Growth of thin oxide films

The transformation of the overlayer into a bulk compound is an important area of study. This aspect is discussed in detail for the oxidation of nickel and refractory metals Cr, Mo, and W [26]. Chapter 5 of this book is essentially focused on the oxide growth of thin oxide films. It is well accepted that relaxation or surfece reconstruction plays an important role in this transformation by loosening the metal-metal bonds. When the reconstructed layer presents strong structural similarities to a dense plane of the bulk compound, it can be considered as a transition layer between the metal lattice and the bulk compound lattice. According to atheoretical model [76], the penetration of the nonmetal atoms into the metal and its conversion into bulk compound would be assisted by the electric field resulting from the charge transfer from the metal to the adsorbed atom. The magnitude of this field depends on the density of adsorbed atoms. As a consequence, the onset of the bulk compoimd formation occurs only beyond a critical concentration of adsorbed atoms in the overlayer. 

The growth of thin oxide films on metals and semiconductors can be a useful as well as a destmctive phenomenon. The study of the reactions involved leads to a measure of control. Specific oxide thickness and properties can be achieved for electronic... 

The rapid development of solid state physics and technology during the last fifteen years has resulted in intensive studies of the application of plasma to thin film preparation and crystal growth The subjects included the use of the well known sputtering technique, chemical vapour deposition ( CVD ) of the solid in the plasma, as well as the direct oxidation and nitridation of solid surfaces by the plasma. The latter process, called plasma anodization 10, has found application in the preparation of thin oxide films of metals and semiconductors. One interesting use of this technique is the fabrication of complementary MOS devices11. Thin films of oxides, nitrides and organic polymers can also be prepared by plasma CVD. 

The reaction between a metal and oxygen is one of the most common phenomena encountered. Yet the mechanism of the formation and growth of the oxide film is still not understood, particularly in the region of very thin oxide films. There remains, too, a lack of sufficient, reliable data for the initial stages of oxidation. 

B. Goldstein and D.J. Szostak. Growth of Thin Platinum Films on Hydrogenated Amorphous Silicon and Its Oxide. J. Vacuum Sci. Technol. 17 718 (1980). 

Growth of surface oxide films takes place only if cations, anions, and electrons can diffuse through the oxide layer. The growth kinetics of very thin films ( 10-50 A) often follow the Mott or Cabrera-Mott mechanisms in which electrons tunnel through the film and associate with oxygen atoms to produce oxide ions at the surface. A large local electric field (10 -10 V/cm) results at the surface which facilitates cation diffusion from the metal-oxide interface to an interstitial site of the oxide. The film thickness Z at time t is given by... 

A further, more detailed, radiochemical investigation of growth of anodic oxide films on tantalum and aluminum has recently been made by Davies et Two types of experiments were performed. First, transport number experiments using Xe markers as described previously, and second, experiments with neutron-activated metals to determine the amount of metal lost to the solution during the anodizing process. In the former, trace amounts of Xe were incorporated into a thin surface layer of anodic oxide. The specimen was then anodized, the depth of the embedded tracers being measured at intervals with a 5-spectrometer. Metal foils identical to those used in the transport number experiments were neutron activated, and then anodized at constant current. The amount of activity in the anodizing solution was then determined by comparison with solutions in which a known quantity of activated material had been dissolved. 

Electrochemical growth of thin La203 films on oxide and metal surfaces. Materials Science and Engineering C, Vol. 23, No. 1-2, pp. 123-128 ISSN 0928 - 4931... 

The generally accepted model for passive film growth, illustrated in Fig. 3-14, is of field-assisted film formation, which is essentially a modified Cabrera-Mott model originally established for gaseous oxidation and the formation of thin oxide films in a gas at low temperature (Cabrera and Mott, 1948-1949 Fehlner and Mott, 1970). This classical theory describes the growth, in the direction perpendicular to the surface, of an oxide layer completely covering the substrate surface, by a hopping mechanism. The... 

Czerwinski, F. 2000. On the nse of the micromarker technique for studying the growth mechanism of thin oxide films. Acta Mater. 48(3) 721-723. 

If K = 1 K, a = 0.25 nm, and z = 3, X = 30nm at 300 K, so that for a film 1 nm thick, the field increases the rate of growth by a factor of about 10 The term in the growth law due to the field, namely exp (K/X), is large only when X is small. Because of this a thin oxide film can form even at low temperatures where the ordinary rate of entry of ions into the oxide, is negligible. As the film thickens, the factor exp /X) decreases rapidly, and the rate of growth soon falls to such a low value that, for practical purposes, oxidation has ended. 

The terms hot corrosion or dry corrosion are normally taken to apply to the reactions taking place between metals and gases at temperatures above 100 C i.e. temperatures at which the presence of liquid water is unusual. The obvious cases of wet corrosion at temperatures above 100 C, i.e. in pressurised boilers or autoclaves, are not considered here. In practice, of course, common metals and alloys used at temperatures above normal do not suffer appreciable attack in the atmosphere until the temperature is considerably above 100 C. Thus iron and low-alloy steels form only the thinnest of interference oxide films at about 200 C, copper shows the first evidence of tarnishing at about 180 C, and while aluminium forms a thin oxide film at room temperature, the rate of growth is extremely slow even near the melting point. 

The fabrication of such a system can be accomplished only by nanofabrication, and different routes can be imagined in this context. We will focus in the following section on the template-controlled growth of metal clusters on thin oxide films, which has proven to give excellent results in terms of low complexity. This approach has been successfully employed for metal-on-metal systems (for a comprehensive review see [6]) and has recently been extended to metal growth on oxide films. 

If measurements are made in thin oxide films (of thickness less than 5 nm), at highly polished Al, within a small acceptance angle (a < 5°), well-defined additional maxima and minima in excitation (PL) and emission (PL and EL) spectra appear.322 This structure has been explained as a result of interference between monochromatic electromagnetic waves passing directly through the oxide film and EM waves reflected from the Al surface. In a series of papers,318-320 this effect has been explored as a means for precise determination of anodic oxide film thickness (or growth rate), refractive index, porosity, mean range of electron avalanches, transport numbers, etc. 

In addition to examining the structure and growth of thin passivating oxide films, the process of film reduction and breakdown promises to be a fruitful area of research. Some preliminary results involving SECM as well as STM have been published [257,270,271]. 

The surface oxidation of a metal such as copper is accompanied by the growth of an oxide layer, the thickness of which may be measured by the method of colour interference when due allowance is made for the refractive index of the oxide formed, or by the decrease in electrical resistance of a thin wire or tube of the metal as oxidation ensues. Investigations have been made on the rate of such oxidations by Tammann (Zeit.f anorg. GJiem. cxi. 78 cxxiii. 196 cxxiv. 196), Hinshelwood (Proc. Mog. Soc. A, cii. 318), Palmer Proo. Roy. Soc. A, cm. 444) and Dunn (unpublished, see also Pilling and Bedworth, Jour. Inst. Metals, xxix. 629, 1923). It is found that the rate of increase in thickness of the oxide film oc) obeys under ideal conditions the ordinary diffusion law or CG = kt. ... 

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