(Invited) Reduction Behavior of NTBC and MTT: Relevance to Cu Deposition via Superconformal Filling of Through Glass Vias

Tuesday, 15 May 2018: 16:20
Room 211 (Washington State Convention Center)
J. Li and N. Dimitrov (Department of Chemistry, Binghamton University, SUNY)
Copper superconformal filling of through glass Holes (TGH) of aspect ratios (AR) 6 and 10 with the use of the additives Tetranitro Blue Tetrazolium Chloride (TNBT), Nitro Tetrazolium Blue Chloride (NTBC), and Thiazolyl Blue Tetrazolium Bromide (MTT) is presented and comparatively discussed as focal point of interest in the development of glass-interposer technologies. The presentation introduces firstly the challenge associated with initial glass metallization and describes briefly the developed by our group “hybrid” method as one of the feasible solutions for coating the glass surface with a continuous and conductive "commoning" layer. Next, the discussion proceeds with consideration of the proposed by W.P. Dow et al “butterfly” mechanism for nucleation of a plug in the center of the TGH to be filled with electrodeposited copper. Different experimental scenarios resulting in the formation hemispherical or X-shaped Cu nuclei that seed the success of the entire superconformal deposition process are also described in this section. The presentation then continues with a critical comparison of the use of all three additives of interest for achieving best results in implementing the superconformal growth mode. Main point of emphasis in that part is the use of MTT for achieving of fastest Cu growth rates. These are manifested by greatly reduced TGH filling times of about 1h and 4h for AR 6 and 10, respectively, compared to about 3h and 12h, respectively, as best results when TNBT and NTBC are tested as additives. In specifying the experimental details,, it is mentioned that constant current electroplating is carried out in the presence of the additives in acidic (pH 2 to 4) copper sulfate or methane sulfonate formulations containing also Cl- ions. In the best plating conditions achieved by the use of MTT, Cu plugs are nucleated in the TGH’s centers while keeping the thickness on external surfaces low. Plugs are formed in AR 6 TGH in as little as 12 min, after which the plating can be viewed as the filling of double-sided blind vias. Issues with imperfections trapped in the plugs have been addressed by decreasing the current around the time of plug formation, then increasing it for the rest of the process. In addition it is specified that the filling of AR 10 holes takes much longer times at lower current density are needed. It is also shown that larger diameter AR 4 TGH have been also successfully filled with Cu. At the end of the presentation a glimpse is made into the reduction behavior of the additives of interest to this work. Also in the conclusive part a cost analysis is presented to show that lab scale quantities of MTT are 8 to 14 times less expensive than the competing additives NTBC and TNBT, respectively.