1. Field of the Invention
This invention relates to an electroless copper plating solution having improved stability characterized by the addition of a stabilizer comprising a source of gallium alone or in combination with an additional stabilizer.
2. Description of the Prior Art
Electroless metal deposition refers to the chemical plating of a metal over an active surface by chemical reduction in the absence of an external electric current. Processes and compositions useful therefor are known, are in substantial commercial use, and are described in numerous publications, including U.S. Pat. Nos. 2,938,805; 3,011,920; and 3,383,224 included herein by reference.
Known electroless copper deposition solutions generally comprise four major ingredients dissolved in solvent, usually water. They are (1) a source of the copper ions, (2) a reducing agent such as formaldehyde, (3) an acid or hydroxide pH adjuster to provide required pH, and (4) a complexing agent for copper ions sufficient to prevent their precipitation in solution.
In addition to the aforesaid major components of an electroless copper solution, it is known that other additives are needed for a commercially useful formulation, for example, to stabilize the solution and improve deposit properties. As to stabilizing the solution, it is known that certain additives added to an electroless copper solution in properly controlled trace quantities act as stabilizers and retard the rate of bath decomposition. Generally, these additives, or stabilizers as they are referred to in the art, are catalytic poisons. The concentration of the stabilizer in solution is usually critical. Trace quantities, typically in the range of a few parts per million, provide stability. An excess of stabilizer may partially or totally stop deposition of the electroless copper.