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Nanometals and Laser Patterning
Jin-Shan Wang, Lee W. Tutt, and Mitchell S. Burberry
Particles 2007
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Overview
• Ag Nanoparticle Material– Synthesis– Coating properties– Sintering properties
• Laser Patterning– Direct thermal– Novel Lift-off
• Summary and Conclusions
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Size-Dependent Melting Point of Nanoparticles
232b m s
s lm s s l
T TT L R
ργ γρ ρ
⎡ ⎤⎛ ⎞− ⎢ ⎥= − ⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦
Ph. Buffat and J-P. Borel, Phys. Rew. A, 13, 1976, 2287
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Attributes
• Solvent Coated– Easy– Cheap– Extension to flexible polymeric substrates
• Easily Patterned– Inkjet– Direct printing; gravure or litho– Laser direct patterning– Conventional UV photoresist
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Ag Nano Potential Applications
• TFT Source/Drain and Conductive Traces– But probably not as bottom gate material
• Due to surface roughness– Contact pads
• Scratch resistance may be limiting
• Conductive Grids for Transparent Conductors– OLED pixel common electrodes– OLED lighting panels – Fiducial marks on transparent electrodes
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Synthesis of Ag(0)
• Temperature: 30–60°C (60°C)• R-NH2: dodecylamine, cyclooctylamine, 2(2-aminoethoxyl)-ethanol
• Reducing agent (RA): NaBH4,C6H5NHNH2
• Time: 1 h
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Purity Analysis of Ag(0) Nanosphere Using NaBH4 as Reducing Agent a
Element Mass wt%
Ag 108 98.5 wt%B 10.8 1.2 wt%Na 23 0.3 wt%
a by Inductively Coupled Plasma Atomic Emission Spectroscopy
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92.87°C
81.55°C26.23(63.42)J/g
120.07°C
13.20J/g
161.42°C
24.00J/g~107°C ~129°C
292.50°C
402.94°C
Ag scinter
-0.4
-0.2
0.0
0.2
0.4
0.6
Hea
t Flo
w (W
/g)
0 50 100 150 200 250 300 350 400 450 500
Temperature (°C)
Sample: CC0793-178 Ag nanoparticlesSize: 10.4700 mgMethod: DSC 500°C 10°C/minComment: 1st heat 10°C/min N2 2C
DSCFile: C:\TA\Data\DSC\lewis0406.11Operator: roger moodyRun Date: 6-Apr-06 07:51
Exo Up Universal V2.6D TA Instruments
Ag(0) Powder Is Soluble in Organic Solvent
?
DSC of 60°C Synthesized Ag Nanoparticles
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348.06°C
466.98°C
155.41°C
151.07°C111.8J/g
probable scintering
-1
0
1
2
3
4
5
Hea
t Flo
w (W
/g)
0 50 100 150 200 250 300 350 400 450 500
Sample: CC0793-169 Ag nanoparticlesSize: 6.5400 mgMethod: DSC 500°C 10°C/minComment: 1st heat 10°C/min N2 2C
DSCFile: C:\TA\Data\DSC\lewis0310.11Operator: roger moodyRun Date: 10-Mar-06 08:57
2 6
Powder Is Insoluble in Organic Solvent
DSC of 30°C Synthesized Ag Nanoparticles
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• Surface capped with dodecylamine• Broad size distribution but all smaller than 10 nm• Well dispersed in nonpolar solvents, such as toluene and cyclohexane, etc. (NOT CHCl3)
TEM of Kodak vs Cabot Ag(0)
Kodak Cabot
(Cabot AG-IJ-G-100-S1)
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RMS 2.1 nm
RMS 8.2 nm
Dynamic AFM
Same sample monitored as heatedºC ºC ºC
ºC ºC
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100°C 120°C
150°C 180°C
AFM @ Different Sintering Temperatures Different samples sintered at temperatures for extended periods
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SEM Images Before and After Sintering @ 150°C for 30 min
Before After
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Resistivity of Coated Ag Film on Glass Sintered @ Different Temperatures a
temp, oC Cabot CC0793-173 bulk Ag100 1042 2316 1.6120 129 138 1.6150 35 7.5 1.6180 32 4.7 1.6
a μΩ.cm, film was coated at 500 rpmb CC0793-173, synthesized @ 60°C and coated from 10% solution incyclohexane
Kodak
Kodak
(AG-IJ-G-100-S1)
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Effects of Sintering Temp and Time on Resistivity of Coated Nano Ag(0) Film a
a μΩ.cm
Time, min 100°C 120°C 150°C1 NA NA NA2 NA NA NA5 NA NA 11.610 NA NA 4.530 1042 130 7.5
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Solvent bp (c) %Ag500 rpm 1000 rpm
Cyclopentane 50 10 Comet CometCyclohexane 80.7 10 Comet Comet
Toluene 110 10 Comet CometEthylbenzene 136 10 Comet CometEthylbenzene 136 5 Comet GoodEthylbenzene 136 15 Comet CometEthylbenzene 136 20 Comet CometCyclooctane 151 10 Comet Comet
Propylbenzene 159 5 Comet GoodPropylbenzene 159 10 Comet GoodPropylbenzene 159 15 Comet GoodPropylbenzene 159 20 Comet CometButylbenzene 183 10 Very Good Very GoodButylbenzene 183 15 Very Good Very GoodButylbenzene 183 20 Very Good Very Good
Coating Quality
Solvent Effect on Spin Coating Quality
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Thickness vs Concentration, in Propylbenzene
0200400600800
100012001400
0 5 10 15 20 25
Concentration,Ag(0)%
Thic
knes
s, A
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Solvent bp,oC Ag(0)%Sheet resistance
(Ohm/square) Thickness (nm) Resistivity (uOhm.cm) Conductivity (S/cm)
Propylbenzene 159 5 no contact 47Propylbenzene 159 10 4.21 62 26 3.8E+04Propylbenzene 159 15 1.78 74 13 7.6E+04Propylbenzene 159 20 0.83 116 10 1.0E+05
Resistivity vs Thickness, in Propyl Benzene
0
5
10
15
20
25
30
0 20 40 60 80 100 120 140
Thickness,nm
Resi
stiv
ity, m
ohm
.cm
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Nanoparticle Material Summary
• Ag particles smaller than 10 nm have been formed reliably.
• The temperature of formation is critical.
• Sintering occurs at <200°C, compatible with many polymeric supports.
• Coating quality is strongly affected by solvent.
• Thickness >100 nm is necessary for uniform film and hence lowest resistivity.
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Laser Patterning
• Direct Laser Thermal Sintering– Infrared
• Novel Lift-off Process– Photoresist– Laser ablation
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Laser Sintering
Before Laser
After Wash (oven optional)
hν
After Laser
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Absorption Spectrum of Ag Nano
Corrected %A
0
10
20
30
40
50
60
70
80
90
100
300 400 500 600 700 800 900 1000 1100
nm
%A
1000 rpm Ag Nano on glass
Glass
100 rpm on glass
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Laser Writer
• ~810 nm CW bar laser
• ~42 mW/channel (max)
• Up to 256 channel modulator
• Up to 0.7 m/s (fast scan)• X-Y servo translation stages
• 2.5" x 2.5" image size
• ~5 μm spot size
• ~2 μm placement• Vacuum collection (active area of investigation and
refinement)
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Cross-section SEM Images
Anneal at ~2 J/cm2, 3 Ω/sq or ~ 90 μΩ-cm
Unannealed
Anneal at ~3 J/ J/cm2, 0.2–0.3 Ω/sq or ~7 μΩ-cm
Anneal at ~1 J/J/cm2, nonconductive
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Laser Sintering Followed by an Annealing Series @ 120°C
2 Point Probe Resistance Measurements Annealing at 120 C
After Laser Sintering
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100 120 140 160
min
Ohm
s
Tau = 7.63 min
Laser Writer 32.5 W, 0.1 m/s, ⇒ ~9 J/cm2
80 Ω⇒ 1.2 Ω/ρbulk Ag = 1.59 μΩ cmρlaser Ag = 25 μΩ cmρoven Ag = 22 μΩ cm
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Coating Thickness and Exposure Series
Ag Nano Laser Sintered Bulk ResisivityThickness and Exposure Series
0.00E+00
5.00E-08
1.00E-07
1.50E-07
2.00E-07
2.50E-07
3.00E-07
3.50E-07
4.00E-07
4.50E-07
5.00E-07
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0
Exposure (J/cm2)
Ohm
*m
10001000b500300Bulk Ag
PtFeWAlAuAg
Bulk
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Coating Thickness and Exposure Series
Laser Sintered Ag Nano Reflection Micrographs; SCW; 32.5 W; 192 Ch’s
1000 rpm 300 rpmΩ/
0.7
1.2
1.5
Off-Scale
Off-Scale
Ω/
0.38
0.56
1.5
1830
Off-Scale
J/cm2
16.9
8.4
4.2
2.8
2.1
1.7
1.4
1.2
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Profilometry of Ag Traces1000 RPM Ag Nano on Glass
The average of 3 scans (6 mg of force) = 176 nm ± 14 nm; rms ~70 nm; peak-to-peak ~177 nm
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Laser Sintered Ag Patterns
ReflectionTransmission
• Laser current 40 A and Velocity 0.1 = 11 J/cm2
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Novel Lift-off Process
• No Etching Required
• Compatible with Photoresists
• Compatible with Laser Ablation Resists
• High Resolution
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Standard Lift-off Process
Requires re-entrant profile (undercut)
Requires asymmetric coating process
Dissolve photoresist
Deposit material
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The Process
• Coat Photoresist and Pattern
• Coat Ag Nanoparticles
• Dissolve Photoresist with Acetone
• Anneal Nanoparticles
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Novel Lift-off Patterning Process
Expose resist
Coat nanoparticles
Dissolve resist
Anneal
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Requirements for Novel Lift-off Patterning Process
• Nanoparticle solvent must not be solvent for resist.
• Stripping solvent must not be solvent for nanoparticles.
• Thickness of nanoparticle layer must not be so large that the cohesive force becomes substantial and allows bridging.
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Why It Works with Our Ag Nanoparticles
• Ag nanoparticle solvent of cyclohexane does not attack the polymer resist.
• Resists are soluble in acetone and the Ag nanoparticles are not.
• Silver nanoparticles are relatively porous to solvents before annealing.
• Silver nanoparticles adhere to substrates and each other.
• Cohesive energy of the silver nanoparticles is relatively low.
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Nanoparticle Ag TFT Channel
Transmission on glass Reflection on glass
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High-Resolution Ag Lines
Reflection micrograph
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Not Everything Is Perfect!
Some things that can go wrong
1. Drying artifacts
2. Thickness cracking and bridging
3. Interfacial work function and chemical effects
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Drying Effects on Coating
Transmission micrograph showing drying artifacts
300 rpm spin coat
Reflection micrograph
after annealing
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Laser Lift-off TFT
Before Ag Nanoparticles
After Ag Nanoparticles
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ZnO Transistor with DS of Ag Nanoparticles
1 .0E-13
1 .0E-12
1 .0E-11
1 .0E-10
1 .0E-09
1 .0E-08
1 .0E-07
1 .0E-06
1 .0E-05
1 .0E-04
1 .0E-03
-10 0 10 20 30 40
[LT_4_19_s amp le4_3 ]V DS=10 ID
[LT_4_19_s amp le4_3 ]V DS=20 ID
[LT_4_19_s amp le4_3 ]V DS=30 ID
Vds = 10
Vds = 20
Vds = 30
2.9E+0514.46.49E-0230
1.2E+0413.14.13E-0220
6.3E+027.79.38E-0210
Ion/IoffV th
MobilityVds
32μm channel length
Vg
IDS
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Conclusions
• Solvent lift-off with resist is a simple and effective means of patterning Ag nanoparticles.
• Direct laser sintering is a simple and effective means of patterning Ag nanoparticles into conductive traces.
• Resistivity approaching 3x bulk silver has been achieved without post-process heating.
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Acknowledgments
Seung Baek Roger Moody
Peter Cowdery-Corvan Shelby Nelson
Therese Feller Glenn Pearce
Jill Fornalik Kay Phillips
Diane Freeman Larry Rowley
Janet Heyen Todd Spath
Craig Lewis
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