Photolithography and Applications in Nanotechnology
[Music]
[Laughter]
[Applause]
let’s think
big they said no on the contrary
let’s think small imagine a blueberry
and a strand of our hair
the difference in scale between the
width of that blueberry
and the diameter of that strand of hair
is the same difference in scale
between the diameter of that strand of
hair and the scale that i’ll be talking
about today
did you know that the world’s first
computer first digital computer
filled up a 30 by 50 foot room
probably but still look how far we’ve
come
pull out the average smartphone from
your pockets today and they fit snugly
into our hands
compact and convenient in fact
your phone could very well have been
fabricated with the so-called
seven nanometer process to put that into
perspective
if that seven nanometer mobile processor
was turned into a seven centimeter
diameter baseball
which is about the average size of a
baseball
the average 171 centimeter male
would become a giant with a height of
almost three times
the radius of the earth not to mention
today’s smartphone’s capabilities far
surpass
those of the world’s first computer
so the million dollar question
how on earth did we go from the world’s
first computer that spanned an entire
basement
to today’s hand-held smartphone
that still outperforms its ancestor
in other words how did we manage to get
these computers so dang small
while still stuffing even more functions
into it than the world’s first computer
ever had a partial answer to this
question
involves photolithography
but what is photolithography well i’ll
start with a simple explanation of
lithography
one of the ancestors of photolithography
if you will
lithography is an older process invented
in 1796
and it was a process that takes
advantage it is a process that takes
advantage of the fact that oil and water
don’t mix very well and
it was a process that was originally
intended for the reproduction of
sheet music so that one could write down
their sheet music once and then be able
to copy and reproduce it
first one would draw their desired
design or write their desired writing
onto a lithographic
grainy stone slab then by covering
the stone slab with solutions such as a
solution of gum arabic and nitric
acid and other solutions we can
selectively kind of
recreate images with lithography
especially like when you
place paper onto the stone slab when you
kind of
press down the paper onto its own slab
you can copy
all of the original designs by new
details because at that point
photography
is a process that can selectively
recreate images onto
a different mediums indefinitely
and it turns out that photo lithography
is rather similar to lithography
even if it doesn’t involve kind of uh
oil and water as
its main materials the key difference
between the two processes
lies in photolithography’s prefix photo
which means light and this new
involvement of light is rather analogous
to the solution of gum arabic and nitric
acid
used in lithography and so
photolithography
event was invented in 1952 by jay
lathrop and james nall
at the predecessor of today’s army
research laboratory
it was a solution as to how they could
better fit the necessary circuits
in the limited space available inside a
proximity fuse
and so photolithography is a process
that involves
the selective opening or closing of
the surface of a semiconductor typically
silicon
and this is quite similar to how we can
recreate selectively recreate
images using lithography
imagine cutting cake and instead of
cutting it
into eight uniform slices like doing
going with a conventional route
say you want to cut a square right from
the middle of the cake
you’d probably trace and cut the outline
first and then carefully remove it
leaving a square shaped hole in the
middle of the cake
this is essentially how photolithography
works but however
because it is very difficult for us to
try and cut a square out from a
piece of silicon wafer we leave it up to
something called a mask
to be your knife and uv light or
ultraviolet light
to be the mouth that ravedously consumes
the square piece of cake but you might
ask what is a mask
well a mask is something that contains
your design
in the form of negative space meaning
that if you shine a flashlight through
this mask onto a wall
you’ll get the opposite of a shadow
puppet or a light puppet
and that would be your design
and so instead of the wafer slowly being
eaten away
by the uv light exposure the mask helps
protect
all areas that you want to keep and the
uv light would only be able to pass
through the negative space
in the mask and so essentially the mask
helps block light where you don’t want
it to go alternatively treat the
silicone wafer
like the entire cake say you wanted to
eat the entire cake
but your mother says absolutely not and
restricts you limits you
so you can only eat the square piece of
cake in the middle in that case
your mother is what we would call the
mask in photolithography
and you would be the mouth that
ravenously consumes
the square piece of cake and with this
actually comes a crucial
step of the process a crucial material
called photoresist
which is something that we spread in a
uniform layer onto the silicon
wafer and this and it kind of acts as an
intermediate between the silicon wafer
and the uv light
and so there are two kinds of
photoresist one part being positive
and other being negative but we’ll stick
with positive
the positive photoresist helps carve
your design
in the form of negative space which is
exactly like how you
carve the square piece of cake from the
entire cake
another way of describing this
phenomenon is to say that
positive photoresist melts away or
clears wherever it is exposed to uv
light and of course i am oversimplifying
this process quite a bit
but it’s often valuable to see the
general picture before being bombarded
by specific details
so the general four-step process of
photolithography that you should at
least try to take away from today
is as follows one
surface treatment where you treat the
wafer to a multitude of materials that
drive out water
in order to encourage adhesion between
the silicone wafer and the photoresist
because photoresist is actually an
organic
material that does not like sticking to
water and so you’ll also probably heat
the silicone wafer on a hot plate at
about 100 degrees celsius or so
to dehydrate it even further two
spin coating we use spin coat
photoresist onto the wafer using a
device called a spin coater which
actually takes advantage of centrifugal
force which is
but in reality inertia to spread a
uniform
layer a uniform coating of photoresist
onto the silicon wafer
three exposure which involves
uv light to melt away certain areas of
photoresist
so that you can etch the silicon wafer
later because in reality
photolithography doesn’t actually etch
or kind of eat away
at the wafer itself instead it does that
with the photoresist
and so photolithography actually
prepares the silicon wafer
for a process called etching that
actually cuts away equal thickness of
the photoresist and the silicone wafer
to create the actual indent in the
silicon wafer
you’ll also probably want a long pass
filter on the mask
to avoid the t-topping which is a
phenomenon
where a t-shape forms due to due to
the diffraction and reflection of the uv
light causing too much photoresist to
clear out
four development where
you clean the wafer by drenching it in
various solutions to develop and clean
it washing away any excess photoresist
and actually this step involves a post
exposure bake
on the hot plate to watch to avoid
stress cracks due to
sudden changes in temperature and so
congratulations
all of this was to create your very own
master the equivalent of lithography
stone
slab a mold or template
that contains your circuit design and so
your silicon wafer can now transfer that
design
reproduce it just like how lithography
reproduces your grease drawing
so why photolithography what’s the point
of all these silicon wafers
well first off photolithography is much
much more efficient at producing circus
than trying to manually tweezer
resistors onto breadboards
and plus photo lithography lets us
create smaller and smaller devices
chips more specifically that almost
impossibly fit even more complexity
practicality and functionality
into onto their tiny compact surfaces
and so this is especially helpful for
creating
devices called mems devices or
microelectro
mechanical systems such as micro
sensors micro actuators micro
electronics and micro structures which
are small enough
for small squeeze and stack into tinier
and tinier spaces
it’s what allows apple to keep trying to
make their phones thinner
every year how about lab on a chip
an example of a mems device
lab on ship takes advantage of certain
fluids is exactly as its name suggests
it’s a system that involves multiple
laboratory techniques
onto a small chip only about a few
square centimeters or so
in size and so how does labana chip take
advantage of these different properties
of these certain fluids
well water at a microscale is actually
acts the same as honey ketchup and blood
does at a macro scale which is the skill
that we’re all used to living in in our
daily lives
and water at a macro scale actually acts
as a newtonian fluid
meaning that its viscosity is
not affected by stress and viscosity
being how easily a
fluid can flow however ketchup
honey and blood are all examples of
non-newtonian fluids
meaning that its viscosities are
affected by stress
and you can see this when you squeeze
ketchup out of a plastic ketchup bottle
how it comes out in a steady stream and
it almost acts
like water becoming more runnier
until it kind of starts sputtering of
course and water at a microscale
actually acts just like a non-newtonian
fluid
meaning that its viscosity is affected
by stress and so we can mix water
using a different technique than we
usually do we can mix water
with a process called diffusion where
water kind of moves from areas of higher
concentration to areas of lower
concentration
sort of like how the carbon dioxide we
breathe out diffuses
into the air and we can mix by diffusion
instead of
how we usually mix it in a process
called turbulence such as how we
vigorously stir
instant hot chocolate powder into hot
water to create hot chocolate
photo lithography actually has many
other applications as well
two photon lithography can create 3d
shapes and a micro
sometimes even nano scale
and so you can see this because
it literally almost lets us create an
entire new dimension of design and this
is actually a miniature statue of
liberty
being recreated using two photon
lithography and you can see the two
photons
right here and photo lithography can
also let us create proper biomimetic
designs
such as materials that mimic gecko ct or
the little bristles
that let lizards run up trees and walls
but what about further into the future
what potential
does this relatively new technique have
in our ever-changing world
well it’s worth noting that
photolithography stands at the root
of most nanotechnology it’s what allows
us to explore new ideas as
our capability to create smaller and
smaller devices
increases potential ideas still being
explored
include nano artery robots that can
clean clogged arteries
and inject medicinal drugs straight into
our bloodstream
skipping the process of external
injections
or how about nano-insulin pumps that can
be inserted into people diagnosed with
diabetes
and internally inject insulin avoiding
the current hassle
of external insulin pumps that are still
unwieldy devices
that you attach on your belt it’s a
world of endless possibility
given enough time patience and research
let’s take a look at bcis what what in
the world are bcis
a bci is an acronym for
brain computer interface and as the name
suggests it’s an interface that connects
a brain and a computer
they represent technologies designed to
communicate with the central nervous
system
such as the brain the spinal cord and
neural sensory retinol
depending on the design and intent of
this technology we can record and
interpret neural signals
designed to complete an internal neural
action
externally perhaps on a computer
and so if a patient has a disease or
trauma that inhibits their neural
functions
we can use a bci to assist their brains
and stimulate neural activity
combined with ai and machine learning to
other growing fields
photolithography can be used to create
smaller and smaller bcis
which can potentially significantly
augment their capabilities
and we would be able to use this
technology for generations to come
the point of this talk today was to
introduce you all to one of the
fundamental processes driving the
development of nanotechnology
one of the obstacles facing this field
however is the looming end to moore’s
law
which stated that every two or two years
or so we would be able to double the
number of transistors
on integrated circuits such shrinking in
size
not only led to faster operation speed
but also lower energy consumption of and
of course higher device density
which are all vanishing as moore’s law
is slowing down
the reason is that such shrinking is
reaching its physical limits
the amount of heat that the chips
generate doesn’t decrease
by the same proportions as we scale
these chips down
and it has become impossible for us to
try and cool them down fast enough
to gain the processing payoff that we
hope to see
such shrinking in size that shrinking a
device even further doesn’t result in
lower energy consumption either
and the number of defects are increasing
as we continually try to produce
smaller and smaller devices using our
current technology
to those of you still undecided on your
career paths maybe you can use this
information as a starting point
because as we graduate high school and
into college and beyond
we’ll be the ones in the labs and
classrooms looking for a way to overcome
this hurdle we’ll be the ones
looking forging solutions that will
allow us to better the world
even if the difference is microscopic
we’ll be the ones exploring the balance
of nanotechnology
leveraging processes like
photolithography adventuring into worlds
of ever decreasing scale
thank you
you