The selectivity of electroless Ni plating onto submicrometer-scaled Cu patterns less than 200 nm in width was examined at 50°C using a plating solution containing Ti(III) ions as a reducing agent. The composition of the solution was 0.04 M NiSO4, 0.08 M TiCl3, 0.24 M citric acid (sodium salt, Na3Cit), and 0.04 M nitrilotriacetic acid (sodium salt, Na3NTA). A continuous and uniform Ni layer of about 10 nm in thickness was found to be epitaxially deposited only on the patterned Cu area, suggesting that the plating method is applicable to a metallizing process for ultrafine devices. The reactions that occurred in the plating solution were discussed from a thermodynamic standpoint. The plating rate was strongly influenced by the immersion potential and pH of the solution. The reduction of water proceeded in the bulk of the plating solution, and this reaction was partially responsible for the lowering of pH and the depression of the plating. Using the stability constants for the Ni2+-citrate and Ni 2+-nitrilotriacetate systems, we calculated the distribution of Ni(II) complexes in the solution with various concentrations of Na 3NTA: the analytical composition was 0.04 M NiSO4-0.24 M Na3Cit-0-0.12 M Na3NTA at 50°C. The concentration of Na3NTA affected the stability of the plating solution and the plating rate, since the abundances of Ni(II) complexes, i.e., NiCit2 4-, NiNTA-, and NiNTA24-, depended on the concentration of Na3NTA; the presence of NiNTA 24-, which is more stable than the others, depressed the nickel plating significantly.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Renewable Energy, Sustainability and the Environment
- Surfaces, Coatings and Films
- Materials Chemistry