Simulation of an Atmospheric Pressure Direct Current Microplasma Discharge in He/N2

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October 13, 2011

Keisoku Engineering System Co., Ltd., JAPAN

Dr. Lizhu Tong

COMSOL CONFERENCE BOSTON 2011

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Contents

1. Atmospheric pressure glow discharge

2. Plasma simulation using COMSOL Multiphysics

3. He/N2 dc microplasma model

4. Results

5. Conclusions

Current-voltage (I-V) characteristics

Discharge structure such as cathode dark space (CDS) region

Effect of cathode temperature

Atmospheric pressure glow discharge (1)

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Recently, interest has grown toward atmospheric pressure plasmas to reduce

the cost by moving away from vacuum equipment.

Microplasmas are characterized by their small size (characteristic dimensions,

of tens to hundreds of microns) and high gas pressure (100 Torr1 atm),

yielding nonequilibrium plasmas.

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Atmospheric pressure glow discharge (2)

Application fields

• Etching for semiconductor devices

• Formation of Diamonds • Formation of carbon

nanotube

• Flue gas treatment • Ozone formation • Sterilization, etc.

• Biocompatibility granted to medical materials

• Control of physicochemical properties on substrate

In this work,

The atmospheric pressure direct current (dc) microdischarge, one of the easy methods

of generating an atmospheric pressure nonequilibrium plasma, is studied.

Types of plasma involved in COMSOL Multiphysics

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The common types of plasma: • Inductively coupled plasma (ICP) • DC discharge • Microwave plasma • Electrical breakdown • Capacitively coupled plasma (CCP) • Combined ICP/CCP reactor

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Plasma module physics interfaces

• The drift diffusion interface • The heavy species transport interface • The Boltzmann equation, Two-term approximation interface • The inductively coupled plasma interface (ICP) • The microwave plasma interface • The capacitively coupled plasma interface (CCP) • The DC discharge interface

Plasma module format in COMSOL Multiphysics

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Plasma interfaces Plasma model library

Neutrals (2)

He, N2

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The reactions included in the model

Plasma chemistry

No. Reaction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

e− +He→e− +He(21S) e− +He→e− +He(23S) e− +He→2e− +He+ e− +He(21S)→2e− +He+ e− +He(23S)→2e− +He+ 2e− +He+→e− +He 2e− +He+→e− +He* e− +He2

+→He +He* e− +N2→2e− +N2

+ e− +N2

+→N2 He+ +2He→He2

++He He(21S)+He→2He+hv He(23S)+2He→He2+He He(23S)+He(23S)→He++He+e− He+ +N2→N2

++He He2

+ +N2→N2++2He

He+ +N2+He→N2++2He

He2+ +N2+He→N2

++3He He(23S)+N2→N2

++He+e− He(21S)+N2→N2

++He+e− He(23S)+N2+He→N2

++2He+e−

Ions (3)

He+, He2+, N2

+

Excited species (3)

He(21S), He(23S), He*

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COMSOL Multiphysics solves a pair of drift diffusion equation for the electron density and electron energy density.

Electron transport

Source term Source term

Rate coefficient

Electron transport boundary conditions

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There are a variety of boundary conditions available for the electrons: Wall which includes the effects of : Secondary electron emission Thermionic emission Electron reflection Flux which allows you to specify an arbitrary influx for the electron density and electron energy density. Fixed electron density and mean electron energy Insulation

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Heavy species transport

Transport of the heavy species (non-electron species) is determined from solving a modified form of the Maxwell-Stefan equations :

The multiphysics interfaces contain an integrated reaction manager to keep track of the electron impact reactions, reactions, surface reactions and species.

where

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Heat transfer

Heat transfer inside the computational domain is modeled by the below equation:

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Electrostatic field

The plasma potential is computed from Poisson’s equation:

The space charge is computed from the number densities of electrons and other charged species.

Discharge voltage

Supplied voltage

Discharge voltage

Current density Ballast resistor

Electrode area

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He/N2 dc microplasma model (1)

DC microplasma model Computational conditions Gas： pure He and He/N2 mixtures N2 fractions: 0.002-0.02% Pressure：760 Torr Power source：DC Operating voltage: 232-450V Ballast resistor: 10 k Electrode area: 0.006 cm2

Cathode temperature：350, 450, 550 K Anode temperature：350 K

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Coupled simulation of plasma and heat transfer

Heat transfer

Plasma

He/N2 dc microplasma model (2)

Heat source

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Plasma PDE equation

Discharge voltage

He/N2 dc microplasma model (3)

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Results (1)

Current-voltage (I-V) characteristics:

The boundary condition for electric field calculation is specified as Vc = 0 on the cathode and Va = Vd on the anode.

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Results (2)

0

2E+19

4E+19

6E+19

8E+19

1E+20

1.2E+20

0 50 100 150 200

Den

sity

(m-3

)

Distance from cathode (m)

electronHe+He2+N2+

Te

V

Discharge structure in a He/0.02%N2 microdischarge at V = 420 V

Number density Electrical potential and electron temperature

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Results (3)

The effect of cathode temperature in He/0.02%N2 microdischarges

The electrical conductivity is strongly dependent on temperature, which can be approximated by

300

400

500

600

700

0 50 100 150 200

Gas

tem

pera

ture

(K)

Distance from cathode (m)

He/0.02%N2 (350K)He/0.02%N2 (450K)He/0.02%N2 (550K)

Gas temperature

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Results (4)

0

50

100

150

200

250

0 50 100 150 200

Pot

entia

l (V

)

Distance from cathode (m)

He/0.02%N2 (350K)He/0.02%N2 (450K)He/0.02%N2 (550K)

0

3E+19

6E+19

9E+19

1.2E+20

1.5E+20

0 50 100 150 200

Ele

ctro

n de

nsity

(m-3

)

Distance from cathode (m)

He/0.02%N2 (350K)He/0.02%N2 (450K)He/0.02%N2 (550K)

Electrical potential Electron density

Discharge structure in He/0.02%N2 microdischarges at different cathode temperatures

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Results (5)

130

150

170

190

210

230

250

0 4 8 12 16 20 24

Dis

char

ge v

olta

ge (V

)

Discharge current (mA)

He/0.02%N2 (350K)He/0.02%N2 (450K)He/0.02%N2 (550K)

I-V characteristics in He/0.02%N2 microdischarges at different cathode temperatures

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Conclusions

The simulations of atmospheric pressure direct current micro-plasma discharges in He/N2 were performed by coupling plasma simulation with heat transfer calculation.

A simple circuit model was used to obtain the discharge voltage regarded as the boundary potential condition in the plasma simulation.

The effect of a small amount of N2 added to He as well as the effect of cathode temperature on the I-V characteristics were studied.

It could be concluded that by using COMSOL Multiphysics, the simulations would be very beneficial in finding the design parameters of atmospheric pressure plasma sources for surface modification.

Thank you for your attention