Atomic-scale simulation of ALD chemistry

Published papers on atomic-scale simulation of the atomic layer deposition (ALD) process are reviewed. The main topic is reaction mechanism, considering the elementary steps of precursor adsorption, ligand elimination and film densification, as well as reactions with substrates (particularly Si and SiO2) and CVD-like decomposition at the surface. Density functional theory is the first principles method generally applied to these mechanistic questions. The most popular subject for modelling is the ALD of oxides and nitrides, particularly the high-k dielectrics HfO2, ZrO2 and Al2O3, due to their importance in semiconductor processing

The Building Stones of Charleston: Sandstone and Limestone, Pre-1800

A range of preservation tools are available to homeowners and architectural conservators caring for historic buildings with stone, guiding preservation treatment choices for the material. These tools include historical research, material testing, and comparison to other buildings where treatments have been tested on a similar stone. This thesis seeks to establish a basic understanding of two stone types used in the construction of buildings in an area of the oldest district in Charleston, South Carolina built prior to 1800. This study will utilize the basic preservation tools of research and material analysis to create a catalogue of stone with both known and unknown origins. This is intended to advance the general understanding of what stone, imported from what quarry, and used in what applications are present in Charleston’s early building culture, to provide a set for comparison for interested parties to learn more about the origin of stone found in their buildings. This study included a survey of exterior stone in Charleston used in buildings dating prior to 1800. From this survey, several case studies were selected based on the prominence and availability of historic resources. Physical analysis was conducted including hardness, state of aggregation, rift and grain, and color. These basic properties create a baseline for further research, as well as providing a quantifiable dataset for comparison of samples that could not be determined through documented records. Microscope slides were prepared when samples could be taken for testing while others were tested in situ using non-destructive methods. The results of this thesis indicate that there is significantly more sandstone in Charleston than limestone. Much of this stone is believed to be from subsequent nineteenth century alterations. English Portland limestone is the oldest stone used in Charleston, and Aquia Creek sandstone from Virginia is the earliest domestic stone used, seen as early as 1788. The physical analysis serves as the beginning of a catalogue of stone in Charleston to help determine the best preservation treatments for each type

Product Placement and its Influence on Children

Despite the growing body of research concerning the practice of product placement, a notable research gap exists pertaining to the influence of product placement on children, and its link to childhood obesity. This study will provide an insight to the influence of product placement on children. Its objective is to analyze the placement of food and beverage products on children of various ages. An experimental approach will be taken whereby child-respondents will be divided into groups and each group will see the same television clip but with different placements (digitally inserted) of various types of food and beverages. All groups will be shown a short clip (20 minutes) from Pop Idol – the British equivalent of American Idol. Explicit and implicit responses to these product placements will be measured in order to understand their influence

The influence of fire retardant additives on the properties of HIPS and PBT

Halogen compounds and antimony-based synergists are used at low loading levels in many polymers systems to impart high levels of flame retardancy. This study used a range of brominated flame retardants (BFR) and the most commonly used synergist for halogen-based flame retardance, antimony trioxide (Sb2O3), to investigate the effects on mechanical and physical properties of flame retardants in HIPS and PBT. The polymers used were High Impact Polystyrene (HIPS) and Polybutylene Terephthalate (PBT). Initially each of the additives was used individually, before being combined to study the effect of the complete package. This was achieved by producing a series of compounds using a twin-screw extruder, and then an injection moulder to produce impact, tensile and fracture toughness specimens. The compounds were also analysed using rheological testing and thermal analysis. Also the effects of Stereon impact modifier and Fyrebloc masterbatches were determined in HIPS. [Continues.

Markerless tracking of tennis racket motion using a camera

This research is concerned with tracking tennis racket movements. Previously, stereo camera systems have been used to track markers attached to rackets, which allows for racket movements to be obtained in three-dimensions. Typically, markers are manually selected on the image plane but this can be time consuming and inaccurate. This paper discusses a markerless method to measure three-dimensional racket movements using a camera. The method relies on a silhouette of a racket captured with a camera whose relative pose (rotation and translation) is unknown. A candidate relative pose is used to measure the inconsistency between the silhouette and a set of racket silhouettes captured with a fully calibrated camera. The measure of inconsistency can be formulated as a cost function associated with the candidate relative pose. By adjusting parameters of the pose to minimise the cost, an accurate estimation for the true pose of the racket can be made. A validation scheme was developed to compare pose estimates with data obtained using camera calibration software. Rotation about the axis of x, y, z' were accurate to within 2.5° for 88, 90 and 86 % of estimates respectively and resultant translation to within 5 mm for 72% of estimates. This research is the first step in a process to fully validate a novel method for measuring tennis racket movements in real play conditions

Mechanism for the atomic layer deposition of copper using diethylzinc as the reducing agent – a density functional theory study using gas phase molecules as a model

We present theoretical studies based on first-principles density functional theory calculations for the possible gas-phase mechanism of the atomic layer deposition (ALD) of copper by transmetalation from common precursors such as Cu(acac)(2), Cu(hfac)(2), Cu(PyrIm(R))(2) with R = (i)Pr and Et, Cu(dmap)(2), and CuCl(2) where diethylzinc acts as the reducing agent. An effect on the geometry and reactivity of the precursors due to differences in electronegativity, steric hindrance, and conjugation present in the ligands was observed. Three reaction types, namely, disproportionation, ligand exchange, and reductive elimination, were considered that together comprise the mechanism for the formation of copper in its metallic state starting from the precursors. A parallel pathway for the formation of zinc in its metallic form was also considered. The model Cu(I) molecule Cu(2)L(2) was studied, as Cu(I) intermediates at the surface play an important role in copper deposition. Through our study, we found that accumulation of an LZnEt intermediate results in zinc contamination by the formation of either Zn(2)L(2) or metallic zinc. Ligand exchange between Cu(II) and Zn(II) should proceed through a Cu(I) intermediate, as otherwise, it would lead to a stable copper molecule rather than copper metal. Volatile ZnL(2) favors the ALD reaction, as it carries the reaction forward

Quantum chemical study of the effect of precursor stereochemistry on dissociative chemisorption and surface redox reactions during the atomic layer deposition of the transition metal copper

Using quantum chemical calculations, we investigate surface reactions of copper precursors and diethylzinc as the reducing agent for effective Atomic Layer Deposition (ALD) of Cu. The adsorption of various commonly used Cu(II) precursors is explored. The precursors vary in the electronegativity and conjugation of the ligands and flexibility of the whole molecule. Our study shows that the overall stereochemistry of the precursor governs the adsorption onto its surface. Formation of different Cu(II)/Cu(I)/Cu(0) intermediate complexes from the respective Cu(II) compounds on the surface is also explored. The surface model is a (111) facet of a Cu55 cluster. Cu(I) compounds are found to cover the surface after the precursor pulse, irrespective of the precursor chosen. We provide new information about the surface chemistry of Cu(II) versus Cu(I) compounds. A pair of CuEt intermediates or the dimer Cu2Et2 reacts in order to deposit a new Cu atom and release gaseous butane. In this reaction, two electrons from the Et anions are donated to copper for reduction to metallic form. This indicates that a ligand exchange between the Cu and Zn is important for the success of this transmetalation reaction. The effect of the ligands in the precursor on the electron density before and after adsorption onto the surface has also been computed through population analysis. In the Cu(I) intermediate, charge is delocalized between the Cu precursor and the bare copper surface, indicating metallic bonding as the precursor densifies to the surface

Thomas-Fermi Theory Revisited

We show that the Thomas-Fermi theory is exact for atoms, molecules, and solids as Z→∞.

Cooperation between adsorbates accounts for the activation of atomic layer deposition reactions

Atomic layer deposition (ALD) is a technique for producing conformal layers of nanometre-scale thickness, used commercially in non-planar electronics and increasingly in other high-tech industries. ALD depends on self-limiting surface chemistry but the mechanistic reasons for this are not understood in detail. Here we demonstrate, by first-principle calculations of growth of HfO2 from Hf(N(CH3)2)4–H2O and HfCl4–H2O and growth of Al2O3 from Al(CH3)3–H2O, that, for all these precursors, co-adsorption plays an important role in ALD. By this we mean that previously-inert adsorbed fragments can become reactive once sufficient numbers of molecules adsorb in their neighbourhood during either precursor pulse. Through the calculated activation energies, this ‘cooperative’ mechanism is shown to have a profound influence on proton transfer and ligand desorption, which are crucial steps in the ALD cycle. Depletion of reactive species and increasing coordination cause these reactions to self-limit during one precursor pulse, but to be re-activated via the cooperative effect in the next pulse. This explains the self-limiting nature of ALD

Multiple proton diffusion and film densification in atomic layer deposition modeled by density functional theory

To investigate the atomic layer deposition (ALD) reactions for growth of HfO2 from Hf(NMe2)4 (TDMAHf) andH2O, a density functional theory (DFT) slab model was employed. We inspected all energy steps, from the early stage of adsorption of each ALD precursor to the densification of multiple atoms into bulk-like HfO2 layers. The activation energy calculations show that repeated proton diffusion from the surface to amide ligands and rotation of the protonated amine is more energetically accessible than the simple elimination of the amine in the initial stage. We therefore propose that multiple protons diffuse to the amide ligands of the Hf precursor before desorption of protonated ligands takes place. Loss of a proton from surface oxygen frees it up for bonding to Hf of the precursor. Protonation of ligands, and especially desorption of ligands, frees up Hf for bonding to surface oxygen. These effects are termed “densification”, as they bring Hf−O packing closer to the bulk scenario. Densification is associated with substantial release of energy. During the metal pulse, saturation of the surface by remaining fragments HfX causes adsorption of further metal precursor to stop. The presence of these fragments prevents further chemisorption of HfX4, since this requires the creation of a strong dative bond between Hf and O. Next, during the H2O pulse, Hf exchanges its remaining ligands with OH groups. The exchange occurs due to the decomposition of adsorbed H2O molecules in clusters of HfX. Decomposition of H2O when adsorbed onto a (Hf(NMe2))x (x ≥ 2) cluster (e.g., dimers) also increases the coordination of Hf and O. Simultaneously, low-coordinated oxygen atoms appear at the surface, which are reactive sites for the next metal pulse. With saturation of the surface by OH groups, H2O molecules begin to appear. This detailed description of ALD chemistry allows us to make qualitative predictions about how the process depends on temperature. The data can also be inputted into kinetic simulations for a quantitative view of the complex film growth process