So having described some of the basics of double patterning, I wanted to show you a
layout of SRAM cells for across different technology generations.
So this one is for say a 90 nanometer technology
and this one is for, you know, a 32 nanometer technology.
And surprisingly you know the statement that was
made in this case by Intel. That you know, they have been able to
improve the fidelity of their lines going forward from each generation.
So you know keeping the lithography wave length staying the same.
The fidelity of the lines in these
SRAM cells has actually proved at every generation.
That seems, you know,
a very that seems to be a surprisingly you know, a very surprising statement.
How is that possible?
So what I have done in this case is essentially taken
those two images and I have made them the same size.
So these are SRAM cells at, layout of SRAM cells at 65 and 32 nanometer technology.
And I've made them the same size.
note they are not the same scale but they are the same size.
So what you immediately see is the 32 nanometer cell
has a, you know, a much better fidelity of the lines.
So look at the lines here, they look very, very high fidelity as compared to say, 65
nanometer technology. Where you see, you know, fatter lines of,
you know, so, you see a fatter line at the
edge and then you see a skinnier line in the middle.
So, actually the fidelity of these lines has improved.
So how is that, you know, how's
that even possible because everything that, you know.
Variability is increasing as you go down the technology generations you
have, you are keeping the wave length same so how you
improving the fidelity?
Also you know this line is exactly if you see
it has a dumbell shape so it's it's you know fatter at
the edge and it's skinnier in the middle.
So the reason why this usually happen is because if you
have, you know, if you want a pattern, something like a rectangle.
due to, you know, edge effects and proximity
effects it will actually, you know, broaden out
on the edges and it would skinny down on the center.
And you, what you'll actually print is this dumbbell kind of a shape.
So to avoid that, you know what, people have
been doing recently that they actually just print straight lines.
So what they d to to enable this pattern is
you start with essentially a set of straight lines.
So what they do is enable this pattern is
you start with essentially a set of straight lines.
So what they do is enable this pattern is
you start with essentially a set of straight lines.
So that is shown here.
And, you know, these are not, these are you konw
long straight lines.
And where you want to, where you want to,
you know, cut them off, you do another lithography step.
So you do, you do a multiple number of other lithography steps, which
are perpendicular to these set of lines so you do all these lithography steps.
In this case, you know, you do one and maybe
one or more than one lithography steps, which is perpendicular.
And so you pattern one set of
resist
and you just pattern it in one direction. And then you pattern the same layer of
resist
in the perpendicular direction to this original direction.
And combined, you essentially get this pattern
So, you know, I, I find it, you know, very interesting
that, you know, the fidelity of the lines has actually improved.
And this is so I wanted to point out how it's actually achieved
So let me take the liberty of showing you one more image.
So this image corresponds to the SRAM layout.
from the latest technology generation node from Intel.
So this was announced in May of last year
and what it showed is essentially that 22 nanometer technology.
Which was using
FinFET transistors
And so, you know, the question I want to ask is, you
know, depending upon, you know, what we have covered so far.
Can you tell me, you know, which,
which which double patterning technique does it use
So it's, it's clear that it uses double patterning technique.
And which one? Is it LELE, is it LFLE or is it SADP?
And, you know you, looking at it, you
can actually tell what they have done, you know.
You can, first of all, you can tell whether they have used
double patterning or whether and, which type of double patterning that they have used.
So you know to point that out let me label some of the things in this picture.
So let me actually, I've already done that for you so let me bring that up.
So what I have done here is essentially, you know, I
have you know, what I've done is I have labeled these lines.
And I have actually tried to measure
the spacing between these alternate set of lines.
So what I measured here is essentially if these
two set of adjacent lines, they have a spacing of x between them.
And the next set of adjacent lines has a spacing
of 1.2 x between them. So you know there,
the, between adjacent lines, the spacing between them is
actually consistently either x or 1.2x.
So what it, you know, first off all what it means is okay, they have used
actually
double pattorning
Because you know if you you just use single patterning you will have uniformly spaced lines.
And what it tells again, is that you know, they have It gives
you some clue to what kind of double patterning technique they have used.
So, you know, as, as I mention, you know,
the spacing between alternate set of lines is actually varying.
So you could, this, you know, this could happen in two ways.
One by error and one by design.
So the way it
can happen by error is, suppose say, you are doing a technique, which involves
overlays, say. LELE or LFLE you
would at this pattern define one set of
lines and you would overlay another set of lines between them.
And this was actually, let's say it was supposed to land dead in the center.
But due to this overlay issue it didn't so it, you know, it landed
these other set of lines they landed somewhere displaced from the center.
So, in this case, you know, you could, you
could have a spacing x and then another spacing 1.2x.
But this is, you know, not by design. The only way you can achieve
this by design is suppose, is if you had done SADP.
So if you had done using a self-aligned double patterning technique and
you had a mandrel, which had
a certain pitch and then you
had spacers around it and you know Use these
spacers and you had this and what you could
then be left with essentially is one set of spacing, which is defined by a mandrel.
And another set so you know this spacing between these
two adjacent spacers is defined by this Mandrel layer.
And the spacing between these set of lines is defined
by the pitch of your Mandrel layer.
So one is defined, you know, one set of spacing is defined
by the width of this Mandrel layer, the another set of spacing between.
So this spacing is defined by the width.
And, you know, the spacing is defined by the pitch of your Mandrel layer.
So you, you can have, you know, two different set of spacings here.
So this could be 1x and this could be 1.2 x.
If you use self aligned
double patterning.
So then we reach to this conclusion that probably, you know, Intel had used
SADP for their 22 nanometer technology around FinFET transistors.
Another advantage you get by doing SADP is CD uniformity or
uniformity in the width of your feature. What I mean by that is suppose
you had, you know, since you pattern in SADP, you pattern your Mandrel layer.
So you know, suppose you had roughness in your Mandrel layer, so you know
you would expect, you know, some line edge roughness in your mandrel layer.
And when you pattern when you define a spacer around this Mandrel layer,
it deposits conformally.
So you know, even though you had this roughness,
you would get this roughness in the spacer too.
But since it's conformal, you know, the width of
this line would still be constant even though it's rough.
You would, you know, have more uniformity in your CD.
because it's self-aligned and conformal spacer around your mandrel.
So that, that's something, you know, which also
helps in your device performance, so that's another.
You know, another advantage, advantage in favor of SADP technique.
So what we've discussed so far is how using double patterning, in particular I
spent a lot of time how using self-aligned double patterning.
Using one exposure step you could still obtain,
you know, twice the amount of number or lines or
you could reduce your pitch into half using this technique.
So the next obvious question that comes to your
mind and you know, can you extend this further?
Can you use this to you know obtain a 4x doubling of your density
or, or you know going forward, you know. Can you obtain a 8x increase in your density?
Or can you decrease your pitch by 1/4 or 1/8 or does it only
happen in multiples of 2? Can you actually do it, another you know
can you do it three times? Can you obtain a tripling of
your density or you know can you reduce your pitch by
1/3rd? And the answer is yes, and let me show you
a few ways you can do it, and you know it's a lot of fun.
Let me show you a, a one way you can
actually triple your pitch density by using just one exposure.
So, you know, this technique over here
this is self aligned triple patterning or SATP.
So again, you know, what you do is, you start with one mandrel.
And you know this is,
your initial layer and you define a set of spacers around it
so this is you know these are self aligned spacers around your madrel.
But in an SADP step you know we stopped here and we and we actually removed this
mandrel and we etched this pattern Into our hard mass but let's not do that here.
So here what we do is, you know we have this first spacer material.
And now this around this first spacer material
you put the second spacer material so you know.
You put another second set of spacer around this
first set of spacer, so which is shown over here.
And what you do next, what you can do next is, you know,
take out this first spacer material, you
know, take this first spacer material out.
And you still have your first mandrel and you also have this second spacer.
So, in effect, what you get now, if you compare, as compared to this,
you get a three times of your you know, three times the number of lines.
So you, we have effectively tripled our density or reduced our pitch into 1/3rd.
And we have used, you know,
we have used just one exposure step, which was used to define this mandrel
layer. So can you do more can you do four times ?
Yes of course, you can do four times as well and this is I have drawn it over here and
so this is using so what this is called over here is Self Aligned
Quadruple Patterning or SAQP.
And what you do here is essentially you start again with your mandrel layer here.
And you define you know, your first
spacer, which is self-aligned to the mandrel.
You, you take the mandrel out, so you
are left with this pattern from the self-aligned spacer.
Then you put another spacer around it. So, you know, you put this another second spacer
around. So this first Spacer layer now acts as
the another Mandrel layer. So now, using
this spacer as a Mandrel layer, so put another spacer around it.
Then you take this first spacer out and now what you essentially have done
is you have what you have done is now you have four times.
So from here to here you have 2 times increase in the
number of lines and from here to here we have another 2.
So overall you have, you know, you have quadrupled your
area density or, you know, you have reduce your effective pitch by one fourth.
And you now, we can keep on doing this.
You know, we, there's you know, your imagination is
the limit there and to put a test on that limit
We'll have a few questions in the homework, which will quiz you on.
You know, different ways you can, you know, quadruple or you
know, are five times your first density and so on.
So, you know you, then, you know,
so, the sense I'm trying to give you here is, you know.
Whether or not EUV comes in, people will keep on, you know, reducing their.
So you know, whether or not, EUV lithography comes in.
people will continue Moore's Law and you know Moore's Law will keep on continuing.
The down side of you know doing quadruple
patterning or doing quintuple pattering or doing triple patterning is
that, you know, your number of steps keeps on increasing.
Your complexity is again increasing.
You know, you have multiple etch steps, you know, you have multiple
You know, you are adding more and more number of process steps.
So you know your yield associated with each lithography definition is going down.
And you need, you know, you need control at of each of these steps.
So, you know, you, whenever EUV comes in it
will act to, you know, reduce these number of steps.
And it will help but whether it happens or not the sense
I wanted to give you, you know, is Moore's law will continue on marching.
And you know, we have covered how double patterning,
and going forward, even quadruple patterning will,
allow you to double your pitch density or you know, quadruple your areal density.