Atomic layer deposition of platinum : from surface reactions to nanopatterning

Abstract

Platinum is a material that finds many applications in the fields of nanoelectronics and catalysis due to its catalytic activity, chemical stability, and high work function. The thin film deposition technique of atomic layer deposition (ALD) is gaining increasing interest for the deposition of Pt ultrathin films and nanoparticles, since it is able to deposit on demanding surfaces such as high-aspect-ratio structures and porous materials. In this dissertation, ALD of Pt was studied, aimed at the development of a novel bottom-up nanopatterning approach. Conventional patterning by lithography involves resist-films and lift-off steps that may yield compatibility issues with the envisioned nanoscale building blocks of future nanodevices, e.g. nanowires, carbon nanotubes, and graphene. The main goal was to develop a nanopatterning approach that enables direct and local fabrication of high-quality nanostructures without the need for additional lithography steps. Since ALD film growth depends critically on the properties of the surface, it is possible to chemically tailor the surface properties to achieve area-selective deposition. For the development of the nanopatterning technique, detailed understanding of the surface reactions of the ALD processes of noble metals turned out to be crucial. The reaction mechanism of Pt ALD was studied by evaluating which surface reactions take place at the catalytically active Pt surface during ALD, based on analogous surface reactions reported in surface science literature. This study led to new insights into the surface reactions that take place during the growth, the saturation of the half-reactions, and the temperature dependence of the process. Inspired by the conclusions drawn from the reaction mechanism study, an approach for plasma-assisted ALD at low substrate temperatures was developed. It was demonstrated that this new process enables the deposition of Pt at temperatures down to room temperature. Consequently, the Pt can be deposited on various temperature sensitive substrates such as polymers, textile and paper, which significantly broadens the possibilities for applications of Pt ALD. Furthermore, the nucleation behavior of Pt ALD was studied using spectroscopic ellipsometry and transmission electron microscopy. It was established that the pressure employed during the O2 half-reaction of the ALD process governs the nucleation behavior, which can be exploited for controlling the nucleation of the Pt. This control enables nanoparticle deposition, thin film deposition with minimal nucleation delay, and areaselective ALD for nanopatterning. The developed nanopatterning approach is based on a combination of ALD with electron beam induced deposition (EBID). EBID is a direct-write patterning technique with nanometer scale resolution but its main drawback is that it gives material of poor quality. The newly developed approach comprises the deposition of a thin seed layer by EBID, followed by area-selective ALD. It was established that this so-called direct-write ALD technique yields high-quality Pt material (~100% pure, 12 µOcm), and an enhanced throughput comparable to that of electron beam lithography (EBL), while it allows for patterning of nanoscale line deposits of only ~10 nm in width. To validate whether direct-write ALD is suitable for contacting applications, it was demonstrated that contacts can be patterned on multi- and single-walled carbon nanotubes. Additionally, it was evaluated whether direct-write ALD is a suitable technique for the fabrication of carbon nanotube field effect transistors (CNTFET). CNTFETs were synthesized by patterning of Pt contacts using direct-write ALD on single-walled carbon nanotubes. It was demonstrated by electrical characterization that these devices behave as a p-type transistors. In conclusion, in this work a novel bottom-up nanopatterning approach has been developed that is completely resist-free, and is especially suitable for the patterning of contacts on sensitive nanomaterials. In addition, the reaction mechanisms studies led to atomic level understanding of the surface reactions of Pt ALD, and thereby will contribute to the use of Pt ALD in a wide variety of applications.