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E SC 412
Nanotechnology: Materials, Infrastructure, and Safety
Wook Jun Nam
Lecture 12 Outline
• Overview
• Exposure Systems
• Optical Lithography Parameters
• Photoresist
• Lift-off Process
• EUV Lithography / X-ray Lithography
Copyright 2014 by Wook Jun Nam
Optical Lithography Overview
Copyright 2014 by Wook Jun Nam
1. High Resolution
2. Precise Alignment Accuracy
3. Throughput (number of wafers/hr)
Lithography Performance Measure
Copyright 2014 by Wook Jun Nam
1. Reliable (e.g., low
defect density)
2. Reproducible
3. High throughput
But, it is
1. Expensive (e.g., >
35% of chip
manufacturing costs)
2. Complicated (e.g.,
light source,
photoresist, optical
elements)
Why optical lithography is the preferred
technique in microelectronic industries ?
cost of exposure tool
http://www.sematech.org/meetings/archives/litho/euvl/7470/Poste
r/FinalS1/1-CO-01%20Goodwin_IFX%20Poster.pdf
Copyright 2014 by Wook Jun Nam
http://commons.wikimedia.org/wiki/File:Spectrum_of_lithography_lights.PNG
International Technology Roadmap for
Semiconductors (ITRS) Lithography Roadmap
(2007)
Copyright 2014 by Wook Jun Nam
Exposure Systems
Copyright 2014 by Wook Jun Nam
Types of Exposure Systems
Contact Proximity Projection
Copyright 2014 by Wook Jun Nam
The Karl Suss MJB3 The Karl Suss MA6
Public Domain: Image Generated by CNEU Staff for free use
Contact/Proximity Exposure Systems:
Aligners
Copyright 2014 by Wook Jun Nam
Contact Lithography
Optical source
Photomask
Photoresist
/substrate:
• Relatively good resolution:
no gap between a mask
and a resist
• 1x image transfer
• Fast, simple, inexpensive
• Mask
damage/contamination
Copyright 2014 by Wook Jun Nam
Proximity Lithography
Optical source
Photomask
Photoresist
/substrate
• Less mask
damage/contamination
• 1x image transfer
• Fast, simple, inexpensive
• Relative bad resolution:
the gap between a mask
and a resist causes more
light diffraction
Copyright 2014 by Wook Jun Nam
http://www.cnf.cornell.edu/cnf5_tool.taf?_function=detail&eq_id=5>itle=PHO
TOLITHOGRAPHY&area=PHOTOLITHOGRAPHY&cacName=Autostep%20i-
line%20Stepper&labUser=&_UserReference=..
Projection Exposure Systems: Stepper
Copyright 2014 by Wook Jun Nam
Projection Lithography
Optical source
Reticle
Photoresist
/substrate
• No mask
damage/contamination
• Excellent resolution
• 4~10x image reduction:
defect size in a mask is
also decreased
• expensive
Copyright 2014 by Wook Jun Nam
Ideal Systems
• Ideal Exposure System: 100% modulation of light over 0
distance.
• Ideal Positive Photoresist: 100% retention if exposed below
Dcrit, 100% removal if exposed above Dcrit.
Perfect exposure system Perfect positive resist
Copyright 2014 by Wook Jun Nam
Modulation Transfer Function (MTF)
I max
I min
• MTF describes quantitatively
the relationship between
source and Image.
• MTF is a measure of an
exposure tool’s ability to
modulate the intensity of light
at the wafer surface and
decreases with decreasing
diffraction grating period (due
to more destructive
interference).
S. A. Campbell, Fabrication Engineering at the micro- and nanoscale, Oxford University Press, 2013
Copyright 2014 by Wook Jun Nam
Modulation Transfer Function (MTF) -
continued
• Considering a diffraction
grating instead of a single
square aperture, the
Fraunhofer limited (far
field) intensity pattern
(non-normalized intensity
in W/cm2) is shown.
S. A. Campbell, Fabrication Engineering at the micro- and nanoscale, Oxford University Press, 2013
Copyright 2014 by Wook Jun Nam
Optical Lithography Parameters
Copyright 2014 by Wook Jun Nam
Resolution, R (Rayleigh equation)
For small (good) resolution,
k1 , NA , λ
λ : wavelength of light sourcek1 : a constantNA : numerical aperture
θ
photoresist
Projector
lens
substrate
DOF
Copyright 2014 by Wook Jun Nam
Depth of Focus (DOF)
2
2
)(2 NA
kDOF
DOF
DOF
For better DOF,
k2 , NA , λ
Resolution
DOF
Copyright 2014 by Wook Jun Nam
Depth of Focus (DOF)
• DOF is the range that light is in focus and can achieve
good resolution of projected image.
• Smaller numerical aperture, larger DOF
• High resolution, small DOF
• Focus at the middle of photoresist layer
2
2
)(2 NA
kDOF
Copyright 2014 by Wook Jun Nam
Improvement in Wavelength (λ )
• Need develop light source, PR
and equipment
• Limitation for reducing
wavelength
• UV to DUV, to EUV, and to X-
Ray
θ
photoresist
Projector
lens
substrate
DOF
Copyright 2014 by Wook Jun Nam
Spectrum of Light Source (Hg lamp)
http://zeiss-campus.magnet.fsu.edu/articles/lightsources/metalhalide.html
Copyright 2014 by Wook Jun Nam
Spectrum of Light Sources (extended)
http://commons.wikimedia.org/wiki/File:Spectrum_of_lithography_lights.PNG
Light sources should have:
• Short wavelength
• High intensity
• Stable
Copyright 2014 by Wook Jun Nam
Improvement in NA
• High NA optics with very low
aberration levels.
• Bigger diameter of lenses
• Immersion lithography (NA>1).
θ
photoresist
Projector
lens
substrate
DOF
Copyright 2014 by Wook Jun Nam
Chemical Mechanical Polishing (CMP)
http://www.scsolutions.com/chemical-mechanical-planarization-
cmp-controllers-0
Copyright 2014 by Wook Jun Nam
Immersion Lithography
http://www.kitguru.net/components/cpu/dragan/intel-insider-lifts-the-lid-on-advanced-processes-exclusive-interview/
With immersion lenses smaller CDs can be resolved
Resolution enhancement !
Copyright 2014 by Wook Jun Nam
Improvement in k1 : Off Axis Illumination
(OAI)
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
Mask Image with Diffraction
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
Improvement in k1 : Phase Shifting Mask
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
Phase Shift Mask
http://www.tf.uni-kiel.de/matwis/amat/admat_en/kap_5/backbone/r5_3_2.html
Copyright 2014 by Wook Jun Nam
Improvement in k1 : Optical Proximity
Correction
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
Improvement in k1
• Resolution Enhancement Techniques (RET)s
− Self-aligned Double Pattern
− Double Patterning Technique
Litho.-Etch-Litho.-Etch (L-E-L-E)
Litho.-Freezing-Litho.-Etch (L-F-L-E)
(Please read the suggested reading L5-2 for more
details)
− Dual Tone Development (DTD)
− Double Exposure (DE)
• Phase shift mask
Copyright 2014 by Wook Jun Nam
An Example of the RETs
http://spie.org/x35993.xml
Self-Aligned Double Pattern (SADP)
Copyright 2014 by Wook Jun Nam
Photoresist
Copyright 2014 by Wook Jun Nam
1. High Resolution
2. High Etch Resistance
3. Good Adhesion
4. Conformal Coating (good step coverage)
5. Good Contrast and Sensitivity
6. Easy of Removal : i.e., strip
Requirement of Photoresist
Copyright 2014 by Wook Jun Nam
1. Solvent
Controls a resist layer thickness at a certain spin sped.
2. Resin
Material physically maintaining the transferred pattern
after a lithography step.
3. Photoactive molecules
Controls weather UV exposed area is removed or
stayed after a development process step.
Components of Photoresist
Copyright 2014 by Wook Jun Nam
Positive & Negative Photoresists
Positive photoresist Negative photoresist
Copyright 2014 by Wook Jun Nam
Photoresist Sensitivity
• Sensitivity identifies energy
needs of exposure : i.e.
threshold energy
• Dose = light source power
density (mW/cm2) x
exposure time (sec)
ET
No
rma
lize
d R
esis
t T
hic
kn
ess
S. A. Campbell, Fabrication Engineering at the micro- and nanoscale, Oxford University Press, 2013
Copyright 2014 by Wook Jun Nam
Photoresist Contrast
• Contrast is PR’s ability to
distinguish between light
(exposed) and dark
(unexposed) area.
• D100 : the minimum dose for
which the PR will completely
dissolve when developed.
• D0: as the maximum energy
density (mJ/cm2) for which the
PR will not dissolve at all when
developed.Positive PR
S. A. Campbell, Fabrication Engineering at the micro- and nanoscale, Oxford University Press, 2013
Copyright 2014 by Wook Jun Nam
Photoresist Contrast (continued)
D0 = D0, D100 = Df
Norm
aliz
ed
Resis
t T
hic
kne
ss
No
rma
lize
d R
esis
t T
hic
kn
ess
S. A. Campbell, Fabrication Engineering at the micro- and nanoscale, Oxford University Press, 2013
Copyright 2014 by Wook Jun Nam
• Most common g-line and i-line
photoresists are diazonaphtho-
quinone-sulphonates (DNQ)/
novolac based.
• Novolac resin is soluble in
aqueous base due to the acidic
phenolic OH functionality.
• DNQ dispersed in the phenolic
matrix inhibits the dissolution in
base developer by a factor of
10 or more.
Photoresist Reactions (positive resist)
http://www3.ul.ie/~childsp/CinA/Issue66/TOC25_Discworld.htm
Copyright 2014 by Wook Jun Nam
• UV irradiation of photoactive compound liberates N2 from
the C ring, forming a highly reactive carbon site
(carbene).
• The carbene intermediate undergoes the Wolff
rearrangement (C atom moves outside of the ring, O2 is
then covalently bonded) to ketene.
Photoresist Reactions (positive)
http://www3.ul.ie/~childsp/CinA/Issue66/TOC25_Discworld.htm
Copyright 2014 by Wook Jun Nam
• Ketene reacts with ambient water in which double bond
of external C is converted to a single bond with an OH
group –carboxylic acid (RH Cleanroom Stability Crucial).
• Consequently, the exposed areas of the resist film
dissolve much faster than the unexposed in an aqueous
base developer.
Photoresist Reactions (positive)
http://www3.ul.ie/~childsp/CinA/Issue66/TOC25_Discworld.htm
Copyright 2014 by Wook Jun Nam
• Negative resist contain a cross linker which is activated
during exposure and thermally activated crosslinks the
resin during a subsequent baking step.
• Higher exposure doses increase the degree of cross
linking, which improves the thermal and chemical
stability, and makes an impact on the attained resist
profile after development.
Photoresist Reactions (negative)
Copyright 2014 by Wook Jun Nam
• Resist molecules are photoacid generators (PAG).
• PAGs produce acid when exposed to light.
• Acid acts like a catalysts in dissolving the molecule
chains.
Chemically Amplified Photoresist
http://www.smtbook.com/popups/resist.htm
Copyright 2014 by Wook Jun Nam
Post Exposure Bake (PEB)
SILVACO ATHENA Simulation
PEB :45 min at 115CNo PEB
More exposure More exposure
Standing waves
http://www.utdallas.edu/~walter.hu/teaching/EE6322/Lecture%206%20Lithography%202.pdf
Copyright 2014 by Wook Jun Nam
Post Exposure Bake (PEB)
http://www.cnf.cornell.edu/cnf_process_photo_resists.html
Copyright 2014 by Wook Jun Nam
Lift-off Process
Copyright 2014 by Wook Jun Nam
1. Thermal Stability
2. Chemical Stability
3. Good Adhesion
Requirement of Lift-off Resist
Image resist
Lift-off resist
Copyright 2014 by Wook Jun Nam
Lift-Off Process (Process Steps)
Lift-off using a single resist layer Lift-off using double layers
Copyright 2014 by Wook Jun Nam
Lift-Off Process (Rabbit Ear)
Lift-off using a single resist layer Lift-off using double layers
Copyright 2014 by Wook Jun Nam
Lift-Off Process (Photoresist)
Surface modified single PR layer Double PR layers
Copyright 2014 by Wook Jun Nam
Lift-Off Process (Inspection)
Optical Microscope Image FESEM Image
Copyright 2014 by Wook Jun Nam
EUV Lithography / X-ray Lithography
Copyright 2014 by Wook Jun Nam
Spectrum of Light Sources
http://commons.wikimedia.org/wiki/File:Spectrum_of_lithography_lights.PNG
Light sources should have:
• Short wavelength
• High intensity
• Stable
Copyright 2014 by Wook Jun Nam
http://commons.wikimedia.org/wiki/File:Spectrum_of_lithography_lights.PNG
International Technology Roadmap for
Semiconductors (ITRS) Lithography Roadmap
(2007)
Copyright 2014 by Wook Jun Nam
Light Source Shorter than 193nm
• Nature of light is too absorptive:
– Lenses, pellicles, photomasks (reticles) are the sources
of the light wastes.
– Pellicle is a protective layer coated on a
photomask/reticle.
– Resists designed for 248nm or 193nm is too absorptive
for 157nm or shorter wavelength.
• No more light transmission for pattern transfer: reflection
is used for 13.5nm EUV technology !!
Copyright 2014 by Wook Jun Nam
Extreme UV Exposure Tool
No more transmission !
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
Bragg Reflector
• Called as a distributed Bragg reflectors (DBR) or Bragg
mirror.
• It consists of alternating layers of high- and low-
refractive index films, each having approximately a
quarter-wavelength thickness.
• Resonant reflectivity is achieved when the light whose
wavelength is close to four times the optical thickness of
the layers, the many reflections combine with
constructive interference, and the layers act as a high-
quality reflector.
http://en.wikipedia.org/wiki/Distributed_Bragg_reflectorNanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
Bragg Reflector (continued)
Copyright 2014 by Wook Jun Nam
Bragg Reflector (continued)
• Alternating layers of Mo and Si are reported to offer the
highest reflectivity at the wavelength of about 13nm.
• 50 pairs of Mo(2.76nm)-Si(4.14nm) multilayer mirror
provides higher than 70% (numerical modeling) and ~68%
(experimental) of reflectivity around 13nm wavelength.Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
EUV Masks
• EUV mask is fabricated on a EUV mirror (Bragg reflector).
• Patterned absorber layer (e.g., Al, Cr, Ta, W) is transferred
using ebeam lithography and reactive ion etching (RIE).
• Reduction ratio of the pattern is about 4:1.Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
Requirements for EUV Resist
• High Sensitivity:
– low intensity of source power
– Chemically amplified resist is desirable
• High Resolution
• Low Edge Roughness
– Chemically amplified resist causes high line edge
roughness
• Low Outgassing of Resist
– High energetic light exposure generates gaseous
molecules from a resist which will contaminate EUV
optics (e.g., mirrors).
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
X-ray Lithography
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
X-ray Lithography
• X-ray is the radiation wavelength spanning from 1nm at
the soft X-ray ends toward 0.1nm of hard X-ray.
• X-ray can penetrate majority of materials
• Materials (e.g., Au, W, Ta) with high atomic number
should be used for mask
• There is no refraction in X-ray, so the image in a mask is
transferred to a resist with 1:1 ratio.
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
X-ray Lithography Challenges
• X-ray mask consists of a membrane of low atomic number
material (e.g., Si, SiC) with patterned high atomic number
material as the absorber on top.
• Typical thickness of the Si membrane and absorber layer is
1~2um and 300~500nm, respectively.
• Because of the poor mechanical strength of thin-membrane
mask, the proximity gap is required.
• It is 1:1 ratio pattern transfer
• Local stress caused by heavy metal absorber and internal
stress of supporting membrane can influence on the pattern
resolution Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
X-ray Lithography Challenges (continued)
• X-ray irradiation generates photoelectrons and Auger
electrons in a resist.
• Scattering of these low energy electron in a resist results in
very effective interaction with resist polymer, and causes
degradation of image resolution.
• It is very hard to control the gap between a mask and a
substrate.
• X-ray also has diffraction effects. Highly coherent x-ray
source (e.g., synchrotron radiation source (SRS)) is
needed.
Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)
Copyright 2014 by Wook Jun Nam
Synchrotron Radiation Source
http://sni-portal.uni-kiel.de/kfs/Infos/Quellen/synchrotronradiationsource.php
Copyright 2014 by Wook Jun Nam
What Technology will Continue the Moore’s
Law ?
• Ebeam lithography
• Nano-imprinting lithography
• Extreme Ultra Violet (EUV) lithography : 13.5nm
• Ion Beam lithography
• X-ray lithography
• Something new ??
Copyright 2014 by Wook Jun Nam
Lecture 12 Outline
• Overview
• Exposure Systems
• Optical Lithography Parameters
• Photoresist
• Lift-off Process
• EUV Lithography / X-ray Lithography
Copyright 2014 by Wook Jun Nam