**1.2 Preferential sputtering**

In the sputtering yield description of a multicomponent system, the preferential erosion influence and surface segregation must be included. For a homogeneous specimen with two components A and B, the surface concentrations, Ns, are equal to the volume concentrations, Nv, in the absence of segregation to the surface, which must occur due to thermal processes. At the sputtering onset.

$$\mathbf{N}\_{\rm A}^{s} \;/\mathbf{N}\_{\rm B}^{s} = \mathbf{N}\_{\rm A}^{v} \;/\mathbf{N}\_{\rm B}^{v} \tag{1}$$

The partial Sputtering Yield of the atomic species A and B is defined by: YA, B = (atoms eliminated amount A, B)/incident particles.

The sputtering yield of species A, YA, is proportional to the surface concentration, Ns A, and similarly, YB is proportional to Ns B.

The partial sputtering yield ratio is given by:

$$\mathbf{Y}\_{\mathbf{A}} \; \prime \; \mathbf{Y}\_{\mathbf{B}} = \mathbf{r} \left( \mathbf{N}\_{\; \; \mathbf{A}}^{\*} \; \prime \; \mathbf{N}\_{\; \; \mathbf{B}}^{\*} \right) \tag{2}$$

Where the erosion factor, r, considers the differences in surface binding energies, escape depths of the ejected atoms and energy transfer within the cascade. Measured values of, r, are generally in the range 0.5–2.

On the other hand, there are several mathematical models that describe the sputtering Yield, it is important to mention that all of these models are based on the work of P. Sigmund. His work "Theory of Sputtering I. Sputter yield of Amorphous and Polycrystalline targets" published in Physical Review is a benchmark in this field [10].
