Electrophotography The Photocopying Process

This is the process used in most photocopying machines. It is largely a mechanical process which uses one key photophysical step based on photo-induced electron transfer of a rather special kind. 

The metal (M usually Al) is in fact the centre of the drum. This is coated with a thin layer of semiconductor (Sc) which is an insulator in the dark but becomes conductive when exposed to light. The whole photocopying process takes place in several stages. 

In a plastic the polymer chains can be either intertwined simply at random to form a three-dimensional pattern, or they can be linked together by chemical bonds. This latter type is called a crosslinked polymer, and it 

A distinction can be made between a positive and a negative resist, according to the action of light. In a positive working resist the monomer is deposited on the copper surface in the form of a viscous liquid. It is then irradiated through a mask (this is simply a drawing of the required pattern on a transparent sheet) and polymerization takes place only at the exposed places. The unirradiated liquid monomer is then washed away in a suitable solvent, and the exposed copper can be dissolved in an etching bath. Finally the protective polymer layer is removed by chemical or mechanical means, and the printed circuit is ready. 

In a negative working resist the monomer is first polymerized over the entire copper surface, then the protective polymer layer is irradiated through the mask. In the irradiated areas the polymer is degraded into smaller units and becomes soluble it can then be removed by treatment with a suitable solvent and the etching bath will attack this exposed copper. 

---