NTBC and MTT reduction electrochemistry: Impact on Cu superconformal plating for fabrication of glass interposers

Nitro blue tetrazolium chloride (NTBC) and thiazolyl blue tetrazolium bromide (MTT) are the organic additives applied successfully in Cu superconformal filling of through glass vias (TGVs) with aspect ratios 6:1 and 10:1. However, traces of purple inclusion were observed through the cross-sectioning of electroplated samples indicating that a reduction could likely be occurring during the plating process. In this work, the reduction propensity of NTBC and MTT in the course of Cu superconformal plating was assessed in the absence of Cu(II) ions by cyclic voltammetry, controlled-potential electrolysis (CPE), and UV-Vis spectroscopy. A CPE potential of -0.25 V vs Ag/AgCl reference electrode was selected aiming at studying the reduction process as close as possible to the actual potential range of Cu deposition. The CPE and UV-Vis results suggest that both additives reduce steadily with time generating different (by)products. Also, it is shown that the MTT reduces itself at a rate that is substantial at lower pH, producing both surface precipitates and soluble reduction products. Unlike that, the NTBC reduction exhibits weaker pH dependence and generates lower quantity of surface precipitates only. Thus, the reduction of MTT is expected to impact considerably stronger than NTBC the Cu superconformal plating process in TGVs.