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SEPTEMBER 2018 I PCB007 MAGAZINE 65 W a and its mathematical predecessor, the Ta- fel slope b, both basically represent the slo- pe in the potential/current diagram of a given electrolyte and control in the end the domain where a leveller can be successfully applied (Figure 9). When designing and synthesizing new level- ler compounds, chronopotentiometry (Figu- re 10) allows for rapid characterization and— much more importantly—a reliable prediction of the bottom-up-fill capability (go or no-go di- scrimination). Needless to men- tion, a VMS suitable for the re- levant geometry/copper feature has already been composed. Any leveller reducing the elec- trochemical potential at high agi- tation as compared to low agita- tion will be basically suited for bottom-up fill because of "direc- ting" the plating current into the feature to be filled (i.e., prefe- rential deposition at low current density areas). Dialing in the Application Once the VMS has been chosen for and the needed/suited additi- ves have been designed and pre- qualified, the subsequent task is now to determine the optimum concentrations of brightener, leveller and carrier. Figure 11 shows an example of a leveller´s characteristic. Its growing concentration does not impact the limiting current but consider- ably influences the bump/pillar uniformity— in particular within die (WID) and within fea- ture (WIF) numbers of bump heights. Again, different bath concentrations yield different polarographs and need to be dia- led into the envisaged plating tool and the Figure 9: Characterizing designed levellers by voltammetry. Figure 10: Leveller selection and qualification (chronopotentiometry).