Electron microscopy metal evaporation

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

The characterization of evaporated alloy films can be carried out at widely different levels of sophistication. At the very least, it is necessary to determine the bulk composition, probably after the film has been used for an adsorption or catalytic experiment. Then various techniques can be applied, e.g., X-ray diffraction, electron diffraction, and electron microscopy, to investigate the homogeneity or morphology of the film. The measurement of surface area by chemisorption presents special problems compared with the pure metals. Finally, there is the question of the surface composition (as distinct from the bulk or overall composition), and a brief account is given of techniques such as Auger electron spectroscopy which might be applied to alloy films. 

The decomposition of formic acid over evaporated Pd-Au alloy films has been studied by Clarke and Rafter (69) the same reaction on Pd-Au alloy wires was studied by Eley and Luetic (128). The alloy films were prepared in a conventional high vacuum system by simultaneous evaporation of the component metals from tungsten hairpins. The alloy films were characterized by X-ray diffraction and electron microscopy. The X-ray diffractometer peaks were analyzed by a method first used by Moss and Thomas (SO). It was found that alloys deposited at a substrate temperature of 450°C followed by annealing for one hour at the same temperature were substantially homogeneous. Electron microscopy revealed that all compositions were subject to preferred orientation (Section III). 

Fig. 26. High -resolution transmission electron microscopy (HRTEM) image of perfluoropoly-ether-modified G3 PPI dendrimers containing Pd metal nanoclusters prepared on aholey carbon copper grid by evaporation of a dilute solution of the dendrimer composite. Reprinted with permission from Ref. 100 Copyright 2000 WUey-VCH...

Non-porous amorphous alumina films (several hundred Angstroms thick) can be prepared by the anodic oxidation of clean, high-purity Al. The oxide film is separated by dissolving any unoxidized metal in a mercury chloride solution. The oxide films are then washed in distilled water and collected on suitable electron microscopy grids. They are dried and heated to 800 "C to obtain amorphous AI2O3. High-purity wires of the desired metals can then be vacuum evaporated on to the films in an evaporator. These films can also be prepared using Al-nitrate,... 

Characterization of the Surfaces of Catalysts Measurements of the Density of Surface Faces for High Surface Area Supports. - It has always been a tenet of theories of catalysis that certain reactions will proceed at different rates on different surface planes of the same crystal. Experiments with metal single crystals have vindicated this view by showing that the rate of hydrogenolysis of ethane on a nickel surface will vary from one plane to another. In contrast the rate of methanation remains constant for the same planes.4 Because of this structure sensitivity of catalytic processes there is a requirement for methods of determining the number of each of the different planes which a catalyst and its support may expose at their surfaces. Electron microscopy studies of 5nm Pt particles supported upon graphite show them to be cubo-octahedra with surfaces bound by (111) and (100) planes.5 Similar studies of Pd and Pt prepared by evaporation reveal square pyramids of size 60-200 A bounded by incomplete (111) faces.6... 

The writer is greatly indebted to C. S. Smith, C. S. Barrett, and E. A. Gulbransen for many fruitful discussions pertinent to surface studies pursued over an extended period. Much helpful assistance in the preparation and characterization of sample surfaces from the viewpoint of surface structure was provided at one time or another by Joseph Cerny, Walter Bergmann, Donald Clifton, Kaye Ikeuye, and James Hess. Without their assistance the tedious assignment involved in the meticulous preparation of sample surfaces could not have been achieved. The useful collaboration with L. P. Schulz on the characterization of metal surfaces by electron microscopy and electron diffraction techniques was an essential part of the surface studies. Assistance in the design and construction of techniques for the controlled evaporation of metals was provided by H. E. Shaw. 

Ladas et al. measured the rate of CO oxidation by Oj on a variety of Al203-supporicd Pd catalysts at 445 K. CO pressure equal to 1.2 X 10 " Pa. and PqJPco equal to 1.1 [S. Ladas, H. Poppa, and M. Boudart, Sutf. Sci., 102 (1981) 151]. The alumina was a flat single crystal onto which Pd particles were deposited by a metal evaporation method. The resulting Pd particle si/c was measured directly by transmission electron microscopy. 

In order to prepare oxide model systems well-suited for characterization by high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), AFM or X-ray photoemission spectroscopy (XPS), as well as for kinetic studies by gas chromatography (GC), oxide films and oxide nanoparticles were vacuum-grown on a crystalline soluble substrate (e.g., NaCl(OOl)) via oxide (or metal) evaporation in a low background pressure ( 10" Pa) of oxygen. 

Figure 6.13 Scanning electron microscopy image of an Au SSV structure. Note the smooth electroplated metal walls and top surface. The rough circular areas at the bottom of each cavity are the evaporated Au substrate.

Samples for high-angle scanning electron microscopy (SEM) (to verify PGDTEM images) were fastened onto metal stubs with conductive glue, placed in a Denton DV7502 vacuum evaporator, and pumped down to less than 10 6 Torr. The surfaces... 

Catalysts were deposited on the char particles by evaporation from solution. Catalyst concentrations were 5 wt % metal. Catalyst distribution on the char was examined by electron microprobe and scanning electron microscopy. 

The surface structure of thicker samples can be examined using scanning electron microscopy (SEM). Here a fine beam of electrons is scanned across the surface of the specimen, and the scattered secondary electrons emitted from the surface of the sample are detected electronically. Secondary electrons are best produced by electron collision with conduction electrons in a metal surface (by analogy with the photoelectric effect), so samples for SEM are usually coated by evaporation with a thin film of gold in order to make them more visible. This naturally limits the fine detail that can be seen. 

Metal films evaporated onto glass can be examined by using molecular spectroscopy and methods such as field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The characterization of organic surfaces has been reviewed by Perry and Somorjai. ... 

Single shell carbon nanotubes were produced in over 70 percent yields by condensation of a laser-vaporized carbon-nickel-cobalt mixture at 1200°C [12] [81]. No multishell nanotubes were detected in the VLS process. X-ray diffraction and electron microscopy showed that the single shell nanotubes have uniform diameters and self-organize into metallic ropes (mats or arrays) of 100-500 nanotubes having a single-rope resistivity of <10 ohm-cm at 300 K. The particulate mixed-metal Ni-Co catalyst exists at the live end of the growing nanotube and leaves the end by evaporation. 

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