Ultrasmooth cobalt films on SiO₂by chemical vapor deposition using a nucleation promoter and a growth inhibitor

Very smooth thin films of Co are deposited on SiO2 by chemical vapor deposition from the precursor dicobalt octacarbonyl Co2(CO)8 by augmenting the process conditions in two ways: (1) pretreating the surface to promote nucleation and (2) adding a coflow of a growth inhibitor. The surface pretreatment involves brief exposure of the SiO2 surface at 70 °C to a tetrakis(dimethylamido) metal complex M(NMe2)4, where M = V, Ti, or Hf. This pretreatment affords a self-limiting, submonolayer coverage of a metal-containing intermediate. When the pretreated surface is then exposed to 0.025 mTorr of the Co2(CO)8 precursor, nucleation occurs to afford a high density (1 × 1012 cm2) of nanoscale islands; the rms surface roughness for a sample with an areal density of 100 Co atoms/nm2 is 1.0 nm. Comparisons show that the M(NMe2)4 pretreatment has three benefits: It accelerates the nucleation process, it increases the nucleation density by about a factor of 3, and it reduces the surface roughness by about a factor of 2. The rms surface roughness of a sample with ~100 Co atoms/nm2 can be further reduced to only 0.3 nm by adding up to 4 mTorr of ammonia NH3 to the feed gas along with the Co2(CO)8 precursor. Ammonia serves as a growth inhibitor that reduces the steady-state growth rate of Co by a factor of 4. This slower growth allows additional nucleation to occur on bare areas, and the growth of these nuclei affords coalescence with a smaller roughness.