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When the pressure in the container is high, the sputtered atoms collide with the inert gas a lot more, causing them to scatter.
This means they approach the substrate at a wider range of angles, making them far more likely to stick to protruding atoms.
This leads to a greater degree of shadowing, as fewer atoms reach the substrate surface.
When the pressure in the container is low, the sputtered atoms undergo fewer collisions.
This means they approach the substrate at a smaller range of angles, so there are fewer shadowed regions.
This leads to a more densely packed film.
When the average angle of incidence is not normal to the substrate, the shadowing pattern changes.
The columns and shadowed regions 'tip over' in the direction of the incoming flux.
This can also lead to a lower density material.