1. 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. 2 Chemical vapour deposition of diamond films Activation of gas mixture by Hot filament (T fil ~2450 K) Microwave plasma DC arc jet Polycrystalline films grow on Si, Mo, W, … substrates; T sub >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. 3 Recent gas phase diagnostics studies in Bristol Hydrocarbon / H 2 mixtures in a hot filament (HF) reactor Molecular beam mass spectrometry of stable species REMPI laser probing H atoms and CH 3 radicals Modelling CH 4 / H 2 and C 2 H 2 / H 2 gas mixtures (Ashfold et al., Phys. Chem. Chem. Phys. (2001), 3, 3471) Probing and modelling CH 4 /NH 3 /H 2 gas mixtures (Smith et al., J. Appl. Phys. (2002), 92, 672) CO 2 / CH 4 mixtures in a microwave (MW) reactor Molecular beam mass spectrometry Modelling H/C/O gas phase chemistry CH 4 / H 2 / Ar mixtures in a DC arc jet reactor Cavity ring down measurements of C 2 H 2 and of C 2 and CH radicals Modelling plasma activated CH 4 / H 2 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. 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%CH 4 in Ar/H 2 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. 5 DC arc jet in operation diamond film growing on Mo substrate plasma jet CH 4 injection ring

6. 6 Diamond films grown with DC arc jet SEM images of polycrystalline diamond films grown in the DC arc jet

7. 7 Film characterisation by Raman spectroscopy Raman Shift / cm -1 60080010001200140016001800 Intensity / arb. units 1332 cm -1

8. 8 Optical emission from the arc jet plume What are primary growth species in this highly activated environment? C atoms? C 2 radicals? Latter show strongly in optical emission. C 2 Swan system (d 3  g  a 3  u ) Spatially resolved C 2 (d-a) emission

9. 9 Proposed mechanism for diamond growth by C 2 C 2 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. 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 CH 3 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. 11 In situ diagnosis of the arc jet plume, II Absorption spectroscopy Beer-Lambert behaviour I = I 0 exp{-  [X] L} Advantages: Straightforward General Quantitative Disadvantages: Insensitive Non-selective Fractional absorption per pass  I = (I 0 – I)/I 0  10 -4

12. 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) = I 0 exp{-k 0 t -  ct} ;  =  [X] ;  I min ~ 10 -8 Change in ring-down rate as a function of excitation wavelength gives the absorption spectrum

13. 13 Cavity Ring Down Spectroscopy in the DC arc jet Variables include: - CH 4 flow rate - power into plasma - distance from substrate

14. 14 C 2 (a) radical detection Portion of C 2 d 3  g  a 3  u (0,0) band integrated absorption coefficient of measured line A 00 = Einstein A coefficient for vibronic transition of interest. p = fraction of total oscillator strength within probed rovibrational transition (T dependent).  C 2 (a, v = 0) column density.  C 2 (a) number density IF we know T gas (and thus q vib ) and the absorbing column length, L (from OES).  [C 2 (a 3  u )] ~ 1.1  10 13 cm -3 for 3.3%CH 4 /H 2 gas mixture, 6 kW input power, assuming T gas = 3300 K and L = 1 cm. (

15. 15 C 2 (a) radical detection – gas temperature determination Boltzmann plots of C 2 (a) rotational state population distribution measured in the plume (2 < z < 25 mm) give T rot = 3300  200 K. ‘Doppler’ linewidth analyses give similar T gas for z > 5mm, but overestimate T gas close to the substrate – a consequence of plume flaring in the boundary layer. probe

16. 16 C 2 (X) radical detection T vib = 3000  500 K [C 2 (X 1  g )] = (3.0  0.9)  10 12 cm -3 again assuming L = 1 cm. = 0.27  0.08 c.f. 0.23 if the a and X states of C 2 were in thermal equilibrium at 3300 K – implies intersystem crossing is faster than reaction (with e.g. H 2 ) under operational conditions. Portion of C 2 (D 1  u  X 1  g ) spectrum recorded in free plume at ~ 235 nm

17. 17 CH(X) radical detection Portion of CH A 2  X 2  (0,0) band ~ 427 nm Non-zero absorbance between peaks probably attributable to C 3 radicals. [CH(X )] = (7.0  1.3)  10 12 cm -3 in the free plume under normal operating conditions. (again assuming L = 1 cm).

18. 18 C 2 (a) and CH(X) radical column densities as fn(z) 3% CH 4 /H 2, 6 kW input power, range of probe transitions C 2 (a)CH(X)

19. 19 C 2 (a) and CH(X) radical column densities as fn[CH 4 ] x sccm CH 4 / 1.8 slm H 2 / 12.2 slm Ar Arc jet power 6 kW, range of probe transitions C 2 (a)CH(X)

20. 20 cw CRDS probing of C 2 H 2 in the DC arc jet reactor ECDL: Littman configuration extended cavity diode laser AOM: acousto-optic modulator

21. 21 Diamond film growth in a hot filament reactor R(22) line of 1 + 3 combination band of C 2 H 2  = 0.022  0.003 cm -1 (650  90 MHz). pressure broadening: ~200 MHz at 50 Torr laser bandwidth: ~4 MHz  T gas = 550  150 K C 2 H 2 present along whole viewing column? [C 2 H 2 ] = 1.2  0.2  10 14 cm -3 for 0.83% CH 4 /H 2 feed (i.e. only 25% of our ‘standard’ CH 4 flow rate) and assuming L = 100 cm

22. 22 CRDS in DC arc jet: summary of experimental findings Probing in the free plume region of the arc jet, with a CH 4 flow of 60 sccm: [C 2 (a)] ~ 1.1 x 10 13 cm -3, [C 2 (X)] ~ 3 x 10 12 cm -3, [CH(X)] ~ 7 x 10 12 cm -3 (all assuming L = 1 cm ) T gas = 3300  200 K [C 2 H 2 ] ~ 1.2 x 10 14 cm -3 (using a reduced (15 sccm) CH 4 flow, assuming L =100 cm) T gas ~ 550 K There is a boundary region close to the substrate, where C 2 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 T gas relatively insensitive to z. [C 2 H 2 ], and T gas value (average over all L?) is insensitive to z in range 2 – 25 mm.

23. 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, H 2 dissociation in arc jet and intermediate chamber, H atom loss and H 2 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, H 2, Ar, C, CH, 3 CH 2, 1 CH 2, CH 3, CH 4, C 2 (X), C 2 (a), C 2 H x (x = 1-6), C 3 H x (x = 0-2), C 4 H x (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 T gas, the flow field, and the various species number densities.

24. 24 Modelling of the DC arc jet plume: T gas Gas temperature distribution, T gas substrate methane injection ring H 2 /Ar plasma enters here

25. 25 Modelling of the DC arc jet plume: H T gas H: H 2 >90% dissociated; high [H] at substrate.

26. 26 Modelling of the DC arc jet plume: CH 4 T gas H: H 2 >90% dissociated; high [H] at substrate. CH 4 injected through ring

27. 27 Modelling of the DC arc jet plume: C 2 H 2 T gas H: H 2 >90% dissociated; high [H] at substrate. CH 4 injected through ring rapidly converted to C 2 H 2

28. 28 Modelling of the DC arc jet plume: C 4 H 2 T gas H: H 2 >90% dissociated; high [H] at substrate. CH 4 injected through ring rapidly converted to C 2 H 2 and to larger C x H y compounds (e.g. C 4 H 2 )

29. 29 Modelling of the DC arc jet plume: C 3 T gas H: H 2 >90% dissociated; high [H] at substrate. CH 4 injected through ring rapidly converted to C 2 H 2 and to larger C x H y compounds (e.g. C 4 H 2 ) and C 3 radicals

30. 30 Modelling of the DC arc jet plume: C 2 H T gas H: H 2 >90% dissociated; high [H] at substrate. CH 4 injected through ring rapidly converted to C 2 H 2 and to larger C x H y compounds (e.g. C 4 H 2 ) and C 3 radicals larger C x H y species break down as [H] and T gas increase in the vicinity of the plume  C 2 H

31. 31 Modelling of the DC arc jet plume: C 2 T gas H: H 2 >90% dissociated; high [H] at substrate. CH 4 injected through ring rapidly converted to C 2 H 2 and to larger C x H y compounds (e.g. C 4 H 2 ) and C 3 radicals larger C x H y species break down as [H] and T gas increase in the vicinity of the plume  C 2 H, C 2,

32. 32 Modelling of the DC arc jet plume: CH T gas H: H 2 >90% dissociated; high [H] at substrate. CH 4 injected through ring rapidly converted to C 2 H 2 and to larger C x H y compounds (e.g. C 4 H 2 ) and C 3 radicals larger C x H y species break down as [H] and T gas increase in the vicinity of the plume  C 2 H, C 2, CH radicals

33. 33 Modelling of the DC arc jet plume: C T gas H: H 2 >90% dissociated; high [H] at substrate. CH 4 injected through ring rapidly converted to C 2 H 2 and to larger C x H y compounds (e.g. C 4 H 2 ) and C 3 radicals larger C x H y species break down as [H] and T gas increase in the vicinity of the plume  C 2 H, C 2, CH radicals and C atoms C 1 H y formation on axis requires high [H] and T gas, and sufficient time for diffusion into core of plume

34. 34 Summary of results from modelling Gas temperature and flow velocity distributions show a cylindrical hot plume with T gas ~3000-4000 K, in good accord with optical emission studies. Highly activated gas mixture. [H]/[H 2 ] 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, C 2, C 2 H, C 2 H 2 and C 3 incident on the growing diamond surface are all >10 12 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 CH 4 input.

35. 35 Comparison with experiment: CH and C 2 (a) Model confirms that CH and C 2 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. C 2 H 2 predicted to be present throughout reactor – consistent with observed number densities and low (~550 K) associated average ‘temperature’.

36. 36 Acknowledgements Royal Society NATO Yuri Mankelevich Nikolay Suetin (Moscow State Univ.) Andrew Orr-Ewing Paul May Colin Western Keith Rosser Jon Wills Chris Rennick James Smith William Boxford Alistair Smith Steve Redman

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