When I attend a conference like ECTC, packed with densely detailed technical presentations, I realize just how much work has to happen behind the scenes to bring all these technologies to production. The devil is clearly in the details. In the 3D space alone, there were dozens of papers and posters being presented from across the spectrum of university and research organizations, although proving possible in laboratory settings, many of the processes being discussed will likely never see the light of day in volume production, while others will provide the Eureka moments we’ve all been waiting for. However all of this work is critical to the ultimate mainstreaming of processes, and subsequent development of standards.
As Toshiaki Itabashi of Dupont WLP Solutions, explained to me while leading me through his poster session, Integrated Materials Enabling TSV/3D-TSV, thousands of man hours go into to developing these processes and perfecting the materials. Once the right recipe is achieved, it makes sense for it to become standard going forward.
I had one of those Eureka moments listening to Juergen Leib, research consultant to AAC Microtec, describe a silicon interposer with TSV developed for a space application. For this application, he said the decision was made to merely line a TSV with copper, rather than fill it. Lining it is all that’s required to achieve electrical conductivity, and reduces the potential stress put on the silicon by filling the via with copper. Additonally, the potential for delamination is reduced. “Wait a minute,” I asked. “If it’s not necessary to fill the via, why do it?” Both Leib and Eric Beyne of IMEC provided the answer. It all depends on the application and the diameter of the via. Beyne explained that for vias smaller than 25µm, filling is necessary because lining only can create voids. Another reason to fill is in stacking chips, which requires a microbump on top of each via. In that case, you need to fill to create the bump. But other TSV applications, lining the via is sufficient, and may ultimately create a stronger structure. So there you have it. I learn something new every day. – F.v.T.