So I want to start this video by asking you a question.
So as you know the Argon Fluoride Laser which is currently
used in the industry, it has a wave length of 193 nanometer.
And using a variety of enhancement techniques such as,
you know, optical proximity correction or using immersion lithography.
We have been, you know, able to extend it down
to 65 nanometer node.
But even using, you know, pushing it to its very
limit, it still runs out of stream at 65 nanometer node.
And you know, the next generation of wave length which is you know, in the extreme
ultra-violet range is has a wave length of 13 nanometers.
Which is not supposed to be ready' til you know, it's not supposed to intersect the
industry road map. Till the 10 or 7 nanometer node.
So but, so the question is you know, still
people have been you know, we have all heard
announcements of you know 45 nanometer node, 32 nanometer
node 22 nanometer node and even down here, nobody is
none of the major player is raising a red flag saying that you know.
Lithography is not ready, and we are
not ready for fifteen nanometer nodes.
So the question is, you know, how have been, people been able
to print dimensions lower than what is possible with the given lithography.
and they, you know, they see a road map forward, so how is that possible?
So the answer to that is, using you know.
A technique called double patterning and this it's a very, very well established
technique and there is a roadmap forward for this
technique to enable you know future technology nodes.
So which I'll talk about in this video.
>> So what does double patterning into mean?
To put it very simply what it means is suppose
I have a set of, I have a pattern of lines.
Which has a pitch of 60, in this case, 60 nanometers.
So the spacing between
these set of lines which is the minimum spacing allowable
by you know, 130 nanometer lithography is say, 60 nanometers.
What double patterning means is now, if I overlay these set of lines, If I
overlay them with another pattern of line which has a pitch of 60 nanometer.
So I overlay this green lines, where the,
these set of blue lines, the key term being overlay.
So if I, you know If I pattern another set of lines which have again,
again the minimum distance between these blue lines is again 60 nanometer.
But now I get the distance between these green
and blue lines to be you know, half of that.
So I get a pitch of 30 nanometer between these
two set of lines.
And so that, that's you know in a way a very simple way that's
what double patterning is and there could
be a multiple ways you could accomplish this.
So I'll describe a few of those so you know you could do lithography and then
you can etch that pattern then you can do lithography of another set of lines.
And then you can etch that which'll you know lithography etch,
lithography etch our LELE. You could also, you
know, do a lithography of one set of lines then freeze that pattern, then
put another set of pattern of lines and then etch them together.
So, you know, this is called Litho Freeze Litho Etch.
>> Or what you can do is you can actually pattern only once, and then.
So you can pattern one set of lines and then use the set
of spacers around that pattern to,
you know, self-aligned spaces around that pattern.
To double your spacing, which is known as self-align double patterning, or SADP.
So, all of these techniques essentially, you know, allow you to double your
pitch density and I'll describe each of them for you.
>> So let me start with LELE, or Litho-Etch, Litho-Etch
technique. So the cartoon here describes the, the
process flow for double patterning using Litho-Etch, Litho-Etch technique.
And what it essentially entails is that you have.
two sets of materials let's say one of them is nitride.
The other one is oxide. And what you do is you
pattern one layer of resist and then you pattern them
into these lines and then you etch that pattern into this first layer.
So you etch that pattern into that silicon nitride layer.
And you'll, remove the rest of the silicon nitride so, now you have this pattern of,
silicon nitride lines And what you do
next is put another layer of resist and then
overlay that pattern with this existing silicon nitride lines.
So now you have this pattern from the second layer of resist, that you put on.
And, now you keep this pattern
you also keep the silicone nitride lines that
you had patterned from the first layers of resist
And then you etch them together into this silicon oxide layers.
So now this silicon oxide layer now has twice the pitch density.
Suppose, this was a pitch density of X, now you have, you know, the
spacing between the lines is X by two, or you now have twice the pitch density.
So,
and then, you can, you know, you can remove
the nitride on the top or you can remove the
resist
and you can wash them away and you get this pattern on
silicon oxide, which is now twice the density.
So what we have essentially done, we have doubled our pitch
density, but at the same time you know, now we are.
let's count the number of steps.
So you know, there's one litho step. There's an etch step.
There's another litho
step. And then there's an etch step.
And then there's, you know It's optional.
But,
if you want, you can wash away this,
silicon
nitride layer.
So, you have, instead of, conventional lithography,
where you would have one litho step.
And, then, you know, one etch step. Now you have.
Actually you have in this case, five steps, as we just counted.
So you have increased the number of steps involved.
Another concern, another MAJOR
concern becomes the overlay.
So when you're patterning the second set of lines.
It's very important that they align
, they align, exactly in between the first set of lines.
So a major concern then becomes overlay. So LELE is not the only game
in town, there are you know other techniques that you can use to achieve double patterning
as well. So let's look at another one of them.
So this is called LFLE.
Or Litho-Freeze Litho Etch.
So what you do in this technique is you pattern one set of lines using
You know, one layer of resist and then you make sure that you freeze this pattern.
You don't etch this pattern immediately into the layer below but
you freeze this pattern in the resist. So what this means is that if you do any
further wet processing or any further development steps it should not
affect this set of lines, i.e this set of pattern resist lines.
And what you do is then next apply another layer of resist,
so this is you know the second layer of resist and then
you do another lithography step.
Another exposure step and you pattern another set of lines but
You still have the set of lines set
of resist lines, from the first patterning step.
So since you have frozen that step, it still, it still does not
go away, and you have now you know these two sets of resist lines.
And them you can etch them
together into this hard mass material, lets say it's oxide.
Now you can, you know, etch this set of lines together into this material.
And now again what you have achieved is, you have doubled your
you have doubled your pattern density or you know
You have reduced your pitch by half.
As compared to the Litho Etch, Litho Etch technique that I described
before that had you know five steps. But this one as we
counted
you know it has one litho step and then one freeze step,
then another litho step and then a final etch step.
So it has you know, it has slightly less
one step less as compared to the LELE technique.
And but you know it has its own set of challenges as well.
So you know resist selectivity becomes very important over here.
And you know, as you can imagine overlay was a issue for LELE as well but
It becomes you know, even a greater issue for the litho freeze technique.
So moving on let's talk about uh,the, the hottest,
kid in the town in the field of double patterning.
So that's this technique called self-aligned double patterning or
SADP and what it entails is essentially, you have, you pattern one set of lines
And this is often called a dummy pattern or
another word that people use for this is mandrel
So which comes from a Latin word which means a
molding material to form a pattern.
And that's exactly what it does.
So what you do next is on top of this dummy pattern or mandrel you deposit a
conformal layer of dielectric let's say again silicon nitride.
And then you etch it back, so you know, this
is a conformal layer, so you had a pattern like this.
And you deposited
a conformal layer of dielectric on top, so it became something like this.
So when you etch this off you would Etch
and you know, it's, it will start etching from everywhere.
And you will be, essentially you will etch this off.
You will etch this off.
And what you'd be left with essentially is a set of spacer lines.
So what you would be left with.
These spacer lines, once you do this etch process.
So that's what is shown here.
Once you deposit this confirmal layer, and etch, you form these
spacers. And now you can remove your
mandrel
or your
mold
material, you can remove that out.
And what you're left with essentially is the set of spacers, and
now they have twice the density of as compared to the dummy pattern.
So they have half the pitch or twice the density as compared to this mandrel layer.
And then you can etch these spacer into this, say oxide
hard
mask and then you can etch them further. So, what you essentially achieve
is using this self-aligning spacers which align to your dummy layer or your mandrel.
You can double your pattern density. So contrasting SADP to
the other techniques that we discussed, that is LELE or LFLE
the advantages of SADP is clearly,
you know, it requires only one lithography step.
So, as you know, the most expensive tool in your
fab
is their lithography tool, and and you know, one of
the most expensive step is to do lithography using that
tool. So as compared to LELE, or LFLE which
require you know, two lithography steps, it requires only one lithography step.
So that is an advantage.
And also since it requires only one lithography step you don't
have to worry about overlaying your two lithography steps with each other.
So that's another advantage.
on the disadvantages side you know, you require in
this case again you know if you count the
number of steps you require one, two, three, four, five
so you require a large number of steps again
which is you know, true for all double patterning techniques.
But you know, over all you know, that, that's
given these advantages, manufacturers both in the memory and.
And as well as in the logic
space people are commercially using this SADP technique.