By Ed Sperling
System-Level Design sat down to discuss the future of stacked die with Riko Radojcic, director of engineering at Qualcomm; Prasad Subramaniam, vice president of design technology at eSilicon; Mike Gianfagna, vice president of marketing at Atrenta; and Herb Reiter, 3D/TSV working group chair for the GSA. What follows are excerpts of that conversation.

SLD: Where are we with 2.5D and 3D?
Radojcic: I think 2.5D was a misnomer, because that implies they are sequential. It’s clear that what we call 2.5D and 3D are going to co-exist for a long time. Some things make sense with an interposer and some make sense to be 3D.
Reiter: I agree—2.5D is a parallel effort to 3D. Lots of things will not use 3D because it’s too expensive. In 2.5D we will see production this year. With 3D it will take until next year for the first ones. I would guess computing or networking would be the first.
Radojcic: I would think those guys will pursue 2.5D.
Subramaniam: Memory makers are already offering 3D solutions today. If you look at just the memory chip, to increase the size of the memory rather than the die they’re stacking it vertically. That kind of 3D is already in production. It’s the question of co-mingling logic and memory that will take time. The advantage of 2.5D is that it allows afterthought. It allows you to take an existing design and to create a new set of I/Os and put in a 3D type of application.
Radojcic: I see no value in doing that. You’re creating an expensive solution to something you can do more cheaply. If you add the 3D interposer you’re adding another wafer. That’s cost. We can solve that problem with a flip chip. It’s cheaper.
Subramaniam: I disagree. We’ve done the analysis. It allows us to take an existing design, like an ARM subsystem in 28nm, even though surrounding logic doesn’t have to be at that 28nm process node. It can be 40nm or 65nm. Rather than building a new chip at 28nm, I can take my existing design, use it as one component of my 3D IC, and build a second chip in a cheaper, older technology.
Radojcic: Yes, as long as you’ve architected your chip like that, such that you can partition it.
Subramaniam: You can’t take any design, no. There has to be some partitioning in the architecture and some forethought. It’s not 100% an afterthought, but there is still some afterthought there.
Radojcic: You have to architect for it. If you haven’t done that, taking an existing chip will just cost you more. If you have done that, of course there is an avenue to doing things better and more flexibly.
Subramaniam: There is enough flexibility in designs that allow you to partition it in some manner.
Radojcic: True, but before 3D came along most of us wouldn’t have partitioned. We wouldn’t have architected it that way. To be able to leverage that value proposition, you must have 3D in mind.
Gianfagna: That’s true. It’s a premeditated act. If you don’t think it through way up front it doesn’t work.
Subramaniam: Because the SoC has a well-defined architecture, it lends itself to this type of application.
Radojcic: But only if you plan for it ahead of time.

SLD: Is this true in all cases?
Reiter: That’s the view of a high-volume supplier. I see low-volume solutions where they use an existing die, put it face down on an interposer, and connect memory to it. So for low to medium volume, 2.5D works. You call it an afterthought. I call it a customized solution.
Radojcic: Why wouldn’t you do that in a traditional multichip package?
Subramaniam: Because you don’t get the interconnectivity. The advantage of a silicon interposer is that you get thousands of interconnects.
Radojcic: But you have to design it sufficiently so you can leverage the interconnects from die to die. If you had designed for a traditional design, though, you would say, ‘I can’t have thousands of interconnects so I’m going to make a serial interface with 100 pins.’ If you take that design for a 100-pin interconnect and stick in an interposer it’s an expensive way of doing things.
Subramaniam: You may be able to take some internal signals out, which you are not able to do with a traditional MCM (multi-chip module) approach.

SLD: Let’s do a reality check. How far along are we toward stacking?
Gianfagna: Last year we had a hot-wired 3D system that was 2D with a bunch of scripts and manual effort. The customer base had strange, contrived designs and they were trying to see what they could and couldn’t do, and the foundries didn’t know what they wanted to do. A year later we have native 3D planning capability, the customer base has specific designs for implementation this year and next, and the foundries have a laser-sharp focus on process learning, mostly around 2.5D initially. If that’s a metric, things are clearer this year than last year. From an EDA perspective, I still think the market is two years away. But we still think this is big.
Reiter: If you look at the Atom chip with the FPGA from Altera, that’s basically a 2.5D solution. The FPGA is for customizing things. The Atom chip was not designed for this application.
Radojcic: But why use an interposer? Why not use a substrate and a multichip package?
Reiter: You could do that.

SLD: What’s missing from the tools side to make all this work?
Reiter: The ability to demonstrate what this technology can do is the most important capability. If you look at big corporations, top management is still hesitant to invest in this technology. If we could demonstrate in a credible way what it can do, people will be more successful in getting money to start programs using this technology.
Gianfagna: The way that happens is the early adopters blaze the trail, everyone tries to follow and the market heats up. What’s needed are commercial drivers. The tools aren’t there, but they’re close enough.
Subramaniam: The tools are not the issue. The development needed to support 3D is incremental. It can be done with the existing infrastructure. It’s really the end application.
Radojcic: Other than path-finding, which is hard to do with traditional tools. And the analysis.
Gianfagna: The complexity is higher. We’ve discovered that, too. RTL prototyping for a single chip has a certain set of challenges. When you go to 3D the modeling requirements are much greater, the constraint generation is more complicated. And we need standards. We can generate all the constraints, but we don’t know where to put them and how to express them because there is no agreed upon way to do that.

SLD: Do the standards organizations know where to start with all of this?
Radojcic: Standards are on a good track. We’ve worked with Si2 and Sematech to propose initials blasts for standards so we can feed them into Si2 and the EDA community and accelerate the process. The bits and pieces are moving, and we are on track to have a set of design exchange format standards by early next year.
Reiter: And Wide I/O.
Radojcic: Yes. The standards are channeled and the engine is revving.
Reiter: We have a bunch of players in a 3D enablement center participating. There are 15 companies listed, including Intel, IBM, TSMC, GlobalFoundries, and so on.
Radojcic: The way this was set up was Sematech said we are going to start a 3D enablement center initiative driven by the SIA. All the members of Sematech were mapped into this. Then a number of companies like Qualcomm, LSI and ASE joined.

Tags: 2.5D stacking, 3D stacking, Atrenta, eSilicon, GlobalFoundries, GSA, IBM, Intel, Qualcomm, Sematech, Si2, SIA, TSMC, Wide I/O

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