Another processing challenge, which is very unique to the
3D nature of the FinFET device is these, spacer stringers.
So remember when we did our planar transistor process flow.
We had these spacers around, We had these spacers around our gate.
And they helped us in defining these source drain extension,
and defining these source drain implant regions, such that you get an extension.
And you get a much higher doping, source drain implant.
And they're also helpful when we were landing contacts the, to our device.
So spacers are essentially, essential part of our
transistor flow but, when you have a 3D device.
So forming these spacers around your gate becomes
very complicated. So let me demonstrate that using This
figure this a 3D figure over here, so its a, its only,
you can only understand it when you look at this in a 3D manner.
If you are, if you are coming from a 2D frame of mind it's become really
difficult to understand it so I will do my
best to use this 3D figure to illustrate it.
So what the way this spacers are defined if you put a conformal layer of your
nitride
around.
And then you etch it off on the top and you're left with these side wall spacer.
The the problem or one of the challenges
with this FinFET device is that you have
multiple of
these sidewalls.
So you have this first sidewall, which is from your gate.
But you also have these other side wall which are
from your fins, so you have these fins for sidewalls as
well
And each of them will form a spacer. How?
So let's let's take a look at it.
So if we defined a nitride around it and we
you know, etched it off from the top to make spacer.
So we will get one spacer, which will aligned like this way.
So I'm looking around this plane. So I'll get I'll get.
Let me draw it more carefully.
So I'll get one spacer, which is around here.
So I'll get
one spacer, which is around around, wrapping around this vertical sidewall.
So I'll get one of these spacer around here.
So this is one of my, the spacer that I actually want.
So this is my spacer wrapping around my gate, which I actually
want to keep but I also get this spacer around this vertical side wall so I also
get Spacer. If I draw a little on my x axis and if I
draw along the y axis, I get another spacer, which is around
my Fin. So this becomes, again I want to keep
this particular spacer, which is on my gate side wall, so I want to
Keep this spacer but I don't want this spacer.
This spacer around
my fin, it prevents me from implanting this fin.
It's also bad if I want to make a contact because I don't, I can't
make contact on the side walls because if they are covered with this spacer.
So, what I want is essentially I want to keep
this first spacer, which is on my gate side walls.
But again I want to get rid of the spacer on
along my fin sidewall. So this requires, this complicates my
process flow quite a bit and it requires very special etches or very very
precision etch control to etch off this this
this spacer on the side while keeping the spacer on my vertical sidewalls.
So here I'm illustrating this same phenomenon so
illustrating it better I have.
What I have done is I've borrowed this figure from this paper and
what I've that they have illustrated this
problem by taking these two cross sections.
So what they have done is they have taken a cross-section along this plane.
Which is the plane where I want to make a spacer
so, I want to make a spacer around my gate, side walls.
And then they have taken a cross section along my fin.
And I don't want to make a spacer around this side wall.
So the way I do it is, I define my gate
to be taller than my fin, so my gate is wrapping around.
So it's, it's much more taller. So this gate the gate height is much
higher than my fin height. So what I get is I get a spacer
over here, and so I get a spacer, which is wrapping around my
gate and I also get a spacer wrapping around my fin.
But remember my gate height is now much taller as compared to my fin height.
Then what I do is I do a etching process so I
try to etch the spacer from the top. So I drew
this etch process.
And I continue that process such that I remove
this spacer on my fin side wall
So I remove this spaces on my fin side wall
but I'm still left with the spacer on my gate side wall.
So again, it's, it's requires a very precision, a very good etch process to
remove this stringer on your side wall. But to keep your
spacer on your gate side wall.
So, this is again something very unique because unique processing
challenge, which arises because you now have this 3D device.
The third challenge, which I want to touch upon in this video is related to forming
of these source and drain junctions. So, when we talked about planar device
I described to you that how do we it becomes very hard to control these short
channel effects and we require these multiple of
these implants so we require a halo implant.
which is heavily dosed and of the same type as the substrate.
We require source drain extension implant to keep this
depletion region from moving away into the channel.
So how does that change in the context of this FinFETdevice as we go
from this planar to this 3D structure, so how does that change.
One of the advantages of going to this 3D devices, or this FinFET device.
That I have labelled upon multiple times is that you get a much a better gate control.
So your gate is
now wrapping around this thin fin and shown here, this electric potential,
so if you apply a voltage of let's say one volt here.
Your gate essentially prevents since and you have 0 volt on your gate,
this gate prevents your drain voltage from penetrating into your fin.
And if you take a cross-section along this direction, which is shown here.
So your drain potential is pretty much contained in this drain region
and it does not does not penetrate through your fin to your source.
So you get a much better gate control.
So the point I want to repeat that is since you have this much better gate
control it kind of helps your, helps your implant design
a little bit.
So what you can do is if you have a much better gate control.
You can what you can do is you can remove this halo
implants, so you don't need this halo implant because we have good gate control.
You can also probably eliminate this source drain extension
implant as well, if you are really smart.
And you can contain your source drain, control the diffusion
of this implant.
Which is your is your source drain main source drain
implant very well so that you produce a nice overlap.
You can eliminate this source drain implant as well
So, this is, this is very good news, right?
So I, you know, I described to you earlier that
we have these multiple implants and how cool it was.
But then, you know, I admit I was really kidding, you know.
It's the
implants in a planer device is really a mess.
And to compete and to, you know, control these multiple implants and the diffusion is really a mess.
So as.
I mean as compared to three implants, I'm managing just with one of these source
drain implants in my FinFET device. So you know that, that's good
news and that should be pretty exciting
another advantage that you get from moving to your.
FinFET device from a planar device is now you
have a entire new way of defining your junction depth.
So, remember when we talked about junctions, we
had this depth of, of our junction region.
And Mr. Denard in his scaling law had required
us to scale the junction depth that every generation by .7x.
And we found first of all it was really hard to do because
we can't we could not prevent the diffusion of this junction.
Secondly, our doping was limited so our resistance from source drain was really going high.
but in, in a FinFET device we have an entire new definition of junction.
So in
a FinFET device, if you think about it, this is your source and drain.
So your junction is essentially just defined by your fin
width or it's defined by the thickness of this Silicon layer.
So if I, if I take a cross-section of my device.
You know, if I cut my device along this direction, what it will
look like. Essentially, you have your silicon fin and
you have these two gates two gates around it.
So this is the
gate from this side and this is the gate from this side.
And my junction is essentially just this region, which I define in my fin.
And this junction depth is just defined as my fin thickness.
So this is, you know, another this is an entirely
new paradigm shift in the way we define junction depth.
And but it's again, you know, it's, it's something that should make
me happy.
Because I can now control my junction depth just by
controlling the depth or controlling the width of my fin.
So why, why did I list this as a challenge?
You know, things look, there are two smiley faces here, so what's the problem?
The problem is again it comes from the 3-D nature of this device.
So to define this
to make this source and drain region. What I need is I need a very
uniform implant in my in my source and drain part of the fin.
So the problems come from this effects,
I'll call, Which I'm calling here as shadowing.
So what I need in my source and drain part of my fin.
What I need is
that I, since my junction is this entire region.
What I need is I need a constant uniform doping in this region and
to do I need to essentially implant it along multiple directions
So that I know I get dopants from the top, I get dopants from the side.
But what I need is I want to create, I want to dope this entire
fin with the same concentration of dopants.
And that requires me to do this implant along these multiple angles.
So this is this is where things start to become problematic.
And what we, the problem that we get is you have these 3D nature of these fins.
So you have these fins sticking out and when you're implanting.
So let's say these are the P-type fins.
So when you're implanting your P-type fins, you want to
cover your N-type fins, so you cover them with a resist.
And there's a, only a limited amount of spacing that you are supposed
to get between your P-type and N-type fins and you have this resist.
So what you, the problem you encounter is that you cannot implant at very very
high angles, you can only implant at vertical angles.
Because if you implant at the high angle, most of your
implants coming from here, it would be shadowed by this resist.
And that, that's the problem over here.
So the problem comes from shadowing that you
get because of this 3D nature of the device.
And when you are trying to implant doing your implantation at a tilt,
you you can't get it into this fin.
Because that that implantation is shadowed out by this resist.
So the, the, the point I want to drive
home hopefully by you know, describing these three challenges.
Is that the devil is really in the details.
So, you know, we can draw these pretty and nice picture of
these devices but when you get to the
actual processing and you run through your process flow
And you want to make these short channel FinFETs very
close to each other, then you run into all these problems.
and, you know, there, there is an upside, a
positive side to look at is that there will be,
there will be multiple problems then it will require army
of engineers to process and integration engineers to solve them.
So an upside
to it is that, you know, there will always be jobs in this field.