Right now, Extreme Ultraviolet Lithography is working. Foundries are using dozens of
these $150 million machines to create amazing patterns nanometers wide.
The industry labored for decades on this intricate technology. But there is no time to rest.
ASML is already looking ahead at the next generation of EUV:
High-NA EUV. The centi-million dollar machine that will be better than regular EUV.
In this video, we are going to do a deep dive into what’s next in the world of EUV.
ASML's follow-up to the most complicated nanolithography system ever delivered.
Today, EUV technically works. And of course we should not ignore just how impressive that is.
However, at a commercial level, there remain substantial challenges.
The biggest problem has to do with throughput:
How many wafers a single machine is capable of producing over a period of time.
Right now, TSMC's leading edge N5 process has about 14-15 EUV layers. EUV machines are
slower than older machines. So if you want to be able to push out as many wafers as before,
you need to have more machines.
So in 2020, TSMC opened up the checkbook and bought as many EUV machines as ASML could
make. By the second half of 2020, they had 50% of the install base.
It worked, but each N5 wafer is estimated by industry analysts to cost as much as a car.
Nearly $17,000, a theoretical 80% price jump. To get that cost down,
you need to put out more wafers. To put out more wafers, you need faster throughput.
Now on to TSMC's next full-step node, N3. That right now uses more of those slow EUV layers -
about 20-25 as compared to N5’s 14-15. There is a risk for throughput to decline even further,
worsening foundries' ability to produce enough wafers and reach the necessary economies of scale.
Furthermore, all things being equal, TSMC is likely going to have to resort
to a technique called multiple-patterning for N3.
This is where you use another exposure to improve the feature density.
Imagine you got a canvas of dough and you want to cut out some noodles about 13 nanometers thick.
This is so that you can make a nano-bowl
of Taiwanese beef noodle soup for your nano-sized buddies.
But alas, your nano-sized noodle-cutter-mold thing has its metal mold teeth things
26 nanometers apart. You can really tell that I spend a lot of time in the kitchen.
Anyway, so how can you cut a 13-nanometer wide noodle?
It is possible if you are extremely careful. If you position the nano-noodle-cutter-mold thing
exactly right, you can make a second pass on the dough and get your 13-nanometer wide noodle.
As you might expect, this doubles or even triples the amount of time it takes to process
a single layer. Multi-patterning makes EUV, already pretty slow, decidedly uneconomical.
TSMC could try to buy more of those $150 million EUV machines. But ASML is already
selling those as fast as they can make them. The more economical alternative would be to
get EUV machines that do lithography better and faster. And that is what High-NA is all about.
The NA in high-NA stands for "numerical aperture". It is a dimensionless number
measuring how much light an optics system can collect and focus.
To explain why this is a big deal, we go back to Rayleigh's formula:
Resolution = k1 x light-wavelength / NA
Improving the resolution means either shrinking the K1 or the wavelength,
or increasing the NA. Literally math.
The K1 has a physical limit and we are already close to it so that is
out the lithographic window. And the first generation of EUV brought the
light-wavelength down to its current 13.5 nanometers. After all that,
I don't think they want to try pushing that envelope again any time soon.
Thus, the last major factor left would be to raise the NA.
Re-engineer the optics so to collect and focus more of that
sweet EUV light. This is essentially what this entire system is about. MORE LIGHT!
The most advanced EUV machine from ASML is the NXE:3600D. Its NA is 0.33,
and that translates to a 26 nanometer pitch. This meets TSMC's N5 process requirements,
which requires a 28 nanometer pitch. That machine can churn out about 160 wafers per hour.
ASML's first High-NA EUV machine, the EXE:5000, will increase that NA from 0.33
to 0.55, translating to a 16 nanometer pitch.
This is a 67% improvement. Very meaningful and the preferred way to make the N3 process work.
Furthermore, not only can High-NA EUV do lithography with a much smaller pitch.
They can also print at a far higher resolution, resulting in less defects.
Less defects mean more good wafers per day.
And if High-NA EUV is as good as advertised then you can start using it to replace layers that had
been previously done with multi-patterning at lower wavelengths. Why make two or three passes
when you can do only one? Savings like this is what will truly justify the use of High-NA EUV.
Ok enough with economics. Let’s talk engineering.
There are two major challenges associated with High-NA EUV: The optics and the resist. Let us
start with the optics and the Starlith 5000 optics system. From our old pal Carl Zeiss.
Okay. We are going to talk some optics. I really, honestly hope that I do not lose any of you guys.
In my previous video, I broke down the components of the Starlith 3000.
The Starlith 3000 series consists of the collector optics, an aperture, illumination optics,
mask or reticle as it can also be called, and projection optics. Then finally it hits the wafer.
To achieve a higher NA, the Starlith 5000 must
be redesigned to essentially allow light at more angles to hit the wafer.
The old systems with 0.33 NA had a thinner light cone at mirror 6, right above the wafer.
Going to 0.55 means making that cone ... thicker.
As a result of the M6 light cone being thicker,
the first big challenge has to do with the mirror right before it: Mirror 5 or M5.
Because multi-layer mirrors with their 50 EUV-reflecting layers
are limited to reflecting at very specific angles - about 13 degrees - the M5 mirror
has to be positioned very close to the M6 light cone.
Side note, yes. The super mirrors are back! And they are bigger than ever. A meter wide,
with an accuracy of a picometer. If you were to blow it up to the size of the Earth,
the largest aberration would be the diameter of a human hair.
Anyway. Back to the case of the high-NA M6 example.
The way this setup is right now, the two light cones will interfere with each other.
This causes big imaging errors. We do not want that.
So how do you have your bigger light cone and eat it too? With some wicked brilliant
engineering. You drill oval-shaped holes into the mirrors. Holes. In the mirrors.
Zeiss calls them obscurations. And they reduce the angular spread on the mirrors
and significantly increase the transmission of the optics.
How? Iono. The documents don't say. But it works.
And I am sure all the optical PhDs will let us know why in the comments below.
The same effect happens a few stages up the optics system at the reticle or mask.
The mask is essentially a grating in front of a mirror and it contains the chip design pattern.
Redesigning a mask costs millions of dollars, so one of the design requirements for High-NA EUV
is that the mask size cannot change.
In order to achieve a high NA at the wafer stage,
you have to have a high NA at the mask stage. The two are linked by a mathematical formula.
The TWINSCAN NXE:3300B is EUV enabled and like I said, has an NA of 0.33 and
a magnification factor of 4x. The reticle mirror mask reflects EUV light at 6 degrees.
With this we can project a 132 millimeter by 104 millimeter
reticle area onto a 33 millimeter by 26 millimeter wafer area. Got it?
Alright, so what happens when we increase the NA? The same thing as we already talked about.
The two light cones are bigger and thus they overlap
by about 1 degree. What are we going to do about that?
The obvious solution would be to raise the EUV light reflection angle. Like from, say, 6 degrees
to 9 degrees. Nice. However, that is a no go because it creates weird mask shadowing effects.
Like I mentioned, the mask is essentially a grate in front of a mirror.
What happens when you shine a flash light and then move over to the side? The shadows move.
The only other thing left to change is the magnification
factor. And that is what they did. They raised the magnification from 4x to 8x.
And that worked. Kind of. You get the higher NA without fatter cones or compromising mask shadows.
But when you magnify something, the light area at the end gets smaller.
And that is exactly what happens. That 33 millimeter by 26 millimeter wafer area
shrinks to a quarter of that size: 16.5 millimeters to 13 millimeters.
Now if ASML was anything like me then they’d shrug and be like “Oh well, you know”.
But this situation is economically unacceptable for their customers. It would take too long for
the EUV machine to crawl its way across a wafer. TSMC is going to pay ASML hundreds of millions
of dollars for this machine. For all that, do they really want to get a 4x slower product?
So we got to fix this. The first thing that comes to mind would be to make a bigger
mask. Twice as large as the current 6 inches, in fact. But as I said earlier,
that is not feasible. The machine would not be able to handle a 12 inch mask anyway.
So what Zeiss and ASML did was to use something called anamorphic imaging. Magnification factors
can exist on both the X and Y axes. But they are not necessarily linked together one to one.
We only really need the 8x magnification factor along the Y-axis, not the X-axis.
With this new optics technology, the system can project a 16.5 millimeter
by 26 millimeter field - half size, just barely acceptable.
ASML and Zeiss announced that they were working together on this solution back in 2014.
They borrowed anamorphic technologies from the movie industry, where they are used
to project movies with unusual widescreen aspect ratios onto standard sized screens.
The optics make up the majority of the technical challenges involved with High-NA EUV, but not all
of them. There are still a few others. And the other major one has to do with the photoresist.
If you can recall, the photoresist is applied on top of the substrate before
the whole thing gets exposed to the EUV. After that, you can either keep the exposed
portions - a negative photoresist - or the non-exposed portions - a positive photoresist.
A higher NA means that the resist needs to be thinner too. This is to lower the risk
of "pattern collapse" or "line collapse". The concept is easy to comprehend. If the
resist is too thick, then the lines after EUV exposure will be too close together.
After exposure, the wafer is washed and the resist washed away. After that, it is dried.
If the lines do not dry at the same time, then the surface tension of the water distorts the design.
The lines might even fall over or peel off the substrate.
Thus, you are aiming to have a resist layer at least 50% thicker
than your pitch. With a 16 nanometer pitch, that is a 20 nanometer thick resist.
Companies are experimenting with new resists.
ASML is working with a variety of partners like IMEC, PSI and CXRO
to try and find a good candidate. As of this writing, they are still working on this one.
There remain a few other engineering challenges too. For instance, making
bigger mirrors, the mask frame layout, the pellicle, or the metrology system.
Metrology in particular will be challenging. This is a system of sensors and programs that checks
on progress and quality. It is crucial to know what's going on with the lines on your wafer.
But how do you inspect something less than 20 nanometers wide?
Right now, ASML is looking into e-beam inspection techniques but that has yet to be finalized.
As always, the technology exists and is well established. The problem is turning it into
a shipping technology competitive against the next best alternative.
The machine remains in development. The first machines are expected to head
over to customers some time in late 2022. Two years later after that in 2024 or 2025
would be an improved version - the EXE:5200 - with an expected throughput of 220 wafers per hour.
Everything about the EXE:5000 will be chonk. The light cones. The optics themselves. The
whole machine will be massive. It will be as big as a crouched T-rex.
And of course, the machine will sport a massive price tag too, topping $300 million each.
With that, you can buy the second most expensive painting in the world.
Hold 8.8 Squid Games. Or pay gas fees for 1 NFT.
300 million dollars. I can only imagine the Twitter replies.
The EXE:5000 is an engineering marvel but it will have to justify that monstrous price.
Alright that's it for tonight. Thanks for watching. If you enjoyed the video,
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