Hybrid metallization of glass and superconformal filling of through glass vias for interposer applicatio

The rise in demand for smaller, lighter, thinner, shorter, smarter, faster electronic products results in high density interconnections (HDI) on the printed circuit board (PCB). Therefore an effective packaging scheme is required to facilitate these connections. The traditionally used which include package-on-package (PoP) and wire-bonded System-in-Package (SiP) are limited in bandwidth because of their long interconnection lengths hence low input/output (I/O) density. Interposer technology is the packaging scheme used to overcome these challenges. Fabricated with arrays of vertical through holes filled completely with conductive material (mostly copper) the interposer facilitates the HDI's of the integrated circuit (IC) to the PCB and enables for 3D stacking of chips thereby enhancing the speed and the band width. Materials used for interposers have evolved from ceramic to organic then to silicon. Silicon has been the major material for interposer technology but is limited (i) by high processing costs since needs for a barrier layer and (ii) by size from the wafer of origin. In a way to address some of these challenges glass has emerged as alternative material for interposer technology and has recently gained interest because of availability in large panel's size, good dimensional stability, low cost and adjustable coefficient of thermal expansion (CTE). However, glass itself is also subjected to some limitations constituting challenges to be addressed in the course of it's adaptation as an interposer material. Among these challenges, the complicated process of glass metallization due to the poor metal-glass adhesion comes first in any glass interposer's (GI) development effort. In this work we explore strategies of improving adhesion of metal to glass and develop routines and protocols aimed at providing continuous coverage of the through holes. As a result we introduce a hybrid approach for glass metallization based on sputter Ti-Cu and subsequent coating by electroless Ni (EN). The approach takes advantage of the good adhesion of very thin Ti-Cu film on glass surfaces and combines this with the ability of electroless plating to bridge center regions of high aspect ratio TGH that are inaccessible by sputtering. The hybrid method of glass metallization has been demonstrated in this dissertation as a viable method to form an initial conductive layer (commoning layer) that allows for subsequent electrolytic plating on glass with various high aspect ratios TGH, applicable to the development of glass interposers. The good adhesion of this layer was confirmed by scotch tape tests and also by its ability to withstand the stress imposed by addition of relatively thick electrolytic copper layers. The next challenge has been associated with superconformal filling with copper of once metalized through glass holes (TGH). More specifically, this challenge has been addressed by the development of plating strategies aiming at complete filling with copper of all TGH while dramatically suppressing deposition on the outer flat surfaces and near the hole entrances as well. The tetranitro tetrazolium blue (TNBT) and Nitro tetrazolium blue chloride (NTBC) additives that were first reported by Dow.et.al as inhibitors for superconformal filling of through holes on printed circuit board have been studied and applied in this dissertation in the development of protocols for superconformal filling of TGH. As a result of thorough optimization process taking into account the notorious complexity of glass metallization process in general, plating approaches (bath composition and plating parameters) have been developed for superconformal filling of TGV's of aspect ratio (AR) 6:1 and 10:1. Among the most important achievements, 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 CuSO 4, 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. A thorough optimization of bath composition is performed for reaching void-free copper deposition in the holes. Effects of bath composition on overvoltage and Cu surface topography are discussed. Finally, a general overview on future challenges and directions in the development and applications of GI is also presented in conclusion of the dissertation work.