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Research Papers

# H2 Mole Fraction Measurements in a Microwave Plasma Using Coherent Anti-Stokes Raman Scattering Spectroscopy

[+] Author and Article Information
Alfredo D. Tuesta

Nanoscale Transport Research Group,
School of Mechanical Engineering,
Purdue University,
West Lafayette, IN 47907
e-mail: alfredotuesta@gmail.com

Aizaz Bhuiyan

Applied Laser Spectroscopy Laboratory,
School of Mechanical Engineering,
Purdue University,
West Lafayette, IN 47907
e-mail: abhuiyan@purdue.edu

Robert P. Lucht

Applied Laser Spectroscopy Laboratory,
School of Mechanical Engineering,
Purdue University,
West Lafayette, IN 47907
e-mail: lucht@purdue.edu

Timothy S. Fisher

Nanoscale Transport Research Group,
School of Mechanical Engineering,
Purdue University,
West Lafayette, IN 47907
e-mail: tsfisher@purdue.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received July 21, 2015; final manuscript received October 24, 2015; published online December 15, 2015. Assoc. Editor: Don A. Lucca.

J. Micro Nano-Manuf 4(1), 011005 (Dec 15, 2015) (9 pages) Paper No: JMNM-15-1053; doi: 10.1115/1.4031916 History: Received July 21, 2015; Revised October 24, 2015

## Abstract

In an effort to provide insights into the thermochemical composition of a microwave plasma chemical vapor deposition (MPCVD) reactor, the mole fraction of H2 is measured at various positions in the plasma sheath, at pressures of 10 and 30 Torr, and at plasma powers ranging from 300 to 700 W. A technique is developed by comparing the Q(1)01 transition of experimental and theoretical spectra aided by the Sandia CARSFT fitting routine. Results reveal that the mole fraction of H2 does not vary significantly from its theoretical mixture at the parametric conditions examined. Furthermore, the $ν″=1→ν′=2$ vibrational hot band was searched, but no transitions were found. An analytical explanation for the increase in the temperature of H2 with the introduction of N2 and CH4 is also presented. Finally, because the mole fraction of H2 does not appear to deviate from the theoretical composition, the rotational and translational modes of H2 are shown to be approximately in equilibrium, and therefore, the rotational temperatures may be used to estimate the translational temperatures of H2.

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## Figures

Fig. 1

Experimental spectra with a reactor pressure of 10 Torr at (a) room temperature and (b) plasma temperature. The plasma generator power was 500 W and the feed gas was entirely composed of H2.

Fig. 2

Theoretical fits to experimental data by use of CARSFT code set to a mole fraction equal to one for data at (a) room temperature and (b) plasma temperature. Fitting of the spectra with the CARSFT code resulted in a temperature of 277 and 1133 K for the room temperature and plasma spectra, respectively.

Fig. 3

Experimental Rex and theoretical Rth ratios at 10 Torr. Experimental ratios, shown by the solid symbols, were produced by heating the susceptor stage of the MPCVD reactor. Theoretical ratios, shown by the empty symbols, were produced by use of the CARSFT code. The dashed line connecting the dots illustrates the results from the spline interpolation. Measurements were taken approximately 3 mm above the puck surface.

Fig. 4

Percent relative error of the measurements from the verification procedure with temperature

Fig. 5

Experimental and theoretical ratios converted to mole fractions

Fig. 6

Theoretical spectrum generated by the CARSFT code for a thermal system at 1000 K. Transitions from the cold band are labeled Q01(J). For clarity, only the odd J transitions are labeled.

Fig. 7

Theoretical spectrum generated by the CARSFT code for a thermal system at 2000 K. Transitions from the cold band are labeled Q01(J) and transitions from the hot band are labeled Q12(J). For clarity, only the odd J transitions are labeled.

Fig. 8

Horizontal temperature profile from the center of the plasma at approximately 3 mm above the molybdenum puck surface. Triangles correspond to measurements with a plasma. Circles correspond to measurements without a plasma at room temperature.

Fig. 9

Square-root of the intensity of the H2 Q01(1) line as a function of beam intensity. The Stokes beam energy was 4.8 mJ/pulse.

Fig. 10

Square-root of the intensity of the H2 Q01(1) line as a function of reactor pressure

Fig. 11

Experimental mole fraction of H2 (50 sccm) in microwave plasma at various temperatures with CH4 (10 sccm) and N2 (20 sccm) for a reactor pressure of 10 Torr and a generator power of 300 and 500 W. Dashed lines indicate the theoretical mixture mole fractions for the corresponding mixture.

Fig. 12

Experimental mole fraction of H2 (50 sccm) in microwave plasma at various temperatures with CH4 (10 sccm) and N2 (20 sccm) for a reactor pressure of 30 Torr and generator power of 500 and 700 W. Dashed lines indicate the theoretical mixture mole fractions for the corresponding mixture.

Fig. 13

CARS signal near spectral region of vibrational hot band

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