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In the last video I talked about benefits
of air gap technology and we established that
it's a, pretty cool thing whether it's in
your airsole or whether it's in semi-conducter technology.
So the next million dollar question is how do we make these air gaps.
You know, how do I form these air gaps.
So this is, you know the one of the typical damascene, process
flow.
And the way it would work is I would have a dielectric and I would create these mul
and then I would fill them or electroplate them with copper with copper.
So being the dark that I am, you know the way simple the first target comes to your
mind, how to create this, air gap, is, to
essential why not you know just, Etch this dielectric
out, so you know, we could do a very selective etch, which etches this
dielectric out, and we are left with this air gap between our copper lines.
And this is this would have been a fine approach if there was only one
layer of inter-connect but the problem is that
we have multiple layers of these inter-connects and
what I want is I want on the top another set of inter-connects, and then I'll
have, I'll be having vias which were lined on these first layer of inter-connects.
So these vias can
these other, all these other
[INAUDIBLE]
on the top cannot be made on top of air.
So this clearly doesn't work so let's look at something that does.
So I want to talk about three different ways of
making these end gaps and the first one is really one
which uses this approach which is I'm labeling this approach which
is I'm labeling it as using as a Sacrificial Polymer and
Porous Dielectric. To give you some reference, this was a
approach which was really publicized by IBM back
in 2007 using their self assembly to form this
Porous Dielectric. So the way this approach is that
you have these copper lines and I clear up the dielectric between
them and now what I do is I fill it up with this polymer, which I'll sacrifice,
hence the name of Sacreficial polymer. So I intend to not.
Keep this, but remove it.
And then I cap my cap this structure with this dielectric.
And so I cap the structure with the dielectric and my pol, my sacrificial
polymer is below this dielectric.
And the neat thing or the unique thing
about the dielectric is that it's a porous dielectric.
it has you know, in some way it has a way where either you can
you, you know, you can let things into it or you can let things out of it.
So it's a two-way street. You can simply let things in.
And you can let
them out.
And the way IBM formed this was using self assembly, and
they were creating these tiny little holes using self assembly matter.
So what you do after you have these pores, is you can do either of these two things.
What you can do is you can heat your substrate And,
what, you can heat your whole wafer and these, this sacrificial
polymer can, go out. And it would essentially
remove this sacrificial, polymer from in between these copper wires.
the other thing you can do is you can weight etch this polymer.
So you can, You can let some chemical in and that again
would achieve the same end that you remove
this polymer in, in between these copper wires.
So these are two ways of the and then the
[UNKNOWN]
what you would mess with is this air gap between
your copper wire which is what you wanted to start with.
The challenges with this approach is that you, you have to heat it up, so
you have to go to higher temperature, and which might damage your transistor below.
Or if you wet etch it then you can,
you know, carrode, there's always a chance you can corrode
these metal line up here.
But this is a, you know, a very viable approach of making these inter-connects.
So let's look at another approach of farming this air gap and
this one utilizes the non-conformal nature of our CVD process.
So if the last approach was cooler, this one is even more cool.
It's super cool.
And it's really approach which converts, which takes
a bug and converts it into a feature.
So we talked about this non-conformal nature of our CVD process.
We discussed that, you know, we always have a higher growth rate on the solid
ridge to see the, on the corners because it receives a larger amount of flux.
And that li,
results in this uh,closure of these step profiles,
or a pinch up of these step profiles.
And if we, you know, continue on this part, what you
get essentially you, you start to, of your two of this
[INAUDIBLE]
start to meet each other and eventually, they meet and they seam left in between.
And if you notice, what's happening below,
essentially below, you are forming this air gap.
The next important thing to do is you know, to seal off this seam, and
what people do is, once this seam has
formed, they switch to a slightly different processor
plasma, so they can start giving a high density plasma, which results in a.
More conformal deposition and then you deposit this this
conformal layer once we have formed this air gap.
So that pinches of these seam over here, so the seam does
not propagate and what you're left with is essentially is these air gap.
So we have formed our air gap between
our metal lines. And you know, this is a, what I really
like about it is this was something which was a bug that we had this pinch off,
but now that we have, sealed this bug inside, to you know, to
sort of make a fan, now we have converted it into a feature.
And the next thing to do is essentially, and once you have formed this
air gap, what we can do is then, you know come off and, we can
essentially, polish or do a CMP step. And then a play rise.
And, what we are left with.
Which essentially this air gap which is in our chapter.
Between, these copper, metal, lines.
And two important things to, you know, keep in, mind, or two
challenging things is, is to really, you know, seal off that, seam.
Because, if that seam exists, over here, then if you do any further
wet processing, that wet chemical can go into and, damage this, bag over here.
So it will again you know, we create this bag so we don't want that.
The other important thing to another caveat is again when you're doing CMP.
When you're doing this CMP step you don't want to expose your air gap.
So you want to very carefully control your CMP
so that you know, you don't polish it off.
So over here's then you will be again exposing this
air gap and this bag can get out and you know
[INAUDIBLE]
kind of chemicals can get in. So those are two important things to keep
in mind when if you are forming this air gap using this non-conformal CPD process.
So another good thing about this using this a non confromal CPD is that
it results into is that it results in air gap where we need them the most.
So this is this is, you know, I point this out
using this image over here.
So clearly, you know, if we use
a non-conformal CPD process, it would form this
air gap between this these two metal lines which have the closest base between them.
But if there are these two other metal lines which are far apart, this level
of nonconformity might not be enough. And over there you
might you might get, you know, a fill without without a air gap.
So which is, which is fine because we
wanted the air gap, or this capacitance between
these two alternate metal lines was most significant
there when they were close to each other.
So it formed the air gap where we needed it the most, and that's that's,
you know, kind of good, because
We can get the air gaps where we need them most, right?
So to finish up I want to just point out another approach and since you know we
talked about approaches are cool we talked about approach which was super cool.
And since we are talking about cool things.
I consider this is a approach which is uber cool.
And the way it works is you
know you want to create all these other approaches you
created this 11 of air gap at each level of inter-connect.
So if you had you know, one if
you had multiple levels of this inter-connect you would
essentially you know, create one air gap here, create
another air gap here, create another air gap here.
And each would require a separate Process, flow for creating this, air
gap.
What this approach, which was, proposed by, Toshiba
does is, again, it creates a maintainable air gap.
So what you do is you form all
these, different inter-connects without, worrying about the air gap.
And then you essentially create this via which,
would would goes through all these inter-connect levels.
And then you perform a edge, a very
selective edge, and that basically removes those dielectric
among all these levels.
So what you're left with is this multi-level air gap.
So, you form these air gaps are different levels all at the same time.
Again, it's something which uber cool, but it's not practically implemented,
because again it's very tough to prevent the collapse and other thing that if
you form air gap this way but nonetheless if we are talking about
cool thing, this is a pretty cool approach to do that as well.
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