[MUSIC]
Hi, welcome to Coursera's Nanotechnology,
The Basics.
My name, my name is Daniel Mittleman.
Thanks for joining me.
Today, we want to talk about some of the
spin-off technologies,
that have been enabled by rapid advances
in the semiconductor in.
So, as we have seen, earlier,
in the lectures this week, semiconductor
electronics
industry has been driven to maintain their
pace on the Moore's Law of prediction.
as a market forecast, really
and in order to stay on that, they've
really been driven
to innovate in a number of areas,
associated with processing silicon wafers.
So, what you're seeing here is a picture
of a
bare silicon wafer, which is really just a
mirrored surface.
And, how it gets turned into a processed
wafer with, nanoscale components and
patterns on it.
In order to do that, you need a lot of
different process tools, and techniques,
and technologies, that enable laying
down layers with fine precision, removing
pieces with fine precision.
And, all of this has been developed, with
the idea of enabling advances in the
semi-conductor industry.
But, of course, all of these ideas, also,
can be spun off
and applied in areas that have nothing to
do with the electronics industry.
And that's really what we want to talk
about in today's lecture.
Some of those spin-off technologies, were
really there are too many to
mention, but I can highlight a few of them
in these pictures here.
So, for example microsurgery or micro
machines or
micro fluids, all of these are ideas
which.
on the face of them have very little to do
with electronics, in
the sense, that they are not about
creating new electronic circuits, or
integrated circuits.
But, they leverage the advances made by
the electronics industry, to
enable the construction of devices which,
which have impact in all of
these other areas.
And, I'd like to highlight just a few of
those advances, some of
those innovations in the industry which
have done all, all of this enabling.
slow, the ability to lay down thin layers
of materials with fine precision.
The ability to carve away small channels,
or narrow channels in silicon.
And, the ability to create more
complicated structures, such as, for
example, cantilevers.
So, the first one I want to talk about, is
the ability to lay down layers
of material on the substrate, with very
high precision.
There are quite a number of ways of
doing that, I'm only illustrating two of
them here.
On the left, you're seeing a picture of a
device,
that was fabricated with a technique known
as physical vapor deposition.
And what you see here is a cross-section
of the
device, where you can see alternating
layers of different materials.
In this case, two different kinds of
oxides,
it's a silicon dioxide and a tantalum
oxide.
And when you would lay down alternating
layers,
with precisely controlled thickness and
precisely controlled morphology and, and
chemistry,
then what you can create are optical
components with extraordinary properties.
And, and, of course, nanometer scale size,
or anyway, micron scale size.
if you need finer precision than that, you
can go to this other technique that's
illustrated by the cartoon on the right
here, which is known as molecular beam
epitaxy.
And this is, somehow, the gold standard or
the,
the highest level of precision that one
can obtain.
This is done inside an ultra high vacuum,
and one can really lay down atomic layers
of almost any material on a substrate
with,
with precision of one atomic layer at a
time.
So, you can control the thickness of these
layers, you can control their composition.
their atomic composition by putting down
different materials at different times.
And you can really create very
sophisticated and complicated structures
using these sorts of techniques.
these ideas are pervasive in our daily
lives anybody
for example, who wears eyeglasses that
have anti-reflection coating
or uses a reasonably good camera that has
anti-reflection
coating lenses coating on the lenses or
anti-scratch coatings.
These are all created by putting down
multiple layers of oxide materials.
a little
bit less visible to the consumer but
perhaps equally important are things like
the laser in your DVD player, or the hard
drive in your computer.
what you're seeing in, on the cartoon on
the bottom here, is a
cross section of what the laser in a DVD
player typically would look like.
And you can see again, there are multiple
layers of
different materials layed down in some
sort of alternating pattern.
With typically nanometer sort of
thickness,
so there's an eight nanometer thickness
layer,
and you see in this particular example the
materials are semi-conductors,
they are gallium arsenide and indium
gallium arsenide and aluminium gallium
arsenide.
And these are laid down with exquisite
precision in terms of the
composition, the smoothness of the
interfaces,
and the thickness of each layer.
Typically one can lay down dozens of
layers, or hundreds, or in some
cases even thousands of layers to create
structures that have all sorts of
functionality.
So putting down layers is good,
but removing material is also good.
Obviously, it's not enough to be able to
put stuff down, you
also have to be able to take stuff away in
selective selectively.
So, there're again quite a few different
technologies that can be used to do this.
Here, I'm illustrating probably one if the
most commonly
used of those, which is called reactive
ion etching,
and on the left you see some text which
describes a three-step process as to how
this works.
Briefly speaking, you use microwaves
to create a plasma, which is just a gas of
charged atoms, charged ions.
with positive charge and you create those
near a positive
electrode and then they are accelerated
towards a negative electrode.
And those accelerating ions, in between
you place
the sample that you want to etch, and
those
ions run into the sample, and because they
are charged they will react chemically
with the surface.
And create some sort of reactive species
that is then removed by your vacuum pump.
So, you can remove material.
And because these ions are accelerated in
one particular direction, they have a, a,
the
etch is highly anisotropic so, etch in
one direction, but not in the sideways
direction.
So, you can create for example deep
trenches using this sort of technique.
It's obviously not enough to simply remove
everywhere on a surface.
You might want to material from some parts
of the surface
but not others, so you need to be able to
protect.
Areas of the surface that where from which
you do not want to remove
materials, and one typical to do this
would be
put down a layer of what's called a
photoresist.
Which is basically just a thin plastic
layer
that protects the surface from the etching
ions
and so the etching would not happen there
and it would happen in the other
locations.
In order to create back that photoresist
pattern, which may
have nanometer sized features, or
apertures or structures in it.
You would typically use photolithography.
So you can see we're going to use a
combination of the tools that have
been developed for creating integrated
circuits, photolithography, etching, and
so on.
In order to create other sorts of
structures as well.
And, and I didn't mention, but this
photoresist would be a temporary
protective structure.
It could be subsequently removed.
By simply dissolving it in acetone or
something like that.
So, the kinds of structures that can be
made with reactive
ion etching are, for example, trenches,
where you could flow liquids.
So, this gives rise to a whole
area of endeavor known as microfluidics,
the idea of having
microliter or smaller volumes of liquids
flowing in thin channels.
And this enables a whole range
of interesting applications in biology and
medicine.
I want to particularly highlight this one
here which
is called the lab on a chip application.
The idea here is that in the chemistry lab
you would
be mixing liquid components and creating
reactant products and so on.
And in microfluidic lab on a chip, you
would be doing
the same thing but on a, on a much smaller
scale.
it would be small, portable, chip, and so
on and so that would be really
advantageous.
there's a laboratory at Rice University,
the McDevitt Lab, who
are doing exactly this sort of thing and
who are
developing diagnostics for for medical use
in the third world,
where a big chemistry is simply too
expensive and not practical.
And so, I would,
I would also highlight this webpage here.
Rice 360 is a group here.
Rice that is pushing the development of
technologies.
Many of which are nanotechnology enabled
for applications in the third world.
Another interesting application is
illustrated by
this photograph on the right here.
Which is a, a cell biology application.
Here we have yeast cells growing inside of
a serpentine shaped channel.
And what's interesting here
is that in ordinary media, the yeast cells
grow straight.
But in a curved channel the yeast grow
curved.
So, to take this one step further we can
be even
more sophisticated than simply drilling
trenches
we can create three dimensional
structures.
Again, out of silicon simply by etching
selectively,
and protecting, and then under-etching,
and so on.
And so, what you see here is a, is a, an
array of cantilevers.
A cantilever is, after all, just a diving
board,
which is anchored on one end and can
vibrate.
And we can imagine taking a diving board
and
shrinking it down to a micron size or
smaller.
And then we have something that will
vibrate much more rapidly.
But it's the same idea.
the ability to make a cantilever really
enables a lot
of ideas in sensing, and I want to
highlight one of those
by pointing out this photo here which
shows the underside of
a cantilever with a sharp tip fabricated
on to the structure.
This is all still silicon.
So, this is
just carving pieces out of silicon,
essentially.
That sharp tip tapers down to a size that
can be only a few nano-meters in size.
And this really, this development really
enabled a whole new way of
thinking about ways to image the
topography of a surface with incredible
sensitivity.
This is known as atomic force microscopy,
as it's illustrated by this cartoon here.
The idea here is to take your cantilever,
which has
a, a very sharp needle tip on the end of
it,
and essentially just drag it across a
surface.
And when it comes to a bump, it hops up
and over.
And, if you can sense the motion of
that cantilever with nanometer scale
resolution, then you
can make a topography map of your surface,
and measure bumps that are only a
nanometer high.
Or even smaller, in fact, the, the limit
of this is less than a nanometer.
The way we measure that.
This is sort of another innovation.
The way we actually measure that is by
using a laser,
and bouncing it of off the surface of the
cantilever and
then reflecting it onto a detector that is
very sensitive to the position of the
laser.
So, as the cantilever removes, the laser
beams move back and
forth and you can measure that change in
position and correlate that.
With change in position of the cantilever.
So, atomic force microscopy is really
opened up a whole
new world of how to image surfaces and
their topography.
it's also spun off it's own interesting
technological ideas.
For example you can take your cantilever,
your sharp tip and
bind it to a single molecule which is then
attached to the surface.
And by moving the cantilever up and down
you can stretch
one molecule and measure its spring
constant or measure its tensile strength.
So measuring the tensile strength of
single
molecule is something that ordinarily you
would imagine
would be very hard to do, but we can do
that using these sorts of techniques.
The other thing that's interesting about
the cantilever idea, is that this
vibration
frequency, they have a natural vibration
frequency, is incredibly sensitive to the
mass
of the cantilever.
So, if we functionalize the top surface of
that cantilever
so that it binds for example, selectively
to a specific protein.
Then when that protein binds there, even
if it's just one protein molecule, the
change
in mass of the cantilever will change
that resonant frequency and one can
measure this.
So, you can actually measure the binding
event of a single molecule on a surface.
That's extraordinary sensitivity, which is
enabled
by our ability to fabricate these
complicated
structures in three dimension using
etching of silicon, essentially.
So, the ideas of this lecture are really
two fold.
Number one the need for smaller and
smaller devices, which is to
say the need to remain on the Moore's Law
curve has driven the
electronics industry to innovate in the
development of new tools, and new
techniques,
and new approaches to structures silicon
on the macro and eventually nano scale.
And, those new tools and techniques have
then, led to
a whole range of spin-off technologies
which
have applications in areas far outside of
electronics.
I hope this has been an interesting
lecture for you.
Thanks for joining.