Photo-Assisted Atomic Layer Deposition of Pt—An In Situ X-Ray Scattering and Fluorescence Study of the Nucleation and Growth

Atomic layer deposition (ALD) has emerged as a powerful method to grow nanostructured noble metals such as Pt. Its capabilities to tailor the morphology from nanoparticles to thin films and to coat 3D substrates conformally are highly desired in electrical, catalytic and electrochemical applications. For ALD of Pt with the (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe₃) precursor, previous work has shown that the choice of reactant has considerable impact on the nucleation [1]. The commonly used O₂ gas leads to the formation of mobile PtO_x species, responsible for the coarsening of nuclei into larger particles during the initial growth regime. In contrast, surface diffusion is suppressed with N₂ plasma as reactant and a high density of small islands is obtained, leading to films with a smooth surface when the islands coalesce into a continuous layer [2].<br/><br/>Photo-assisted ALD is a variant of conventional thermal ALD in which additional energy is provided to the reactions by exposing the sample to UV light [3]. Absorption of photons by precursor molecules in the gas phase or adsorbed on the surface can lead to excitation and even dissociation of the molecules, yielding active species that enable or speed up thin film growth. Photo-activation may also allow for lower deposition temperatures, increased growth rates and modified film properties. However, despite the fact that promising advantages of photo-assisted ALD have indeed been demonstrated, the number of reports using photo-assisted ALD is limited.<br/><br/>In this work, photo-assisted ALD of Pt is investigated for the first time by implementing UV-illumination during MeCpPtMe₃-O₂ ALD. The nucleation and growth are investigated <i>in situ</i> via X-ray fluorescence (XRF) and grazing incidence small angle X-ray scattering (GISAXS) at the BM26 beamline of the ESRF synchrotron, yielding the Pt surface density (#Pt atoms/cm²) and size and coverage (#nuclei/cm²) of the nuclei, respectively.<br/><br/>For all temperatures tested, the <i>in situ </i>growth curves reveal a significant increase in Pt deposition when the UV-illumination is turned on continuously during the ALD process. To understand the effect of photo-assistance during each ALD step, the timing of the illumination in the ALD cycle is varied, and it is found that illumination during the MeCpPtMe₃ exposure triggers the growth enhancement. This suggests that the precursor is activated in the gas phase prior to adsorption, potentially via dissociation of the Pt-Me bonds [4], enabling a larger Pt uptake on the surface.<br/><br/>Remarkably, depositions in which a number of photo-assisted ALD cycles is followed by thermal ALD also show a drastic increase in Pt uptake, even if only few photo-assisted ALD cycles are carried out. This confirms a crucial role for the UV-light in the nucleation process. The <i>in situ</i> GISAXS patterns revealed that a larger amount of smaller islands is formed with photo-assisted ALD compared to conventional thermal ALD, indicative of an increased nucleation density. This shows that UV-light is a promising external trigger to control nucleation during noble metal ALD.<br/><br/>[1] Dendooven et al. Nat. Commun. 2017, 8, 1074.<br/>[2] Longrie et al. ECS J. Solid State Sci. Technol. 2012, 1, Q123.<br/>[3] Miikkulainen et al. ECS Trans. 2017, 80, 49.<br/>[4] Engmann et al. PCCP 2012, 14, 14611. Egger et al. J. Organomet. Chem. 1970, 24, 501.