The scope of electroless copper plating technology is evolving to applications with smaller structures and smoother substrates, where adhesion of the copper film can be hard to achieve. The extent to which different factors, such as film stress and hydrogen evolution, contribute to delamination and blister formation on smooth organic substrates is a topic of active discussion. Stress and strain evolution is reported here for a nickel-and formaldehyde-free formulation for a range of plating bath temperatures. X-ray diffraction, with the sin 2 ψ method, was used to monitor the strain within the copper crystallites during and after their growth. Substrate bending measurements were used to detect the average stress of the film. This technique indicates a 60 nm wide zone of tensile stress at the interface between substrate and film. During and immediately following deposition, the average substrate stress is more compressive than implied by the strain of the Cu crystallites. After deposition, the additional compressive stress seen by substrate bending relaxes until it matches the Cu crystallite strain. Furthermore, the thermal substrate contraction effect upon termination of plating is considered, since it may influence the film adhesion.Electroless copper films, widely used in the printed circuit board industry, are currently being optimized for providing interconnects for future generations in integrated circuit (IC) substrates. For such applications, the industry is moving toward specifying limits for stress in electroless copper deposits. Nickel is frequently added to electroless copper baths because it controls the film stress 1 and because it reduces hydrogen evolution during deposition. 2,3 However, co-deposited nickel reduces the conductivity of the copper deposits, which may be a problem in high-end applications that require confined current paths during the subsequent galvanic copper plating. Further, nickel as an additive and formaldehyde as the reducing agent are incompatible with the desire to employ green chemistry processes wherever possible. In order to produce highly reliable films within these constraints, one has to monitor the factors that are known to limit film adhesion, the film stress 4 and co-deposited hydrogen. 2 Hydrogen, co-deposited at typical pressures of 70 MPa in grain boundaries, 5 reduces the film ductility and adhesion. 2,6-8Here we present X-ray diffraction and substrate curvature measurements that are sensitive to the strain of the copper crystallites and the average stress of the copper film, respectively. These measurements were carried out during and after film deposition from a bath formaldehyde-free and nickel-free at different temperatures. For chemical vapor deposition, physical vapor deposition, and sputtering the film stress found by these two methods usually agrees. 9-11 In the present case, significant differences can be observed in the more complex electroless deposition process.
ExperimentalThe Cu films were produced from an electroless nickel-free bath containing 3 g/...
