Photolithography
PEOPLE Program
July 8, 2013
How is Lithography Related to Computers?
• Computer chips are
made using
photolithography
Packaging
• Instead of drawing
with a sharp tip, it
uses light to transfer
a tiny pattern from a
photomask to a lightsensitive chemical
Central Processing Unit (CPU)
How is Lithography Related to Computers?
ENIAC (Electronic
Numerical Integrator and
Computer)
First general-purpose
electronic computer
Apple II
Apple MacBook Air
An early personal
computer
Modern laptop
computer weighing
only 3 pounds!
65 years… and many advances in photolithography!
Photolithography
• Role of the Resist:
Light exposure changes
solubility and allows mask
formation
• Applications: Making
tiny, detailed stuff! For
example, microelectronics,
microfluidics, MEMS,
bioanalytics, sensing, and
many more
• Main Benefit: High
throughput!
Geissler, et al. Adv. Mater. 2004, 16, 1249-1269
Transferring the Pattern: Make a Mask!
Substrate that will
“receive” the pattern
Mask that controls
the passage of light
Silicon wafer
Transparency with desired pattern
Transferring the Pattern to the Silicon Wafer
1. Coat silicon wafer with photoresist
Photoresist
Silicon Wafer
2. Tightly place mask on top
3. Expose to UV light
Mask
Silicon
wafer
4. Develop and Rinse
?
Depends on the type of photoresist
Components of Conventional Photoresist
Solvent:
gives the resist its flow
characteristics
Resin: mix of polymers that
hold the resist together; gives
the resist its mechanical and
chemical properties
Sensitizers:
sensitive to light; these will
react when exposed to light
Additives:
chemicals that control other
aspects of the resist material
What it mean if something can
dissolve?
What does it mean if something
can’t dissolve?
What could be some
examples of things that can
dissolve in H2O?
What could be some
examples of things that
cannot dissolve in H2O?
Our definition
Dissolve: A solid loses its structure and becomes
dispersed into a liquid.*
Examples:
•Table salt (NaCl) dissolves in
water.
•Sand (SiO2) does not dissolve
in water.
•Gasoline does not dissolve in
water.
•A breath-strip dissolves in your
saliva.
*Dissolving can take other forms, as well. For example a gas can
dissolve into another gas, or a gas can dissolve into a liquid.
Positive vs. Negative Photoresist
• Positive Photoresist: Exposed areas
become able to dissolve in the developer (so
the exposed portion is dissolved by the
developer)
• Negative Photoresist: Exposed areas
become not able to be dissolved in the
developer (so the unexposed portion is
dissolved by the developer)
– We will be using SU-8, a common negative
photoresist
Positive vs. Negative Photoresist
3. Expose to UV light
POSITIVE
Photoresist
NEGATIVE
Photoresist
4. Develop and rinse
4. Develop and rinse
5. Etch patterns into wafer
5. Etch patterns into wafer
6. Remove residual photoresist
6. Remove residual photoresist
Make sure you get it! Fill in the blanks:
In a _____ photoresist, _____ light reacts with the photoresist to
make it unable to be dissolved, so it is rinsed away by the _____.
3. Expose to UV light
POSITIVE
Photoresist
NEGATIVE
Photoresist
4. Develop and rinse
4. Develop and rinse
5. Etch patterns into wafer
5. Etch patterns into wafer
6. Remove residual photoresist
6. Remove residual photoresist
A Common Photoresist: SU-8
• “Traditional” epoxy resin
negative photoresist
• UV sensitized with a
cationic polymerization
initiator
• Low near-UV
optical
absorption 
high aspect
ratio (>15)
features
EPON® Resin SU-8 (Shell Chemical)
Hg “I-line”,
365 nm
Lee, et al. JVSTB 1995, 13, 3012-3016; Bertsch, et al. Sens. Actuators 1999, 73, 14-23; Lin, et al. J. Micromech. Microeng.
2002, 12, 590-597; Liu, et al. Adv. Mater. 2007, 19, 3783-3790
Review!
What is cross-linking?
Do you think that a substance that has a
lot of cross-links would be more easily or
less easily dissolved? Why do you think
so?
Negative Photolithography
Unexposed areas remain
able to be dissolved by
developer chemical.
Areas exposed to light
become crosslinked and
don’t dissolve in the
developer chemical.
UV
Photoresist
Oxide
Substrate
Unexposed
Exposed
Soluble
Pre-exposure
- photoresist
Crosslinks
Post-exposure
- photoresist
Post-develop
- photoresist
Today’s Activity: Photolithography
• Bucky masks have been
created for you
• Glass slides have been
primed and coated SU-8
photoresist
Your Mask:
• You will be performing
photolithography using
these masks
PEOPLE Program 2013
Your Name
Today’s Activity: Photolithography
UV Light

Resist
Mask

1) Vapor prime
2) Spin coat
3) Soft bake
5) Post-exposure
bake
6) Develop
7) Hard bake
4) Alignment
and Exposure
8) Inspect
Step 1: Vapor Prime
The First Step of
Photolithography:
• Wafer “primed” for photoresist
deposition by cleaning with
isopropanol and acetone
• Promotes good photoresist-towafer adhesion
• Followed by dehydration bake
(200 °C, 5 min)
Silicon
Wafer
 Ensures wafer surface is
clean and dry
Step 2: Spin Coat Photoresist on Wafer
1. Wafer secured to vacuum
chuck of spin coater
2. Dispense ~3 mL of
photoresist onto wafer
3. Slow spin: 500 rpm for 15 s
4. Fast spin: 6000 rpm for 60 s
• Quality measures:
– time
– speed
– thickness
– uniformity
– particles and defects
Photoresist
dispenser
Vacuum chuck
To vacuum
pump
Spindle connected
to spin motor
Steps of Photoresist Spin Coating
1) Resist dispense
3) Spin-off
2) Spin-up
4) Solvent
evaporation
Step 3: Soft Bake
• Improves photoresist-towafer adhesion
• Promotes resist uniformity
on wafer
• Drives off most solvent in
photoresist
• Improves linewidth control
during subsequent
development/etching steps
• Our soft bake procedure:
– T = 95 °C on hot plate
– t = 2 min
– Cool to room temperature
Silicon wafer
coated with
photoresist
Hot plate
(T = 95 °C)
Step 4: Alignment and Exposure
• Exposure transfers the
mask pattern to the
photoresist-coated wafer
• Light activates the
photosensitive
components of
photoresist
• Quality measures:
– linewidth resolution
– overlay accuracy
– particles and defects
UV light source
Mask

Resist
Step 5: Post-Exposure Bake
Silicon wafer coated with
exposed photoresist
• Performed immediately
after exposure
• Completes the photo
reaction initiated during
exposure
• Same procedure as the
pre-exposure soft bake:
– T = 95 °C on hot plate
Hot plate
(T = 95 °C)
– t = 2 min
– Cool to room temperature
Step 6: Photoresist Development
• Soluble areas of photoresist
are dissolved by developer
solvent
• Can be performed on the
spin coater or simply by
immersing in the developer
• Visible patterns appear on
wafer: windows, islands,
lines, text, images, …
• Quality measures:
- line resolution
- uniformity
- particles and defects
Developer
dispenser
Vacuum chuck
To vacuum
pump
Spindle connected
to spin motor
Steps 7 and 8: Hard Bake and Inspect
• Hard bake step
increases thermal,
chemical, and
mechanical stability of
developed resist
features.
• Developed resist
features can be
inspected (usually with
magnification) to assess
the quality of pattern
transfer.
Summary
1. Coat wafer with photoresist
Photoresist
Silicon Wafer
5. Etch patterns into wafer
2. Tightly place mask on top
3. Expose to UV light
6. Remove residual photoresist
4. Develop and rinse
Today’s Activities
Group 1
•
•
•
•
•
Jake & Zahmere
Ruby & Tyrice
Roselena & Manuel
Paola & Shealyn
Terry & Mario
Group 2
•
•
•
•
•
Champange & Jazmin
Alvaro & Karen
Tashiana & Tabitha
Mariah & Zantasia
Alma & Sara
Today’s Activities
Si Wafers
(Diane, Ben & Kelly)
Photolithography
(Matt & Alfonso)
Session 1
Group 1
Group 2
Session 2
Group 2
Group 1