Etching (microfabrication) - Figures of Merit

Figures of Merit

If the etch is intended to make a cavity in a material, the depth of the cavity may be controlled approximately using the etching time and the known etch rate. More often, though, etching must entirely remove the top layer of a multilayer structure, without damaging the underlying or masking layers. The etching system's ability to do this depends on the ratio of etch rates in the two materials (selectivity).

Some etches undercut the masking layer and form cavities with sloping sidewalls. The distance of undercutting is called bias. Etchants with large bias are called isotropic, because they erode the substrate equally in all directions. Modern processes greatly prefer anisotropic etches, because they produce sharp, well-controlled features.

Selectivity Yellow: layer to be removed; blue: layer to remain
  1. A poorly selective etch removes the top layer, but also attacks the underlying material.
  2. A highly selective etch leaves the underlying material unharmed.
Isotropy Red: masking layer; yellow: layer to be removed
  1. A perfectly isotropic etch produces round sidewalls.
  2. A perfectly anisotropic etch produces vertical sidewalls.

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