Area-selective chemical vapor deposition of cobalt from dicobalt octacarbonyl: Enhancement of dielectric-dielectric selectivity by adding a coflow of NH₃ Available to Purchase

The authors describe the enhancement of the area-selective chemical vapor deposition of cobalt films on one oxide surface over another from the precursor Co₂(CO)₈ by addition of the nucleation inhibitor ammonia (NH₃). In the absence of an NH₃ coflow, the Co₂(CO)₈ precursor exhibits a weak intrinsic selectivity: at 70 °C, Co nucleates quickly on Al₂O₃ but more slowly on SiO₂. The addition of an NH₃ coflow, however, greatly amplifies the selectivity between different oxide surfaces. Thus, NH₃ significantly inhibits nucleation on acidic oxides such as SiO₂ and WO₃ but has little effect on more basic oxides such as Al₂O₃, HfO₂, and MgO. Comparison of growth on fully hydroxylated and dehydroxylated SiO₂ suggests that hydroxyl groups are the nucleation sites that are affected by the addition of NH₃. The mechanism of nucleation appears to be disproportionation of Co₂(CO)₈ to Co²⁺ (the intermediate that leads to nucleation) and Co(CO)₄⁻: this disproportionation occurs readily on basic oxides but not on acidic oxides. The addition of NH₃ has little effect on Co nucleation on basic oxides, probably because ammonia binds poorly to such surfaces, but NH₃ greatly retards nucleation on acidic oxides such as SiO₂; the authors propose that the latter result is either a site blocking effect or the result of conversion of Co²⁺ to inactive Co(NH₃)_x²⁺ species. Nucleation of cobalt is facile on gold (a very unreactive metal) even in the presence of NH₃. The authors have found, however, that the deposition of Co on tungsten can be inhibited by exposing the surface briefly to ozone; no deposition occurs on the resulting thin tungsten oxide overlayer from Co₂(CO)₈ in the presence of NH₃. In other words, this thin oxide overlayer affords the same selective inhibition behavior as seen on bulk WO₃. In this way, both metal-on-metal and metal-on-oxide selectivity can be achieved. Cobalt films grown in the absence and presence of ammonia have resistivities of 11–20 and 15–25 μΩ cm, respectively.