7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Numerical Investigation on Ar/CH4/H2 Induction Thermal Plasma Field withArbitrary Power Modulation at Reduced Pressures for Diamond Film Growth
Yamato Hosoi, Taizo Higashi, Yasunori Tanaka, Yusuke Nakano, Tatsuo Ishijima
Faculty of Electrical Engineering and Computer Science, Kanazawa University
e-mail (speaker): lxoxl@stu.kanazawa-u.ac.jp
The authors have been investigating the application ofRF modulated induction thermal plasma (MITP) todiamond film growth. This MITP offers time-controlled
heat and radical particle fluxes on the substrate holder.
Our previous research shows that irradiation of Ar/CH4/H2
plasma at reduced pressures increases the growth rate of
diamond film in the experiments [1]. In the present study,
we numerically calculated the temperature, gas flow
velocity fields, and mole fraction distributions of CH4/H2
with different modulated input power waveforms. The
radical particle fluxes above the substrate holder were also
calculated to consider suitable modulated input power
waveforms for diamond growth at reduced pressures.
Figure 1 shows a schematic cross-sectional view of theplasma torch (z < 330 mm) and the chamber (z > 330 mm).
The wall temperature was fixed at 300 K. Argon wassupplied from the end of the torch (z = 0 mm) at 20 slpm
each in the axial and swirl directions. The water-cooled
tube was inserted from the top of the torch to the axial
position z = 160 mm. Through this tube, the feedstock
CH4/H2 gas was supplied with a flow rate of 0.02/2 slpm
as a feedstock gas. The pressure in the plasma torch andchamber was set to 20 torr. Input coil power wasmodulated to 4 waveforms as shown in Fig. 2. Waveform(a) 7.9→7.9 kW has the rectangular waveform whose duty
factor is 25%, and waveforms (b)7.9→5.6 kW,(c)6.9→4.6 kW, (d)5.9→3.6 kW have different maximum
power Pmax and minimum powers Pmin1 and Pmin2 in Fig.2.
Table 1 lists the different Pmax, Pmin1 and Pmin2. First, thetemperature, flow velocity, and mole fraction of CH4/H2
fields of thermal plasma were obtained by solving thecoupled equations of conservation of mass, momentum,and energy, Maxwell’s equation, and the transport
equation of CH4/H2 gas on the local thermal equilibrium
assumption. Then, the particle flux on the substrate (z =386 mm) was calculated using the calculated temperature,
flow velocity, and mole fraction of CH4/H2 fields, and thecalculated thermal equilibrium particle composition [2].
Figure 3 shows the calculated results for temperature
above the substrate holder (z = 386 mm). Compared towaveform (a), the temperature for waveforms (b)-(d)become kept around 3000-4000 K during lower power
duration at which the high carbon radical fluxes arepresent according to the calculated thermal equilibriumcomposition [2]. Figure 4 shows that time-averaged fluxesof neutral particle above the substrate holder (z = 386 mm).
C2, C2H and CH3 are known as key species for diamond
growth. Compared to waveform (a), C2, C2H and CH3
fluxes were high in waveform (b)-(d) during the lower
power duration.
This result shows possibility that the radical particle flux
on the substrate holder contributing diamond film growthincreases by decreasing input power gently from falling torising on pulse wave whose duty factor is 25%.

Fig.1 Calculation space.

Fig.2. Modulated input power waveforms.
Table 1. Waveform condition.

Fig.3. Temperature above the substrate holder (z = 386mm) versus time.

Fig.4. Time-averaged fluxes of neutral particle above thesubstrate holder (z = 386 mm).
References[1] K. Hata, et al.: J.Appl.Phys., 126, 223302 (2019)[2] K. Hata, et al.: J.Phys.D:Appl.Phys., 54, 195105(2021)