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