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

Parametric Study of Degree of Dissociation in a Low Pressure ECR based
Hydrogen Plasma Source
Surajit Kuila, Debaprasad Sahu and Ramesh Narayanan
Department of Energy Science and Engineering
Indian Institute of Technology Delhi
e-mail (speaker): dpsahu@dese.iitd.ac.in
Knowledge of atomic Hydrogen (H) concentrations in apartially ionized hydrogen plasmas is essential for various
applications such as cleaning and reduction of oxidationfrom the metal surface, passivation of silicon surface,
growth of diamond film, negative hydrogen ion
production etc. [1,2]. The dissociation of the feed gas H2
by electron impact is the primary source of H atoms in alow pressure hydrogen plasma source. The Degree ofDissociation (DoD) refers to the ratio of the numbr
densities of the dissociated molecules to the total numberof molecules.
In this work parametric study of DoD of hydrogen plasma
in a microwave (2.45 GHz) electron cyclotron resonance(ECR) source is reported. Gas pressure was varied from 2– 10 mTorr while the input microwave power wasmaintained in the range of 300- 600 W. Two ECR field
topologies were used to investigate its effect ondissociation. The basic experimental set up is shown inFig. 1(a). In first configuration the ECR source [3] asshown in the extreme left is encapsulated by three
permanent ring magnets (NdFeB) that provide therequired field (875 G) for the resonance heating to takeplace. In the second configuration only one ring magnet isused which allow to have two ECR surfaces inside thesource compared to former where the back ECR layer
lied in atmosphere behind the quartz plate[Fig1(b),(c)], thus did not attribute to heating. Plasma wasproduced in the source and expanded into an adjoining
cylindrical chamber of length 55 cm and diameter 15cm.

The line intensity ratio method was used to determine theDoD [4]. For this purpose the Balmer-α and Balmer-βlines were captured using an optical emission
spectrometer which was integrated with the vacuum
chamber through a lens and multimode fiber.
The DoD was found to be always higher in the single-ring
configuration. At 2 mTorr pressure and 600 W power
9% DoD was obtained for this configuration compared to
7% in the three-ring configuration. However, at higher
operating pressure dissociation became lowered. Similar
trend was observed when the input power is reduced [Fig1(d), (e)]. To investigate further the experimental results
were compared with that obtained from a volume
averaged global model of a low pressure hydrogen plasma.
The model results showed reasonable agreement with theexperimental results.
References[1] Vesel, A. et al., Applied Surface Science, 387, 11401146 (2016)[2] Bacal, M.et al., Applied physics reviews, 2,021305(2015)[3] A Ganguli et al, Plasma Sources Sci. Technol. 25025026(2016)[4] Lavrov, B. P et al, Plasma Sources Sci. Technol., 15(1),135.

Figure 1. (a) Schematic of the experimental set up with three-ring magnets (b) Magnetic field simulation ofthree-ring magnets and (c) single ring magnet. COMSOL Multiphysics tool is used. (d) Plot of DoD withpressure for both the experimental set up; Power: 600 W (e) Variation of DoD with input power in the single
ring experimental set up; pressure: 2 mTorr.