So the next, next generation lithography technique I want to talk to
you about is this technique called Directed Self Assembly, or DSA.
And it it uses the, it leverages
this concept of self assembly of co-block polymers.
It's essentially a mixture of two polymers.
Shown here is essentially a mixture of two polymers, one blue in
color and another one yellow in color. And when you heat up these mixtures
of this polymer, it orients itself so that it forms these blocks.
And there are, essentially forms these blocks with have short range order.
And so in this case it forms these patterns of holes.
And the size of these holes is defined
by essentially a property of the polymer. So it could be anywhere from five
to 30 nanometer, and depending upon how big those polymers are.
And then you could selectively etched out one, etch out one of the polymers.
In this case, you can remove this yellow polymer to leave these stencil of holes.
So it relies on the concept of
block co-polymer and self assembly of these
block co-polymers. So it's showing here is one such example,
it uses this mixture of polystyrene and PMMA
and both of them essentially form co-polymer.
And they can arrange themselves either in the straight
lines, which is the lamellar phase, and or
they can arrange themselves in the cylindrical phase.
And depending on lamellar phase, you might get
these series of, in this case spiral lines.
Or you can get these holes.
So important thing to note here is that you get from using this self assembly,
what you get is a short range order, but you don't get a long-range order.
So you can get these features these very nice small features, which are five
to 30, anywhere from five to 30 nanometer depending on the size of the polymer.
But there's no long range order of these of these features.
So that's where the D in in DSA comes in. So, what directed self assembly involves
is you start you start with a set of
lines, which you can define in this case by say 193 nanometer.
Are you in higher wavelength of light, and then you
fill this space with your with this co-polymer,
so you in this case say fill the space with the PMMA and
this polystrene mixture.
And then you heat it up, and then you form this set of lines.
But they are directed or they are given direction by this template which
was essentially put in place for us by using a higher wavelength photography.
So in many ways its similar to the double patterning and so on.
But the only thing is now you can achieve so
in double patterning remember the, the pitch was determined by the
mandrel.
But in this case, your pitch is essentially determined by this co-polymer.
And you can have any, any times of multiplication of your feature size.
So you could have 4x, you know, 20x, 100x multiplication.
And you keep the number of the process steps the same.
So you don't, you don't add, you don't add more process steps.
So that's one of the main difference between DSA and
double patterning in case, you are.
It's, it's easy to you know, it looks it's actually
good that if, if you had this confusion in your
mind that it looks similar to to double patterning that
means you, you know you're following the material well so far.
So, shown here is a shown here is a one of this example using DSA.
In this case people have used
this directed template, and they have using this
very thick line, they have been not able to pill, print very fine features.
And you can see in this case, they have a feature size of 22 nanometer.
Again, you can have, you are not restricted to
just printing straight lines, you can print holes also
in this case, showing how they have used this template to print these set of holes.
And again the main challenge here is, is defects.
So, if you want to print millions of these features without any
of these polymer collapsing, or any of these defects occurring.
So by the way, I, though I have described DSA as
a next generation lithography technique, it seems
like some people are already using it.
Especially the Bit Patterned Media or the hard disk drive that you use.
So they use these very small holes to define each of these bit.
And in this case, I'm showing hard drive from Hitachi,
and they use a DSA to define these these bits.
And so they can print these very small pitch holes by using DSA.
And at a very, at a very negligible cost as compared to if
you're use, they were using double
patterning or any of some other techniques.
So the next the next in the final, the next generation in
lithography technique, I want to talk to you about is this mask-less lithography.
And the most common, or the most dominant
form of mask-less lithography is using electron beams.
So again, the last two techniques I discussed that is the nano
imprint and there's DSA, one of the main problems was these defects.
So electron beam lithography relatively is once you get rid of the
mask and you use this electron beam to write your pattern, you don't
suffer from defects.
But one of the main challenges with this
electron beam lithography, or mask-less lithography is throughput.
So the time it takes to print any feature using this electron beam can be simply
described by this formula, where the time is
essentially proportional to the dose of your resist required.
It's proportional to the area you want to
print, and then it's divided by the current
of the electron beam that you are using.
So let's say you have a dose of one milliJoule per centimeter squared.
And then you have a current of, you want to print
the as, you want to print the area of 1 centimeter squared.
And then you have a current of, let's say 10 to power minus
9 Ampere, or one nano Ampere.
So if you if you take all these number, how much time will it take to print you?
So it, it turns out, it will take you 10 to power 6 seconds to print this.
Which will equivalent to 12 days to print this 1 centimeter squared of Area.
And you know, you, you see, you see, any of you have used this
electron beam machine, they are very common in university labs.
The SNF lab has a couple of them,
and anybody who has used these machines they know,
you know, one of the main problems is
it takes so long to print a set of small features.
So can you guess how long will it take to, to print 300 mm wafer?
So if you do that calculation, it, it will essentially take you 22 years.
If you, if you, if you use this beam current of 1 nanoamp.
But the only way you can you can decrease this amount of time required or you
can increase the throughput is increasing your increasing your beam current.
So, let's look at how we can do that.
So, one of the ways we can apparently do that is you
know, you're using this electron beam for for
printing your feature, so there are two ways you can do it.
Firstly, you can increase the energy of your electrons, or you can increase dose.
So you put more electrons in the same beam size.
And each of them essentially hits hits a brick wall if you put too much
energy into it you will be essentially damaging your resist.
Then if you put even higher energy, you'll be damaging your wafer.
If you increase the number of dose, so this seems like a smart idea.
Right?
Why not you know, put more electron in the same beam size.
So again, the limitation of this
comes from these electrons, are charged particles.
If you put them very close to each other, they will repel
each other off and it increases your resolution.
So, again, both of these techniques seem to hit a road block.
So the only way you can increase the throughput
of your E-beam lithography machine is shown over here.
So one thing you can do is increase, to
increase the throughput is to use multiple of these E-beams.
So in, in the current
system that people are using, they use thousands
of these E-beams to essentially increase the throughput.
So shown over here is one such
commercial one such machine which is being in prototype.
So this machine essentially has 13,000 electron beams
per chamber.
And that gives it roughly throughput of 10 wafer per hour.
So remember now, instead of years, we're talking about 10 wafers per hour.
So you know, one one 10, 10, 6 minutes per wafer instead of 22 years.
And you can of course then it's not a very big
machine also.
You can see that one of these chambers is just 1 meter by 1 meter in size.
And then you can also add additional
parallelism here by using multiple of this chamber.
So, this two row has ten of these chambers.
And if you have two rows of these each having 5 chambers,
you can combine to get a throughput of 100 wafer per hour.
That's similar to what you get from your
deep UV
and it gets the same amount of area as awell. So E-beam
lithography is, is a serious contender to to deep UV.
And the challenge there is essentially getting the same throughput.