Plasma-Assisted Atomic Layer Deposition of Palladium

Abstract

A method is presented for the atomic layer deposition (ALD) of palladium using remote hydrogen plasma as the reducing source and agent. Palladium was deposited on iridium, tungsten and silicon at 80 C using a remote inductively coupled hydrogen plasma with palladium(II) hexafluoroacetylacetonate as the precursor. In the case of the Pd film grown on Ir, the carbon and fluorine content were significantly reduced compared to previous thermal ALD results. Use of remote plasma eliminated the noble metal substrate requirement needed for thermal ALD, enabling films to be grown on W and Si. Ultra-thin Pd films grown on W and Si possessed a nearly random texture from reflection high-energy electron diffraction (RHEED) measurements. Atomic force microscopy (AFM) images showed very different surface morphologies for the different substrates suggesting very different substrate film interactions. X-ray photoelectron spectroscopy (XPS) measurements indicate high quality Pd films for all substrates, suggesting the substrate temperature was low enough to prevent dissociation of the hfac ligand and adequate C and F scavenging by the atomic hydrogen. The remote hydrogen plasma source results in the loss of selectivity but growth is evident on every surface used including surfaces that do not react strongly with the Pd precursor and are not catalytic towards the dissociation of molecular hydrogen.

Atomic Layer Deposition of Pd on an Oxidized Metal Substrate

“…As can be seen in the Figure, the peak positions for Pd are shifted by approximately 0.1 eV relative to the XPS analysis on Pd films performed by Militello et al [28] In experiments with noble metal substrates, the thermal ALD tended to result in Pd 3d spectra that had many convoluted peaks. This effect has been generally observed for thermal ALD, and is not present for identical conditions but with the use of plasma, as seen previously with Ten Eyck et al [16] This is potentially an artifact of the deposition rate, or more specifically a function of the H availability on the surface. Even when chemisorption of the precursor occurs across the entire wafer, if the initial Pd layers do not have consistent atomic H available, the incorporation of hfac ligand components into the Pd film would be expected since hfac ligands have been shown to remain bound and unfragmented on the surface at the deposition temperatures used in these experiments.…”

“…Attempts to grow on oxidized Si were unsuccessful under any of the tested conditions in the absence of plasma, attributed to the lack of atomic H to facilitate ligand desorption from the surface. The apparatus used for this deposition has been described previously, [16] with the exception that for the present work there was no plasma utilized at any point of the ALD process. A control dataset using plasma conditions is, however, presented as a benchmark for the ALD performance.…”

“…[16] As shown in Figure 2, the use of a plasma does enable the deposition on oxidized Si, and as reported it was believed that the plasma was responsible for the deposition on the oxidized W surface as well. [16] Tungsten samples used at points in this work, however, did show that some deposition is possible provided there is sufficient time allowed during the H 2 pulse for ligand removal. However, the data were inconsistent and further work is needed to better understand the root of this variability.…”

“…[13] There are many coordination compounds based on b-diketonate chemistries that are suitable for metal ALD; [14,15] however, to date there has been fairly limited success with ALD using metal-organic compound precursors and low process temperatures when deposited on oxide surfaces because of the lack of a suitable reducing agent. Previous work has been undertaken with a plasma reduction scheme by this group [16] and others [1,[17][18][19][20] to enable deposition on various substrates, including oxidized metals at relatively low temperatures. Unfortunately, the use of a plasma eliminates key attributes of an ALD approach, namely the ability to deposit a conformal coating onto three-dimensional structures and the simplicity of the ALD tool.…”

Atomic Layer Deposition of Noble Metals – New Developments in Nanostructured Catalysts

“…11 To increase the substrate generality, Ten Eyck et al used a remote hydrogen plasma for Pd ALD on Ir, W, and Si surfaces. 12 However, plasmas are not well suited to catalyst synthesis because they require line-of-site between the plasma and the substrate surface which is impractical for nanoporous catalyst templates. The lack of a suitable reducing reagent for Pd ALD on porous oxide surfaces has limited the development of ALD Pd catalysts.…”