Enhanced Deposition Selectivity of High-k Dielectrics by Vapor Dosing and Selective Removal of Phosphonic Acid Inhibitors.

Area-selective atomic layer deposition (AS-ALD), which provides a bottom-up nanofabrication method with atomic-scale precision, has attracted a great deal of attention as a means to alleviate the problems associated with conventional top-down patterning. In this study, we report a methodology for achieving selective deposition of high-k dielectrics by surface modification through vapor-phase functionalization of octadecylphosphonic acid (ODPA) inhibitor molecules accompanied by post-surface treatment. A comparative evaluation of deposition selectivity of ZrO2 thin films deposited with the O2 and O3 reactants was performed on SiO2, TiN, and W substrates, and we confirmed that high enough deposition selectivity over 10 nm can be achieved even after 200 cycles of ALD with the O2 reactant. Subsequently, the electrical properties of ZrO2 films deposited with O2 and O3 reactants were investigated with and without post-deposition treatment. We successfully demonstrated that high-quality ZrO2 thin films with high dielectric constants and stable antiferroelectric properties can be produced by subjecting the films to ozone, which can eliminate carbon impurities within the films. We believe that this work provides a new strategy to achieve highly selective deposition for AS-ALD of dielectric on dielectric (DoD) applications toward upcoming bottom-up nanofabrication.

Efficient access to general α-tertiary amines via water-accelerated organocatalytic multicomponent allylation.

A tetrasubstituted carbon atom connected by three sp3 or sp2-carbons with single nitrogen, i.e., the α-tertiary amine (ATA) functional group, is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. The synthetic approach toward ATA structures is intricate, therefore, a straightforward catalytic method has remained a substantial challenge. Here we show an efficient water-accelerated organocatalytic method to directly access ATA incorporating homoallylic amine structures by exploiting readily accessible general ketones as useful starting material. The synergistic action of a hydrophobic Brønsted acid in combination with a squaramide hydrogen-bonding donor under aqueous condition enabled the facile formation of the desired moiety. The developed exceptionally mild but powerful system facilitated a broad substrate scope, and enabled efficient multi-gram scalability.

Advanced Atomic Layer Deposition: Ultrathin and Continuous Metal Thin Film Growth and Work Function Control Using the Discrete Feeding Method.

The ultrathin and continuous ruthenium (Ru) film was deposited through an improved atomic layer deposition (ALD) process with a discrete feeding method (DFM), called DF-ALD, employing a cut-in purge step during the precursor feeding. The excess precursor molecules can be physically adsorbed onto the chemisorbed precursors on the substrate during precursor feeding, which screens the reactive sites on the surface. Using DF-ALD, surface coverage of precursors was enhanced because the cut-in purge removes the physisorbed precursors securing the reactive sites beneath them; thus, nucleation density was greatly increased. Therefore, the grain size decreased, which changed the microstructure and increased oxygen impurity concentration. However, a more metallic Ru thin film was formed due to thermodynamic stability and improved physical density. Consequently, DF-ALD enables the deposition of the ultrathin (3 nm) and continuous Ru film with a low resistivity of ∼60 µΩ cm and a high effective work function of ∼4.8 eV.

Synthesis of a Hybrid Nanostructure of ZnO-Decorated MoS2 by Atomic Layer Deposition.

We introduce the synthesis of hybrid nanostructures comprised of ZnO nanocrystals (NCs) decorating nanosheets and nanowires (NWs) of MoS2 prepared by atomic layer deposition (ALD). The concentration, size, and surface-to-volume ratio of the ZnO NCs can be systematically engineered by controlling both the number of ZnO ALD cycles and the properties of the MoS2 substrates, which are prepared by sulfurizing ALD MoO3. Analysis of the chemical composition combined with electron microscopy and synchrotron X-ray techniques as a function of the number of ZnO ALD cycles, together with the results of quantum chemical calculations, help elucidate the ZnO growth mechanism and its dependence on the properties of the MoS2 substrate. The defect density and grain size of MoS2 nanosheets are controlled by the sulfurization temperature of ALD MoO3, and the ZnO NCs in turn nucleate selectively at defect sites on MoS2 surface and enlarge with increasing ALD cycle numbers. At higher ALD cycle numbers, the coalescence of ZnO NCs contributes to an increase in areal coverage and NC size. Additionally, the geometry of the hybrid structures can be tuned by changing the dimensionality of the MoS2, by employing vertical NWs of MoS2 as the substrate for ALD ZnO NCs, which leads to improvement of the relevant surface-to-volume ratio. Such materials are expected to find use in newly expanded applications, especially those such as sensors or photodevices based on a p-n heterojunction which relies on coupling transition-metal dichalcogenides with NCs.

Analysis of Defect Recovery in Reduced Graphene Oxide and Its Application as a Heater for Self-Healing Polymers.

Reduced graphene oxide (RGO) obtained from graphene oxide has received much attention because of its simple and cost-effective manufacturing process. Previous studies have demonstrated the scalable production of RGO with relatively high quality; however, irreducible defects on RGO deteriorate the unique intrinsic physical properties of graphene, such as high-mobility electrical charge transport, limiting its potential applicability. Using the enhanced chemical reactivity of such defects, atomic layer deposition (ALD) can be a useful method to selectively passivate the defect sites. Herein, we analyzed the selective formation of Pt by ALD on the defect sites of RGO and investigated the effect of Pt formation on the electrical properties of RGO by using ultrafast terahertz (THz) laser spectroscopy. Time-resolved THz measurements directly corroborated that the degree of the defect-recovering property of ALD Pt-treated RGO appearing as Auger-type sub-picosecond relaxation, which is otherwise absent in pristine RGO. In addition, the conductivity improvement of Pt-recovered RGO was theoretically explained by density functional theory calculations. The ALD Pt-passivated RGO yielded a superior platform for the fabrication of a highly conductive and transparent graphene heater. By using the ALD Pt/RGO heater embedded underneath scratched self-healing polymer materials, we also demonstrated the effective recovery property of self-healing polymers with high-performance heating capability. Our work is expected to result in significant advances toward practical applications for RGO-based flexible and transparent electronics.

Effects of Ar Addition to O2 Plasma on Plasma-Enhanced Atomic Layer Deposition of Oxide Thin Films.

A method for significantly increasing the growth rates (GRs) of high- k oxide thin films grown via plasma-enhanced atomic layer deposition (PE-ALD) by enhancing the plasma density through the addition of Ar gas to the O2 plasma oxidant was developed. This approach led to improvements of ∼60% in the saturation GRs of PE-ALD ZrO2, HfO2, and SiO2. Furthermore, despite the significantly higher GR enabled by PE-ALD, the mechanical and dielectric properties of the PE-ALD oxide films were similar or even superior to those of films grown via the conventional O2 plasma process. Optical emission spectroscopy analyses in conjunction with theoretical calculation of the electron energy distribution function revealed that adding Ar gas to the O2 plasma increased the density of high-energy electrons, thereby generating more O2 plasma species, such as ions and radicals, which played a key role in improving the GRs and the properties of the films. This promising approach is expected to facilitate the high-volume manufacturing of films via PE-ALD, especially for use as gate insulators in thin-film transistor-based devices in the display industry.

Effects of a Nb nanopin electrode on the resistive random-access memory switching characteristics of NiO thin films.

We report the effects of bottom electrode shapes on resistive random-access memory (RRAM) devices composed of Nb (bottom electrode)/NiO (dielectric)/Nb (top electrode) structures. By adopting a nano-fabrication process using an anodic aluminum oxide (AAO) nanotemplate, a well-aligned Nb nanopin array bottom electrode was formed on the surface of a Si substrate. For comparison, a Nb thin film was employed as a different type of bottom electrode. Then, a NiO thin film dielectric was prepared on both the Nb bottom electrodes via a spin coating method, followed by Nb sputtering for the Nb top electrode. Both the RRAM devices with Nb nanopin and thin film bottom electrodes exhibited typical unipolar resistive switching behavior. However, a lower SET/RESET voltage was observed for the Nb nanopin electrode compared to the Nb thin film electrode by virtue of an enhanced electric field induced by the nanopin-shaped electrode. More significantly, on the basis of endurance and retention characteristics, the Nb nanopin electrode played a key role in minimizing the dispersion of the low- and high-resistance state currents and the variation in the SET/RESET voltage by developing more-concise conducting filaments in the conducting path. Therefore, we foresee that this approach can provide an insight into the optimal design of RRAM devices.

Area-Selective Atomic Layer Deposition of Metal Oxides on Noble Metals through Catalytic Oxygen Activation.

Area-selective atomic layer deposition (ALD) is envisioned to play a key role in next-generation semiconductor processing and can also provide new opportunities in the field of catalysis. In this work, we developed an approach for the area-selective deposition of metal oxides on noble metals. Using O2 gas as co-reactant, area-selective ALD has been achieved by relying on the catalytic dissociation of the oxygen molecules on the noble metal surface, while no deposition takes place on inert surfaces that do not dissociate oxygen (i.e., SiO2, Al2O3, Au). The process is demonstrated for selective deposition of iron oxide and nickel oxide on platinum and iridium substrates. Characterization by in situ spectroscopic ellipsometry, transmission electron microscopy, scanning Auger electron spectroscopy, and X-ray photoelectron spectroscopy confirms a very high degree of selectivity, with a constant ALD growth rate on the catalytic metal substrates and no deposition on inert substrates, even after 300 ALD cycles. We demonstrate the area-selective ALD approach on planar and patterned substrates and use it to prepare Pt/Fe2O3 core/shell nanoparticles. Finally, the approach is proposed to be extendable beyond the materials presented here, specifically to other metal oxide ALD processes for which the precursor requires a strong oxidizing agent for growth.

Enhanced Light Stability of InGaZnO Thin-Film Transistors by Atomic-Layer-Deposited Y2O3 with Ozone.

We report the effect of Y2O3 passivation by atomic layer deposition (ALD) using various oxidants, such as H2O, O2 plasma, and O3, on In-Ga-Zn-O thin-film transistors (IGZO TFTs). A large negative shift in the threshold voltage (Vth) was observed in the case of the TFT subjected to the H2O-ALD Y2O3 process; this shift was caused by a donor effect of negatively charged chemisorbed H2O molecules. In addition, degradation of the IGZO TFT device performance after the O2 plasma-ALD Y2O3 process (field-effect mobility (µ) = 8.7 cm2/(V·s), subthreshold swing (SS) = 0.77 V/dec, and Vth = 3.7 V) was observed, which was attributed to plasma damage on the IGZO surface adversely affecting the stability of the TFT under light illumination. In contrast, the O3-ALD Y2O3 process led to enhanced device stability under light illumination (ΔVth = -1 V after 3 h of illumination) by passivating the subgap defect states in the IGZO surface region. In addition, TFTs with a thicker IGZO film (55 nm, which was the optimum thickness under the current investigation) showed more stable device performance than TFTs with a thinner IGZO film (30 nm) (ΔVth = -0.4 V after 3 h of light illumination) by triggering the recombination of holes diffusing from the IGZO surface to the insulator-channel interface. Therefore, we envisioned that the O3-ALD Y2O3 passivation layer suggested in this paper can improve the photostability of TFTs under light illumination.

Reaction Mechanism of Area-Selective Atomic Layer Deposition for Al2O3 Nanopatterns.

The reaction mechanism of area-selective atomic layer deposition (AS-ALD) of Al2O3 thin films using self-assembled monolayers (SAMs) was systematically investigated by theoretical and experimental studies. Trimethylaluminum (TMA) and H2O were used as the precursor and oxidant, respectively, with octadecylphosphonic acid (ODPA) as an SAM to block Al2O3 film formation. However, Al2O3 layers began to form on the ODPA SAMs after several cycles, despite reports that CH3-terminated SAMs cannot react with TMA. We showed that TMA does not react chemically with the SAM but is physically adsorbed, acting as a nucleation site for Al2O3 film growth. Moreover, the amount of physisorbed TMA was affected by the partial pressure. By controlling it, we developed a new AS-ALD Al2O3 process with high selectivity, which produces films of ∼60 nm thickness over 370 cycles. The successful deposition of Al2O3 thin film patterns using this process is a breakthrough technique in the field of nanotechnology.

Incomplete elimination of precursor ligands during atomic layer deposition of zinc-oxide, tin-oxide, and zinc-tin-oxide.

For atomic layer deposition (ALD) of doped, ternary, and quaternary materials achieved by combining multiple binary ALD processes, it is often difficult to correlate the material properties and growth characteristics with the process parameters due to a limited understanding of the underlying surface chemistry. In this work, in situ Fourier transform infrared (FTIR) spectroscopy was employed during ALD of zinc-oxide, tin-oxide, and zinc-tin-oxide (ZTO) with the precursors diethylzinc (DEZ), tetrakis(dimethylamino)tin (TDMASn), and H2O. The main aim was to investigate the molecular basis for the nucleation delay during ALD of ZTO, observed when ZnO ALD is carried out after SnO2 ALD. Gas-phase FTIR spectroscopy showed that dimethylamine, the main reaction product of the SnO2 ALD process, is released not only during SnO2 ALD but also when depositing ZnO after SnO2, indicating incomplete removal of the ligands of the TDMASn precursor from the surface. Transmission FTIR spectroscopy performed during ALD on SiO2 powder revealed that a significant fraction of the ligands persist during both SnO2 and ZnO ALD. These observations provide experimental evidence for a recently proposed mechanism, based on theoretical calculations, suggesting that the elimination of precursor ligands is often not complete. In addition, it was found that the removal of precursor ligands by H2O exposure is even less effective when ZnO ALD is carried out after SnO2 ALD, which likely causes the nucleation delay in ZnO ALD during the deposition of ZTO. The underlying mechanisms and the consequences of the incomplete elimination of precursor ligands are discussed.

A Process for Topographically Selective Deposition on 3D Nanostructures by Ion Implantation.

Area-selective atomic layer deposition (AS-ALD) is attracting increasing interest because of its ability to enable both continued dimensional scaling and accurate pattern placement for next-generation nanoelectronics. Here we report a strategy for depositing material onto three-dimensional (3D) nanostructures with topographic selectivity using an ALD process with the aid of an ultrathin hydrophobic surface layer. Using ion implantation of fluorocarbons (CFx), a hydrophobic interfacial layer is formed, which in turn causes significant retardation of nucleation during ALD. We demonstrate the process for Pt ALD on both blanket and 2D patterned substrates. We extend the process to 3D structures, demonstrating that this method can achieve selective anisotropic deposition, selectively inhibiting Pt deposition on deactivated horizontal regions while ensuring that only vertical surfaces are decorated during ALD. The efficacy of the approach for metal oxide ALD also shows promise, though further optimization of the implantation conditions is required. The present work advances practical applications that require area-selective coating of surfaces in a variety of 3D nanostructures according to their topographical orientation.

An atomic layer deposition chamber for in situ x-ray diffraction and scattering analysis.

The crystal structure of thin films grown by atomic layer deposition (ALD) will determine important performance properties such as conductivity, breakdown voltage, and catalytic activity. We report the design of an atomic layer deposition chamber for in situ x-ray analysis that can be used to monitor changes to the crystal structural during ALD. The application of the chamber is demonstrated for Pt ALD on amorphous SiO2 and SrTiO3 (001) using synchrotron-based high resolution x-ray diffraction, grazing incidence x-ray diffraction, and grazing incidence small angle scattering.

Confinement of ferroelectric domain-wall motion at artificially formed conducting-nanofilaments in epitaxial BiFeO3 thin films.

We report confinement of ferroelectric domain-wall motion at conducting-nanofilament wall in epitaxial BiFeO3 thin film on Nb-doped SrTiO3 substrate. The BiFeO3 film exhibited well-defined ferroelectric response and unipolar resistive switching behavior. We artificially formed conducting-nanofilaments in the BiFeO3 via conducting atomic force microscope techniques. The conducting-nanofilament wall, which does not possess any ferroelectric polarization, is then able to block domain propagation. Consequently, we demonstrate that the domain-wall motion is effectively confined within the conducting-nanofilament wall during polarization switching. This significant new insight potentially gives an opportunity for the artificial manipulation of nanoscale ferroelectric domain.

Thickness and post-annealing effects of the sputtered La-capping layer inserted between the TiN gate and Hf-based dielectrics.

We investigated effects of the sputtered La-capping layer inserted between TiN and Hf-based dielectrics, HfO2 and HfSiO4/HfO2, mainly focusing on effective work function (EWF) and equivalent oxide thickness (EOT) changes by modulation of its thickness and post-metal annealing (PMA). The use of thin La capping up to 5 Å showed a linear flatband voltage (V(FB)) shift of -60 mV/Å, regardless of gate dielectrics. However, with the increase of the La thickness, a slight increase in EOT was observed for HfO2, whereas a negligible change in EOT was shown for the HfSiO4/HfO2 bilayer. It might be ascribed to the facile La oxidation, which acts as an additional oxide layer on both of the gate dielectrics. Meanwhile, high-temperature PMA exhibited slight reduction in V(FB) as well as an EOT increase for both of the Hf-based dielectrics. On the basis of X-ray photoelectron spectroscopy (XPS) results, the reason for the slightly decreased EWF resulted from two competing dipoles formed by movements of oxygen vacancies (V(O)) and La atoms during the PMA. Additionally, oxygen affinity and diffusion of the La-capping layer on both of the gate dielectrics are further discussed in conjunction with thermodynamic analyses, and thereby, schematic energy band diagrams were proposed by taking into account competing dipole layers by VO movement and La diffusion.

Ru nanostructure fabrication using an anodic aluminum oxide nanotemplate and highly conformal Ru atomic layer deposition.

We fabricated metallic nanostructures directly on Si substrates through a hybrid nanoprocess combining atomic layer deposition (ALD) and a self-assembled anodic aluminum oxide (AAO) nanotemplate. ALD Ru films with Ru(DMPD)(EtCp) as a precursor and O(2) as a reactant exhibited high purity and low resistivity with negligible nucleation delay and low roughness. These good growth characteristics resulted in the excellent conformality for nanometer-scale vias and trenches. Additionally, AAO nanotemplates were fabricated directly on Si and Ti/Si substrates through a multiple anodization process. AAO nanotemplates with various hole sizes (30-100 nm) and aspect ratios (2:1-20:1) were fabricated by controlling the anodizing process parameters. The barrier layers between AAO nanotemplates and Si substrates were completely removed by reactive ion etching (RIE) using BCl(3) plasma. By combining the ALD Ru and the AAO nanotemplate, Ru nanostructures with controllable sizes and shapes were prepared on Si and Ti/Si substrates. The Ru nanowire array devices as a platform for sensor devices exhibited befitting properties of good ohmic contact and high surface/volume ratio.