Ultrasmooth cobalt films on SiO2 by chemical vapor deposition using a nucleation promoter and a growth inhibitor

Abstract

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.