Alex.A. Samarian and Brian.W. JamesSchool of Physics, University of Sydney, NSW 2006, Australia

Sheath edge locationThe charge of dust particles in sheath region

Dust Motion in rf-Discharge Plasma

2

Sheath model

Different features of the Different features of the sheath and plasma areassheath and plasma areas

Sheath edge location is Sheath edge location is one of the most one of the most important parameters in important parameters in many sheath modelsmany sheath models

PlasmaPlasma

SheathSheath

ElectrodeElectrode

ne(z)ne(z)

ni(z)ni(z)(z)(z)

ze

zb

Cross section of sheath in z-dirCross section of sheath in z-dirperpendicular to electrodeperpendicular to electrode

E(z)E(z)

3

Charging of dust particle

Ion and electron fluxesIon and electron fluxes

a) electrons

b) ions

4

Equilibrium of dust particles

FG

rav

FG

rav

FEl

ec

FEl

ec

FIon

Drag

FIon

Drag

FThermo

phoretic

FThermo

phoretic

FNeut

Drag

FNeut

Drag

Force Force DiagramDiagram

FGra

v

FGra

v

FElecFElec FIon DragFIon Drag

FThermophor

etic

FThermophor

etic

FNeut

Drag

FNeut

Drag

Force DiagramForce Diagram

TemperatureGradient

TemperatureGradient

Neutral FluxNeutral Flux

avm

eZ

si

d2

2

1

21

d

2

2

2

2

2

22

2

2

2

2

1

/

ln2

avm

eZ

avm

eZa

avm

eZ

si

d

si

dD

si

d

is the screening length

Where

and

zElectric Field

GradientElectric Field

Gradient Ion FluxIon Flux

5

0

0,1

0,2

0,3

0,4

0,5

0 0,5 1 1,5 2 2,5 3

Dust radius, m

(hb-heq)/hb

(z)(z)E(z)E(z)

zezb

ElectrodeElectrodePlasmaPlasma SheathSheath

2

222 1

i

diiii

amv

eZvmnaF

for i=107 s-1

for i= 5x106 s-1

for i= 106 s-1

Normalised difference between grain equilibrium position & sheath edge position as a function of grain radius for different values of ion plasma frequency

6

Front ViewFront ViewExperimental setup

7

Back ViewBack ViewExperimental setup

8

Experimental SetupExperimental chamber and image of test dust particles levitated above the electrode

The test grains are generated in the discharge (power up to 200W, pressure up to 1 torr) by electrode sputtering.

Dust particles

Grounded electrode

Powered electrode

9

Main parameters

RF discharge 15 MHz

Pressure from 10 to 400 mTorr

Input power from 15 to 200 W

Self-bias voltage from 5 to 180V

Carbon (C) particles’ diameter ~ 1 μm

Melamine formaldehyde - 2.79 μm ± 0.06 μm

Melamine formaldehyde - 6.13 μm ± 0.10 μm

Argon plasma Te ~ 2 eV, Vp =50 V & ne ~ 109 cm-3

10

Result of Measurements

0

0,5

1

1,5

2

2,5

3

0 2 4 6 8 10 12 14 16 18 20

h, mm

T, eV I, a.u.1

0.5

heq

Measured electron temperature Te (points with error bars) and discharge emission I

(solid line) for P = 90m Torr, W = 80 W. The dashed line shows the equilibrium position (heq =10.8 mm) of test grains (a~350 nm).

Rf-compensated Rf-compensated

single Langmuir single Langmuir

probeprobe

Electron temperature Electron temperature

TTee

Ocean optics Ocean optics

spectrometer spectrometer

PC2000 PC2000

discharge emission Idischarge emission I

11

Result of Measurements

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100

Pressure, mTorr

hb

, mm

Position of sheath edge (Position of sheath edge (hhbb) vs pressure at different rf-input powers) vs pressure at different rf-input powers

– 35W

– 60W

– 100W

12

Result of MeasurementsEstimation of dust charge Estimation of dust charge

from vertical equilibriumfrom vertical equilibrium

hq

E

Eeq

heq hb

2(Vb-Vp)/hb

V(h)=h2Vb/hb2+Vp-Vb

E=2(Vp-Vb)(hb-heq)/hb2

Zd=2 a3 hb2/3e (Vp-Vb)(hb-heq)

Dust particle radius, (m)

Charge on dust particle (e)

From vertical equilibrium

From probe data

0.95 5.3x103 3.6x103

1.39 1.1x104 5.8x103

3.07 4.2x104 1.2x104

hb

13

Summary

Spatial location of sheath edge in planar rf-Spatial location of sheath edge in planar rf-discharge has been determined using test dust discharge has been determined using test dust grains.grains.

The diagnostic technique is based on measurement The diagnostic technique is based on measurement of the equilibrium position of fine (<500nm) dust of the equilibrium position of fine (<500nm) dust grains levitated above the powered electrode of the grains levitated above the powered electrode of the rf-discharge. rf-discharge.

Using the resulting value of sheath dimension the Using the resulting value of sheath dimension the charge on dust particles in the sheath region has charge on dust particles in the sheath region has been estimated. been estimated.