Paper TF-TuA12
Atomic Layer Deposition of Co-Al Films Studied by In-Situ Infrared Spectroscopy

Tuesday, October 19, 2010, 5:40 pm, Room Ruidoso

Cobalt-aluminum alloys are of great interest due to their unique properties such as corrosion resistance, high thermal stability, and unusual magnetic properties. In a search for appropriate precursors, we have investigated the growth of Co-Al thin films by atomic layer deposition (ALD) using CCTBA (μ²-h²-(^tBuacetylene)dicobalthexacarbonyl) and DMAH (Dimethylaluminumhydride) on H-terminated Si(111) and on 2-nm TaN films. In-situ infrared absorbance spectra show that upon the first CCTBA pulse on H/Si(111), alkynes (CC triple bonds) bound to Co₂(CO)₆ are converted to a benzene ring, as evidenced by the ring C=C stretching mode at 1475 and 1610 cm^-1. This transformation is not completely unexpected because cobalt carbonyl complexes (Co_x(CO)_y) are used to catalyze cyclotrimerization reactions in organotransition-metal chemistry. IR spectra also show that the presence of hydrogen enhances the adsorption of carbonyl groups on the H/Si(111) surface. After the first CCTBA pulse which reacts almost completely with H–Si bonds (2083 cm^-1), the amount of adsorbed CO on the surface (1970 cm^-1) is found to decrease. The subsequent DMAH pulse is effective to remove the surface carbonyl groups, leaving Al–CH₃ and/or Al–H bonds on the surface. In contrast, the adsorption of carbonyl on a TaN surface where H is absent is negligible after the first CCTBA pulse.

In all cases, CH_x ligands are not removed during CCTBA or DMAH cycles, leading to accumulation of carbon species in the film. The growth of metallic Co-Al films is hindered due to Al–O bond formation during deposition. The source of oxygen is likely associated to a surface Fischer-Tropsch (FT) process. It appears that reaction of cobalt particles with hydrogen (originating from the DMAH precursor) generates water as a by-product through a FT-like process, thus forming Al–O bonds. This formation of Al-O bonds through a FT process is greatly suppressed (~ 50%) by annealing the sample to 300 ^oC in N₂ ambient before exposure to DMAH, which removes the carbonyl group from the surface and therefore the source of oxygen. Although the growth pattern is similar for both H/Si(111) and TaN substrates, a part of Ta atoms in the original TaN films are reduced to metallic Ta⁰ during growth, according to XPS Ta 4f core level spectrum after deposition of Co-Al(O_x) films.