Processes at the Target Surface

The properties of the emitted material depend on the bombarding ions, their kinetic energy, incidence angle, atomic mass as well as on the target material and its structure. The most important parameter of the sputtering process is the so-called sputter yield Y which defines the number of emitted target atoms Ze per incident particles Z.  

The dependence of sputtering yield on ion energy is shown in Fig. 5.3 for different elemental targets. At moderate ion energies in the order of 30 to 1,000 eV, sputtering is characterized by knock on effects where the incident ions collide with a surface atom and these atoms further react with additional atoms. These events may eventually lead to a release of target material atoms. 

At low energy, a threshold is observed. This is due to the fact that the surface binding energy barrier has to be overcome for sputtering to occur. An important consequence is that the sputter yield of an oxidized target surface is much smaller than for a metallic surface. 

At high energy, the impinging ion has enough energy to break all bonds in its local environment. This is the collision cascade regime, which can be accurately modeled. 

For the application of ZnO sputtering using Ar as sputter gas and typical deposition conditions with impinging ion energy in the order of 100-1,000 eV, it is remarkable that Zn has the highest sputtering rate known for any elemental target material. 

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