Why is it critical that this structure in your smartphone be only 75 atoms thick?: Exploring Quantum Mechanics in your Smartphone. By Branch Education
In your smartphone there are millions of
memory cells that store all your phone’s data,
and in each memory cell there’s a structure
whose dimensions are only around
75 to 100 atoms wide.
If this structure were thicker or thinner,
the memory cells wouldn’t work, and in this episode,
we’ll explore why it's only around
75 to 100 atoms thick,
and how quantum mechanics is involved.
But before we get there,
let’s move onto familiar ground.
Here’s a smartphone and whenever you take a picture,
receive a message, or download an app,
it saves that data into this grey, rectangular, and overall,
rather boring looking microchip.
But this memory storage microchip is far from
mundane because the level
of technology inside it will astound you.
To begin, let’s open this microchip and investigate.
First, we see that this one component holds
a stack of 8 chiplets.
Let’s zoom in further to a nanoscopic view
of one of these chiplets,
and in it we find a massive array of
memory cells stacked 100 layers tall,
by 40,000 columns wide, and 50,0000 rows down.
These memory cells are called charge trap flash,
and each memory cell contains a charge trap
in which different levels of electrons
are used to store 3 bits of information.
But here’s the issue:
how do scientists and engineers design billions
of nanoscopic memory cells that can
reliably trap electrons for years on end?
This is crucial because if the electrons
are not properly trapped,
your saved pictures and files would become corrupt
and you would lose entire sections of data.
To solve this and prevent electrons from leaking out,
scientists and engineers surrounded the charge trap
with dielectric materials,
which are non-conductive insulators that
stop electrons from passing through.
You can think of this charge trap as a valley
and the insulating dielectric as the walls of the valley,
and when electrons are moved onto the charge trap,
they become trapped in the valley
with no way out and they can stay there for years on end.
This is the basic idea of how your
smartphone stores its information.
However, this setup poses a new issue.
You see, the channel,
where the electrons come from is over here,
on the other side of the dielectric barrier
in a valley of its own. Now we have two valleys
with a rather high barrier in the middle.
Electrons can now comfortably stay
in one valley or the other. But, here’s the problem.
How do we move the electrons we want from the channel,
across the dielectric and into the charge trap valley?
Or in other words, how do we write information
to a memory cell?
Well, that’s where quantum mechanics comes into play.
In classical mechanics this electron is
a localized point charge with an energy level.
The dielectric is an electrical insulator
and the minimum energy level required for electrons
to get over the barrier is so high
that doing so would take a lot of energy
which would damage the barrier itself.
And thus, the electron can’t pass over the dielectric.
However, in quantum mechanics,
the electron’s location is not a point,
but rather it’s a probability density
or a cloud that depicts where the electron
is most likely to be found.
When we apply a positive voltage
on the gate over here,
the positive electric field attracts
the negatively charged electron’s probability cloud
from the channel and pulls it towards the charge trap valley.
If the dielectric barrier is thin enough,
and the pull from the positive voltage on the gate is strong enough,
then the electron’s probability cloud
is pulled far enough across the dielectric barrier
so that there becomes a sizable probability
that the electron will find itself on the other side
of the dielectric barrier and into the charge trap.
This phenomenon is called quantum tunneling,
since it can be imagined that the electron tunnels
through the barrier, instead of going over it
and every time you take a picture your smartphone
uses this phenomenon to write information
to the charge trap flash memory cells.
Scientists and engineers use a set of quantum mechanical equations
developed by Ralph Fowler and Lothar Nordheim in the 1920s
for figuring out exactly how thin the
dielectric barrier should be,
and how strong the gate voltage should be
in order to tunnel electrons from the channel
across the dielectric and into the charge trap.
In fact, perhaps the most impressive detail
is how thin the dielectric barrier is.
These are some of the smallest things
humans have ever mass manufactured and as you see,
the dimensions shown here are in nanometers.
If we were to zoom in on the dielectric,
we find that it’s only 75 to 100 atoms thick.
The dielectric walls can’t be thicker, because if they were,
the required voltage would be considerably higher, and
the extra voltage would cause damage
to the memory cells. And, if the walls were thinner,
there would be a higher chance that
the electrons would leak out.
It’s amazing that these structures are found in
billions of identical charge trap
flash memory cells in your smartphone
and in every smartphone in the world.
Of course, to save a picture onto your phone
you don’t have to understand quantum mechanics,
but it’s interesting to know that it’s happening.
One quick note before we wrap up.
These charge traps do in fact lose their charge overtime.
It takes about a decade of inactivity
before files may potentially become corrupt.
Also, these memory cells have a limited number
of write and erase cycles.
A good rule of thumb is that you should
always back-up your important
files into multiple locations.
And that’s it!
This episode is a part of a series of episodes
detailing how solid-state drives
and 3D NAND or VNAND memory cells work.
If you’re curious take a look!
You can also watch this video a second time
and read through the creator’s comments
in the English-Canadian subtitles.
In there we added additional notes and details such as exact values,
materials, terminology, caveats, and much more.
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Thanks for watching!
And remember to consider the conceptual simplicity
yet structural complexity in the world around you!