Area selective CVD of metallic films from molybdenum, iron, and ruthenium carbonyl precursors: Use of ammonia to inhibit nucleation on oxide surfaces

The authors demonstrate that the addition of an ammonia coflow during the chemical vapor deposition of MoC_xN_y, Fe, or Ru thin films at ≤200 °C from the metal carbonyl precursors Mo(CO)₆, Fe(CO)₅, or Ru₃(CO)₁₂ affords area-selective growth: film grows readily on titanium metal or vanadium nitride substrate surfaces, but no nucleation occurs on air-exposed SiO₂, TiO₂, Al₂O₃, or MgO within the investigated times of 1-2 h. By contrast, in the absence of ammonia, nucleation and deposition on these oxide surfaces can either be slow or rapid, depending strongly on the oxide surface preparation. NH₃ is also the source of N in MoC_xN_y, which has a resistivity of 200 μΩ cm and becomes superconducting at a critical temperature of 4 K. The authors hypothesize that the passivating effect of NH₃ on oxide surfaces involves site blocking to prevent precursor adsorption, or an acid-base interaction to stabilize surface-bound metal subcarbonyl intermediates, or a combination of these mechanisms. A key finding is that surface selective growth is often crucially dependent on the sample history of the substrate, which must be specified in detail if reproducible results are to be obtained.