Dr. Zhang, I think we have reached a verdict.
Bob Patti wrote in to confirm Anonymous Caller’s statement regarding polymer-filled, copper-lined TSVs, and also provided some additional data to support his comments. I’ve taken the liberty of paraphrasing his comments here:
With the caveat that he is not a “big fan” of conformal vias, and merely providing data, Bob stated:
“Certainly filling vias with a polymer makes sense to keep material from being trapped and depending on what processing may need to follow, it may be required. A via-last interposer might be a better solution. However, Dr. Zhang’s question was is it possible to metalize both sides without filling, apparently it is, shown here in this ZyCube presentation (see slides 29-30, the CIS pictured is from Oki.)
At Tezzaron, we only use filled tungsten via first (middle) processes. I don’t think copper via-first is a good alternative, except for interposers. Tungsten TSVs are, at best, limited to ~20µm depth, so copper is (at least currently) the only alternative for vias needing to go more than ~20µm deep. So interposers really must use copper. The issue with filled copper at high aspect ratios and/or large diameters is the exposure to thermal cycling. An interposer will not be exposed to numerous 300-400°C temperature cycles, as a copper via-first chip would be. So interposers are far more likely to tolerate filled vias.
Most memories that I am familiar with, other than our own, are via-last copper. I think they are also conformal, but I could be wrong. This makes them look like the Oki sensor.”
So in general, the consensus is that as long as the via formation occurs after the metallization layers, as in via last for CIS, or certain interposer applications, a conformal via is fine. Otherwise, the via should be filled with something: copper, tungsten, or polymer. Got it. — F.v.T.