Let's start with some fun facts.
So, I'm showing here, a cartoon of the Mariana Trench.
And the Mariana Trench as some of you
might know, it's the deepest trench on the planet.
And it's really deep.
So if you look at the how deep it is, this Mariana Trench is is
approximately 2,550 kilometers deep. So
that's really, really deep. if you look at how wide it is
it kind of varies from the top to the bottom but on an average
is So Mariana Trench has rate of 30
kilometers. So if I look at the aspect ratio and
I define the aspect ratio as my height divided by
my weight, so for my Mariana Trench it has a aspect
ratio of this 2,550 kilometers divided
by 30 kilometers, which is a aspect ratio of approximately 37.
I've already worked it out for you. So, this Mariana Trench has
aspect ratio of, 37 and it's, so deep that, there have
been only two human exploration down the Mariana Trench
and more recently it was in the news. Because David Cameron,
the very famous film producer, he wanted to go down there.
And really check out what's
in there.
Didn't really find anything new but anyway,
it was in news recently, because of that.
On the other hand on this other side what I'm showing is the
tallest skyscraper in the world, which is the Burj Khalifa in
Dubai. And this this Burj Khalifa
has has a height of 828 meters.
It is the tallest building on the planet and
it has a weight of around 175 meters,
on the bottom. In the middle or on the average it has a
weight of lets say 100 meters so if I look at the aspect ratio
of my Burj Khalifa, this has the aspect ratio of
828 meters divided by a 100 meters or around 8.2.
So another interesting fact about the Burj
Khalifa that it cost around US$1.5 billion to make.
And to put that in perspective with semiconductor
technology, a semiconductor state of the art factory cost
$7 billion dollars to make.
So one semiconductor fab is more than four Burj Khalifas that you can, make.
So, that sometimes makes you wonder why anybody in with some sane
mind would spend this much amount of money to make a semiconductor fat.
But, nonetheless, the point I want to bring home is
that, you know the tallest building in the world has aspect
ratio of 8.2.
The deepest trench in the world have aspect ratio of 37.
And I want to be these numbers in you know, I want to put these numbers
in perspective, because increasingly we are seeing a
similar deep trenches and skyscrapers in semiconductor technology.
So why, why, why is that?
So, let's look at this stacked capacitor DRAM,
which is you'll find it in all of your iPhone
and your iPads. The DRAM potentially uses this stacked
capacitor technology, and if you, shown here is a CM picture of
your DRAM chip. So, if you take a CM picture of your DRAM
array, it looks like these skyscrapers.
Or it looks like these stacked capacitors, which are essentially way tall, and
they remind you of these tall buildings that you see in any American city.
Downtown, it reminds me of the Manhattan downtown.
And but there's somewhere a capacitor over here,
which is used to access and reach there's
somewhere a transistor over here.
Which is used to access and read this capacitor.
But the point I want to make is,
it, it's increasingly looking like those sky scrapers.
And so why does this this stacked capacitor has to be so tall?
So that brings me to this the way this
[UNKNOWN]
one transistor, one capacitor, DRAM works is that you have
star charge stored on your stacked capacitor.
And when you access it using this word line when
you turn this transistor on, that that charges up your
baseline. And that increases the voltage on
your bitline and you detect that increase in voltage using a sense amplifier.
So the the amount this this bitline voltage goes up can
be describe by this simple formula where you are amount of the
[UNKNOWN]
the voltage by this bitline voltage will go up would be proportional
to your, your overall operating voltage. Let's
say let's fix it at 1 volt and then it suggests a capacitor and divider so
its depends on how large your bitline
capacitance this capacitor, has to
drive up. So to pick a number
[UNKNOWN]
at least since this increase in voltage, you have since amplifier needs a voltage
increase of at least 150 millivolts. And this bitline
capacitance is somewhere in the order of 100 femtofarads.
So if you put that number in here, what you get is
your capacitor has to be at least has to have a capacitance
of more than 15 femtofarad.
And femtofarad is, you know, 10 to par minus 15 farad.
So you think that you might be thinking that this is a very
low number but in fact, to achieve this number at these very large geometry.
You need to make a very tall structure. So this is another view of of
this this same stack capacitor but this is showing a 1D array of these capacitors
[UNKNOWN].
And to essentially achieve that capacitance of 15 femtofarads.
The, the, the diameter of this of this
of this stack capacitor is essentially driven by the
[UNKNOWN]
of Moore's law, that you want to pack them as closely as possible.
So this diameter can in current day technology, is a around 30 nanometer.
Or the width of this, structure is around 30 nanometer.
And to, to essentially achieve this cap, to
achieve this capacitance which is proportional to the area.
So this capacitance would be proportionate
to your, barometer of this of this of this thing hanging up in the sky, which
would be essentially proportionate to the diameter, and proportional to the height.
So the diameter is essentially, currently 30 nanometers, projected to go even down,
so to achieve this Achieve this 15 femtofarad capacitance.
The height
that you require on these things has come
out to actually become somewhere around 2,000 nanometer.
And if you again come back to our aspect ratio, which we, in this case can be
defined as our, our height divided by this diameter.
So as you can see in this case it's 2000 nanometers or
2 micron divided by 30 nanometers.
Which gives you an aspect ratio of you know this would be somewhere around 66.
So you can already see that this this stack capacitor and the capacitor
in this stack capacitor has an aspect ratio of 66, which is
higher than what you get in your Mariana Trench, and much higher than what
you get in your Burj Khalifa.
Let's look at another chip where again you see this high aspect ratio come into play.
And in this case I want to look at an example where you have a deep trench.
So looking at the NAND flash memory. Chips on our iPad and our iPhone
have these I, this flash memory chips. If you
open it up and take a close look at it. it looks something like this.
So you have shown here are these this cross-section
along my wordline, so I have these, different uh,
[UNKNOWN]
different flash memory cell along a wordline.
And what I am seeing here is essentially again these deep
trenches found in my STI edge. So
again the Moore's, the dictum of Moore's law requires that
I pack as many of these of these flash
memory sets in a given area.
And it requires me to bring these different sets as close to each other,
which requires which puts a really stringent requirement on
The distance I can have between my STI and you know, current
technology, current technology the distance is between 20 to 30 nanometer.
And on the other hand, we discussed about
flash memory, so we saw that, you know, you
really need to apply very high voltages, you apply
voltages of 20 volts to program anything these cells.
So, you need a very good isolation between these
two cells which requires you to have a very deep dig a very deep trench,
so you can isolate the voltage between these two adjacent cells.
So, these STI, if you look at the dimension over here.
So, this STI has is is 217 nanometer deep in this case.
And I just describe
you that the, it's between 20 to 30 nanometer wide.
So again, if I look at the aspect ratio over here, it's 217 divided by 20.
So again a aspect ratio of 10, that is an aspect ratio of greater than that
of the Burj Khalifa. So similar story repeating again when
you have when you make a FinFET device, again, you need a STI edge, and show here
is this FinFET from TSMC, from IEDM 2010.
So, again, you see that, you know, you need to define this Fin, you again
get aspect ratios, which can be anywhere between 5 to 10.
An even bigger aspect ratio, something we'll talk about when we talk
about packaging. So when we'll talk about this through
silicon via technology that is used to connect stack
up these different chips, say, let's have DRAM or micro process chips.
So stack them on top of each other. This is achieve
by this electrical contact which is through silicon via.
And again the, these etches are anywhere between 100 to 200
micron deep and you require them to be 5 to 10 micron wide.
So again, aspect ratio of anywhere between 20 or even higher.
So what we're seeing is that you,
increasingly we are seeing in our in our in our semiconductor
technology. These high aspect ratio structure,
and this is this causes a lot of process challenges.
How to etch these structure, how to fill these structures
without creating a pinch off or air gap. How to prevent them from collapsing or
falling on top of each other.
And these are you know, increasingly
becoming very, challenging, issues in semiconductor technology.