A cap layer for a metal feature such as a copper interconnect on a semiconductor wafer is formed by immersion plating a more noble metal (e.g. Pd) onto the copper interconnect and breaking up, preferably by mechanical abrasion, loose nodules of the noble metal that form on the copper interconnect surface. The mechanical abrasion removes plated noble metal which is only loosely attached to the copper surface, and then continued exposure of the copper surface to immersion plating chemicals leads to plating at new sites on the surface until a continuous, well-bonded noble metal layer has formed. The method can be implemented conveniently by supplying immersion plating chemicals to the surface of a wafer undergoing CMP or undergoing scrubbing in a wafer-scrubber apparatus.