**1.3 Sigmund mechanism**

The sputtering process nature is discontinuous due to each one ion initiating an atomic collision cascade. The sputtering yield is dependent on the deposited energy fraction at the surface, which is the function of surface topography local and ion beam incidence angle. From atomic level point of view, the surface sputtering has a random nature as a result the sputtering yield varies markedly from one event to another. These two phenomena have important consequences with respect to the surface topography when considering dimensions similar to those of atomic collision cascade.

A particularly violent event can produce a crater by removing a large number of atoms, either by erosion or by sublimation from a hot spot. The sputtering yield variation with position leads to the prediction that in the absence of a surface smoothing mechanism such as diffusion; flat surfaces are unstable, against increasing roughness at scales similar to the atomic collision cascade size. This instability can lead to the growth of small irregularities of atomic dimensions, such as surface protuberances.
