Influence of the plasma current to Ti-melt on the plasma parameters and microstructure of TiN coatings in the triode ion plating system

Abstract

TiN coatings have been deposited by triode ion plating in a mixture of Ar and N-2 with a varying total current through the melt. The observed energy spectra of ions and neutrals are discussed with respect to the plasma parameters measured with Langmuir probes. The current I-pm from the plasma to the Ti-melt in the crucible was modified from 0 A up to 100 A. As I-pm is increased, major effects on the plasma and on the film microstructure (preferred orientation, stress-free lattice parameter, residual stress, thickness and microhardness) were observed. The positive voltage of the melt U-R with respect to the ground increased from 0.1 to 29 V. The plasma potential (positive voltage with respect to ground) of the main plasma body increased from 4 to 16 V, while the electron temperature ranged from 2.5 to 4 eV. Plasma density and floating potential is also discussed. A high energy tail appeared in the energy distribution function of some neutral and ionic species, especially for neutral and ionized evaporated Ti. At high values of I-pm the energy distribution of both highly energetic Ti+ and neutral Ti goes up to 25 eV and 30 eV. At low values of I-pm the majority of the Ti+ ions are thermalized. The microstructure of the deposited TiN films develops from porous structures, with low compressive or even tensile stresses at I-pm=0 A, up to dense structures, with high compressive stresses of more than 15 GPa at the highest value of I-pm. Possible explanations of the observed effects are discussed, based on plasma ionization processes close to the melt and in the plasma body. (C) 1997 Elsevier Science S.A.