Local passivation

As shown in Fig. 22, since the dissolved metal ions are locally enriched near the surface, the fluctuation in concentration takes a positive value. In this fluctuation process, the passive film does not provide the absolute condition for protecting the substrate dissolution because, as shown in the preceding section, a breakdown in local passivity prior to... 

A process sequence schematic of the principal steps is given in Fig. 4.14. The process sequence is similar and compatible to the fabrication of the circular hotplate in Sect. 4.1.2. The most important modifications include the additional process steps to apply the local passivation. The modifications of the other steps are described in the following section [115]. 

It was observed for chloride-breakdown of the passive film on metallic iron in neutral borate solution that the amount of chloride ions required for initiating the local passivity breakdown is dependent on the film thickness, film defects, and electric field in the film as well as on the solution pH [41,42]. It was also observed that at the initial stage of the passivity breakdown the passive film locally dissolves and becomes thinner around the breakdown embryo before the underlying metal begins to dissolve in pitting at the passivity breakdown site [42,43]. From these observations, it is likely that the passivity breakdown is not a mechanical rupture of the passive film but a localized mode of dissolution of the passive film accelerated by the adsorption of aggressive anions on the film. 

A 3D structuration can be achieved, if [EMT] 1, that is, at high / el- If the structure resistance / po > / el> the field delocalizes, and 2 D-structures are obtained only. Highest / EL-values can be realized in monomolecular condensation films. For example, localized passivation of silicon or Ti may be important in nanotechnology. This can be done applying Scanning X Microscopy (SXM) techniques in moist gas atmosphere [164,165]. This allows the production of oxide patterns of some nm height and width of 20 to 100 nm. 

Some of them (for example, zinc) are corroded, with hydrogen evolution leading to a local desiccation of the electrode which makes the cell swell, if it is sealed. An interfacial pH increase can also occur leading to a local passivation of the electrode. 

The corrosion initiates by local passivity breakdown in the crevice as a result of (a) microeontacts due to wall asperity or (b) pitting inside the ereviee. 

Due to their very narrow gap, these microcrevices could be very efficient in promoting a very local pH change (see Section 10.5) and a local passivity breakdown by the mechanism presented in the former paragraph. As soon as active dissolution occurs somewhere inside the macrocrevice, the breakdown of passivity spreads out progressively to the whole crevice provided the potential inside the crevice is high enough. 

The passive state of a metal can, under certain circumstances, be prone to localized instabilities. Most investigated is the case of localized dissolution events on oxide-passivated surfaces [51, 106, 107, 108, 109, 110, ill, 112, 113, 114, 115, 116, 117 and 118]. The essence of localized corrosion is that distinct anodic sites on the surface can be identified where the metal oxidation reaction (e.g. Fe —> Fe + 2e ) dominates, surrounded by a cathodic zone where the reduction reaction takes place (e.g. 2Fi + 2e —> Fi2). The result is the fonnation of an active pit in the metal, an example of which is illustrated in figure C2.8.6(a) and (b). 

In an electrochemical polarization experiment on a passive system tire onset of localized dissolution can be detected by a steep current increase at a very distinct anodic potential (tire pitting potential, —see figure... 

From an electrochemical viewpoint, stable pit growtli is maintained as long as tire local environment witliin tire pit keeps tire pit under active conditions. Thus, tire effective potential at tire pit base must be less anodic tlian tire passivation potential (U ) of tire metal in tire pit electrolyte. This may require tire presence of voltage-drop (IR-drop) elements. In tliis respect the most important factor appears to be tire fonnation of a salt film at tire pit base. (The salt film fonns because tire solubility limit of e.g. FeCl2 is exceeded in tire vicinity of tire dissolving surface in tlie highly Cl -concentrated electrolyte.)... 

These processes are considerably more complex in actual CMOS fabrication. First, the lower layers of a CMOS stmcture typically have a twin-tub design which includes both PMOS and NMOS devices adjacent to each other (see Fig. 3b). After step 1, a mask is opened such that a wide area is implanted to form the -weU, followed by a similar procedure to create the -weU. Isolation between active areas is commonly provided by local oxidation of sihcon (LOCOS), which creates a thick field oxide. A narrow strip of lightly doped drain (LDD) is formed under the edges of the gate to prevent hot-carrier induced instabiUties. Passivation sidewalls are used as etch resists. A complete sequence of fabrication from wafer to packaged unit is shown in Figure 10. 

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

Copper-containing lead alloys undergo less corrosion in sulfuric acid or sulfate solutions than pure lead or other lead alloys. The uniformly dispersed copper particles give rise to local cells in which lead forms the anode and copper forms the cathode. Through this anodic corrosion of the lead, an insoluble film of lead sulfate forms on the surface of the lead, passivating it and preventing further corrosion. The film, if damaged, rapidly reforms. 

Precipita.tingInhibitors. As discussed earlier, the localized pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions. Precipitating inhibitors form complexes that are insoluble at this high pH (1—2 pH units above bulk water), but whose deposition can be controlled at the bulk water pH (typically 7—9 pH). A good example is zinc, which can precipitate as hydroxide, carbonate, or phosphate. Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

The CBD diagram can provide various lands of information about the performance of an aUoy/medium system. The technique can be used for a direc t calculation of the corrosion rate as well as for indicating the conditions of passivity and tendency of the metal to suffer local pitting and crevice attack. 

Silt, sand, concrete chips, shells, and so on, foul many cooling water systems. These siliceous materials produce indirect attack by establishing oxygen concentration cells. Attack is usually general on steel, cast iron, and most copper alloys. Localized attack is almost always confined to strongly passivating metals such as stainless steels and aluminum alloys. 

Passive attack involving underdeposit corrosion tends to involve large system surface areas and, hence, accounts for the greatest amount of metal loss, by weight, in cooling water systems. Active attack tends to produce intense localized corrosion and, as such, a greater incidence of perforations. 

There are no films or protective surface films on active metals, e.g., mild steel in acid or saline solutions. Passive metals are protected by dense, less readily soluble surface films (see Section 2.3.1.2). These include, for example, high-alloy Cr steels and NiCr alloys as well as A1 and Ti in neutral solutions. Selective corrosion of alloys is largely a result of local concentration differences of alloying elements which are important for corrosion resistance e.g., Cr [4],... 

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