EUV With Fewer Mirrors

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A mirror inside ASML's EUV lithography machine  reflects just 70% of the EUV light it receives. With 10-12 reflections in the machine,  this can get inefficient. Just 1% of the   photons hit the wafer. Electrical power  efficiency is said to be less than 0.2%. It also contributes to  troublesome stochastic defects,   since not enough EUV photons hit the  resist to overcome quantum effects.

So a recent paper from Professor  Tsumoru Shintake at the Okinawa   Institute of Science & Technology caught my eye. It proposes a simplified setup with radically  fewer mirrors. But Shintake makes it clear   to me that his system no way challenges  ASML's. In fact, it should complement it. I think this thing can work. In today’s video,   I want to walk you through this interesting  new thing cooking up in beautiful Okinawa.

## Beginnings We should begin with a brief overview of  a commercial EUV lithography system. I   am not going to cover everything, just  enough to get you through this video. First, we need EUV light, 13.5 nanometer  wavelength light. The light source creates it in   a number of ways - lasers hitting tin droplets,  particle accelerators, whatever you want. A mirror then collects the light and  sends it through the Illumination module,   which spreads out the light and makes  it as uniform as possible for the mask. That light then bounces off a photomask, a special  mirror with the chip design printed onto it.

The reflected light then goes through an  Optical Projection module that reduces   the size of the pattern on the mask's  field by some ratio and focuses it. Finally, the light hits the resist-coated  wafer. Ideally at the exact same angle across   the machine's whole wafer exposure  field so to avoid distortions.

I must pause briefly to explain  Wafer Exposure Field again.   It is a fancy word meaning the size area of the  wafer exposed at once by the lithography machine. We want the field to be as big as  possible. A smaller field means we  

need more exposures to cover the whole  wafer. That both hurts productivity and   offers more chances for things to  go wrong like alignment errors. The industry standard has been 26  millimeters by 33 millimeters. It   is important to be backwards compatible with  that, or else your solution is not economically   viable. The bare acceptable minimum would  be a width of half that - 13 millimeters.

That can barely print an Apple SOC chip.  The A18 Pro's longest side is just under 13   millimeters. Thanks to Max of the YouTube  channel High Yield for the help on that. Anyway. ASML offers two variants  of the EUV machine - a Low and   High-NA. The Low-NA machine's NA  is 0.33. The High-NA machine, 0.55. NA stands for Numerical  Aperture. Roughly speaking,   it describes the range of angles at which a  lens can accept light. So a higher NA lets  

you pattern at higher resolution. Though as we  will discuss, this benefit does not come free. Let us count the mirrors for the Low-NA machine.  One near the light source to collect the light,   four in the illuminator, the mask-mirror itself,  and then six mirrors in the projection module.   That makes 12 total reflections - each  of them absorbing 30% of the photons.

## Schwarzschild Optics Why so many? Over the 20+  years of EUV development,   teams have experimented  with the number of mirrors. One of the first EUV systems was designed  by EUV pioneer Professor Hiroo Kinoshita. His system had a flat reflective photomask. And   for the projection module, there was a  two-mirror Schwarzschild optics system. Schwarzschild optics have been used  before in microscopes. The classic system  

has two sphere-shaped mirrors - a primary and  secondary - aligned on a single axis. Thusly,   the secondary mirror is in the light's path. Professor Kinoshita's main goal at the time was to  prove that you really can use mirrors to reflect   EUV light onto a chip design pattern and then  onto the wafer. This two-mirror arrangement   succeeded in doing that, but also presented  serious limitations for high volume production.

The mirrors’ spherical, curved  shapes mean that they reflect   the chip design image in a spherical,  curved manner. This curved reflection   gets more pronounced the further away  you get from the center, or off-axis. But since both the wafer and the mask are flat,   the curved image is not uniformly in  focus. This "field curvature" distortion   as it is called limits how large we  can make our wafer exposure field. It gets worse when we start scaling the NA -  which per the famous Rayleigh Criterion is one   way to improve patterning resolution. Because a  higher NA means capturing light at more angles,   it also worsens the field curvature effect  and amplifies the optical distortions.

## NTT's Aspherical Systems As a result, people moved away from  Schwarzschild spherical mirrors. In 1992, the Japanese telecom company  NTT presented an early EUV system   they had been working on for the  past three years. Its projection   module used two aspherical, equal-radii mirrors. Aspherical, meaning not spherical. Note that these   are sometimes also referred  to as Schwarzschild as well.

And Equal radii, meaning that they share the same  radius of curvature, but in different directions. One primary convex mirror curving  outwards and a secondary concave   mirror curving inwards. This lets them cancel  out each other’s field curvature distortions. Both mirrors had holes in them to allow light  to enter and exit the larger projection system.

This setup was called the "equal  radii two-mirror" projection module. In addition to the two mirror projection module,  the NTT system had a reflecting photomask,   two-mirror illuminator, and a synchrotron - a type  of particle accelerator - for its light source. ## The MET Tools If you recall your EUV history, and it should be  part of the high school curriculum if you ask me.

Then you know that in the late 1990s a consortium   of American IC makers led by Intel  greatly helped refine the EUV technology. The EUV LLC consortium's product was the  Engineering Test Stand. Its 4-mirror projection   system had an NA of about 0.1, which was low  compared to a production machine. But its exposure   field of 24 by 32.5 millimeters was large enough  to meet high volume industrial requirements. But that 0.1 NA. Afterwards,  the American national labs  

and the famous technology consortium  SEMATECH decided that if we wanted to   test the other aspects of the EUV ecosystem,  we should do it on a small scale exposure tool   with an NA more comparable to that  of a production machine. Like 0.3 So in 2004, they joined together to build  the Berkeley Micro or Microfield Exposure   Tool or MET, which had an 0.3  NA. It adopted the two-mirror   equal radii projection system and had an  exposure field of 0.6 by 0.2 millimeters. The Berkeley lab has continued working  on the MET for many years since,   upgrading the optics and mechanics to  produce MET5 in 2020. MET5 has an NA of 0.5,   a field size of 0.2 millimeters by 0.03  millimeters, and a magnification factor of 5. By then, it was decided that if we wanted  a bigger field size and better resolution,   then we had to have more than  just two mirrors for projection.  

ASML's first EUV Alpha Demo Tool - shipped in 2006  after six long years of development - had six. More mirrors seemed the only way to  achieve a suitably large field. However,   the additional mirrors necessitated a  more powerful light source to deal with   the substantially greater power loss  - from something like 10-20 watts to   200+. Doing that took roughly another  ten years and it got dodgy at times. ## The Shintake System I asked Professor Shintake about  how he came up with his system. Professor Shintake began studying the nuclear   sciences before switching to  electron accelerator science.

He has spent thirty years researching such systems   and their related technologies  at places like Stanford. In 2011, he helped build the world's  second X-ray free electron laser. The Spring-8 Angstrom Compact Free-Electron  Laser. It is Japan's first such device. He has also designed an underwater  propeller for generating electricity   from tidal currents. That is very  cool. Anyway what I am trying to  

say is that he - unlike me - is not  some random dude on the Internet. In 2022, he came across a diagram of the ASML EUV  system in the newspaper and wondered why the EUV   light takes such a strange path. Such an off-axis  path would ordinarily degrade optical performance. Over the next two years, he studied over a hundred  papers and books, and spoke with various people in   the industry. During his research, he came  across the Petzval field curvature theory. Some background. The Petzval field curvature  theory was made by the Slovak mathematician Joseph   Petzval. Joseph is perhaps most well known for  inventing the first photographic portrait lens. Petzval's formulas suggested a way to use  the two-mirror equal-radii projection system   to increase the field size. To confirm  his hunch, the Professor purchased some  

optical simulation software and ran it for three  months using a number of parameter combinations. ## How It Works Shintake humbly says that his system evolves from the existing ASML 6-mirror projection  system. Let me give a brief overview. So the stages are largely the same -  light source to illumination to mask to   projection to wafer. So let us trace the  path the light takes through the system. First, we collect light from the light source with   the goal of sufficiently illuminating  the flat EUV photomask with EUV photons.

After reflecting off the photomask, the  light enters the projection module chamber   with the M1 and M2 mirrors inside  through a hole in the M1 mirror. Once inside the chamber, the  light goes down to the M2 mirror. Then it reflects back up to the  M1 mirror, and reflects off that. Finally after that, it goes  down to the resist-coated wafer,   exiting the chamber through  a hole in the M2 mirror. In total, we have five reflections   just like the old NTT EUV prototype  system back in the early 1990s. ## Projection Did that make sense? I hope it did. Because now

we are gonna do a deep dive  into the projection system. A presentation from Carl Zeiss and optics  legend Dave Shafer first inspired Shintake   to make this projection module. One particular  slide showed how a two-mirror, equal-radii setup   can project a perfectly flat field - as in no  field curvature - while avoiding other issues. But his presentation also noted that  the mask and wafer would need to be   positioned inside the center of the two  mirrors. So most everyone presumed that   this arrangement would be impossible  or impractical to achieve in practice. Shintake played around with this and  found that we can get similar-ish   results by making the M2 mirror as thin  as possible and as close as possible to   the wafer. It still leaves us with some field  curvature distortion but within parameters.

The MET also had to put the mirror  as close as possible to the wafer.   It can be bit tricky to execute, since  it can be hard to make that mirror as   thin as possible while keeping  it rigid. But this can be done. To enlarge the small field size, we make the  tool much larger. In the case of the first MET,   the distance between the photomask - the object  producing the image - and the wafer - where the   image is being formed - was about 276  millimeters or about 10.8 inches. Which   as you undoubtedly remember, gave us a field  size of 0.6 by 0.2 millimeters. Too small.

Shintake's proposal increases the distance  between the object and the image - and   thusly also the tool's height - to the very limit  allowed by modern semiconductor fabs' ceilings: About 2 meters or 6 foot, 6 inches. Or just a bit taller than a Michael  Jordan, the semiconductor industry's   widely-accepted standard for height increment. The semiconductor fab people do not  seem particularly phased by this,   at least nowadays. I feel like ASML's  High-NA EUV tool has sort of reset   people's expectations for tool sizes. But  I do wonder about the second-order effects   of such a big device. Shintake will  likely rejigger this down the line.

## Illumination So this two-mirror projection system  has been known. Based on the history   we just reviewed, it is not particularly novel. So I think what is actually most special about the   Shintake system is the module located  above the projection, the illuminator. The EUV lithography tool is a  modern scanner device. So the   mask is exposed through a fixed "exposure slit". You "scan" or, like, drag the mask through the  light beam while moving the wafer at the same   time in the opposite direction. Once the scan is  done, you step on to the next part of the wafer.

It makes more sense to be honest when you look   at a transparent mask rather  than a reflective mirror one. The challenge with the illumination system is that  the reflection geometries are complicated. The EUV   light must be positioned in such a way to bounce  off the mirror and into the projection chamber.

This was a great challenge. Professor  Shintake told me that he designed three   previous illumination systems that did  not work. Then, while cleaning his room,   he came up with a clever something  he calls "dual line field".

With dual line field, four smaller light  sources bundled together into two larger   light sources are positioned to  illuminate the mask. But how to   keep the sources from reflecting onto each  other? As Shintake says in an interview: > If you hold two flashlights, one in  each hand, and aim them diagonally at a   mirror in front of you at the same angle,  then the light from one flashlight will   always hit the opposite flashlight,  which is unacceptable in lithography. > But if you move your hands outward  without changing the angle of the   flashlights until the middle is  perfectly lit up from both sides,   the light can be reflected without colliding  with the light from the opposite flashlights What makes this really clever is that unlike the  ASML machine, there are two fixed exposure slits. So as the lithography machine does  its scan, a spot on the mask gets   exposed to the light cone from the first  slit, projecting a spot onto the wafer. But the mask is still scanning. That spot on  the mask moves out of the first light cone   and then gets exposed to that second  light cone, projecting to the wafer.  

Such an arrangement can only work with a  scan-and-step machine, with a moving mask. This is very clever. This two-slit arrangement  lets the two illumination cones to be offset,   thus avoiding a reflection back at the sources.  I am guessing this is why the Okinawa Institute   has a patent application out on the dual  line field. It’s key to making this all work. ## The Possibilities So what is possible with this simplified design?  Let us first look at the critical dimension. Run the variables through the famous Rayleigh  Criterion equation and you find that a 0.2 NA  

and a 13.5 nanometer wavelength gets you  a 24 nanometer half-pitch - assuming that   the K(1) process factor is the same 0.35  like as with traditional EUV machines. To compare, a leading edge 193-nanometer  immersion lithography machine has an NA   of 1.35 and a K(1) of 0.27. This works  out to about a 40 nanometer half-pitch.

Now, there is more to consider than just  resolution - throughput, power cost, overlay,   maintenance, and so on. But just based  on critical dimension, this looks good. How about ASML's Low-NA EUV machines?  They have about a 13-nanometer half pitch.   Their High-NA EUV machines' theoretical half pitch   is around 8 nanometers. So the ASML EUV  machines can still do much better here. But if we raise the NA of  the Shintake system to 0.3,  

then the critical dimension half pitch works  out to 16 nanometers. That is interesting. 0.3 is probably where this will  go. Shintake told me that the   0.2 NA machine proposed in his first  paper did not match up with industry   standards. Namely, its field scan size of  20 millimeters and its reduction ratio of 5.

So he is rewriting the paper with more  industry-standard parameters - a 13-millimeter   field scan size and reduction ratio of 4.  The device might also be smaller somewhat. This would require the adoption of field-stitching   techniques like those the industry is  making in preparation for High-NA EUV.   But we should note that most smartphone  SOC chips might already be printable. ## The Bent Mask So what are the catches? There are a few  complications and the paper lays them out. But one that caught my eye is that with  two mirrors we cannot avoid the projected   photomask image having some bit of field  curvature - which can lead to print errors.

So for additional help, he suggests  that we slightly curve the mirrored   photomask to correct the remaining curvature. Yes, as in taking a regular flat  mask and mechanically bending   it (Bend It Like Beckham!) by anywhere from 30-120   microns - depending on NA and tool size  - when mounting the thing onto the chuck. I asked someone in the mask industry about  trying to bend these quarter million dollar   EUV photomasks, and he sounded  concerned. The mask blanks are  

made from a very stiff substrate of ultra  low expansion glass and they might crack. My friend suggested making curved blanks  and then printing the chip design onto   that. This will also be challenging  since the multi-electron-beam mask   writers aren't equipped for that right  now. The depth at which they can write  

features - depth of focus - is limited,  and will need to be made variable. More consultation is needed, but  the Professor assured me that   the curved mask is not absolutely  necessary for the system to work. ## An Imminent Breakthrough? This paper was somewhat covered in the  overseas media, but I think in an unfair way.

Overseas commentators exclaimed in typical  bombastic, uneducated format: "Japan on   edge of EUV lithography chip-making revolution",  with the subtitle "Okinawa Institute of Science   and Technology claims breakthroughs that could  break ASML’s monopoly on advanced chip-making   equipment" - based on what I presume to be a  five minute reading of the first paragraph. I abhor such statements because  if you read the whole paper,   you can see what it actually is. It is a  creative re-imagining that builds upon the   decades of work already done in the EUV  ecosystem. It complements, not displaces. Right now, the machine only exists in a  computer simulation. I think it can work,  

theoretically, but we are far away  from a real device. There remain many   more technical hurdles to overcome,  including those not yet foreseen. If someone wanted to found a startup around  this from scratch, there is so much more they   would need to figure out. Like how to  make the EUV light source, the mirrors,   the photomask plus their blanks, the precision  mechatronics to manipulate the wafer stage with   nanometer-accuracy, the special new photoresists,  and the software to control all that. So Shintake intends for his system to help,   not challenge the ASML systems. He re-iterated  this several times during our interview. ## Conclusion Before we end, I want to thank Professor  Shintake and Dr. Patrick Naulleau of EUV  

Tech for taking the time to speak with  me, and answering my uninformed questions. Let me wrap this up. I admire Shintake's  system because it represents a rethinking   of EUV as we know it today. He went  back to the drawing board and came   up with something wonderful. I don't  see any reason why this can't work,   though a commercial machine  needs a lot more than just that. So what do we do with this? The paper  proposes that next we should do a   proof-of-principle experiment.  A real-ish lithography machine  

with requirements more in line  with the semiconductor industry,   like a 13 millimeter-wide field for instance.  I think someone should try to help him make it.

2024-10-22

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