AL 2012 – Day 3
I continue to focus on line-edge roughness in my own research. This means that I attended papers in every conference in the symposium, since LER is an issue that cuts across all topics in lithography. (To be truthful, I meant to go to a paper in the new etch conference that talked about LER, but never made it.) LER is finally, in my opinion, getting the attention it deserves. I believe, and say to anyone who will listen, that LER is the ultimate limiter of resolution in optical lithography (e-beam as well). In fact, that was the title of my talk on Wednesday. I think that LER is a core component of Tennant’s Law, that it is killing EUV (in the same way that EUV source power is killing EUV), and that it will limit how far 193-nm lithography can be pushed. And the many difficulties of LER is one reason that directed self-assembly (DSA) so attractive.
Wednesday began for me with another tour-de-force paper by Chris Bencher and coauthors (Applied Materials and IBM) on continued progress on defectivity for DSA. Their work showed that defect inspection and review tools were capable of enabling progress for DSA, and that defect levels, while not zero, are low enough to do serious work on finding and eliminating the defects that are there. This is good news. Many people are scared that DSA defects are somehow thermodynamically inevitable, or that the statistics of DSA defectivity scale in some ugly way. That doesn’t look to be the case. Among other things, Bencher inspected 550 million contact holes on a DSA wafer and found 22 were missing (one of the fears of DSA, as well as for most lithography schemes, is missing contacts). This is a rate that makes finding defects hard, but getting to sufficiently low defect rates probable.
The next step is to get semiconductor-grade block-copolymer materials into the fabs for testing on real processes. And that is starting to happen. Yuriko Seino of Toshiba showed some amazing results of a DSA contact hole shrink process that looked almost ready to be used in manufacturing. Contact holes were printed in a guide material of spin-on carbon (CD = 72 +/- 8 nm, LER = 3.9 nm) on 300-mm wafers. A PMMA-Polystyrene block copolymer was spun on, filling the holes with the self-assemblying polymer (a ring of polystyrene forms along the outside of the contact, with PMMA in the middle). A DUV flood exposure made the PMMA soluble in an organic developer. After development, the DSA holes had a CD of 28.5 +/- 1.4 nm, with an LER (or CER, contact edge roughness) of 0.7 nm. Amazing results – but this is what DSA does. Still to come are electrical via chain yield tests – an essential test of the overall process capability.
An interesting problem that must be tackled before DSA can be used in manufacturing is the impact of this process on design. In the contact hole shrink process (the most likely place DSA will first appear in manufacturing fabs), arbitrary contact holes on arbitrary grids are not possible. Instead, DSA will assembly to produce a specific contact hole size, and will be in the right spot only if those holes are on a proper grid. Both of these issues will significantly impact chip layout. Which is why I was excited to see a paper from Stanford on DSA-aware layout for random logic. With the right design approach, the limited range of contact hole features that can be printed with DSA can be a big advantage.
Unfortunately, on Wednesday I had to reprise my role as self-appointed ethics policeman for papers. A company (that should have known better) gave a paper presenting a new model they had developed. They kept all aspects of how the model worked secret, revealing not even the least detail (for competitive reasons, no doubt). Further, the model was not commercially available – it was for internal use only. As a result, after listening for 20 minutes I could come away from that talk with absolutely nothing. The minimum (and foundational) ethical principle of scientific publication is that sufficient detail be given so that others can reproduce the work. Otherwise, the paper is not a scientific one – it cannot be used to build our shared body of knowledge. I used the question period at the end of the presentation to explain this basic principle to the author.
After another poster session and several beers at KLA-Tencor’s PROLITH party, the third day of Advanced Lithography came to an end. Tomorrow morning will bring the EUV tool and source status review papers. I predict a full house at that session.