Film formation and dissolution

Film Formation and Dissolution Behavior. In order to study the formation of film and the dissolution behavior of films of 1 and 2 toward an alkaline solution, 1 was dissolved at 25 wt% in 2-methoxyethanol at room temperature, then spin-coated on Si wafer. The wafer was prebaked (100 C for 5 min) to remove the residual solvent. The resulting clear and tough transparent film was obtained. Such films were also obtained from 2 by dissolving it in diglyme (30wt %) at room temperature, followed by spin-coating on Si wafer, and then prebaking at 80 C for 10 min. The films of 1 and 2 dissolved completely in 0.1 % tetramethylammonium hydroxide (TMAH) aqueous solution at room temperature within 5 sec. 

The purpose of this section is to review recent literature on the kinetics and mechanisms of iron oxide dissolution. Particular emphasis is placed on pure iron oxides and iron oxides on steel surfaces. The limited scope of this review precludes complete coverage of the vast amount of work on passive film formation and dissolution. However, some aspects of passivation that apply directly to chemical clening are reviewed in Chapter 4. 

The situation may also occur where a dense oxide film is formed on the surface, as is shown in Fig. 2.2(c). Such dense films are formed in the case of corrosion-resistant materials such as Cr and Ni and some valve metals. In this case the kinetics may be controlled by the movement of ions or electrons through the film (Sehmuki, 2002), or the dissolution rate of the film at the film-electrolyte interface, as depicted in Fig. 2.2(c). Since the film thickness, film composition and thereby film formation and dissolution can change with time (Yu and Scully, 1997), it is difficult to predict the rate of corrosion in this case. More importantly, the difference between active and passive dissolution rates, i.e. corrosion loss, is due to the presence of the film, be it porous or compact... 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

Thus films can be divided into two groups according to their morphology. Discontinuous films are porous, have a low resistance and are formed at potentials close to the equilibrium potential of the corresponding electrode of the second kind. They often have substantial thickness (up to 1 mm). Films of this kind include halide films on copper, silver, lead and mercury, sulphate films on lead, iron and nickel oxide films on cadmium, zinc and magnesium, etc. Because of their low resistance and the reversible electrode reactions of their formation and dissolution, these films are often very important for electrode systems in storage batteries. 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

The previous section discussed the structure at the junction of two phases, the one a solid electron conductor, the other an ionic solution. Why is this important Knowledge of the structure of the interface, the distribution of particles in this region, and the variation of the electric potential in the double layer, permits one to control reactions occurring in this region. Control of these reactions is important because they are the foundation stones of important mechanisms linked to the understanding of industrial processes and problems, such as deposition and dissolution of metals, corrosion, electrocatalysis, film formation, and electro-organic synthesis. 

On most corroding metals, the above reactions occur at an oxidized surface and, depending on the peroperties of the surface layer, passivation may occur by which the kinetics of metal dissolution are substantially supressed either by ohmic, ionic, or electronic transport at a surface passivating film or by electrocatalytic hindrance. In passivation phenomena, a steady state with a balance between the formation and dissolution of the surface film takes place. As a result, the ionic flux of metal ions dissolving through the passivating film is highly reduced. 

Because acoustic wave devices are sensitive and respond rapidly, they are ideally suited for real-time monitoring of chemical and physical systems. As discussed in the introduction to this chapter, thin films represent a growing industrial and technological concern for a variety of applications. The use of acoustic devices to characterize the physical properties of these films has been dealt with in the previous sections. Here we describe how these devices can be used to monitor film formation or dissolution processes, or to observe and characterize film properties as a function of time (similar to the monitoring of diffusion in polymers described in Section 4.2.2). 

A metal CMP process involves an electrochemical alteration of the metal surface and a mechanical removal of the modified film. More specifically, an oxidizer reacts with the metal surface to raise the oxidation state of the material, which may result in either the dissolution of the metal or the formation of a surface film that is more porous and can be removed more easily by the mechanical component of the process. The oxidizer, therefore, is one of the most important components of the CMP slurry. Electrochemical properties of the oxidizer and the metal involved can offer insights in terms of reaction tendency and products. For example, relative redox potentials and chemical composition of the modified surface film under thermodynamically equilibrium condition can be illustrated by a relevant Pourbaix diagram [1]. Because a CMP process rarely reaches a thermodynamically equilibrium state, many kinetic factors control the relative rates of the surface film formation and its removal. It is important to find the right balance between the formation of a modified film with the right property and the removal of such a film at the appropriate rate. 

Thus, surface film formation and metal dissolution occur via an oxidation reaction with a balancing reduction reaction to sink the electrons generated. 

The value of dissolution valence in the electropolishing region is somewhat lower than 4 as shown in Figs. 5.20 and 5.21. This indicates that hydrogen reaction, which is a chemical reaction, still occurs at such high anodic potentials, where the reactions are characterized by the formation and dissolution of an anodic oxide film. 

A New Model. The results of the studies on anodic oxide films (see section 5.9 and chapter 3 on passive film and anodic oxides) show that anodic oxide properties (oxidation state, degree of hydration, 0/Si ratio, degree of crystallinity, electronic and ionic conductivities, and etch rate) are a function of the formation field (the applied potential). Also, they vary from the surface to the oxide/silicon interface, which means that they change with time as the layer of oxide near the oxide/silicon interface moves to the surface during the formation and dissolution process. The oxide near the silicon/oxide interface is more disordered in composition and structure than that in the bulk of the oxide film. Also, the degree of disorder depends on the formation field which is a function of thickness and potential. The range of disorder in the oxide stmcture is thus responsible for the variation in the etch rate of the oxide formed at different times during a period of the oscillation. The etch rate of silicon oxides is very sensitive to the stmcture and composition (see Chapter 4). 

In a given solution, the rate of oxide growth is a function of the field, that is, Ta =fiyid). The dissolution rate of oxide is a function of solution composition, formation field, and the time lapse between formation and dissolution of the oxide, Ia-b. that is, i B =y([F]. pH, VId, fA-B). At a steady state, the rate of oxide growth equals the rate of its dissolution, ta = r, and the thickness of the oxide film is constant. When dJr Ia-b, that is, the oxide formed at A has different properties and a different etch rate from that at point B when oxide A reaches the surface, r, oscillation can occur when the following condition exists. 

The direct dissolution is sensitive to surface geometrical factors such as surface curvature and orientation, while the indirect dissolution through the formation and dissolution of oxide is insensitive to the surface geometrical factors. Formation of an oxide film, which generally shifts the rate limiting process to inside the oxide, masks the semiconductor properties of silicon. 

The observed current-potential behavior is a function of the simultaneous processes of film formation, its dissolution, and metal dissolution. The latter seems to be mostly responsible for the magnitude of the current at all potentials. In the active potential region dissolution is hindered by a decrease in the free electrode area, and in the passive region dissolution depends entirely on the properties of the passivating film. 

Investigated examples include film formation on lead electrodes, various metal dissolution processes, redox electrochemistry of electrochromic films of IrOa [272] and various intrinsically conducting polymers [273-276]. A review covering experimental aspects and results pertaining to ion adsorption, hydride and oxide film formation and hydrophilicity of metals has been provided elsewhere as well as further reports [277-284],... 

The electrochemical behaviour of metals in anhydrous HF has been reviewed by Vijh, with particular attention to anodization, open-circuit corrosion, film formation, anodic dissolution, and evolution of F2. The dependence of the F2 overpotential at Ni in anhydrous HF on the current density has been investigated. At low current densities the overvoltage was mainly due to the potential difference across the anodic barrier film, whereas at high current density the electronic conduction of the film increased appreciably, resulting in a decrease in the potential drop. Other workers have shown that the process of H2 discharge in HF is affected by the addition of NaF, presumably by reducing the overvoltage on nickel. 

Methylbenzyl alcohol can be used like benzyl alcohol, and can be employed advantageously in stoving enamels. In cellulose nitrate and acetyl cellulose lacquers methylbenzyl alcohol helps to improve flow and film formation and prevents blushing at relatively high atmospheric humidity levels. It has also proved to be a useful additive in paint-removal agents on account of its dissolution properties and long evaporation time. The solvency of methylbenzyl alcohol for colorants is similar to that of benzyl alcohol. 

Finally, if Eq. (1.13) gives < 0, it does not necessarily mean the absence of the film in the system. The situation is also possible (e.g., very small a or k) when the film is formed however, its positive stationary thickness is impossible. These non-stationary unsteady systems can exist either at partial covering of the surface or in the regime of periodic formation and dissolution of the film (type 3). The behaviour of such systems is somewhat similar to the unsteady electrochemical systems (see Chap. 5). 

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