Growth and morphology of electroless cobalt thin films deposited onto palladium pre-activated silicon surfaces

In this work, it is investigated the growth and morphology of cobalt thin films deposited by electroless onto (100) P silicon wafer surfaces pre-activated with palladium. Cobalt electroless deposition was performed in a solution composed by sodium citrate (Na 3 C 6 H 5 O 7 ), ammonium sulfate ((NH 4 ) 2 SO 4 ), cobalt sulfate (CoSO 4 .7H 2 O) and sodium hipophosphite (NaH 2 PO 2 .H 2 O). Cobalt films were characterized by Rutherford Backscattering Spectrometry (RBS), Atomic Force microscopy (AFM) and Scanning Electron Microscopy (SEM). After pre-activation in a diluted hydrofluoric solution, Pd stays sparsely distributed over the Si surface as local Pd deposits and the longer the immersion time, the larger the size of these deposits. For small immersion times in Co bath, total planar concentration and RMS roughness progressively increases and tends to saturate for large immersion times. This saturation is accompanied by the appearing of oxygen on the surface when Pd tends to be buried or disappear from the surface. In this case, Co deposition stops and surface Co starts to oxidize in the alkaline media of the Co bath.