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Thread: EUV was never going to be single patterning

  1. #1
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    EUV was never going to be single patterning

    An investor presentation by ASML near the end of 2014 showed that 7nm was never going to be a single-patterning node even with EUV, but the start of a multipatterning spiral similar to what immersion is going through now. Hence, the need to develop high-NA with Zeiss. Even then, there would be no recovery to single patterning, as shown in the overlay tree below.

    From slide 49:
    https://staticwww.asml.com/doclib/in..._MvdBrink1.pdf

    EUV was never going to be single patterning-asml-euv-multipatterning-roadmap.jpg

    So, from this consideration, the throughput and corresponding source power target needs to be doubled, even without considering shot noise yet.

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    Is this REALLY the case? Suppose that EUV were reliable "enough" and cheap "enough" say in 2019 or so. Obviously the leading edge customers are at that point champing at the bit to get to EUV, whatever it costs.

    BUT there's also a whole lot of much lower volume customers stuck at 28nm or so because their volumes don't justify double patterning or worse. If TSMC, GloFo, or Samsung were to offer them 14nm with single-patterning, wouldn't that be an appealing proposition?
    Is there some reason why EUV will ALWAYS be so expensive (even after the aggressive pent-up demand of the first year or three is satisfied) that it wouldn't make sense in the sort of scenario I suggest, as just the natural successor to ArF for many (even less demanding) litho tasks?

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  3. #3
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    Quote Originally Posted by name99 View Post
    BUT there's also a whole lot of much lower volume customers stuck at 28nm or so because their volumes don't justify double patterning or worse. If TSMC, GloFo, or Samsung were to offer them 14nm with single-patterning, wouldn't that be an appealing proposition?
    Is there some reason why EUV will ALWAYS be so expensive (even after the aggressive pent-up demand of the first year or three is satisfied) that it wouldn't make sense in the sort of scenario I suggest, as just the natural successor to ArF for many (even less demanding) litho tasks?
    EUV has many issues on its own already, to justify its use it needs to be able to demonstrate single patterning beyond that of immersion double patterning. Especially with more advanced SADP, double patterning is not something prohibitive but it has been matured, with the help of the memory industry. DRAM uses many double patterning layers, for example. As of today, immersion with double patterning is still the only way for HVM at 40-50 nm pitch. EUV is not fast or clean enough at this point. On the other hand, double patterning also forces foundries to push to the smallest pitches possible in order to reduce cost per transistor. 2 masks to achieve one-third the pitch (yes, it's possible*) is hard to beat. EUV therefore needs to show single patterning scalability below 20 nm half-pitch, which is the 7nm territory. This is where the EUV double patterning question comes in.

    *http://www.cerc.utexas.edu/utda/publications/C111.pdf see Fig. 12 wide U-bend. A similar concept is described in US Patent 7846849.

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    Layer sub-patterns with exclusive SMO

    A dense 10nm Metal1 clip has more points further from the optimized pupil center due to higher density, while a much looser cut hole pattern with a 4x-5x nm half-pitch (resolvable by immersion) looks different in its optimized illumination (EUV pupil optimization still needed for this loose pitch!), due to looser pitch and higher symmetry. Likewise, 32 nm pitch lines and spaces will be illuminated differently from the cut pattern.

    It means that mixtures of different sub-pattern layouts at different locations with different optimized (pupil) sources can occur in the same layer, as expected for random logic BEOL layers with same minimum pitch. That could mean EUV needs multi-patterning at even looser design rules than 7nm, e.g., 10nm.

    The effect of multiple included layer sub-patterns in SMO is demonstrated in another paper from SPIE 2017. Here the anchor pitch is 32 nm while the overlay mark is a 200 nm pitch 100 nm feature; it already took some optimization to get them together with the 2-bar in the same focus window without shifting more than 0.8 nm. However, including more sub-pattern clips from the same layer into the optimization resulted in some sub-patterns shifting up to 1 nm in the same focus window.

    Wikipedia recently had a nice figure to show some examples of different SMO cases: Fileifferent EUV SMOs.png - Wikipedia
    EUV was never going to be single patterning-different_euv_smos.jpg


    References:

    J. Mulkens, J. Karssenberg, H. Wei, M. Beckers, L. Verstappen, S. Hsu, and G. Chen, "Across Scanner Platform Optimization to enable EUV Lithography at the 10-nm Logic Node," Proc. SPIE vol. 9048, 90481L (c) 2014 SPIE.

    X. Liu, R. Howell, S. Hsu, K. Yang, K. Gronlund, F. Driessen, H-Y. Liu, S. Hansen, K. van Ingen Schenau, T. Hollink, P. van Adrichem, K. Troost, J. Zimmermann, O. Schumann, C. Hennerkes, and P. Graupner, "EUV source-mask optimization for 7 nm node and beyond," Proc. SPIE vol. 9048, 90480Q (c) 2014 SPIE.

    C. Tabery, J. Ye, Y. Zou, V. Arnoux, P. Raghavan, R-H. Kim, M. Cote, L. Mattii, Y-C. Lai, and P. Hurat, "In-design and signoff lithography physical analysis for 7/5nm," Proc. SPIE vol. 10147, 1014705 (c) 2017 SPIE.

    W. Gillijns, L. E. Tan, Y. Drissi, V. Blanco, D. Trivkovic, R. H. Kim, E. Gallagher, and G. McIntyre, "Reticle enhancement techniques towards iN7 Metal2," Proc. SPIE vol. 10143, 1014314 (c) 2017 SPIE.

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    Through-slit SMO (LithoVision 2017)

    An interesting update at LithoVision 2017 by Mentor Graphics (Director Dr. John Sturtevant), among other interesting things, referred to variation of aberrations (indicated by Zernike coefficients) across tools and across slit positions. Such aberrations require SMO (source-mask optimization) corrections, per tool, per slit position (essentially multiple patterning by one mask per corrected slit position).

    Mentor Graphics Director Details Challenges for Edge Placement Control in 2020

    This in fact could have been anticipated years ago: Modeling and experiments of non-telecentric thick mask effects for EUV lithography | Chiew-Seng Koay and Greg McIntyre - Academia.edu and cross-slit aberration variations in EUV tools are generally acknowledged now:

    EUV was never going to be single patterning-euv-aberrations-across-slit.jpg(Patent US9715170)

    NXE 3400 aberrations are still comparable to older NXE 3350 models: http://pfwww.kek.jp/PEARL/EUV-FEL_Wo.../01_Lercel.pdf (slide 9) The thermal aspects have not even been considered yet.


    At different slit positions, there would be different 3D shadowings of the same feature thick mask patterns. Thus, the SMO would need to be correspondingly different at different slit positions.

    The different slit position exposures also would need to be stitched together.

    Background details: An aberration is a deviation of the wavefront from the target, in other words, the illumination direction is off. This will impact the depth of focus (for k1<0.5), unless the pitch is retargeted. OPC involving feature resizing or repositioning does not affect this. But it is not tolerable for design for the minimum metal pitch to vary from 36 to 42 nm at different slit positions, differently for each EUV tool. So the illumination must be varied for different slit positions, different for each EUV tool. On each tool, the stitching of multiple mask exposures should be expected.

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    Common use of Quasar illumination in EUV is bad sign for random logic single exposure

    A basic lithographic principle might also give a hint when the application of EUV single exposure is not possible. When the pitch is well above wavelength/NA, then conventional on-axis illumination can be feasibly used; usually a high sigma is preferred to accomodate a wide range of pitches. However, once the pitch goes below wavelength/NA (in the current NXE:3400 tools, this is 13.5 nm/0.33~41 nm), only off-axis illumination, such as Quasar, can be applied to produce any image. Unfortunately, off-axis illumination is very picky about pitch (https://www.rit.edu/~w-lith/research...40-36_full.pdf), due to depth of focus differences, and pitches above wavelength/NA do not mix well with pitches below wavelength/NA on the same mask. What's worse is that pitches less than 2x the minimum pitch cannot be helped by assist features either; there is no room to fit the assist feature to match the minimum pitch. We know from the SPIE papers that 7nm node MMP is running at somewhere less than 41 nm and Quasar is the most common illumination mode, as expected for this pitch range. The use of Quasar illumination is the warning that not all pitches can be accomodated in the exposure.

    Added note
    : I had recently also found out about the pitch-dependent best focus position (shift of Bossung curves, to be precise), which was discovered early on, and now has been attributed to 3D Mask effects across pitch (phase effects from the absorber thickness and refractive index). It seems to directly impact the pitch compatibility. This is still different from depth of focus differences, which is linked to the preferred illumination (angle distribution). Best focus position is essentially the center of the focus window, while depth of focus is the width of that window.

    Some references on this:

    https://www.linkedin.com/groups/3672...07464847622146

    https://www.spiedigitallibrary.org/conference-proceedings-of-spie/4343/1/Impact-of-the-EUV-mask-phase-response-on-the-asymmetry/10.1117/12.436666.short?SSO=1

    https://www.spiedigitallibrary.org/conference-proceedings-of-spie/4562/1/Understanding-Bossung-curve-asymmetry-and-focus-shift-effect-in-EUV/10.1117/12.458302.short

    https://www.degruyter.com/view/j/aot.2017.6.issue-3-4/aot-2017-0019/aot-2017-0019.xml

    https://www.spiedigitallibrary.org/c...69.short?SSO=1

    http://goldberg.lbl.gov/papers/Burkh...4220X_2015.pdf

    https://avs.scitation.org/doi/pdf/10.1116/1.3697718

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    Trend of decreasing k1 for EUV

    The k1 metric is a widely used indicator for difficulty of lithographic imaging. One introduction is given here on p. 10: http://hilase.h51.eu/wp-content/uplo...ion-101112.pdf

    EUV used to claim the advantage of "high k1", but that is no longer the case. For the projected 7nm node metal pitch of 36 nm, k1 = 18 nm half-pitch * 0.33 NA/ 13.5 nm wavelength = 0.44. This is <0.5, so that resolution enhancement techniques (RETs) such as off-axis illumination (most commonly quasar offered by ASML), subresolution assist features (SRAFs), phase-shift masks (which don't exist for EUV, at least not yet), and restricted design rules (i.e., unidirectional, restricted pitch) should apply. It is difficult to accommodate bi-directional layouts at low k1 because the illumination is not favorable toward imaging tip-to-side or tip-to-edge gaps as it is customized for tightly imaging the tightest pitch lines and spaces in between. Even for unidirectional layouts, a single exposure cannot tightly image the tip-to-tip gaps at the line breaks, because the same limitations apply. The following reference: Line end shortening and corner rounding... (PDF Download Available), shows the impact of off-axis illumination on severe line end shortening and corner rounding, for k1 of ~0.5 and below.

    So it is not surprising that at least one foundry offers "cut metal" or "cut poly" even at 28nm node. It would be a break from single exposure patterning, though not widely known as such. Of course, with the infamous 3D mask effects of EUV, the lines, tip-to-tip gaps, and tip-to-edge gaps are not even at the same best focus at the same time, aggravating the situation further.

    I only know of two companies which probably get this, the third probably needs to catch up.

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