Step coverage
Due to the the fact that deposition is highly directional in case of evaporation processes, the step coverage is very bad. The film is nor conformal anymore for structures with aspect ratios higher than 1. Another cause for the bad step coverage is the fact that the substrate is not heated (T < 500oC) and therefore the adsorbed atoms do not have sufficient energy to diffuse on the surface and fill in the vacancies. As a consequence, the deposited film looks like in the following picture, exhibiting poor sidewall coverage and even totally lack of film in some regions.

Methods to improve step coverage:

• rotate the wafer to avoid shadowing; the film will be axial uniform (see figure below)

• collimation (interposing a grid with high aspect ratio) so that particles arrive perpendicular on the substrate
• heating the wafer, so that atoms can diffuse on the surface before being incorporated in the film
  • the length for which the atoms can diffuse is proportional with the temperature and can be comparable with the width of the structure:, where D is the diffusion constant, τ is the time that molecules have for diffusing before being incorporated in the layer 
  • disadvantages of heating
    • diffusion constant depends on the material type, thus for alloys there are problems with the film composition
    • the high temperature affects the film morphology and grain size (films deposited at higher temperature can be rougher, due to larger grain formation)
• ion beam during or after evaporation to redistribute the atoms on the surface by sputtering or diffusion, due to an increase of energy ( ion energy is ~ 1keV, compared to the atom energy of 0.1 eV)