Mutual control of critical temperature, residual resistance ratio, stress, and roughness for sputtered Nb films

TL;DR

This study investigates how sputtering parameters influence the stress and superconducting properties of niobium films, revealing a critical pressure point that optimizes film quality and proposing a new predictive model for film stress.

Contribution

It introduces a comprehensive analysis of the relationship between sputtering conditions and Nb film properties, including a modified kinetic model for stress prediction.

Findings

Identified a critical sputtering pressure where film stress relaxes and properties degrade.

Achieved control of film stress from -400 MPa to +600 MPa while maintaining high-quality superconducting properties.

Developed a new exponential-based kinetic model reducing prediction error from 20% to 8%.

Abstract

Superconducting single quantum logic integrated circuits traditionally exploit magnetron sputtered niobium thin films on silicon oxide substrates. The sputtering depends on multiple process parameters, which dramatically affect mechanical, electrical, and cryogenic properties of Nb thin films. In this work, we focus on the comprehensive relationship study between 200-nm Nb film characteristics and their intrinsic stress. It is shown that there is a critical value of the working pressure pcritical at the fixed sputtering power above which stress in the film relaxes whereas the film properties degrade significantly. Below pcritical one can control intrinsic stress in the wide range from -400 MPa to +600 MPa maintaining perfect film surface with a 0.8 nm roughness (Rq), electrical resistivity less than 20 uOhm*cm, critical superconducting transition temperature above 8.9 K and residual resistance ratio over 6.4. We suggest a modified kinetic model to predict Nb films stress with the linear dependence of high-energy parameters on the working pressure replaced with an exponential one, which allowed reduction of the approximation error from 20 to 8%.