The rate of deposition is known to
control mainly the thin films microstructure. The deposition rate depends
mainly on the sputtering parameters such as the working gas pressure (in our
case the Ar pressure), the distance between the substrate and the target and the
sputtering applied power. To simplify the present study, applied power and the
distance between the substrate to the target were fixed to be 300 W and 100 mm,
respectively. Hence, the parameter that was used to change the deposition rate
was the sputtering pressure (working Ar pressure). The operated sputtering
pressures ranged from 0.1 to 0.8 Pa. The results presented in Table 1 indicate
that the Ar pressure has an effect on the chemical composition and thickness of
the thin films.

The deposition rate and the
thickness of the thin films are strongly influenced by the sputtering pressure
and it is proportional to the sputtering yield. An optimum pressure exists for
high deposition rates. At too low Ar pressure, there are not enough collisions
between atoms and electrons to sustain the plasma. At too high pressures, there
so many collisions that electrons do not have enough time to gather energy
between collisions to be able to ionize the atoms. Hence, the thicknesses of
thin films were deposited at too low and too high Ar pressure (sample S1 and S4)
are less than those that were sputtered at medium Ar pressure (sample S2 and S3).

 As it was explained earlier, the thin film chemical
composition depends on the sputtering parameters when an alloy target consists of
several elements with different sputtering yields. The compositional change with
the variation of Ar pressure is due to the different motion velocity of elements
from the target to the substrate. Table 1 illustrated that the high elemental
deviation between the chemical concentration of the target and the sputtered
film was observed by increasing the Ar pressure. The reduction of the Ti
concentration in the thin films is due to the dissipation of the kinetic energy
of the sputtered Ti atoms as result of their collisions with Ar atoms. Titanium
is the lighter element compared with the Nickel in the target. Hence, its
initial energy is more and thus it thermalizes faster than Ni. Hence, the
compositions of the sputtered films always have a less Ti content in comparison
with the target.

The kinetic energy of the incident
atoms at the film surface can be estimated by a Boltzmann distribution: