AVS 54th International Symposium
    Thin Film Tuesday Sessions
       Session TF-TuA

Paper TF-TuA2
Understanding the Long Nucleation Period for Pd ALD on Alumina Substrates

Tuesday, October 16, 2007, 2:00 pm, Room 613/614

Session: Surface Chemistry for Atomic Layer Deposition
Presenter: D.N. Goldstein, University of Colorado
Authors: D.N. Goldstein, University of Colorado
S.M. George, University of Colorado
Correspondent: Click to Email

Palladium (Pd) atomic layer deposition (ALD) can be performed on alumina substrates using Pd(hfac)2 (hfac= hexafluoroacetylacetone) and formalin. However, this system requires a nucleation period of 50-100 cycles before Pd grows steadily at 0.2 Å/cycle. To understand the long nucleation period, this study explored the details of the surface reactions occurring during Pd ALD nucleation. This research utilizes in situ Fourier Transform Infrared (FTIR) spectroscopy on nanopowder substrates to observe the surface species. Pd(hfac)2 dissociatively adsorbs on the Al2O3 substrate to yield surface Pd(hfac) and Al(hfac) during the initial Pd(hfac)2 exposure. During the first formalin exposure, only the hfac ligands attached to Pd are removed and the Al(hfac) species remain on the substrate. In addition, the formalin exposure adds formate species to the available Lewis acid sites on Al2O3. After the first cycle, no more surface Pd(hfac) is observed and formalin exposures remove some of the Al(hfac) species. These Al(hfac) species are believed to be the cause for the long nucleation period. Surface Al(hfac) is continually produced from Pd(hfac)2 exposures and competes with Pd metal for the same surface sites. The fate of Al(hfac) is either thermal decomposition to trifluoroacetate groups or removal from the surface as a volatile enol isomer. Formalin provides hydrogen to release the enol of hfac but cannot remove the trifluoroacetate groups. Trifluoroacetate groups may be a source of fluorine contamination in the films. The nucleation period ends when the volatile hfac products desorb and Pd can cover the original Al2O3 surface. At this point, only Pd(hfac) species are observed in the FTIR spectra. Transmission electron microscopy studies reveal Pd nanocrystallites that increase in number with increasing number of ALD cycles. These crystallites eventually coalesce to a conformal Pd film.