SELECTIVE AND BLANKET ELECTROLESS COPPER DEPOSITION FOR ULTRALARGE SCALE INTEGRATION

Electroless Cu thermodynamics, electrochemistry, mechanism, kinetics, and mass transport are reviewed. Electroless Cu deposition is a thermo dynamically favorable and kinetically inhibited process, with two elec trochemical reactions including anodic oxidation of a reducing agent a nd cathodic reduction of metal ions occurring simultaneously. with a m ultistep 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). Selec tive and blanket electroless Cu deposition from formaldehyde-based sol utions with ethylenediaminetetraacetic acid as a complexing agent was investigated for trench/via filling applications. A single-wafer elect roless Cu deposition system with up to 8 in. wafer capab ility has bee n designed and manufactured. An electroless Cu deposition solution and operation conditions have been optimized to obtain electroless Cu fil ms at high deposition rate (similar to 75 to 120 nm/min), with low res istivity (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 electric al uniformity (std dev <3% for 6 in. wafers and 5 to 7% for 8 in. wafe rs). A novel dry seeding method on sputtered Cu/Al bilayers has been d eveloped to provide protection of Cu catalytic properties from passiva tion 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 seed ing method has been developed with Cu contact displacement deposition on a TiN diffusion barrier to provide selective and blanket electroles s 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 blan ket and selective electroless deposition modes.