In the metallization process, a layer of metal is deposited on the substrate surface to provide electrical contact to the devices.

There are three major types of metallization schemes:

• subtractive
• fully-additive
• semi-additive

Subtractive metallization involves the build-up of a blanket layer of metal on the substrate. This process uses a photoresist and metal etch to define traces.

Fully-additive metallization involves the direct build-up or plating of metal traces on the substrate.

Semi-additive metallization uses a blanket seed layer of metal. Traces are built-up using photoresist and plating. The seed layer is then removed.

The metallization process can either be performed through electroless or electrolytic plating.

• Electroless plating uses Cu deposition from solution by chemical activation of the surface.
• Electrolytic plating uses current-induced deposition of Cu through an electrolyte.

A semi-additive metallization process using electroless plating is described below.