Plasma-Assisted Atomic Layer Deposition of Palladium

Atomic layer deposition of rhodium and palladium thin film using low-concentration ozone

Rhodium (Rh) and palladium (Pd) thin films have been fabricated using an atomic layer deposition (ALD) process using Rh(acac)₃ and Pd(hfac)₂ as the respective precursors and using short-pulse low-concentration ozone as the co-reactant. This method of fabrication does away with the need for combustible reactants such as hydrogen or oxygen, either as a precursor or as an annealing agent. All previous studies using only ozone could not yield metallic films, and required post treatment using hydrogen or oxygen. In this work, it was discovered that the concentration level of ozone used in the ALD process was critical in determining whether the pure metal film was formed, and whether the metal film was oxidized. By controlling the ozone concentration under a critical limit, the fabrication of these noble metal films was successful. Rhodium thin films were deposited between 200 and 220 °C, whereas palladium thin films were deposited between 180 and 220 °C. A precisely controlled low ozone concentration of 1.22 g m⁻³ was applied to prevent the oxidation of the noble metallic film, and to ensure fast growth rates of 0.42 Å per cycle for Rh, and 0.22 Å per cycle for Pd. When low-concentration ozone was applied to react with ligand, no excess ozone was available to oxidize the metal products. The surfaces of deposited films obtained the RMS roughness values of 0.30 nm for Rh and 0.13 nm for Pd films. The resistivities of 18 nm Rh and 22 nm Pd thin films were 17 μΩ cm and 63 μΩ cm.

Rh and Pd metallic thin films were fabricated by atomic layer deposition using Rh(acac)₃ and Pd(hfac)₂ precursors, and only low-concentration ozone as co-reactant.

The co-reactant role during plasma enhanced atomic layer deposition of palladium

Atomic layer deposition (ALD) of noble metals is an attractive technology potentially applied in nanoelectronics and catalysis. Unlike the combustion-like mechanism shown by other noble metal ALD processes, the main palladium (Pd) ALD process using palladium(ii)hexafluoroacetylacetonate [Pd(hfac)2] as precursor is based on true reducing surface chemistry. In this work, a thorough investigation of plasma-enhanced Pd ALD is carried out by employing this precursor with different plasmas (H2*, NH3*, O2*) and plasma sequences (H2* + O2*, O2* + H2*) as co-reactants at varying temperatures, providing insights in the co-reactant and temperature dependence of the Pd growth per cycle (GPC). At all temperatures, films grown with only reducing co-reactants contain a large amount of carbon, while an additional O2* in the co-reactant sequence helps to obtain Pd films with much lower impurity concentrations. Remarkably, in situ XRD and SEM show an abrupt release of the carbon impurities during annealing at moderate temperatures in different atmospheres. In vacuo XPS measurements reveal the remaining species on the as-deposited surface after every exposure. Links are established between the particular surface termination prior to the precursor pulse and the observed differences in GPC, highlighting hydrogen as the key growth facilitator and carbon and oxygen as growth inhibitors. The increase in GPC with temperature for ALD sequences with H2* or NH3* prior to the precursor pulse is explained by an increase in the amount of hydrogen species that reside on the Pd surface which are available for reaction with the Pd(hfac)2 precursor.

Atomic layer deposition of Pd and Pt nanoparticles for catalysis: on the mechanisms of nanoparticle formation

The deposition of Pd and Pt nanoparticles by atomic layer deposition (ALD) has been studied extensively in recent years for the synthesis of nanoparticles for catalysis. For these applications, it is essential to synthesize nanoparticles with well-defined sizes and a high density on large-surface-area supports. Although the potential of ALD for synthesizing active nanocatalysts for various chemical reactions has been demonstrated, insight into how to control the nanoparticle properties (i.e. size, composition) by choosing suitable processing conditions is lacking. Furthermore, there is little understanding of the reaction mechanisms during the nucleation stage of metal ALD. In this work, nanoparticles synthesized with four different ALD processes (two for Pd and two for Pt) were extensively studied by transmission electron spectroscopy. Using these datasets as a starting point, the growth characteristics and reaction mechanisms of Pd and Pt ALD relevant for the synthesis of nanoparticles are discussed. The results reveal that ALD allows for the preparation of particles with control of the particle size, although it is also shown that the particle size distribution is strongly dependent on the processing conditions. Moreover, this paper discusses the opportunities and limitations of the use of ALD in the synthesis of nanocatalysts.

Nano/subnanometer Pd nanoparticles on oxide supports synthesized by AB-type and low-temperature ABC-type atomic layer deposition: growth and morphology

The synthesis of uniformly dispersed nano/subnanometer Pd nanoparticles on oxide supports with atomic layer deposition (ALD) has been studied in terms of growth and morphology. In situ quartz crystal microbalance (QCM) measurements showed that AB-type Pd ALD grew more favorably on TiO(2) than on Al(2)O(3) at 200 °C by the sequential exposure of Pd(II) hexafluoroacetylacetonate (Pd(hfac)(2)) and formalin. The growth rate of AB-type Pd ALD decreased on the Al(2)O(3) surface at a lower deposition temperature, and there was negligible growth at 110 °C. However, a new ABC-type Pd ALD, which we developed recently, operates at significantly lower temperature by growing both protected Pd nanoparticles and the support simultaneously. Additionally, these two types of Pd ALD demonstrated very different growth behaviors. Scanning transmission electron microscopy (STEM) studies showed that the size of the Pd nanoparticles could be well controlled by varying AB-type Pd ALD cycles at 200 °C, and low-temperature ABC-type Pd ALD provides a novel way to synthesize highly uniform, ultrafine, supported Pd nanoparticles directly on high-surface-area supports, regardless of loading. Both types of Pd ALD indicate that ALD is a promising technique for synthesizing advanced catalysts with precise control.