A Dispatch from SEMICON West – Applied Materials Launches Epi System Focused on NMOS Strain
By coincidence I flew through Heathrow a week after another 777 did a belly-flop there a few years ago, and again I was impressed at the strength of the airframe – an engine had been ripped off a wing but otherwise it was pretty well intact – and fortunately in that case there was no fire, and no fatalities.
That’s hardly relevant to SEMICON West of course, but it’s hard not to get involved when we get that close to the statistics of travel accidents, be they road, rail or air.
Anyway – back to the show – or at least the pre-show events. Applied Materials (AMAT) had an analyst day on Monday, and in the morning they invited a few of us to some product launches. The one that caught my eye and ear was a new epi system focused on NMOS epitaxial source/drains to create channel strain, since that has been mooted as a next step for several years now, but not shown up in a production context.
The theory is that if you can get carbon and phosphorus to replace silicon atoms in the crystalline structure, because they are smaller than silicon, they will generate tensile stress in the crystal lattice. When it is deposited in cavities etched in source/drains the stress is applied to the channel. (Putting the larger germanium atoms in the lattice has the opposite effect, and creates compressive stress, an effect used since the 90nm node.)
|Schematic of e-SiGe in PMOS (left), and e-Si:CP in NMOS (right) source/drains|
The problem (as I understand it) has been that the carbon does not like staying in such substitutional positions, and it will abandon them as soon as it sees anneal temperatures, thus losing the stress effect. Phosphorus is happy to be substitutional, and has of course been used as a n-type dopant for decades, so I suspect the problem there is simply getting the concentration to a level sufficient to stress the lattice.
So on Monday AMAT launched the Applied Centura RP Epi system with an NMOS transistor application. To quote: “This capability supports the industry's move to extend epi deposition from PMOS transistors to NMOS transistors at the 20nm node, enabling chipmakers to build faster devices and deliver next-generation mobile computing power.”
The Applied folks seem confident that once the epi is formed, the carbon can be kept stable and capable of applying the strain at the end of the manufacturing process. I quizzed them as to how this is done and apparently the keys are the quality of the clean after cavity etch (i.e. AMAT’s Siconi dry clean), plus millisecond annealing to minimize the thermal budget.
There is plenty of literature documenting the effect; at last year’s IEDM conference, IBM announced their 22nm server process, which uses embedded strain for both N- and P-MOS. Together with nitride stress, they claim a 10 percent performance increase over the 32nm equivalent. I also asked the speaker there about the carbon stability, and he confirmed that they regard it as a manufacturable process.
|Cross-section of NFET showing embedded Si:C Source/Drain Stressor |
I’ve been waiting for epi-strained NMOS for the last couple of process generations, and had almost been convinced that it wouldn’t happen. Now we have to watch for it when we get the next 20nm parts!
 S. Narasimha et al., “22nm High-Performance SOI Technology Featuring Dual-Embedded Stressors, Epi- Plate High-K Deep-Trench Embedded DRAM and Self-Aligned Via 15LM BEOL”, IEDM 2012, pp 52 – 55.