Atomic layer deposition derived routes for the synthesis of nanostructured materials

Nithin Poonkottil
Abstract & Cover

Atomic Layer Deposition (ALD) is a promising method to deposit thin films on 3D−substrates without compromising high uniformity and conformality. In ALD, the substrate is repeatedly exposed to a sequence of reactive gases. Between each gas exposure, a pump/purge step is implemented to reach very low pressure in the ALD chamber (typically in the order of 10−6 mbar). The gases will react with the available surface groups of the substrate, adding atomic amounts of material to the substrate. A distinct feature that sets ALD superior to other methods is its self-limiting character. This means, when all the surface groups have been reacted with the introduced gas, the reaction stops. The high conformality and uniformity of ALD are benefited from its self-limiting character. Another gas exposure is followed after the pump/purge step to refresh the surface before the growth can continue. One ALD cycle will thus consist of a loop of alternated exposures of the two reactive gases. Often, the reactant in the first half cycle is called precursor, which is the source of metal. The reactant in the second half cycle is called the co−reactant, which assists the transformation of the precursor adsorbed on the substrate into the target material. The ALD cycles can be repeated until the desired thickness of the material has been achieved. Thanks to these unique features, ALD finds a lot of commercial applications including microelectronics, photovoltaics, and batteries. It is possible to deposit a wide variety of materials, including oxides, nitrides, sulphides, phosphates, and (noble) metals via different ALD chemistries. The first part of this thesis deals with the development of a novel ALD chemistry for deposition of ruthenium dioxide (RuO2) which is an interesting material for several applications, including (electro)catalysis and microelectronics. A new thermal ALD process for RuO2 deposition was developed and its ALD characteristics were determined. The process uti[1]lizes ruthenium tetroxide (RuO4) as the precursor and alcohols as a (mild) reductant to deposit RuO2. The deposition was feasible at temperatures as low as 60 ◦C and had a temperature window from 60−120 ◦C. The growth per cycle (GPC) of the process can be tuned by changing the alcohol counterpart. For instance, the use of methanol resulted in a GPC of 1 Å/cycle, ethanol in 1.5 Å/cycle, while in the case of 1−propanol and 2−propanol it was around 2 Å/cycle. The films were as−deposited amorphous from lab−based XRD. An anneal in helium or in air transformed the films into crystalline rutile RuO2 around 420 ◦C. The process also allowed for the deposition of smooth and conductive RuO2 films. Insights into the reaction mechanism were obtained by using several in situ techniques including in situ Fourier transform infrared spectroscopy (FTIR), mass spectrometry, and in vacuo X−ray photoelectron spectroscopy (XPS). Hence, we put forward the following mechanism. During the alcohol-containing pulse, the top RuO2 layer is partially reduced to RuOx (x<2), and consequently the alcohol is oxidized on the RuO2 surface into CO2 and H2O. The alcohol pulse also leaves carbon monoxide (CO) residues on the surface. During the RuO4 pulse two reactions occur: 1) oxidative removal of CO to CO2, 2) surface oxidation back to RuO2. In this reaction additional RuO2 is deposited on the surface. As discussed before, ALD offers conformal coatings on the substrate, however, for certain applications including the creation of nanostructures, (nanoparticles, nano lines etc.) limiting ALD growth in the lateral direction is very important. In this aspect, we show the potential of area−selective atomic layer deposition (AS−ALD) to derive nanostructures in a bottom[1]up fashion. Area-selective deposition takes place during the initial ALD cycles if an ALD process induces immediate growth on the surface of one particular material while there is a delay before the growth commences on the surfaces of other materials that are present on the same sample. This results in the formation of nanostructures that resemble the pattern of the growth surface. In this work, area−selective ALD of RuO2 is demonstrated using the ALD process consisting of RuO4 and alcohols as discussed before. Selective RuO2 deposition was achieved on SiO2 with inhibition on poly methyl methacrylate (PMMA). There was no deposition on PMMA blanket films even up to 200 ALD cycles, resulting in around 25 nm of selective RuO2 deposition on SiO2. Different parameters affecting the selectivity of the process were studied, including polymer thickness and deposition temperature during ALD. The feasibility of selective deposition with other co-reactants such as ethanol and iso-propanol was investigated, and we found that the growth per cycle can be increased by using a higher chain alcohol without compromising the selectivity. Other polymer layers were also studied as potential inhibition layers for AS−ALD of RuO2. Polymers with C=O functional groups effectively inhibit RuO2 growth. The developed area selective method was demonstrated by selectively depositing RuO2 on patterned SiO2/PMMA samples, followed by PMMA removal, resulting in RuO2 nanopatterns on the surface as demonstrated by transmission electron microscopy (TEM), and scanning electron microscopy with energy dispersive X−ray spectroscopy (SEM/EDX). Furthermore, we demonstrate sequential infiltration synthesis (SIS), an[1]other ALD−derived method that is quite promising for synthesizing inor ganic nanopatterns. This technique is based on the block selective infiltra[1]tion of ALD precursors and reactants to one of the blocks of a di−block copolymer (di−BCP) template. This results in inorganic material growth inside one of the domains of the di-BCP. The polymer template can be re[1]moved by appropriate post-treatments to generate the desired nanopattern. In this work, Ru and RuO2 nanostructures are prepared by SIS. Using a self assembled polystyrene−block−polymethylmethacrylate (PS−b−PMMA) template, Ru and RuO2 nanostructures resembling the PS domain have been synthesized. RuO4 and H2 gas were used as reactants for Ru SIS and RuO4 and methanol for RuO2 SIS. Selective and strong interaction of RuO4 molecules with PS domains has been achieved, without affecting the PMMA domains. The aromatic C=C and C-H bonds present in the PS domain were consumed as a result of RuO4 infiltration. Density functional theory calculations also supported the favorable interaction with PS and a plausible mechanism for Ru infiltration was put forward. A single SIS cycle was found to considerably enhance the contrast of the PS domain in the template as found from morphological assessments. Finally, infiltrated PS−b−PMMA was subjected to an H2 plasma treatment to remove the organic template and to generate Ru nanopatterns. The crystalline nature of the samples was confirmed by grazing incidence wide angle X−ray scattering measurements and the samples after plasma showed a superior crystallinity to the as−infiltrated samples. Finally, we show a novel selective decomposition strategy to obtain BMNPs, exemplified for Pt-Sn BMNPs. Monometallic nanoparticles with ALD are typically obtained by exploiting the island growth mode during the initial stages of metal ALD growth. In island growth, the deposited material tends to settle on the grown material, resulting in island like particles. With subsequent ALD cycles, the particles grow big enough to coalesce, resulting in a closed or continuous metal layer. Therefore, one can limit the number of ALD cycles before a closed layer is formed if the goal is to deposit metal nanoparticles. However, it is more challenging to deposit bimetallic nanoparticles and regulate their composition using ALD.In this thesis, Pt-Sn bimetallic nanoparticles (BMNPs) are prepared by the selective doping of Pt NPs with Sn. This is based on the selective de[1]composition of tetrakis(dimethylamino)tin (TDMASn), a Sn ALD precursor on Pt. There was no decomposition observed on other substrates such as SiO2, Al2O3 and TiN. Although the selective decomposition on Pt resulted in Pt-Sn BMNPs, the morphology revealed significant coarsening after BMNP formation. The original size of the Pt NPs was preserved by the introduction of an extra H2 pulse after each TDMASn pulse, resulting in a cyclic TDMASn-H2 process. The differences between the TDMASn only and TDAMSn-H2 process were investigated using in situ characterizations. The H2 pulse performs a dual role in the process: removing the NCH3CH3 ligands from the TDMASn precursor on the Pt surface by the fromation of volatile NHCH3CH3 and removing superflous Sn from the Pt surface. The Sn uptake showed saturation as a function of TDMASn-H2 cycles, and the Sn content in the BMNPs was tuned by changing the substrate temperature. The formation of Pt-Sn BMNPs using the TDMASn-H2 process was also shown on high surface area SiO2 supports. In summary, creating nanostructures by ALD requires limiting the lateral growth of the ALD process. There are several ALD−related techniques that can be exploited for the synthesis of such structures. In the framework of this thesis, the potential of three different routes such as selective deposition, selective infiltration, and selective decomposition, is exploited for the tailored synthesis of nanostructured materials 

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Nithin Poonkottil
Ghent University
(Ghent, Belgium)
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