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Lithography In the Top-Down
Process – New Concepts
• Learning Objectives
– 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
(exciplex) 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
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
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 has been
studied for some time.
• An electron beam is
exceptionally
“narrow”, and does
not require a mask
• λ=h/P
Phase Shift Mask
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
Nano-Imprint Lithography
• Concept – To use a “stamp” of precise
dimension to create features in resist
• Advantages
– High Throughput
– No issues in diffraction
– No secondary emission
– Can be carried out in non-vacuum
environment
Nano-Imprint Lithography (2)
• T-NIL (Thermal NanoImprint 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
can be removed
http://www.nilt.com/Default.asp?Acti
on=Details&Item=219
UV-Nano Imprint Lithography
• UV-NIL (Ultra-violet
Nano-Imprint
Lithography)
– A UV sensitive resist is
used
– The stamp must be
UV-transmissive
– UV light is applied
through the stamp
– Pressure is applied to
“stamp” imprint
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
microcscope
cantilever writes a
pattern in resist
– Extremely precise
– “Scratching the
surface”
http://www.pacificnanotech.com/afm
-modes_active-modes.html