The rate of deposition is known tocontrol mainly the thin films microstructure. The deposition rate dependsmainly on the sputtering parameters such as the working gas pressure (in ourcase the Ar pressure), the distance between the substrate and the target and thesputtering applied power. To simplify the present study, applied power and thedistance 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 ratewas the sputtering pressure (working Ar pressure). The operated sputteringpressures ranged from 0.1 to 0.8 Pa. The results presented in Table 1 indicatethat the Ar pressure has an effect on the chemical composition and thickness ofthe thin films.
The deposition rate and thethickness of the thin films are strongly influenced by the sputtering pressureand it is proportional to the sputtering yield. An optimum pressure exists forhigh deposition rates. At too low Ar pressure, there are not enough collisionsbetween atoms and electrons to sustain the plasma. At too high pressures, thereso many collisions that electrons do not have enough time to gather energybetween collisions to be able to ionize the atoms.
Hence, the thicknesses ofthin 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 chemicalcomposition depends on the sputtering parameters when an alloy target consists ofseveral elements with different sputtering yields. The compositional change withthe variation of Ar pressure is due to the different motion velocity of elementsfrom the target to the substrate. Table 1 illustrated that the high elementaldeviation between the chemical concentration of the target and the sputteredfilm was observed by increasing the Ar pressure. The reduction of the Ticoncentration in the thin films is due to the dissipation of the kinetic energyof the sputtered Ti atoms as result of their collisions with Ar atoms. Titaniumis the lighter element compared with the Nickel in the target. Hence, itsinitial energy is more and thus it thermalizes faster than Ni.
Hence, thecompositions of the sputtered films always have a less Ti content in comparisonwith the target.The kinetic energy of the incidentatoms at the film surface can be estimated by a Boltzmann distribution: