Felipe Iza and Jeffrey A.HopwoodElectrical and Computer Engineering Department, Northeastern University, Boston MA (USA)

ATMOSPHERIC MICROPLASMA SOURCE BASED ON A MICROSTRIP SPLIT-RING RESONATOR

THE MICROSTRIP SPLIT-RING RESONATOR (MSRR) MICROWAVE INDUCED PLASMA (MIP) SOURCE

GOALTo create a microplasma source suitable for portable applications. Desired characteristics include:

v Low power – Low voltage

v Operation at atmospheric pressure

v Operation in noble gases and air

v Long lifetime

APPLICATIONS: v Chemistry and Environmental Science: Mass, Ion mobility and Optical Emission spectrometers

v Biomedical: Sterilization, cell treatment, bio-compatible coatings.

v Localized Material Processing: Etch, deposition, cleaning, surface modification

vLight Sources: Visible and UV light sources

CHALLENGES: Device:

v Impedance-matched low-loss device

v Optimization of gap size and geometry

vMaterial selection and electronics integration

Plasma:

v Internal plasma efficiency in air discharges at atmospheric pressure.

INPUT IMPEDANCE AND PLASMA IGNITION

Device III45 µm gap

Device II120µm gap

Device I500µm gap

DC Paschencurve

Pressure x gap size (torr-cm)

0

200

300

400

10-3 10-2 10-1 100 101

Air

100

RM

S B

reak

dow

n V

olta

ge (V

)

AIR

Pressure (torr)

Device III45 µm gap

Device II120µm gap

Device I500µm gap

0.0

1.0

2.0

3.0

10-1 100 101 102 103

1.5

2.5

0.5

Air

Igni

tion

Pow

er (W

)

Breakdown Voltage (V)Ignition Power (W)

Device III45 µm gap

Device II120µm gap

Device I500µm gap

DC Paschencurve

Pressure x gap size (torr-cm)

0

100

150

200

10-3 10-2 10-1 100 101

Argon

50

RM

S B

reak

dow

n V

olta

ge (V

)

ARGON

10-1

100

101

102

103

0

0.5

1

1.5

2

2.5

3

Pressure (torr)

Device III45 µm gap

Device II120µm gap

Device I500µm gap

0.0

1.0

2.0

3.0

10-1 100 101 102 103

1.5

2.5

0.5

Argon

Igni

tion

Pow

er (W

)

Ignition Power and Breakdown Voltage

2p pk = ß - ja = - j? Q ?

oin

p po 1 o 2

p po 1 o 2

ZZ = Z Z

Z + tanh(jkl ) Z + tanh(jkl )2 2

Z Z+ Z tanh(jkl ) + Z tanh(jkl )

2 2

+

pZ

2pZ

2

l1 l2

Zin

Zp

Zin

θ pZ2

pZ2

Z2Z1

l2l1

Zin

ZoZo

ogap in

Z QV = 4 P

p

( )oin o

o

ZZ 1-cos?

a?≈

oin in in o

?V = 2P Z =V sin

2

gap oV = 2V Compromise:Zo↑ ⇒ narrow microstrip ⇒ Q ↓

Device Input Impedance and Voltage Across the Gap

-150 -100 -50 0 50 100 1500

2

4

6

8

10

Angle θ (degrees)

Z in

(kΩ

)

Increasing Q(10 to 1000)

Angle θ (degrees)

10

120

230

3401000 …

o

pQa?

=

Odd mode

3 Designs

Diel. Permittivity: εr=10.8Diel. Thickness: 635 µmMicrostrip width: 2.5 mm

Zo=20ΩGap: 500 µm

Diel. Permittivity: εr=10.8Diel. Thickness: 635 µmMicrostrip width: 2.5 mm

Zo=20ΩGap: 120 µm

Diel. Permittivity: εr=10.2Diel. Thickness: 2.54 mmMicrostrip width: 1 mm

Zo=70ΩGap: 40 µm

DEVICE I DEVICE II DEVICE IIISMA

connector

DischargeGap

λ/4

~ 2 cm

~ 5

cm

λ/2

Matching network

Split ring resonator

AlCuDielectricCuAl

Hole drill

Photolithography

Cladding removal

Photoresist strip

Cu etch

Fabrication

Principle of Operation

Groundplane

Lineplane

Section AA’

Groundplane

Lineplane

Section BB’

A’A B’

B

Magnitude of the electric field |E|Simulation using HFSS from Ansoft

Discharge gapGround Plane

Dielectric

g

h

Microstrip

Eg

Eo

g oh

E 2 Eg

≈V, I

Split

-Rin

gR

eson

ator

Line

arR

eson

ator

Voltage CurrentV, I

DEVICE I:Ar plasma @ 1W, 900 MHz

Glass tube

Microstrip

Glass tube

Microstrip

Glass tube

0.1 torr 10 torr 760 torr100 torr

Pressure Range of OperationExperiment Setup

Coaxial Probe

Glass tube(chamber)

Manifold

PlasmaSourceGas outlet

To pressure gauges

Gas inlet

Needlevalve

30dB

900.000 -7.3

MKS

0.53

0.53- - -

Self-ignition at atmospheric pressure with <3W in argon and air has been accomplished with devices with gap sizes of 45 µm and 25 µm respectively.

45 µm120 µm500 µmGap Size190 V115 V110 VVgap (@1W)

~4 MV/m~1 MV/m~0.2 MV/mEgap (@1W)

104138130Q70 Ω20 Ω20 ΩZo

Device IIIDevice IIDevice I

λ / 2

λ λα

αλo

θo

θo

Vol

tage

(a.u

.)

Angle θ (degrees)-150 -100 -50 0 50 100 150

o?V V sin2

≈

Vo

-Vo

θ0

θ

αλo

π

i e floatingE 6T V= + e eE 2T=Collisional energy loss per electron-ion pair created

Ions (Ei)

Excitation:Electronic, vibrational and rotational

Dissociation

Elastic collisions

Acknowledgements:Related Publications:v Iza F. and Hopwood J., “Rotational, vibrational and excitation temperatures of a microwave- frequency microplasma,” IEEE Transactions on plasma science, Vol. 32 No. 2, pp. 498-504, April 2004

v Iza F. and Hopwood J., “Low-power microwave plasma source based on a microstrip split-ring resonator,” IEEE Transactions on plasma science, Vol. 31, No. 4, pp 782-787, August 2003

v “Low-Power Plasma Generator,” F. Iza and J. Hopwood. US Patent Application

Filament, fan and striation formationFilament geometry

Self-arrangement of parallel filamentsOther non-uniformities

Grant No. DMI-0078406 Grant No. 15995805

PLASMA SOURCE POWER EFFICIENCY

( )

MSRR p-Q

p-2 2 Q

1?

1-e1+

1- G - T e

=

0.4

0.5

0.6

0.7

0.8

Frequency (MHz)

Ref

lect

ion

coef

ficie

nt (s

11)

790 800 820795 805 810 815

0.25 W0.50 W0.75 W1.00 W1.25 W

Symbols: measurement Lines: Fitted dataDevice II – Argon 400mtorr

Device Power Efficiency

660 – j1424460 – j1701432 – j1923407 – j2146405 – j2223

12954 – j54626508 – j23764808 – j20024105 – j18323236 – j1348

274 – j 800227 – j 900223 – j 996221 – j1067223 – j1150 40

0 m

torr

760

torr

Zp = Rp+jXp

Device IIIDevice II

0.250.500.751.001.250.500.751.001.251.50

Power(W)

400

mto

rr76

0 to

rrRp

Rp

Xp

Xp

Xp>Rp

Rp>Xp

Data not available

p

oin

o 1 o 2p

op p

1 o 2

ZZ =

Z + tanh(jkl ) Z + tanh(jkl )2 2

+ Z tanh(jkl ) + Z tanh(

Z Z

Z2Z

jkl )2

+

in11

in

Z -50s =

Z +50

Zp

Zin

Discharge Gap

Microstrip

Sheath SheathBulk

Microstrip

pXj

2pX

j2

pR

Zp=Rp+jXp

s: Sheath widthA: Contact areaVs: Sheath voltage

po

1 1 sX =

C? e ? A=

0.5~0.750.5e ss n V−∝

p bulke

?R K

n=

ν: Electron-neutral collisional frequency

ne: Electron density

At low pressure the plasma impedance (Zp) is dominated by the sheath reactance (Xp)

At atm. pressure Zp is dominated by the resistance of the bulk of the plasma (Rp)

Maximum efficiency is achieved when Zp=2Zo

Power efficiencies of ~60% are attainable both at low and high pressures

MSRR-MIP Efficiency – Device II and III

Power (W)0 10.5 1.5

60

70

50

40

30

Effic

ienc

y (%

)

MSRR-MIP II400mtorr

MSRR-MIP III400mtorr

MSRR-MIP III760torr

Zo 20→70

Plasma Impedance

The plasma impedance (Zp) is determined by fitting the reflection coefficient (s11) measurements.

p

o p

Z G =

2Z +Zo

o p

2Z T =

2Z +Z

Normalized Plasma Impedance (Zp/Zo)

OptimumZp=2Zo

10-6 10-4 10-2 100 102 104 1060

20

40

60

80

100

Effic

ienc

y (%

)

Q increasing

Qincreasing

Q=10

Q=120

Q=230

Q=1000

…

p

o p

Z G =

2Z +Zo

o p

2Z T =

2Z +Z

ΓEoTEo

Zp

PlasmaEo

Zo

Resonator Quality factor

Q

MSRRPower dissipated in the plasma

?Power dissipated in the device

=

INTERNAL PLASMA EFFICIENCY

CONCLUSIONS

Losses in the sheath

Collisional losses (Ec)

Energy absorbed by the plasma

Electrons (Ee) Inelastic

collisions

Ionization(Eiz)

Collisional energy loss per electron-ion pair created1000

100

Te (eV)

Ar[2]

E c(e

V)

1 10 10010

Ionization Potential (Eiz)Ar 15.8 eVO2 13.6 eVN2 14.5 eV

O2[2]

N2[3]

Ar @ Te=2eV ~ N2 @ Te=7eV

ex dis e mc iz ex dis e

iz iz iz

? ? 3m ?E =E +E +E T

? ? M ? +

10-1 100 101 102 1030

1

2

3

4

Te

(eV

)

ni=1010cm-3

ni=1011cm-3

ni=1012cm-3

ni=1013cm-3

ni=1014cm-3

ni=1015cm-3

ni=1016cm-3

Experimental datafrom a mICP[1]

Finite Difference model of MSRR-MIP I

Electron Temperature

Pressure (torr)

Pressure (torr)0.01 0.1 1 10 100 1000 10000

Floa

ting

Pote

ntia

l (V

)

-5

0

5

10

15

20

251250 mW1000 mW750 mW

250 mW500 mW

150 mW

Probe: Thin gold wire (d=50µm)

Floating Potential - Device I - Argon

Low Pressure Regime

High Pressure Regime

AIRARGON

~60000eV~700eV~700eV~60eV2%

~10eV

High Pressure(Te ~ 1eV)

1%

~40eV

Low Pressure(Te ~ 2.5eV)

~10eV~40eVSheath Ee+ Ei

0.02%15%ηplasma

High Pressure(Te ~ 1eV)

Collisions Ec

Low Pressure(Te ~ 2.5eV)

eic

izplasma EEE

Ecretedpair ion -electronper absorbedenergy Total

energyIonization?

++==

25 µm

Feedback Loop

Ring Resonator

Amplifier

After operation in atm. air

Before operation in atm. air

NON-MAXWELLIAN SEMI-BALLISTIC ELECTRONS

Device:Gap size ~25 µm

Gap Voltage ~ 35V

Atmospheric air conditions:Electron mean free path ~ 5 µm (~5 collisions crossing the gap)

Electron-neutral collisional frequency 1THz >> 1GHz

Energy gained between collisions:

5 = 7 eV !!35

25

[1] Iza F., Hopwood J., Plasma Sources Sci. Technol., 2002, Vol. 11, No. 3, pp. 229-235 [2] Lieberman M.A. and Lichtenberg A.J., “Principle of plasma discharges and materials processing,” 1994, p. 81 [3] Tao K. Tao K., Ph.D. Thesis, Northeastern University, Boston - Massachusetts, 2001, pp. 75

0

1

2

1 10 100 1000Pressure (torr)

Tem

pera

ture

(eV

)

1eV =11605K

Excitation Temperature0.3W - 1W

Electron Temperature ni =10 10 to 10 16cm -3Vibrational Temperature

0.3W-1WN2 1st positive system

N2 2nd positive system

Device II - 99.9% Ar 0.1%N2

370360350340330320310300290280

Device III @ 1W – Argon and Air

1 10 100 1000Pressure (torr)

Gas

Tem

pera

ture

(K)

Air

Argon

300 310 320 330340 350 360 370

Device II - 99.9% Ar 0.1%N2

Pressure (torr)

0.5

10.90.80.70.6

0.40.3

Pow

er (W

)

10 100 7601

Trot

PLASMA CHARACTERISTICS

Temperatures

Microstrip

Microstrip

400µm

900µm

Electron Density

Ion

Den

sity

ni(c

m-3

)

0

2·1010

4·1010

6·1010

8·1010

1·1011

1.2·1011

1.4·1011

Power (W)

mICP[1]

250% increase!!

0 0.4 10.2 0.6 0.8 1.2

MSRR-MIP:Device IDevice IIDevice III

Arg

on 4

00 m

torr

Estimated electron density in Ar @1W : ~2⋅1014cm-3A

rgon

760

torr

e2

ebulk ne

?mArea

LengthR =

Measured plasma impedanceEstimated geometry

The MSRR-MIP source creates high density non-thermal discharges even at atmospheric pressure

Plasma efficiency is limited by collisional losses due to low electron temperature . How to increase electron temperature? ⇒ semi-ballistic electrons!!

THE MSRR MICROPLASMA SOURCE IS SUITABLE FOR PORTABLE APPLICATIONS:v Low costv Low-Power: < 3W Argon and Airv Atmospheric operation: Ar and airv Long lifetime

PERFORMANCE EVALUATION:v MSRR efficiency: up to 70%v Self-started operation and high-density dischargesü 1011 cm-3 Argon 400mtorr @ 0.5Wü 1014 cm-3 Argon 760torr @ 1W

v Small sheath voltage at pressures > 3 torrv Non-thermal plasma: Argon Tgas ~100Cv Reducing gap size improves discharge efficiency:ü Non-maxwellian semi-ballistic electrons (Te>1eV)ü Increase power densityü Reduce plasma impedance (improved matching

from the microstrip to the plasma)

FUTURE RESEARCH

New material selection for operation in air: Au, Al2O3Gap optimization

Power supply integration

Device Engineering

Plasma Physics

νω ω

η

η π

π

Γ

η

ν

νex νdis νm

νizνizνiz