Deposit thickness

Decorative chromium plating, 0.2—0.5 ]lni deposit thickness, is widely used for automobile body parts, appHances, plumbing fixtures, and many other products. It is customarily appHed over a nonferrous base in the plating of steel plates. To obtain the necessary corrosion resistance, the nature of the undercoat and the porosity and stresses of the chromium are all carefliUy controlled. Thus microcracked, microporous, crack-free, or conventional chromium may be plated over duplex and triplex nickel undercoats. 

Electrolytic plating rates ate controUed by the current density at the metal—solution interface. The current distribution on a complex part is never uniform, and this can lead to large differences in plating rate and deposit thickness over the part surface. Uniform plating of blind holes, re-entrant cavities, and long projections is especiaUy difficult. 

Electrodeposits of Pt can only be applied as relatively thin coatings that are porous. Although the porosity decreases with increase in deposit thickness, so does the internal stress and if the platinum adhesion is poor the coating may exfoliate. As a consequence, thicknesses of 2-5 to 1-5 fim Pt... 

Deposit thicknesses appropriate to various conditions of service are laid down in a number of specifications. Formerly, those of most general application were the standards issued by the British Standards Institution and listed in Table 11.6. These standards are still valuable documents, but those that have recently been revised now omit specific thicknesses of deposit and have... 

It will be seen that the design of articles to be electroplated can have a considerable effect on the corrosion resistance of the electrodeposited coating. The chief effects are the result of variations in deposit thickness, but also important are features which can influence the adhesion, porosity and physical properties of the deposit. Good design will also avoid features of the plated article capable of trapping liquids or solid contaminants which might cause more rapid corrosion. 

With heat exchangers, cleaning should be considered as an option when the efficiency has fallen off to some specific level e.g. a terminal temperature difference. With boilers, unless there has been some occurrence which may be alleviated by a particular clean, periodic cleaning to pre-empt corrosion by limiting deposit thickness should be considered... 

Back-pressure also increases as generator load increases. Where scale deposition takes place on the condenser cooling-water side, a similar increase in back-pressure can be observed. The level of increase is based on a combination of deposit thickness and the particular mineral present. 

NOTE For any boiler, the maximum recommended tolerance for deposit thickness can be related to a weight of deposit per unit area clearly, the weight will vary dependent on the density of the deposit. Typically, deposit densities vary between 2.3 and 5.7 g/cm3. The densities of calcite and magnetite (which are two common mineral components of deposits) are 2.71 and 5.17 g/cm3, respectively. Table 4.2 assumes an average deposit density of 3.5 g/cm3. 

All deposits contain various ratios of scale and corrosion products, but often one material predominates, such as calcite or magnetite. These materials have different densities and thermal factors that influence the allowable deposit thickness or weight per unit area before cleaning becomes necessary. Practical allowances usually are between the limitations for each of these two materials. These allowances may be perhaps 50 to 100 mg/cm2 of surface area for lower pressure boilers and 25 to 50 mg/cm2 of surface area for higher pressure boilers. (For a more precise allowance, see the information below.)... 

Referring to Table 4.2 (boiler heat transfer surface cleanliness) and the supporting italicized notes in Chapter 4 concerning tolerance for deposit thickness, it can be seen that ... 

Historically, EC-ALE has been developed by analogy with atomic layer epitaxy (ALE) [76-82], ALE is a methodology used initially to improve epitaxy in the growth of thin-films by MBE and VPE. The principle of ALE is to use surface limited reactions to form each atomic layer of a deposit. If no more than an atomic layer is ever deposited, the growth will be 2-D, layer by layer, epitaxial. Surface limited reactions are developed for the deposition of each component element, and a cycle is formed with them. With each cycle, a compound monolayer is formed, and the deposit thickness is controlled by the number of cycles. 

Fig. 15. Graph of deposit thickness as a function of the potential used to deposit Te. Each point represents a deposit formed with 200 cycles. Adapted from ref. [142],...

CdSe DEPOSIT THICKNESS as a FUNCTION of Se DEPOSITION POTENTIAL... 

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. 

Although electroless deposition seems to offer greater prospects for deposit thickness and composition uniformity than electrodeposition, the achievement of such uniformity is a challenge. An understanding of catalysis and deposition mechanisms, as in Section 3, is inadequate to describe the operation of a practical electroless solution. Solution factors, such as the presence of stabilizers, dissolved O2 gas, and partially-diffusion-controlled, metal ion reduction reactions, often can strongly influence deposit uniformity. In the field of microelectronics, backend-of-line (BEOL) linewidths are approaching 0.1 pm, which is much less than the diffusion layer thickness for a... 

The metal ion in electroless solutions may be significantly complexed as discussed earlier. Not all of the metal ion species in solution will be active for electroless deposition, possibly only the uncomplexed, or aquo-ions hexaquo in the case of Ni2+, and perhaps the ML or M2L2 type complexes. Hence, the concentration of active metal ions may be much less than the overall concentration of metal ions. This raises the possibility that diffusion of metal ions active for the reduction reaction could be a significant factor in the electroless reaction in cases where the patterned elements undergoing deposition are smaller than the linear, or planar, diffusion layer thickness of these ions. In such instances, due to nonlinear diffusion, there is more efficient mass transport of metal ion to the smaller features than to large area (relative to the diffusion layer thickness) features. Thus, neglecting for the moment the opposite effects of additives and dissolved 02, the deposit thickness will tend to be greater on the smaller features, and deposit composition may be nonuniform in the case of alloy deposition. 

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