Advanced Manufacturing
Choices
ENG 165-265
Spring 2015, Class 6 Photolithography
3/14/2016
Content
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Lithography definitions
Resist tone
Introduction to the lithography process
Surface Preparation
Photoresist Application
Soft Bake
Align & Expose
Develop
Hard Bake
Inspection
Etch Layer or Add Layer
Resist Strip
Final Inspection
C-MEMS
LIGA
Clean- Room, Wafer Cleaning
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•CD and Tg
•Making a Mask
•Moore’s ‘Law’
Photolithography -- Definitions
 Photolithography is used to produce 2 1/2-D images using light
sensitive photoresist and controlled exposure to light.
 Microlithography is the technique used to print ultra-miniature
patterns -- used primarily in the semiconductor industry.
Photolithography -- Definitions
Patterned
wafer
Test/Sort
Diffusion
Thin Films
Polish
Photo
Etch
Implant
Photolithography is at the Center of *the Wafer
Fabrication Process
Resist Tone
Negative:
Prints a pattern that is opposite
that is on the mask.
of the pattern
Positive:
on the mask.
the pattern
Prints a pattern that is the same as
Resist Tone
Ultraviolet Light
Chrome island
on glass mask
Areas exposed to light
become polymerized and
resist the develop chemical.
Island
Exposed area
of photoresist
Window
photoresist
Shadow on
photoresist
photoresist
oxide
oxide
silicon substrate
silicon substrate
Resulting pattern after
the resist is developed.
Negative Lithography
Resist Tone
Areas exposed to light
become photosoluble.
Ultraviolet Light
Chrome island
on glass mask
Island
Shadow on
photoresist
Window
photoresist
Exposed area
of photoresist
photoresist
oxide
oxide
silicon substrate
silicon substrate
Resulting pattern after
the resist is developed.
Positive Lithography
Introduction to the Lithography Process
Ten Basic Steps
of
Photolithography
1. Surface Preparation
2. Photoresist Application
3. Soft Bake
4. Align & Expose*
5. Develop
6. Hard Bake
7. Inspection
8. Etch
9. Resist Strip
10. Final Inspection
* Some processes may include a Post-exposure Bake
1. Surface Preparation
(HMDS vapor prime)
• Dehydration bake in
enclosed chamber with
exhaust
• Clean and dry wafer surface
(hydrophobic)
• Hexamethyldisilazane
(HMDS)
• Temp ~ 200 - 250°C
• Time ~ 60 sec.
HMDS
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1. Surface Preparation
(HMDS vapor prime)
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1. Surface Preparation
(HMDS vapor prime)
2.
Photoresist Application
• Wafer held onto
vacuum chuck
• Dispense ~5ml of
photoresist
• Slow spin ~ 500 rpm
• Ramp up to ~ 3000 5000 rpm
• Quality measures:
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photoresist
dispenser
time
speed
thickness
uniformity
particles & defects
vacuum chuck
to vacuum
pump
spindle
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2.
Photoresist Application
• Resist spinning thickness T depends on:
▫ Spin speed
▫ Solution concentration
▫ Molecular weight (measured by
intrinsic viscosity)
• In the equation for T, K is a calibration
constant, C the polymer concentration in
grams per 100 ml solution, h the intrinsic
viscosity, and w the number of rotations
per minute (rpm)
• Once the various exponential factors (a,b
and g) have been determined the equation
can be used to predict the thickness of the
film that can be spun for various
molecular weights and solution
concentrations of a given polymer and
solvent system
3.
Soft Bake
• Partial evaporation of photoresist solvents
• Improves adhesion
• Improves uniformity
• Improves etch resistance
• Improves linewidth control
• Optimizes light absorbance
characteristics of photoresist
4.
Alignment and Exposure
UV Light Source
• Transfers the mask image
to the resist-coated wafer
• Activates photo-sensitive
components of
photoresist
• Quality measures:
Mask
▫ linewidth resolution
▫ overlay accuracy
▫ particles & defects
l
Resist
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4.
Alignment and Exposure
• Alignment errors (many
different types)
• Mask aligner equipment
• Double sided alignment
especially important in
micromachines
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4.
Alignment and Exposure
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4.
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Alignment and Exposure
Contact printing
Proximity printing
Self-aligned
Projection printing : R = 2bmin =
0.6 l/NA
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4.
Alignment and Exposure
• The defocus tolerance (DOF)
• Much bigger issue in
miniaturization science than
in ICs
http://www.newport.com/tutornew/optics/
Optics_Reference_Guide.html
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4.
Alignment and Exposure
Photolithography-DOF
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The defocus tolerance (DOF)
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Much bigger issue in miniaturization
science than in ICs
A small aperture was used to ensure the foreground
stones were as sharp as the ones in the distance.
What you need here is a use a telephoto lens at its widest aperture.
Photolithography-DOF
5. Develop
• Soluble areas of photoresist
are dissolved by developer
chemical
• Visible patterns appear on
wafer
developer
dispenser
▫ windows
▫ islands
• Quality measures:
▫ line resolution
▫ uniformity
▫ particles & defects
vacuum chuck
to vacuum
pump
spindle
6. Hard Bake
 Evaporate remaining
photoresist
 Improve adhesion
 Higher temperature
than soft bake
7. Development Inspection
• Optical or SEM metrology
• Quality issues:
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particles
defects
critical dimensions
linewidth resolution
overlay accuracy
8. Plasma Etch-Or Add Layer
• Selective removal of upper layer of
wafer through windows in
photoresist: subtractive
• Two basic methods:
CF4
▫ wet acid etch
▫ dry plasma etch
• Quality measures:
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defects and particles
step height
selectivity
critical dimensions
• Adding materials (additive)
• Two main techniques:
▫ Sputtering
▫ evaporation
Plasma
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8. Plasma Etch-Or Add Layer
9. Photoresist Removal (strip)
• No need for photoresist following
etch process
• Two common methods:
▫ wet acid strip
▫ dry plasma strip
O2
• Followed by wet clean to remove
remaining resist and strip
byproducts
Plasma
10. Final Inspection
• Photoresist has been
completely removed
• Pattern on wafer matches
mask pattern (positive
resist)
• Quality issues:
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defects
particles
step height
critical dimensions
30
Two variation on the lithography process:
1.C-MEMS Process, 2. LIGA
(b) UV exposure
(a) Spin-coating
photoresist
UV light
Mask
SU-8
Si
(c) Developing
(d) Pyrolysis
SU-8
post
Carbon
post
•Park B, Taherabadi L, Wang C, Zoval J, Madou M. Electrical properties and shrinkage of carbonized photoresist films and the implications for carbon
microelectromechanical systems devices in conductive media. Journal of the Electrochemical Society 2005;152(J136).
•Ranganathan S, McCreery R, Majji S, Madou M. Photoresist-derived carbon for microelectromechanical systems and electrochemical applications.
Journal of the Electrochemical Society 2000;147(1):277-82.
•Wang C, Jia G, Taherabadi L, Madou M. A novel method for the fabrication of high-aspectratio C-MEMS structures. Journal of Microelectromechanical
Systems 2005;14(2):348-58
Two variation on the lithography process:
1.C-MEMS Process, 2. LIGA
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Clean-rooms, Wafer Cleaning
• Yellow light and low particle
size/density curves
• Cleaning steps
▫ RCA1-peroxides and NH3removes organics
▫ RCA2-peroxide and HClremoves metals
• Dry vs. wet cleaning
• Supercritical cleaning-no liquid
phase
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Clean-rooms, Wafer Cleaning
Clean-rooms, Wafer cleaning
• Yield is the reason for the cleanrooms-the smaller the features the
more important the cleanroom
• In the future people will work
outside the cleanroom and only
wafers will be inside the clean
environment
• At universities, modularity (many
different materials and processes)
is more important than yield
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CD and Tg
• CD (e.g. 90 nm) i.e. critical
dimension (the smallest feature
made in a certain process)
• Glass transition temperature,
above Tg the resist picks up dirt
quite readily and the profile
might get degraded
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Making a Mask
• Software Mask
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Moore’s ‘Law’
• Observation and self
fulfilling prophecy --not
a physical law
• Is it running out of
steam?