**3.2 Surface protrusions evolution under ion irradiation**

**Figure 2** shows an MFA image of the surface before being irradiated in which it is observed that there are protuberances with angstroms heights without a defined pattern in its surface distribution, it has been established that such protrusions as an irradiation consequence result in the peaks formation with nanometer dimensions, which can be seen in **Figure 8**.

The roughness enhancement mechanism proposed by Sigmund [15] explains the formation of these nanostructures in terms sputtering yield variation generated as a consequence of the local incidence angle change of the ion beam.

In the case of three-dimensional, cone-like protrusions, the sputtering yield is about 0.2 in the cone zones with dimensions similar to or larger than the displacement cascades size. For protrusions with dimensions of the order of displacement cascade size the sputtering yield has values < 1, although this model predicts the nanoparticle's formation from protrusions with angstrom dimensions, its major contribution is that it predicts the most important geometrical feature, the nanoparticles top will always tend to have a conical shape. As an above result and in the absence of any surface smoothing mechanism by migration, pre-existing surface protrusions on the

*Surface Microstructure Changes Induced by Ion Beam Irradiation DOI: http://dx.doi.org/10.5772/intechopen.112015*
