Photolithography

ECE614: Device
Modelling and Circuit
SimulationSimulation
Unit 2 Photolithography
By Dr. Ghanshyam Singh
Sharda University

Outline
• Photolithography
– An introduction
– Photoresist (PR/resist)
• PR application• PR application
• Positive and Negative PR
– Soft and hard baking
– Alignment and Resist Exposure
– PR Development

Introduction
• Photolithography literally meaning light-stone-
writing in Greek, is the process by which patterns
on a semiconductor material can be defined using
light.
• Photolithography is an optical means for
transferring patterns onto a substrate.transferring patterns onto a substrate.
• Patterns are first transferred to an imagable
photoresist layer.
• Photoresist is a liquid that can be applied onto a
substrate, exposed with a desired pattern, and
developed into a selectively placed layer for
subsequent processing.
• Photolithography is a binary pattern transfer: there
is no gray-scale, color, nor depth to the image.

UV Exposure
Developed
PR Removed
Soluble
(UV) Insoluble

Basic of Photolithography
• Microfabrication processes:
• Additive (Deposition, Metallisation,
Oxidisation)
• Subtractive (Etching)
• Modifying (Doping, Annealing)• Modifying (Doping, Annealing)
• Two Primary Techniques for Additive and
subtractive processes:
– Etch-back
– Lift-off

Etch-Back
Q:What is the thickness of the substrate?

Lift-off
Making metal
contacts

Photolithography
processes
• Surface Preparation
• Coating
• Soft baking
• Alignment
• Exposure
• Resist Development• Resist Development
• Hard Baking
• Processing Using the Photoresist as a
Masking Film
– (Etching/Film Developing)
• Stripping
• Cleaning

Wafer Cleaning
• RCA clean: use for new silicon wafers out
of the box
– 1. SC1 @70 ºC for 15 min
– 2. DI water rinse for 5 min
– 3. SC2 @70 ºC for 15 min
– 4. DI water rinse for 5 min
– 5. Spin and rinse dry
• Standard degrease
– 5 min soak in Acetone > Methanol > DI water
with Ultrasonic > spin dry + N2 blow off
• Others:
– Refer to the Cleaning section

Wafer Priming
• To promote adhesion (PR)
• Method:
– 15 min in 80-90ºC in convention
ovenoven

Photoresist
• It is a must for Photolithography
• Photoresist is an organic polymer which
becomes soluble/insoluble when exposed to
ultraviolet light. It contains a light-sensitive
substance whose properties allow image
transfer onto a PCB board. Using photoresisttransfer onto a PCB board. Using photoresist
prevents etching or plating of the area it covers
(this is also known as resist).
• Goals
– is to deposit a uniform, adherent,
defect-free and etch resistance
photoresist (PR) film on a wafer.

What is a Resist?
• There are 4 basic ingredients in PR
– Polymers
– Designed to react with UV / laser sources / X-ray /e-
Beam
– Large and heavy molecules
– Photosensitive
– Solvents:
– As a form of liquid and allows application to wafer– As a form of liquid and allows application to wafer
surface by spinning
– Sensitisers
– to control the reaction (broaden or narrow)
– Additives
– to control light rays in the resist film(-PR), inhibit
dissolution of the non-exposed parts(+PR)

Positive and Negative
Resist
• Positive PR
• After exposure to the proper light
energy, the polymers are converted
into a more soluble state.
• Advantages:• Advantages:
» The unexposed region do not swell
much
» Higher resolution than -PR
» Response to 300-400 nm spectral
range
» More etch resistance, thermal
stability
» Reliable
» 1-3 micron thick (Typical after
baking 1 µm)

Positive and Negative
PR
• Negative PR
• The first photoresist to be used
• After exposure to proper light
energy, the polymers are converted
into a less soluble stateinto a less soluble state
• Limited resolution
• Health/Environment hazard
• Cheap

Performance factors for selection of
+PR and -PR
• Resolution capability
– The smallest opening or line that can be
produced in a photoresist patterned layer
– Positive PR > Negative PR
– Polymer components are smaller
• Adhesion capability• Adhesion capability
– A PR must have good adhesion property.
Lack of adhesion normally results in
distorted images.
– Positive PR < Negative PR
– Can use adhesion promoter (wafer priming)

+PR and -PR
• Pin-holes
– Pin-holes are microscopically small voids
in the resist bulk layer which can allow the
etchants to seep through the resist layer and
etch small holes in the wafer surface.
– Use thicker PR coating. +PR has higher
aspect ratio.
– Positive PR > Negative PR– Positive PR > Negative PR
• Exposure speed
– An important selection factor for a PR is
the response time to an energy radiation.
– Faster means more productivity
– +PR: 20-60s, while -PR 3-4 times faster
– Positive PR < Negative PR
Aspect ratio is the ratio between the image opening to the resist thickness

+PR and -PR
• Exposure sensitivity
– An energy required to initiate the
polymerisation (for -PR) or
photosolubilisation ( for +PR)
– Ensure all parts of resist are fully exposed
– Depend on wavelength/energy
• Step coverage• Step coverage
– An ability of the PR to cover the side-edge
of the surface steps produced by previous
lithography process with enough thickness
for the PR to still act as an etch barrier
– the ratio of the thickness of a film over a
step edge to the thickness in a flat area.
Most metalization processes result in a
thinner metal layer over a step than in a flat
area.
– Positive PR > Negative PR

+PR and -PR
• Thermal flow
– This refers to the fluidity of the PR during
heating process
– Must be able to maintain shape and
structure during baking
• Contrast• Contrast
– The ability of a PR to distinguish between
the light and dark portion of a mask.
– The higher the contrast, the sharper the
edge profiles of developed lines
– D0 = incident dose, D100 = completion dose
(completely dissolved, +PR)
0
100
log
1
D
D
=γ

Contrast
Fraction of
resist remained
A B C
A: low exposure
where almost all
-PR is removed1
Depend on resist thickness, soft bake, hard bake, radiation,
development, water, surface reflectivity and etc.
D10
0
D0
Energy dose
(mJ/cm2)
1
0
0
1
0
B: Transition
C: High
exposure where
almost all -PR
remains

+PR and -PR
• Etching resistance
– The ability of a resist to withstand etching
process.

Storage and Control of
PR
• Light and heat
– Any light or heat can activate the
transformation
– Store in brown bottles
– UV-filtered room
– Temperature control– Temperature control
• Viscosity control
– resist container must be kept capped to
prevent the evaporation of resist
solvent, which will result in high
viscosity
• Cleanliness
– As clean as possible, filter

Spin coating
• A resist is applied to the surface using a spin-
coating machine. This device holds the wafer of
semiconductor, using a vacuum, and spins it at
high-speed (3000-6000 rpm) for a period of 15-30
seconds. A small quantity resist is dispensed in the
centre of the wafer. The rotation causes the resist to
be spread across the surface of the wafer withbe spread across the surface of the wafer with
excess being thrown spun off. Close to the centre of
the wafer, the variation in the thickness of resist is
around 30 nm. Preparation of the resist is concluded
by a soft baking, where the wafer is a gently heated
in a convection oven and then a hotplate to
evaporate the resist solvent and to partially solidify
the resist.
Vacuum chuck Soft baking

PR spin-coating thickness
• Resist thickness is set by
• viscosity
• rotational speed
• Resist thickness is given by t=kp2/w1/2,
wherewhere
• k=spinner constant (80-100)
• p=resist solids content in percent
• w=spinner rotational speed in
rpm/1000
– Typically 1-2 µm for Si processes
– Have to be measured
Problem? Edge
Bead

Soft Baking
• Why?
– After spin coating and air-drying, the polymer
film has two important characteristics:
– It contains about 1-3% residual solvent
» Affect exposure and development
– The film may have built-in stresses
» loss of adhesion and erratic etching» loss of adhesion and erratic etching
• To ensure reproducible processing by removal of
excess solvent from the resist
• Improve adhesion by reducing stress
• The thickness of the resist is usually decreased by
25%

Baking methods
Convention Oven: takes 30 min, good
temperature control (Tc), 400 wafer/hr,
Queuing (Q) (Solvent trapping)
Moving Belt: 5-7 min, Average Tc, 90
wafer/hr, No Qwafer/hr, No Q
Vacuum Oven: 30 min, Poor Tc, 200
wafer/hr, Queuing
Hot Plate: 45 s, Good Tc, (No Solvent
trapping)
Microwave and IR lamps are also
available for commercial use.

Hot plate baking
Fast, suitable for automation, need extremely smooth surface
Get rid of Water and improve adhesio

Overview of
Align/Expose/develop steps

Methods of
Photolithography
Cont
act
Proximit
y
Projecti
on
Light
source
Lens
syste
m
Mask
PR
Wafer
Gap
Contact for
expose
Separate for
align
Less wear on
mask, but
poorer
resolution
Need excellent
imaging optics

Alignment and Exposure
– For simple contact, proximity and projection
systems, the mask is the same size and scale as
the printed wafer pattern. i.e. 1:1
– Projection systems give the ability to change
the reproduction ratio. Going to 10:1 reduction
allows larger size patterns on the mask, whichallows larger size patterns on the mask, which
is more robust to mask defects.
– Mask size is a problem for large wafers
– Most wafers contain an array of the same
pattern, so only one cell of the array is needed
on the mask. This system is call Direct Step on
Wafer(DSW). These machines are also called
Steppers
– The mask must be perfect!

More Advanced
Lithography
Method Feature
Size (µµµµm)
UV Photolithography 1
Laser Direct Write Wafer 1-2
Electron Beam 0.25-0.1
Ion Beam 0.05-0.1
X-Ray Lithography <0.1
Smallest feature size for lithographic
techniques.
Will be discussed later……….

Photomasks
• Master patterns which are transferred to
wafers
• Types:
• Photographic emulsion, Fe2O3, Cr on glass
• Cr on quartz glass (Expensive for deep• Cr on quartz glass (Expensive for deep
Uvlitho)
• Dimensions:
• 4”x4”x0.060” for 3 inch wafers
• 5”x5”x0.060” for 4 inch wafers
• Polarity:
• Light field= mostly clear, drawn feature=opaque
• Dark field=mostly opaque, drawn feature=clear

Alignment (contact type)
vacuum
mask
lev
el
lev
el
wafer
Levelling
lens
Alignin
g
lev
el
UV
Contacted+
Exposure

Resist Development
• After exposure, the pattern is developed by the
chemical dissolution of the unpolymerised regions,
leaving the polymerised region intact on the wafer.
• Important parameters:
– Developer strength– Developer strength
– pH
– Temperature
– Humidity
• Problems: Resist Swelling and Distortion
• Positive developer : KOH+H2O; Negative
developer: Organic Solvents

Developing Methods
• Immersion Development
– Simple
– Immerse in a tank of developer solution for a specific
time, then rinsing and cleaning
– Reuse several times
• Spray Development• Spray Development
– Preferred
– less chemical
– Improve image definition (spray pressure)
– Cleaner (Always fresh)
– resist coating, spin dryer, and spray
developer can be integrated to one system.
developi
ng
rinsing drying

Developers
• Negative developer
• xylene;
• Rinser: n-butyl
• Positive developer:
• NaOH, KOH, TMAH• NaOH, KOH, TMAH
• Rinser: DI water
• Problems
– Incomplete development (leave a thin
resist layer)
– Over development (too much edge
removed)

Development Inspection
• The first quality check after the
photolithography process is performed
after developing and baking.
• Checking:
• Pattern Start• Pattern
• Misaligned pattern
• Surface problems
(contamination/holes/lines..)
• Distorted pattern
Start
again!
After
developing
mask
Short,
contact

Hard Bake
• Used to stabilise and harden the
developed photoresist.
• Hard Bake removes any
remaining traces of the coatingremaining traces of the coating
solvent or developer
• Introduces some stress into the
photoresist
• Some shrinkage
• Longer or hotter hard bake
makes resist removal much
more difficult

Hard Bake
• Hard Bake is needed for acid
etching. E.g. GaAs in H2SO4-H2O2-
H2O
• Hard Bake is not needed for• Hard Bake is not needed for
metal liftoff patterning (Metal
Contact)
• Some Flows..
100ºC 110ºC 120ºC 140ºC

Photoresist Removal
• Want to remove the photoresist and any of
its residues
• Positive PR:
• Acetone,
• solvents• solvents
• Negative PR:
• CH3COC2H5(MEK),
• CH4COC4H9 (MIBK)

Etching
• Is to remove unwanted regions,
where are not protected by resist
• Two main Etching Methods
• Wet Chemical Etching• Wet Chemical Etching
– Difficult to control
– Cheapest
• Dry Etching
– Ion Beam
– RIE
– Plasma
More accurate, but
expensive

Final Inspection
• Checking for
• Dimensions
• Misaligned pattern
• Distorted shape
• Surface problems
» Contamination
» holes
» scratches
• Electrical properties: resistivity
etc.

Photolithography

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Photolithography