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64 PCB007 MAGAZINE I SEPTEMBER 2018 density and plating equipment (hydrodyna- mics) is basically being performed for any type of interconnect geometry (µm to mm range) by using voltammetry and/or chronopotentiome- try as the most powerful tool for any start-up qualification (Figure 8). Typical state-of-the-art basic copper elect- rolytes (e.g., virginal makeup solution or VMS) consist of copper sulfate, sulfuric acid, plus chloride ion; the latter may also be regarded as an accelerating active additive controlling the lattice and grain buildup of the electropla- ted copper. Concentrations in a particular VMS need to be adjusted to any geometry's require- ments; they govern the magnitude of the limi- ting current and thus the final plating speed as well as the geometrical perfection of the nee- ded copper feature. The Butler-Volmer-based equation applies: n = Ion charge F = Faraday constant D = Diffusion coefficient C*= Ion concentration d = thk diffusion layer Typical additives—influencing lattice geomet- ries—to a VMS are suppressors (wetting copper surfaces, controlling diffusion), activators (refi- ning grain sizes by proliferation of nuclei) and so-called levellers caring for feature uniformity, feature shape (Table 4). Details of Kirkendall-void aspects will be presented later in this article. For any particular copper feature/geomet- ry the best suited leveller—in concert with the other bath constituents and additives (Table 4)—needs to be designed to its best molecular constitution and qualified by said electroche- mical methods. The achievable throwing power (i.e., degree of uniformity of the macroscopic current distri- bution) can be predicted and application spe- cific characterized by the Wagner number W a : κ = electrolyte conductivity l = distance between anode and cathode b = Tafel slope Figure 8: Schematic of a voltammeter. Table 4: Bath constituents controlling feature attributes.