nanoimprint lithography and applications -...

Nanoimprint Lithography and Applications

Wei Wu

Department of Electrical Engineering

University of Southern California

[email protected]

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/

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

*Lithography for flash, ITRS roadmap 2013 update

20239.5

Source: Britney Spears guide to Semiconductor Physics

http://britneyspears.ac/lasers.htm

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.



Problem of Photolithography: of Light Used Has Not Scaled with Resolution

Moore’s law feature size shrinks 0.7 times every two years

0

100

200

300

400

500

600

700

0100200300400500600700Node/Half-pitch (nm)

Wave

length

(nm

)

Rayleigh's equation

I-line

DUV

193 nm

NA

λk_ 1SizeFeature

Quartz is opaque

157 nm is dead

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

Resist

Mold

Substrate

Nanoimprint Lithography (NIL)

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

Substrate

Resist

Mold

Substrate

Nanoimprint Lithography (NIL)

Nanoimprint Lithography

High resolution-not limited by wavelength

High throughput -parallel process

Low cost

UV-curable NIL with Double-layer Spin-on Resist

UV

W. Wu, G. Y. Jung, D. L. Olynick, et al., Applied Physics a-Materials Science & Processing 80, 1173 (2005).

1. Prepare substrate, spin transfer layer and liquid imaging layer on

2. Alignment

3. Press and exposure

4. Mold and substrate separation

5. Residue layer and under layer etching

6. Metal evaporation and lift-off or etching

“Ls” in Resist at 12 nm Half-pitch by NIL

Dense lines

Isolated lineSharp corners

W. Wu, W. M. Tong, J. Bartman, et. al., Nano Lett. 8 (11), 3865-3869 (2008).

5nm

Nanoimprinted Dot array in Resist at 10 nm Half-pitch

“Ls” in Resist at 8 nm Half-pitch by NIL

Hua et al, Nano Lett., 2004 Austin et al, Nanotech. 2005

Low Information Content Patterns

Using a Carbone nanotube mold Using a supper lattice mold

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. 2009

The Limiting Factor of EBL

Resolution limiting factors of electron beam (with a perfect resist):

Beam scattering (forward and backward)

Second electron generation

Proximity effect 10 nm ~ microns

Spot size

~ 4 nm

Overall beam spot diameter

2222dcsg ddddd

nmV

d

V

VCd

Cd

M

dd

d

cc

ss

vg

2.1,22.1

2

1 3

dv: virtual source diameterM: demagnefication

Spherical aberration

Chromatic aberration

Diffraction

(assume no astigmation)

Beam spot: 3.5 Å

~$2M

Latest Helium Ion Microscope

Typical trimer image on HP HIM

B.W. Ward et al, J. Vac. Sci. Technol. B, 2006

Rearranged tungsten atoms for bright helium ion source

0.24 nm record imaging

resolution was demonstrated

using HIM From Carl Zeiss

Overall beam spot diameter

2222dcsg ddddd

nmV

d

V

VCd

Cd

M

dd

d

cc

ss

vg

2.1,22.1

2

1 3

dv: virtual source diameterM: demagnefication

Spherical aberration

Chromatic aberration

Diffraction

(assume no astigmation)

35 KeV Electrons 35 KeV He+

He Ion is Scattered Over Shorter Ranges

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!

D. Winston et al, J. Vac. Sci. Technol. B, 2009V. Sidorkin et al, J. Vac. Sci. Technol. B, 2009

Helium Ion Beam Lithography

HIBL for Sub-5 nm Patterning on HSQ Resist

5 nm half pitch 4 nm half pitch

5 nm half-pitch

4 nm half-pitch


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 nm

Nanoimprint Using HIBL Template

HSQSilicon

Template after HIBL and development

Short exposure to O2 plasma and coating of mold release agent

Mold release agent

UV-curable NIL resist

Fused silica

UV nanoimprint using HIBL template

Imprinted Resist with 4-nm Half-pitch Lines

12 nm half pitch

5 nm half pitch

4 nm half pitchSample coated with 2 nm platinumand imaged under XL30 SEM at 20kV


5 nm Half Pitch Lines Patterned in 10 nm Thick Chromium

5 nm half pitch

8 nm half pitch

20 nm Si3N4 membrane

10 nm chromium

He+

beam

Nanoimprint of 3D Patterns

W. Wu, M. Hu, F. S. Ou, Z. Y. Li and R. S. Williams, Nanotechnology 21 (25), -(2010)

Si polymer with Au

• NIL duplicates 3-D nanostructures into a transparent UV-cured polymer

• High aspect ratio and resolution patterning (<10 nm in tip radius, ~ 2 mm in height)

• Si master is fabricated Bosch etching process

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)

Rigid polymer

Elastic polymer

Anti-adhesion layer

Hybrid Soft PDMS/Hard Crest Mold for UV-NIL

Mold Imprinted resist

Roll-to-roll Nanoimprint for Low-cost Large-Area Patterning

Home-built roll-to-roll NIL system in my lab

Cost 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 Nanophotonics

Ambient light only

Reflective Display

https://kindle.amazon.com/

• Sunlight readability

• Printing-like

• Low power consumption

No commercial color reflective display on market

Single-layer architecture

Color filter insideColor filter inside

Display Architectures

RGB additive model

• Red, green and blue sub-pixels

• Parallel arrangement

LCD RGB Display

A single pixel 33 %

RGBW

Color filters

Brightness

Not working yet

Electrophoretic Color Display

Kwak, Y., Park, J., & Park, D. S. (2008).

Qualcomm Mirasol

Interferometric Modulation

MEMS

Brightness

Mirasol Display

https://www.qualcomm.com/products/mirasol/technology

Stack Architectures

Three-layer Architecture

Ideal efficiency: 100 %

Brightness

• Performance of filters

Saturation (Gamut)

• Reflection spectra of filters

Contrast

• Method to tune reflectanceThree-layer architecture

Color filters inside100 %

Ideal Reflection Spectra

Brightness

• Reflectance

• Efficiency

• Loss

Saturation (Gamut)

• overlapping

Contrast

1

0400 500 600 700

Wavelength (nm)R

efle

ctan

ce

Reflection spectra of color filters

Three-layer

Colored ink

Challenge?

• No Ink hassquarefunction likespectrum

Electrowetting Display

Electrowetting Displays A paper by Johan Feenstra & Rob Hayes

High Contrast Grating

Resonance

Re-radiate

Interfere

HCG

A.S.P. Chang, H. Tan, S. Bai, W. Wu, Z. Yu, S.Y. Chou, “Tunable External Cavity Laser With a Liquid-Crystal SubwavelengthResonant Grating Filter as Wavelength-Selective Mirror”, Photonics Technology Letters, IEEE, 19(2007) 1099-1101M. Shokooh-Saremi, “Physical basis for wideband resonant reflectors,” Optics Express, vol. 16, 2008.

Schematic diagram of GMR device

A surface-emitting laser incorporating ahigh-index-contrast subwavelength

Grating (HCG)

Huang, M. C. Y., Y. Zhou, et al. (2007). "A surface-emitting laser incorporating a high-index-contrast subwavelength grating." Nat Photon 1(2): 119-122.

Resonance Grating Reflector

2D grating

Polarization independent

Double-layer pillar

High index contrast

Less angle sensitivity

Red, Green and Blue Filters

Three-layer architecture

Gamut Chart

Color reproduce

Comparable with IP 5

Saturation of red color

Resonance

• Grating dimensions

• Index contrast

Change background index

• Electrowetting

Operation to Tune the Reflectance

Electric Field Distribution

Background index = 1 Background index = 1.8

Reflection Cancellation Average reflectance < 5 %

On & Off

ON

OFF

Reflection vs Background Index

Water1.32

1D grating

in Si

1D PDMS mold

1D PDMS mold

2D hole array

in Si

Si

Interference Lithography

MoldDuplication

LinewidthAdjustment

2D PDMS Mold

MoldDuplication

QuartzTiO2

On Quartz

TiO2

Growth

DoubleImprint

Nanoimprint, Lift-off and etch

2D TiO2 and Quartz Array

Fabrication Flow Chart

Fabrication Process

(a) Imprint (c) Release (e) Etching

(b) Imprint (d) Lift-off (f) Etching

Device Images

Blue & Green filter

TiO2

Photos of Blue Filter

20 mm

In Air In Liquid

Photos of Green Filter

20 mm

In Air In Liquid

On 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

Video

Important specification

Grating Unit Counts

Optical Localization

Resolution

Simulation

8 µm • 8 µm

Grating UnitCount

Resolution

Far Beyond Perception Limit

Pixel Size(µm)

4x4 7.5x7.5 15x15 30x30

DPI 6350 3386 1692 846

Photos of Color Mixing

Blue, Green Filters

Black & Red Background

Blue, Purple

Green, Yellow


Blue, Green Filters



Blue, Green Filters


Zoomed-in View


Blue Filter

White & Black Background

Low loss

Blue filter regionWhite -> Low loss

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 forunprecedented 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 semiconductor

Acknowledgement

R. Stan Williams, Doug Ohlberg

Wen-Di Li

Yuhan Yao, Yifei Wang, He Liu, Yuanrui Li, Boxiang Song

Thank you…

[email protected]://www.usc.edu/dept/ee/wugroup/

mailto:[email protected]

http://www.usc.edu/dept/ee/wugroup/

nanoimprint lithography and applications -...

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