The present invention relates generally to electroless plating and in particular to the activation of oxide surfaces for electroless plating.
Electroless plating is a wet chemical plating technique utilized by the semiconductor industry to deposit thin films of metal or metal alloy over a substrate during the fabrication or packaging of semiconductor devices. Electroless plating can be accomplished with relatively low cost tooling and materials as compared to electroplating. Further, electroless plating is selective, provides excellent step coverage, and good filling capabilities, even when filling high aspect ratio trenches and vias. Accordingly, electroless plating is suitable for the construction of submicron feature devices.
Electroless plating is a controlled autocatalytic chemical reduction reaction of aqueous metal or metal alloy ions to a base substrate. That is, the metal or metal alloy being deposited serves to catalyze the reaction. Basically, a device such as a semiconductor structure is placed in an electroless plating bath. The electroless bath typically includes an aqueous solution of metal ions, complexing agents, and reducing agents. The bath may also include stabilizers, various additives, and buffers, as well as rate promoters to speed up or slow down the deposition process. As such, the particular composition of the plating bath typically varies based upon the specific application to account for the desired parameters of the plating process. Unlike conventional electroplating however, no electrical current or power supply, anodes, batteries, or rectifiers are required to perform an electroless plating deposition.
Electroless plating is of interest in the fabrication of semiconductor devices because electroless plating deposition provides a substantially uniform conductive layer that can conformally plate a substrate, even if the substrate has an irregular shape or deep openings. The rate of deposition of the metal layer may also be successfully controlled. The electroless plating process can be controlled to generally avoid plating material buildup up at the edges and corners of the substrate. As such, high aspect ratios, sharp edges, holes, trenches, and vias may all be successfully plated. Electroless deposition thus provides an attractive alternative processing technique when conventional processing such as using physical vapor deposition (PVD) cannot provide satisfactory step coverage. Also, electroless plating layers are virtually nonporous, which allows for improved corrosion resistance.
During electroless plating, the metal ions in the plating bath are reduced on a catalytic surface by a reducing agent. Accordingly, the portions of a substrate to be plated generally must be of the same material, or exhibit an affinity for the plating metal or metal alloy. This is advantageous from the perspective that plating may occur at the same time on electrically isolated areas of the device being plated. This also allows selectivity to the deposition process. However, certain nonconductive substrates such as oxides and nonconductive metal oxides are not catalytically active.
In order to utilize electroless plating techniques over an oxide surface according to a conventional approach, one or more nucleating layers or seed layers are deposited over the oxide layer. Optionally, one or more barrier layers may also be required, depending upon the materials involved. The seed layer may comprise for example, a conductive paste or a catalytic film applied by evaporation or sputtering. The seed layer is selected such that the seed layer adheres to the underlying oxide layer, yet exhibits a catalytic surface receptive to electroless plating.