Variations in different masks, alignment problems and the physical limits of immersion add up to serious issues at 7nm and 5nm.
Continuing to rely on 193nm immersion lithography with multiple patterning is becoming much more difficult at 7nm and 5nm.
With the help of various resolution enhancement techniques, optical lithography using a deep ultraviolet excimer laser has been the workhorse patterning technology in the fab since the early 1980s. It is so closely tied with the continuation of that it is difficult to think of one without the other. But how much longer this technology can continue isn’t clear, given the magnitude and breadth of the problems expected at upcoming nodes.
The semiconductor industry has been banking on extreme ultraviolet (EUV) lithography for the past decade to circumvent the problems that 193i is beginning to encounter. For example, immersion lithography requires double patterning at 16nm/14nm and quadruple patterning at 7nm. Both schemes work, but they present some new and major challenges. Extra patterning increases the cycle time and cost in both the photomask shop and in the fab. And that’s just the tip of the iceberg.
But if chipmakers extend immersion/multi-patterning to 5nm, they may need to resort to the unthinkable—octuple patterning, described almost universally as a nightmarish scheme that is considered unwieldy and too costly.
At 7nm and/or 5nm, the alternative is EUV, which supposedly simplifies the patterning flow. With a 13.5nm wavelength, EUV would be able to pattern even the finest detail with a single pass at a 22nm half-pitch. If EUV is ready, chipmakers likely would use EUV to pattern some of the critical features, such as contacts and vias, at 7nm with a single exposure. But at 5nm, they would require EUV, plus a multiple patterning scheme.
Still to be seen, however, is when or whether EUV will become commercially viable. And given the uncertainty with EUV, the industry must continue to develop multi-patterning as a backup plan at 7nm and/or 5nm. It might even end up as the primary option at those nodes. If so, octuple patterning would become a reality.
Already, more problems are beginning to surface with 193nm immersion. For example, when wires and devices are split into two or more masks using quadruple or octuple patterning, it can affect the overall performance of a chip as well as the yield.
Variation is another issue. “The challenge at every new node is to understand how the specs are printed,” said Carey Robertson, product marketing director for Calibre extraction at Mentor Graphics. “They are never printed at the exact number. And as you entertain other structures, the deltas will be non-intuitive. The problem is understanding what the metal will actually look like because one mask has a different variation than another.”
That issue grows worse with each new node after 22nm. With quadruple patterning, for example, printing wires or devices is more difficult than with double patterning. There are more masks required. Then, the masks must be broken into different pieces, which in turn are layered on top of each other.
