In conventional methods of forming circuitization by electroless plating onto dielectric materials, a seeding or catalyst layer is first blanket deposited on the substrate. Next a negative resist, typically a photoresist, is applied, over the seed layer and then the photoresist is exposed to light to form polymerized select portions of the resist. The photo resist is then developed using a developer, that is, a solvent which dissolves the unexposed portions of the photoresist and leaves the polymerized portions of the photoresist atop the seed layer. The portions of the photoresist that are removed by the developer correspond to the metal pattern of the circuitization for the circuit board. Typically, several processing steps are performed to accelerate the catalyst, modify the seed layer, etc. Thereafter, metal is electrolessly plated onto the bare, exposed portions of the seed layer, that is, the portions of the seed layer that are not covered by the photoresist. Thus metal is plated in the pattern of the bare seed layer not covered by the photoresist. The polymerized photoresist is then stripped using stripping solvents. As a result, portions of the seed layer, are covered by metal circuitization, while portions of the seed layer are exposed.
After plating, the processor is left with the decision whether to strip the exposed portion of the seed layer, that is the portion of the seed layer not covered by the electrolessly plated metal or permit the entire seed layer to remain. If the entire seed layer remains, the metal in the seed layer often conducts current from one electrical line in the circuit to another electrical line thereby creating a short circuit.
If the exposed portions of seed layer are to be stripped, this will necessitate another processing step. Moreover, the solvents needed to remove the seed layer, for example, alkaline cyanide solutions are typically not environmentally friendly, and such solvents often present disposal problems. Furthermore, such solvents often attack and degrade the circuitry and the dielectric materials.
It would be desirable to have a method for forming circuitization by electrolessly plating that does not leave a blanket seed layer, that is simple and does not require stripping of the seed layer.