Imaging and Modeling C2 Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from chemiluminescent reactions and investigations of higher-pressure effects and plasma constriction

Joseph P P  Gore; Edward J D Mahoney; James A Smith; Michael N R Ashfold; Yuri A Mankelevich

doi:10.1021/acs.jpca.1c01924

Imaging and Modeling C₂ Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from chemiluminescent reactions and investigations of higher-pressure effects and plasma constriction

Joseph P P Gore, Edward J D Mahoney, James A Smith, Michael N R Ashfold, Yuri A Mankelevich

Research output: Contribution to journal › Article (Academic Journal) › peer-review

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Abstract

Wavelength and spatially resolved imaging and 2D plasma chemical modeling methods have been used to study the emission from electronically excited C2 radicals in microwave-activated dilute methane/hydrogen gas mixtures under processing conditions relevant to the chemical vapor deposition (CVD) of diamond. Obvious differences in the spatial distributions of the much-studied C2(d3Πg–a3Πu) Swan band emission and the little-studied, higher-energy C2(C1Πg–A1Πu) emission are rationalized by invoking a chemiluminescent (CL) reactive source, most probably involving collisions between H atoms and C2H radicals, that acts in tandem with the widely recognized electron impact excitation source term. The CL source is relatively much more important for forming C2(d) state radicals and is deduced to account for >40% of C2(d) production in the hot plasma core under base operating conditions, which should encourage caution when estimating electron or gas temperatures from C2 Swan band emission measurements. Studies at higher pressures (p ≈ 400 Torr) offer new insights into the plasma constriction that hampers efforts to achieve higher diamond CVD rates by using higher processing pressures. Plasma constriction is proposed as being inevitable in regions where the local electron density (ne) exceeds some critical value (nec) and electron–electron collisions enhance the rates of H2 dissociation, H-atom excitation, and related associative ionization processes relative to those prevailing in the neighboring nonconstricted plasma region. The 2D modeling identifies a further challenge to high-p operation. The radial uniformities of the CH3 radical and H-atom concentrations above the growing diamond surface both decline with increasing p, which are likely to manifest as less spatially uniform diamond growth (in terms of both rate and quality).

Original language	English
Pages (from-to)	4184-4199
Number of pages	16
Journal	Journal of Physical Chemistry A
Volume	125
Issue number	19
Early online date	9 May 2021
DOIs	https://doi.org/10.1021/acs.jpca.1c01924
Publication status	Published - 20 May 2021

Bibliographical note

Funding Information:
The Bristol authors are grateful for financial support from the Engineering and Physical Sciences Research Council (EPSRC) through grant no. EP/K018388/1, the EPSRC Centre for Doctoral Training in Diamond Science and Technology (EP/L015315/1), and Element Six Ltd. The authors are also very grateful to Dr. Colin Western for expert guidance with the PGOPHER simulations and to Dominic Palubiski for photographing the p-dependent plasma emission. The work was performed within the Cooperation in Science and Technology agreement between the Skobeltsyn Institute of Nuclear Physics at Lomonosov Moscow State University and the University of Bristol. All underlying data are available at the University of Bristol data repository, data.bris, at DOI: 10.5523/bris.28n44fl84kgbp2o12ugep52ark.

Funding Information:
The Bristol authors are grateful for financial support from the Engineering and Physical Sciences Research Council (EPSRC) through grant no. EP/K018388/1, the EPSRC Centre for Doctoral Training in Diamond Science and Technology (EP/L015315/1), and Element Six Ltd. The authors are also very grateful to Dr. Colin Western for expert guidance with the PGOPHER simulations and to Dominic Palubiski for photographing the p-dependent plasma emission. The work was performed within the Cooperation in Science and Technology agreement between the Skobeltsyn Institute of Nuclear Physics at Lomonosov Moscow State University and the University of Bristol. All underlying data are available at the University of Bristol data repository, data.bris at DOI: 10.5523/bris.28n44fl84kgbp2o12ugep52ark.

Publisher Copyright:
© 2021 American Chemical Society.

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Metrology concepts for a new generation of plasma manufacturing with atom-scale precision
Ashfold, M. N. R. (Principal Investigator)
1/07/13 → 21/12/18
Project: Research

Cite this

Gore, J. P. P., Mahoney, E. J. D., Smith, J. A., Ashfold, M. N. R., & Mankelevich, Y. A. (2021). Imaging and Modeling C₂ Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from chemiluminescent reactions and investigations of higher-pressure effects and plasma constriction. Journal of Physical Chemistry A, 125(19), 4184-4199. https://doi.org/10.1021/acs.jpca.1c01924

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title = "Imaging and Modeling C2 Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from chemiluminescent reactions and investigations of higher-pressure effects and plasma constriction",

abstract = "Wavelength and spatially resolved imaging and 2D plasma chemical modeling methods have been used to study the emission from electronically excited C2 radicals in microwave-activated dilute methane/hydrogen gas mixtures under processing conditions relevant to the chemical vapor deposition (CVD) of diamond. Obvious differences in the spatial distributions of the much-studied C2(d3Πg–a3Πu) Swan band emission and the little-studied, higher-energy C2(C1Πg–A1Πu) emission are rationalized by invoking a chemiluminescent (CL) reactive source, most probably involving collisions between H atoms and C2H radicals, that acts in tandem with the widely recognized electron impact excitation source term. The CL source is relatively much more important for forming C2(d) state radicals and is deduced to account for >40% of C2(d) production in the hot plasma core under base operating conditions, which should encourage caution when estimating electron or gas temperatures from C2 Swan band emission measurements. Studies at higher pressures (p ≈ 400 Torr) offer new insights into the plasma constriction that hampers efforts to achieve higher diamond CVD rates by using higher processing pressures. Plasma constriction is proposed as being inevitable in regions where the local electron density (ne) exceeds some critical value (nec) and electron–electron collisions enhance the rates of H2 dissociation, H-atom excitation, and related associative ionization processes relative to those prevailing in the neighboring nonconstricted plasma region. The 2D modeling identifies a further challenge to high-p operation. The radial uniformities of the CH3 radical and H-atom concentrations above the growing diamond surface both decline with increasing p, which are likely to manifest as less spatially uniform diamond growth (in terms of both rate and quality).",

author = "Gore, {Joseph P P} and Mahoney, {Edward J D} and Smith, {James A} and Ashfold, {Michael N R} and Mankelevich, {Yuri A}",

note = "Funding Information: The Bristol authors are grateful for financial support from the Engineering and Physical Sciences Research Council (EPSRC) through grant no. EP/K018388/1, the EPSRC Centre for Doctoral Training in Diamond Science and Technology (EP/L015315/1), and Element Six Ltd. The authors are also very grateful to Dr. Colin Western for expert guidance with the PGOPHER simulations and to Dominic Palubiski for photographing the p-dependent plasma emission. The work was performed within the Cooperation in Science and Technology agreement between the Skobeltsyn Institute of Nuclear Physics at Lomonosov Moscow State University and the University of Bristol. All underlying data are available at the University of Bristol data repository, data.bris, at DOI: 10.5523/bris.28n44fl84kgbp2o12ugep52ark. Funding Information: The Bristol authors are grateful for financial support from the Engineering and Physical Sciences Research Council (EPSRC) through grant no. EP/K018388/1, the EPSRC Centre for Doctoral Training in Diamond Science and Technology (EP/L015315/1), and Element Six Ltd. The authors are also very grateful to Dr. Colin Western for expert guidance with the PGOPHER simulations and to Dominic Palubiski for photographing the p-dependent plasma emission. The work was performed within the Cooperation in Science and Technology agreement between the Skobeltsyn Institute of Nuclear Physics at Lomonosov Moscow State University and the University of Bristol. All underlying data are available at the University of Bristol data repository, data.bris at DOI: 10.5523/bris.28n44fl84kgbp2o12ugep52ark. Publisher Copyright: {\textcopyright} 2021 American Chemical Society.",

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doi = "10.1021/acs.jpca.1c01924",

language = "English",

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Gore, JPP, Mahoney, EJD, Smith, JA , Ashfold, MNR & Mankelevich, YA 2021, 'Imaging and Modeling C₂ Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from chemiluminescent reactions and investigations of higher-pressure effects and plasma constriction', Journal of Physical Chemistry A, vol. 125, no. 19, pp. 4184-4199. https://doi.org/10.1021/acs.jpca.1c01924

Imaging and Modeling C₂ Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from chemiluminescent reactions and investigations of higher-pressure effects and plasma constriction. / Gore, Joseph P P ; Mahoney, Edward J D; Smith, James A et al.
In: Journal of Physical Chemistry A, Vol. 125, No. 19, 20.05.2021, p. 4184-4199.

Research output: Contribution to journal › Article (Academic Journal) › peer-review

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AU - Gore, Joseph P P

AU - Mahoney, Edward J D

AU - Smith, James A

AU - Ashfold, Michael N R

AU - Mankelevich, Yuri A

N1 - Funding Information: The Bristol authors are grateful for financial support from the Engineering and Physical Sciences Research Council (EPSRC) through grant no. EP/K018388/1, the EPSRC Centre for Doctoral Training in Diamond Science and Technology (EP/L015315/1), and Element Six Ltd. The authors are also very grateful to Dr. Colin Western for expert guidance with the PGOPHER simulations and to Dominic Palubiski for photographing the p-dependent plasma emission. The work was performed within the Cooperation in Science and Technology agreement between the Skobeltsyn Institute of Nuclear Physics at Lomonosov Moscow State University and the University of Bristol. All underlying data are available at the University of Bristol data repository, data.bris, at DOI: 10.5523/bris.28n44fl84kgbp2o12ugep52ark. Funding Information: The Bristol authors are grateful for financial support from the Engineering and Physical Sciences Research Council (EPSRC) through grant no. EP/K018388/1, the EPSRC Centre for Doctoral Training in Diamond Science and Technology (EP/L015315/1), and Element Six Ltd. The authors are also very grateful to Dr. Colin Western for expert guidance with the PGOPHER simulations and to Dominic Palubiski for photographing the p-dependent plasma emission. The work was performed within the Cooperation in Science and Technology agreement between the Skobeltsyn Institute of Nuclear Physics at Lomonosov Moscow State University and the University of Bristol. All underlying data are available at the University of Bristol data repository, data.bris at DOI: 10.5523/bris.28n44fl84kgbp2o12ugep52ark. Publisher Copyright: © 2021 American Chemical Society.

PY - 2021/5/20

Y1 - 2021/5/20

N2 - Wavelength and spatially resolved imaging and 2D plasma chemical modeling methods have been used to study the emission from electronically excited C2 radicals in microwave-activated dilute methane/hydrogen gas mixtures under processing conditions relevant to the chemical vapor deposition (CVD) of diamond. Obvious differences in the spatial distributions of the much-studied C2(d3Πg–a3Πu) Swan band emission and the little-studied, higher-energy C2(C1Πg–A1Πu) emission are rationalized by invoking a chemiluminescent (CL) reactive source, most probably involving collisions between H atoms and C2H radicals, that acts in tandem with the widely recognized electron impact excitation source term. The CL source is relatively much more important for forming C2(d) state radicals and is deduced to account for >40% of C2(d) production in the hot plasma core under base operating conditions, which should encourage caution when estimating electron or gas temperatures from C2 Swan band emission measurements. Studies at higher pressures (p ≈ 400 Torr) offer new insights into the plasma constriction that hampers efforts to achieve higher diamond CVD rates by using higher processing pressures. Plasma constriction is proposed as being inevitable in regions where the local electron density (ne) exceeds some critical value (nec) and electron–electron collisions enhance the rates of H2 dissociation, H-atom excitation, and related associative ionization processes relative to those prevailing in the neighboring nonconstricted plasma region. The 2D modeling identifies a further challenge to high-p operation. The radial uniformities of the CH3 radical and H-atom concentrations above the growing diamond surface both decline with increasing p, which are likely to manifest as less spatially uniform diamond growth (in terms of both rate and quality).

AB - Wavelength and spatially resolved imaging and 2D plasma chemical modeling methods have been used to study the emission from electronically excited C2 radicals in microwave-activated dilute methane/hydrogen gas mixtures under processing conditions relevant to the chemical vapor deposition (CVD) of diamond. Obvious differences in the spatial distributions of the much-studied C2(d3Πg–a3Πu) Swan band emission and the little-studied, higher-energy C2(C1Πg–A1Πu) emission are rationalized by invoking a chemiluminescent (CL) reactive source, most probably involving collisions between H atoms and C2H radicals, that acts in tandem with the widely recognized electron impact excitation source term. The CL source is relatively much more important for forming C2(d) state radicals and is deduced to account for >40% of C2(d) production in the hot plasma core under base operating conditions, which should encourage caution when estimating electron or gas temperatures from C2 Swan band emission measurements. Studies at higher pressures (p ≈ 400 Torr) offer new insights into the plasma constriction that hampers efforts to achieve higher diamond CVD rates by using higher processing pressures. Plasma constriction is proposed as being inevitable in regions where the local electron density (ne) exceeds some critical value (nec) and electron–electron collisions enhance the rates of H2 dissociation, H-atom excitation, and related associative ionization processes relative to those prevailing in the neighboring nonconstricted plasma region. The 2D modeling identifies a further challenge to high-p operation. The radial uniformities of the CH3 radical and H-atom concentrations above the growing diamond surface both decline with increasing p, which are likely to manifest as less spatially uniform diamond growth (in terms of both rate and quality).

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Gore JPP, Mahoney EJD, Smith JA , Ashfold MNR, Mankelevich YA. Imaging and Modeling C₂ Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from chemiluminescent reactions and investigations of higher-pressure effects and plasma constriction. Journal of Physical Chemistry A. 2021 May 20;125(19):4184-4199. Epub 2021 May 9. doi: 10.1021/acs.jpca.1c01924