Characterization and in-situ monitoring of sub-stoichiometric adjustable superconducting critical temperature titanium nitride growth

Abstract

The structural and electrical properties of Ti-N films deposited by reactive sputtering depend on their growth parameters, in particular the Ar:N_2 gas ratio. We show that the nitrogen percentage changes the crystallographic phase of the film progressively from pure α-Ti, through an α-Ti phase with interstitial nitrogen, to stoichiometric Ti_2N, and through a substoichiometric TiN_X to stoichiometric TiN. These changes also affect the superconducting transition temperature, T_c, allowing, the superconducting properties to be tailored for specific applications. After decreasing from a T_c of 0.4 K for pure Ti down to below 50 mK at the Ti_2N point, the T_c then increases rapidly up to nearly 5 K over a narrow range of nitrogen incorporation. This very sharp increase of T_c 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.