EUV is a tough problem to solve.
So there are a lot of challenges still remaining in the field
which which needs to be solved
for a commercial commercialization of this technology.
So what I want to do in this video is point to you the grand challenges.
I want to focus I want to focus on three main three of the biggest challenges.
So the first one relates to the source. That is how do we generate
these these photons with the extreme
ultraviolet wavelength or wavelength of 13 nanometer.
And so that's, that's the first challenge I want to talk about.
The second challenge I want to talk about is how to then take these electrons.
Focus them, take them through this path and take them to the resist.
And the second thing I want to
focus on is essentially the optical component of it.
So how to design the optics, how to design the mask.
And the third biggest challenge in the field that
I want to mention is relating to the resist.
So relating to the resist that is required for EUV.
So let's start first with the source.
So as I mentioned previously that if you operate in
this extreme ultraviolet wavelength, you require very high energy
photons which get absorbed at pretty much everything that comes in its way.
So first of all, first thing's
first, how do we generate these high-energy photons?
So let's discuss a little bit about the
physics of these Excimer lasers and how these high-energy
excitations are generated. So even for deep UV
lithography, the photon energy that we require
are 5ev for this 248 nm
deep UV and 6.4ev for
this 193nm wavelength that is currently
being used. And how they are generated, are
they are generated using this thing called Excimer Laser.
And what it means is that you essentially have a excited state of matter so
you have a plasma in this case consisting of essentially noble
gases or noble halide gases. And you excite them to to
have some positive charge on these species.
So, for example, say, you have argon fluoride.
So you would excite it, so that it already has a positive charge.
So you'd generate this excited state and when
you already have positive charge on your, on
your atom, these electrons in your orbitals are
now more close, closely bound with your nucleus.
And now
when you have electron transition, you get this high energy.
So to generate this EUV wavelength, you actually need a very excited state.
So what they commonly uses tin and that, to a very excited state.
So tin with three or four electrons
already removed from it so it's highly positively charged.
And then these d
level of electrons, when they transition back, they generate this
photon which has this very high energy of 92 electron volts.
So first of all since at this energy your photon gets absorbed by
everything, to generate this plasma or to generate this light,
everything has to be in vacuum.
So one of the challenge in this thing to generate this EUV
photon or actually transporting this EUV photon to your to your all the way
to your wafer. Is that all your all your optics,
all your all your mask, all your generation source
have to be vacuum. So let's let's look at two possible ways
we can generate these EUV photons using tin.
So the first one is actually using what is called a laser
pulsed plasma and this is, this figure here describes how it works.
the company which has been the most active, most active in
this field is Cymer, which is based in San Diego in California.
So, the way this works is you focus this laser and then you fire a
pulse of this laser, you fire a pulse of this laser onto your incoming droplet.
And you hit each of these incoming droplets with this
laser which puts this tin droplets into the excited state.
And when it relaxes from
there, it generates this EUV photon. So it generates this EUV photon
and then you have this mirror over here which focuses them into a intermediate
focus point and then you pass it on to the next next
level of optics. So one of the challenges,
with this technology is you need to have a very
synchronized fashion of these droplets falling.
And then you need to hit each of them using this pulse laser.
So it reminds me of you know, one of those games that you
play like Pong or Angry Birds or Pac-Man. Where essentially you need
to hit your incoming target
to score points. And that's exactly how it works.
So this one it shows you image of these droplets falling
and each of these droplets is between fifty and a hundred microns in diameter.
So it's a very small target to hit and you need
to hit them, each of them, with this pulse of laser.
And they're falling at a rate of 40 kilohertz.
So, you need to be very
precise and you need to keep on hitting them
with a very high precision. otherwise, it's game over.
And another challenge is once you have this tin droplets,
you know, a lot of tin which ionizes and which, you know, there's
a debris of you know, another problem is collecting this debris of tin.
So there's a, there's a challenge that what if this thing, it falls
on this mirror or it falls on some of the optical components over here.
And that's another challenge with this technology.
the other way to generate this EUV photon is this
other competing technology which is also a plasma but generated not by a laser but
instead using a discharge, discharge pulse.
And what it means is essentially you have these two tin
baths so you have two of these baths, which are filled with molten tin.
And you have these disc rotating and they essentially
pick up this tin on its surface whenever it it dips into
the bath and then you hit one of these circles.
You, first of all you apply a voltage here and then you hit this with a laser pulse.
And again, that generates tin in this excited state
and when it relaxes, you get these EUV photons.
And here's a company called Extreme
Technology which is generating this competing
technology, for generating your EUV photons.
So the main challenge, a common challenge for both of these is
that it needs to be operated in a, in a vacuum environment.
And the plasma itself that you use for generating these EUV
photons, that same plasma can absorb this EUV photons as well.
So again the efficiency
you get for your photon generation is quite low.
So awesome!, at least we know how to generate these high-energy photons now.
So what's next?
So what's next is now taking these high-energy photons,
so taking these high-energy photons and then focusing them onto
your wafer and also using this mask, running them
through this mask to generate the pattern on your wafer.
And again a challenge there
is how to to
all of this optics, which is needed for focusing your photons or the mask.
It has to be all in vacuum
because anything, any air in between will essentially
absorb these high-energy photons and it will just
reduce the number of photons you have available.
And the, the other challenge is that you
can't use any, any lenses. So typically for for deep UV or
immersion lithography, you essentially run your, your incoming photons through a
series of lenses. And that focuses them, but you can't use
it, you can't do this if you have photons of energy of this high energy.
Then essentially, all of them will get absorbed
within this lens and you won't get anything out.
So, you can't
use any lens optics. So lens optics is completely ruled out.