Formation of Al-Cr-N coatings by DC reactive sputtering

It is expected that an Al-Cr-N composite nitride film may have a higher resistance to oxidation at elevated temperatures than the well-known Ti-Al-N films because both AlN and CrN possess good oxidative resistance. Al-Cr-N films were formed by a de reactive sputtering process, while the plasma condition being monitored with an optical emission spectroscope and a mass spectrometer. The effect of the sputtering power and applied probe current on the emission intensity and ion current of reacting species in the plasma were investigated by using the monitors. The formed films were characterized by X-ray diffraction, electron microprobe analysis and auger electron spectroscopy. The results are summarized as follows. (1) AlN+ and CrN+ were observed in the plasma by mass spectroscopy, suggesting that nitrides formed partly in the plasma before depositing on a substrate. (2) The emission intensity of reacting species increases with increasing sputtering power and applied probe current. (3) The atomic ratio of Al/Cr in the Al-Cr-N films is found to be proportional to the ratio of the emission intensities of aluminum and chromium measured by optical emission spectroscopy. (4) The crystal structure of Al-Cr-N films is B1(NaCl) type in the range up to 50 at% of aluminum and B4(Wurtzite) type over 75 at% of aluminum. (5) The distribution of aluminum, chromium and nitrogen is very uniform in the films compared with the film formed by an activated reactive evaporation process.