ANISOTROPIC DEPOSITION OF ELECTROLESS NICKEL - BEVEL PLATING

Wet chemical processes such as etching or plating are generally isotropic. Although anisotropic wet etching can be achieved using single-crystalline substrates or a duplex resist, anisotropic metal deposition seems a difficult task. However, we show here that anisotropic deposition of Ni has been accomplished in an acid electroless Ni bath to which an appropriate level of Pb2+ has been added. Since the edges of a given pattern experience an additional contribution from nonlinear diffusion, mass transport to these edges is enhanced compared to the bulk of a substrate. As a result the surface concentration of the adsorbed-stabilizer is also higher at the edges. By choosing the proper Pb2+ concentration, the edges can be selectively poisoned: bevels can be grown with an angle that is a function of the stabilizer concentration in the electrolyte. That the Pb2+ stabilizer inhibits the cathodic partial reaction (metal deposition itself), but not the oxidation of the reducing agent has been verified with cyclic voltammetry at a rotating-disk electrode, carried out in the vicinity of the plating potential. The process can be used to deposit metal pyramids onto the Al bond pads of an integrated circuit. This morphology is useful for the direct mounting of an integrated circuit onto a conductor pattern (flip chip).