Critical Properties of Submicrometer-Patterned Nb Thin Film

Abstract

We have studied transport properties of submicrometer-patterned Nb thin films. Critical parameters, such as transition temperature and critical current density, were measured as functions of the film width, ranging from 50 nm to 5000 nm, and thickness, from 10 nm to 150 nm. Nb films were deposited by dc magnetron sputtering on Si substrates and patterned by lift-off with e-beam lithography. For a given film thickness, superconducting transition temperature, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , decreased with decreasing film width below 200 nm. In the thickness ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> ) dependence, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> dropped drastically for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> les 20 nm due to proximity effect of surface layers, which are formed by strain or oxidation. The critical current density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for a given film thickness increased gradually with decreasing width and decreased sharply below 200 nm. The gradual <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> increase for wide strips is analyzed to be due to edge barrier effect for flux entry near the transition. The sharp drop below 200 nm is ascribed to the width variation of the size of about 20 nm along the strip and contamination of the film edge. These results are useful for designing and analyzing submicron-line-based superconducting electronic devices.