Principles of electrodeposition

as are the other components necessary for the preparation of practical resist emulsions (section 2.4). Finally, in section 2.5 the different applications of ED resists are described in more detail. 

Electrodeposition, which also encompasses metal electroplating and the coating of various dispersed solids, is used here specifically with reference to the deposition of emulsified organic material onto a conductive substrate. The two methods of electrodeposition are cataphoretic coating, in which the part to be coated is made the cathode, and ana-phoretic coating, in which the part to be coated is made the anode. 

In both cataphoretic and anaphoretic resists an ionized polymer acts as a surfactant to emulsify itself and the other resist ingredients in water. The resulting micelles are typically of the order of 50-200 nm in diameter and bear a surface charge. A typical micelle in a cataphoretic emulsion, surrounded by a diffuse layer of counter-ions, is shown in Fig. 2.1. 

Coulombic repulsion of like charges keeps the particles sufficiently separated to avoid flocculation and settlement. 

The following discussion concentrates on the mechanism of cataphoretic coating for simplicity. The principles of anaphoretic coating are basically the same, the only difference being reversal of the charges on the micelles and electrodes. 

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R.J. Dibert, Symposium on Electrodeposition of Coating, Introduction and principles of electrodeposition of coating, J. Paint Technol., 1966, 43, 421-423 G.E.F. Brewer, Electrodeposition of film-forming macroions, J. Macromol., Sci.-Chem., 1973, A7, 709-713. 

All the basic principles of electrodeposition of alloys were presented by Brenner [25] the alloy is produced by the reduction of the ions of each component of the alloy. 

The electrophoretic priming of motor vehicles was introduced in 1963, some years after the principles of electrodeposition of paint were first established. The process is now widespread indeed, almost all mass-produced, steel car bodies are pretreated in this manner. 

Following this introduction, the chapter continues with a description of the principles of electrodeposition (section 2.2). The various types of polymers employed in ED resist formulations are then described (section... 

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

Electrodeposition presents, in principle, several advantages for the investigation and production of layered alloys. Among these are the tendency of electrodeposited materials to grow epitaxial and thus to form materials with a texture influenced by the substrate. Electrodeposition can be used in systems that do not lend themselves to vacuum deposition. The electrodeposition process is inexpensive and can be upscaled with relative ease for use on large parts further, it is a room-temperature technology. This last point may be important for systems in which undesirable interdiffusion between the adjacent layers may readily occur. 

In 1986 Yahalom and Zadok (4) pointed to methods to produce composition-modulated alloys by electrodeposition, initially for the copper-nickel couple. They obtained modulation to thicknesses down to 8 A. The principle of the method is as follows. 

Comparing this approach with previous work - except the studies on solid electrolytes - ionic liquids have two distinct advantages over aqueous or organic solvents (i) Due to their extremely low vapor pressure ionic liquids can be used without any problem in standard plasma vacuum chambers, and the pressure and composition in the gas phase can be adjusted by mass flow controllers and vacuum pumps. As the typical DC or RF plasma requires gas pressures of the order of 1 to 100 Pa, this cannot be achieved with most of the conventional liquid solvents. If the solvent has a higher vapor pressure, the plasma will be a localised corona discharge rather than the desired extended plasma cloud, (ii) The wide electrochemical windows of ionic liquids allow, in principle, the electrodeposition of elements that cannot be obtained in aqueous solutions, such as Ge, Si, Se, A1 and many others. Often this electrodeposition leads to nanoscale products, as shown e.g. by Endres and coworkers [60]. 

In Chapter 1 we explain the motivation and basic concepts of electrodeposition from ionic liquids. In Chapter 2 an introduction to the principles of ionic liquids synthesis is provided as background for those who may be using these materials for the first time. While most of the ionic liquids discussed in this book are available from commercial sources it is important that the reader is aware of the synthetic methods so that impurity issues are clearly understood. Nonetheless, since a comprehensive summary is beyond the scope of this book the reader is referred for more details to the second edition of Ionic Liquids in Synthesis, edited by Peter Wasserscheid and Tom Welton. Chapter 3 summarizes the physical properties of ionic liquids, and in Chapter 4 selected electrodeposition results are presented. Chapter 4 also highlights some of the troublesome aspects of ionic liquid use. One might expect that with a decomposition potential down to -3 V vs. NHE all available elements could be deposited unfortunately, the situation is not as simple as that and the deposition of tantalum is discussed as an example of the issues. In Chapters 5 to 7 the electrodeposition of alloys is reviewed, together with the deposition of semiconductors and conducting polymers. The deposition of conducting polymers... 

The autocatalytic deposition of Ni and Co was intensively studied, although the most realistic mechanism has not yet been proposed. In the explanation of the mechanisms of autocatalytic deposition, the authors use the principles of the electrodeposition, although there are significant differences among the two processes. 

The first subdiscipline of chemistry in which the QCM was widely applied was electrochemistry. In 1992 Buttry and Ward published a review entitled Measurement of interfacial processes at electrode surfaces with the electrochemical quartz crystal microbalance , with 133 references [8]. This is the most widely cited paper on quartz crystal microbalances. After presenting the basic principles of AT-cut quartz resonators, the authors discuss the experimental aspects and relation of electrochemical parameters to QCM frequency changes. In their review of the investigation of thin films, they discuss electrodeposition of metals, dissolution of metal films, electrovalency measurements of anion adsorption, hydrogen absorption in metal films, bubble formation, and self-assembled monolayers. The review concludes with a brief section on redox and conducting polymer films. 

Figure 7.25 Principles of anodic voltammetric stripping to study the influence of additives on electrodeposition.

Although this chapter is limited to electrodeposition of semiconductors, it is only fair to mention, even if briefly, some examples of electrodeposition of metal nanostructures. This is important because the principles and techniques used in electrodepositing metals are essentially the same as those used for depositing semiconductors - the main difference is that almost all studies on electrodeposition of nanocrystalline semiconductors involve compound semiconductors, with the added comphcations this entails. Examples include pulsed electrodeposition of metal multilayers [1, 2], porous membrane-templated electrodeposition of gold nanotubes [3], and Ni nanowires [4]. 

Here, we develop the principle of the potential step method for a particularly simple reaction, namely, the electrodeposition of a metal under conditions where the deposition rate is controlled by mass transport. 

Separation procedures are based on the principles of volatilization, liquid-liquid distribution, adsorption, diffusion, chromatography, ion exchange, electrophoresis, precipitation, coprecipitation, and electrodeposition. In all of these, radio-tracers provide the best tool for methodological investigations, determination of equilibrium constants, kinetic data, and optimization of applied analytical data (yield, interference levels, etc.) [54], Use of radiotracers in complex multielement separation schemes is reviewed in [4], [17], [20]. [41], [54], radiochromatography is reviewed in [551. [61], [93], 197],... 

At present, in electrochemistry, the most commonly studied ionic liquids belong to the family of the imidazolium-based ones. This is due to their low melting points and high electrochemical stability. The potential application of these ionic liquids in electrodeposition processes [26] and development of some electrochemical devices, such as biosensors [28] and lithium batteries [29], have been largely considered. Evidently, when accounting for toxicity and environmental persistence, these solvents do not fully address these important principles of green chemistry. 

The principle of electrophoresis, the most straightforward method for electrodeposition is based on the electric field-driven charged particles (silicate, organosilicate, metal oxides, micelles, or polymer composite particles) to an electrode at an electrophoretic velocity, v, which is determined by Stoke s law. 

Such electrodeposition and synthesis methods are based on multielectron processes of metal and nonmetal deposition from ionic melts. The absence of information on the theoretical basis and principles of the control of both multielectron processes and HES processes did not allow one to conduct electrometallurgic synthesis in practice. However, systematic data accumulated at the turn of the century regarding multielectron processes of refractory metal and nonmetal electrodeposition served as the scientific basis and impact for the revived interest in the problem of electrolytic deposition and HES from melts. 

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