Stripline Measurements of Surface Resistance: Relation to HTSC Film Properties and Deposition Methods

Abstract

ABSTRACT We describe the measurement of microwave surface resistance, Rs, of thin films of high -transition- temperature superconductors by the stripline resonator method. This method allowsmeasurement of Its as a function of frequency from 0.5 GHz to 20 GHz and has a sensitivity ofgreater than 1x10 -7 O. RS for films deposited by various methods has been measured, and alimited temperature dependence for one YBCO film is also reported. The results are related to thedeposition methods and film properties. 1. INTRODUCTION The discovery of the high -Tc superconducting materials has heightened interest in manypassive microwave device applications, including resonators, filters, delay lines and other signalprocessing components that can operate at temperatures as high as that of liquid nitrogen. Inparticular, planar stripline circuit implementations of these devices are desirable because they arecompact, have low radiation losses and can be fabricated using conventional photolithographictechniques. Superconductors are of interest because conventional low -Tc materials havedemonstrated much lower microwave losses than conventional conductors and on theoreticalgrounds we expect the same properties for the high -Ta materials but at higher temperatures.Furthermore, for most superconducting materials, transmission lines are dispersionless because,for frequencies small compared with the bandgap energy, the London penetration depth isindependent of frequency. In a normal conductor dispersion results from the frequencydependence of the skin depth.The parameter which reflects the loss in a superconductor is the microwave surfaceresistance, RS, which, although potentially low, is not identically zero as it is at dc.1 At any acfrequency and finite temperature, resistive losses are present because the inductance which resultsfrom the inertia of the superconducting electrons allows penetration of the microwave electric fieldinto the superconductor. This field couples to the normal electrons, resulting in resistive powerdissipation. Thus a superconducting material is characterized by a surface resistance that can beexpressed as1