High-NA EUV is an evolutionary step forward in lithography. It
unlocks a great deal of benefits, but at a cost.
ASML has worked on High-NA for over a decade. Right now,
it is about 2 years away from hitting fab floors.
Today, we are going to update progress with High-NA EUV.
After that, we will take an irresponsible look ahead at the insanity that is Hyper-NA EUV.
## Why Higher NA?
I note that today's video is a bit more advanced than
normal. We will also be going over previous work.
We should begin with a recounting of the intention of High-NA and Hyper-NA EUV lithography.
The NA refers to "Numerical Aperture". It is a dimensionless number measuring how
much light an optics system can collect and focus.
There are about 160 EUV systems in operation around the world today.
All of them have an NA of 0.33, which works out to a 13 nanometer half pitch line.
Good for them. But 13 nanometers is insufficient
for future process nodes. TSMC and others are producing their latest
3-nanometer generation nodes using EUV multi-patterning which is not ideal.
Raising the NA to 0.55 would shrink the size of the lithography machines' half
pitch lines some 64% to 7.5 or 8 nanometers. At least in theory.
High-NA is scheduled to enter the supply chain in 2025 and its performance is said
to be sufficient for the N2 or N1.8 process nodes as well as the next 2-3 nodes after that.
## Photomasks
ASML is not vertically integrated so there needs to be consensus with the critical suppliers.
Some of the most important of which are those for the photomasks,
the mirror glass, and inspection tools.
In 2013, SEMATECH, imec, and others held two meetings with
these guys about what a High-NA EUV transition might look like.
For instance, one of the possible scenarios would be to set the NA at 0.5,
but increase the size of the EUV mask from its current 6 inches to 7 inches.
The upside of this option would be faster throughput. We would be
able to print some 70% more pattern features onto the wafer per hour.
However the suppliers said that they would not pursue larger EUV
mask sizes without agreement from the chipmakers - Intel,
TSMC and the like. Those guys would be the ones paying for the bigger masks, after all.
The chipmakers declined an invitation to present their views at that first
2013 meeting, which was construed as a sign that they were waiting to see
how things would shake out with regards to first-generation EUV.
At the second 2013 meeting, GlobalFoundries presented a cost-of-ownership analysis.
They argued that it does not make sense to increase the mask size.
This is assuming that a 7 inch mask is 30% more expensive,
a 9 inch mask is 40% more expensive, and a 12 inch mask 50% more expensive.
So in the end, it was decided that the EUV mask sizes would not change
for High-NA EUV - leading to significant consequences for ASML's machine design.
## Bigger Mirrors
First things first. Higher NA means bigger mirrors.
In an EUV system, the NA is proportional to the opening angle - the angle between
the mirror's axis and its surface. So increasing
the NA means increasing the mirror's opening angle - and thus its width.
All of the mirrors have to get larger, but the final one is an absolute unit.
Zeiss reports that it is about 1.2 meters wide and weighs 360 kilograms.
This is nearly twice as large and 10 times as heavy as the final mirror for the 0.33 NA system,
which was 0.65 meters wide and weighed a mere 40 kilograms.
"Bigger" is far easier said than done. Zeiss had to build a very
large autonomous robot just to handle these mega-mirrors. Furthermore, they had to build
custom 20 foot high vacuum vessels for running optics metrology on the mirrors.
To fund this, Zeiss and ASML sealed a deal worth $1.9 billion in November 2016. $1.1
billion of that is for stock and the rest are pledges for capital expenditures and R&D.
## Optics Focus Point 1
These mirrors are multi-layer Bragg reflectors.
Because they have so many layers, their reflectivity is dependent on the angle
of the incoming light beam. Unmodified, this reflectivity rapidly declines at higher angles.
This restriction plus the need for larger mirrors has several major
ramifications throughout the optics system. ASML discussed two focus areas in particular.
The first area is the mask.
The mask itself is a mirror and the higher NA means that the incoming light cone interferes
with the outgoing one. The obvious solution would be to increase the reflection angle so
to put more space in between the two cones. But sadly the mask does not allow this.
Not only because of the declining reflectivity with rising angles but also because the chip
design pattern is printed onto the masks using lines of absorbers.
If the angle of the incoming light cone is anywhere higher than about 6 degrees,
then the design's absorber lines get unacceptably distorted.
So we magnify the light cone to shrink its width. But if you do a full magnification
on all axes then this also shrinks the mask field size to a fourth of its previous size. Mask field
size is the size of the area transferred from mask to wafer in a single exposure.
To keep it from becoming unacceptably small,
they use anamorphic lenses to magnify just one of the mask's two axes.
The major consequence of this is that High-NA EUV systems have a field size
about half as large as that of low-NA EUV. This means that chipmakers at the start
will only use it for chip layers of exceptionally high complexity.
Stitching together print features runs the risk of discontinuities at the borders.
Ameliorating these effects will require new computational lithography techniques.
## Optics Focus Point 2
The second major engineering focus point in
the system occurs with the last two mirrors prior to the wafer.
Because that last mirror is so wide, the light coming from the
second to last mirror hits it at an angle that is wider than ideal.
So Zeiss drilled a hole or obscuration into the last mirror in order to lower
the angles again and improve productivity. This is
a well-known feature used for telescopes to avoid stray light and improve contrast.
## Depth of Focus
Furthermore, the higher NA in High-NA
EUV significantly reduces the chip design's depth of focus.
Depth of focus is the vertical distance at which the image quality is still acceptable.
In other words, it is the range at which the chip design's image
remains acceptably in focus. That focused image has to be maintained
throughout the whole resist layer. The larger that distance is, the better.
So ideally, we want a low resolution limit and a high depth of focus.
The problem is that if we raise the NA to achieve this lower resolution limit,
that inherently shrinks the depth of focus.
The equation dictating this is a second Rayleigh equation - one
not as well known as its more glamorous cousin:
> k2 * (wavelength / numerical aperture^2)
Where, K2 is a function of the process same as K1 with the resolution equation.
With a 0.55 NA, that depth of range is 45 nanometers,
which is over 60% thinner than how it was for the 0.33 NA tools.
This may be made even worse by refraction effects from
the resist layer and the vacuum above it depending on the resist material.
Per Snell's Law, if the resist material's index of
refraction is greater than 1 then it helps increase the depth of focus.
But so far the tests seem to show refractive indices less than 1 - though pretty close to
it. This means that we should expect to see refractive effects hurting our depth of focus.
So this super thin depth of focus has several wide-ranging effects. High-NA EUV wafers must
now be incredibly flat to stay within that depth of focus range. Focus control will be essential.
This in turn places tight requirements on the
whole semiconductor processing chain from wafer substrates up onwards.
For instance, etching and deposition steps can easily add new topologies to the wafer
that push it out of the focus range. So we need to do flattening and polishing.
And within the stepper tools themselves, new attention has to be paid to the machines
handling the wafers - even the chuck holding the wafer inside the machine.
## Light Source
The Exposure Tool’s light source is one of the most commonly cited parts of the EUV system.
And I have to admit that it is quite charismatic. How can you not be in awe of lasers hitting
50,000 tin droplets each second ... twice? ASML is working on new, more powerful light
sources for High-NA. It will largely work the same way, but with a few modifications.
First, they will increase the frequency of tin droplets from 50,000 a second to 60,000.
Second and more significantly, ASML will swap out the old 10-micron
laser used for the first of the two laser blasts with a 1-micron one.
Oh and they also turned it on its side this
time to help improve the light transmission. So there you go.
Right now, the NXE 3400 EUV light source achieves about 300 watts of
power. ASML predicts that if these modifications work out then they can
scale all the way to as high as 800 watts someday in a glorious future.
It also means however that High-NA EUV will use even more electricity than the original
EUV machines - which are already estimated to use as much electricity as 3 Walmart Supercenters.
In light of this, I should note the existence of EUV light sources using methods other
than laser-produced plasma. One particularly interesting approach is the Free-Electron Laser.
Here, we use a particle accelerator to accelerate electrons to near the speed
of light. We then inject them into an array of magnets. Inside, they oscillate and emit light.
## Resists
We should talk resists now. Resists have long been
one of the limiters in EUV technology - a critical portion of the EUV puzzle.
EUV resists work different from DUV resists. When a DUV photon hits a
resist molecule, that same molecule gets excited and that is about it.
But when an EUV photon hits a resist molecule,
there is so much energy that the molecule ejects an electron.
This ejected electron can then go on to create further chain reactions in
the molecules surrounding that first molecule - causing a sort of "blur".
Calculating and minimizing this effect is difficult. And it is not helped by the
fact that we are now printing features the same size of individual resist molecules.
Nor by the fact that quantum stochastic effects cause random photons to show up
at unpredictable spots, leading to errors.
And with the depth of range being so small, the resist layer must be
very thin. And thus the resist material has to absorb EUV light extremely well.
Resist progress is very proprietary, costly and time consuming. And thus far we have yet to come
across one capable of satisfactorily meeting all the requirements. The search continues.
## Hyper-NA
High-NA is two years away and supposedly meets our goals for five years more.
Yet we are already mulling over the next conceptual step after High-NA.
Which would be to raise the NA from 0.55 to 0.75 or beyond - a level called "Hyper-NA".
The first time most of us may have heard of it is during a
Bits and Chips interview with ASML CTO Martin Van den Brink. He says about it:
> Theoretically, it can be done. Technologically, it can be done. But how much room is left in the
market for even larger lenses? Could we even sell those systems? ... If the cost of hyper-NA
grows as fast as we’ve seen with high-NA, it will pretty much be economically unfeasible.
Saying this apparently got him some grief but he isn't exactly wrong. Each of these High-NA
machines will cost somewhere in the range of $300-350 million. Even being charitable
with the trend, a Hyper-NA EUV machine can cost over half a billion dollars.
That is about the cost of 1 Barcelona contract with Messi.
## Hyper-NA Engineering Challenges
There will be substantial engineering challenges associated with the larger mirrors for Hyper-NA.
So imagine the massive robots for mirror handling and vacuum chambers
built for mirror metrology, but even larger.
As well as the resist issues with High-NA. The depth of focus decline
is proportional to the square of NA so it’s even worse with Hyper.
We go back to the mirrors at the mask area. A larger NA means even
wider light cones coming on and off of the mask.
But even with magnification, simulations show that the reflection angles average
about 6.5 degrees for 0.75 NA and 7.5 for 0.85 NA.
At the higher end of the range, the reflection angles are about 10-12 degrees,
which is beyond the acceptable reflectivity value.
So we may have to modify the thickness of the mirror multi-layers to accommodate
this - but this seems possible if we are just going with 0.75 NA.
## Hyper-NA Polarization
Another thing to consider is polarization. In other words, the direction in which the
light is oscillating - like as if it were a vibration on a guitar string.
Light as it comes out of the EUV light source is unpolarized,
meaning that it goes in a variety of directions. Therein lies the issue.
Imagine light going through a grate - which is kind of
like what an EUV photomask is. Just a big grate.
When the gaps between the grate are wide, things are good. Light oscillating in
all of the directions act largely the same as it passes through the gaps or reflect off the lines.
But now shrink the gaps. When the width of the gaps get to be about twice the
width of the light's wavelength, you start seeing strange effects.
Light oscillating in the direction orthogonal to the grating's direction
becomes much more likely to reflect away and not pass through the grating.
The scientific phrase for this orthogonal light is "transverse-electric" or TE or "S".
The scientific phrase for the light oscillating in the direction parallel
to the grating is "transverse-magnetic" or TM - or just "P" for parallel.
This effect was first observed in 1902 but analyzed by - who else? - Rayleigh
in 1907. Anyway at the end of the day, this is what it means for the chipmaker.
In a chip's design, lines going in one direction (the TE polarized light) will have better contrast
and thus print quality than the lines going in the other direction (TM polarized light).
This contrast is measured by a figure known as "Normalized image log-slope"
or NILS and is dependent on the incoming light's angle. Anyway,
polarization was already a significant concern with High-NA EUV, but it will be
a huge problem with Hyper-NA EUV when the gaps between the lines get even smaller.
ASML might consider using a polarizer, which filters out light in certain directions. But
by definition, a polarizer filters out some light which weakens power and slows throughput.
Perhaps new light sources like the aforementioned Free-Electron
laser - which emits polarized light - can help with this.
## Conclusion
I have said before that the semiconductor industry is pretty terrible at predicting anything beyond
five years. Hyper-NA is just too far into the future for us to know how it will shake out.
Right now, there remains big challenges with High-NA EUV.
While the optics engineering and light source seem to be on track,
we lack significant information on other, critical parts of the system.
Most significantly, the resists and the polarization issues. These may
have significant effects on High and Hyper-NA's real world performance.
These limitations make me wonder whether it makes
sense for a chipmaker to shell out for a future $500 million machine.
That being said, I do see pathways towards more powerful, effective
lithography without the need to raise to Hyper-NA. Better resists, larger masks,
new and more powerful light sources, magic fairy dust from the Martian moon of Phobos and so on.
Right now there is no wavelength beyond EUV. But that doesn’t mean
we have done all we can with this wonderful 13.5 nanometer light. ￼