14.6: Growth Modes
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When using evaporation, the flux travels in a straight line and so the deposition is line-of-sight only. This can lead to macroscopically shadowed regions, which are reduced by rotating the substrate during the deposition process.
When sputtering, the deposition is not line of sight (due to scattering in the intervening gas) but shadowing still occurs on an atomic scale. Variables which affect the degree and orientation of the shadowing are, among others, the background pressure in the chamber and the average angle from which the sputtered atoms approach.
If the atoms have low energies then they will remain in their initial positions. If the atoms have higher energies, then their thermal mobility may be high enough for surface diffusion. This allows the atoms to rearrange themselves into a lower energy conformation. A typical energy threshold to mobilise atoms on the surface is around 5eV. Hence, sputtered atoms typically have enough energy for surface diffusion to take place, whilst evaporated ones don’t.
If the energy is high enough for surface diffusion to take place then once the stable nuclei form on the substrate surface, they may coalesce. This leads to 3 possible modes of film growth.