When TSMC initially introduced its N2 (2 nm class) process technology earlier this month, the company outlined how the new node would be built on the back of two new cutting-edge fab techniques: gate-all-around transistors, and backside power rails. But, as we’ve since learned from last week’s EU symposium, TSMC’s plans are a bit more nuanced than first announced. Unlike some of their rivals, TSMC will not be implementing both technologies in the initial version of their N2 node. Instead, the first iteration of N2 will only be implementing gate-all-around transistors, with backside power delivery to come with a later version of the node.

So far, TSMC has mentioned two distinctive features of N2: nano sheet gate-all-around (GAA) transistors, and backside power rails. GAA transistors have two unique advantages over FinFETs: they solve many challenges associated with the leakage current since GAAFET’s channels are horizontal and are surrounded by gates around all four sides. Meanwhile, backside power rail enabled improved power delivery to transistors, which increases performance and lowers power consumption.

But, as it turns out, TSMC is not planning to start with both nanosheet GAA transistors and backside power rails in the initial generation of its N2 process technology. As disclosed by the company last week at their EU symposium, the first generation of N2 will only feature gate-all-around transistors. Backside power delivery, on the other hand, will come later with more advanced implementations of N2.

At this point the company hasn’t said too much as to why they’re not rolling out backside power delivery as part of their initial N2 node. But, in discussing the bifurcation, TSMC has noted that backside power delivery will ultimately add additional process steps, which the company is seemingly looking to avoid on their first try with GAAFETs.

The lack of backside power delivery in the original version of the N2 fabrication technology perhaps explains rather moderate performance improvement of N2 when compared to N3E node. While for high-performance computing (CPUs, accelerators, etc.) a 10% to 15% performance improvement at the same power and complexity does not seem to be impressive, a 25% to 30% power drop at the same speed and complexity seems to be very good for mobile applications. 

Advertised PPA Improvements of New Process Technologies
Data announced during conference calls, events, press briefings and press releases

 
TSMC

N5
vs
N7
N3
vs
N5
N3E
vs
N5
N2
vs
N3E

Power
-30%
-25-30%
-34%
-25-30%

Performance
+15%
+10-15%
+18%
+10-15%

Chip Density*
?
?
~1.3X
>1.1X

Volume
Manufacturing
Q2 2022
H2 2022
Q2/Q3 2023
H2 2025

*Chip density published by TSMC reflects ‘mixed’ chip density consisting of 50% logic, 30% SRAM, and 20% analog. 

Considering that TSMC always offers multiple versions of its nodes, that TSMC has several variants planned for N2 is not all that surprising. Nonetheless, it is a bit odd to see that TSMC is taking a rather long road to backside power delivery.

Compared and contrasted to the competition, this will end up being a notable difference from how rival Intel is planning to handle their own GAAFET/backside power transition with the Intel 20A process. Intel intends to introduce its GAA RibbonFET transistors and PowerVia interconnects together in mid-2024 – going so far as to create an internal pseudo node just to focus on RibbonFET development. TSMC, on the other hand, is taking a more cautious approach to risks and innovations, one which potentially has TSMC moving at a slower pace, but is also an approach that has traditionally been a better fit for TSMC’s need to deliver more constant and consistent updates to its fab offerings.

And while we’re still a few years out, it will be interesting to see what this means for the competitiveness of TSMC’s first-generation N2 node. Will a GAAFET process without backside power delivery be at a meaningful disadvantage? Per current schedules, we’ll find out the answer to that in the second half of 2025, when TSMC’s first N2 node is slated to enter high-volume manufacturing (HVM).

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