So last few videos we talked about double patterning
which was essentially introduced at the 65 nanometer.
We also talked about EUV which which scales this
wavelength from 193 nanometer to down to 13.5 nanometer.
An important question to ask looking at this picture,
is that how did, how the heck did we continue scale and continue to print these
pitches down to 65 nanometer from a wavelength of 248 nanometer.
so how did we continue to scale the pitch in this in this era.
And certainly, we must have developed some tricks to to manipulate
light, to print at much smaller features than the wavelength.
And, are we continuing to use those tricks as we go forward?
So what I want to describe in this video are these resolution
enhancement techniques. Which are essentially a set of tricks
which allow you to print prints spacing or achieve a resolution
of what is much smaller than the wavelength of light you're using.
So all of that is governed by this simple equation, which is
called Rayleigh equation, given by Mr. Lord Rayleigh himself.
And what it describes is that the minimum feature size that you can print is
denoted by this letter d, is proportional to
the wavelength of the light you are using.
It's inversely proportional to the numerical aperture.
And this, it has a proportionality constant, which
is called k, and which essentially just relates, for
a given wavelength and a given, numerical aperture,
what is, the minimum distance that you can print.
So most of the resolution enhancement techniques, they involve
playing with the one of these three vectors.
So lamda scaling we already mentioned, that.
you know, it's basically stuck.
And if you want to scale it further you need to go to EUV.
So what you are left with are essentially these two, two, two parameters
that you can play with.
You can either play with this proportionality constant.
Or you can play with this numerical aperture.
So in so these are some of the
resolution enhancement techniques that I'll describe to you.
So, the first of them is immersion lithography, and that essentially deals
with Increasing this numerical aperture so that
you can reduce your reduce the minimum dimension you can print.
The other of these techniques are related to,
how do you manipulate the source of the light?
or the mask that you're using for lithography
to essentially allow you to operate at lower k.
So they allow you to operate at lower k. So lets
lets look at them one by one. Lets look at immersion lithography first.
Shown here is a typical setup that you'll see in
any lithography system, where you have a source of a light.
And it passes through a mask, and when
the light is passed through the small feature of
the mask, the diffraction of light which occurs over here
And there are different peaks associated with this diffraction.
There's a first order diffraction, there's a second order diffraction peak.
And then you have a lens.
The purpose of this lens is to capture as many diffraction peaks you
can, and then focus them back together to your wafer, where you Print on your die.
So the numerical aperture is is given by this very simple equation where
numerical aperture is defined as N times sin theta were N
is your refractive index.
And the sine theta or this theta is a reflection of how
much how many of these diffraction patterns are you able to capture.
So, higher the more the number of the diffraction pattern you capture that is more
bigger you make your lense bigger is your theta and correspondingly bigger
is your sin theta.
So one way to increase your numerical aperture is
to increase this theta or increase the sin theta.
But sin theta being a bound variable, it only stays between zero and one.
So people did, in fact, increase
this numerical aperture by making a bigger lens,
so you can capture higher orders of diffraction,
but you can only make this theta go
up to one or more.
Practical limit is 0.93, so your numerical aperture is essentially then
defined by 0.93 times your refractive index, so this is a even simpler equation.
And looking at it, the only way you can increase
your numerical aperture now is to increase your refractive index.
And that's where immersion lithography comes into picture.
So what immersion lithography involves, it's a very
simple concept.
What you do is, you insert water between your lens and your wafer.
So then the refractive index of this medium, instead of being one for air,
it becomes now 1.43, which is the refractive index of water.
And now your numerical aperature, instead of being 0.93, it can go up
to 1.35.
So you might have seen when you put things in water,
you see them more clearly, it's the same concept over here.
You immerse your wafer, immerse basically
water between your lens and your wafer.
So you'll have a puddle of water here, and
you'll have water coming in from here, and water
going out of here, and that's how it's designed in the
current lithography system. And this image over here,
it's just Illustrates, again the advantage of
immersing into water, and what it shows, essentially,
is that if you immerse in water, your refractive
index now changes from 1 to 1.43, and so,
for a given depth of focus, for a given depth of focus.
Now you can print a much smaller resolution or
for the same resolution now you can have a much higher depth of focus.
And so this is what what immersion lithography
helps in doing and it has been in production for a number of years now.