffer layer supported by the substrate, and   depositing by reactive coevaporation a thin film on the substrate having the nominal composition of RE 2 Ba y Cu 3 O x      wherein RE is a rare earth, wherein the ratio of y/z is 1.65±10% and x is between 6 and 7 inclusive,   wherein the thin film has a superconducting transition temperature of <95K, and   wherein the article including 45-degree grain boundaries in a concentration of <1%.   
     
     
         2 . The method for forming a superconducting article of  claim 1  wherein the thin film has a superconducting transition temperature ≦87K. 
     
     
         3 . The method for forming a superconducting article of  claim 1  wherein the substrate is lattice matched to the thin film. 
     
     
         4 . The method for forming a superconducting article of  claim 1  having an RMS surface roughness of less than about 10 nm. 
     
     
         5 . The method for forming a superconducting article of  claim 1  wherein the topmost buffer layer is lattice matched to the thin film. 
     
     
         6 . The method for forming a superconducting article of  claim 1  wherein one of the buffer layers is MgO. 
     
     
         7 . The method for forming a superconducting article of  claim 1  wherein one of the buffer layers is Al 2 O 3 . 
     
     
         8 . The method for forming a superconducting article of  claim 1  wherein the substrate is a single crystal. 
     
     
         9 . The method for forming a superconducting article of  claim 1  wherein the substrate is selected from the group consisting of: MgO, Al 2 O 3 , LaAlO 3 , NdGaO 3 , (La 0.18 Sr 0.82 )(Al 0.59 Ta 0.41 )O 3 , and SrTiO 3 . 
     
     
         10 . The method for forming a superconducting article of  claim 1  wherein the substrate has a thermal expansion match to the thin film. 
     
     
         11 . The method for forming a superconducting article of  claim 1  wherein the substrate has a surface area >3 square inches. 
     
     
         12 . The method for forming a superconducting article of  claim 1  containing a-axis-oriented grains in a concentration of >1% relative to c-axis-oriented grains.