  W-B-C films were deposited on Si(100) substrates held at elevated temperature
by reactive sputtering from a W target in Kr/trimethylboron (TMB) plasmas.
Quantitative analysis by X-ray photoelectron spectroscopy (XPS) shows that the
films are W-rich between ~ 73 and ~ 93 at.% W. The highest metal content is
detected in the film deposited with 1 sccm TMB. The C and B concentrations
increase with increasing TMB flow to a maximum of ~18 and ~7 at.%,
respectively, while the O content remains nearly constant at 2-3 at.%. Chemical
bonding structure analysis performed after samples sputter-cleaning reveals C-W
and B-W bonding and no detectable W-O bonds. During film growth with 5 sccm TMB
and 500 oC or with 10 sccm TMB and 300-600 oC thin film X-ray diffraction shows
the formation of cubic 100-oriented WC1-x with a possible solid solution of B.
Lower flows and lower growth temperatures favor growth of W and W2C,
respectively. Depositions at 700 and 800 oC result in the formation of WSi2 due
to a reaction with the substrate. At 900 oC, XPS analysis shows ~96 at.% Si in
the film due to Si interdiffusion. Scanning electron microscopy images reveal a
fine-grained microstructure for the deposited WC1-x films. Nanoindentation
gives hardness values in the range from ~23 to ~31 GPa and reduced elastic
moduli between ~220 and 280 GPa in the films deposited at temperatures lower
than 600 oC. At higher growth temperatures the hardness decreases by a factor
of 3 to 4 following the formation of WSi2 at 700-800 oC and Si-rich surface at
900 oC.
