recent progress in low-temperature, atmospheric pressure ...€¦ · by surfx technologies llc. •...

©2007 UCLA. All rights reserved.

Recent Progress in Low-Temperature, Atmospheric Pressure Plasmas

Michael Barankin and Robert HicksChemical Engineering and Materials Science

University of California, Los Angeles, CA


Outline

• Radio frequency, low-temperature, atmospheric plasma sources

• Discharge physics & chemistry• Plasma applications• Conclusions


Atomflo™ Plasma Tool

Atomflo™ applicator

• Handheld system developed by Surfx Technologies LLC.

• Generates intense beam of radicals at low temperature.

Control Unit


High-Speed Linear Sources

• Beam widths from 1.0 to 12 inches.

• Activates plastic at up to 1.0 m/s.

• Gas temp.150 – 300 oC.• Treats 3D objects.

5.0 cm

Atomflo™-500R by Surfx


• Atomflo™ may be fed with up to 5.0 vol.% O2, H2, N2, CF4, N2O, NH3, etc, in inert gas.

• Up to 20% of molecules are dissociated into atoms, O, H, N, F, etc.

• Chemicals may be injected downstream to deposit thin coatings.

Versatile Chemistry


Plasma Physics

• Need to determine the properties of the plasma:– Breakdown voltage, VB

– Electron density, ne

– Electron temperature, Te

– Neutral gas temperature, Tn


Current-Voltage Curvesfor Atomflo™ Source

Current (A)0.0 0.2 0.4 0.6 0.8 1.0 1.2

Vol

tage

(V)

50

100

150

200

250

300

760 Torr

500 Torr

300 Torr200 Torr

100 Torr

Breakdown

Helium breaks down at 170 V.

Townsend region before breakdown.

Abnormal glow discharge.


Current and Voltage Waveforms

Time (ns)

0 50 100 150 200

Vol

tage

(V)

-400

-200

0

200

400

Current (A

)-2

-1

0

1

2• Smooth waveforms

without spikes.• Capacitive – current

precedes voltage in time.• Phase angle:

Argon ~ 80º.Helium ~ 83o.

RF at 13.56 MHz


Electron Density

EenJ eeμ−=J = current density, A/cm2

ne = plasma density, cm-3

μe = electron mobility (α 1/P), cm2/V.s

E = electric field (V/d), V/cm


Electron Temperature

Total power input

Power loss due to ionization Loss from

electron-ion collisions

Loss from electron-atom collisions

Power balance on free electrons:

J. Jonkers et al., Plasma Sources Sci. Technol., 12, 30 (2003).

( )geBe

eeagB

eeieegB

e TTKMm

TKPnnIk

TKPn υσυσε −

⎥⎥⎦

⎤

⎢⎢⎣

⎡++=

232

11

Electron temperature


Physics of Atomflo™Plasma Source

• Break down voltages: helium = 170 V; argon = 550 V.

• Electron density (ne) = 1.0 x1012 cm-3.• Electron temperature = 1.2 eV.• Neutral temperature = 150 – 300 oC.


Plasma Chemistry

• Need to determine:– Reaction mechanism.

– Concentrations of radicals in plasma and afterglow.


Experimental Apparatus

MCT Detector

FTIR Source

TubeFlow Cell

To Vacuum

SapphireWindow

SapphireWindow

Plexiglass Box

IR Beam

Powered Electrode

Grounded Electrode

Quartz TubeCeramic Collar

Titration GasInlet

Gas Inlet

Titration with NO:NO + O + M → NO2 + M


Oxygen Atom Density

Conditions: 5.0 L/min Ar, 6.0 vol.% O2, 150 W/cm3, and 300±30 °C.

[O] = 1.2±0.6 ×1017 cm-3

Gas density = 1.3×1019 cm-3

⇒1.2 vol.% O atoms!

NO Concentration (x1017cm-3)

0 1 2 3 4 5 6 7

Inte

nsity

(a.u

)

0.00

0.01

0.02

0.03 Titration Point


Plasma Model

• Model inputs:– ne

– Te

– Tg

– Feed gases

• Mechanism:– 32 rxns plasma– 11 rxns afterglow

Time (ms)

0 1 2

Con

cent

ratio

n (c

m-3

)

1010

1011

1012

1013

1014

1015

1016

1017

1018

1019

Distance (cm)

0 1 2 3 4 5

O2

O

O3

O-

O2-

O(1D)

O2(1Δg)

O2+

O2

O

O2(1Δg)

O3

Plasma Afterglow


Comparison ofModel and Experiment

• Oxygen atoms– Exp.:

1.2±0.6 ×1017 cm-3

– Model:1.0×1017 cm-3

• Ozone– Exp.*:

4.3±0.5 ×1014 cm-3

– Model:2.9×1014 cm-3

*Determined experimentally by UV absorption.


Comparison ofAtmospheric Plasmas

Type of Discharge Plasma Density (cm-3)

O Atom Density (cm-3)

Torch Corona Dielectric barrier discharge RF capacitive discharge

1016 109 109 1012

1017- 1018 1012 1013

1016- 1017

Atomflo™ 1000x more powerful than coronas!


Applications

• Activating polymers for bonding.

• Metal etching.

• Depositing thin films:– SiO2, Si3N4, a-Si:H, ZnO, DLC.


Surface Activation


Adhesion toCarbon-Fiber Composites

Without plasma,adhesive failure.

With Atomflo™ plasma,cohesive failure.


Treatment of 3-D Parts

• Robot-mounted plasma sources scan – any 3-D object.– any size of flat panel

display.


Plasma Etching Results

• Rates exceed those obtained in low-pressure plasmas.

• Chemistry selection for disparate materials:– Organics etched with

O2 plasma.– Metals etched with

CF4/O2 plasma.

Etch

ing

Rat

e (μ

m/m

in)

0.01.02.03.04.05.06.07.08.0

Kapton SiO2 Ta W

Etch

ing

Rat

e (μ

m/m

in)

0.01.02.03.04.05.06.07.08.0

Kapton SiO2 Ta W


Tantalum Etching inCF4/O2 Plasma Afterglow

• Tantalum is a surrogate for plutonium.

• Etch rates up to 6.0 μm/min observed.

• Rate most sensitive to applied power.

200 300 400 500 600 7000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Etch

Rat

e ( μ

m/m

in)

RF Power (W)


Surface Morphology After Tantalum Etching

x30000

1μmSEI 5.00kV X30000

1μmSEI 5.00kV X30000

Surface is heavily fluorinated: TaF3and TaF4 by XPS


Plasma-Enhanced Chemical Vapor Deposition

PlasmaZone

CeramicSpacer

RF Power

SampleStage

Substrate

PlasmaGas Mixture

MetalPrecursorMetal

Precursor

Heater

PlasmaZone

CeramicSpacer

RF Power

SampleStage

Substrate

PlasmaGas Mixture

MetalPrecursorMetal

Precursor

Heater


Plasma-Enhanced Chemical Vapor Deposition

Argon & O2

Volatilechemicalprecursor

RF Power

Afterglow Chemistry

Thin Film Materials

PLASMAPlasma Physics


Organosilane Precursors

TMCTS

TEOS

HMDSO

TMDSO

HMDSN


Low Porosity Glass Grown with HMDSN

Three-dimensional image of 650-nm-thick film grown at 0.24 μm/min using HMDSN.

-35 nm

50 μm


Dielectric Strength

0.50.9

1.5

0

50

100

150

200

250

300

Vol

ts

Thickness (μm)

HMDSN Dielectric Strength HMDSN Breakdown

Glass film on 316 SS


Diamond-Like Carbon

• Process conditions:– 0.1 L/min C2H2, 0.5

L/min H2, 30 L/min He, 180 W & 155 oC.

• Dep. rate = 0.2 μm/min• Confirm DLC film by

C13 NMR with magic angle spinning.

sp2 sp3


Conclusions

• RF atmospheric plasmas are powerful tools for surface treatment.

• Ideal for automated processing of 3-D plastics.

• New processes are being developed to greatly enhance the functionality of materials.


Acknowledgements• Surfx Technologies

Peter Guschl, Steve Babayan, Joel Penelon, Quoc Truong, & Jason Orsborn

• U. Delaware, Center for Composite MaterialsJoe Dietzel & Jack Gillespie

• UCLAAngela Ladwig, Eleazar Gonzalez, Greg Nowling,

Xiawan Yang, Maryam Moravej, Vincent Tu, Sarah Habib, Franky Chan, Melanie Yajima, Vu Le, JarradGhirdalucci, Guowan Ding, Andreas Schutze


Numerical Model of thePlasma Chemistry

• One-dimensional “plug-flow” simulation.

• Mechanism includes 39 elementary steps among 19 species: neutrals, ions and metastables.

• Results compared to titration experiment.


Profiles of the Charged and Metastable Species

0 5 10 15 20106

107

108

109

1010

1011

1012

1013

1014

Den

sity

(cm

-3)

Distance (mm)

He+

He2+

He*

He2*

CF3+

CF2+

F_ CF3_

Electronegative plasma: nF– = 5 x ne.

Gas flow

Te = 2.5 eV


Profiles of the Neutral Species

Fluorine atom density: 1.3x1015 cm-3.

Good agreement with H2 titration.

0 10 20 30 40107

108

109

1010

1011

1012

1013

1014

1015

1016

Den

sity

(cm

-3)

Distance (mm)

F

CF2

CF3

C2F6

CF

C2F4

C2F5

C2F3

F2

Plasma Afterglow

Gas flow


H2 Titration of F Atoms in CF4 Plasma Afterglow

F atoms = 2x1015 cm-3

1.2 cm downstreamof electrodes.

12.8 Torr CF4, 2.3 Torr O2, 745

Torr He, 73 W/cm3 and 100 °C.0 1 2 3 4 5

0.002

0.003

0.004

0.005

HF

infr

ared

abs

orba

nce

Hydrogen concentration (x1015cm-3)

Titration point


Effect of Process Conditions on Tantalum Etch Rate

0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

2.5

3.0

Etch

Rat

e ( μ

m/m

in)

O2 Partial Pressure (Torr)

μEt

ch R

ate

(m

/min

)0 5 10 15 200.0

0.5

1.0

1.5

2.0

2.5

CF4 Partial Pressure (Torr)


Plasma Etching of UO2

Process conditions:

– Total Flow: 42 L/min, He/O2/CF4

– O2: 6 Torr – CF4: 15 Torr – RF Power: 300 W– Temperature: 200 oC– Nozzle-to-sample distance: 3 mm


Uranium Oxide Surface Morphology

0.5 μm 0.5 μm

Scanning electron micrographs of UO2 film before and after etching with CF4/O2/He plasma:

Before After a 2-min etch After a 5-min etch0.5 μm


Surface CompositionX-ray photoelectron spectra of UO2 film

1000 800 600 400 200 0

Cou

nts

Binding Energy (eV)

Ni A

uger

Fe 2

pU

4d 5/

2

U 4

d 3/2

F A

uger

U 4

f 7/2

O A

uger

F 1s

1/2

O 1

s 1/2

U 5

d 5/2

C 1

s 1/2

U 4

f 5/2

After a 2-min etch

Before etching


Etch time (min)0 2 4 6 8

Are

a (a

. u)

0.0

4.0e+4

8.0e+4

1.2e+5

1.6e+5

Dependence of U 4f Peak Intensity on Time

• Rate accelerates due to increased film porosity.

• Re-deposition may result in tail at >3 min.

recent progress in low-temperature, atmospheric pressure ...€¦ · by surfx technologies llc. •...

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