H. Henry Lamb

Supported metals and supported organometallics

Abstract Organometallic chemistry and heterogeneous catalysis are developing into common territory, illustrated by catalysts incorporating structures ranging from mononuclear metal complexes to metal aggregates (crystallites) on metal oxide supports. Reactions of precursors such as metal carbonyls and metal allyls with metal oxide surfaces have been understood on the basis of solution organometallic chemistry combined with functional group chemistry of the supports. The supported species are transformed by processes including reductive carbonylation (whereby metal complexes and salts are converted into metal clusters) and oxidative fragmentation (whereby metal clusters and aggregates are converted into mononuclear metal complexes). Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used in conjunction with other techniques to characterize bonds between mononuclear metal complexes and metal oxide surfaces and also to provide structural information about metal-support interfaces in highly dispersed supported metal aggregates. EXAFS results are also helping to define the limitations of standard methods for estimating surface areas of highly dispersed metals by measurements of chemisorption of H 2 and CO. Some supported ‘molecular’ organometallics are structurally and catalytically novel, indicating possibilities for design of surface catalytic sites. Ensembles consisting of three Re complexes on MgO are active for the structure-sensitive cyclopropane hydrogenolysis, whereas a catalyst consisting of the same complexes (evidently isolated on the support surface) is inactive. ‘Molecular’ osmium carbonyl clusters stabilized on the basic MgO surface and in basic zeolites catalyze CO hydrogenation, and 10-atom aggregates of osmium, formed by removal of CO ligands from a ‘molecular’ precursor, are active for n-butane hydrogenolysis. These structurally well-defined catalysts offer good opportunities for understanding metal-support interactions and structure sensitivity in metal catalysis.

Reaction pathways in remote plasma nitridation of ultrathin SiO2 films

Low-temperature nitridation of 3 nm SiO2 films using He/N2 and N2 remote radio frequency (rf) plasmas was investigated. On-line Auger electron spectroscopy and angle-resolved x-ray photoelectron spectroscopy (ARXPS) were employed to determine the concentration, spatial distribution, and local chemical bonding of nitrogen in the resultant films. Experiments were performed using a substrate temperature of 300 °C and 30 W rf power. Nitridation using an upstream He/N2 remote plasma at 0.1 Torr incorporates nitrogen at the top surface of the SiO2 film. In contrast, a lower concentration of nitrogen distributed throughout the film is obtained when the process pressure is increased to 0.3 Torr. ARXPS indicates a N–Si3 local bonding configuration, irrespective of the spatial distribution of N atoms. Slightly more nitrogen is incorporated using a downstream He/N2 plasma at each process pressure. By comparison, nitridation of SiO2 films using a N2 remote plasma at 0.1 Torr is very slow. Optical emission spectroscop...

Tungsten chemical vapor deposition using tungsten hexacarbonyl: microstructure of as-deposited and annealed films

Abstract Tungsten (W) films were deposited on Si(100) from tungsten hexacarbonyl, [W(CO)6], by low-pressure chemical vapor deposition (CVD) in an ultra-high vacuum (UHV)-compatible reactor. The chemical purity, resistivity, crystallographic phase, and morphology of the deposited films depend markedly on the substrate temperature. Films deposited at 375°C contain approximately 80 at.% tungsten, 15 at.% carbon and 5 at.% oxygen. These films are polycrystalline β-W with a strong (211) orientation and resistivities of >1000 μΩ cm. Vacuum annealing at 900°C converts the metastable β-W to polycrystalline α-W, with a resistivity of approximately 19 μΩ cm. The resultant α-W films are porous, with small randomly oriented grains and nanoscale ( 95 at.%) polycrystalline α-W, with low resistivities (18–23 μΩ cm) and a tendency towards a (100) orientation. Vacuum annealing at 900°C reduces the resistivity to approximately 10 μΩ cm, and results in a columnar morphology with a very strong (100) orientation.

Electron cyclotron resonance plasma reactor for SiO2 etching: Process diagnostics, end‐point detection, and surface characterization

An electron cyclotron resonance plasma reactor for low‐pressure etching of SiO2 layers on Si is described. Under typical operating conditions of 1.1 mTorr neutral pressure, 5 sccm CF4/46 sccm He, and 400 W net microwave power, an average etch rate of 35–40 nm/min is obtained. Ion densities were measured by using a quadrupole mass analyzer (QMA) immersed in the downstream plasma; an optical multichannel analyzer was used for actinometric measurements of atomic F and O concentrations. The results are correlated with process variables and SiO2 etch rates. Real‐time end‐point detection was effected by actinometry and by monitoring the concentrations of etch products using a differentially pumped QMA. H2 addition inhibits SiO2 etching but improves SiO2/Si etch selectivity. For selective etching, in situ, off‐line surface analysis by Auger electron spectroscopy demonstrated an increase in the surface C concentration during SiO2 etching and deposition of a fluorocarbon layer at the end point which strongly inhib...

Low-temperature Ar/N2 remote plasma nitridation of SiO2 thin films

Low-temperature nitridation of SiO2 thin films by Ar/N2 remote plasma processing was investigated using on-line Auger electron spectroscopy, angle-resolved x-ray photoelectron spectroscopy (ARXPS), and optical emission spectroscopy (OES). Nitridation experiments were performed at 300 °C using 30 W Ar/N2 remote plasmas at 0.1 and 0.3 Torr. Ar/N2 remote plasma exposure of 5 nm SiO2 films for 30 min results in nitrogen incorporation throughout the films, independent of process pressure and plasma reactor configuration (i.e., upstream versus downstream N2 injection). ARXPS indicates a N–Si3 local bonding configuration with second nearest neighbor oxygen atoms. Ar/N2 remote plasma exposure at 0.1 Torr results in higher nitrogen concentrations (8–10 at. %). Reactor configuration has a negligible effect at 0.1 Torr; conversely, downstream N2 injection results in higher nitrogen concentrations (5–6 at. %) than upstream injection (3–4 at. %) at 0.3 Torr. OES indicates that the Ar/N2 remote plasmas contain N2 tripl...

Real-time monitoring of high-gravity corn mash fermentation using in situ raman spectroscopy.

In situ Raman spectroscopy was employed for real‐time monitoring of simultaneous saccharification and fermentation (SSF) of corn mash by an industrial strain of Saccharomyces cerevisiae. An accurate univariate calibration model for ethanol was developed based on the very strong 883 cm−1 C–C stretching band. Multivariate partial least squares (PLS) calibration models for total starch, dextrins, maltotriose, maltose, glucose, and ethanol were developed using data from eight batch fermentations and validated using predictions for a separate batch. The starch, ethanol, and dextrins models showed significant prediction improvement when the calibration data were divided into separate high‐ and low‐concentration sets. Collinearity between the ethanol and starch models was avoided by excluding regions containing strong ethanol peaks from the starch model and, conversely, excluding regions containing strong saccharide peaks from the ethanol model. The two‐set calibration models for starch (R2 = 0.998, percent error = 2.5%) and ethanol (R2 = 0.999, percent error = 2.1%) provide more accurate predictions than any previously published spectroscopic models. Glucose, maltose, and maltotriose are modeled to accuracy comparable to previous work on less complex fermentation processes. Our results demonstrate that Raman spectroscopy is capable of real time in situ monitoring of a complex industrial biomass fermentation. To our knowledge, this is the first PLS‐based chemometric modeling of corn mash fermentation under typical industrial conditions, and the first Raman‐based monitoring of a fermentation process with glucose, oligosaccharides and polysaccharides present. Biotechnol. Bioeng. 2013; 110: 1654–1662.

Nitrogen incorporation in ultrathin gate dielectrics: A comparison of He/N2O and He/N2 remote plasma processes

Ultrathin Si oxynitride films grown by low-temperature remote plasma processing were examined by on-line Auger electron spectroscopy and angle-resolved x-ray photoelectron spectroscopy to determine the concentration, spatial distribution, and chemical bonding of nitrogen. The films were grown at 300 °C on Si(100) substrates using two radio-frequency remote plasma processes: (i) He/N2O remote plasma-assisted oxidation (RPAO) and (ii) two-step remote plasma oxidation/nitridation. A 5 min He/N2O RPAO process produces a 2.5 nm oxynitride film incorporating approximately 1 monolayer of nitrogen at the Si–SiO2 interface. The interfacial nitrogen is bonded in a N–Si3 configuration, as in silicon nitride (Si3N4). By comparison, a 90 s He/N2 remote plasma exposure of a 1 nm oxide (grown by 10 s He/O2 RPAO) consumes substrate Si atoms creating a 1 nm subcutaneous Si3N4 layer. The nitrogen areal density obtained via the two-step process depends on the initial oxide thickness and the He/N2 remote plasma exposure time. Moreover, as the oxide thickness is increased (by increasing the He/O2 remote plasma exposure), the nitrogen distribution shifts away from the Si–SiO2 interface and into the oxide. More nitrogen with a tighter distribution is incorporated using He versus Ar dilution. Insight into the remote plasma chemistry was provided by optical emission spectroscopy. Strong N2 first positive and second positive emission bands were observed for He/N2O and He/N2 remote plasmas indicating the presence of N2 metastables and ground-state N atoms.Ultrathin Si oxynitride films grown by low-temperature remote plasma processing were examined by on-line Auger electron spectroscopy and angle-resolved x-ray photoelectron spectroscopy to determine the concentration, spatial distribution, and chemical bonding of nitrogen. The films were grown at 300 °C on Si(100) substrates using two radio-frequency remote plasma processes: (i) He/N2O remote plasma-assisted oxidation (RPAO) and (ii) two-step remote plasma oxidation/nitridation. A 5 min He/N2O RPAO process produces a 2.5 nm oxynitride film incorporating approximately 1 monolayer of nitrogen at the Si–SiO2 interface. The interfacial nitrogen is bonded in a N–Si3 configuration, as in silicon nitride (Si3N4). By comparison, a 90 s He/N2 remote plasma exposure of a 1 nm oxide (grown by 10 s He/O2 RPAO) consumes substrate Si atoms creating a 1 nm subcutaneous Si3N4 layer. The nitrogen areal density obtained via the two-step process depends on the initial oxide thickness and the He/N2 remote plasma exposure time...

Ultrathin silicon oxynitride films grown by Ar/N2O remote plasma processing

On-line Auger electron spectroscopy, secondary ion mass spectroscopy, and angle-resolved x-ray photoelectron spectroscopy (ARXPS) were employed to determine the concentration, spatial distribution, and local chemical bonding of nitrogen in ultrathin oxynitride films grown by Ar/N2O remote plasma processing. Nitrogen incorporation in the films occurs primarily at the Si–SiO2 interface irrespective of rf power (5–50 W); however, the interfacial nitrogen concentration increases with rf power. Up to 0.6 monolayers of nitrogen atoms, bonded as (Si–)3N, are incorporated at the interface. Atomic oxygen generated in N2O plasma decomposition scavenges nitrogen from the bulk of the film, effectively confining nitrogen to a very narrow region near the substrate. A very small concentration of nitrogen, bonded as (Si–)2N–O, is detected on the top surface of the film by ARXPS.

Engineering catalyst surfaces with metal carbonyl clusters

Abstract This article reviews recent progress in the application of metal carbonyl clusters to preparation of supported catalysts and suggests opportunities for future research. Catalysis by surface-bound metal carbonyl clusters and catalysis by structurally simple supported metals derived by decarbonylation of surface-bound clusters have been achieved. A catalytic cycle for ethene hydrogenation by triosmium clusters on silica has been proposed, and molecular metal clusters have been stabilized on basic metal oxides under conditions of CO hydrogenation catalysis. In these examples, molecular metal clusters are the only organometallic surface species detected by in situ spectroscopy. Surface-bound metal carbonyl clusters have been used to prepare well-defined catalytic entities, ensembles of supported mononuclear complexes and supported metal atom clusters, which retain the nuclearity of the precursor. The latter are structurally simple supported metals which provide an opportunity to explore structure sensitivity in catalysis by nano-scale metal clusters. Of technological importance, metal carbonyl clusters and mononuclear organometallics are being used to engineer multi-layer catalysts with advanced architectures and novel catalytic properties.

Kinetics of UV/O 2 Cleaning and Surface Passivation: Experiments and Modeling

Removal of adsorbed organic contaminants from Si surfaces by reaction with molecular O 3 and photo-generated atomic oxygen species in a UHV-compatible photochemical reactor was investigated. Treatment of contaminated wafers with externally generated O 3 at 25°C was effective in removing adsorbed organics, but surface cleaning rates were enhanced by simultaneous 254-nm UV irradiation of the reactor contents. In situ photo-generation of O 3 and atomic oxygen species by 185- and 254-nm irradiation of O 2 gave comparable results. A simplified gas-phase kinetics model describing O 3 generation by a low-pressure Hg lamp was developed and used to gain insight into the effects of relative humidity and O 2 partial pressure on steady-state O 3 concentrations.