|
Tungsten/carbon (W/C) multilayer thin films with a nominal d spacing varying
from 2.5 to 14 nm were prepared by magnetron sputtering technique. The thicknesses of
the W and C layers were varied from 0.5 to 12 nm. The multilayers were subjected to
isochronal anneals in a quartz tube furnace in the range of 300 to 1000 C under high
purity Ar flow conditions. X-ray diffraction, Raman scattering and Auger depth
profiling were used to characterize the structure of the as-prepared and annealed
films. It is found that an overcoat layer of silicon nitride (30-50 nm) prevents the
multilayers from oxidation during the 1 hr heat treatment at temperatures as high as
1000 C in Ar flow. In all studied W/C multilayers, the carbon layers are amorphous
(up to 12 nm). The tungsten layers are also amorphous when their thicknesses are less
than 5 nm. Tungsten layers thicker than 5 nm show crystalline W peaks in addition
to the amorphous W feature. Annealing of samples with a silicon nitride protective
layer results in several structural changes which depend on annealing temperature, d
spacing, the as-deposited W layer structure and the layer thickness ratio of W to C.
For W layer thicker than C layer and W layer thickness > 4 nm and/or C layer thickness
< 1 nm, the multilayers show the initial crystal formation of microcrystalline W2C
occurring at C-W interfaces (that interface in which C was deposited on W) after 600 C
anneal, followed by a second crystallization of a-W or a-W and WC at W-C
interfaces (W was deposited on C) at the annealing temperature of 900 C. They reveal
a relatively small (< 5 %) or essentially no layer expansion. For those multilayers
having thin W layers (2 nm) and the same or thicker C layer thicknesses, the
initial crystallization takes place at both W-C and C-W interfaces at 900 C or
higher. The crystal formed is a-W or a-W and WC. The layer pair period of the
multilayers in this group increases monotonically with increasing annealing
temperature. Expansion is up to 16 % of the original d spacing and occurs in both W
and C layers at approximately equal rates. The expansion in all multilayers is
interpreted to be associated mainly with the structural ordering processes in the
amorphous W and C layers.
|
