Nanoimprint Lithography and Applications - Wei Wu Department of Electrical Engineering
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Nanoimprint Lithography
and Applications
Wei Wu
Department of Electrical Engineering
University of Southern California
wu.w@usc.edu
IEEE Nanotechnology Council
The IEEE Nanotechnology Council (NTC) is a
multi-disciplinary group whose purpose is to
advance and coordinate work in the field of
Nanotechnology carried out throughout the IEEE in
scientific, literary and educational areas.
• The IEEE Nanotechnology Council is part of Division I – Circuits and
Devices and is made up of 22 member societies
• Publications:
• IEEE Transactions on Nanotechnology (T-NANO)
• IEEE Nanotechnology Magazine (INM)
• IEEE Nanotechnology Express (ENANO)
• IEEE Transactions on NanoBioscience (T-NB)
• IEEE Transactions on Molecular, Biological, and Multi-Scale
Communications (T-MBMC)
• IEEE Journal on Exploratory Solid State Computational Devices and
Circuits (xCDC)
• And several conferences
http://sites.ieee.org/nanotech/
Outline
• Motivation
• Nanoimprint lithography
• Helium ion beam lithography
• Nanoimprint lithography + Helium ion beam
lithography
• Color reflective display based on nano-
photonics
• Working principle
• Fabrication
• Characterization
• Summary
Single-digit Nanometer Era
2023
9.5
*Lithography for flash, ITRS roadmap 2013 update
Photolithography
• Process used to transfer a
pattern from a photomask to
the surface of a substrate
• Formation of images with
visible or ultraviolet radiation
in a photoresist
• Most widely used lithography
system.
Source: Britney Spears guide to Semiconductor Physics
http://britneyspears.ac/lasers.htm
Problem of Photolithography: of Light Used
Has Not Scaled with Resolution
Moore’s law feature size shrinks 0.7 times every two years
λ 700
Rayleigh's equation
Feature_ Size k1
I-line
NA 600
DUV
193 nm
500
Wavelength (nm)
400
300
200
100 157 nm is dead
0
Quartz is opaque
700 600 500 400 300 200 100 0
Node/Half-pitch (nm)
Next generation lithography (NGL) tools:
Extreme UV lithography (EUV)
–Extremely expensive
(Light source, complex optical system, expensive and
fragile mask, defects count)
X-ray lithography
–Expensive light source (synchrotron preferred)
–Mask material
E-beam direct write lithography (EBL)
–Extremely slow (serial process)
E-beam projection lithography (EPL)
–Mask material
–Distortion due to heat
Nanoimprint Lithography (NIL)
Mold
Resist
Substrate
Chou, Krauss, and Renstrom, APL, Vol. 67, 3114 (1995); Science, Vol. 272, 85 (1996)
M. Colburn, A. Grot, G. Wilson’s et al SPIE 2000
Nanoimprint Lithography (NIL)
Mold
Resist
Substrate
Substrate
Nanoimprint Lithography (NIL)
Mold
Resist
Substrate
SubstrateNanoimprint Lithography High resolution -not limited by wavelength High throughput -parallel process Low cost
UV-curable NIL with Double-layer Spin-on Resist
1. Prepare substrate, spin transfer 4. Mold and substrate
layer and liquid imaging layer on separation
2. Alignment 5. Residue layer and
under layer etching
UV
3. Press and exposure 6. Metal evaporation and
lift-off or etching
W. Wu, G. Y. Jung, D. L. Olynick, et al., Applied Physics a-Materials Science & Processing 80, 1173 (2005).“Ls” in Resist at 12 nm Half-pitch by NIL
5nm
Sharp
Isolated line corners
Dense lines
W. Wu, W. M. Tong, J. Bartman, et. al., Nano Lett. 8 (11), 3865-3869 (2008).Nanoimprinted Dot array in Resist at 10 nm Half-pitch
“Ls” in Resist at 8 nm Half-pitch by NIL
Low Information Content Patterns Using a Carbone nanotube mold Using a supper lattice mold Hua et al, Nano Lett., 2004 Austin et al, Nanotech. 2005 Great demonstration of resolution, but they have low information content and not enough for most applications
How to Achieve Better Resolution than Electron
Beam Lithography?
Smallest half-pitch patterned by EBL in HSQ: 4.5 nm
Yang et al, J. Vac. Sci. Techno. 2009The Limiting Factor of EBL
Resolution limiting factors of electron beam (with a perfect
resist):
Spot size Beam scattering Second electron
(forward and backward) generation
~ 4 nm Proximity effect 10 nm ~ micronsOverall beam spot diameter
2 2 2 2
d d d d d
g s c d (assume no astigmation)
dv dv: virtual source diameter
dg M: demagnefication
M
1 Spherical aberration
ds Cs 3
2
V Chromatic aberration
dc Cc
V
1.2
dd 1.22 , nm Diffraction
VBeam spot: 3.5 Å
~$2MLatest Helium Ion Microscope
Rearranged tungsten atoms
Typical trimer image on HP HIM
for bright helium ion source
B.W. Ward et al, J. Vac. Sci. Technol. B, 2006
0.24 nm record
imaging
resolution was
demonstrated
using HIM
From Carl ZeissOverall beam spot diameter
2 2 2 2
d d d d d
g s c d (assume no astigmation)
dv dv: virtual source diameter
dg M: demagnefication
M
1 Spherical aberration
ds Cs 3
2
V Chromatic aberration
dc Cc
V
1.2
dd 1.22 , nm Diffraction
VHe Ion is Scattered Over Shorter Ranges 35 KeV Electrons 35 KeV He+ W.-D. Li, W. Wu and R. S. Williams, Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 30 (6), 06F304 (2012).
He Ion: Much Less Proximity Effect
Beam scattering
Small spot + little proximity effect --> better beam for lithography!Helium Ion Beam Lithography V. Sidorkin et al, J. Vac. Sci. Technol. B, 2009 D. Winston et al, J. Vac. Sci. Technol. B, 2009
HIBL for Sub-5 nm Patterning on HSQ Resist
5 nm half pitch 4 nm half pitch
4 nm
5 nm half-pitch
half-pitch
W.-D. Li, W. Wu and R. S. Williams, Journal of Vacuum Science & Technology B:
Microelectronics and Nanometer Structures 30 (6), 06F304 (2012).He Ion: Much Less Proximity Effect
Beam scattering
•Small spot + little proximity effect --> better beam for lithography!
Issues with He ion beam:
•Slow (low beam current)
•He ion beam is not for every substrate (He bubble formation)Helium Bubbles
Combination of HIBL and NIL to Reach Single-digit Design at
Low-cost and High Throughput
1. Fabricate NIL template using a scanning helium ion
beam
Expecting superior resolution compared with EBL based
fabrication
2. NIL to transfer high-resolution patterns
Molecular resolution; low cost; and high throughput
3. Device fabrication at sub-10 nmNanoimprint Using HIBL Template
Template after HIBL HSQ
and development
Silicon
Mold
Short exposure to O2 plasma and
release
coating of mold release agent
agent
UV nanoimprint using
HIBL template UV-curable
NIL resist
Fused silicaImprinted Resist with 4-nm Half-pitch Lines
12 nm half 5 nm half
pitch pitch
4 nm half pitch
Sample coated with 2 nm platinum
and imaged under XL30 SEM at
20kV
W.-D. Li, W. Wu and R. S. Williams, Journal of Vacuum
Science & Technology B: Microelectronics and Nanometer
Structures 30 (6), 06F304 (2012).5 nm Half Pitch Lines Patterned in 10 nm Thick Chromium
He+
beam
20 nm Si3N4 membrane
10 nm chromium
8 nm half 5 nm half
pitch pitchNanoimprint of 3D Patterns
• NIL duplicates 3-D nanostructures
into a transparent UV-cured
polymer
• High aspect ratio and resolution
patterning (Patterning on Non-flat Substrate
200nm pitch gratings on the surface of an optical
fiber with 125 μm diameter
Li, Z. W., Gu, Y. N., Wang, L., Ge, H. X., Wu, W., Xia, Q. F., . . . Williams, R. S.,
Nanoletters 9(6), 2306-2310 (2009)Hybrid Soft PDMS/Hard Crest Mold for UV-NIL
Elastic polymer
Rigid polymer
Anti-adhesion layer
Mold Imprinted resistMold
Roll-to-roll Nanoimprint for Low-cost
Large-Area Patterning
Home-built roll-to-roll NIL system in my labCost Study Example: Bit-patterned Magnetic Media According to the study by hard drive industry: • NIL is the only economically practical approach to pattern BPM • The cost of patterning both side of a 3.5” disk is less than $2 in mass production
Nano-crossbar
Circuits
Color Reflective Display
Based on NanophotonicsReflective Display
Ambient light only
• Sunlight readability
• Printing-like
• Low power consumption
https://kindle.amazon.com/
No commercial color reflective
display on marketDisplay Architectures
RGB additive model
• Red, green and blue sub-pixels
• Parallel arrangement
A single pixel
33 %
Color filter inside
LCD RGB Display Single-layer architectureElectrophoretic Color Display
RGBW
Color filters
Brightness
Not working yet
Kwak, Y., Park, J., & Park, D. S. (2008).Mirasol Display
Qualcomm Mirasol
Interferometric Modulation
MEMS
Brightness
https://www.qualcomm.com/products/miraso
l/technologyStack Architectures
Three-layer Architecture
Ideal efficiency: 100 %
100 % Color filters inside
Brightness
• Performance of filters
Saturation (Gamut)
• Reflection spectra of filters
Contrast
• Method to tune reflectance
Three-layer architectureIdeal Reflection Spectra
Brightness
• Reflectance
Reflectance
• Efficiency 1
• Loss
Saturation (Gamut)
• overlapping 0
400 500 600 700
Contrast Wavelength (nm)
Reflection spectra of color filtersElectrowetting Display
Three-layer
Colored ink
Challenge?
• No Ink has
square
function like
spectrum
Electrowetting Displays A paper by Johan Feenstra & Rob HayesHigh Contrast Grating
Resonance
Re-radiate
Interfere
HCG
A surface-emitting laser incorporating a
high-index-contrast subwavelength
Grating (HCG)
Schematic diagram of GMR device
A.S.P. Chang, H. Tan, S. Bai, W. Wu, Z. Yu, S.Y. Chou, “Tunable External Cavity Laser With a Liquid-Crystal Subwavelength
Huang, M. C. Y., Y. Zhou, et al. (2007). "A surface-emitting laser incorporating a high-index-
Resonant Grating Filter as Wavelength-Selective Mirror”, Photonics
contrast subwavelength grating." Nat Technology Letters, IEEE, 19(2007) 1099-1101
Photon 1(2): 119-122.
M. Shokooh-Saremi, “Physical basis for wideband resonant reflectors,” Optics Express, vol. 16, 2008.Resonance Grating Reflector 2D grating Polarization independent Double-layer pillar High index contrast Less angle sensitivity
Red, Green and Blue Filters
Three-layer architectureGamut Chart Color reproduce Comparable with IP 5 Saturation of red color
Operation to Tune the Reflectance Resonance • Grating dimensions • Index contrast Change background index • Electrowetting
Electric Field Distribution Background index = 1 Background index = 1.8
Reflection Cancellation Average reflectance < 5 %
On & Off
ON
OFFReflection vs Background Index
1.32
WaterFabrication Flow Chart
Interference Mold Linewidth
Lithography 1D Duplication 1D Adjustment 1D
Si grating PDMS PDMS
in Si mold mold
Mold Double
TiO2 Duplication Imprint
Growth TiO2 2D 2D
Quartz On PDMS hole array
Quartz Mold in Si
Nanoimprint, Lift-off and etch
2D TiO2 and
Quartz ArrayFabrication Process (a) Imprint (c) Release (e) Etching (b) Imprint (d) Lift-off (f) Etching
Device Images
Blue & Green filter
TiO2Photos of Blue Filter
In Air In Liquid
20 mmPhotos of Green Filter
In Air In Liquid
20 mmOn and Off of Blue and Green Filters
Angle Dependence
• Resonance in HCG is more
localized
• It depends more on the
resonance of each unit cell than
periodicity
• The reflection is less
dependent on the incident
angle
• No significant change of color
with incident angle up to 39o
You do not believe me?Movie of Blue Filter
Improved Viewing angle with Diffuser
Covered with a Bulk Diffuser
Diffuser
VideoResolution Important specification Grating Unit Counts Optical Localization
Simulation
Grating Unit
Count
8 µm • 8 µmResolution Far Beyond Perception Limit Pixel Size 4x4 7.5x7.5 15x15 30x30 (µm) DPI 6350 3386 1692 846
Photos of Color Mixing Blue, Green Filters Black & Red Background Blue, Purple Green, Yellow
Photos of Color Mixing Blue, Green Filters Black & Red Background
Photos of Color Mixing Blue, Green Filters Black & Red Background Zoomed-in View
Photos of Color Mixing
Blue Filter
White & Black Background
Low loss
Blue filter region
White -> Low lossColor-mixing of Two Filters Blue and Green Overlaying Area
Color-mixing of Two Filters Blue and Green Overlaying Area
Summary
• Nanoimprint lithography
• High resolution
• Low cost
• Large area
• 3D structure
• Non conventional substrates
• Invented a color reflective display with potential for
unprecedented vivid color
• Developed the fabrication processes
• Proved the feasibility
• Bright color
• Large Viewing Angle
• Colored & Clear States
• High resolution
• Color-mixing
• Nanoimprint has great potential beyond semiconductorAcknowledgement
Yuhan Yao, Yifei Wang, He Liu, Yuanrui Li,
Boxiang Song
R. Stan Williams, Doug Ohlberg
Wen-Di LiThank you…
wu.w@usc.edu
http://www.usc.edu/dept/ee/wugroup/You can also read
