What I want to do in, these next couple of
videos is to give you an introduction about the process technology.
That is, the different process steps which
are involved in, making, this, microprocessor chip.
So in a modern-day microprocessor, there're more than 150 steps involved
in making these making and inspecting these different layers.
And there as you can see, they're 10 plus of
these layers in one, and going all the way from
your transistors
which are on a nanometer scale. And all the way up to your microbonds
which you use to connect to your PCB and these are in the micron scale.
So, we see that there's a three orders change in dimensions
going from from the transistors all the way up to the microbond.
And that approximately translates
into these ten layers that you have. So if you have ten layers and you
have doubling of your dimensions at each of these layers.
That should give you two to power ten, or approximately 1000 X increase in your
dimensions going from your transistors all the way up to
the way top level of your inter connector or your microbond.
So we
want to classify these processes into two broad classes, so we want to
classify them as a either front end or back end process.
So, all the process steps which are involved in making,
this microprocessor up till the point where you, insert a plug.
So all the process test which are involved in making the transistors and making the
gate dilelectric, doing implants, they're commonly referred as front-end processes.
And all the process steps involved after that, that is depositing and
patterning these metal layers and these interconnects, they are also known as.
Back end process steps.
So these are the two broad classes that are
used to describe or classify these process steps.
And I'm not exactly aware where the origin of this
front end and back end came from. But one guess is that
traditionally, all of these process steps maybe were done in a production line where
all the tools were arranged in a, in a, you know, in a
conveyor belt kind of a, of a, conveyor belt kind of a fashion.
And these front end processes, they were started the very first of the.
Of this of this process belt
, and that's why they're called the front end, and this back end would be processed.
That's what you do towards the end, so they're called as back end.
Some, another analogy that people give is another reason that people give is
that this front end processes they involve things that you etch into the silicon.
So these are patterns are etched steps that
you do to your silicon substrate. And these back-end process
steps, they involve primarily the deposition of layers on top of the silicon.
So these typically involve Growing things on top of silicon and so that, that's
one reason that people give. Another another another way to classify
them is based on thermal budget
So most of your high temperatures processes, such as activating
your implants they are, are done in the, in the front end process steps.
So, these are the process steps where you can operate at high temperature.
Where as, where as in the back end of the line, you you have already made your
transistors you don't want to damage. the, the
transistors which have
already made in the front end process.
So the, most of the process steps here they tend to be
done at low temperature, or temperatures of less than 400 degrees centigrade.
So these are the different ways we can classify the process steps.
Let's get started with the front end of the line process flow.
So, the front end of the line process flow
involves many process steps.
For the sake of being brief, I want to describe five of them to you.
The first one is called STI or Shallow Trench Isolation.
This is the process where we dig trenches into the
Silicon to separate the p transistors from the n transistors.
And then I want to talk about making the well implants
to define the active regions are making these, p and n transistors.
Then I want to describe to you, how to make gate
stack or how to make the gate of the transistor.
Then I want to talk to you about how to
make the junction or how to make the source and drain
And then finally, how do we contact or how
do we make contact to our gate and source and drain
So
First things first. So the first thing we need to start this
process flow with silicon substrate. So we can either go with this bulk
silicon substrate as shown in over, over here or we can use this SOI substrate.
And this SOI substrate that essentially
involves growing insulating layer on top of.
your silicon,
so this a insulator here, insulator layer typically silicon oxide and then this
thing thin layer of silicon is placed on top of this insulator.
Hence the name SOI, which stands for silicon on insulator.
So this silicon layer is now on top of insulator.
Another way to look at it is separated from
this So, bulk of the wafer by this insulator.
So either way, we can either start with a bulk or a SOI substrate.
Some people prefer to use bulk substrates.
some niche application and some other players, they prefer to use SOI as their
starting material. So anyway we can pick either of those.
And then the first step that is done is the shallow trench isolation.
Or the first module I should say is
the shadow trench making this shallow trench isolation.
And the process flow for that is first you deposit a
layer of nitride. And this nitride layer act as a hardmask.
And by hardmask I mean is a material
which is harder relative to the photoresist material.
And it's typically deposited before you deposit your photo
resist And it's essentially a mask for etching as
we will just describe later. And then the next thing you do is once
you have put this nitride, you cover your wafer with Photoresist.
And this Photoresist itself is consisting of two coatings.
So you first have an anti-reflection coating.
And then you coat your wafer with photo resist.
And the reason why this anti-reflective coating is used, is because
we want to avoid a reflection coming down from the surface of your wafer.
So this is depicted in this diagram over here.
So if you say if you place your photoresist
of this on top of your wafer, then you
shine light, when you shine light, when you shine light.
It will reflect back from the surface, and then it reflects
back again from the surface, and it will create these standing waves in the resist
which are bad for the fidelity.
And they'll lead to essentially feature loss, or loss of resolution in your resist.
So to avoid that, you place the anti reflective coating, so you could place
this anti reflective coating either below your resist or on the top of the resist.
So when it is placed below the resist, it's, it is called Bottom Anti
Reflecting Coating or BARC and then you coat it with the photoresist. So this photoresist
is
essentially consisting of two materials.
One is the anti-reflective coating and then the actual photoresist.
And this is common
for all process
flows
for all process where you do a patterning
using a resist you always use a anti-reflective coating.
So I want to define it the first time we are using it
but in any subsequent process flow when I show you resist and patterning.
By default we'll be talking about essentially
this bilayer of anti reflective coating and the
photoresist
And then the next step we'll do after we have our
photoresist
is we expose it to light. So, the part which is exposed, it gets
developed and, then essentially you will remove the part which is undeveloped or
depending upon the tone of the
photoresist
and what you get is essentially this patterned photoresist
So you pattern your
photoresist and you get pattern
of your photoresist
as shown over here.
The next step involved is essentially we etch.
We etch this pattern into first our nitride
and then use that nitride as a hard
mask
because it's a harder material as compared to photoresist
to etch that feature into our, into our, into our
.