Large-Area Interference Lithography Exposure Tool

Large-Area Interference Lithography
Exposure Tool Development
John Burnett1, Eric Benck1 and James Jacob2
1Physical
Measurements Laboratory,
NIST, Gaithersburg, MD, USA
2Actinix, Scotts Valley, CA
2011 International Symposium on Lithography
Extensions, Miami, October 20-21, 2011
Maskless Hybrid IL Concept
LONG COHERENCE LENGTH FAR-UV LASER
PLMA
Seeder
LOW COST NANO-SCALE ASICS
DARPA GRATE
NLO
Project Goals
Demonstrate feasibility of large-size interference lithography
Demonstrate:
1) New 197 nm laser appropriate beam characteristics to enable
die-field size patterns (~33 mm x 26 mm). The issues are
power, spatial mode quality, band width, and stability.
2) Can deliver beams to wafer with phase properties to enable
pattern fidelity.
3) Metrology concept to control interference pattern pitch and
pattern registration. Concept based on moiré patterns.
4) Develop grating fabrication tool verify complete concept.
IL Approach
Mach-Zehnder design based on plate beam splitter with
large-diameter beam (40 mm)
Advantages:
Large interference pattern possible (~33 26 mm).
Large DOF.
COTS optics meet requirements (no special gratings).
Challenges:
Interference from opposite sides of beam – requires
high spatial coherence.
Temporal Coherence:
Large pattern requires long temporal coherence Lc.
Lc = /
Large Lc poses coherent scattering issues (unwanted
@ 197 nm:
= 0.130 pm
reflection patterns, coherent defect scattering).
Lc = 30 cm
Interference pattern is ray mapping rather than imaging
– need to polish P-V (not RMS).
Laser Concept
Generating narrow-band, high-power, sub 200 nm light
• Use a stable, coherent, tunable CW IR fiber laser seeder
• Chop into two-nanosecond pulses at 1-4 MHz rep rate
• Amplify in large mode area fiber amps
• EO phase modulator to compensate chirp pulses
• Frequency upshift IR to UV using efficient non-linear
optical processes
Infrared Front End of Light Source
Diode
Pump
<10 W
Pulse timing, chirp compensation
and drive electronics
BP filter
CW Fiber Laser
1055 or 1550 nm
25 mW, 5 kHz LW
Isolator
Fiber
pre-amp
Fiber power amp
Modulator
1-2 ns PW
1-4 MHz PRF
Phase
compensator
[Approved for Public Release, Distribution Unlimited]
•
2.5 W Avg Power
•
1 MHz PRF, 2 ns PW
•
400 MHz BW
197 nm System
Fiber Laser 1055 nm
Freq
Doubler
Freq
Tripler
Freq
Mixer
Freq
Mixer
•
•
•
•
Fiber Laser 1550 nm
Freq
Doubler
197 nm
250 mW
0.75 ns
0.13 pm BW
High conversion efficiency relaxes dependence on fiber
front ends to produce high peak power, which in turn
reduces the amount of SPM needed to be compensated
[Approved for Public Release, Distribution Unlimited]
Beam Shaping
Must convert small  Gaussian laser beam to uniform collimated beam.
Keplerian-type anamorphic telescope
collimated beam with
planar wavefront
Gaussian input
beam
first lens redistributes rays to
transform the intensity profile
Gaussian input
beam distributions
(before homogenizing lens)
Actinix 197 nm laser
Beam diam. at 1st lens: ~1 mm
Divergence: ~0.7 mrad
(FWHM -1/2 angle)
second lens corrects the wavefront
distortion due to the first lens
Ray trace model for 197 nm laser
homogenizing lens
(asphere)
Flat top output distribution
(before collimating lens)
40 mm
Collimating/wave flattening
Aspheric group
Beam Shaper Aspheric Surfaces
collimated beam with
planar wavefront
Gaussian input
beam
Issues
Geometric Optics – 2 convex aspheric surfaces can be used to exactly
transform a Gaussian beam to an arbitrary output profile, e.g. top hat.
Top hat profile gives substantial diffraction due to edges
– affects beam parallelism and uniformity on propagation < 1m.
Need to roll off output profile, e.g., Fermi-Dirac.
R0
governs range,
governs roll off
Details depends on precise characteristics of laser – to determined.
Beam shaper ray-mapper - light from each source point tracks through
1 point on asphere surface must figure asphere to P-V specifications !
For interferometer, figure errors result in fringe positioning errors.
For control of fringe positions:
Angle tolerance ~ ~0.05 mrad, P-V tolerance ~ 50 nm.
Interferometer - Modeling
Realistic ray trace model
Input beam from beam shaper.
40 mm
deformable mirror
Actinix 197 nm
solid-state laser
fine frequency control
- pitch control
beam shaping
optics
plate beam splitter
beam splitter
compensator
For Lc = 30 cm in model:
no significant loss in image
contrast at edge of field (16.5 mm).
CCD
imaging
optics
HR mirror
- pitch control
70 nm pitch
coupling
prism
HR mirror
- pitch control
Started modeling use of adaptive
optics to correct effects of
aberrations on pattern.
wafer/
metrology grating
33 mm
Interference pattern
at edge of field
Trace beam to wafer plane
- gives 35 nm HP interference
pattern.
26 mm
How do you insure pattern has
correct pitch, orientation, position?
Real-time pattern metrology.
Interference pattern
at center of field
Moiré Interferometry – Concept
Input Beam 2
Input Beam 1
• 2 beams with half angle
pattern P= /2 sin
1, -1
2, +1
• Superpose on grating with spacing d
• Condition that 1st orders diffract
normally: d= /sin or
d = 2p
• Two waves have constant
phase relation
d = grating pitch
If conditions are not precisely met, a moiré beat pattern is produced.
Modulation envelope with period pmod = -p(p/ p).
Basis for metrology scheme to characterize pitch deviations from reference grating
and correct with feedback control.
Moiré Interferometry
40 mm
deformable mirror
Actinix 197 nm
solid-state laser
fine frequency control
- pitch control
Projection of interference pattern on
reference grating with line spacing = 2
gives ±1 diffraction orders in vertical
direction for both beams.
plate beam splitter
beam splitter
compensator
pitch
Image beams on CCD. If interference
pattern/grating lines not commensurate
moiré pattern.
Moiré pattern gives deviation from perfect
overlay of interference pattern on reference
grating.
Eliminate moiré nulls with stage rotation,
translation, control, and adaptive optics.
Can be used in feedback mode to correct
overlay error.
beam shaping
optics
CCD
imaging
optics
HR mirror
- pitch control
coupling
prism
HR mirror
- pitch control
wafer/
metrology grating
reference grating w/
2 interference pitch
at wafer plane
Moiré Interferometry - Modeling
Quantitative simulation of optical effect projection of 197 nm
interference pattern (35 nm HP) on reference grating
=8.7 rad,
lens aberration
=8.7 rad, =3 pm,
no lens aberration
=3 pm,
Remove wavefront
distortion w/ adaptive
optics
=0.87 rad,
Reduce horizontal
component of moiré
beats w/ adjustments
=0.1 pm,
=3 pm,
=0 rad,
=0 pm
Reduce vertical
component of moiré
beats w/ stage
rotation
=0.87 rad, =0.1 pm,
stage translation=35 nm
Establish correct phase
with stage translation
=0 rad,
Maintain no moiré
nulls with feedback
control
Complete elimination of moiré nulls guarantees registration across field!
Moiré Interferometry – Measurements
266 nm MZ Interferometer with grating at interference plane.
mirror 1
spatial filter/beam expander 50 mm
266 nm laser
imaging optics
2400 gr/mm grating
(416 nm pitch)
50-50 beam splitter
CCD
Beams projected
at 39.7 from normal.
mirror 2
Images of interference pattern projected on grating for various mis-registrations.
0,
0
Moiré null region
0,
0
0,
reduced
0,
reduced
0,
0
1 cm
interference pattern (104 nm half pitch) commensurate with grating lines.
Verifies basic metrology concept at 266 nm
Summary
Program underway to explore viability of full-field Interference Lithography.
Actinix/Sandia building the laser which can be used for IL and inspection projects.
Have modeled some of the key optical issues – coherence, beam shaping.
Have devised viable approach to metrology to ensure registration.
Have modeled the metrology concept and demonstrated the principle at 266 nm.
Plan and Prospects
• NIST continue building metrology system; Actinix completing laser breadboard.
• Incorporation into lithography demonstration tool – pending funds and partners.
• We strongly believe the concept is viable for low volume, low cost nano-fabrication.
Acknowledgments
We gratefully acknowledge Michael Fritze for pioneering the hybrid lithography concept
and supporting of our efforts, John Hoffnagle for key contributions with the beam shaping
and coherence length modeling, and Darrell Armstrong for his work on the fiber laser
development. This work is supported in part by DARPA contract W91CRB-10-C-0080.