TSVs: just the tip of the 3D ICeberg

At ECTC last week, I counted at least 21 presentations dedicated to TSVs alone, and 13 dedicated to other processes for 3D IC integration. The sheer volume and depth of research required around bringing these technologies to market is sometimes lost on those of us who sit outside the circle of academia and research, and only hear about those that make it to marketability. And although TSVs have become the poster child for 3D IC integration, sometimes we forget that there are other steps of equal importance to achieving these 3D stacks; namely backgrinding, thinning and dicing; temporary bonding and debonding for both wafer-to-wafer (W2W) chip-to-wafer processes (C2W); and chip stacking.

According to IMEC’s Eric Beyne, while many companies are involved in developing materials and equipment for TSV processes (etch, seed layer, fill, etc.) there are only 2 equipment manufacturers (EV Group and SUSS MicroTec) that offer tools for temporary bonding and debonding processes, and only a few materials companies (Brewer Science, 3M, and Dupont) developing temporary bonding materials. Beyne described one temporary bonding material that when heated, allows the device wafer to slide apart from the carrier wafer; and another that vaporizes the material holding them together. Unfortunately, both of these materials involves high temperatures, which, when used in sequential processes to achieve multiple chip stacks, can stress the ultrathin device wafers. Therefore, IMEC is working on a parallel process to overcome address this. One approach is to stack the chips using a sort of stencil to maintain alignment, and then perform the bond step all at once.

Chip stacking is another step still requiring some solutions in C2W processes, where the perfect combination of speed and accuracy is yet to be achieved. S.E.T has developed a high-precision, flexible die bonder that reportedly achieves a throughput of 150pph, earning the company installs at IMEC, SEMATECH and CEA-Leti; but according to Mike Thompson, CTO of Replisaurus, S.E.T’s parent company, that number needs another zero (1500) to make it volume-production-capable. Solving this alignment plus throughput conundrum is the focus of TNO’s Bluebird project with Datacon. As part of the EMC3D consortium, EV Group is also working with Datacon to solve the C2W stacking issue.

What’s not being revealed is likely to be more important than what is being revealed. As was pointed out to me on more than one occasion last week, companies aren’t likely to talk about what they’re working on until it’s a done deal. And they never talk about what they’ve tried that has failed, which could ultimately be as useful for competitors to know about as the successes are. In any case, it appears that there’s still work to do before all the stars align. – F.v.T.