After 20+ years of development, extreme ultraviolet lithography has become a commercial reality.
As I write these words, multi-million dollar machines from ASML use EUV light to create
impossibly small patterns in wafers.
This technological magic requires a powerful heart inside of it. And indeed, there is an
amazing system driving ASML's $150 million lithography machine: The EUV Light Source.
In this video, we are going to look at the lasers firing pulses at tin droplets to create
the powerful, 13.5 nanometer wavelength light for our latest, greatest microprocessors.
Before we move on, I have to note something. This video will mostly focus on the EUV approach
taken by Cymer, an ASML subsidiary.
There is another semiconductor laser company called Gigaphoton, a subsidiary of Komatsu,
that is also attempting to reliably generate EUV light with their own unique approach.
I think what they are doing is pretty cool too. We can cover them in a future video but
not in this one. Maybe if this video gets twenty thousand likes or something like that.
Every so often, the best engineers across all of the top semiconductor firms get together
to try and decide what they might use to etch the next generation of chips. They hash out
the technical aspects of the candidates, coordinate the necessary R&D, and then cooperate, despite
usually being rivals.
For a long time, the industry was etching with light in the 193 nm wavelength. This
generation was getting long in the tooth, but engineers kept finding ways to extend
its usefulness in the industry.
For instance, they realized that shooting the light through ultrapure water through
to the wafer allowed them to focus more sharply and etch yet smaller features. This technique
is called immersion lithography.
Then they started using techniques like double-patterning. This is where you use additional exposures
to improve the feature density. In other words, kind of like doing a double pass through the
machine.
These helped the industry push ahead. But at the end of the day, they needed to move
on from 193 nm. And EUV light would be the 13.5 nanometer, $150 million dollar raft to
take them there.
Scientists and engineers identified two simple but Big Requirements for the next generation
EUV light source. We have advanced far beyond simple light bulbs.
The first Big Requirement is that the source needed to have a high conversion efficiency.
That means it has to be able to generate enough EUV light in the correct wavelength - 13.5
nanometers.
Furthermore, the light needs to be emitted in the right direction - backwards towards
the light collector optics. Then the mirror - roughly shaped like a headlamp - can collect
this light and send it towards the rest of the optics system.
And of course, it needs to generate this light relatively efficiently. You don't want to
put an obscene amount of energy into this light source.
The second Big Requirement has to do with the debris. Firing lasers at things creates
a lot of debris. That stuff will find its way to the critical optics system and dampen
their performance. To meet cost of ownership requirements, a way has to be found to mitigate
this debris.
These two major issues are surprisingly simple to explain. But they turned out to be incredibly
difficult to overcome.
The EUV laser story begins all the way back in 1986. Japanese researcher Hiroo Kinoshita
began experimenting with new wavelengths of light, essentially X-rays, to etch transistors
onto silicon.
His work allowed him to print 500 nm half pitches patterns. Half pitch, meaning half
the minimum distance spacing between lines. Bell Labs soon followed with a 50 nm half
pitch print in 1990.
These early experiments were done with synchrotron radiation. A synchrotron is a class of donut-shaped
particle accelerator descended from the cyclotron. It keeps a particle beam continuously circulating
for many hours, emitting radiation.
Essentially a relative of the Large Hadron Collider. Industrialists envisioned having
one synchrotron linked up to multiple lithography tools. But researchers quickly realized that
these light sources were not practical for the industry.
Not because building an Iron Man style arc reactor to etch chips would cost too much
or take up too much space. TSMC would do it.
But because a synchrotron emitted EUV light over too broad a field of view. Not enough
of it would go into the collector mirror, thus failing the First Big Requirement.
At the same time, the goalposts moved farther backwards. EUV had originally been scheduled
for the 100nm node, which meant the light source and print resolution did not have to
be all that powerful.
But then it got penciled in for the 70nm node, and then the 50 nm node. Later on, it would
fall even further as delays piled up.
This had cascading effects throughout the entire design. The EUV optics system - painstakingly
crafted by German optics maker Carl Zeiss - needed to change. To increase the print
resolution, Zeiss had to add more mirrors into the projection and illumination optics
systems.
Each mirror, even when perfectly made, is only capable of reflecting at most around
70% of the EUV light that hits it. The rest gets absorbed.
These two factors together meant that for EUV lithography to work on the shop floor,
ASML needed a stronger, single-r (is that a word?), more brilliant point of plasma light.
Capable of at least 250 watts to be competitive with current 193 nanometer laser technology.
Something to note. It is not just good enough for the technology to work. Anyone can slap
together a bunch of lab equipment and shove it in front of the users. It also has to be
appreciably better than what is already on the market - or else there is no point for
it to be. Customer commercial requirements constantly set the bar for these engineering
wonders.
Thus, another breakthrough in EUV light generation was required before industry leaders could
start to take it seriously.
Researchers for a long time had been experimenting with something called laser-produced plasma
or LPP. This is where you point a laser at a gold plate or wire target so to create plasma.
The plasma then presumably emits EUV light in the proper wavelength required.
This might work, but the prevailing issue everyone thought would be the dealbreaker
was debris. Bits and pieces of the target would fly off and contaminate the optics system,
degrading performance.
Researchers tried a variety of different targets, narrowing it down to three: Lithium, Tin,
and Xenon.
At first, the most promising target seemed to be rare Xenon gas - in either gaseous or
ice form - and YAG lasers. YAG stands for Yttrium Aluminum Garnet lasers. They are powered
by a crystal.
It kind of reminds me of what they use for lightsabers.
Early experiments had researchers firing 1 kiloWatt YAG lasers at a jet of Xenon at a
rate of 10,000 cycles per second. Scientists at Sandia Laboratory were eventually able
to suppress Xenon ice particles from getting to the optics using a gas jet. But the Xenon
ice plasma also emitted high energy ions that degraded the optics anyway. Nobody knew how
to fix that.
That last paragraph sounds like a line of dialogue from a Star Trek movie.
The second most plausible option was tin. Tin plasma certainly emits a lot of EUV light
in the required wavelength. But the conversion efficiency you got from firing lasers at a
tin plate was terrible. More on that later.
Furthermore, using tin as an EUV light source notoriously generates a whole lot of tin debris.
As I mentioned before, a tin layer just 1.2 nanometers thick will cause a 20% decline
in collector efficiency and a 10% decline in total tool output. How are people going
to deal with that?
Thus, tin did not seem to hold all that much promise. Regardless, people continued experimenting.
Various companies in the industry began tinkering with firing lasers at tin droplets rather
than a tin plate with good results.
Then in 2002 all the leading minds got together in Dallas to talk about it. They debated the
future of the Xenon laser option and realized that Xenon did not offer a very promising
technical future.
The Xenon option had a fundamental roadblock. The conversion efficiency was too low - less
than 1%. It cannot generate more than several tens of watts of EUV light.
The Tin option would create a debris nightmare up ahead in the development roadmap. But maybe
with enough engineering this could be overcome. There would be no fundamental physics issue
ahead. Thus, the industry made a massive turn and embarked on a journey towards tin plasma.
The original target material had been a tin plate. However, the conversion efficiency
with the plate was rather low.
The reason for this is because of a low level of what is called spectral efficiency. This
refers to the ratio of 13.5 nm light as compared to the energy put into it. With the tin plate,
the spectral efficiency was very low. Like around 1%.
The plasma made on a tin plate shape had too high an opacity, which blocked 13.5nm wavelength
light from reaching the mirror. So a new shape was proposed to increase the spectral efficiency
- one that is more concave rather than flat.
Fire a laser onto this and it will create 13.5 nm light with far more intensity. The
overall spectral efficiency and thus overall conversion efficiency of the light source
improves to an acceptable level.
On an entirely separate issue, having a big plate of tin or even a tin wire as the target
created too much debris or fast ions. Obviously we want to avoid this. So we want the target
to be as small as possible. Thus, the droplets that companies had already started to experiment
with.
Engineers looked at this double challenge: Small as possible. Concave shaped. And the
double solution they eventually came up with was pretty wicked.
How do we get the shape we want, but as small as humanly possible? With two shots from two
lasers. Hitting a falling bulls-eye not once, but twice. Lasers akimbo. 100,000 times per
second.
The first shot comes from a low-energy laser pulse - called a "prepulse".
Hitting the tin droplet with this prepulse creates strong pressure waves that reshape
the tin droplet from a sphere into a concave sheet.
The prepulse needs to hit the droplet at the exact right location.
If not, the concave sheet tilts in the wrong direction and messes up the second shot. Furthermore,
it could reflect the second shot back at the laser machine, damaging it.
But when done right, it can properly and consistently set the table for what comes next.
The second shot is the main pulse - generated by an amplified carbon dioxide laser operating
in the 10 micrometer wavelength.
This more powerful blast again remakes the droplet into an acorn shape and vaporizes
it, creating the dense plasma from the liquid target.
This plasma is what emits the 13.5 nm light that we so desperately crave. It is hotter
and denser than the plasma created by lightning. But it does fall short of the plasma created
within the Sun's core. Room for improvement.
The prepulse laser was the critical ingredient. Systems installed at customer sites as recently
as 2010 struggled to get past the 10 watt EUV power level. Adding the prepulse in 2016,
along with drastically more powerful CO2 lasers, allowed ASML and Cymer to drastically scale
up its EUV light source power levels thereafter from 10 watts to 200+.
Teams around the world studied and modeled the physics and dynamics of the tin droplet
when struck by the two lasers. There are papers and papers studying the expansion dynamics,
cavitation, and fragmentation of a single falling tin droplet. Yet there remains so
much more to know.
So we talked on the physics. Now how does the actual machine work? The machine itself
is rather simple and has a few main components:
The high powered CO2 laser, which includes a master oscillator and a power amplifier.
The latter two together are referred to as MOPA.
The beam transport system, which handles the laser's focusing and the beam positioning.
Then there is the vacuum vessel, filled with low-pressure hydrogen gas. It houses the droplet
generator, metrology modules, for monitoring performance within the system ...
And the EUV collector mirror. This collects the light and sends it on its merry little
way. There is a hole in the middle of this ellipsoid-shaped mirror for the laser to fire
through.
The CO2 laser needs to reach a power level of over 20 kilowatts, which requires a lot
of amplification.
Since it is so big and bulky, it typically has to be installed under the fab cleanroom
floor.
The droplet generator shoots tin droplets into the vessel at a very high rate - 50 kiloHertz
or 50,000 cycles a second. Despite the name, they are not dropped but shot through the
vessel at a surprisingly high speed - 80 meters per second. That is about 288 kilometers per
hour (178 miles per hour).
The reason for this is because we want to have a large separation space between each
30 micrometer-wide droplet - about 1.6 millimeters. This is to make sure that the tin plasma from
one droplet does not interfere with another.
Literally hitting 50,000 bullets a second with a laser. Twice.
The resulting final conversion efficiency from this setup? 6% - a massive leap forward
from the 1% that we saw at the very start with those lightsaber YAG lasers.
This finally allowed ASML's EUV machines to reach the stable 250 watt power level needed
to achieve the 125 wafers per hour operation level that its clients desire. There even
exists a pathway towards 450 watts in the future.
When researchers and industrialists embarked on the Tin Journey, they realized that they
needed to figure out some way to have the collector optics survive the over 100 billion
pulses or 30,000 hours throughout its lifetime.
The multilayer collector mirror is at threat not only from tin debris particles but also
high energy ions and neutrals from the plasma.
What Zeiss and ASML eventually decided upon was to pump hydrogen into the vacuum chamber
as a buffer gas. The hydrogen gas would cool the plasma, block a percentage of tin ions,
and etch layers of tin from the mirrors' surface.
I also briefly covered this in my previous video about Carl Zeiss's EUV optics.
Nikola Tesla once said about his frenemy Thomas Edison:
> Edison was by far the most successful and, probably, the last exponent of the purely
empirical method of investigation ... everything he achieved was the result of persistent trials
and experiments often performed at random [but with] extraordinary vigor and resource
> His method was inefficient in the extreme, for an immense ground had to be covered to
get anything at all unless blind chance intervened ... a little theory and calculation would
have saved him 90% of labor
Backhanded compliment if I have ever heard of one. But it demonstrates the difference
between creating the Argon Fluoride lasers for 193nm and the EUV laser.
With the first two, the most necessary basic technologies had already been developed. The
semiconductor industry merely had to grind through the many variations in a trial-and-error
manner to find what worked. Not to say that the effort wasn't admirable. It most certainly
was!
But with EUV, such trial-and-error cannot be done. Now, we are in green grass territory.
Only a deep understanding of the physics and its fundamental dynamics can help uncover
the best way to deal with the thorny energy efficiency and debris mitigation issues, while
achieving what is commercially needed in the industry.
They literally first calculated the exact opacity, temperature, droplet size, ion density,
emission duration, and such years ahead of time. Then they built a machine to achieve
it in real life. The modern-day moon landing.
Thus, in other words. One was Edison. The other was Tesla.
Alright that's it for tonight. Thanks for watching. If you enjoyed the video, consider
subscribing. The feed will show you a bunch of other videos from this channel that might
fit your interest.
Want to send me an email? Drop me a line at jon@asianometry.com. I love reading your emails.
Introduce yourself, suggest a topic, or more.
Until next time, I’ll see you guys later.