Microstructure control of CrNx films during high power impulse magnetron sputtering

The microstructure and composition of CrNx (0 <= x <= 1) films grown by reactive high power pulsed magnetron sputtering (HIPIMS or HPPMS) have been studied as a function of the process parameters: N-2-to-Ar discharge gas ratio, (f(N2/Ar)), negative substrate bias (V-s), pulsing frequency, and energy per pulse. The film stoichiometry is found to be determined by the composition of the material flux incident upon the substrate during the active phase of the discharge with no nitrogen uptake between the high power pulses. Scanning electron microscopy investigations reveal that for 0<= 150 V is dominated by the CrN and hexagonal beta-Cr2N phases and shows a high sensitivity to V-s. As the amplitude of V, decreases to 40 V and self-biased condition, the film morphology evolves to a dense columnar structure. This is accompanied by an increase in the average surface roughness from 0.25 nm to 2.4 nm. CrNx samples grown at f(N2/Ar)>= 0.3 are columnar and show high compressive stress levels ranging from -7.1 GPa at f(N2/Ar)=0.3 to -9.6 GPa at f(N2/Ar)=1. The power-normalized deposition rate decreases with increasing pulse energy, independent of f(N2/Ar). This effect is found to be closely related to the increased ion content in the plasma as determined by optical emission spectroscopy. The HIPIMS deposition rate normalized to DC rate decreases linearly with increasing relative ion content in the plasma, independent of f(N2/Ar) and pulsing frequency, in agreement with the so-called target-pathways model. Increasing frequency leads to a finer grain structure and a partial suppression of the columnar growth, which is attributed to the corresponding increase of the time-averaged mean energy of film-forming ions arriving at the substrate. (C) 2010 Elsevier B.V. All rights reserved.