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PROBING THE GAS PHASE CHEMISTRY
INVOLVED IN DIAMOND CHEMICAL
VAPOUR DEPOSITION (CVD).
Mike Ashfold
School of Chemistry
University of Bristol
Bristol BS8 1TS
http://www.chm.bris.ac.uk/pt/laser/
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Chemical vapour deposition of diamond films
Activation of gas mixture by
• Hot filament (Tfil ~2450 K)
• Microwave plasma
• DC arc jet
Polycrystalline films grow on Si, Mo,
W, … substrates; Tsub >950 K.
Growth of single crystal diamond by
CVD demonstrated (Isberg et al,
Science, 297, 1670 (2002))
Properties:
• High thermal conductivity
• Optical transparency (UV  mid IR)
• Chemically inert
• Electrical insulator – can be doped
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Recent gas phase diagnostics studies in Bristol
Hydrocarbon / H2 mixtures in a hot filament (HF) reactor
• Molecular beam mass spectrometry of stable species
• REMPI laser probing H atoms and CH3 radicals
• Modelling CH4 / H2 and C2H2 / H2 gas mixtures
(Ashfold et al., Phys. Chem. Chem. Phys. (2001), 3, 3471)
• Probing and modelling CH4/NH3/H2 gas mixtures
(Smith et al., J. Appl. Phys. (2002), 92, 672)
CO2 / CH4 mixtures in a microwave (MW) reactor
• Molecular beam mass spectrometry
• Modelling H/C/O gas phase chemistry
CH4 / H2 / Ar mixtures in a DC arc jet reactor
• Cavity ring down measurements of C2H2 and of C2 and CH radicals
• Modelling plasma activated CH4 / H2 gas mixtures
(Wills et al., J. Appl. Phys. (2002), 92, 4213
Rennick et al., Chem. Phys. Lett. (2004), 383, 518
Rennick et al., Diam. Rel. Mater. (in press))
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Diamond film growth in a DC arc jet
10 kW DC arc jet
1%CH4 in Ar/H2 at 50 Torr
Growth rates ~100 m hr-1
Aggressive activation:
much higher gas
temperatures and flow
rates than in HF or MW
reactors.
How to probe gas phase chemistry and composition?
Optical emission spectroscopy (OES) and
cavity ring down spectroscopy (CRDS).
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DC arc jet in operation
diamond film
growing on
Mo substrate
plasma jet
CH4 injection ring
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Diamond films grown with DC arc jet
SEM images of polycrystalline
diamond films grown in the
DC arc jet
Film characterisation by Raman spectroscopy
Intensity / arb. units
7
1332 cm-1
600
800
1000
1200
1400
Raman Shift / cm-1
1600
1800
Optical emission from the arc jet plume
What are primary growth species in this highly activated environment?
C atoms? C2 radicals? Latter show strongly in optical emission.
C2 Swan system
(d3
g

a3
Spatially resolved
C2(d-a) emission
u)
25000000
Emission intensity / arb. units
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20000000
15000000
H
10000000
5000000
0
350
400
450
500
550
Wavelength / nm
600
650
700
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Proposed mechanism for diamond growth by C2
C2 addition to H-terminated
and to bare diamond (110)
surfaces has been
calculated to be barrierless
and exothermic.
(D.A. Horner et al.
Chem. Phys. Lett. 233 (1995) 243)
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In situ diagnosis of the arc jet plume, I
Optical emission spectroscopy (OES)
• Only fluorescent species can be observed.
• Provides information about the (minor) electronically excited components in
plume – how to relate to ground state concentrations, properties, etc?
• Spatially resolved measurements difficult.
Resonance enhanced multiphoton ionisation (REMPI)
• Used successfully to probe ground state H atoms and CH3 radicals in HF
reactor, but ion probe will not survive harsh plasma environment and
background ion/electron signal would be a problem.
Laser induced fluorescence (LIF)
• Species of interest must have fluorescent excited state.
• Need to quantify excited state quenching characteristics in order to relate
measured LIF signal intensities to ground state populations of interest.
• Detector likely to be overwhelmed by intense spontaneous emission from
plume.
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In situ diagnosis of the arc jet plume, II
Absorption spectroscopy
Beer-Lambert behaviour
I = I0 exp{-s [X] L}
Advantages:
• Straightforward
• General
• Quantitative
Disadvantages:
• Insensitive
• Non-selective
Fractional absorption per pass
I = (I0 – I)/I0  10-4
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In situ diagnosis of the arc jet plume, III
Intra-cavity absorption spectroscopy
Build cavity around sample
•Multipass a light pulse
•Detect rate of loss of light
•Cavity ring-down spectroscopy
I(t) = I0 exp{-k0t - ct} ;  = s [X] ; I min ~ 10-8
Change in ring-down rate as a function of excitation
wavelength gives the absorption spectrum
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Cavity Ring Down Spectroscopy in the DC arc jet
Variables include:
- CH4 flow rate
- power into plasma
- distance from substrate
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C2(a) radical detection
2 gd
   d  8c g C2 (a, v  0)A 00p
a
line
Portion of C2 d3ga3u (0,0) band
integrated absorption
coefficient of measured line
A00 = Einstein A coefficient for vibronic
transition of interest.
p = fraction of total oscillator strength
within probed rovibrational transition
(T dependent).
C2(a, v = 0) column density.
 C2 (a) number density IF we know Tgas (and thus qvib)
and the absorbing column length, L (from OES).
 [C2(a3u)] ~ 1.1  1013 cm-3 for 3.3%CH4/H2 gas mixture,
6 kW input power, assuming Tgas = 3300 K and L = 1 cm.(
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C2(a) radical detection – gas temperature determination
Boltzmann plots of C2(a) rotational state
population distribution measured in the plume
(2 < z < 25 mm) give Trot = 3300  200 K.
‘Doppler’ linewidth analyses give similar Tgas
for z > 5mm, but overestimate Tgas close to
the substrate – a consequence of plume
flaring in the boundary layer.
probe
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C2(X) radical detection
Portion of C2 (D1uX1g) spectrum
recorded in free plume at  ~ 235 nm
Tvib = 3000  500 K
[C2(X1g )] = (3.00.9)  1012 cm-3
again assuming L = 1 cm.
[C (X)]
2
= 0.270.08
[C (a)]
2
c.f. 0.23 if the a and X states of
C2 were in thermal equilibrium at
3300 K – implies intersystem
crossing is faster than reaction
(with e.g. H2) under operational
conditions.
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CH(X) radical detection
Portion of CH A2X2 (0,0) band ~ 427 nm
[CH(X )] = (7.0 1.3)  1012 cm-3
in the free plume under normal
operating conditions.
(again assuming L = 1 cm).
Non-zero absorbance between peaks
probably attributable to C3 radicals.
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C2(a) and CH(X) radical column densities as fn(z)
C2(a)
CH(X)
3% CH4/H2 , 6 kW input power, range of probe transitions
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C2(a) and CH(X) radical column densities as fn[CH4]
C2(a)
x sccm CH4 / 1.8 slm H2 / 12.2 slm Ar
Arc jet power 6 kW, range of probe transitions
CH(X)
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cw CRDS probing of C2H2 in the DC arc jet reactor
V
w
ave
m
eter
isolator
E
C
D
L
AOM
piezom
ount
trigger
photodiode
C
V
D
reactor
fibreoptic
V
C
H/H/(A
r)
4
2
ECDL: Littman configuration extended cavity diode laser
AOM: acousto-optic modulator
fibreoptic
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Diamond film growth in a hot filament reactor
R(22) line of 1 + 3 combination band of C2H2
 = 0.022  0.003 cm-1
(650  90 MHz).
pressure broadening: ~200 MHz
at 50 Torr
laser bandwidth: ~4 MHz
Tgas = 550  150 K
C2H2 present along whole viewing column?
[C2H2] = 1.2  0.2 1014 cm-3 for 0.83% CH4/H2 feed
(i.e. only 25% of our ‘standard’ CH4 flow rate) and
assuming L = 100 cm
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CRDS in DC arc jet: summary of experimental findings
Probing in the free plume region of the
arc jet, with a CH4 flow of 60 sccm:
• [C2(a)] ~ 1.1 x 1013 cm-3,
• [C2(X)] ~ 3 x 1012 cm-3,
• [CH(X)] ~ 7 x 1012 cm-3 (all assuming L = 1 cm )
• Tgas = 3300  200 K
• [C2H2] ~ 1.2 x 1014 cm-3 (using a reduced
(15 sccm) CH4 flow, assuming L =100 cm)
• Tgas ~ 550 K
• There is a boundary region close to the substrate, where C2 and CH column
densities increase – due to plume flaring and the longer L?
• Increased linewidths at small z mainly due to plume flaring. Internal
quantum state population distributions of radical species suggest Tgas
relatively insensitive to z.
• [C2H2], and Tgas value (average over all L?)
is insensitive to z in range 2 – 25 mm.
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Modelling of the DC arc jet plume (Mankelevich)
•
2-D (r,z) model, comprising of three blocks, describing:
(i) activation of the reactive mixture (i.e. gas heating, ionisation, H2 dissociation
in arc jet and intermediate chamber, H atom loss and H2 production on
nozzle exit walls),
(ii) gas-phase processes (heat and mass transfer, chemical kinetics),
(iii) gas-surface processes at the substrate.
•
Thermochemical data and the reduced chemical reaction mechanism builds
on Yu.A. Mankelevich et al., Diam. Rel. Mater. (1996), 5, 888.
•
Chemical kinetics scheme involves 23 species (H, H2, Ar, C, CH, 3CH2,
1CH , CH , CH , C (X), C (a), C H (x = 1-6), C H (x = 0-2), C H (x = 0-2))
2
3
4
2
2
2 x
3 x
4 x
and 76 reversible reactions.
•
Set of conservation equations for mass, momentum, energy and species
concentrations, with appropriate initial and boundary conditions, thermal
and caloric equations of state, are integrated numerically in cylindrical (r,z)
coordinate space until attaining steady state conditions.
•
Model output includes spatial distributions of Tgas, the flow field, and the
various species number densities.
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Modelling of the DC arc jet plume: Tgas
• Gas temperature distribution, Tgas
H2/Ar plasma enters here
methane injection ring
substrate
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Modelling of the DC arc jet plume: H
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
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Modelling of the DC arc jet plume: CH4
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
• CH4 injected through ring
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Modelling of the DC arc jet plume: C2H2
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
• CH4 injected through ring
• rapidly converted to C2H2
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Modelling of the DC arc jet plume: C4H2
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
• CH4 injected through ring
• rapidly converted to C2H2
• and to larger CxHy compounds
(e.g. C4H2)
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Modelling of the DC arc jet plume: C3
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
• CH4 injected through ring
• rapidly converted to C2H2
• and to larger CxHy compounds
(e.g. C4H2) and C3 radicals
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Modelling of the DC arc jet plume: C2H
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
• CH4 injected through ring
• rapidly converted to C2H2
• and to larger CxHy compounds
(e.g. C4H2) and C3 radicals
• larger CxHy species break down
as [H] and Tgas increase in the
vicinity of the plume  C2H
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Modelling of the DC arc jet plume: C2
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
• CH4 injected through ring
• rapidly converted to C2H2
• and to larger CxHy compounds
(e.g. C4H2) and C3 radicals
• larger CxHy species break down
as [H] and Tgas increase in the
vicinity of the plume  C2H,
C 2,
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Modelling of the DC arc jet plume: CH
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
• CH4 injected through ring
• rapidly converted to C2H2
• and to larger CxHy compounds
(e.g. C4H2) and C3 radicals
• larger CxHy species break down
as [H] and Tgas increase in the
vicinity of the plume  C2H,
C2, CH radicals
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Modelling of the DC arc jet plume: C
• Tgas
• H: H2 >90% dissociated; high
[H] at substrate.
• CH4 injected through ring
• rapidly converted to C2H2
• and to larger CxHy compounds
(e.g. C4H2) and C3 radicals
• larger CxHy species break down
as [H] and Tgas increase in the
vicinity of the plume  C2H,
C2, CH radicals and C atoms
• C1Hy formation on axis requires
high [H] and Tgas, and sufficient
time for diffusion into core of
plume
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Summary of results from modelling
• Gas temperature and flow velocity distributions show a cylindrical hot
plume with Tgas~3000-4000 K, in good accord with optical emission studies.
• Highly activated gas mixture. [H]/[H2] ratio just above the substrate is
~ 0.25 (cf ~0.01 in typical low power HF or MW PECVD reactors). Surface
chemistry is dominated by H abstraction and addition reactions.
• Gas pressure is not uniform throughout the chamber - encouraging the
recirculation needed to transfer hydrocarbon from injection ring into the hot
plume.
• Numerous chemical transformations occur during this transport.
Predicted number densities of C, CH, C2, C2H, C2H2 and C3 incident on the
growing diamond surface are all >1012 cm-3.
• Most, if not all, of these species must contribute to film growth given the
high (~3%) utility of carbon source gas deduced experimentally by
comparing observed film growth rates with the metered CH4 input.
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Comparison with experiment: CH and C2(a)
• Model confirms that CH and C2 species are localised in the hot plume.
• Quantitative agreement between observed and modelled column densities
and rotational temperatures (~3300 K). Larger Doppler width seen at small z
due to flaring of plume along observation axis.
• C2H2 predicted to be present throughout reactor – consistent with observed
number densities and low (~550 K) associated average ‘temperature’.
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Acknowledgements
Andrew Orr-Ewing
Paul May
Colin Western
Keith Rosser
Yuri Mankelevich
Nikolay Suetin
(Moscow State Univ.)
Jon Wills
Chris Rennick
James Smith
William Boxford
Alistair Smith
Steve Redman
Royal Society
NATO
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