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 2.2 Stopping power The stopping power represents the capacity of a material to slow down and stop an incidental ion. It is defined as the kinetic energy lost by the ion per distance unit. It can break up into three terms on which the relative values depend, among others, on the kinetic energy of the ion and on its chemical nature. These three terms are: - nuclear stopping power which is related to the direct collisions between ion and target atoms nuclei, - electronic stopping power the origin of which is due to a coupling between ion and target atoms electrons, - exchange stopping power (negligible) which is related to an exchange of electric charges between ion and target. As illustrated in figure 2, for the primary ions energy range used for SIMS analysis, the main contribution is due to the nuclear stopping power.  The sputtering phenomenon is the result of the transfer of ion momentum toward target atoms. As illustrated by figure 3, three sputtering modes are generally distinguished. In the simple collision mode, the energy transferred by an incidental ion (E<100eV) may eject atoms but is too weak to cause a collision series. For higher primary energies (100<E<1000eV), collision series occur between primary ions and target atoms. For energies higher than 1000eV, the incidental ion momentum is such that all atoms near from the impact area are moved, then resulting in a local warming. For SIMS analysis, except in particular cases (low energy profiles for high depth resolution), the primary ions kinetic energy is higher than 1keV. As all the atoms near from the impact area are moved, there is under surface (instantaneous surface since it is constantly eroded) a species redistribution. This phenomenon, called "collisional mixing", limits the depth resolution during the measurement of concentration profiles. The disturbed zone thickness depends on the penetration depth of primary ions, which depends itself on impact energy and angle of incidence. For Cs+ ions the following relation can be used:
l: penetration depth (nm). E: impact energy (keV). q: incidence angle. In the case of Cs+ ions with an energy about 10keV, the penetration depth reaches approximately 9nm in normal incidence. 
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