Superconformal Filling of Through Vias in Glass Interposers

Abstract

The additive tetranitroblue tetrazolium chloride (TNBT) has been applied successfully in this work to superconformal filling (SCF) of through-vias with an aspect ratio (AR) of 6 in glass interposers (TGVs). The TGVs were initially coated with thin conformal Cu using an earlier developed hybrid method. Optimization of plating parameters and bath composition led to SCF of these TGVs in only 3 h at a current density of 2.5 mA/cm 2 in a stagnant bath containing 0.88M CuSO 4 , 40 ppm TNBT, 40 ppm Cl − , 0.1M CH 3 COOH, and 0.6M Na 2 SO 4 . This is the shortest reported filling time for this AR using DC plating and a single-additive system. © 2014 The Electrochemical Society. [DOI: 10.1149/2.0081408eel] All rights reserved.Manuscript submitted April 28, 2014; revised manuscript received May 28, 2014. Published June 19, 2014 The increase in interconnection density in electronic packaging has led to the addition of interposers to package structures with Si being the interposer material of choice. Methods have been developed for creating Cu-filled through vias (TSVs) in Si interposers based on (i) blind via plating followed by back side wafer thinning until the via barrel is reached, 1 or (ii) for small diameter holes, "bottom up" filling after blocking one end of the hole by plating at high current density (CD).2 Recently, glass interposers (GIs) have been attracting increased interest because of their lower cost and certain performance advantages.3-7 Due to its lower thermal conductivity compared to Si, it is even more important to fill holes in glass with Cu to improve heat dissipation, and it would be desirable to do so by less cumbersome means than used for TSVs. W.P. Dow has reported successful super conformal filling (SCF) of through-vias (TVs) in printed circuit boards (PCBs) having aspect ratios (AR) of about 1.5 to 2.5 using DC plating and formulations containing the single organic additives tetranitroblue tetrazolium chloride (TNBT) or nitrotetratetrazolium blue chloride (NTBC) and Cl − ions. [8][9][10][11] These bulky cationic additives and their reduction products, e.g. diformazan derivatives, formed at typical Cu electroplating potentials, adsorb on Cl − covered Cu surfaces and inhibit further Cu deposition.10,11 Establishment of a concentration gradient of these inhibitors leads to a substantially faster plating rate at the middles of the vias than at their entrances or on the flat surfaces. [8][9][10][11] Thickening plating eventually forms a blockage at the centers of the holes transforming them into two blind vias. [8][9][10][11] In cross section this blockage sometimes has the appearance of a "butterfly", from which the term butterfly plating originates. [8][9][10][11][12] Continued plating under certain conditions uniformly fills the two resulting blind vias without development of voids, while plating thickness on the surface, where additive inhibition is most effective, is kept relatively low. 8,9,11,12 To date, the shortest time reported in the literature to fill TVs of 6:1 AR (in Si) using this app...

Superconformal Filling of High Aspect Ratio through Glass Vias (TGV) for Interposer Applications Using TNBT and NTBC Additives

“…3E in Ref. 18). These observations suggest that the presence of CH 3 COOH in the plating solution results in linearization of the gradient in the amount of adsorbed TNBT from the TGV opening to the middle of the hole thus leading to a clear X-shaped Cu nucleation and growth during the deposition process.…”

“…18). The X-character remains more or less preserved not only for 2 hours when a plug first forms in the middle of the hole and then keeps growing toward the TGV openings ( Figures 3B and 3C, respectively), but remnants of it (two "V"s) can be seen until the very end of the filling process (as already shown in Fig.…”

“…The preferential growth at the middles of the holes leads to the formation of plugs that completely block the through-holes, effectively creating two blind holes. 15,18 Based on the appearance of cross sections shortly after plugging was achieved, Dow called this the "butterfly" effect. Plugging is then followed by continued filling of the 'blind' holes formed on either side of the plugs.…”

“…This was accomplished in 3 hours at a current density of 2.5 mA cm −2 under stagnant conditions in both "acid-free" and 0.1 M CH 3 COOH-containing Cu sulfate solutions. 18 Stirring was found to lead to voids in the TGVs. 18 In the present work we extend the use of TNBT in 0.3 M CH 3 COOH under stagnant conditions to the filling of 10:1 AR holes by optimization of current density, and report on the use of the related additive NTBC for filling of both 6:1 and 10:1 AR holes.…”

“…18 Stirring was found to lead to voids in the TGVs. 18 In the present work we extend the use of TNBT in 0.3 M CH 3 COOH under stagnant conditions to the filling of 10:1 AR holes by optimization of current density, and report on the use of the related additive NTBC for filling of both 6:1 and 10:1 AR holes. An optimum level of solution agitation was found to be necessary to ensure continuous, void-free filling in the case of NTBC.…”