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Step coverage refers to the state of film coverage when a film is deposited on an uneven surface of a substrate (the surface perpendicular to the evaporation source) by sputtering, vapor deposition, etc.
When a film is applied on top of an uneven or bumpy surface, the film becomes partially thinner or nonexistent in certain parts. This means “the step/surface coverage is low” and causes issues such as yield loss.
A film is considered to have good step coverage, if the film thickness is uniform throughout the sides and the bottom. CVD (chemical vapor deposition) tends to provide better coverage than PVD (physical vapor deposition).