Superconformal Filling of High Aspect Ratio through Glass Vias for Interposer Applications Using Tnbt and Ntbc Additives

The drive toward higher circuit density and higher operating frequencies in electronic packaging has led to the employment of interposers between the Si chip and the underlying package. Currently Si or glass, both are considered materials feasible for the fabrication of interposers.   Through-holes are needed to make connections between the top and bottom surfaces of the interposer.  This can be done with conformal Cu plating, but especially in the case of glass it is also desirable to fill the through glass vias (TGVs) with Cu to compensate for its poorer thermal conductivity compared to Si.  Filling also enables formation of a flat surface for subsequent layers or for component attachment.  Techniques have already been developed for filling “blind” holes/vias by Cu electroplating using combinations of suppressors and accelerators.  In the case of Si, through-vias have been generated by first filling holes as blind ones. Then thinning the back side of the wafer by CMP is applied until the bottoms of the filled vias are intersected, thereby producing filled through-holes. Alternatively, sealing one end of the through vias could be made and then one could fill them as blind vias.  In the case of glass, TGVs may be formed directly into the glass without the need for mechanical thinning, but this leaves the problem of filling the holes as through-holes, open on both ends.  The aforementioned plating methods for filling of blind vias do not work well when directly applied to filling of TGVs, and a different approach is needed. 

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 H₂SO_4, 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.