Electroplating deposit distribution

For large Wa, the current distribution is uniform. For example, when electroplating (qv) an object, usually a uniform deposit is desirable. Equation 35 suggests that a larger piece, ie, low Wa, would be more difficult to plate uniformly than a smaller one. 

The sheer amount of material deposited on a cathode is of less practical significance than the distribution of the deposit over the cathode and its thickness. Indeed, it ought to be understood that in practice metal ions cannot be expected to and do not deposit as continuous sheets from one edge of the cathode to the other. Rather, metal ions become attached to the cathode at certain favored sites. The result of this is the possible presence of discontinuities in the form of pores, cracks, or other irregularities. Thus, in electroplating, current density and its distribution play a centrally important role in determining the quality of the final deposit. Defined in terms of the actual electrodeposition setup or process, the current density is the total current divided by the area of the electrode. 

Actinides were determined at the ultratrace level in moss samples collected from the eastern Italian Alps (1500 m a.s.l.). The frozen samples were cut into 1-2 cm sections and analyzed separately to obtain the distribution curves of the vertical concentrations. For plutonium and americium isotope analysis, 1-2 g of the samples were ashed, leached, separated with respect to analytes and analyzed by alpha spectrometry and LA-ICP-MS after the plutonium or americium had been electroplated on a stainless steel disk.23 Estimated limits of quantification of LA-ICP-MS for actinide radionuclides deposited on stainless steel plates after chemical separation are summarized in Table 9.45. For most of the long-lived radionuclides in moss samples, lower limits of determination were found at the 10 15gg 1 concentration level compared to those of a - spectrometry 23... 

The type and nature of the current density distributions are very important when electroplating is used for uniform deposits, appropriate conditions e.g., electrolytes of high throwing power are needed. 

Current density is defined as current in amperes per unit area of the electrode. It is a very important variable in electroplating operations. It affects the character of the deposit and its distribution. 

Fodisch et al. [238] deposited palladium by electroplating on a porous alumina layer, which had been obtained by anodic oxidation of aluminum. Electroplating can be seen as an alternative to impregnation. The authors observed a rather inhomogeneous (bimodal) deposition within the pores of the alumina layer palladium was mainly concentrated at the pore mouths and at the end of the pores, whereas no palladium was detected in the middle section of the pores. Possibly as a result of this distribution, the performance of the catalyst was found to be rather poor in comparison with that of a catalyst prepared by impregnation. 

The shape and typical dimensions of this cell are shown in Figure 26.24 [94]. Due to the nonuniform primary current distribution on the inclined cathode (with a local current density that decreases with increasing anode-cathode separation distance), a single electroplating experiment is used to study the effect of current density on deposit morphology (and deposit composition, for the case of alloy plating). 

Deposit thickness distribution in wafer electroplating must be considered in terms of two separate scales. (1) Macroscopic distribution, on the wafer scale (cm) and (2) microscopic distribution, on the length scale of the features (microns). Because of the large variation (4-5 orders of magnitude) between the scales, these distributions are controlled by different mechanisms. Furthermore, the design objectives for the two scales are quite different. While it is important to obtain uniform deposit thickness on the wafer scale, a bottom-up fill is desired on the features scale, since uniform deposition leads to the formation a center seam. 

Mehdizadeh and Dukovic (5) expanded the theoretical treatment and included mass transport effects in an axisymmetric system as well as a 3-D geometry. In the 3-D geometry, they assumed four peripheral low-contact-area terminals and have shown the effect of peripheral point contacts on the thickness distribution of a 200 mm wafer. Initially, the thickness near the four point contacts is very high, whereas between the contacts is very low. A time series of a growing deposit with four peripheral point contact terminals is shown in (6). Point contacts result in azimuthal nonuniformity. However, the nonuniformity in the vicinity of the contacts becomes appreciably better as the plated thickness builds up. In applications such as Damascene electroplating where the final plated thickness is usually not more than l/im, azimuthal nonuniformity can be a problem. Our solution was to implement an almost continuous peripheral contact terminal and to assume that the system is axisymmetric and that only the radial nonuniformity needs improvement. 

Cells with a small cathode and a large anode are often used in electroplating technology. In this case, a homogeneous distribution of the deposit over the entire cathode is required. 

Fig. 8.18 Modification of extreme current densities to provide a more uniform current distribution, (i) use of a radiused corner to reduce throwing power deficiency in a recess (ii) use of a conforming anode to overcome poor throwing power in a recess (iii) use of inert shields to prevent build-up of deposits on edges and (iv) use of cathodic burners or robbers to minimize edge build-up. (After Ashby (1982) Electroplating for Engineering Technicians, National Physical Laboratory, London and Courtesy Ionic Plating Ltd.)...

---