Betting On Glass TSVs

By Ed Sperling
There are two big issues when it comes to through-silicon vias. One involves cost. The second involves heat—in particular, how to get heat out of a stacked die and what the thermal coefficient of the TSV will be to make sure it expands at a rate consistent with the SoCs in a package.

To address these issues, System-Level Design caught up with Rao Tummala, professor of electrical and computer engineering and material sciences, as well as the director of the 3D Systems Packaging Research Center at Georgia Institute of Technology, where work has been under way for several years to address these issues. What follows are excerpts of that conversation.

SLD: Why use glass?
Tummala: There are a number reasons. One is that it can be done pinless. A second reason is that it’s highly insulating, with extremely high resistivity, as opposed to silicon. We also know how to handle thin glass for embedded applications. The infrastructure is already available. And we know how to metallize glass. So it’s the best material except for one problem.

SLD: What’s the problem?
Tummala: We have to make holes in glass that are very small, with very high throughput, at very low cost. That’s the main problem we see with glass. But if you solve that problem, then it becomes an ideal material for semiconductor applications.

SLD: So how close are you to solving that?
Tummala: We’ve actually solved most of it. Like everything else, we know how to make glass thin—from 30 to 75 microns in thickness. We developed the process in partnership with the companies we work with to make small holes very fast. We can make more than 1,000 holes in one step. And we know how to metallize. We actually formed an electronic substrate by putting in thin wires and other metal layers, and through-via metallization so we can add components on both sides.

SLD: Who’s behind this effort?
Tummala: We have about 15 companies funding this research. Now we are looking to replace organic packages that are used by companies like Intel, AMD and IBM and almost everyone else. All the smart phones are going to very high-speed images, which will require extremely high logic-to-memory bandwidth. Everyone is moving toward through-silicon vias in every chip. All the semiconductor companies are betting on that technology. I’ve been promoting interposer technology. With glass we think we can substitute for silicon with no TSV in the logic chip and interconnect that with an interposer using extremely high bandwidth. We are looking at other applications, too. I cannot go into the details. But we are running an IEEE workshop here in November on this topic.

SLD: What’s the difference in the thermal coefficient of glass versus silicon?
Tummala: In the case of silicon it’s fixed. It’s 3ppm (parts per million per degree Celsius), plus or minus. In the case of glass, you have options depending on the type of glass you pick. You can go from 3ppm to 9ppm. We think that picking glass at 8ppm, which is between the 3ppm of chips and the organic board at 17 ppm, would put the interposer right in between. That’s the best way to solve that problem.

SLD: Doesn’t that vary depending upon the packaging, as well?
Tummala: Yes, in the case of 3D chips, if you take a lot of real estate with copper vias, you could end up with maybe 6 to 8 ppm for that 3D stack. If you put 5 micron vias on 16 micron centers, which is roughly a third of that area, that’s about 8 ppm.

SLD: Can glass also be a channel for heat or ESD?
Tummala: You can use glass in two ways. One is to isolate, so if you put logic and memory all in one stack you end up heating the memory chips, as well. You don’t want to heat the memory, but you have no choice. If you put logic on one side of glass and memory on the other, the glass works as an insulator. You also can use glass for conductivity. Right now you get rid of heat with heat sinks. We expect our technology of making holes will be so chip compared with silicon that we should be able to metalize a lot of those holes with copper and be able to use that for thermal conductivity. It will be even better than a silicon chip. In theory we should be able to get very high conductivity locally, if you need it, by having copper vias through glass.

SLD: So the glass becomes an insulator around the copper via?
Tummala: Yes, exactly. You end up with a better signal.

SLD: What’s the timing for glass TSVs?
Tummala: We have demonstrated the technology. We know how to make holes and metallize. Now we’re dealing with some of the liabilities and demonstrating them. I would say a two-year time frame is realistic for it to be commercially available.

SLD: Are all the major foundries and chip companies looking at this?
Tummala: Yes. In the last six months, we have moved all these technologies into glass. The next step will be to use glass for chips. We think wafers are good, but they’re too expensive, so we’re looking at panels that are 700mm to 900mm. That will provide hundreds or thousands of interposers. We started looking at glass for cost, but we’re also seeing performance improvements. You get both with glass.

SLD: Is defect density easier to control in glass than silicon?
Tummala: Glass is super smooth. Unlike silicon, which needs to be polished, glass comes out smooth.

SLD: So you don’t need CMP?
Tummala: No, that’s not necessary.