The effect of bias on d.c. and r.f. sputtered WC-Co coatings

• Published in: Thin solid films 1980-01, Vol.67 (2), p.265-277
• Main Authors: Eser, E., Ogilvie, R.E., Taylor, K.A.
• Format: Article
• Language: English
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/0040-6090(80)90459-9

The structure and chemistry of WC-Co coatings, r.f. and d.c. sputter deposited at various bias levels, were analyzed using techniques such as electron microprobe analysis, Auger electron spectroscopy, scanning electron microscopy and X-ray diffraction. These analyses, together with the measurement of the ion density in the plasma surrounding the substrate, showed that the coating characteristics are controlled by the substrate bias-plasma density combination. An increasing negative bias was shown to inhibit the preferential nucleation defects as well as the columnar growth in r.f. sputter deposited coatings but not in d.c. sputter deposited coatings. The reason for this is that, although the plasma density was only 3 × 10 10 cm -3 in the d.c. discharge, it was 8 × 10 10 cm -3 in the r.f. case. As a result, in r.f. sputtering, not only the substrate temperature but also the re-sputtering from the growing film relative to the sputtering from the target was higher. A higher carbon and cobalt deficiency observed in r.f. deposited coatings also resulted from the higher level of ion bombardment during r.f. sputtering. The carbon deficiency is believed to be associated with the temperature rise, causing re-evaporation of the carbon atoms before they reach equilibrium with the substrate. The decrease in the cobalt content is the result of the increased re-sputtering. A carbon-to-tungsten ratio of less than unity encountered under zero bias conditions, for both r.f. and d.c. sputtering, indicates the reflection of some portion of the carbon atoms. With an increasing level of ion bombardment the crystal structure of the coatings was found to change from f.c.c. β-WC 1− x to h.c.p. α-W 2C.