Studies of structure and morphology of sputter-deposited stainless steel-nitrogen films

Stainless steel films doped with nitrogen were deposited on heated and unheated (100) silicon substrates by radio-frequency magnetron sputtering of an austenitic stainless steel target in argon and nitrogen gas mixtures, containing a range of nitrogen compositions. The evolution of phases, morphologies and grain structures in the resultant films was studied by X-ray diffraction and field emission scanning electron microscopy. It was found that, with increasing nitrogen composition in the gas mixture, the crystalline structure of the films deposited on the heated substrates changed from bcc ferrite (alpha), to nitrogen-stabilized fcc austenite (gamma), then to distorted expanded austenite phase (gamma(N)) with nitrogen supersaturation, and finally to the newly discovered fcc 'MN' phase with ideal cubic symmetry and further enlarged lattice. On the unheated substrates, the phase-evolution trend was found to be different for % N-2 above 10. For the 25% N-2 film, amorphous phase formed along with the crystalline austenite and ferrite phases, while the percentage of amorphous content decreased when % N-2 was increased to 50. This different trend was understood to be due to the role of increase in % N-2 in decreasing the energy loss of sputtered species through collisions. The dependence of crystalline phase formation on the energy of sputtered species is less severe on the heated substrates. Although all the films deposited experienced three-dimensional fibrous growths, they exhibited different surface morphology and grain structure. There exists a correlation between film morphology and phase constituents, while grain size was influenced by the nucleation density and the energy and mobility of adatoms that are reduced due to nitrogen incorporation.