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

Adsorption and Reaction Mechanisms of Direct Palladium Synthesis by ALD Using Pd(hfac)2 and Ozone on Si (100) Surface

Palladium nanoparticles made by atomic layer deposition (ALD) normally involve formaldehyde or H2 as a reducing agent. Since formaldehyde is toxic and H2 is explosive, it is advantageous to remove this reducing step during the fabrication of palladium metal by ALD. In this work we have successfully used Pd(hfac)2 and ozone directly to prepare palladium nanoparticles, without the use of reducing or annealing agents. Density functional theory (DFT) was employed to explore the reaction mechanisms of palladium metal formation in this process. DFT results show that Pd(hfac)2 dissociatively chemisorbed to form Pd(hfac)* and hfac* on the Si (100) surface. Subsequently, an O atom of the ozone could cleave the C–C bond of Pd(hfac)* to form Pd* with a low activation barrier of 0.46 eV. An O atom of the ozone could also be inserted into the hfac* to form Pd(hfac-O)* with a lower activation barrier of 0.29 eV. With more ozone, the C–C bond of Pd(hfac-O)* could be broken to produce Pd* with an activation barrier of 0.42 eV. The ozone could also chemisorb on the Pd atom of Pd(hfac-O)* to form O3-Pd(hfac-O)*, which could separate into O-Pd(hfac-O)* with a high activation barrier of 0.83 eV. Besides, the activation barrier was 0.64 eV for Pd* that was directly oxidized to PdOx by ozone. Based on activation barriers from DFT calculations, it was possible to prepare palladium without reducing steps when ALD conditions were carefully controlled, especially the ozone parameters, as shown by our experimental results. The mechanisms of this approach could be used to prepare other noble metals by ALD without reducing/annealing agents.

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.