Atomic layer deposition chemistry: recent developments and future challenges

New materials, namely high-k (high-permittivity) dielectrics to replace SiO(2), Cu to replace Al, and barrier materials for Cu, are revolutionizing modern integrated circuits. These materials must be deposited as very thin films on structured surfaces. The self-limiting growth mechanism characteristic to atomic layer deposition (ALD) facilitates the control of film thickness at the atomic level and allows deposition on large and complex surfaces. These features make ALD a very promising technique for future integrated circuits. Recent ALD research has mainly focused on materials required in microelectronics. Chemistry, in particular the selection of suitable precursor combinations, is the key issue in ALD; many interesting results have been obtained by smart chemistry. ALD is also likely to find applications in other areas, such as magnetic recording heads, optics, demanding protective coatings, and micro-electromechanical systems, provided that cost-effective processes can be found for the materials required.     

Motion of nanodroplets near chemical heterogeneities

We investigate the dynamics of nanoscale droplets in the vicinity of chemical steps which separate parts of a substrate with different wettabilities. Due to long-ranged dispersion forces, nanodroplets positioned on one side of the step perceive the different character of the other side even at a finite distance from the step, leading to a dynamic response. The direction of the ensuing motion of such droplets depends not only on the difference between the equilibrium contact angles on these two parts but in particular on the difference between the corresponding Hamaker constants. Therefore, the motion is not necessarily directed toward the more wettable side and can also be different from that of droplets which span the step.     

Molecular dynamics simulation of two-sided chemical modification of carbon nanoribbons on a solid substrate

The position of a graphene nanoribbon on a solid substrate allows the chemical modification of only one of the nanoribbon sides. A method was proposed that enables the chemical modification of the other side, too. It was numerically modeled how a nanoribbon separated from a substrate rolls up into a roll and how the roll unrolls on a flat substrate. The dependences of the number of coils and the radii of rolls forming by hydrogenation on the nanoribbon length and width were determined.     

Inherently Area-Selective Atomic Layer Deposition of Manganese Oxide through Electronegativity-Induced Adsorption

Manganese oxide (MnO_x) shows great potential in the areas of nano-electronics, magnetic devices and so on. Since the characteristics of precise thickness control at the atomic level and self-align lateral patterning, area-selective deposition (ASD) of the MnO_x films can be used in some key steps of nanomanufacturing. In this work, MnO_x films are deposited on Pt, Cu and SiO₂ substrates using Mn(EtCp)₂ and H₂O over a temperature range of 80–215 °C. Inherently area-selective atomic layer deposition (ALD) of MnO_x is successfully achieved on metal/SiO₂ patterns. The selectivity improves with increasing deposition temperature within the ALD window. Moreover, it is demonstrated that with the decrease of electronegativity differences between M (M = Si, Cu and Pt) and O, the chemisorption energy barrier decreases, which affects the initial nucleation rate. The inherent ASD aroused by the electronegativity differences shows a possible method for further development and prediction of ASD processes.     

Chemical patterning of ultrathin polymer films by direct-write multiphoton lithography

We applied 2-photon laser ablation to write subdiffraction nanoscale chemical patterns into ultrathin polymer films under ambient conditions. Poly(ethylene glycol) methacrylate brush layers were prepared on quartz substrates via surface-initiated atom-transfer radical polymerization and ablated to expose the underlying substrate using the nonlinear 2-photon absorbance of a frequency-doubled Ti:sapphire femtosecond laser. Single-shot ablation thresholds of polymer films were ~1.5 times smaller than that of a quartz substrate, which allowed patterning of nanoscale features without damage to the underlying substrate. At a 1/e(2) laser spot diameter of 0.86 μm, the features of exposed substrate approached ~80 nm, well below the diffraction limit for 400 nm light. Ablated features were chemically distinct and amenable to chemical modification.     

Site‐Targeted Interfacial Solid‐Phase Chemistry: Surface Functionalization of Organic Monolayers via Chemical Transformations Locally Induced at the Boundary between Two Solids

Effective control of chemistry at interfaces is of fundamental importance for the advancement of methods of surface functionalization and patterning that are at the basis of many scientific and technological applications. A conceptually new type of interfacial chemical transformations has been discovered, confined to the contact surface between two solid materials, which may be induced by exposure to X‐rays, electrons or UV light, or by the application of electrical bias. One of the reacting solids is a removable thin film coating that acts as a reagent/catalyst in the chemical modification of the solid surface on which it is applied. Given the diversity of thin film coatings that may be used as solid reagents/catalysts and the lateral confinement options provided by the use of irradiation masks, conductive AFM probes or stamps, and electron beams in such solid‐phase reactions, this approach is suitable for precise targeting of different desired chemical modifications to predefined surface sites spanning the macro‐ to nanoscale.     

Atomic layer deposition of metal-oxide thin films on cellulose fibers

Abstract

Atomic layer deposition (ALD) is a vapor-phase technique capable of producing inorganic thin films with precise control over the thickness of the film. The ALD method offers high precision in the design of advanced 3D nanostructures. In this article, silica and alumina thin films have been grown over fibers of cellulose by the ALD process. The morphology and the chemical composition of the fabricated thin films are characterized, as well as their thermal durability through elevated temperatures. Moreover, XPS is used to confirm the phases of the alumina nanofilms and to further understand the deposition process on the cellulose microfibers.     

Nanocomposite ta-C films prepared by the filtered vacuum arc process using nanosized Ni dots on a Si substrate

Recently, structural manipulation of tetrahedral amorphous carbon (ta-C) film at the nanometer scale has attracted much attention. We demonstrate a novel method to obtain a nanocomposite film where nanoscale columns of graphitic phase are embedded in a tetrahedral amorphous carbon matrix. When using a Si substrate with nanosized Ni dots on the surface, graphitic columns grew selectively on the Ni dots, while a dense ta-C film was deposited on the bare Si surface. The growth of the graphitic columns is closely related to the nanosized Ni dots that catalyze the graphitic-carbon formation in a filtered vacuum arc deposition condition.     

Superior Photostability and Photocatalytic Activity of ZnO Nanoparticles Coated with Ultrathin TiO2 Layers through Atomic‐Layer Deposition

Atomic‐layer deposition (ALD) is a thin‐film growth technology that allows for conformal growth of thin films with atomic‐level control over their thickness. Although ALD is successful in the semiconductor manufacturing industry, its feasibility for nanoparticle coating has been less explored. Herein, the ALD coating of TiO₂ layers on ZnO nanoparticles by employing a specialized rotary reactor is demonstrated. The photocatalytic activity and photostability of ZnO nanoparticles coated with TiO₂ layers by ALD and chemical methods were examined by the photodegradation of Rhodamine B dye under UV irradiation. Even though the photocatalytic activity of the presynthesized ZnO nanoparticles is higher than that of commercial P25 TiO₂ nanoparticles, their activity tends to decline due to severe photocorrosion. The chemically synthesized TiO₂ coating layer on ZnO resulted in severely declined photoactivity despite the improved photostability. However, ultrathin and conformal ALD TiO₂ coatings (≈0.75–1.5 nm) on ZnO improved its photostability without degradation of photocatalytic activity. Surprisingly, the photostability is comparable to that of pure TiO₂, and the photocatalytic activity to that of pure ZnO.     

Substrate- and time-dependent photoluminescence of quantum dots inside the ultrathin polymer LbL film

The photoluminescence of CdSe/ZnS quantum dots (QDs) in different configurations at solid surfaces (glass, silicon, PDMS, and metals) is considered for three types of organization: QDs directly adsorbed on solid surfaces, separated from the solid surface by a nanoscale polymer film with different thickness, and encapsulated into a polymer film. The complete suppression of photoluminescence for QDs on conductive metal surfaces (copper, gold) indicated a strong quenching effect. The temporal variation of the photoluminescent intensity on other substrates (glass, silicon, and PDMS) can be tuned by placing the nanoscale (3-50 nm) LbL polymer film between QDs and the substrate. The photooxidation and photobleaching processes of QD nanoparticles in the vicinity of the solid surface can be tuned by proper selection of the substrate and the dielectric nanoscale polymer film placed between the substrate and QDs. Moreover, the encapsulation of QD nanoparticles into the polymer film resulted in a dramatic initial increase in the photoemission intensity due to the accelerated photooxidation process. The phenomenon of enhanced photoemission of QDs encapsulated into the ultrathin polymer film provides not only the opportunity for making flexible, ultrathin, QD-containing polymer films, transferable to any microfabricated substrate, but also improved light emitting properties.     

Initial surface reactions of TiO2 atomic layer deposition onto SiO2 surfaces: density functional theory calculations

We present a density functional theory (DFT) study of the initial surface reactions of TiO2 deposition onto a SiO2 substrate using atomic layer deposition (ALD). The precursors for the deposition process were chosen to be TiCl4 and H2O, and several cluster models were used for the SiO2 substrate. We predict the activation barriers, transition states, and reaction pathways of the surface reactions, and we investigate the effect of surface heterogeneity (such as the presence of siloxane bridges) on the reactivity of the SiO2 surface. Our study suggests that the concentration and arrangement of different reactive groups on the substrate will strongly dictate the process of film growth during ALD, including the film morphology and the growth rate.     

Study of the Interface Microstructures of CVD Diamond Films by TEM

 The characteristics of the interface microstructures between a CVD diamond film and the silicon substrate have been studied by transmission electron microscopy and electron energy loss spectroscopy. The investigations are performed on plan-view TEM specimens which were intentionally thinned only from the film surface side allowing the overall microstructural features of the interface to be studied. A prominent interfacial layer with amorphous-like features has been directly observed for CVD diamond films that shows a highly twinned defective diamond surface morphology. Similar interfacial layers have also been observed on films with a <100> growth texture but having the {100} crystal faces randomly oriented on the silicon substrate. These interfacial layers have been unambiguously identified as diamond phase carbon by both electron diffraction and electron energy loss spectroscopy. For the CVD diamond films that exhibit heteroepitaxial growth features, with the {100} crystal faces aligned crystallographically on the silicon substrate, such an interfacial layer was not observed. This is consistent with the expectation that the epitaxial growth of CVD diamond films requires diamond crystals to directly nucleate and grow on the substrate surface or on an epitaxial interface layer that has a small lattice misfit to both the substrate and the thin film material.     

Ultrathin film characterization: photoreactive polyacrylamide

The structure and properties of photoreactive polyacrylamide thin films suitable for medical devices are presented. Using a solution deposition process, we report on the influences of polymer concentration, substrate residence time in solution and UV illumination upon the film structure, wettability and frictional properties. Ellipsometry, atomic force microscopy and lateral force microscopy show that increasing polymer concentration and illumination increased the film thickness and uniformity. Dynamic contact angles and frictional coefficients of the modified surfaces depend upon the film structure and thickness for films less than 40Å thick. We also demonstrate the potential of lateral force microscopy for investigating tribology at the nanoscale level.     

Preparation and Properties of Sol–Gel Thin Film Containing Rhodamine B Derivative with Ethoxy Silano Group on Organic Substrate and its Application to Surface-Treatment Thin Film for Display

A thin film containing rhodamine B derivative with ethoxy silano group was formed on organic film substrate using the sol–gel method. Rhodamine B derivative with a triethoxysilano group, SiO₂ sol and acrylic polymer having a triethoxy group were reacted in alcohol to give a coating solution for film formation, followed to be roll-coated on polyethylene terephthalate (PET) film and heat-treated at 130°C. This thin film consists of inorganic polymer (SiO₂), organic polymer (acrylic resin) and organic dye. These component parts become interconnected through mutual chemical bonding. This thin film has an absorption peak at 578 nm and superb water resistant characteristics (almost no dye elution in 50°C water for 150 min) as a result of chemical bonding between the organic dye and the matrix skeleton. It also has good flexibility. The film can be used as a wavelength-selective absorption film for displays to improve contrast.     

Multiple keys for a single lock: the unusual structural plasticity of the nucleotidyltransferase (4')/kanamycin complex

The most common mode of bacterial resistance to aminoglycoside antibiotics is the enzyme-catalysed chemical modification of the drug. Over the last two decades, significant efforts in medicinal chemistry have been focused on the design of non- inactivable antibiotics. Unfortunately, this strategy has met with limited success on account of the remarkably wide substrate specificity of aminoglycoside-modifying enzymes. To understand the mechanisms behind substrate promiscuity, we have performed a comprehensive experimental and theoretical analysis of the molecular-recognition processes that lead to antibiotic inactivation by Staphylococcus aureus nucleotidyltransferase 4'(ANT(4')), a clinically relevant protein. According to our results, the ability of this enzyme to inactivate structurally diverse polycationic molecules relies on three specific features of the catalytic region. First, the dominant role of electrostatics in aminoglycoside recognition, in combination with the significant extension of the enzyme anionic regions, confers to the protein/antibiotic complex a highly dynamic character. The motion deduced for the bound antibiotic seem to be essential for the enzyme action and probably provide a mechanism to explore alternative drug inactivation modes. Second, the nucleotide recognition is exclusively mediated by the inorganic fragment. In fact, even inorganic triphosphate can be employed as a substrate. Third, ANT(4') seems to be equipped with a duplicated basic catalyst that is able to promote drug inactivation through different reactive geometries. This particular combination of features explains the enzyme versatility and renders the design of non-inactivable derivatives a challenging task.     

Nanoscale patterning of two metals on silicon surfaces using an ABC triblock copolymer template

Patterning technologically important semiconductor interfaces with nanoscale metal films is important for applications such as metallic interconnects and sensing applications. Self-assembling block copolymer templates are utilized to pattern an aqueous metal reduction reaction, galvanic displacement, on silicon surfaces. Utilization of a triblock copolymer monolayer film, polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP-b-PEO), with two blocks capable of selective transport of different metal complexes to the surface (PEO and P2VP), allows for chemical discrimination and nanoscale patterning. Different regions of the self-assembled structure discriminate between metal complexes at the silicon surface, at which time they undergo the spontaneous reaction at the interface. Gold deposition from gold(III) compounds such as HAuCl4(aq) in the presence of hydrofluoric acid mirrors the parent block copolymer core structure, whereas silver deposition from Ag(I) salts such as AgNO3(aq) does the opposite, localizing exclusively under the corona. By carrying out gold deposition first and silver second, sub-100-nm gold features surrounded by silver films can be produced. The chemical selectivity was extended to other metals, including copper, palladium, and platinum. The interfaces were characterized by a variety of methods, including scanning electron microscopy, scanning Auger microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy.     

Rapid micropatterning of mesoporous silica film by site-selective low-energy electron beam irradiation

Rapid microfabrication of mesoporous silica film at low temperature was achieved with low-energy electron beam (LEEB) irradiation. A mesostructured film (thickness approximately 200 nm), which was prepared through hydrolysis and condensation of tetramethoxysilane in the presence of hexadecyltrimethylammonium chloride, was irradiated with LEEB at 25 kV and 300 microA under pressures of 10 and 1000 Pa. The surfactant molecules can be eliminated completely at temperatures less than 40 degrees C after only 10 min (10 Pa) and 5 min (1000 Pa) of irradiation, resulting in conversion to a highly ordered mesoporous silica film without cracking. The LEEB-irradiated film also showed reasonable chemical resistance toward dilute hydrofluoric acid solution due to sufficient consolidation by cross-linking of silicate networks during the irradiation. The unirradiated regions were etched away preferentially to the irradiated areas; therefore, rapid micropatterning of the mesoporous silica film was possible by area-selective LEEB irradiation followed by chemical etching.     

Atomistic simulation of the formation of nanoporous silica films via molecular chemical vapor deposition on nonporous substrates

To distinguish thin deposited film characteristics clearly from the influence of substrate morphological properties, the growth mechanism and the macroscale and nanoscale properties of nanoporous SiO(2) films deposited on nonporous silica (SiO(2)) substrates from chemical precursors Si(OH)(4) and TEOS (tetraethoxysilane) via low-pressure chemical vapor deposition are the primary targets of this study. This work employs a kinetic Monte Carlo (KMC) simulation method coupled to the Metropolis Monte Carlo method to relax the strained silica structure. The influence of the deposition temperature (473, 673, and 873 K) on the properties of the SiO(x) layers is addressed via analysis of the film growth rates, density profiles of the deposited thin films, pore size distributions, carbon depth profiles (with respect to TEOS), and voidage analysis for layers of different thicknesses (8-18 nm). A comparison of simulation with experimental results is also carried out.     

Atomic layer deposition of TiO2 from TiI4 and H2O onto SiO2 surfaces: ab initio calculations of the initial reaction mechanisms

The initial surface reactions involved in the atomic layer deposition (ALD) of TiO2 from TiI4 and H2O onto a SiO2 substrate have been investigated using electronic structure calculations based on cluster models. The detailed atomic growth mechanisms on different types of functionalities on the SiO2 substrate have been proposed. The effects of quantum tunneling and hindered rotations of adsorbates on the rate of surface reactions have been investigated. The effects of tunneling were found to be negligible for all reactions, because typical ALD temperatures range from 150 to 450 degrees C. However, the rotational contributions to the rate constants must be taken into account in certain cases. All of the three surface functionalities investigated exhibit high chemical reactivity toward TiI4 precursors at typical ALD temperatures. The rate constants of the second half-reactions between Ti intermediates and H2O are 5-8 orders of magnitude smaller than the first half-reactions between TiI4 and the surface functionalities. Although the iodine release reaction has been used to explain previous experimental measurements, it is predicted to be unfavorable (kinetically and thermodynamically) and is unlikely to occur at typical ALD temperatures. Substitution of TiI4 with TiCl4 as the metal precursor can increase the binding energies of the absorbates onto the surface due to the high electronegativity of the Cl ligands. However, the activation barriers are not significantly different between these two metal precursors. More importantly, our calculations predict that TiI4 precursors tend to produce TiO2 films with fewer impurities than the TiCl4 precursors.     

Characterization of chemical heterogeneity in polymer systems using hydrolysis and tapping‐mode atomic force microscopy

Characterization of polymer coatings microstructure is critical to the fundamental understanding of the corrosion of coated metals. An approach for mapping the chemical heterogeneity of a polymer system using chemical modification and tapping‐mode atomic force microscopy (TMAFM) is demonstrated. This approach is based on the selective degradation of one of the phases in a multiphase polymer blend system and the ability of TMAFM to provide nanoscale lateral information about the different phases in the polymer system. Films made of a 70:30 polyethyl acrylate/polystyrene (PEA/PS) blend were exposed to a hydrolytic acidic environment and analyzed using TMAFM. Pits were observed to form in the PEA/PS blend films, and this degradation behavior was similar to that of the PEA material. Using these results, the domains in the 70:30 blend were identified as the PS‐rich regions and the matrix as the PEA‐rich region. This conclusion was confirmed by Fourier transform infrared‐attenuated total reflection analyses that revealed the hydrolysis of the PEA material. TMAFM phase imaging was also used to follow pit growth of the blend as a function of exposure time. The usefulness of the chemical modification/AFM imaging approach in understanding the degradation process of a coating film is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci B Part B: Polym Phys 39: 1460–1470, 2001