Lithography - The University of Hong Kong

ELEC 7364 Lecture Notes
Summer 2008
Lithography
by STELLA W. PANG
from The University of Michigan, Ann Arbor, MI, USA
Visiting Professor at
The University of Hong Kong
The University of Michigan – Visiting Prof. HKU p. 1 S. W. Pang
Lithography Requirements
Resolution down to 24 nm
Depth of Focus better than 0.5 µm
Linewidth Control better than 4 nm
Low Defect Density of 30/m 2 and Large Chip Size of 1600 mm 2
The University of Michigan – Visiting Prof. HKU p. 2 S. W. Pang

Lithography Projection
Photons with Short Wavelength
EUV - Similar to X-Ray with High Energy Photons
Masks are Difficult to Make Even with 4:1 or 10:1 Reduction
Alternative Lithography Difficult to be Accepted
The University of Michigan – Visiting Prof. HKU p. 3 S. W. Pang
Lithography - Mask to Patterns
Mask Design and Layout with CAD Tools
Mask Making by Direct Write: 1:1 up to 10:1
Full Wafer Printing or Projection Printing of 1 Die at a Time
High Throughput Needed (>50 Wafers/hr)
The University of Michigan – Visiting Prof. HKU p. 4 S. W. Pang

Optical Lithography
The University of Michigan – Visiting Prof. HKU p. 5 S. W. Pang
Optical Lithography Process
Mask Design - CAD
Files
Optical Mask (Ni
/Quartz)
Photoresist Coating
Exposure and
Development
The University of Michigan – Visiting Prof. HKU p. 6 S. W. Pang

Photoresist
Mostly organic polymer
- properties change after exposure to radiation
(photons, electrons, ions)
- selective removal of exposed (positive) or
unexposed (negative) area by development
Photoresist Requirements
- Resolution (L min )
- Sensitivity (Throughput)
- Etch Resistance (Pattern Transfer)
- Spectral response, contrast, adhesion, pin hole
density, thickness variation, stability (Pattern
Quality)
The University of Michigan – Visiting Prof. HKU p. 7 S. W. Pang
Resist Preparation
Cleaning – remove surface contamination; Organic
solvents, Oxide Etch
Dehydration – remove moisture on surface; Bake at

HMDS on Si
Si Surface is Terminated with Si-O-Si(CH 3 ) 3 instead of Si-OH
Polymer Layer has better Adhesion to –CH 3 group
More Uniform Coating and Better Adhesion of Photoresist
The University of Michigan – Visiting Prof. HKU p. 9 S. W. Pang
Spin Coating of Resist
Liquid applied, changed to uniform coating after
spinning and baking
Balance between Centrifugal Force, Viscosity
Resistance, and Solvent Evaporation
The University of Michigan – Visiting Prof. HKU p. 10 S. W. Pang

Photoresist Thickness after Spinning
T = KS α ( ηβ
ω γ R 2 )1/ 3
T - Photoresist Thickness
S - % Solid in photoresist (affects η)
η - Viscosity
ω - Spin speed in angular velocity (2000-6000 rotation per min – rpm)
R – € Radius of wafer
S and η could vary as solvent is evaporated
Spin time ~30 to 60 s
The University of Michigan – Visiting Prof. HKU p. 11 S. W. Pang
Photoresist Spin Curve
T~ω -n where 0.4

Pre-Bake or Soft-Bake
Remove solvents (80-90 % removal)
Release stress
Improve adhesion
Excessive: Reduce resist sensitivity
Insufficient: Reduce Developer Contrast
The University of Michigan – Visiting Prof. HKU p. 13 S. W. Pang
Exposure
Control Exposure Does – e.g. 10-100 mJ/cm 2;
depends on photoresist sensitivity and affects
profile and feature size
Optical Radiation: g-line: 436 nm; i-line: 365
nm; Excimer Laser: KrF – 248 nm; ArF – 193
nm;
F 2 – 157 nm; Immersion Lens with n>1
Exposure modes: Contact printing; Proximity
printing; Projection printing (step and scan
with reduction); Direct Write
The University of Michigan – Visiting Prof. HKU p. 14 S. W. Pang

Projection vs. Contact/Proximity Printing
Research Labs - Contact and Proximity Printing Since
Systems are Cheaper, Smaller, and Easier to Use
Industry - Projection Printing with High Throughput
for High Reproducibility, Low Defects, and More
Automation
The University of Michigan – Visiting Prof. HKU p. 15 S. W. Pang
Contact and Proximity Printing
1X Mask - High Resolution Features more Difficult to
Make
Contac Printing Could Generate Defects on Masks and
Substrates
Resolution is Reduced by Gap
The University of Michigan – Visiting Prof. HKU p. 16 S. W. Pang

Projection Printing
4-10X Mask -
Easier to Make
Optimized for
Resolution,
Depth of Focus,
Throughput
Resolution
improves by
shorter
Wavelength (λ)
and Higher
Numerical
Aperture (NA)
while Depth of
Focus becomes
Worse
The University of Michigan – Visiting Prof. HKU p. 17 S. W. Pang
Optics for Optical Projection
Simple Ray Optics, assume light travels in straight rays
Numerical Aperture (NA)
€
(n=1 in air)
NA = D 2 f = n sinα
NA determines how much light is transmitted or collected
at the image plane, influence exposure time and
resolution
The University of Michigan – Visiting Prof. HKU p. 18 S. W. Pang

Resolution
Resolution and Depth of Focus
l min = k λ
NA
k: 0.5 to 0.7; experimental
parameter
€
Depth of Focus
λ
DOF = k '
NA 2 k'~0.5
€
For λ=365 nm; NA=0.45; k=0.7; k'=0.5
l min =0.57 µm; DOF=0.9 µm
Tradeoff between l min and DOF, with better resolution, DOF
is less
The University of Michigan – Visiting Prof. HKU p. 19 S. W. Pang
Fraunhofer
Diffraction Pattern
Through Circular
Aperture
Light Intensity on Wafer
Modulation Transfer
Function - High on
Mask, Lower on Wafer
Due to Optics
Diffraction
The University of Michigan – Visiting Prof. HKU p. 20 S. W. Pang

Resist Development
Typical Condition: 1 min, 20 °C in developer
Selective resist removal based on
- Chemical Transformation (e.g. carboxylic acid
formation during positive resist exposure)
- Change in MW (faster development for low MW
polymer)
The University of Michigan – Visiting Prof. HKU p. 21 S. W. Pang
Hard Bake or Post Bake
Typical Condition: 10 min, 120 °C on Hot Plate
Improves Etch Resistance and Adhesion, Prevents
Swelling
Resist Flows and Profile becomes Tapered
The University of Michigan – Visiting Prof. HKU p. 22 S. W. Pang

2 Component Positive Resist
Binder + Sensitizer; acid formed due to radiation
which can be dissolved in base developer
Resin/Binder (Novolac Co-polymer), >80 %
- MW 500-20000, doesn't interact with radiation
- Phenolic group, acidic, can be dissolved in base
solution (~15 nm/s)
- Structure and composition of polymer determine
viscosity and developer solubility
The University of Michigan – Visiting Prof. HKU p. 23 S. W. Pang
Sensitizer in 2 Component Positive Resist
Sensitizer/Inhibitor (200 nm/s)
Changes in Sensitizer due to exposure (chemical
transformation)
The University of Michigan – Visiting Prof. HKU p. 24 S. W. Pang

Chemical Transformation of Positive Resist
Sensitizer Activated by Photons - Chemical
Transformation
Release of N 2
Carboxylic Acid Form in Presence of H 2 O
The University of Michigan – Visiting Prof. HKU p. 25 S. W. Pang
One Component Positive Resist
PolyMethylMethacrylate (PMMA)
MW 200-1000K, high resolution, low sensitivity
Bond breaking (chain scission) due to exposure to radiation
MW reduced and low MW polymer is removed during
development
The University of Michigan – Visiting Prof. HKU p. 26 S. W. Pang

2 Component Negative Resist
Cross linking to generate longer polymer chain with
higher MW
Non-planer solvent (e.g. Tolune, Xylene) removes low
MW polymer (~100 nm/s); leaves behind high MW
polymer (~1 nm/s)
Rubber Matrix (~Resin)
- Soluble in non-planar solution (e.g. Tolune,
Xylene, ~100 nm/s)
The University of Michigan – Visiting Prof. HKU p. 27 S. W. Pang
Sensitizer in 2 Component Negative Resist
Sensitizer (R-N 3 )
Free radicals generated when exposed to radiation,
causing cross-linking of polymer. MW increased and
low MW polymer is removed during development,
leaving behind high MW polymer (exposed area)
The University of Michigan – Visiting Prof. HKU p. 28 S. W. Pang

Cross Linking of Negative Resist
Reactive Radicals are generated by Radiation
Release of N 2
Radicals Link up with Other Polymer Chains and Form
new Compounds with Higher Molecular Weight
The University of Michigan – Visiting Prof. HKU p. 29 S. W. Pang
One Component Negative Resist
PolyMethylMethacrylate (PMMA)
Cross linking occurs when dosage is
high enough due to free radical
accumulation that starts up chain
reaction for cross linking
The University of Michigan – Visiting Prof. HKU p. 30 S. W. Pang

Improvements in Resist Technology
Chemical Amplification (CA) Resist
- High Sensitivity, High Contrast
- Process harder to control, sensitive to
contaminants in air, less etch resistance
- Photon absorption causes decomposition of
Photoacid Generator (PAG), forming Acid,
during post bake, inducing cascade of Chemical
Transformations in Resist
Multilayer Resist (Bilayer, Trilayer)
- High Resolution, Better Etch Resistance with
Thicker Resist, Planarization
- More Complicated process with additional dry
etching of bottom layer resist
The University of Michigan – Visiting Prof. HKU p. 31 S. W. Pang
Silylated Resist
Silylated Resist (Introduce Si in Polymer)
- Use to increase etch resistance during O 2 plasma
etching of bottom layer resist
- Only a small amount of Si needed (

Resist Contrast and Sensitivity
Resist Contrast depends on photoresist and UV property, affects
resist profile
Development Curve (Contrast) For Positive Photoresist
γ p
=
1
log( D o
D 1
)
e.g.
Positive resist (AZ 1450J), 0.5 µm thick
€
D o =33 mJ/cm 2 ; D 1 =21 mJ/cm 2 ; γ=5.1
Large γ - more vertical resist profile
3 regions on development curve:
A. Low Exposure - small but finite development rate (e.g. 1 nm/s)
B. Medium Exposure - solubility increases with exposure dose
C. High Exposure - resist fully removed when D≥ D o (e.g. 100 nm/s)
The University of Michigan – Visiting Prof. HKU p. 33 S. W. Pang
Negative Resist Contrast
Resist Contrast depends on photoresist and UV property, affects
resist profile
Development Curve (Contrast) For Negative Photoresist
γ n
=
1
log( D 1
D 0
)
Similar to Positive Photoresist development curve, but Reversed:
A. Low Exposure - Fast development € rate
B. Medium Exposure - solubility decreases with exposure dose
C. High Exposure - resist will not be removed
The University of Michigan – Visiting Prof. HKU p. 34 S. W. Pang

Light Absorption and Sensitivity
I = I o e -αz where α is absorption coefficient
Less dose near bottom of resist, Could affect resist
profile
Resist Sensitivity - minimum dose required to define
patterns in resist
- positive resist: S=D o , minimum dose needed to remove
exposed resist
- negative resist: S=D 2 , minimum dose needed to keep
exposed resist at 50% thickness
The University of Michigan – Visiting Prof. HKU p. 35 S. W. Pang
Resist Profile
Depends on resist contrast and exposure profile
Ideal profile - Vertical
dT
dx = ∞
€
The University of Michigan – Visiting Prof. HKU p. 36 S. W. Pang

€
Calculate Resist Profile
From Development Curve
T(D,x) = γT o
log
€
Let Exposure Dose D(T ,x) = I(x)te −α (T o −T )
Where I(x) = Light Intensity; t = Exposure Time
Slope of Resist (Profile) =
€
D o
D(T, x)
T(D,x)
T o
= γ log
T(D,x) € = γT o
[log
I(x)te ]
−α (T o −T )
D o
D(T ,x)
The University of Michigan – Visiting Prof. HKU p. 37 S. W. Pang
dT
dx
D o
€
To Get Vertical Profile
Large contrast (γ)
Small absorption (α) - get more light down to
bottom of resist
Little light scattering (large dI(x)/dx)
High D o so that some of the scattered light will
not expose resist
Short exposure time to reduce scattering
Thinner resist for less light scattering
The University of Michigan – Visiting Prof. HKU p. 38 S. W. Pang

Modulation Transfer Function (MTF)
Ratio of modulation of an optical image to that
of an object - represent image quality
Depends on optical system and resist -
wavelength, numerical aperture, contrast,
light coherence, spatial frequency
MTF (λ, NA, γ, S, f)
The University of Michigan – Visiting Prof. HKU p. 39 S. W. Pang
Light Distribution and MTF
Mask Modulation
M mask
= I m −max − I m −min
I m −max
+ I m −min
Wafer Modulation
If I min = 0, then M = 1; If I min ≠ 0, then M

MTF and Linewidth
If γ = ∞ (vertical profile), MTF affects linewidth
Exposure Time Affects Dose - Linewidth for Positive
Photoresist is Reduced for Longer Exposure Time
The University of Michigan – Visiting Prof. HKU p. 41 S. W. Pang
Light Coherence and Resolution
Resolution also depends on Light Coherence (S)
Coherent Light (S = 0) – Light passes slit in 1 angle
(point source)
Incoherent Light (S = 1) – Light with different angles
and phases
f = spatial frequency; line and space, or period of
gratings = 1/2L
f c = cutoff frequency, minimum feature size that can be
defined
The University of Michigan – Visiting Prof. HKU p. 42 S. W. Pang

MTF - Pattern Transfer Quality
MTF = 1: Sharp Transition from Light to Dark
MTF = 0: No difference between I max and I min , can't
form any patterns
f
Poor image quality at low MTF c
= 2NA
λ
For Coherent light S=0: Lower resolution, better image
quality (High MTF); MTF=1 for f≤0.5 f c
For Partial Coherent or Incoherent light S>0: Higher
€
resolution, worse image quality; MTF>0 for f≤f c ;
MTF=0 for f=f c
The University of Michigan – Visiting Prof. HKU p. 43 S. W. Pang
MTF Needed For Given Feature Size
Smallest features that can be patterned with
λ=200 nm, NA=0.5:
Can't work with MTF=0
If resist requires MTF=0.6 MTF = 0.6 =
or 2L=0.33 µm; f=3 lines/µm; or l min = 0.165 µm
Below this resolution, image quality is so poor
€
that it is not useful (e.g. can't do 5 lines/µm at
f c ; or l min = 0.1 µm)
1
2L ( f )
2NA
λ ( f c )
The University of Michigan – Visiting Prof. HKU p. 44 S. W. Pang

Phase Shifting Mask (PSM)
Traditional
Mask
Phase Shifted
Mask
Make adjacent phases to be 180° out of phase with one another.
This provides destructive interference and reduce diffraction,
therefore reducing I min and increasing MTF with better contrast
and resolution
Resolution can be increased by 2 times or more depending on S
The University of Michigan – Visiting Prof. HKU p. 45 S. W. Pang
Contrast Function
I max
− I min
Contrast Function = I max
+ I min
at phase shifted image
Improve Resolution using Same Optical System
The University of Michigan – Visiting Prof. HKU p. 46 S. W. Pang
€

Phase Shifted Mask Formation
d(n −1) = λ 2 ;n = c v
d =
λ
2(n −1)
Add a transparent layer (e.g. quartz/polymer) such that the
adjacent apertures are 180° out of phase - Optical thickness
shifted by λ /2 €
Need to take into account all adjacent features
For phase shifted mask
- Higher light intensity in exposed area, less residue left
behind after development
- Lower light intensity in unexposed area, larger process
window for over-exposure, better resist profile, and less
undercutting
- Intense computation needed for mask design to account for
all adjacent apertures.
- Significant improvement in resolution
The University of Michigan – Visiting Prof. HKU p. 47 S. W. Pang
Next Generation Lithography
Non-Optical Lithography
Resolution limit - 100 nm and below
Extreme UV (EUV) Reflective Projection
Xray Lithography (XRL)
Electron Beam Lithography (EBL)
Nanoimprint Lithography (NIL)
The University of Michigan – Visiting Prof. HKU p. 48 S. W. Pang

Extreme UV (EUV) Reflective Projection
Soft Xray - λ=10-15 nm; photon energy ~100 eV
All Vacuum, all reflective optics
EUV Source - High power laser (Nd-YAG) to heat up
supersonic Xe jet to produce EUV
Reflective Optics - >50 multiple layers of thin films of
dissimilar n and α to form optical interference and
high reflectivity (e.g. Mo/Si). Layer thickness=λ/4
- e.g. Mo/Si with Layer thickness = λ/4
- Need better than 0.01 nm thickness control and
0.3 nm smoothness
- improved reflectivity (from ~50% to >70%)
- no damage for long exposure time
The University of Michigan – Visiting Prof. HKU p. 49 S. W. Pang
EUV SYSTEM AND MASK
All Reflective Optics with Multiple Mirrors for
Reduction
Mirrors and Mask with Alternating Layers of High and
Low Mass Materials for Constructive Interference
The University of Michigan – Visiting Prof. HKU p. 50 S. W. Pang

EUV System
Projection system with 4-10X reduction
Scanning mask and wafer
Small NA (e.g. 0.25) with large DOF
Could have large process window such that
Phase Shifting Mask (PSM) or Optical
Proximity Correction (OPC) is not needed
Closer to Optical Lithography
The University of Michigan – Visiting Prof. HKU p. 51 S. W. Pang
EUV vs. Optical Lithography
193 nm DUV is Used - Pushed As Far As
Possible
- Phase-Shifted Masks
- Double Exposure
- High Index Lens Immersion
- Below 32 nm
EUV Concerns
- Control of Mo/Si Layer Thickness (

Xray Lithography (XRL)
Short wavelength Xray - λ=1 nm; photon energy
~10 KeV
All Vacuum, membrane mask based on xray
absorption
Xray Sources - Laser or Plasma Generated Xray
source
- Heat up target (e.g. Fe-Ni) by high intensity laser
or heat up gases by electromagnetic wave to
generate high temperature plasma that emits
xray. Pulsed laser or EM power is often used
- ~10% efficient. Need to increase source
brightness
- Need to reduce debris from plasma using
shielding or off center excitation
- Xray pulses with very high power for short time
can damage mask/sample. Better to have lower
power but high repetition rate
The University of Michigan – Visiting Prof. HKU p. 53 S. W. Pang
Compact Synchrotron or Electron Storage Ring
Particle accelerator with magnetic
confinement and electrons are accelerated in
a ring close to speed of light (~1 GeV)
Xray emits from tangent of curved trajectory.
Can get multiple exposure stations (~25) from
1 ring
Emitted Xray: Broad Spectrum emission, need
filtering for exposure
Bright source with high throughput and high
resolution
Expensive (>25 M) and needs backup system
Large floor space, concern about downtime
and reliability
The University of Michigan – Visiting Prof. HKU p. 54 S. W. Pang

Xray Mask
Materials for absorber and membrane depend on α
Concerns about Xray Mask and Exposure
- Thick absorber with high α and thin membrane with low α
for high mask contrast. Difficult to pattern thick absorber,
thin membrane is less stable
- Membrane need to be transparent in visible light for
alignment
- Need resist with high sensitivity for high throughput and
low α to expose thick layer
Advantages of XRL
- High resolution due to short λ
- High throughput compared to direct writing
- No proximity correction needed
- Immune to low MW particles with low α (e.g. dust)
The University of Michigan – Visiting Prof. HKU p. 55 S. W. Pang
High Resolution Xray Mask
Angle Evaporation on Sidewalls and Top Surface with Alternative
Layers of High/Low MW
Linewidth Controlled by Layer Thickness
The University of Michigan – Visiting Prof. HKU p. 56 S. W. Pang

Electron Beam Lithography (EBL)
Very short wavelength λ=0.01 to 0.05 nm,
depending on electron energy
λ(nm) =
0.039
V(keV )
Electron energy~10-50 KeV
€
All Vacuum, either direct write or projection
electron beam through membrane mask
The University of Michigan – Visiting Prof. HKU p. 57 S. W. Pang
Direct Write EBL
Input computer designs and generate patterns by
scanning electron beam
Need electron source with high brightness (e.g. field
emission source rather than thermionic emission
source) for high throughput
Need good lens system to focus down to small beam
size
The University of Michigan – Visiting Prof. HKU p. 58 S. W. Pang

Distortions in EBL
Astigmation - misalignment of lenses that causes
beam shape to change; Can be corrected by
magnetic lenses
Spherical Aberration - electrons land at different spots
(focus points) due to their entry through lens from
different point off central axis. Increase minimum
spot size
The University of Michigan – Visiting Prof. HKU p. 59 S. W. Pang
Distortions in EBL
Astigmation - misalignment of lenses that causes
beam shape to change; Can be corrected by
magnetic lenses
Spherical Aberration - electrons land at different spots
(focus points) due to their entry through lens from
different point off central axis. Increase minimum
spot size
The University of Michigan – Visiting Prof. HKU p. 60 S. W. Pang

Additional Distortions in EBL
Chromatic Aberration - due to energy spread of
electrons. Slower electrons are brought to focus
closer to lens than faster electrons; Increase
minimum spot size
Spot size also depends on Mutual Columbic Repulsion
of electrons and Diffraction limit of electrons
The University of Michigan – Visiting Prof. HKU p. 61 S. W. Pang
Feature Size by EBL
Once a beam gets minimum spot size through lenses,
feature size limitation depends on electron scattering
in resist and substrate
Electrons scatter around until energy is lost or exit
through resist surface by backscattering - Resolution
depends on resist thickness due to scattering
The University of Michigan – Visiting Prof. HKU p. 62 S. W. Pang

Electron Penetration Range
Monte Carlo simulation is used to track
electron paths - Most electron energy is
absorbed in substrate
Maximum electron penetration range
R(cm) = 4.6x10−6
ρ
V(keV ) 1.75
e.g. ρ=1 g/cm 3 ; V=25 KeV; R g =12.9 µm
€
The University of Michigan – Visiting Prof. HKU p. 63 S. W. Pang
Proximity Effect
Amount of Dose depends on position and shape of
features and their neighbors - Proximity correction by
exposure dose adjustment using computer software
to calculate dosage from all adjacent features
The University of Michigan – Visiting Prof. HKU p. 64 S. W. Pang

Early Imprint Lithography
• ~1040 Movable Clay, Ceramic, and Wooden type Invented in
China
• ~1440 Gutenberg in Germany Commenced work on his
Printing press with Metal moving type
Woodblock
Edition of
Chinese Play
Gutenberg
Press
The University of Michigan – Visiting Prof. HKU p. 65 S. W. Pang
Types of Imprint Lithography
Hot Embossing – Thermoplastic
– Polymer Flows above Glass Transition
Temperature under Pressure
Step and Flash Imprint Lithography (SFIL) – UV
Assisted
– UV Curable Polymer Flows and Solidifies after
UV Exposure
Reversal Nanoimprint Lithography – with and without
UV, Mold Coating, and Transfer to Substrate
– Multiple Imprints to Generate 3D Nanostructures
– Low Temperature and Low Pressure
– Flexible: no Residual Layer, no Liquid Fill
The University of Michigan – Visiting Prof. HKU p. 66 S. W. Pang

Applications of Imprint Lithography
Semiconductor Integrated Circuits
– Mix and Match with Optical Lithography
Hard Disk Drives
– High Resolution, High Density, Double-Sided,
and Low Cost
Light Emitting Diodes and Flat Panel Displays
– High Resolution, High Density, Large Area, and
Low Cost
Biomedical Systems
– High Resolution, High Density, Large Area, 3D
Fluidic System, and Low Cost
The University of Michigan – Visiting Prof. HKU p. 67 S. W. Pang
Full Wafer Imprint
Imprint Throughput
– Imprint Large Area (8 inch diameter) with High
Throughput
– Need Uniform Pressure between Mold and
Substrate such as Air Cushion Pressure
Step and Repeat Imprint
– Similar to Optical Lithography, Easier to Control
– Easier to make Imprint Mold with Smaller Area
– Lower Throughput
The University of Michigan – Visiting Prof. HKU p. 68 S. W. Pang

Concerns of Imprint Lithography - I
Patterning
– High Resolution Tools Needed
– Line Width Roughness Induced by Lithography
and Etching
– Uniform Residual Layer
1X Mold
– CD Control
– High Cost for Master – Multiple Molds can be
Duplicated by Imprinting
The University of Michigan – Visiting Prof. HKU p. 69 S. W. Pang
Concerns of Imprint Lithography - II
Overlay
– 3 Sigma of 10 nm Needed
– Easier with Step and Scan than Full Wafer
Imprint
Defects
– Contact between Mold and Substrate
– Mold Cleaning and Defect Removal
– Avoid Trapped Air by Using Vacuum, Flat Molds
/Substrates, Spin-Coating rather than
Dispensing
– Lower Pressure to Avoid Mold Damage
The University of Michigan – Visiting Prof. HKU p. 70 S. W. Pang

MODES OF REVERSAL IMPRINTING
NIL
T>>T g
CONVENTIONAL
IMPRINT
Embossing Inking
Whole-Layer Transfer
T>>T g
T~T
T~T g , non-planar g , planarized
COATING ON MOLD
3 IMPRINT MODES
DEPENDS ON TEMPERATURE, PRESSURE,
AND PLANARIZATION
X. D. Huang, L.-R. Bao, X. Cheng, L. J. Guo, S. W. Pang, and A. F. Yee, J. Vac. Sci. Technol. B 20,
2872-2876 (2002).
The University of Michigan – Visiting Prof. HKU p. 71 S. W. Pang
HARD INERT SiC MOLDS FOR NANOIMPRINTING
C 2 F 2 /O 2 30/10 sccm, 150 W rf POWER, -350 V, 6 Pa
ETCH RATES: SiC 30 nm/min; Cr/Ni 0.8 nm/min; RESIST 200 nm/min
ARRAYS OF DOTS IN SiC
DEPTH = 320 nm 600 nm
DEPTH = 800 nm
GAP = 50 nm
WIDTH = 100 nm
High Aspect Ratio 8:1
S. W. PANG, T. TAMAMURA, M. NAKAO, A. OZAWA, AND H. MASUDA, J. VAC. SCI.
TECHNOL. B 16, 1145 (1998).
The University of Michigan – Visiting Prof. HKU p. 72 S. W. Pang

IMPRINTING ON METAL AND CERAMIC
Nanoimprint Directly into Al
Combine Lithography and Pattern
Transfer
SiC Mold with 40 nm Nanostructures
4 µm Wide, 25 µm Deep CERAMIC Imprinted
at 20 MPa, 30 s Using Polyimide MOLD
Ceria-Zircronia Ceramic For Ultrasonic
Imaging With High Resolution
1 µm
40 nm DOTS
75 µm
SINTERED at 1500 °C, 60 min
DEPTH = 25 µm
J. A. Bride, S. Baskaran, N. Taylor, J. W. Halloran, W. H. Juan, S. W. Pang, AND M. O'Donnell, Appl.
Phys. Lett. 63, 3379-3381 (1993).
The University of Michigan – Visiting Prof. HKU p. 73 S. W. Pang
MULTIPLE STEPS IN Al BY SINGLE
NANOIMPRINTING
THREE DEPTHS OF 100, 200, AND 300 nm
500 µm
x 20 µm/div; z 500 nm/div
x 5 µm/div; z 500 nm/div
S. W. Pang, T. Tamamura, M. Nakao, A. Ozawa, and H. Masuda, J. Vac. Sci. Technol.
B 16, 1145-1149 (1998).
The University of Michigan – Visiting Prof. HKU p. 74 S. W. Pang

IMPRINTING ON FLEXIBLE SUBSTRATE
50 µm THICK FLEXIBLE KAPTON FILM
IMPRINT AT 175 °C
ESPECIALLY SUITABLE FOR SUBSTRATES THAT
CANNOT BE EASILY SPIN-COATED
FLEXIBLE POLYMER FILMS
PLASTIC ELECTRONICS
The University of Michigan – Visiting Prof. HKU p. 75 S. W. Pang
THREE MODES OF REVERSAL IMPRINTING
200
Imprinting Temp. ( o C)
180
160
140
120
Embossing
Transition
Whole-layer
Inking
Whole-Layer
Transition
Embossing
Inking
Tg
100
80
60
0 50 100 150 200 250 300 350
R max
(nm)
The University of Michigan – Visiting Prof. HKU p. 76 S. W. Pang

PATTERNING OVER TOPOGRAPHY
SUSPENDED OR SUPPORTED NANOSTRUCTURES
DEPENDING ON POLYMER PROPERTIES, IMPRINT
CONDITIONS, AND PATTERN DIMENSIONS
5-10 µm Gaps 2-5 µm Gaps
POLYCARBONATE
PMMA
TOUGHER POLYMER MORE BRITTLE POLYMER
L.-R. Bao, X. Cheng, X. D. Huang, L. J. Guo, S. W. Pang, and A. F. Yee, J. Vac. Sci. Technol. B 20,
2881-2886 (2002).
The University of Michigan – Visiting Prof. HKU p. 77 S. W. Pang
THREE-DIMENSIONAL NANOSTRUCTURES
MULTIPLE IMPRINTS OF POLYMERS PATTERNS
CAN TO USED TO FORM STACKED POLYMER LAYERS
WITH SIMILAR OR DIFFERENT PROPERTIES
The University of Michigan – Visiting Prof. HKU p. 78 S. W. Pang

MULTIPLE LAYER 3D NANOSTRUCTURES
3D PATTERNING BY REPEATED IMPRINTING
L.-R. Bao, X. Cheng, X. D. Huang, L. J. Guo, S. W. Pang, and A. F. Yee, J. Vac. Sci. Technol. B 20,
2881-2886 (2002).
The University of Michigan – Visiting Prof. HKU p. 79 S. W. Pang
SELECTIVE SU-8 POSTS TRANSFER BY
REVERSAL UV IMPRINT
2 s Exposure; Bake: 65 °C 1 min, 95 °C 5 min; 1 min Development
Reversal UV Imprint: 80 °C, 5 MPa, 1 min; 1 s UV Exposure; 80 °C, 5 MPa, 1 min
1.3 µm Diameter Posts Selectively Transferred to 5 µm Wide, 300 nm Deep SU-8 Gratings
1 µm 1 µm
Polymer Posts Transferred Selectively only to Top of Gratings
Allows Complex Structures Formation on Selected Areas
C. Peng and S. W. Pang, J. Vac. Sci. Technol. B 24, 2968-2972 (2006).
The University of Michigan – Visiting Prof. HKU p. 80 S. W. Pang

LOW TEMPERATURE AND LOW PRESSURE REVERSAL UV
NANOIMPRINT TO FORM SEALED CHANNELS
Quartz
SU-8
UV
Si
Si
a. Coating SU-8 on Quartz b. Imprint with UV
Quartz
Cured SU-8
Si
c. Coating Thin SU-8 Adhesion
Layer on Si Substrate
u
Quartz
Cured SU-8
SU-8
UV
Quartz
Cured SU-8
Cured SU-8
Si
u
Si
d. Imprint with UV e. Formation of Channels
in SU-8
Sealed Channels Formation by Reversal Imprint with UV Exposure
Low Temperature (55 °C) and Low Pressure (2 MPa) Used for Reversal
Imprint
Top SU-8 Layer UV-Cured before Reversal Imprint, Resulting in Good
Channel Profile Control Because No SU-8 Flow
The University of Michigan – Visiting Prof. HKU p. 81 S. W. Pang
4 LAYER OF STACKED NANOCHANNELS
1. Pattern SU-8 Coated Glass by UV Imprint: 55 °C, 1 MPa, 2 s UV
2. Transfer Patterned SU-8 to Si Substrate with Thin SU-8 Adhesion Layer by
Reversal UV Imprint: 55 °C, 2 MPa, 2 s UV
3. Repeat Above Processes to Fabricate 3D Multiple Level Nanochannels
400 nm Channels 200 nm Channels
400 nm Channels 200 nm Channels
Good Channel Profile with 4 Repeated Reversal Imprints
Uniform Array and Multiple Levels of 3D Nanochannels Formed by
Repeating Reversal UV Nanoimprint
B. Yang and S. W. Pang, J. Vac. Sci. Technol. B 24, 2984-2987 (2006).
The University of Michigan – Visiting Prof. HKU p. 82 S. W. Pang

REVERSAL IMPRINT TO TRANSFER METAL
PATTERNING OVER NANOSTRUCTURES
- COAT SU-8, METAL,
AND PO LYMER
ON GLASS MOLD
(a )
Glas s
SU-8
Metal
Polym er
Glass
SU-8
Metal
Polymer
- PATTERN POLYMER AND
METAL BY LITHOGRAPHY
AND ETCHING
(b)
A: IMPRINT TO Si
B: IMPRINT TO SU-8
Glass
SU-8
Metal
Polymer
Si
Glass
SU-8
Metal
SU-8
Si
- REVERSAL IMPRINT
(c1)
- REV ERSAL IMPRINT
(c2)
Metal
Polym er
Si
Metal
SU-8
Si
- SE PARATE MOLD
(d1)
- SEP ARATE MOLD
(d2)
Glass Mold with Cured SU-8 Layer and Etched Metal Patterns
Metal Patterns Directly Transferred to Substrate Structures
The University of Michigan – Visiting Prof. HKU p. 83 S. W. Pang
PATTERNED 3D METAL STRUCTURES FORMING
CHANNELS WITH SUBSTRATE
Glass Mold: SU-8 Gratings with 5 µm Period, 270 nm Deep, 2.5 µm Thick and
0.5 µm Al Patterned by Lithography and Etching
Reversal UV Imprint: 80 °C, 5 MPa, 1 min; 1 s UV Exposure; 80 °C, 5 MPa, 1 min
SU-8 Gratings on Si Substrate: 5 µm Period, 340 nm Deep, 3.5 µm Thick
Al Gratings
SU-8 Gratings
1 µm
3D Channels with Interconnects Fabricated Using Structures in Mold,
Patterning and Etching of Metal, and Structures on Substrate
C. Peng and S. W. Pang, IEEE Inter. Conf. Nanotechnology, Cincinnati, Ohio, July 2006.
The University of Michigan – Visiting Prof. HKU p. 84 S. W. Pang

IMPRINTED 3-D GRATING WITH SEALED CAVITIES
Duo Mold Imprinting
Polymer Coating on Low Surface Energy Mold
Surface Area also Determines Surface Adhesion
Grating
Grating
Closed
Cavities
Si
Closed Si
1 µm
Cavities
1 µm
FDTS Treated
(Low Surface
Energy)
FDTS Treated
(Low Surface
Energy)
Material: PMMA (Mw ∼15k)
Both Molds (700 nm Period
Mold and Square Mold)
Treated With FDTS
Imprinted at 5 MPa and
150 °C
Transferred to Substrate at 2
MPa and 80 °C
Y. P. Kong, H. Y. Low, S. W. Pang, and A. F. Yee, J. Vac. Sci. Technol. B 22, 3251-3256 (2004).
The University of Michigan – Visiting Prof. HKU p. 85 S. W. Pang
SURFACE ENERGY MODIFIED BY SEQUENTIAL SILANE
TREATMENTS
Polymer Adheres to Mold With Higher Surface Energy
PEDS- FDTS
Treated Mold
(Medium Surface Energy)
Surface Treatment
FDTS b
PEDS c -FDTS
PEDS
Surface Energy a
(MJ/m 2 )
11.8 ± 0.3
18.3 ± 0.4
37.6 ± 0.9
a By contact angles and the geometric-mean method
b Perfluorodecyltrichlorosilane
FDTS
Treated Mold
(Low Surface Energy)
c Phenethylmethyldichlorosilane
Adhesion - Higher Surface Energy
Release - Lower Surface Energy
Surface energy difference
~6.5 mJ/m 2
Y. P. Kong, H. Y. Low, S. W. Pang, and A. F. Yee, J. Vac. Sci. Technol. B 22, 3251-3256 (2004).
The University of Michigan – Visiting Prof. HKU p. 86 S. W. Pang

SURFACE PROPERTIES OF Si TREATED WITH SILANES
Sample
Lower Surface Energy for FDTS Treated Surface
than PETS or MOPTS Treatment
Contact angle
for H 2
O
Contact angle
for CH 2
I 2
Surface energy (mJ/m 2 )
γ s
O 2
Plasma 26.9 47.7 65.2
PETS 108.1 69.9 23.4
MOPTS 73.6 58.7 34.6
FDTS 103.0 99.8 11.7
PETS (PhenEthyl TrichloroSilane)
MOPTS (Methacryl Oxypropylene TrichloroSilane)
FDTS (1H,1H,2H,2H-perFluoroDecyl TrichloroSilane)
Si C 2 H 4
Cl
CF 3
(CF 2 ) 7 C 2 H 4 Si
Cl
O
CH 2 C C O C 3 H 6
CH 3
Cl
Surface Energy Can be Controlled by Various Surface Treatments
Silanes Can be Mixed in Various Ratio for Desired Surface Energy
L.-R. Bao, L. Tan, X. D. Huang, Y. P. Kong, L. J. Guo, S. W. Pang, and A. F. Yee, J. Vac. Sci. Technol. B 21, 2749 (2003).
The University of Michigan – Visiting Prof. HKU p. 87 S. W. Pang
Cl
Cl
Cl
Si
Cl
Cl
Cl
Example of
Step and Flash Imprint Lithography (SFIL)
at Molecular Imprints
Courtesy of Dr. D. Resnick
Applications in Memory and Hard Disk Drives
Imprint Technology Issues
The University of Michigan – Visiting Prof. HKU p. 88 S. W. Pang

Large and Diverse Market Opportunities
from Molecular Imprints
Semiconductor ICs
Hard Disk Drives
Light Emitting Diodes
Future Applications
CMOS Image Sensors
(CIS)
Flat Panel Displays
Bio-Medical
89
The University of Michigan – Visiting Prof. HKU p. 89 S. W. Pang
Step & Flash Imprint Lithography (S-FIL)
from Molecular Imprints
High resolution fused silica
template, coated with release layer
Template
Step 1: Dispense
drops
Planarization layer
Substrate
Imprint fluid
dispenser
Low viscosity fluid (Si
-containing for S-FIL,
Organic for S-FIL/R)
Step 2: Lower
template and fill
pattern
Step 3: Polymerize
imprint fluid with UV
exposure
Template
Planarization layer
Substrate
Planarization layer
Substrate
very low imprint pressure < 1/20
atmosphere at room temp
Same Process Used for
Step & Repeat and
Whole Substrate
Patterning
Step 4: Separate
template from
substrate
Template
Planarization layer
Substrate
Step & Repeat
The University of Michigan – Visiting Prof. HKU p. 90 S. W. Pang

Stepper Technology for Non-Volatile Memory
from Molecular Imprints
Tool is being used for process development, device
prototyping, and for integrating the imprint process into
the CMOS process flow
- Toshiba using tool for device prototyping (MNE, 2007)
– 18nm lithography with