Aluminas film deposition techniques

By applying the template technique, Kyotani et al. and Che et al. succeeded in preparing Pt and Pt/Ru metal-filled uniform carbon nanotubes in which the metal is present as either nanorods or nanoparticles. It should be noted that no metal was observed on the outside wall of the tubes. This is due to the preparation procedure, in which the metal precursor was loaded into the carbon-deposited alumina film before the dissolution of alumina by HF (see Fig. 10.1.9). Thus, there is no other space for metal to be loaded except in the channels. 

A carbon-deposited film was prepared from the alumina film with 30-nm channels by the CVD technique using propylene. Fluorination was carried out by direct reaction of the film with dry fluorine gas (purity 99.7%). The film was placed in a nickel reactor and was allowed to react with 0.1 MPa of fluorine gas for 5 days at a predetennined temperature in the range of 50 to 200°C. Then the fluorinated carbon nanotubes were separated by dissolving the alumina film with HF. A schematic drawing of the fluorination process is given in Fig. 10.1.15. 

A novel CD technique used metallic A1 to reduce Se. Elemental Se and metallic A1 foil, together with ZnCE, were dissolved in NaOH solution and heated to 80°C in an autoclave with substrates of Teflon or alumina [145]. (Metal chalcogenide films have been chemically deposited onto Teflon in a number of reports.) It is notable that the films deposited onto alumina were reported to be 1.8 p,m thick— much thicker than ZnSe CD from other solutions. The films were sphalerite ZnSe... 

Different methods have been used to deposit microporous thin films, including solgel, pyrolysis, and deposition techniques [20], Porous inorganic membranes are made of alumina, silica, carbon, zeolites, and other materials [8], They are generally prepared by the slip coating method, the ceramic technique, or the solgel method (Section 3.7). In addition, dense membranes are prepared with metals, oxides, and other materials (Chapter 2). 

The fullerenes, Cgo and C70, are produced in the laboratory by the contact arc-evaporation of 6 mm graphite rods (e.g. Johnson Matthey, spectroscopic grade) in 100 torr of helium in a water-cooled stainless steel chamber described previously [5]. The soluble material in the soot produced from the arc-evaporation is extracted with toluene using a Soxhlet apparatus. The pure fullerenes are obtained by chromatography on neutral alumina columns using hexanes as the eluant, or by the use of a simple filtration technique using charcoal-silica as the stationary phase and toluene as the eluant [5]. The fullerenes so prepared are characterized by UV/Vis spectroscopy and other techniques. FT-IR spectra of vacuum deposited fullerene films on KBr crystals also provide a means of characterization, just as do Raman spectra of films deposited on a silicon crystal. Ultraviolet and X-ray photoelectron spectra of fullerene films on... 

A study of dielectric characteristics of alumina thin films deposited on silicon substrates from Al(acac)3 dissolved in dmf by spray pyrolysis between 450 and 650 °C was recently reported by Falcony and coworkers. The addition of water vapor significantly improved the dielectric characteristics and smoothness of the deposits. In comparison to the CVD technique described above (see Section m.A.l) this procedure lead to considerable carbon impurities in the films. The overall resisfivify of fhe alumina layers decreases, when both the concentration of the solution and the deposition temperature increase, which is explainable with the increase of carbon residues in the films. 

The preparation of model catalyst films suitable for investigation by microscopic techniques has been described by Wanke and Bolivar. The most common technique of preparing alumina or silica substrates is oxidation of aluminum or silicon foils. Supp( t films are typically mounted for examination after which a metal film is prepared on the support by vacuum deposition or sputtering to thicknesses ranging from monolayer to 2 nm. Thomal treatment of the sample causes breakup of the metal film into metal crystallites. Table 1 summarizes conditions used by various investigators to convert metal films to crystallites. Apparently, crystallite nucleation is a strong function of metal, atmosphere, temperature, and film thickness and a weak function of support, although these results are only qualitative, since precise conditions for metal film breakup were not available from many of the studies listed. Nevertheless, the more recent studies indicate that breakup of Pt/alumina films to 1.8 nm particles occurs in vacuum at temperatures as low as 473 K. 

Several techniques have been developed to deposit alumina films on different surfaces such as those of semiconductors or metals. These films find apphcation in various areas. In most cases, amorphous alumina films are desired. Depending on the deposition techniques, various precursors may be used the following combinations have been reported plasma-enhanced atomic layer deposition using trimethylaluminum (112), metal-organic chemical vapor deposition using aluminum tri-iso-propoxide (113), and condensation from the gas phase using laser-evaporated alumina (114). Similar evaporation techniques can also be apphed to prepare Y-AI2O3 powders (115,116). 

Jayaraman et aL (1995b) deposited ultrathin Pd films (< 500 mn) on porous ceramic substrates using the sputter deposition technique. The following two parameters were found to be most critical to the synthesis of the gas-tight metal-ceramic composite substrate type (surface roughness) and deposition temperature. Fairly gas-tight Pd films with good adhesion could be coated on sol-gel derived fine pore y-alumina substrates but not on coarse a-alumina substrates. Poor adhesion between the coated film and the... 

Detailed studies were conducted by infrared, TPD, XPS but also by more sophisticated techniques such as CP-MAS solid state NMR or EXAFS, on the various steps by which molybdenum can be deposited on alumina supports starting from [Mo(CO)e]. Indeed, thin films of molybdenum or of its oxides have wide application as gas sensors or solar cell catalysts. 

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

Chemical Vapour Deposition (CVD) of microporous silica films with a thickness of about 1.5 pm onto mesoporous glass or y-alumina substrates are obtained by deposition from TEOS-oxygen mixtures at 300-700°C. Pore sizes are estimated to be 0.4-0.6 nm or are virtually absent. CVD techniques may also be useful for repairing residual defects and for pore narrowing. 

For thin-film metallization, a thin metallic film is first deposited onto the surface of the substrate. The deposition can be accomplished by thermal evaporation, electronic-beam- or plasma-assisted sputtering, or ion-beam coating techniques, all standard microelectronic processes. A silicon wafer is the most commonly used substrate for thin-film sensor fabrication. Other substrate materials such as glass, quartz, and alumina can also be used. The adhesion of the thin metallic film to the substrate can be enhanced by using a selected metallic film. For example, the formation of gold film on silicon can be enhanced by first depositing a thin layer of chromium onto the substrate. This procedure is also a common practice in microelectronic processing. However, as noted above, this thin chromium layer may unintentionally participate in the electrode reaction. 

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