Nanometals and Laser Patterning
Jin-Shan Wang, Lee W. Tutt,
and Mitchell S. Burberry
Particles 2007
Overview
• Ag Nanoparticle Material
– Synthesis
– Coating properties
– Sintering properties
• Laser Patterning
– Direct thermal
– Novel Lift-off
• Summary and Conclusions
2
Size-Dependent Melting Point of Nanoparticles
2
⎡
⎛ ρs ⎞ 3 ⎤
Tb − Tm
2 ⎢
γs −γl ⎜ ⎟ ⎥
=
Tm
L ρ s Rs ⎢
ρl ⎠ ⎥
⎝
⎣
⎦
Ph. Buffat and J-P. Borel,
Phys. Rew. A, 13, 1976, 2287
3
Attributes
• Solvent Coated
– Easy
– Cheap
– Extension to flexible polymeric substrates
• Easily Patterned
–
–
–
–
Inkjet
Direct printing; gravure or litho
Laser direct patterning
Conventional UV photoresist
4
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
5
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
6
Purity Analysis of Ag(0) Nanosphere Using
NaBH4 as Reducing Agent a
a
Element
Mass
wt%
Ag
B
Na
108
10.8
23
98.5 wt%
1.2 wt%
0.3 wt%
by Inductively Coupled Plasma Atomic Emission Spectroscopy
7
DSC of 60°C Synthesized Ag Nanoparticles
Sample: CC0793-178 Ag nanoparticles
Size: 10.4700 mg
Method: DSC 500°C 10°C/min
Comment: 1st heat 10°C/min
N2
2C
File: C:\TA\Data\DSC\lewis0406.11
Operator: roger moody
Run Date: 6-Apr-06 07:51
DSC
0.6
Ag(0) Powder Is Soluble in Organic Solvent
Ag scinter
0.4
Heat Flow (W/g)
92.87°C
?
0.2
120.07°C
161.42°C
402.94°C
0.0
~107°C
81.55°C
13.20J/g
26.23(63.42)J/g
~129°C
24.00J/g
292.50°C
-0.2
-0.4
Exo Up
0
50
100
150
200
250
300
Temperature (°C)
8
350
400
450
500
Universal V2.6D TA Instruments
DSC of 30°C Synthesized Ag Nanoparticles
Sample: CC0793-169 Ag nanoparticles
Size: 6.5400 mg
Method: DSC 500°C 10°C/min
Comment: 1st heat 10°C/min
N2
2C
5
File: C:\TA\Data\DSC\lewis0310.11
Operator: roger moody
Run Date: 10-Mar-06 08:57
DSC
Powder Is Insoluble in Organic Solvent
155.41°C
4
Heat Flow (W/g)
3
2
466.98°C
348.06°C
probable scintering
1
0
-1
151.07°C
111.8J/g
0
50
100
150
200
250
300
350
400
450
500
26
9
TEM of Kodak vs Cabot Ag(0)
Kodak
Cabot
(Cabot AG-IJ-G-100-S1)
• 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)
10
Dynamic AFM
ºC
ºC
RMS 2.1 nm
Same sample monitored as heated
ºC
ºC
RMS 8.2 nm
11
ºC
AFM @ Different Sintering Temperatures
Different samples sintered at temperatures for extended periods
100°C
120°C
150°C
180°C
12
SEM Images Before and After Sintering
@ 150°C for 30 min
Before
After
13
Resistivity of Coated Ag Film on Glass Sintered
@ Different Temperatures a
temp, oC
Cabot
100
120
150
180
(AG-IJ-G-100-S1)
1042
129
35
32
CC0793-173
bulk Ag
Kodak
2316
138
7.5
4.7
1.6
1.6
1.6
1.6
μΩ.cm, film was coated at 500 rpm
b CC0793-173,
Kodak
synthesized @ 60°C and coated from 10% solution in
cyclohexane
a
14
Effects of Sintering Temp and Time on
Resistivity of Coated Nano Ag(0) Film a
Time, min
1
2
5
10
30
a
100°C
NA
NA
NA
NA
1042
μΩ.cm
15
120°C
NA
NA
NA
NA
150°C
NA
NA
11.6
4.5
130
7.5
Solvent Effect on Spin Coating Quality
Solvent
Cyclopentane
Cyclohexane
Toluene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Cyclooctane
Propylbenzene
Propylbenzene
Propylbenzene
Propylbenzene
Butylbenzene
Butylbenzene
Butylbenzene
bp (c)
%Ag
50
80.7
110
136
136
136
136
151
159
159
159
159
183
183
183
10
10
10
10
5
15
20
10
5
10
15
20
10
15
20
16
Coating Quality
500 rpm
1000 rpm
Comet
Comet
Comet
Comet
Comet
Comet
Comet
Comet
Comet
Comet
Comet
Comet
Very Good
Very Good
Very Good
Comet
Comet
Comet
Comet
Good
Comet
Comet
Comet
Good
Good
Good
Comet
Very Good
Very Good
Very Good
Thickness vs Concentration, in Propylbenzene
1400
Thickness, A
1200
1000
800
600
400
200
0
0
5
10
15
Concentration,Ag(0)%
17
20
25
Solvent
bp,oC
Ag(0)%
Sheet resistance
(Ohm/square)
Propylbenzene
Propylbenzene
Propylbenzene
Propylbenzene
159
159
159
159
5
10
15
20
no contact
4.21
1.78
0.83
Thickness (nm)
47
62
74
116
Resistivity
(uOhm.cm)
Conductivity (S/cm)
26
3.8E+04
13
7.6E+04
10
1.0E+05
Resistivity, mohm.cm
Resistivity vs Thickness, in Propyl Benzene
30
25
20
15
10
5
0
0
20
40
60
80
Thickness,nm
18
100
120
140
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.
19
Laser Patterning
• Direct Laser Thermal Sintering
– Infrared
• Novel Lift-off Process
– Photoresist
– Laser ablation
20
Laser Sintering
hν
Before Laser
After Laser
After Wash (oven optional)
21
Absorption Spectrum of Ag Nano
100 rpm on glass
Corrected %A
100
1000 rpm Ag Nano on glass
90
80
Glass
70
%A
60
50
40
30
20
10
0
300
400
500
600
700
nm
22
800
900
1000
1100
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)
23
Cross-section SEM Images
Unannealed
Anneal at ~2
~ 90 μΩ-cm
Anneal at ~1 J/J/cm2, nonconductive
J/cm2,
Anneal at ~3 J/ J/cm2, 0.2–0.3
Ω/sq or ~7 μΩ-cm
3 Ω/sq or
24
Laser Sintering Followed by an Annealing
Series @ 120°C
Laser Writer 32.5 W, 0.1 m/s, ⇒ ~9 J/cm2
2 Point Probe Resistance Measurements
Annealing at 120 C
After Laser Sintering
100
90
80
70
Tau = 7.63 min
Ohms
60
80 Ω ⇒ 1.2 Ω/
ρbulk Ag = 1.59 μ Ω cm
ρlaser Ag = 25 μ Ω cm
ρoven Ag = 22 μ Ω cm
50
40
30
20
10
0
0
20
40
60
80
m in
25
100
120
140
160
Coating Thickness and Exposure Series
Ag Nano Laser Sintered Bulk Resisivity
Thickness and Exposure Series
5.00E-07
1000
1000b
500
300
Bulk Ag
4.50E-07
4.00E-07
3.50E-07
Ohm*m
3.00E-07
2.50E-07
Bulk
2.00E-07
Pt
Fe
W
Al
Au
Ag
1.50E-07
1.00E-07
5.00E-08
0.00E+00
0.0
2.0
4.0
6.0
8.0
10.0
Exposure (J/cm 2)
26
12.0
14.0
16.0
18.0
Coating Thickness and Exposure Series
Laser Sintered Ag Nano Reflection Micrographs;
J/cm2
1000 rpm
SCW; 32.5 W; 192 Ch’s
300 rpm
Ω/
Ω/
16.9
0.7
0.38
8.4
1.2
0.56
4.2
1.5
1.5
2.8
Off-Scale
1830
2.1
Off-Scale
Off-Scale
1.7
1.4
1.2
27
Profilometry of Ag Traces
1000 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
28
Laser Sintered Ag Patterns
• Laser current 40 A and Velocity 0.1 = 11 J/cm2
Reflection
Transmission
29
Novel Lift-off Process
• No Etching Required
• Compatible with Photoresists
• Compatible with Laser Ablation Resists
• High Resolution
30
Standard Lift-off Process
Requires re-entrant profile (undercut)
Requires asymmetric coating process
Deposit material
Dissolve photoresist
31
The Process
• Coat Photoresist and Pattern
• Coat Ag Nanoparticles
• Dissolve Photoresist with Acetone
• Anneal Nanoparticles
32
Novel Lift-off Patterning Process
Expose resist
Anneal
Coat nanoparticles
Dissolve resist
33
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.
34
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.
35
Nanoparticle Ag TFT Channel
Transmission on glass
Reflection on glass
36
High-Resolution Ag Lines
Reflection micrograph
37
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
38
Drying Effects on Coating
Reflection
micrograph
after annealing
Transmission micrograph
showing drying artifacts
300 rpm spin coat
39
Laser Lift-off TFT
Before Ag Nanoparticles
After Ag Nanoparticles
40
ZnO Transistor with DS of Ag Nanoparticles
1 .0 E- 0 3
1 .0 E- 0 4
1 .0 E- 0 5
1 .0 E- 0 6
IDS
Vds = 10
[L T_ 4 _ 1 9 _ s a mp le 4 _ 3 ]V D
S = 1 0 ID
1 .0 E- 0 7
1 .0 E- 0 8
Vds = 20
1 .0 E- 0 9
[L T_ 4 _ 1 9 _ s a mp le 4 _ 3 ]V D
S = 2 0 ID
1 .0 E- 1 0
Vds = 30
1 .0 E- 1 1
[L T_ 4 _ 1 9 _ s a mp le 4 _ 3 ]V D
S = 3 0 ID
1 .0 E- 1 2
1 .0 E- 1 3
-10
0
10
Vg
20
30
40
Vds
Mobility
V
Ion/Ioff
th
32μm channel length
41
10
9.38E-02
7.7
6.3E+02
20
4.13E-02
13.1
1.2E+04
30
6.49E-02
14.4
2.9E+05
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.
42
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
43