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: N2-to-Ar discharge gas ratio, (fN2/Ar), negative substrate bias (Vs), 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 < fN2/Ar < 0.15 and 150 V bias, a columnar film growth is suppressed in favor of nano-sized grain structure. The phenomenon is ascribed to the high flux of doubly charged Cr ions and appears to be a unique feature of HIPIMS. The microstructure of column-less films for 100 V ≤ Vs ≤ 150 V is dominated by the CrN and hexagonal β-Cr2N phases and shows a high sensitivity to Vs. As the amplitude of Vs 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 fN2/Ar ≥ 0.3 are columnar and show high compressive stress levels ranging from −7.1 GPa at fN2/Ar = 0.3 to −9.6 GPa at fN2/Ar = 1. The power-normalized deposition rate decreases with increasing pulse energy, independent of fN2/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 fN2/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.