Circuit Paths are deposited on circuit boards in general by fotolithorgaphic methods:
- First step is to coat the whole circuit board with a meallic layer.
- The whole metallic layer is covered by a fotoresist. A negativ or a positive resist can be used. In the development process the negative resist remains where it has been exposed to illumination while a positive resist is removed.
- By using a fotomask the resist is illuminated. In case of a positive resist these areas are illuminated where the metallic läyer shall be removed and in case of a negative resist the areas where the metal shall remain.
- After the developping process the fotoresist remains on the areas where the circuit path shall remain.
- By etching the metallic layer is removed where not protected by the fotoresist. The fotoresist is not etched or only marginally.Etching can be executed by wet chemical etching or by plasma etching. The etching rate is higher in case of wet chemical etching however as it is allway isotropic also portions of the metal are removed which are underneath the resist. Plasma etching can be strongly anisotropic so that there is no underetching at all and the edge of the circuit path is a sharp copy of the mask (Ion etching IE or Reactive Ion Etching RIE).
- The final step is removing the foto resist from the remaining metallic structure by isotropic plasma etching in an oxygene plasma (plasma stripping plasma ashing)