Nanometals and Laser Patterning

Jin-Shan Wang, Lee W. Tutt, and Mitchell S. Burberry

Particles 2007

2

Overview

• Ag Nanoparticle Material– Synthesis– Coating properties– Sintering properties

• Laser Patterning– Direct thermal– Novel Lift-off

• Summary and Conclusions

3

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

4

Attributes

• Solvent Coated– Easy– Cheap– Extension to flexible polymeric substrates

• Easily Patterned– Inkjet– Direct printing; gravure or litho– Laser direct patterning– Conventional UV photoresist

5

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

6

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

7

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

8

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

9

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

10

• 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)

11

RMS 2.1 nm

RMS 8.2 nm

Dynamic AFM

Same sample monitored as heatedºC ºC ºC

ºC ºC

12

100°C 120°C

150°C 180°C

AFM @ Different Sintering Temperatures Different samples sintered at temperatures for extended periods

13

SEM Images Before and After Sintering @ 150°C for 30 min

Before After

14

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)

15

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

16

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

17

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.

20

Laser Patterning

• Direct Laser Thermal Sintering– Infrared

• Novel Lift-off Process– Photoresist– Laser ablation

21

Laser Sintering

Before Laser

After Wash (oven optional)

hν

After Laser

22

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)

24

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

31

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

33

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

37

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

39

Drying Effects on Coating

Transmission micrograph showing drying artifacts

300 rpm spin coat

Reflection micrograph

after annealing

40

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

42

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.

43

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