1

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/

2 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

3 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))

4 Diamond film growth in a DC arc jet

How to probe gas phase chemistry and composition?

Optical emission spectroscopy (OES) and cavity ring down spectroscopy (CRDS).

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.

5 DC arc jet in operation

diamond film growing on Mo substrate

plasma jet

CH4 injection ring

6 Diamond films grown with DC arc jet

SEM images of polycrystalline diamond films grown in the DC arc jet

7 Film characterisation by Raman spectroscopy

Raman Shift / cm-1

600 800 1000 1200 1400 1600 1800

Inte

nsi

ty /

arb

. un

its 1332 cm-1

8 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.

350 400 450 500 550 600 650 700

0

5000000

10000000

15000000

20000000

25000000

Em

issi

on in

tens

ity /

arb

. un

its

Wavelength / nm

H

C2 Swan system (d3g a3u)Spatially resolved C2(d-a) emission

9 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)

10 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.

11 In situ diagnosis of the arc jet plume, II

Absorption spectroscopy

Beer-Lambert behaviour

I = I0 exp{-[X] L}

Advantages:• Straightforward• General• Quantitative

Disadvantages:• Insensitive• Non-selective

Fractional absorption per pass

I = (I0 – I)/I0 10-4

12 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} ; = [X] ; I min ~ 10-8

Change in ring-down rate as a function of excitation wavelength gives the absorption spectrum

13 Cavity Ring Down Spectroscopy in the DC arc jet

Variables include: - CH4 flow rate - power into plasma- distance from substrate

14 C2(a) radical detection

Portion of C2 d3ga3u (0,0) band

pA)0v,a(Cgg

c8d 2

a

d2

line00

integrated absorptioncoefficient 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(a3u)] ~ 1.1 1013 cm-3 for 3.3%CH4/H2 gas mixture, 6

kW input power, assuming Tgas = 3300 K and L = 1 cm.(

15 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

16 C2(X) radical detection

Tvib = 3000 500 K

[C2(X1g )] = (3.00.9) 1012 cm-3 again assuming L = 1 cm.

= 0.270.08

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.

Portion of C2 (D1uX1g) spectrum recorded in free plume at ~ 235 nm

(a)]2

[C

(X)]2

[C

17 CH(X) radical detection

Portion of CH A2X2 (0,0) band ~ 427 nm

Non-zero absorbance between peaksprobably attributable to C3 radicals.

[CH(X )] = (7.0 1.3) 1012 cm-3 in the free plume under normal operating conditions.(again assuming L = 1 cm).

18 C2(a) and CH(X) radical column densities as fn(z)

3% CH4/H2 , 6 kW input power, range of probe transitions

C2(a) CH(X)

19 C2(a) and CH(X) radical column densities as fn[CH4]

x sccm CH4 / 1.8 slm H2 / 12.2 slm ArArc jet power 6 kW, range of probe transitions

C2(a) CH(X)

20

ECDL AOM

photodiode

wavem eter

isolator

piezo mount fibre optic

fibre optic

trigger

V

V

CH / H / (Ar)4 2

CVD reactor

cw CRDS probing of C2H2 in the DC arc jet reactor

ECDL: Littman configuration extended cavity diode laser AOM: acousto-optic modulator

21 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 Torrlaser 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

22 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.

23 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, 1CH2, CH3, CH4, C2(X), C2(a), C2Hx (x = 1-6), C3Hx (x = 0-2), C4Hx (x = 0-2))

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.

24 Modelling of the DC arc jet plume: Tgas

• Gas temperature distribution, Tgas

substrate

methane injection ring

H2/Ar plasma enters here

25 Modelling of the DC arc jet plume: H

• Tgas

• H: H2 >90% dissociated; high [H] at substrate.

26 Modelling of the DC arc jet plume: CH4

• Tgas

• H: H2 >90% dissociated; high [H] at substrate.• CH4 injected through ring

27 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

28 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)

29 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

30 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

31 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, C2,

32 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

33 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

34 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.

35 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’.

36 Acknowledgements

Royal Society

NATO

Yuri MankelevichNikolay Suetin (Moscow State Univ.)

Andrew Orr-Ewing Paul MayColin WesternKeith Rosser

Jon WillsChris Rennick James SmithWilliam BoxfordAlistair SmithSteve Redman

37