Characterization and In-situ Monitoring of Sub-stoichiometric Adjustable Tc Titanium Nitride Growth

TL;DR

This study investigates how nitrogen content influences the structural and superconducting properties of Ti-N films, demonstrating that spectroscopic ellipsometry can effectively monitor nitrogen incorporation for process control.

Contribution

It introduces a method to in-situ monitor nitrogen content and superconducting transition temperature in Ti-N films using spectroscopic ellipsometry.

Findings

Nitrogen content controls the crystallographic phase of Ti-N films.

Superconducting transition temperature Tc varies sharply with nitrogen incorporation.

Spectroscopic ellipsometry correlates well with nitrogen composition and Tc.

Abstract

The structural and electrical properties of Ti-N films deposited by reactive sputtering depend on their growth parameters, in particular the Ar:N2 gas ratio. We show that the nitrogen percentage changes the crystallographic phase of the film progressively from pure \alpha-Ti, through an \alpha-Ti phase with interstitial nitrogen, to stoichiometric Ti2N, and through a substoichiometric TiNX to stoichiometric TiN. These changes also affect the superconducting transition temperature, Tc, allowing, the superconducting properties to be tailored for specific applications. After decreasing from a Tc of 0.4 K for pure Ti down to below 50 mK at the Ti2N point, the Tc then increases rapidly up to nearly 5 K over a narrow range of nitrogen incorporation. This very sharp increase of Tc makes it difficult to control the properties of the film from wafer-to-wafer as well as across a given wafer to within acceptable margins for device fabrication. Here we show that the nitrogen composition and hence the superconductive properties are related to, and can be determined by, spectroscopic ellipsometry. Therefore, this technique may be used for process control and wafer screening prior to investing time in processing devices.