Deposition of a thin electroless copper films on PVD copper seed layers in interconnect metallization was investigated for formation of a composite seed layer. Issues such as film stress, adhesion, electrical reliability, and hydrogen incorporation were addressed to determine feasibility of the process for the fabrication of real device structures. The formation of blisters between the copper films and underlying barrier layer were observed due to hydrogen incorporation and high level of compressive stress as a result of the electroless deposition process. Optimization of the electroless copper bath and process flow were investigated to minimize blister formation and improve fill enhancement effectiveness. Low temperature postelectroless anneal was found to remove incorporated hydrogen but also decreased overall fill effectiveness. Post-CMP electrical reliability of thin PVD/electroless/copper fill process was found to be equivalent to thick PVD/copper fill process. Potential reliability issues with electroless deposition on poorly seeded features were suggested by via yield degradation following final anneal.