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Page 1: Diagnostic studies of a capacitively coupled RF plasma ... · PDF fileDiagnostic studies of a capacitively coupled RF plasma ... Retroreflector c h 4 s e t u p. D S 4 ... - Commercial

Diagnostic studies of a capacitively coupled RF plasma containing CH

4-H

2-Ar

Part II: On CH4 dissociation and hydrocarbon plasma chemistry

T.Kawetzki, Ch.Lukas, V.Schulz-von der Gathen, H.F.Döbele, M.Käning°, J.Röpcke°Institut für Laser- und Plasmaphysik, Uni GH Essen, 45117 Essen, Germany

°Institut für Niedertemperatur-Plasmaphysik, R.-Blum-Str. 8- 10, 17489 Greifswald, Germany

Institut für Laser-und PlasmaphysikUni GH Essen

TOPIC: The combination of several independent diagnostics allows an insight into the plasma chemistry in the reactive plasma of a methane containing discharge.

KBrWindow Synth.quartz

window

PGS 2

2m spec- trograph

Optical Emission Spectroscopy

Infrared- Diode LaserAbsorpti on Spectroscopy "IRMA"

Microwave Interferometry

13.56 MHzRF-Discharge with Movable Electrode System

ICCD-Camera

Cooler

IR-ILDIR-Detector

Retroreflector

ch4s

etup

.DS

4

Optical Fiber

MicrowaveDetector

0 20 40 60 80 1000

5

10

15

pow

er_d

iss

The degree of dissociation of methane

50% CH4 in Ar 50% CH4 in H2

CH

4 deg

ree

of d

isso

ciat

ion

[%]

Power [W]

0 20 40 60 80 1000

2

4

6

pow

er_c

h3

Concentration of the methyl radical

50% CH4 in Ar 50% CH4 in H2

[CH

3] [1

012 m

olec

ules

cm-3

]

Power [W]

0 20 40 60 80 1000

1x1010

2x1010

3x1010

4x1010

5x1010

pow

er_

ne

50% CH4 in Ar 50% CH4 in H2

Electron density

n e [cm

-3]

Power [W]0 20 40 60 80 100

0

2

4

6

8

10Concentration of acetylene

pow

er_c

2h2

50% CH4 in Ar 50% CH4 in H2

[C2H

2] [1

013 m

olec

ules

cm-3

]

Power [W]

0 20 40 60 80 1000

2

4

6

8

10

Electron density and degree of dissociation of methane

ar%_ne+diss

n e [10

10cm

-3]

Ar [%]

0

10

20

30

CH 4 d

egre

e of

dis

soci

atio

n [%

]

0 20 40 60 80 100

0

1

2

3

pow

er_c

2h6

Concentration of ethane

50% CH4 in Ar 50% CH4 in H2

[C2H

6] [1

014 m

olec

ules

cm-3

]

Power [W]

CH4(methane)

CH CH3(methyle)

CH2

C2H4(ethylene)

C2H6(ethane)

C2H2(acetylene)

+ e-

+ e- + e -, H

+ CH3+ CH2

+ CH4

+ e-

(1)(1)

(1,2)

(3)(4)

(5)

(6)(7)

0 20 40 60 80 1000

1

2

3

4

5

[CH 3]

[1012

mol

ecul

es c

m-3]

Ar [%]

0

2

4

6

Concentration of the methyl radical and acetylene and the relative intensity of the CH band

ar%_ch3+c2h2+ch

rel. CH intensity [C2H

2] [1

013 m

olec

ules

cm-3

]

(1) The EEDF determines the dissociation of CH4

(2) Chemical reaction of CH4 with H at high H density

[ W.L. Hsu, J. Appl. Phys. 72, 3102 (1992) ]

(3) C2H6 is formed by recombination of CH3

[ similar dependence on power ]

(4) The abstraction of H to produce C2H5 and then C2H4 is too slow[ different dependences on the argon mixture of CH3 and C2H2 ]

(5) C2H4 is produced by the rapid reaction of CH with CH4

[ similar dependence on the argon mixture of CH and C2H2 ]

(6) C2H2 is formed by electron impact dissociation of C2H4

[ W.Y. Fan et al., J. Phys. Chem. A, submitted ]

(7) C2H2 does not arise from C2H6 [ W.Y. Fan et al. ]

- No noticeable difference in the electron density with CH4

in Ar or H2, despite a big difference in pure Ar or H2

- The electron density dependence on power exhibits twoalmost linear sections

- The degree of dissociation of CH4 depends nearly linearlyon power

- Degree of dissociation at maximum power: about 15 %

- A higher degree of dissociation with CH4 in Ar than in H2

indicates changes in the EEDF

- The CH3 radical is the most likely precursor for formingcarbon films

- The CH3 density increase with power is less pronouncedthan the increase of the degree of dissociation

- Noticeable differences of about 25 % in the CH3 densitiesin Ar or H2

- CH3 density in H2 at maximum power: about 5.5 x 1012 cm-3

- CH3 density in Ar at maximum power: about 4.0 x 1012 cm-3

- The C2H6 density and the CH3 density behave similarly ( higher densities in H2 than in Ar )

- The C2H2 density also increases with power, but withhigher densities in Ar, just opposite to C2H6 and CH3

- The electron density as a function of the admixture of Arand H2 exhibits a maximum at 1:1 of Ar and H2

- The degree of dissociation is lower at higher electrondensities

=> At high hydrogen partial pressures chemical dissociation byH is important

=> There have to be changes in the EEDF

- In layer forming plasmas it is difficult to measure theEEDF by probes

- Non-intrusive measurements of the EEDF are desirable

- For a first attempt to get information about the EEDF byOES see part I

- The variation of the CH3 density with the Ar mixture iscomparable to the variation of the electron density

- The behaviour of the C2H2 density is qualitatively different

- The maximum C2H2 density is at an Ar admixture of 80 %

- The behaviour of the relative emission density of CH is similarto the C2H2 density

- Most probably CH is the dominant source of C2H4 and C2H2

Diagnostics

Emission Spectroscopy

- The emission of various species is observed using a 2m-spectrograph and an ICCD-Camera

Microwave Interferometry

- A Heterodyn-Interferometer operating at a wavelength ofλ = 1 mm is used to monitor the electron density

For more details of the emission spectroscopy and the microwave interferometry see part I

Tunable IR diode laser absorption spectroscopy (TDLAS)

- powerful technique to measure absolute number densities of molecules and radicals in the electronic ground state

- A new compact and transportable tunable infrared multicomponent acquisition system 'IRMA' has been used

- IRMA consists of 4 independent IR diode lasers

- Concentrations of several molecular species could be measured simultaneously

- The IR beam of IRMA passes the plasma 2 times

- For more details of IRMA: oral presentation of J.Röpcke

Experimental Setup

- capacitively-coupled parallel-plate discharge

- uncooled stainless-steel electrodes

diamater of electrodes: 100 mm

distance of electrodes: 25 mm

- electrode system displaceable in two orthogonal directions

Power supply

- Commercial RF transceiver with matching box

- Output power @ 13.56 MHz: 10 - 100 W

Gas supply

- Mass flow controlled gases: Argon, H

2, CH

4

- Total pressure: 10 - 100 Pa

- Total flow (here): 66 sccm

Setup of the IRMA Optical Table

BC: beam combiner, CS: cold station, D: detector, HC: Herriot cell, IF: intermediate focus, GM: grating monochromator, RC: reference cell.

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0- 4 0

- 2 0

0

2 0

4 0

1 3 0 2 . 7 3 6 c m - 1

1 3 0 2 . 5 9 7 c m - 1

]

][

] [

[

las

er

of

f

e n d f i t

b e g i n f i t

C H 4

C 2 H 2

sig

na

l [

mV

]

c h a n n e l n u m b e r

Measured and fitted absorption spectrum

Summary and Conclusions

- For the first time three independent diagnosticmethods have been applied to a CH4 containingdischarge simultaneously to get an insight intoplasma physical and plasma chemical processeswhich could influence thin carbon film growth rateand homogeneity

- Particle densities of several C-H molecules,electron densities and gas temperatures weremeasured by IR absorption spectroscopy,microwave interferometry and emissionspectroscopy, respectively

- The various plasma parameters were measuredat different RF-powers and different gas mixtures

- In principle all diagnostic methods are lineintegrated, but the special discharge chamberallows to measure radial profils by using Abelinversion (see part I)

- On the basis of these measurements a modelhas been developed that describes the chemicalprocesses and allows to identify the main plasmachemical reaction paths

- Furthermore some informations about the EEDFwere obtained (see part I)

This work was funded by the DFG in the frame of the SFB191 (Essen/Bochum) and SFB198 (Greifswald)

Institut für Niedertemperatur

-Plasmaphysik Greifswald

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