Model for the evolution of Nb–Si–N thin films as a function of Si content relating the nanostructure to electrical and mechanical properties

Abstract

DC reactive magnetron sputtering was used for the deposition of Nb–Si–N thin films at 250 °C substrate temperature. Si content was varied by changing the current applied on the Si target, whereas that on Nb target was kept constant. The evolution of morphology, grain size and lattice constant has been analyzed by Transmission Electron Microscopy and X-rays diffraction investigations. Hardness, stress and resistivity measurements provide complementary information concerning the macroscopic properties of the films. In the Nb–Si–N films, 3 distinct concentration regions have been observed depending on the Si content. For low Si content (CSi≤4 at.%) the Si atoms substitute the Nb atoms in the NbN lattice. Exceeding the solubility limit, a nanocomposite film grows; it is made of NbN:Si nanocrystallites surrounded by an amorphous SiNy layer. Further increase of Si content (CSi≥7 at.%) implies a reduction of the crystallite size, while the thickness of the SiNy layer at the crystallite surface remains constant (∼1.3 monolayer). The amount of the SiNy amorphous phase in the films increases through the increase of the surface to volume ratio of the crystallites. The structure of the films is responsible for the increase of hardness from 25 to 34 GPa through a 2-step mechanism, i.e., by forming a solid solution of Si atoms in NbN lattice and by forming a nanocomposite material. The large variation of the temperature coefficient of resistivity is well correlated with the thickness of SiNy layer on the NbN:Si crystallite surface. Based on the three concentration regions, a three-step model is proposed for the film formation of the Nb–Si–N thin films. This model correlates nanoscale structures with macroscopic properties of the films.