Superconformal Filling of High Aspect Ratio through Glass Vias for Interposer Applications Using Tnbt and Ntbc Additives
In this report, Cu superconformal filling of through glass vias (TGV) of aspect ratios (AR) 6:1 and 10:1 for glass interposer applications is achieved by DC electroplating in the presence of the additives tetranitroblue tetrazolium Cl- (TNBT) and nitroblue tetrazolium Cl- (NTBC) in acidic CuSO4/ Cl- formulations. Filling takes place by the so-called ‘butterfly’ mechanism that nucleates Cu plug in the TGV’s center and keeps low surface coverage. Holes of AR 6:1 are filled in 3 hours at 2.5 mA.cm-2, using a bath composed of 40 ppm TNBT, 80 ppm Cl-, 0.88M CuSO4, and 0.3M CH3COOH under stagnant conditions. Increasing [TNBT] to 80 ppm and [Cl-] to 120 ppm in the presence of 0.3 M CH3COOH allows filling of 10:1 AR holes in 12.5 hours at 1.0 mA.cm-2 while maintaining thin surface coverage. In the case of NTBC, 6:1 holes are filled in 4-5 h at 2.5 – 4.0 mA.cm-2, and 10:1 holes are filled in 12-13 h at 1.0 to 1.5 mA.cm-2 in a slowly stirred bath containing 0.88 M CuSO4, 80 ppm Cl-, 0.6 M H2SO4, and 45-50 ppm NTBC. Bath composition is necessary to avoid void-formation in the holes. Effects of composition on overvoltage and Cu surface topography are discussed. Finally, a critical discussion on some mechanistic aspects of the supeconformal filling process, taking place according to the Figure, is also presented.