During the photochemical etching process any material not protected by the resist is eroded. This means that as the chemicals cut through the metal they are also cutting laterally under the protective coating of photo resist.
When Does Breakout Occur During Photo Etching?
When the process has etched halfway from both sides they reach a breakpoint. When breakout occurs the etchant can run through the holes and smooth the sidewalls.
What is Etch Factor?
During the engineering of the film tool, compensation is required to produce parts within specific tolerances, this is called etch factor.