Transcript Lecture 14 - Bama.ua.edu

Advanced Optical Lithography
Lecture 14
G.J. Mankey
[email protected]
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Resolution Limit
•
The minimum resolution of an optical element is given by the Raleigh criterion:
resolution  k1
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
NA
The basic rule of thumb is that the minimum line width achievable is basically
equal to the wave length of the incident radiation.
This limit is for classical optics, there are three "tricks" which may be applied to
lower this limit:
– Interference lithography
– Phase mask lithography
– Near-field lithography
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Interference Lithography
• Two coherent beams of radiation from a laser interfere to make periodic fringes
with a smaller separation than the optical wavelength.
• By optimizing the sensitivity of the resist, features as small as 1/10 of the
optical wavelength can be obtained.
ref: http://www.el.utwente.nl/smi/
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Interference Lithography Setup
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For large area exposures, the
apparatus is complicated, since
during the exposure, the system must
be mechanically and optically
stabilized to sub-wavelength
precision.
Laser wavelengths for an Ar laser of
350 nm have been applied to produce
100 nm features such as dots and
holes.
We tried this without stabilization-small vibrations completely smear
out the pattern.
ref: P.W. Konkola et al.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Achromatic Interference Lithography
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The mirrors are replaced by gratings to
selectively diffract the light to the
exposure spot which relaxes the
requirements on the coherence of the light
source.
This technique has been applied to
produce 13 nm wide posts of PMMA on
Si using an excimer laser.
This system is under development by
Michael Walsh, James M. Carter, Robert
C. Fleming, Timothy A. Savas, Dr. Mark
L. Schattenburg, and Professor Henry I.
Smith at MIT.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Phase Mask Lithography
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Instead of an intensity mask, light travels
through a polymer mask and accumulates
an additional phase relative to light that
travels through the air gap.
At the corners of the phase mask in
contact with the resist, light interferes
destructively and there is a dip in
intensity.
This can be quite sharp, possibly less
than 1/4 the wavelength of the light, so
100 nm features are possible.
ref. J. Aizenberg et al. Appl. Phys. Lett 71, 3733 (1997).
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Phase Mask Process
Photoresist
Cure PDMS, remove
elastomer mask from master
Si
Photolithography
Expose through
elastomer mask
RIE
PDMS casting
Develop
PDMS
Ref: H. Jiang et al., Spring MRS Meeting ‘99
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Phase Mask Needles
AFM
•
MFM
200 nm permalloy needles were produced with a phase mask. A limitation discovered
was the shape depended on the quality of the initial optical mask used to produce the
phase mask.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Optimized Phase Mask Process
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With a better mask, a periodic array of 300 nm wires was produced.
Optimizing the exposure time using the phase mask resulted in a minimum
linewith of 150 nm.
After ion milling a 150 nm wide wire remained.
Contacting this wire with subsequent processing was quite a challenge.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Near Field lithography
Light illumination
•
Membrane
mask
Vacuum
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Membrane
NiCr absorber
d<<
An intensity mask with subwavelength gaps is placed in close
proximity to the resist.
The evanescent wave emanating
from the gap exposes the resist.
This evanescent wave occurs only in
the near field at a distance much less
than the wavelength of the light.
Photoresist
ref: OPTICAL NEAR FIELD NANOLITHOGRAPHY.htm
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Limitation of Near Field Lithography
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The evanescent field penetrates
only a few tens of nanometers
into the resist.
A two-layer resist process may be
utilized to make this work.
The two-layer process is
frequently used in e-beam and
AFM lithography which will be
discussed next.
Light
Metal Slit
Mask
~10’s nm
Resist
Substrate
Evanescent near field region
Figure 2. Evanescent near field region
ref: OPTICAL NEAR FIELD NANOLITHOGRAPHY.htm
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center