Oxide films porous, metal dissolution

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. 

The current (z) measured during the growth of the porous film can be taken as the sum of the ionic current, due to the oxidation of the metal at the metal/oxide interface, and the electronic current, 4i, due to faradaic processes occurring at the oxide/electrolyte interface. The former can be described as the sum of the formation current density, associated to oxide formation, and the dissolution current, zmetal ions into the electrolyte at the pore bottom. Then,... 

Concrete is a porous media that transmits moisture, oxygen, and aggressive ions (chlorides and sulfates) to the surface [7,67]. Chlorides penetrate the passive oxide film and form active-passive cells. Minute anodes are formed surrounded by a large cathode area, resulting in very high current densities and metal dissolution at the anodes. The metal area immediately surrounding the anode is cathodicaUy protected. 

The properties eharaeterizing a typieal passivating oxide film are low ionie eonductivity and low solubility. Due to these properties the oxide prevents to a large extent the transport of metal ions from sites in the crystal structure of the metal to the liquid, i.e. it prevents the anodie dissolution. However, there is a slow dissolution corresponding to the passive eurrent density ip. It is assumed that sueh oxide films are not formed by deposition of eorrosion products from the liquid, as this usually gives more or less porous surfaee layers, but that the oxide is direetly formed in close connection with the erystal structure of the metal. Because of this and because the films are very thin, many passivating films allow electrons to be transferred, so that electroehemieal reaetions ean occur on the external face of the oxide. In order to understand praetieal eorrosion cases it is often important to know the differences between various kinds of surfaee films. 

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... 

Conducting polymers can be prepared by chemical or electrochemical techniques. Electrochemical synthesis provides easier routes when compared with chemical synthesis and allows control over film formation, especially relevant if polymers are required as thin films deposited on the surface of metallic substrates. However, electrochemically synthesized polymers are usually more porous, a feature that requires consideration when a barrier effect is necessary. Another important aspect in the corrosion field is that the application of potential/current necessary to promote electropolymerization may accelerate dissolution (corrosion) of the metal. In some cases, an oxide pre-layer is deposited between the metal and the polymer to promote adhesion and hinder metal dissolution during the electropolymerization process (Tallman et al., 2002 Spinks et al., 2002). Alternatively, the application of layered coatings based on different conducting polymers can be a strategy to overcome the problem of metal dissolution. In the work of Lacroix et al. (2000), a layer of PPy was firstly deposited on zinc and mild steel in neutral conditions, followed by deposition of PANi in an acidic medium, because the direct deposition of PANi on those metallic substrates was not possible in an acidic medium, causing dissolution of the metal. 

For homogeneously doped silicon samples free of metals the identification of cathodic and anodic sites is difficult. In the frame of the quantum size formation model for micro PS, as discussed in Section 7.1, it can be speculated that hole injection by an oxidizing species, according to Eq. (2.2), predominantly occurs into the bulk silicon, because a quantum-confined feature shows an increased VB energy. As a result, hole injection is expected to occur predominantly at the bulk-porous interface and into the bulk Si. The divalent dissolution reaction according to Eq. (4.4) then consumes these holes under formation of micro PS. In this model the limited thickness of stain films can be explained by a reduced rate of hole injection caused by a diffusional limitation for the oxidizing species with increasing film thickness. 

Mechanical Passivity.—In certain instances the dissolution of an anode is prevented by a visible film, e.g., lead dioxide on a lead anode in dilute sulfuric acid this phenomenon has been called mechanical passivity, but it is probably not fundamentally different from the forms of passivity already discussed. The film is usually not completely impervious, but merely has the effect of decreasing the exposed surface of the electrode to a considerable extent the effective c.d. is thus increased until another process in which the metal is involved can occur. At a lead anode in sulfuric acid, for example, the lead first dissolves to form plumbous ions which unite with the sulfate ions in the solution to form a porous layer of insoluble lead sulfate. The effective c.d. is increased so much that the potential rises until another process, viz., the formation of plumbic ions, occurs. If the acid is sufficiently concentrated these ions pass into solution, but in more dilute acid media lead dioxide is precipitated and tends partially to close up the pores the layer of dioxide is somewhat porous and so it increases in thickness until it becomes visible. Such an oxide is not completely protective and attack of the anode continues to some extent it is, however, a good conductor and so hydroxyl ions are discharged at its outer surface, and oxygen is evolved, in spite of its thickness. 

The anodic partial reaction also involves a charge transfer at the interface because a metal atom loses electrons. It then dissolves in the solution as a hydrated or complexed ion and diffuses towards the bulk. In the vicinity of the metal surface, the concentration generated by dissolution therefore often exceeds that of the bulk electrolyte. Once the solubility threshold is reached, solid reaction products begin to precipitate and form a porous film. Alternatively, under certain conditions, metal ions do not dissolve at all but form a thin compact oxide layer, called passive film. The properties of the passive film then determine the rate of corrosion of the underlying metal (Chapter 6). 

When a negative solubility gradient exists from the metal surface to the outside of the salt layer, dissolution of the NiO film covering the metal is followed by reprecipitation of NiO at some distance away from the surface. The oxide precipitate is highly porous and therefore non-protective [23]. Figure 9.40 schematically shows such a situation. 

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