Thickness of oxide film formed

A number of methods for measuring the thickness of oxide films formed on metals have been described by Kubaschewski and Hopkins (1). Bach method has its special advantages and limitations and it is highly important, wherever possible, to check results obtained by one method with those made by one or more of the other methods. 

The thickness of oxide films formed with increasing time on the 100, ill, l 0, and 311 faces of a copper crys-... 

Determinations of the refractive index and thickness of oxide films formed by various methods on silicon have been made by Archer and by Claussen and Flower " using ellipsometry. Archer found a refractive index of 1.362 for an oxide film formed in an aqueous solution of 0.1 M sodium borate and 0.1 M boric acid. Claussen and Flower found that the refractive index of the oxide formed in 0.0025 N KNO3 anhydrous N-methylacetamide was 1.468 and that of films formed in 0.04 iV KNO3 in N-methylace-tamide with 6-8% water was 1.417. Zobel and Young using Lewis method " with liquids of known refractive index, and ellipsometry, have observed a refractive index for films formed in the above aqueous borate solution of about 1.4. Care has to be taken to check that film breakdown effects do not render the film untypical. 

A layer of thick, protective, strongly cohesive oxide film can be formed on zirconium by a patented process developed at TWV. In this process, a zirconium subject is treated in fused sodium cyanide containing 1-3% sodium carbonate, or in a eutectic mixture of sodium and potassium chlorides with 5% sodium carbonate. Treatment is carried out at temperature ranging from 600 to 800°C for up to 50 h. A treatment time of several hours typically is used. The thickness of oxide film formed in the fused salt bath ranges from 20 to 30 gm. This film has greatly improved resistance to abrasion and galling over thick oxide films grown by many other means. 

Scale a thick visible oxide film formed during the high-temperature oxidation of a metal (the distinction between a film and a scale cannot be defined precisely). 

The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6% from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area to a point where the roughness factor approaches unity. 

Stainless steel is corrosion resistant because a protective oxide layer naturally forms on top of the surface in the presence of oxygen and humidity. This protective oxide layer typically has a thickness in the order of nanometers, depending on the present environmental conditions. XPS studies of oxide films formed in air on AISI 316 revealed that not only oxidation of the material takes place, but also chromium and metallic nickel accumulate at the interface between oxide layer and bulk material [1]. The protective film is, of course, not perfect but contains defects like inclusions and grain boundaries. At these defects the film may locally break down and dissolution of the bulk material may start [2]. This kind of corrosion is called pitting corrosion and is estimated to cause a third of all chemical plant failures in the United States [3]. 

Oxide Films on Conner. Oxide films are worthy of particular mention since thin oxide films are always formed on copper surfaces. When they are thin, these films do not lead to any significant increase in contact resistance [8]. However, the thickness of oxide films increases with time [10]. As the thickness increases, there is an increase in contact resistance [11] in the form of film resistance hence, there is a need for regular contact bar cleaning in every plant. If sufficient cleaning is not carried out, the contact temperatures will increase due to this higher resistance and the rate of oxide formation will also increase since the rate of formation of oxide films is also known to increase with temperature [12]. The rate of oxide formation can become rapid if the temperature is allowed to continue to escalate [11]. 

Metals in the passive state (passive metals) have a thin oxide layer on their surface, the passive film, which separates the metal from its environment. Metals in the active state (active metals) are film free. Most metals and alloys that resist well against corrosion are in the passive state stainless steel, nickel-chromium based superalloys, titanium, tantalum, aluminum, etc. Typically, the thickness of passive films formed on these metals is about 1-3 nm. 

The thicknesses of the films formed at the passive potentials -100 and 500 mV (SCE) were calculated to be 1.0 and 1.5 nm, respectively. It can be seen fiom Figure 2 that the Ni spectra contain almost no detectable oxidized Ni. Therefore, Ni does not contribute directly to the structure of the barrier or deposit films. This is surprising because Ni is well known to contribute to pitting resistance [2], In addition, it can be seen that the film formed at 500 mV (SCE), which is several hundred millivolts above the transpassive potential of Mo, contains contributions fiom Mo (as M0O2), which has been shown to be the main cation in the passive film formed on Mo in 0.1 M HCl [3], Therefore, it would appear that Mo contributes directly to the passive... 

Fig. 5. XPS spectra of As 3d and In 4d in InAs sample during various stages of its processing in oxygen glow discharge plasma at sample voltage of 350 V (a), experimental values of the relation between integrated intensities of peak components and calculated values of thickness of the formed oxide films (b) ( ) - total relation of As 3d / In 4d peak components intensities ( ) - relation of peak components intensities of As 3d / In 4d connected with the substrate (A) - relation of peak components intensities of As 3d / In4d connected with the oxide film ( ) - calculation of the total thickness of oxide films from the absolute attenuation of As 3d peak from the substrate (o) - calculation of the thickness of oxide films from the ratio of As 3d peak components connected with the oxide film and InAs substrate.

There is also a small rise in the Cr O O and Cr 02 signals in the vicinity of the metal/oxide interface which corresponds to the zone where the original 2 nm thick, 0-oxide film formed during surface pretreatment is situated. Expansion of the Cr 02 profile confirms a small but clearly distinct peak in this region (Fig. 4.4(b)) for polycrystaUine chromium oxidized at 825°(3. This result demonstrates that the O/SIMS technique is sufficiently... 

With respect to the UHV-based techniques capable of providing chemical analysis, such as ESCA, AUGER, etc., several such studies have been performed. However, these studies were, by and large, performed on very thick oxide layers, formed after anodic oxidation of the Pt for many hours. Results from these studies thus have little bearing on the nature of the oxides formed on potential cycling. Part of the reason why these studies used such thick films lies in the considerable difficulty of detecting the thin oxide films formed during a potential sweep, even with relatively sensitive techniques. 

The oxide film formed in dry air at room temperature consists of a spinel phase, probably a solid solution of magnetite and maghemite. Such films form on magnetic tapes. They are around 1.5-2.0 nm thick, and in a dry atmosphere, can provide indefinite protection (e.g. the Delhi pillar). Ali and Wood (1969) found that with time and at a relative humidity of 46%, some hematite developed as well. At higher temperatures (200-300 °C) well defined duplex films with an inner layer of magnetite... 

The first step in sample preparation is the deposition of a thin metal film on an insulating substrate (e.g. a glass microscope slide). This base electrode is deposited by conventional vacuum deposition techniques with the electrode geometry defined by a shadow mask. Next, this electrode is oxidized either by exposing the film to room air or oxygen, or by establishing an oxygen plasma within the vacuum chamber. In the case of Al-electrodes, a remarkably uniform oxide layer is formed, typically 1-2 nm thick. The oxide film may then be dosed with the compound of interest this is achieved in one of three ways. 

Several different types of reaction patterns may form on the sphere as the reaction proceeds. In cases where the reaction products actually build up on the surface of the metal, as in oxidation, the oxide film will form more rapidly on one face than on another. When the films are in the range of 200 to 2,500 A. and the sphere is examined by placing a tube of white paper over the crystal, the different thicknesses of oxide on the different faces appear as a regular pattern of interference colors of great beauty. The symmetry of one of these highly colored patterns is shown in Fig. 1. In electrodeposition on a crystal sphere at a low current density the metal will deposit more rapidly on one face than another, and the sphere is converted into a polyhedron, or small facets are formed on the different faces which may be seen under the microscope. 

An HgCdTe layer 2, comprising photodiodes 3, is formed on a first side of a CdTe substrate 11. The detector is bonded to a silicon chip 7 by a flip-chip process. A reflection preventive film is formed on a second side, opposite to the first side, of the CdTe substrate. The film is formed by a cyclotron resonance plasma CVD method by introducing nitrogen, nitrous oxide and silane as reaction gas. The thickness of the film is selected so that the reflectivity is minimized for radiation having a wavelength to be detected by the photodiodes. 

One of the primary advantages of the polycide concept is that the silicide top layer oxidizes readily to form a dense adherent Si02 overlayer, and the polycide structure underneath remains intact. Also, the oxide film forms within a reasonable time. Oxide thicknesses formed by dry 02 oxidation are shown in Figure 10, as a function of time and temperature. 

As another example, oxide films on a vapor-deposited Ag substrate are presented [116]. Detailed XPS investigations show the development of Ag20 already 0.15 V below its equilibrium potential of E = 0.35 V [ 115]. Fig.50a presents the k-weighted Fourier Transform of the reflectivity-EXAFS, FT(ARk). of a 2.5 nm thick oxide film formed in 1 M NaOH at E = 0.40 V at an angle 0 = 0.09° relative to the surface. 

Layer-by-layer Auger spectral analysis of the anodic and cathodic products formed from the TFE solutions has revealed that two groups of spectra show the lines from free carbon (260-280 eV), oxygen ( 500 eV) characteristic for nickel oxide and nickel itself across the whole thickness of the film (Fig. 3). 

Aluminum is an active metal and its resistance to corrosion depends on the formation of the protective oxide film. According to the Pourbaix diagram the metal is passive in the pH range —4-9. The protective oxide film formed in water and atmospheres at ambient temperatures is amorphous and a few nanometres in thickness. The stability of the oxide film and its disruption results in corrosion. 

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