Electroless Cu thermodynamics, electrochemistry, mechanism, kinetics, and mass transport are reviewed. Electroless Cu deposition is a thermodynamically favorable and kinetically inhibited process, with two electrochemical reactions including anodic oxidation of a reducing agent and cathodic reduction of metal ions occurring simultaneously. with a multistep catalytic redox mechanism. an Arrhenius type of rate equation, and mass-transport Limited reaction in narrow and deep features such as subhalf micron trenches and vias of high aspect ratios (>3). Selective and blanket electroless Cu deposition from formaldehyde-based solutions with ethylenediaminetetraacetic acid as a complexing agent was investigated for trench/via filling applications. A single-wafer electroless Cu deposition system with up to 8 in. wafer capab ility has been designed and manufactured. An electroless Cu deposition solution and operation conditions have been optimized to obtain electroless Cu films at high deposition rate (similar to 75 to 120 nm/min), with low resistivity (rho < 2 mu Omega cm), low surface roughness (R(a) similar to 10 to 15 nm for similar to 1.5 mu m thick deposits) and good electrical uniformity (std dev <3% for 6 in. wafers and 5 to 7% for 8 in. wafers). A novel dry seeding method on sputtered Cu/Al bilayers has been developed to provide protection of Cu catalytic properties from passivation by using an Al sacrificial layer and to obtain uniform initiation and blanket growth of electroless Cu by in situ Al dissolution in the plating bath. Electroless Cu films blanket deposited on sputtered Cu/Al seed layer were conformal with 100% step coverage. A novel wet seeding method has been developed with Cu contact displacement deposition on a TiN diffusion barrier to provide selective and blanket electroless copper plating. Subhalf micron (down to 0.3 mu m) trenches and vias of high aspect ratios (up to 5:1) were completely filled for both blanket and selective electroless deposition modes.