Preface: Special Topic on Atomic and Molecular Layer Processing: Deposition, Patterning, and Etching.

Thin film processing technologies that promise atomic and molecular scale control have received increasing interest in the past several years, as traditional methods for fabrication begin to reach their fundamental limits. Many of these technologies involve at their heart phenomena occurring at or near surfaces, including adsorption, gas-surface reactions, diffusion, desorption, and re-organization of near-surface layers. Moreover many of these phenomena involve not just reactions occurring under conditions of local thermodynamic equilibrium but also the action of energetic species including electrons, ions, and hyperthermal neutrals. There is a rich landscape of atomic and molecular scale interactions occurring in these systems that is still not well understood. In this Special Topic Issue of The Journal of Chemical Physics, we have collected recent representative examples of work that is directed at unraveling the mechanistic details concerning atomic and molecular layer processing, which will provide an important framework from which these fields can continue to develop. These studies range from the application of theory and computation to these systems to the use of powerful experimental probes, such as X-ray synchrotron radiation, probe microscopies, and photoelectron and infrared spectroscopies. The work presented here helps in identifying some of the major challenges and direct future activities in this exciting area of research involving atomic and molecular layer manipulation and fabrication.

Effect of Co-Reactants on Interfacial Oxidation in Atomic Layer Deposition of Oxides on Metal Surfaces.

We have examined the atomic layer deposition (ALD) of Al2O3 using TMA as the precursor and t-BuOH and H2O as the co-reactants, focusing on the effects of the latter on both the ALD process and the possible modification of the underlying substrate. We employed a quartz crystal microbalance (QCM) to monitor ALD in situ and in real time, and the deposited thin films have been characterized using X-ray photoelectron spectroscopy, spectroscopic ellipsometry, X-ray reflectivity, and atomic force microscopy. Growth of thin films of Al2O3 using TMA and either t-BuOH or H2O as the co-reactant at T = 285 °C produces thin films of similar physical properties (density, stoichiometry, minimal carbon incorporation), and the growth rate per cycle is similar for the two co-reactants at this temperature. At a lower temperature of T = 120 °C, the behavior is starkly different, where growth occurs with H2O but not with t-BuOH. At either process temperature, we find no evidence for significant coverages of a long-lived tert-butoxy species from the reaction of t-BuOH. Deposition of thin films of Al2O3 on metal surfaces of Cu and Co has been examined for evidence of interfacial oxidation. While growth with either co-reactant does not lead to the oxidation of the underlying Cu substrate, use of H2O leads to the oxidation of Co, but use of t-BuOH as the co-reactant does not. Thermodynamic factors may affect the early stages of growth, as Al species will likely scavenge all free O species. In contrast, at later times, diffusion of species through the deposited Al2O3 thin film could result in oxidation at the Al2O3|metal interface, a process that is strongly hindered in the case of t-BuOH due to its size. This observation highlights the importance of the choice of the co-reactant concerning ALD of oxides on metal surfaces.

Controlling nanocrystal superlattice symmetry and shape-anisotropic interactions through variable ligand surface coverage.

The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the molecular interactions between ligands bound to the NC surface. We illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. We show that PbS NCs with dense ligand coverage assemble into face-centered cubic (fcc) superlattices whereas NCs with sparse ligand coverage assemble into body-centered cubic (bcc) superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chemistry characterization combined with density functional theory calculations suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and leads to the formation of superlattices with translational and orientational order.

A Vacuum Ultraviolet-Enhanced Oxidation Mechanism for Pd: Near-Surface Oxidation for Atomic Layer Etching.

Density functional theory (DFT) is used to better understand the oxidation of Pd metal using vacuum ultraviolet (VUV) light co-exposed with O2, which is known to produce O and O3. The oxidation of Pd metal arising from O, O2, and O3 is assessed on bare Pd, Pd with a 0.25 monolayer of adsorbed atomic O, and Pd with increasing O incorporation into the substrate. DFT calculations are complemented experimentally by co-exposing 20 nm Pd films to 1 Torr of O2 and VUV photons (6.5 < hν < 11.3 eV) from a D2 lamp at temperatures ranging from 50 to 200 °C and times from 30 s to 40 min. Oxidation of Pd is characterized using in situ X-ray photoelectron spectroscopy. Co-exposures at 50 °C and 1 Torr O2 are performed with the Pd illuminated by the VUV light and shadowed from the VUV light in attempting to select for the oxidant that impinges on the Pd surface and causes oxidation. Results suggest that atomic O incident from the gas phase is responsible for oxidation of Pd, as no PdOx formation is observed for the same time period with the sample shadowed. Growth of PdOx via O diffusion is studied with the nudged elastic band method. Atomic O diffusion through Pd has an activation energy barrier of ∼2.87 eV with respect to a surface O. This decreases to ∼1.80 eV once the 0.25 monolayer of O occupies the surface. The extent of Pd oxidation is limited to the near-surface Pd region for all times and temperatures investigated. PdOx formation does not appear to exceed one to two atomic layers of Pd for conditions explored herein.

Competitive Adsorption as a Route to Area-Selective Deposition.

In this work, we have explored the use of a third species during chemical vapor deposition (CVD) to direct thin-film growth to occur exclusively on one surface in the presence of another. Using a combination of density functional theory (DFT) calculations and experiments, including in situ surface analysis, we have examined the use of 4-octyne as a coadsorbate in the CVD of ZrO2 thin films on SiO2 and Cu surfaces. At sufficiently high partial pressures of the coadsorbate and sufficiently low substrate temperatures, we find that 4-octyne can effectively compete for adsorption sites, blocking chemisorption of the thin-film precursor, Zr[N(CH3C2H5)]4, and preventing growth on Cu, while leaving growth unimpeded on SiO2. The selective dielectric-on-dielectric (DoD) process developed herein is fast, totally vapor phase, and does not negatively alter the composition or morphology of the deposited thin film. We argue that this approach to area-selective deposition (ASD) should be widely applicable, provided that suitable candidates for preferential binding can be identified.

Growth of first generation dendrons on SiO2: controlling chemisorption of transition metal coordination complexes.

We have investigated the growth of first generation branched polyamidoamine dendrons on silicon dioxide as a way to tailor and control the subsequent chemisorption of transition metal coordination complexes. Beginning with straight-chain alkyl, amine-terminated self-assembled monolayers as anchors, we find that the efficiency of the dendritic branching step depends on the length of the anchor, it being nearly perfect on a 12-carbon chain anchor. The reaction of these layers, both the anchor layers and the first generation dendrons, with Ta[N(CH3)2]5 and Ti[N(CH3)2]4 have been examined in ultrahigh vacuum using X-ray photoelectron spectroscopy. We find that the saturation coverage increases with the density of terminal -NH2 groups; thus, the branching step has effectively amplified the chemisorptive capacity of the surface. Concerning the spatial extent of reaction we find that it depends on the thickness and structure of the organic layer. The thinnest layer cannot prevent penetration of the metal complex to the organic/SiO2 interface, where it can react with residual -OH, whereas, on the longer straight chain anchor, reaction occurs exclusively at the terminal -NH2 group. On the branched dendrons, the situation is more complex, and reaction occurs not only with the terminal -NH2 group but also likely with functional groups, such as -NH-(C=O)-, on the backbone of the branched dendron.

Tuning of Coupling and Surface Quality of PbS Nanocrystals via a Combined Ammonium Sulfide and Iodine Treatment.

Surface states of colloidal nanocrystals are typically created when organic surfactants are removed. We report a chemical process that reduces surface traps and tunes the interparticle coupling in PbS nanocrystal thin films after the surfactant ligands have been stripped off. This process produces PbS/PbI2 core/shell nanocrystal thin films via a combined ammonium sulfide and iodine treatment. These all-inorganic nanocrystal thin films are air-stable and exhibit bright emission with optimum photoluminescence quantum yield close to that of pristine PbS nanocrystals passivated by oleate ligands. Interparticle coupling of post-treatment nanocrystal thin films is continuously tunable by varying the iodine treatment process. Optical studies reveal that this method can produce PbS nanocrystal thin films superior in both coupling and surface quality to nanocrystals linked by small molecules such as ethanedithiol or 3-mercaptopropionic acid.

Observation from scanning tunneling microscopy of a striped phase for octanethiol adsorbed on Au(111) from solution.

A self-assembled monolayer of 1-octanethiol was prepared on a Au(111) surface via liquid-phase adsorption. An investigation of the surface using ultrahigh-vacuum scanning tunneling microscopy revealed a striped phase of the octanethiol molecules under the conditions examined. This phase resembles the well-known "pinstripe" structure of alkanethiols on Au(111), with a registry that is similar to that of the previously observed p x radical3 structures. We discuss the nature of this structure with respect to those that have been observed for other n-alkanethiols.

Gas-surface reactions between pentakis(dimethylamido)tantalum and surface grown hyperbranched polyglycidol films.

We have investigated the growth of hyperbranched polyglycidol films, and their subsequent reaction with a transition metal coordination complex, pentakis(dimethylamido)tantalum, Ta[N(CH 3) 2] 5 using ellipsometry, contact angle measurements, atomic force microscopy and X-ray photoelectron spectroscopy (XPS). Up to thicknesses of approximately 150 A, the growth of polyglycidol is approximately linear with reaction time for growth activated using either sodium methoxide or an organic superbase. The reaction of Ta[N(CH 3) 2] 5 at room temperature with these layers depends strongly on their thickness--the amount of uptake of Ta by the surface increases with the thickness of the organic layer, and thicker films also lead to more extensive ligand exchange reactions (with the R-OH groups), with as many as 4 ligands being lost on the thicker organic films. Ta penetrates the surface of all films examined (thicknesses 30-84 A), but the average depth of the penetration is nearly independent of the thickness of the organic film, and it is approximately 15-25 A. Modification of the polyglycidol with an aminoalkoxysilane introduces a significant fraction of -NH 2 termination in the organic layer. Reactions of this layer with the Ta complex are quite different than those on an unmodified layer--now on average only a single ligand exchange reaction occurs, while on the unmodified surface as many as four ligands are exchanged.

Ab initio calculations of the reaction mechanisms for metal-nitride deposition from organo-metallic precursors onto functionalized self-assembled monolayers.

An atomistic mechanism has been derived for the initial stages of the adsorption reaction for metal-nitride atomic layer deposition (ALD) from alkylamido organometallic precursors of Ti and Zr on alkyltrichorosilane-based self-assembled monolayers (SAMs). The effect of altering the terminal functional group on the SAM (including -OH, -NH2, -SH, and -NH(CH3)) has been investigated using the density functional theory and the MP2 perturbation theory. Reactions on amine-terminated SAMs proceed through the formation of a dative-bond complex with an activation barrier of 16-20 kcal/mol. In contrast, thiol-terminated SAMs form weak hydrogen-bonded intermediates with activation barriers between 7 and 10 kcal/mol. The deposition of Ti organometallic precursors on hydroxyl-terminated SAMs proceeds through the formation of stronger hydrogen-bonded complexes with barriers of 7 kcal/mol. Zr-based precursors form dative-bonded adducts with near barrierless transitions. This variety allows us to select a kinetically favorable substrate for a chosen precursor. The predicted order of reactivity of differently terminated SAMs and the temperature dependence of the initial reaction probability have been confirmed for Ti-based precursors by recent experimental results. We predict that the replacement of methyl groups by trifluoromethyl groups on the SAM backbone decreases the activation barrier for amine-terminated SAMs by 5 kcal/mol. This opens a route to alter the native reactivities of a given SAM termination, in this case making amine termination energetically viable. The surface distribution of SAM molecules has a strong effect on the adsorption kinetics of Ti-based precursors. Unimolecular side decomposition reactions were found to be kinetically competitive with adsorption at 400 K.

The reaction of tetrakis(dimethylamido)titanium with self-assembled alkyltrichlorosilane monolayers possessing -OH, -NH2, and -CH3 terminal groups.

The reactions of tetrakis(dimethylamido)titanium, Ti[N(CH(3))(2)](4), with alkyltrichlorosilane self-assembled monolayers (SAMs) terminated by -OH, -NH(2), and -CH(3) groups have been investigated with X-ray photoelectron spectroscopy (XPS). For comparison, a chemically oxidized Si surface, which serves as the starting point for formation of the SAMs, has also been investigated. In this work, we examined the kinetics of adsorption, the spatial extent, and stoichiometry of the reaction. Chemically oxidized Si has been found to be the most reactive surface examined here, followed by the -OH, -NH(2), and -CH(3) terminated SAMs, in that order. On all surfaces, the reaction of Ti[N(CH(3))(2)](4) was relatively facile, as evidenced by a rather weak dependence of the initial reaction probability on substrate temperature (T(s) = -50 to 110 degrees C), and adsorption could be described by first-order Langmuirian kinetics. The use of angle-resolved XPS demonstrated clearly that the anomalous reactivity of the -CH(3) terminated SAM could be attributed to reaction of Ti[N(CH(3))(2)](4) at the SAM/SiO(2) interface. Reaction on the -NH(2) terminated SAM proved to be the "cleanest", where essentially all of the reactivity could be associated with the terminal amine group. In this case, we found that approximately one Ti[N(CH(3))(2)](4) adsorbed per two SAM molecules. On all surfaces, there was significant loss of the N(CH(3))(2) ligand, particularly at high substrate temperatures, T(s) = 110 degrees C. These results show for the first time that it is possible to attach a transition metal coordination complex from the vapor phase to a surface with an appropriately functionalized self-assembled monolayer.

Covalent attachment of a transition metal coordination complex to functionalized oligo(phenylene-ethynylene) self-assembled monolayers.

We have investigated the reaction of tetrakis(dimethylamido)titanium, Ti[N(CH(3))(2)](4), with N-isopropyl-N-[4-(thien-3-ylethynyl) phenyl] amine and N-isopropyl-N-(4-{[4-(thien-3-ylethynyl) phenyl]ethynyl}phenyl) amine self-assembled monolayers (SAMs), on polycrystalline Au substrates. The structure of the SAMs themselves has also been investigated. Both molecules form SAMs on polycrystalline Au bound by the thiophene group. The longer-molecular-backbone molecule forms a denser SAM, with molecules characterized by a smaller tilt angle. X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS have been employed to examine the kinetics of adsorption, the spatial extent of reaction, and the stoichiometry of reaction. For both the SAMs, adsorption is described well by first-order Langmuirian kinetics, and adsorption is self-limiting from T(s) = -50 to 30 degrees C. The use of angle-resolved XPS clearly demonstrates that the Ti[N(CH(3))(2)](4) reacts exclusively with the isopropylamine end group via ligand exchange, and there is no penetration of the SAM, followed by reaction at the SAM-Au interface. Moreover, the SAM molecules remain bound to the Au surface via their thiopene functionalites. From XPS, we have found that, in both cases, approximately one Ti[N(CH(3))(2)](4) is adsorbed per two SAM molecules.

A surface modification strategy on silicon nitride for developing biosensors.

A surface modification strategy for the use of giant magnetoresistive materials in the detection of protein-protein interactions is developed. This modification strategy is based on silanization of semiconductive materials. A native silicon nitride surface was treated with concentrated hydrofluoric acid to improve surface homogeneity. Nano-strip was used to oxidize silicon nitride to form a hydrophilic layer. Aminopropyltriethoxysilane was subsequently used to functionalize the treated surfaces to form amine groups, which were further activated with glutaraldehyde to introduce a layer of aldehyde groups. The effectiveness of this modification strategy was validated by chemiluminescence immunoassays of purified 6x His-HrpW of Pseudomonas syringae pv. tomato DC3000 and human transferrin. Signals with intensities related to concentrations of these two immobilized model proteins were observed. The modified surface was also validated by a more complex system: intercellular proteins secreted by DC3000. HrpW in these protein mixtures was successfully recognized by anti-HrpW antibodies when mixed proteins were immobilized onto activated surfaces. This surface modification strategy provides a platform onto which proteins can be directly immobilized for biosensor and protein array applications.