[MUSIC].
So the point that is here, you can scale down your
transistors, but as long as your interconnects are not scaling properly.
And we'll discuss later in the lesson if you scale the interconnect.
It's
[UNKNOWN]
does not scale. The
[UNKNOWN]
with interconnect with scaling remains a constant.
So more important as compared to the transistor
today, what has become more important is the delay coming from your interconnects.
So it's So even though you might have a transistor
below, which is killing all the action, might be killed with what's happening
at the interconnect level.
[SOUND].
So there's always two ways to make any interconnect line,
or in fact there are always two ways to make.
Pattern anything, right. You can do a subtractive method.
That is you deposit a film, you deposit a pattern
I mean you pattern a Photoresist. And then you etch that pattern into your
[UNKNOWN].
you can then deposit this dielectric, and then
you can planarize it, using a CMP process.
And hence you achieve this structure.
Another way of achieving this same structure.
So this was, you know this was something
which was preferred for the different layers of.
In the early times of scaling, in the 80s
and 90s when people used to make interconnects with
aluminum, this was the way it was used to make that.
You deposit alumniumn, you pattern it into lines, and you coat with oxide.
Because and the reason being people know how to etch aluminum.
With more and more
[INAUDIBLE]
some
out of uh,copper because copper has a lower
[INAUDIBLE]
than aluminum, and you don't know a good etch chemistry for copper.
Because whatever you etch it with it's not
so even with all the other
materials that people are using in, even in their
metal gate they don't know how to edge them.
So the other way to make that achieve that same
pattern as you make a stencil so you start with an
oxide and you have that etch top layer which is
that blue layer could be a nitrate, could be another dielectric.
And you etch that stencil into the static electric.
And then you fill
it with the material. And then you polish it off.
And then you, you essentially have that same pattern again.
So, to generate this pattern you could either do it
subtractively or you can do a down the stream process.
And these are.
The reason it's called Damascene is people used to, the city
in Damascus, where people used to make sod using this same process.
So that is why
it's called the Damascene process, so. Right, so, once, once you have these
[COUGH]
we are, we finish the processing thin over here
and then what we do is pattern these plugs.
So we deposit another layer of photoresist, we make these plugs and we So
these are plugs which will connect to
you're, to you're soldering metal or your soldering
[UNKNOWN].
So you edge these plugs into your Oxide and then
you deposit it with a liner. So you deposit ti-nitride, and
it's a CVD processor AMD processor and it. Fills up this entire area.
So the thing with damascene process is you
need these things called liner or barrier materials.
So in this case, the material you'll be filling
it up with, tungsten, so tungsten likes to react.
It's done using the CVD of tungsten is done using this precursor of tungsten
Except for liner tungsten WF4,
and it loves to react with oxygen and damage your oxide.
So you need a liner which will
prevent this precursor from reacting with your oxide.
Similarly later, we'll be filling things up with copper.
And copper is another metal which loves to diffuse into your oxide.
So, you again need a barrier or a liner material.
Some of the space on this barrier or liner material that.
It should first of all be a good
barrier, so it should prevent materials from diffusing in.
more importantly it should a, it should have a good adhesive with
your dielectric, as well as the material your trying to fill in.
Right?
And than it's resistivity should be relatively low as well.
But that's a contradiction because, typically materials
which have low resistivity are not good barriers.
Because low resistivity means you have larger grains,
which on something we have larger, more grain, You have larger
grain means you, you're, it's easier to penetrate your grain boundaries.
So the materials which are typically used
for liners are amorphous materials like tri-nitride
or tantalum nitride or materials like those
and they don't have very good resistivity.
And so what's here done this liner
is you do a CVD of tungsten and the thing is CVD process is If you're doing a
[UNKNOWN]
in a trench, it goes from both directions. So you always, if you're doing a
[UNKNOWN]
process, you are always left with a seam in between.
So that's a characteristics of a
[UNKNOWN]
process, when you're trying to fill a trench with a
[UNKNOWN]
process, many times you are left with a seam.
And that can be problematic sometimes but essentially assuming it was okay here.
You polish that tungsten off and you're left
with these plugs which are now connecting to your
transmitter.
This is another picture of the A6 processor and
you can, it is taken at this level where you
can see the draw nester left form and these
tungsten plugs are coming from above and connecting to the
[UNKNOWN].
So the next
process I will detail on is this process called dual damascene.
So it's, dual damascene process is done at the same time.
So not only do you form so suppose this was metal one.
So this dual damascene process will allow you to form metal two, as well as the
to metal one in this one single-step process.
And it's a way for process through so let me describe it to you.
So that the way we do this is You have
you want to minimize your RC dealer for
this, all these dielectrics over here, these are low k dielectrics.
So low k dielectrics means either they are porous or they have carbon added into the
oxide. So these are silicon or SIOC And me adding
carbon to it if you deposit such a low K film once you're doing
for the processing, you are protect it from reacting with oxygen,
[INAUDIBLE].
So you typically put a capping layer on top of this low
K film. And, then what you do is you pardon
[INAUDIBLE].
And the first thing you pardon is the vias.
Remember this is a metal one, so you want a
set of vias that will connect the metal one to metal.
So for example,
[INAUDIBLE]
going in this direction and then metal too would be going in this direction.
You going
connect your metal going to your metal too.
Your first thing pattern is the bios and you
do this after you make this pattern and you
establish your rear the next thing you do is
you fill the whole thing up with the polarization layer.
So this is an organic layer, something
similar to a resist, but not photosensitive.
So you fill it up with
this organic planarization layer, and it goes out, so it's basically
a liquid solution that goes up and fills up everything, right?
So you are.
You have from this planar layer
again.
Then what you do is you do another set of patterning on top of this.
So you pattern, you deposit another layer of full resist and now you want your,
now you want to pattern your metal two lines, which are doing in this direction.
So this has the pattern for your metal two lines.
And then you do a RIE etch, so you do this etch where you
essentially etch this pattern into
So now you can see, and you also etch that etch extra into that organic.
Then my initial layer, but that doesn't matter, right?
So you, what you're left with is you have, using
two lithography steps, but only one layer, you have transcribed.
And using this planerization organic planerization layer,
you have transfered both of these patterns
together into this one layer, right? So now you can remove your organic
[UNKNOWN]
layer.
And what you are left with is these two patterns,
which are both now patterned into your So your mold
is now ready, so now your template is ready, now
you can come and fill it up with a metal.
But again before filling up after the metal to the
metal we want to fill it up with with copper.
For before filling filling it up with the copper, again you
need a barrier which will prevent copper from refilling into your outside.
So you use a Tantalum nitride barrier.
Some people use just just a Tantalum barrier, and then you also use a seed.
So, I'll show you the reason for that, but the process that is used
for filling your lines with metal is
is this electrochemical deposition of copper, or also known as electroplating.
So the way this copper is deposited used electroplating, it's the same process that
use to plate your silverware, or plate your jewelery.
And, the way it works is you have your wafer
and you have your liner, and you deposit it with a
[UNKNOWN].
Thin layer of your seed material, so you deposit
it with that seed material which is shown over here.
So that seed is copper and then you dip it up in this
bath which is, essentially has copper ions in it and because you have
seed it will form this nice uniform electrical.
equal potential surface on your waveform, on which
your ions can now come and attach themselves, right?
So, your, it's a process of electroplating.
you know, borrowed not from the semiconductor, but borrowed
by the semiconductor industry from, say the silverware industry.
Or the industry but
[INAUDIBLE]
will not
[INAUDIBLE],
but he did a lot of work on on developing this copper electroplating process.
So the, one of the good one of the advantage you
get from doing electroplating is you get a bottom up fill.
So you have a liner and hence your ions are
coming and depositing from below you get a bottom up fill.
So,
as compared to a CVD or PVD process where you're depositing things from
the top and it's a line of side process you might get a void.
So that, that's an advantage of doing electro
plating which is which is a PVD process.
So the next step after that is to
[INAUDIBLE]
the CMP your Your material off so that the CMP and
you polish your copper off, and the question I
have is I described you one way of doing this double damascene process.
There could be another way, this is shown as other option.
Where you can have a hard mask, so you can make your via first.
You can do your hard mask, you can etch the places where
you want your via, and then you put your dielectric, and then you
can etch together, and then use this hard mask to etch that via.
So, you can do one etch, to, instead of doing two etch.
This is another option of making this double dimacine processor.
This is final completed microprocessor. A slightly one to power a PC.
But it's, the reason I like this diagram is
you can clearly see, so these are the low k
layers, and you can also see this gaping layers,
this gaping oxides, and you can see all the different.
Metal layers and so we are almost to, you know this is the homerun, so we are almost
getting to finishing our microprocessor.
So we have then the, you know the next
steps are essentially continuation of this so you did
metal one two, you do the same thing metal
three, four, metal four five six seven eight nine, right?
So the only thing left is now how to connect
this to the outside world right, so we have done this.
And many people don't think you need process class, you
need somewhere to connect this to the outside world to write.
So the way it's done is you know you do your Metal 9 typically that
[INAUDIBLE]
nine levels of metals.
And then you finally, what you do is you put,
cap it with another nitride, nitride is another good barrier.
So, whenever you prevent, you're preventing something filling in
or out, you always cap it with a nitride, right?
Then you put another
[UNKNOWN],
you open up this circular opening. And then you etch it down
to your final copper layer, and then you sorry,
[LAUGH].
So you finally cap it, and you put a solder liner.
And then you put another full resist, and then you fill copper into
this full resist, and then again you dip it into a solar bath.
So it's very similar to that electroplating process, but
essentially you now have this region which forms the solar.
So it's like you have from
the seed
[UNKNOWN].
Solder but again or a solder solution and you
get, you found this bump, and then what you do is you remove off that
photoresist, so what is, you're left with this microbump, and
finally, what you do is you In that chip that we showed
on the very first lecture, the second lecture and
now essentially you have these bumps.
And you take it and connect it to your PC, I mean
basically just flip the chip and mount it on top of your PCB.
So this is that.
These are the small micro bonds, and I just showed you how they are formed.
And then you take your microprocessor, and over here you
can see there is microbumps, and they are connecting it together.
[MUSIC]