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Lithography in the Top Down Method
New Concepts
Lithography In the Top-Down Process
New Concepts
• Learning Objectives
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To identify issues in current photolithography
To quantify the needs of nanomanufacturing
To define improvements in photolithography
To explore new lithography processes
To define the limitations of these new processes
in top-down nanomanufacturing
What are the limitations of current
photolithography processes?
• Light sources from traditional mercury
vapor lamps have little “deep UV” spectra
• Finer feature sizes require shorter
wavelength sources
• Photoresist must be sensitive to appropriate
wavelengths of light
• Lenses and optical components have
limited numeric aperture
Why Shorter Wavelengths?
• Minimum Feature Sizes are dictated by the
following relationship
F = K (λ/NA)
Where F = Feature Size in
nM
λ = Wavelength (nM)
K = Process Constant
NA = Numeric Aperture
Shorter Wavelength Sources
• Replace the mercury vapor lamp an excimer laser
source with shorter wavelength emission
– ArF – 193 nM – Shorter wavelength than so-called “deep
UV” peak of 248nM
– F2 Laser – Low output but at 157nM
• Matching photoresist that is sensitive to this spectra
is also required.
• Laser sources under development – 13.5 nM!
(extreme UV or EUV range)
Numerical Aperture
• Light passing through the mask will be
subject to diffraction. The numerical
aperture of the lens used determines its
capability to bring the diffracted pattern into
a single point of focus.
• NA = n sin θ where n = index of refraction of
the media in which the lens is working (air)
and θ is the angular spacing between
objects making up the image
Numerical Aperture (2)
• sin θ = 1.22 λ/D where θ
is the angular spacing
between objects and D
= diameter of the lens
• A larger diameter lens
helps, but is difficult to
manufacture
• Depth of Field Issues
limit the use of larger
diameter as a solution
Increased NA reduces depth of field
Source: MATEC Module 41
Improving the Index of Refraction
Improving the Photomask
• Sharp edges in photomasks are not well
reproduced as feature
sizes shrink
• Optical proximity
correction techniques
put borders on corners
and edges to correct
for this
Phase Shift Masking
• More complex mask
includes “trim mask”
• Destructive
interference optical
effects attenuate
patterns
• Effectively appears to
be higher resolution
http://www.asml.com/asmdotcom
Double Patterning
• Adjacent features are on
two masks
• Avoids Rayleigh
constraints on resolution
• Requires double the
number of steps
• Requires additional
masks and critical
alignment of masks
Practice Questions
Click once for each question.
1. What limits feature sizes in photolithography?
Wavelength of the light source used
Numerical aperture of the lens
2. What effect causes blurring in photomasks?
Diffraction of the light source
3. What is the limitation that occurs when
numerical aperture is increased?
Depth of field is decreased
Alternative Exposure Methods
Electron Beam Lithography
• Use of exposure
sources other than UV
light have been studied
for some time.
• An electron beam is
exceptionally “narrow”,
and does not require a
mask
• Low throughput limits
use in manufacturing
Electron Beam Lithography (2)
• E-beam lithography
also serves as a tool
for mask making
• Throughput is not an
issue in this case,
since the masks are
made once, and used
many times.
• Sub-50 nM feature
sizes are possible
X-Ray Lithography
• Synchrotron radiation
sources can be used
• Masks use “absorber”
materials on a
membrane
• X-rays pass through
the membrane
• PMMA photoresists
can be used
X-Ray Lithography Issues
• Spacing, mask
dimension, and
wavelength are critical
So-called “sweet spot”
will provide small
feature size for a given
wavelength exposure
and defined mask
feature
[email protected]
Nano-Imprint Lithography
• Concept – To use a “stamp” of precise
dimension to create features in resist
• Advantages
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High Throughput
No issues in diffraction
No secondary emission
Can be carried out in non-vacuum environment
Nano-Imprint Lithography (2)
• T-NIL (Thermal Nano-Imprint
Lithography)
– PMMA resist similar to that
used in X-ray lithography is
spin coated onto surface
– Stamp is pressed into
contact with surface
– Substrate is heated to glass
temperature of resist
– Pressure is applied to
“stamp” imprint
– Substrate cools and stamp
Http://www.nilt.com/Default.asp?
can be removed
Action=Details&Item=219
UV-Nano Imprint Lithography
• UV-NIL (Ultra-violet
Nano-Imprint
Lithography)
– A UV sensitive resist is
used
– The stamp must be UVtransmissive
– UV light is applied through
the stamp
– Pressure is applied to
“stamp” imprint
Http://www.nilt.com/Default.asp?
Action=Details&Item=219
Issues in Nanoimprint Lithography
• Alignment of layers can be more difficult
than with projection lithography
• “Proximity effect” of having large stamp
areas near small features may cause uneven
feature sizes
• Residual layer thickness and profile may
vary
• Patterned areas may “stick” to stamp
AFM Probe Lithography
• An atomic force
microcscope cantilever
writes a pattern in
resist
– Extremely precise
– “Scratching the surface”
http://www.pacificnanotech.com/afm
-modes_active-modes.html
AFM “Dip Pen” Lithography
• An atomic force
microscope
cantilever writes a
pattern on the
substrate
– Extremely precise
– Deposit inks or
conductive coatings
http://nanoink.net/WhatisDPN.htm