In many nano(opto)electronic devices, the roughness at surfaces and interfaces is of increasing importance, with roughness often contributing toward losses and defects, which can lead to device failure. Consequently, approaches that either limit roughness or smoothen surfaces are required to minimize surface roughness during fabrication. The atomic-scale processing techniques atomic layer deposition (ALD) and atomic layer etching (ALE) have experimentally been shown to smoothen surfaces, with the added benefit of offering uniform and conformal processing and precise thickness control. However, the mechanisms which drive smoothing during ALD and ALE have not been investigated in detail. In this work, smoothing of surfaces by ALD and ALE is studied using finite difference simulations that describe deposition/etching as a front propagating uniformly and perpendicular to the surface at every point. This uniform front propagation model was validated by performing ALD of amorphous Al2O3 using the TMA/O2 plasma. ALE from the TMA/SF6 plasma was also studied and resulted in faster smoothing than predicted by purely considering uniform front propagation. Correspondingly, it was found that for such an ALE process, a second mechanism contributes to the smoothing, hypothesized to be related to curvature-dependent surface fluorination. Individually, the atomic-scale processing techniques enable smoothing; however, ALD and ALE will need to be combined to achieve thin and smooth films, as is demonstrated and discussed in this work for multiple applications.
Platinum and platinum oxide films were deposited using thermal and remote plasma ALD. The combination of MeCpPtMe3 precursor and O2 gas or a short (0.5 s) O2 plasma exposure resulted in high purity, low-resistivity (15 microOhm cm), high-density (21 g/cm3), cubic platinum films. The combination of MeCpPtMe3 precursor with a longer (5 s) O2 plasma exposure resulted in semi-conductive PtO2 films with a band gap of ~1.5 eV. The long nucleation delay observed with the thermal process could be overcome by using the remote plasma process where atomic oxygen is provided from the gas phase. The ALD process dependence on substrate temperature was investigated, where both the thermal and the remote plasma Pt process revealed a temperature window from 200 C to 300 C and the remote plasma PtO2 process had a temperature window from 100 C to 300 C.
Atomic layer deposition (ALD) of noble metals has attracted much attention in recent years for the deposition of thin metal films, as well as for the synthesis of supported metallic nanoparticles. Noble metal surfaces and nanoparticles possess catalytic activity for dissociation of metalorganic precursor and O 2 molecules, which has important consequences for the reaction mechanisms of ALD. In this work, a case study is presented with respect to the importance of catalytic surface reactions during the nucleation and growth of Pt ALD, which serves as a model system for the growth of other noble metals by ALD. It is illustrated that atomic level understanding of these processes is vital for the development of novel nanopatterning and nanoparticle synthesis approaches.
In this note it is demonstrated that optical emission spectroscopy (OES) is an easy-to-implement and valuable tool to study, optimize, and monitor thin film growth by plasma-assisted atomic layer deposition (ALD). The species in the plasma can be identified through the analysis of the light emitted by the plasma. OES provides therefore information on the reactant species delivered to the surface by the plasma but it also yields unique insight into the surface reaction products and, as a consequence, on the reaction mechanisms of the deposition process. Time-resolved measurements reveal information about the amount of precursor dosing and length of plasma exposure needed to saturate the self-limiting half reactions, which is useful for the optimization of the ALD process. Furthermore, time-resolved OES can also be used as an easy-to-implement process monitoring tool for plasma-assisted ALD processes on production equipment; for example, to monitor reactor wall conditions or to detect process faults in real time.
The deposition of Pd and Pt nanoparticles by atomic layer deposition (ALD) has been studied extensively in recent years for the synthesis of nanoparticles for catalysis. For these applications, it is essential to synthesize nanoparticles with well-defined sizes and a high density on large-surface-area supports. Although the potential of ALD for synthesizing active nanocatalysts for various chemical reactions has been demonstrated, insight into how to control the nanoparticle properties (i.e. size, composition) by choosing suitable processing conditions is lacking. Furthermore, there is little understanding of the reaction mechanisms during the nucleation stage of metal ALD. In this work, nanoparticles synthesized with four different ALD processes (two for Pd and two for Pt) were extensively studied by transmission electron spectroscopy. Using these datasets as a starting point, the growth characteristics and reaction mechanisms of Pd and Pt ALD relevant for the synthesis of nanoparticles are discussed. The results reveal that ALD allows for the preparation of particles with control of the particle size, although it is also shown that the particle size distribution is strongly dependent on the processing conditions. Moreover, this paper discusses the opportunities and limitations of the use of ALD in the synthesis of nanocatalysts.
Metal fluorides typically have a low refractive index and a very high transparency and find many applications in optical and optoelectronic devices. Nearly stoichiometric, high-purity AlF3 films were deposited by atomic layer deposition (ALD) using trimethylaluminum [Al(CH3)3] and SF6 plasma. Self-limiting growth was confirmed and the growth per cycle was determined to range from 1.50 Å to 0.55 Å for deposition temperatures between 50 °C and 300 °C. In addition, the film density of ∼2.8 g cm−3 was found to be relatively close to the bulk value of 3.1 g cm−3. Vacuum ultraviolet spectroscopic ellipsometry measurements over the wavelength range of 140–2275 nm showed a refractive index n of 1.35 at 633 nm, and an extinction coefficient k of <10−4 above 300 nm, for all deposition temperatures. Optical emission spectroscopy during the SF6 plasma exposure step of the ALD cycle revealed the formation of C2H2 and CF2 species, resulting from the interaction of the plasma with the surface after Al(CH3)3 exposure. On the basis of these results, a reaction mechanism is proposed in which F radicals from the SF6 plasma participate in the surface reactions. Overall, this work demonstrates that SF6 plasma is a promising co-reactant for ALD of metal fluorides, providing an alternative to co-reactants such as metal fluorides, HF, or HF-pyridine.
Remote plasma atomic layer deposition (ALD) of TaNx films from Ta[N(CH3)2]5 and H2, H2-N2, and NH3 plasmas is reported. From film analysis by in situ spectroscopic ellipsometry and various ex situ techniques, data on growth rate, atomic composition, mass density, TaNx microstructure, and resistivity are presented for films deposited at substrate temperatures between 150 and 250°C. It is established that cubic TaNx films with a high mass density (12.1gcm−3) and low electrical resistivity (380μΩcm) can be deposited using a H2 plasma with the density and resistivity of the films improving with plasma exposure time. H2-N2 and NH3 plasmas resulted in N-rich Ta3N5 films with a high resistivity. It is demonstrated that the different TaNx phases can be distinguished in situ by spectroscopic ellipsometry on the basis of their dielectric function with the magnitude of the Drude absorption yielding information on the resistivity of the films. In addition, the saturation of the ALD surface reactions can be determined by monitoring the plasma emission, as revealed by optical emission spectroscopy.
Area-Selective Atomic Layer Deposition, AS-ALD, can be initiated by local area-activation 1,2 or -deactivation. 3 We studied the maskless growth of ZnO from diethyl zinc and water. The method is based on the different nucleation delays that arise from precursor adsorption on OH-terminated versus that on H-terminated Si-substrate. TEOS (tetraethyl orthosilicate) and H 2 O were used to deposit a SiO 2 seed layer by EBID (e-beam-induced deposition) in a pattern of 500x500 nm 2 dots on H-terminated a-Si:H substrate. After this local activation the substrate was coated with a ZnO film in a conventional ALD reactor using 80 cycles of diethyl zinc and water vapor. In situ spectroscopic ellipsometry (Fig. 1), SEM, cross-sectional TEM and energy dispersive X-ray spectroscopy (EDX) showed good areal selectivity with ZnO growth only occurring on the SiO 2 dots. This observation was corroborated by Density functional theory calculations suggesting a kinetically hindered surface reaction between diethyl zinc and H-terminated Si. Atomic Layer Etching, ALE, was also tested on a ZnO case. Current ALE technology is emerging in both thermal, isotropic and plasma-based, anisotropic approaches. 4 In this presentation we will demonstrate a plasma-assisted ALE-process driven by radicals, and therefore being isotropic. The ALE-process was tested at temperatures between 150 and 250 °C. We used alternating doses of acetylacetone (Hacac) and O 2 -plasma intermitted by Ar-purging. The ZnO-layer thickness as measured by spectroscopic ellipsometry decreased linearly with the number of cycles. In a synergy test, we proved that only the alternated dosing of Hacac and O 2 -plasma caused ZnO etching, whereas Hacac and O 2 -plasma alone did not (Fig. 2). Preliminary infrared studies suggest that Hacac forms volatile complexes by metal oxide surface chelation (e.g. Zn(acac) 2 ), whereas the O 2 -plasma step removes non-reactive Hacac fragments to refresh the surface for the next etching cycle. 5 TEM- and XPS-inspection indicated no damage of the ZnO surface, good preservation of the ZnO-stoichiometry throughout the etching process, and no distinct contamination. Furthermore, we demonstrated this ALE-process to be selective over SiO 2 . We believe that this novel plasma-assisted ALE-concept can be extended to several other materials. [1] A. J. M. Mackus et al. , Nanoscale , 4 , 4477 (2012). [2] R. Chen et al., Adv. Mater. , 18 , 1086 (2006). [3] A. Mameli et al. , Chem. Mater. 29 , 921 (2017). [4] D. R. Zywotko, et al. , Chem. Mater. 29 , 1183 (2017). [5] M. A. George, et al., J. Electrochem. Soc., 143 , 3257 (1996). Figure 1
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.
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