So let's come back to our good friend Lord Rayleigh and his equations
So we can check out immersion lithography, which
is essentially a way to increase your numerical aperture.
So what, what else can we do to improve or enhance our resolution.
So the next set of resolution enhancement techniques that
I wanted to talk to you about are essentially.
concerning reducing this k, or allowing you to operate at a lower k.
A good analogy for this technique is that you have a thicker
brush, so you have a thick brush and it has a fixed wavelength.
And you want to.
you want to print fine strokes using that
or you want to print fine features using that.
So, these next set of resolution enhancement techniques.
They allow you
to do that by playing tricks either at, either with the source of
the light you are using or with the mask that you are using.
And they essentially you, what you can do as you
can do a proximity correction, you can introduce a phase shift.
Or you can do a off-axis illumination.
So let me describe these one by one. The first technique in this bag of
tricks that I want to show you deals with interference of
light, and it's it's called a Phase Shift Mask or PSM.
And it involves this very simple concept of physics.
That if you have two features next to each other,
and you have light coming in from both of them.
The electric fields from these the first feature looks like
this, the electric field from the second feature looks like this.
And they, if the, they essentially add up constructively.
So there's a constructive interference in the electric field.
And intensity which is essentially a square of the electric field.
Now it looks like this and the, the, you have a constructive interference between two
neighboring features. What what PSM or Phase Shift Mask involves
is that now you add a shift in the phase
of the light so you add either extra optical path
Or you remove some optical path in this light of in this path of the light
from the second feature, so now there's a fay, there's a shift in the phase.
So let's say there's a 90 degree shift in the phase of these two lights.
And now the electrical fields from these
two neighboring patterns looks like this and when they add up.
You essentially get a pattern of electric field like this, and intensity
again is square of the electric field, so the intensity looks like this.
So now what you see is by adding this phase shift, you have been able
to resolve the the features, neighboring features very well.
And previously there were
interfering with each other, and you were getting
this blurred feature by adding this phase shift.
You are now able to preserve these two features separately,
and there are multiple ways you can add this phase shift.
So if you're using a quartz mask, you can etch
into it to reduce the optical path for this feature.
You can add chrome to it to
cause this phase shift, or you can essentially
have these different depths between two different features.
And these are the multiple ways that you can introduce these phase shifts.
The second of this technique, which is optical proximity correction.
It essentially deals with correcting for
both interference and diffraction of light.
So you have
this well defined optical system where you know the dimensions exactly.
You know the proximity between the different features.
You know the the total dimensions of your system.
And so you already know what is going to occur, so
you know how many others of diffraction you're able to capture.
You know that interf-, there'll be inference.
How about we reverse, we reverse-engineer
that phenomenon?
So that is what this Optical Proximity Correction or OPC does.
So it, it knows that, you know, we have finite capabilities.
We can maybe capture one or two or three or four orders of diffraction.
It knows that there'll be interference.
So we know that, suppose we print a feature like this on our mask.
It, actually in reality because of this interference and
diffraction, it prints like this.
So it what, what we ask ourselves is, can we do better?
Can we actually, since we know what these effects of interference
and diffraction are, can we actually design a mask like this.
So if we design a mask which has this additional features over here.
Which tend to overcome some of these effects.
It helps in printing a better feature.
So, this one
is close to, more close to what we actually wanted.
So that is what OPC or optical proximity correction involves.
And here's a nice picture showing OPC and phase shift mask in action.
So this is what we want to print, so this is our template.
This is what we actually what we want to print.
And, so this is a design layout we
want to achieve.
And, if you look at the mask, which is used to print this.
So this is the mask which is used to print this,
and if you look at this, this looks nothing like this.
So you, you can broadly see some lines that you want to print here.
But you have these so many extra features and if you take a AFM.
If you take Atomic Force Microscope image what you see on you see the topography
of this mask, you also see that it's not a, not a flat mask.
There are a lot of features in this, so these are
probably phase shift, probably for shifting the phase of the light.
So you have a, these OPC features, you also
have these 3D features in your topography of the mask.
And both of them essentially are trying to
correct for this interference and diffraction of light.
And what you see here is actual pattern which is a printed on the wafer.
and though the mask looks nothing like this, the mask looks nothing like this.
But you see a lot of correspondence between
what actually printed, and what you wanted to print.
So there is correspondence between the feature you wanted to achieve,
and what printed but the mask of it looks nothing like it.
It has these
extra features for OPC.
It also has this 3D topography for correcting for this phase
. So,
so far the two techniques that I have described to you, that is optical
proximity correction and phase shift mask. They both are related to the
mask so these are both things you do on the mask to achieve a better resolution.
So we ask ourselves when you look at this optical path,
that can we do something on the source of the light?
Can we do something about the illumination of
the light?
So, one simple thing that we can do is essentially use an off-axis illumination.
So over here we are Shining the light at zero degrees or at normal incidence.
We can also shine light at an at an angle.
And what it does, it now it achieves helps you achieve a.
A better diffraction pattern, so you get this much
higher angular reflection.
And you can you can have multiple of these angles at which
light can come in, and that is essentially known as off-axis illumination.
And that's also a technique that is used to improve our feature.
The other thing that we can do on the, on the
source side is we can now play with the shape of the
source as well.
So, so suppose a conventional source, which has just one hole.
Now your source could have multiple hole and each of
these could be targeting, light coming in at a different angle.
So this could be a quadruple quadruple axis illumination.
Or it could be an annular ring-like illumination
where you have light coming in from all angles.
And more recently what people have started
doing is you can turn on and off individual of these pixels on your
mask
So they have made this pixelated light source.
And you can turn on and off light coming from different pixels,
and all this helps in essentially improving, again, your image quality.
So if I was to use one word to describe what
has happened in this field so far for resolution enhancement techniques.
I would use the term Source Mask Optimization.
And it summarizes whatever I can do on the
source of the light, turn on and off individual features.
Tricks I can play on the mask of the light that is adding extra features.
Or adding phase shift to essentially help me achieve a better resolution.
And it has re, it has given rise to this new field in the semi-conductor industry.
which is known as computational
lithography are using a lot of computations.
All this requires a lot of very high end servers.
In fact a new skill set of people who know a lot of things about computation.
Who know a lot of things about Fourier transforms and so on,
which by the way I used to dread when I was a student.
And they, they essentially use a lot of
these computational power to use to help you
achieve print finer strokes using this thicker brush.
And this field has really taken off in the last 10 years.
And this is, these bag of tricks have
allowed us to use this 193 nanometers of light.
And taken us all the way to 65
nanometers node, using all these set of tricks.