The next semiconductor process node after 20nm promises tremendous power and performance benefits, but also poses some new challenges, according to a presentation by ARM and IBM at the ARM TechCon conference Oct. 30, 2012. The presentation showed how the "second generation" of double patterning and the emergence of FinFET transistors will impact IC designers.
The presentation was authored by Lars Liebmann, IBM fellow and notable lithography expert, and Greg Yeric, principal design engineer at ARM. Unfortunately Liebmann was unable to attend due to Superstorm Sandy, so Yeric gave the entire presentation. Immediately after this presentation, a Cadence-sponsored technical session described a 14nm test chip tapeout using an ARM Cortex-M0 processor and IBM's FinFET process technology (see blog post about this session here).
Although the IBM-ARM talk was titled "Design and 14nm," Yeric first suggested that "10nm" is a more accurate descriptor. "What Lars means by 14nm is a pure node shrink of 20nm," he said. "It's a 70% linear pitch shrink, or a 50% area shrink, which may be renamed 10nm. What we're talking about is the next generation." Yeric referred to this node as "10nm" during most of the presentation that followed.
While many people hoped extreme ultraviolet (EUV) lithography would be ready for "what we'll now call 10nm," Yeric said, that's not going to happen - at least not for the development phase. He noted that EUV still has "an impressive list of things that have to be overcome," including a vacuum environment where particles can go all over the place, masks that can't have pellicles for particle protection, and the development of new resist chemistries, not to mention the need for a ten-ton crane to service an EUV machine on an ongoing basis.
From Double to Triple Patterning
According to Liebmann and Yeric, the "first generation" of double patterning is the Litho-Etch, Litho-Etch (LELE) approach used at 20nm today. Also called "pitch split," it works down to a 50nm to 60nm pitch. It's just like the name sounds - you expose the first mask, then etch; you expose the second mask, then etch. Colorization is used to determine which patterns go on which mask.
LELE double patterning requires "color aware" design tools, a double-patterning aware standard cell library, and new design rules. Even so, problems can emerge. These include what Liebmann calls "three body problems." Yeric explained: "If you have three things in a row that are in a tight pitch, you can't break them into two masks, so you have to identify the problem and understand how to fix it. In some technologies you can do stitching, where you overlap the two masks so they're electrically connected."
At the 10nm node, Yeric noted, pitches will be smaller than those allowed by LELE. So one alternative is triple patterning, which uses three colors and three masks. "It is a difficult problem for EDA, more than 50% harder - more like an exponential problem," Yeric said. "DRC [design rule checking] with 3 masks is an NP-complete problem." Thus, the challenge is to develop heuristics and algorithms that will permit the process to complete in a reasonable period of time, and work is ongoing to accomplish this.
While many potential first-generation double patterning problems could be avoided with cell design, errors will span across cells more frequently at 10nm, Yeric said. Triple patterning, he said, is "more of a systems issue." On the other hand, a third color can resolve many of the "three body problems" that occur when three shapes are too close together.
Self-Aligned Double Patterning
Another alternative that can be used at 10nm is Self-Aligned Double Patterning (SADP), also called Sidewall Image Transfer (SIT). This is a more complicated procedure than LELE - it involves creating relief patterns called "mandrels," depositing sidewall spacers onto the relief features, and trimming away everything that's not needed to produce the desired shape. The advantage is that it can work with pitches as small as 40nm, whereas LELE only goes down to 50nm to 60nm.
But as always, there are tradeoffs. Challenges of SADP include:
- Increased scaling penalty and design complexity
- Complex pitch/coloring restrictions
- No off-track routing optimization (as is possible in LELE)
- No odd-track jogs (possible in LELE with stitching)
- Block mask (the second mask in SADP) width and spacing errors are difficult to prevent without over-constraining
SADP is "still double patterning, but you get to where you want to go in a different manner," Yeric said.
Designing Cells with FinFETS
In a FinFET, the field effect transistor (FET) gate wraps around three sides of the transistor's elevated channel, or "fin." FinFETs provide more control over current than planar transistors, and promise greatly reduced power for a given level of performance. Yeric showed how FinFETs will allow transistor gate length scaling, which stopped around 90nm, to resume. The three-sided FinFET gate allows better control over leakage and provides a "good switch," he said.
Yeric pointed to other advantages of FinFETs, including improved sub-threshold swing, improved drain-induced barrier lowering, and lower channel doping, which enhances electron mobility and greatly reduces random dopant fluctuations. But FinFETs are not without challenges. One is "width quantization." Standard cell designers can make planar transistors any width they want, but they cannot change the height or width of a fin. They can add more fins to increase drive strength, but this, of course, can only be done in discrete increments (adding 1, 2 or 3 fins, not 2.75 fins).
A 12-track standard cell allows 12 fin pitches, but some of those pitches are needed for gate and rail connections, so you actually end up with 4 fins per device. That's probably okay. But an 8-track standard cell only leaves room for 2 fins per device, which is probably not sufficient for performance and variability requirements, Yeric said. Designers can adjust the number of fins by "pushing" the active fin pitch. For example, if the metal pitch is 48nm, fin pitches from 40nm to 48nm could be used, in combination with cell track heights from 8 to 13. This gives designers some flexibility but requires up-front planning.
"This design technology co-optimization discussion is another paradigm shift in this industry for FinFETs," Yeric said. "We didn't have to do any of this before. Now we need to have a discussion up front about the best plan of attack for what kinds of cells and fin pitches we need."
Designer's Cheat Sheet
At the conclusion of the presentation, Yeric offered a "Designer's Cheat Sheet" on FinFETs. Here's a brief summary:
- New variation signatures
--Local variation will decrease
--On-chip variation (OCV) de-rating might be less optimistic
--Less mismatch between device types
--Reduced inverted temperature dependence (where things get slower as they get colder)
- Little or no body effect (because FinFET channels are fully depleted)
--More complex cells are possible
- Paradigm shift in device strength per unit area
--Watch the wires - they're not scaling well
--Expect better power gates
--Pay attention to power delivery
--Electromigration (EM) will be significant and must be understood up front
"The things we're talking about with FinFETs cross boundaries in the ecosystem," Yeric concluded. "We need design technology interaction, and we need design enablement discussions. There has to be a well functioning ecosystem to get the most out of FinFETs."
The Liebmann-Yeric presentation is available to ARM TechCon attendees; see the web site for details. Predictive FinFET models are available at http://ptm.asu.edu/.
Richard Goering