Talk:Extreme ultraviolet lithography

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Illustration[edit]

Finally, an accurate representation of EUV image formation mechanism!218.168.206.105 (talk) 19:55, 15 May 2008 (UTC)

Phase defects[edit]

The newly added phase defect plots are based on some simplifying assumptions, which are stated in the figures' descriptions. They may be updated in the future. Recent experimental data (stated in the article) appears to be more helpful in conveying the information, with less reliance on assumptions. Figures based on these data may in the future replace some of the figures relying on specific assumptions.Guiding light (talk) 12:40, 6 July 2008 (UTC)

General[edit]

Sweet, this article is currently ranked 5th over at google. Good job, everyone.the1physicist 02:54, 14 December 2005 (UTC)

That's good to hear. The nice thing about Wikipedia is, being somewhat encyclopedic, it is a better information source than other internet sites, e.g. news sites, provided users treat it and use it correctly. Guiding light 08:02, 12 January 2006 (UTC)

I think the first paragraph heading should be changed to "Background", "Wavelength Choice", "Limitations of Photolithography" or something similar. Any objections/suggestions?the1physicist 19:28, 4 March 2006 (UTC)

Maybe a "Background of EUV Development" or something like that?Guiding light 15:22, 5 March 2006 (UTC)

Sounds good, I'll make the change. Also, the article seems a tad bit technical. I think the average person would have trouble understanding the article in its current form. Should we put a {{technical}} tag in the article?the1physicist 19:13, 5 March 2006 (UTC)

Generally, EUVL is a highly complex topic with a lot of technical details. If there is a specific point that needs more clarification, it should be good enough just to leave a note here. We can go over the sections in this forum. Thanks. Guiding light 01:01, 6 March 2006 (UTC)

I added a section on mask costs, and put two recent contributions which seemed extremely relevant under that heading. Hope it makes the flow better. Guiding light 06:12, 2 April 2006 (UTC)

Re: the section on throughput, it's considered pretty unlikely that either nanoimprint or maskless lithography will be able to achieve even EUV-level throughput. Nanoimprint seems to mostly appeal to people working in materials other than silicon, and with feature sizes significantly larger than 32 nm. Maskless seems best-suited for prototyping and other short-run applications, where the savings in mask cost outweighs the throughput hit. But all of this is getting pretty deep into industry specialist territory, so I'm not sure whether it belongs in a generalist discussion like this. Opinions? KDerbyshire 18:39, 3 May 2006 (UTC)

Those are good points to bring up. The current applications for nanoimprint and maskless are exactly as you state. The throughput equation gets pretty complicated below 40 nm. Photoresist chemical amplification is expected to become the resolution-limiter at this scale. By taking it out, however, throughput suffers dramatically. Other techniques like nanoimprint or maskless atomic deposition will not be limited in this way, although they can be limited by other issues.Guiding light 00:52, 4 May 2006 (UTC)

Why wasn't EUV light source and optics development started much earlier? That is largely to blame for its repeated delay!218.168.164.152 12:23, 4 May 2006 (UTC)

EUV development has been going on for twenty years. In that time, the industry has discovered that (a) focusing x-rays is really hard and (b) predicting the timing of research advances is even harder.

Guiding Light: Hmmm... Good points. I'm willing to draft a paragraph or two for the rest of you to beat on, but I have some pressing deadlines to deal with first. KDerbyshire 14:39, 4 May 2006 (UTC)

Regarding the history of EUV development, the multilayer and plasma discharge emission technology had been established for soft X-rays (i.e., EUV) and hard X-rays before 1990, but EUV LLC didn't start until 1997. I actually think LLC didn't start years earlier because of the usual two reasons: 'no immediate need' and 'too hard to do'. History is repeating itself..now is the time to consider how to pattern 1 nm objects. Guiding light 22:25, 4 May 2006 (UTC)

The topic of EUV is usually dominated by supporters. Kudos to this article for approaching NPOV. Just to give you guys the big picture, EUV has a shot noise problem. There are few photons used per square nanometer, and since these arrive randomly at the wafer, it makes for noisier printing (Poisson noise). EUV also has a throughput-downtime tradeoff. The more powerful source wears out the mirrors sooner, so they need to be replaced more often. So the cost-effectiveness is still not there.61.61.254.9 00:19, 9 May 2006 (UTC)

Double patterning throughput is not straightforward cutting in half. The rate of final product wafers produced is hardly influenced by an extra exposure step. But restricting the second exposure to the same lithography tool (for example, for better alignment) is a strong manufacturing constraint.Guiding light 14:08, 11 November 2006 (UTC)

Article update[edit]

This article will be updated, mostly to make it more concise while keeping the main points to date.Guiding light 16:52, 20 January 2007 (UTC)

New article on EUV only?[edit]

EUV has uses other than lithography, like photoelectron spectroscopy. Maybe that warrants its own article? Some sections of this article can be subsequently moved over there.Guiding light (talk) 02:32, 23 May 2008 (UTC)

EUV has worse Depth of focus than 193 nm Double patterning[edit]

Using either Burn Lin's rigorous formula or the approximate paraxial formula for depth of focus, it is seen that by using double patterning and immersion lithography, the depth of focus is better than using EUV (see the table). Intel already uses double patterning for 65 and 45 nm, so it has no good reason not to stay with it through its 11 nm nodes, as guided by its SPIE 2006 presentation. Samsung and IM Flash, also using double patterning, probably to ~ 20 nm Flash. Other companies may be very different, not using double patterning right now. So for them it is a matter of can they afford the cost of double patterning.Guiding light (talk) 10:24, 27 August 2008 (UTC)

June 2009 update indicates 22 nm will not be EUV and 15/16 nm node can also be done by 193 nm immersion with double patterning. EUV still behind at 24 nm.218.35.242.119 (talk) 11:53, 21 June 2009 (UTC)

Graphs Removed[edit]

After much consideration, I removed graphs I generated for Wikipedia articles which bordered on possible publication material. I don't think it detracts from the article content, since the referenced contributions are not affected.Guiding light (talk) 16:01, 24 June 2009 (UTC)

Questions regarding 'Unexpected Resolution Limits' Section[edit]

On the whole, I find this page to be very informative about EUVL. The first comment in the General section that this page has many hits from google is well deserved. However, I have several questions regarding the section on Unexpected Resolution Limits. The overall problem that I have with a lot of this is that many of the references that are cited in this section are relevant for e-beam exposures with energies of 70 to 100 keV, for x-ray exposures with wavelengths around 1nm, or e-beam flood exposures (more typical of a SEM), and these results are directly cited to provide precise estimates how horrible the proximity effects in EUV will be. These references are certainly relevant to EUVL because many of the exposure mechanisms are similar, but such results should not be quantitatively applied to EUVL because the energies are often several orders of magnitude higher. For the casual reader (who will likely not read the references in detail), these comparisons will give the incorrect impression that there is a large body of scientific evidence that these proximity effects will make "the practical effective resolution to be at best ~30nm" (this is the last sentance in the first paragraph in the 'Proximity Effect (secondary electrons)' section). Here is a detailed list of comments on some of the references:

  • Ref 29 by Lee: exposure is by 100 keV beam. EUV photons have ~90 eV, so energy is off by 1000x.
  • Ref 30 by Feder: exposure by 0.83nm x-rays (energy ~1.5 keV). There is also no control in the experiment (a PMMA layer coated with ebrium and not exposed).
  • Ref 31 by Carter: Exposure is by 1.3nm x-rays. Second sentance in the citation states that "In a PMMA film, the image blur dur to these [photo and Auger] electrons is on the order of 5nm". However, the more pessimistic interaction length of 50nm is cited, which is observed due to backscattering from a gold substrate. The 50nm number is relevant in the Wikipedia article because a comparison is made with the results by Feder, but a more careful explanation of the experiment would be helpful to the casual reader.
  • Ref 32 by Yamazaki: Exposure by a 70 keV ebeam with a beam diameter of 7nm.
  • Ref 34 by Renoud: exposure by an electron beam. Here they discuss charge build up that is similar to what is seen on a SEM when the substrate is an insulator. I have not done the calculation, but it seems like that the charging due to EUV exposure would not be even wildly close to ebeam exposure doses. (EUVL's big problem is lack of photons, after all.)
  • Ref 36 by Kotera: This article is actually about EUVL (yeah!). However, he states "It is shown that the maximum radial range of those electrons is about 30nm.... If we assume that the proximity effect can be neglected since the intensity is less than 0.001, 5nm is the maximum distance to be patterned by EUV exposure without a proximity effect correction." Oddly, only the 30nm distance is cited in the Wiki article.
  • Ref 38 by Kozawa: This paper is also about EUVL, and a point spread function for acid generation is presented. The text in the article states that the "acid generation probability... has a maximum at approximately 3nm." At the maximum, the probability is about 0.35. Perhaps the 20nm range listed in the Wikipedia article corresponds to 0.01 probability in the tail of the distribution.

I think that there are a lot of people who don't know much about EUVL, and might visit this page to learn about it. On one side, there are a lot of critics, and on the other side, a lot of people dedicated to making it work. I am not actively working on these systems, so I am not an expert on whether it will go into production or not. However, as it stands, this article does not appear to be very even-handed in it evaluation of the challenges faced by EUVL. Bluedog29 (talk)

Bluedog thanks for your active pursuit of the details. Since the distances traveled by the electrons do not follow well-defined formulas such as the Rayleigh diffraction limit, but instead form a wide distribution from ~ nm to ~30 nm, the WORST CASE tail results ~30 nm must be used for the judgment because they result in the manufacturing losses. Kozawa and Kotera's results are still simulated, while the actual results in resist (mostly derived from electron beams) are more indicative. It doesn't matter if the low-energy electrons are generated ultimately by EUV, X-ray or other electrons.Guiding light (talk) 03:34, 16 May 2010 (UTC)


Guiding Light: My main complaint is that there are many results where we have applied a "WORST CASE" rule without letting the reader know that we have made this judgement for them. It would be more appropriate to include a direct quote from Kotera:

  • It is shown that the maximum radial range of those electrons is about 30nm.... If we assume that the proximity effect can be neglected since the intensity is less than 0.001, 5nm is the maximum distance to be patterned by EUV exposure without a proximity effect correction.

and then make a statement like the one in your post: "There is concern that the longer tail would lead to manufacturing losses." Instead, the casual reader is only given the "WORST CASE" and the results of our "judgement" on the viability of EUVL as a technology. Indeed, if we interpret Feder's result of 40nm and Carter's result of 50nm as the resolution limit, then it should be impossible to image 40nm spaces, which have been demonstrated on several different EUV tools.

Much of this information has to be interpreted carefully. For example: the new graph showing the inelastic mean free path in PMMA -- it is the average propagation distance traveled by an energetic electron before an inelastic scattering event that generates another electron. (The mechanism for the electron cascade.) It is not the distance an electron can travel in PMMA. Many other events can occur, such as energy losses due to interactions with the electron clouds of the PMMA (usually described by something like the continuous slowing down approximation, or CSDA, which gives a "breaking energy" per distance travelled, dE/dx) or an event like capture by some chemical species that has an affinity for free electrons (like the cation resulting from generation of the electron, or perhaps a neutral species such as an aromatic ring). If we choose to model with the CSDA, then the Bethe stopping distance gives the distance the electron can travel -- it is the integral of the the inverse of the stopping power, dE/dx, from the initial energy to zero energy. If we are interested in capture of the free electron, Kazawa has used the Smoluchowski equation to model this. Both approaches give average distances much smaller than 30nm.

The loss of energy by an electron is by ionization or else (below ionization potential) by quasi-elastic phonon generation (very low rate of energy loss by electron this way). The continuous slowing down model is a classical, somewhat misleading picture. Electron capture does happen, in the end, when the electron is slowed down to <<1 eV. Guiding light (talk) 06:13, 21 June 2010 (UTC)

Reality is more subtle than portrayed in this article. I think no one knows how far the electrons go. All of these models and experiments have many assumptions (like the CSDA) and extrapolations (from 1.5keV x-rays on an ebrium substrate to ~90 eV EUVL). This is not to say those references are irrelevant -- I would say the opposite -- experiments at low energies in very thin resist films are probably impossible, so we must glean whatever we can from other systems.

EUVL has proven to not be as magnificent as its supporters would claim, but also not as horrible as its detractors think, either. I must admit that "horrible" discoveries often outnumber "magnificent", but a more useful EUVL article would provide the casual reader a more nuanced description of EUV's challenges. I am happy to contribute, but I am new to Wikipedia (at least as a potential contributor), and I am reticent to edit someone else's work without some consensus on the discussion page.Bluedog29 (talk) 17:49, 14 June 2010 (UTC)

Bluedog, regarding just the specific sentence excerpt by Kotera, it only makes reference to the photoelectron energy deposition as drawn in Fig. 4, and has not yet accounted for the secondary electrons generated by that photoelectron (which spreads it further). Still, your point is well taken, it is really not for certain to pin down the resolution at ~30 nm at this point. But as an interesting footnote, the NXE:3100 preproduction tool has the resolution spec of 27 nm, far short of the expected optical resolution.Guiding light (talk) 02:01, 21 June 2010 (UTC)

The X-ray lithography article discusses mostly the same problems but appears more optimistic about them. David R. Ingham (talk) 00:39, 22 January 2012 (UTC)

Other options for source and optics[edit]

David, your points are really good. Although this article has focused on the mainstream EUV development, there is no reason not to mention them as alternatives, if there can be references cited for them. It looks like they should be considered (ifnot already) for X-ray lithography? We can refer to that article, and possibly explain with references why they haven't been practiced for EUV development yet.Guiding light (talk) 07:46, 1 February 2012 (UTC)

Source[edit]

There are at least 3 ways of generating electromagnetic waves in a volume of vacuum: synchrotron radiation, gyrotrons and free-electron lasers (all relativistic vacuum tubes). Gyrotrons probably can't make such short wavelengths. Synchrotron radiation is a continuum of wavelengths, which might not work because the optics tends to be wavelength dependent. Free electron lasers are the the laser version of synchrotron radiation and produce a high quality coherent beam of any chosen wavelength. The main difficulty is that one needs a fairly high energy particle accelerator, probably a linear accelerator to get fast enough electrons (Note that there is at least one very big linear accelerator in Palo Alto and one cynchrotron in Berkeley that are now used mostly as light and X-ray sources). Peak power is good, but I don't know if the right average power and duty factor have been developed yet. The linear accelerator needs to be either long or superconducting, in order to put out high average power without overheating. A synchrotron might not produce enough current, but might work for development. By using a better source, one should need less optics. [Incidentally, one does not need to use quantum mechanics to understand and design a free electron laser, just E&M and special relativity.]

An other possibility is an electron beam hitting a solid target, as in the usual X-ray machines. That would seem to be an improvement on using a laser beam to excite the target.

Optics[edit]

Another option would be to use a Fresnel wave plate. I met a Russian who was developing such a thing for soft X-rays. Its transmission might be even worse than that of a Bragg mirror, but it is less sensitive to material properties and therefore more durable.

An other thing that works for X-rays is a grazing angle reflector, but one might not be able to reflect much light that way.

David R. Ingham (talk) 22:02, 21 January 2012 (UTC)

EUVL light source[edit]

Even with my changes, this is still not quite general enough. Ordinary X-ray machines bombard heavy metal targets with energy a bit higher than the desired photon energy. They produce two kinds of X-rays. Spectral lines are produced by knocking out inner shell electrons. An electron falls in the hole, emitting the energy difference as a photon (fluorescence). Bremsstrahlung X-rays are produced when electrons come near to nuclei and accelerate in the strong electric field. These are a continuum ending at the electron energy. These processes will make euv with lower voltages and lighter (less charge in each nucleus) targets. These processes do not involve heat or multiple ionization, except as by-products.

EUV exposure of photoresist[edit]

Can't one include something in the resist mixture that absorbs the euv with resonant photoelectric effect to shorten the range and modify the spectrum of secondary electrons?

  • I recall reading a paper where doping with Br or some other element had this or similar effect. If I can get that reference, I can add it to this and the X-ray article. I was really hung up on the secondary electrons for some time. But in the end, can't really tell how many or how far. But most recent papers now say the acid diffusion in chemically amplified resists dominates anyway, so the whole issue is blurred (unintentional pun) anyway.Guiding light (talk) 07:51, 1 February 2012 (UTC)