Surface Chemistry during Atomic Layer Deposition of Pt Studied with Vibrational Sum-Frequency Generation.

V Vandalon,W.M.M Kessels,A.J.M Mackus

doi:10.1021/acs.jpcc.1c06947

Abstract

A detailed understanding of the growth of noble metals by atomic layer deposition (ALD) is key for various applications of these materials in catalysis and nanoelectronics. The Pt ALD process using MeCpPtMe3 and O2 gas as reactants serves as a model system for the ALD processes of noble metals in general. The surface chemistry of this process was studied by in situ vibrational broadband sum-frequency generation (BB-SFG) spectroscopy, and the results are placed in the context of a literature overview of the reaction mechanism. The BB-SFG experiments provided direct evidence for the presence of CH3 groups on the Pt surface after precursor chemisorption at 250 °C. Strong evidence was found for the presence of a C=C containing complex (e.g., the form of Cp species) and for partial dehydrogenation of the surface species during the precursor half-cycle. The reaction kinetics of the precursor half-cycle were followed at 250 °C, showing that the C=C coverage saturated before the saturation of CH3. This complex behavior points to the competition of multiple surface reactions, also reflected in the temperature dependence of the reaction mechanism. The CH3 saturation coverage decreased significantly with temperature, while the C=C coverage remained constant after precursor chemisorption on the Pt surface for temperatures from 80 to 300 °C. These SFG results have resulted in a better understanding of the Pt ALD process and also highlight the surface chemistry during thin-film growth as a promising field of study for the BB-SFG community.

Highlights

Ultrathin films and nanoparticles of noble metals have a wide range of applications associated with their catalytic nature, chemical stability, and high work function.[1−6] Atomic layer deposition (ALD) of noble metals is gaining increasing interest for the fabrication of such ultrathin films and nanoparticles.[7−9] This is mainly motivated by the unique combination of properties that ALD offers, including precise control over film thickness, unparalleled conformality over complex 3D structures, and superior uniformity across large substrates
Carbon containing precursor fragments were detected by broadband sum-frequency generation (BB-sum-frequency generation (SFG)), which were associated with surface species containing C C, as pinpointed in a separate series of experiments absorbing various molecules on Pt and SiO2 surfaces
The reaction kinetics and the temperature dependence of the surface coverage of the CH3 and C C containing species during the precursor half-cycle were studied with BB-SFG providing a more detailed picture of the reactions occurring during the precursor half-cycle

Summary

Introduction

Ultrathin films and nanoparticles of noble metals have a wide range of (potential) applications associated with their catalytic nature, chemical stability, and high work function.[1−6] Atomic layer deposition (ALD) of noble metals is gaining increasing interest for the fabrication of such ultrathin films and nanoparticles.[7−9] This is mainly motivated by the unique combination of properties that ALD offers, including precise control over film thickness, unparalleled conformality over complex 3D structures, and superior uniformity across large substrates. The application fields of noble metal ALD include catalysis and nanoelectronics.[5,10−15] Insight into the reaction mechanisms of noble metal ALD processes is essential to extend their operating conditions, to enable new applications, and allow for deposition on challenging substrates (e.g., powders or polymers). Similar mechanistic insights were needed to reliably deposit Ru,[18] Pd, Pt-Ir alloys,[3,15] and Pt coatings on nanoparticles.[7,9] Fundamental insight into the reaction mechanisms aids in understanding aspects such as nucleation and island growth, which are of interest for the controlled growth of nanoparticles.[9,17,20]

Results

Discussion

Conclusion