  We have presented the kinetic study of the very initial growth stages of an
ultra thin film (40 $\AA$- 150$\AA$) of Ag sputter-deposited on Si(001)
substrate containing native oxide using grazing incidence x-ray reflectivity
(GIXR) technique and Atomic Force Microscopy (AFM). We observe that the film
consists of mounds with the presence of voids. The thickness `d_{xray}' and the
packing fraction `\eta ' of the film as a function of time `t' follow a simple
power law, d_{xray} $\thicksim t^{m} and \eta \thicksim $t^{n}$ with the
exponent m = 0.58 and n = 0.37 respectively. We have quantitatively determined
that the voids between the mounds decrease at the initial growth stages with
the increase in mound size using GIXR measurement. The mound size increases
mainly by the coalescence process on the substrate. We have observed that as a
function of time the mound size R(t) increases radially as $\thicksim $t^{z}.
The radial growth exponent z crosses over from z > 0.5 to z $\thicksim $ 0.25
indicating two growth regimes. The GIXR measurement reveals sublinear
dependence of \eta on d and the AFM measurement shows a cross over of the
radial growth exponent, both these indicates that the lateral growth of the
mound is enhanced initially reducing the voids.

Materials Science