2. Definition of a surface analysis

Depending on the discharge conditions, it is possible to create craters with flat, concave or convex bottoms. In the case of metallic samples, a concave crater can be obtained by working under low source impedance, i.e. at relatively high pressure. Conversely, a convex crater can be obtained by working under high impedance, i.e. at relatively low pressure. The optimization consists in working between these two extremes to obtain a crater with as flat a bottom as possible (figure 11 ), which is necessary to ensure good depth resolution in surface characterizations.