Deposit and Current Distribution

In this chapter we consider the flow of electric current in the electrochemical system and the effect it has on the process of electrodeposition. 

Fundamentals of Electrochemical Deposition, Second Edition. By Milan Paunovic and Mordechay Schlesinger Copyright 2006 John Wiley Sons, Inc. 

For the purposes of discussion, we distinguish between two types of electric conductance metallic and electrolytic, the first being a stream of electrons, as in a copper wire, the second being a stream of ions, as in the case of a salt solution in water. In this case, positive ions will drift in the direction of the cathode, whereas negative ions will drift in the direction of the anode. 

The magnitude of the drift velocity vector v of conduction electrons can be calculated rather easily. It is surprisingly low. If we denote the number of conduction electrons per cubic meter n and the electronic charge as e, then 

In copper there are two conduction electrons per atom and n = 8.5 X 10 electrons per cubic meter. For a wire with a cross section of 1 mm carrying a current of 1 A, a value of v = 25 X 10 m/h is obtained. For the sake of comparison, it is interesting to note that in a molar copper sulfate solution, the absolute mobility (mobility in a potential gradient of 1 V/cm) of copper ions is 2.5 X lO mTh. 

For the sake of comparison, it is interesting to note that in a molar copper sulfate solution the absolute mobility (mobility in a potential gradient of one volt per centimeter) of copper ions is 2.5 X 10 2 m/h. 

---

J.T. Keating, Sulfate Deposition and Current Distribution in Membranes for Chlor-Alkali Cells. In F. Hine, W.B. Darlington, R.E. White, and R.D. Vaijian (eds), Electmchemical Engineering in the Chlor-Alkali and Chlorate Industries, PV 88-2, The Electrochemical Society Pennington, NJ (1988), p. 311. 

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. 

On the other hand, the selectivity of the electrochemical deposition of the metal on the substrate must be 100% of the current efficiency, with no interference from the other metal deposition processes. Therefore, the potential distribution needs to be presented for any serious electrochemical reactor study and the electrocatalyst selection problem. The major problem of current distribution depends on the type of the process that controls the entire reaction rate, such as charge transfer, ohmic contributions, or mass transport to or from the electrode. Many parameters have to be evaluated in the course of an electrochemical process to obtain the desired uniform potential and current distributions. One of the conditions that has to be fulfilled is the continuity equation for the current density vector, j ... 

Another difference between classical electrochemistry and electrochemical engineering lies in the size of the electrode. Conventional electrochemistry most commonly employs micro electrodes of well defined area operating under carefully controlled current and mass transfer conditions. Conversely, electrochemical engineering typically employs large surface area electrodes, where, moreover, the surface area and electrode activity varies constantly as metal is deposited. In addition, there are usually difficulties in maintaining uniform potential control and current distribution over the electrode surface. It is also necessary to consider the reverse stripping process of recovering the metal after collection. 

As metal concentration drops along electrolysis time, the 1l decreases, and one strategy would be to operate the process tmder limiting current conditions [26]. Nevertheless, the use of 1l also has important consequences regarding the deposit morphology, which will be rough and powdery [1]. Hence, there should be a compromise between the applied current and deposit morphology. Moreover, it is difficult to maintain the uniformity of iL inside the three-dimensional electrode due to the aforementioned potential and current distribution [12]. 

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. 

Naoi and co-workers [55], with a QCM, studied lithium deposition and dissolution processes in the presence of polymer surfactants in an attempt to obtain the uniform current distribution at the electrode surface and hence smooth surface morphology of the deposited lithium. The polymer surfactants they used were polyethyleneglycol dimethyl ether (molecular weight 446), or a copolymer of dimethylsilicone (ca. 25 wt%) and propylene oxide (ca. 75 wt%) (molecular weight 3000) in LiC104-EC/DMC (3 2, v/v). 

The rotating hemispherical electrode (RHSE) was originally proposed by the author in 1971 as an analytical tool for studying high-rate corrosion and dissolution reactions [13]. Since then, much work has been published in the literature. The RHSE has a uniform primary current distribution, and its surface geometry is not easily deformed by metal deposition and dissolution reactions. These features have made the RHSE a complementary tool to the rotating disk electrode (RDE). 

In many cases mass transfer is not the sole cause of unsteady-state limiting currents, observed when a fast current ramp is imposed on an elongated electrode. In copper deposition, in particular, as a result of the appreciable surface overpotential (see Section III,C) and the ohmic potential drop between electrodes, the current distribution below the limiting current is very different from that at the true steady-state limiting current. 

Fig. 3. Diagrams of electrochemical cells used in flow systems for thin film deposition by EC-ALE. A) First small thin layer flow cell (modeled after electrochemical liquid chromatography detectors). A gasket defined the area where the deposition was performed, and solutions were pumped in and out though the top plate. Reproduced by permission from ref. [ 110]. B) H-cell design where the samples were suspended in the solutions, and solutions were filled and drained from below. Reproduced by permission from ref. [111]. C) Larger thin layer flow cell. This is very similar to that shown in 3A, except that the deposition area is larger and laminar flow is easier to develop because of the solution inlet and outlet designs. In addition, the opposite wall of the cell is a piece of ITO, used as the auxiliary electrode. It is transparent so the deposit can be monitored visually, and it provides an excellent current distribution. The reference electrode is incorporated right in the cell, as well. Adapted from ref. [113],...

On the submicron scale, the current distribution is determined by the diffusive transport of metal ion and additives under the influence of local conditions at the interface. Transport of additives in solution may be non-locally controlled if they are consumed at a mass-transfer limited rate at the deposit surface. The diffusion of additives in solution must then be solved simultaneously with the flux of reactive ion. Diffusive transport of inhibitors forms the basis for leveling [144-147] where a diffusion-limited inhibitor reduces the current density on protrusions. West has treated the theory of filling based on leveling alone [148], In his model, the controlling dimensionless groups are equivalent to and D divided by the trench aspect ratio. They determine the ranges of concentration within which filling can be achieved. 

A second application of current interest in which widely separated length scales come into play is fabrication of modulated foils or wires with layer thickness of a few nanometers or less [156]. In this application, the aspect ratio of layer thickness, which may be of nearly atomic dimensions, to workpiece size, is enormous, and the current distribution must be uniform on the entire range of scales between the two. Optimal conditions for these structures require control by local mechanisms to suppress instability and produce layer by layer growth. Epitaxially deposited single crystals with modulated composition on these scales can be described as superlattices. Moffat, in a report on Cu-Ni superlattices, briefly reviews the constraints operating on their fabrication by electrodeposition [157]. 

---