Paper TF-MoA9
In-situ Quadrupole Mass Spectroscopy Analysis of Low Temperature Cobalt Deposition Reactions using Co₂(CO)₈ and Co(C₅H₅)(CO)₂ in Atomic Layer Deposition Process Sequencing

Monday, October 20, 2008, 4:40 pm, Room 302

Cobalt is of interest as a catalyst in Fisher-Tropsch synthesis, in the Pauson–Khand reaction and for carbon nanotube growth. Cobalt’s high magnetic permittivity also makes it valuable for data storage applications. Cobalt thin films are widely deposited by thermal and plasma ALD and CVD at temperatures between 200 and 400 ^oC. Lower temperature deposition processes are beginning to open new applications, including coating of temperature sensitive polymers and templating of biological materials. However, processes for conformal metal film deposition at less than 150 ^oC are not widely available. Previous studies of CVD Co from Co₂(CO)₈ show a transition in reaction kinetics at temperatures near 100 - 150 ^oC, consistent with a thermally activated increase in gas phase interactions between precursor molecules resulting in a decrease in deposition rate with increasing temperature. For this work, we undertook a study of low temperature cobalt thin film deposition using ALD process sequencing under the hypothesis that avoiding precursor interactions may produce a viable low temperature ALD process. Film growth was studied between 30 and 130 ^oC using Co₂(CO)₈ and H₂ gases using on-line quadrupole mass spectrometry and Auger electron spectroscopy. Similar experiments using cobalt cyclopentadienyl dicarbonyl and H₂ reactants were also performed between 140 and 350 ^oC. For the dicobalt octacarbonyl precursor, film deposition rate and in-situ quadrupole mass spectroscopy results demonstrate that for all gas pulse times studied, film growth proceeds continuously at temperature as low as 60 ^oC, with no indication for self-limiting precursor adsorption. The observed continuous film growth with the Co₂(CO)₈ is related to the zero-valent metal center, where no reduction step is required to produce a reactive surface for adsorption. The cyclopentadienyl dicarbonyl precursor showed evidence for carbonyl cleavage and volatile Co(cyclopentadienyl), without film growth at less than 300 ^oC. In addition, the mass spectroscopy results show evidence for CH₄ and C₂H₆ production during Co film nucleation, with no hydrocarbons present during subsequent processing. This is consistent with an interesting self-catalytic effect of the cobalt nuclei that decreased upon continuous film formation.