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

Experimental and Numerical Investigation of Plasma Sheath Electric Field
Penetration Through a Micro-aperture
Sushanta Barman1, Ashutosh Nayak2 , and Sudeep Bhattacharjee 1
Department of Physics, Indian Institute of Technology Kanpur, India
2
School of Physical Sciences, National Institute of Science Education and Research, Bhubaneswar,
India
e-mail (speaker): sushanta@iitk.ac.in
1

when dP is smaller than the plasma Debye length, theincreased electric field at the center of the plasma
electrode (PE) aperture arises due to inadequate shieldingof electric fields within the Debye sphere in the sheathregion. Detailed experimental and simulation results will
be presented at the conference.
This study will help to understand the effect of plasma
sheaths on the performance of plasma-based FIBs. More
importantly, it helps to focus high-current plasma ion
beams to sub-micron regimes using the phenomenon of
nonlinear magnification of plasma-based FIBs [2,3],
which originates from nonlinearity in sheath electric field
penetration.
References[1] P. Vanraesa and A. Bogaerts, Journal of AppliedPhysics 129, 220901 (2021).
[2] S. Barman, S. K. Maurya, and S. Bhattacharjee,
Plasma Research Express 4, 025003 (2022).
[3] S. K. Maurya, S. Barman, N. Pan, S. Bhattacharjee,
Physics of Plasmas 26, 063103 (2019).
[4] S. K. Maurya, S. Barman, S. Paul, and S.
Bhattacharjee, Journal of Physics D: Applied Physics 52,055205 (2018).

ydp

x
1.0(a)

PE

Plasma Sheathne, ni (m 3)

The plasma sheath has been the subject of extensive
investigation over the years; however, its comprehensiveunderstanding remains an ongoing challenge. Therefore,
it attracts the attention of researchers to investigate theproperties of plasma sheaths under various conditions,
not only for its implications in fundamental physics but
also for its practical applications. The plasma sheathgoverns a range of fundamental processes at the interface
between plasma and surfaces, including electricalcoupling, surface chemical properties, and bidirectional
mass transport [1]. Consequently, it significantly impacts
various plasma applications, including surface
modification, electromagnetic wave excitation, plasmaimmersion ion deposition, plasma etching, and, more
importantly, the performance of plasma-based focusedion beam (FIB) systems [2]. Therefore, investigation of
the penetration of plasma sheath fields through microaperture, which governs the shape of the ion emission
surface, becomes imperative for enhancing the efficiencyof plasma-based FIBs.
In this study, we have performed experiments toinvestigate the penetration of sheath electric fields
through a grounded micro aperture (plasma electrode) ofvariable sizes dP = 20 to 500 µm. In the experiment,continuous mode microwaves of frequency 2.45 GHz areused to ignite Ar plasma and the plasma is confined in an
octupole magnetic multicusp [3,4]. The operatingpressure and microwave power are varied in the range of
0.1 to 1 mTorr and 100 to 400 W, respectively. A plasma
sheath is formed on the plasma-facing side of the plasma
electrode (Fig. 1(a)), allowing the electric fields of thesheath to penetrate through its aperture (O) into theextraction side. An electric field probe has been designed
and fabricated to measure the axial distribution of thepenetrated sheath electric field, as shown in Fig. 1 (a).
The experimental results show that as the size of theplasma electrode (PE) aperture (dP) decreases to below
the Debye length (~100 microns), the penetration of theelectric field through the aperture increases nonlinearly.
Furthermore, the strength of electric field penetration isobserved to be dependent on dP and experimental
operating conditions, such as pressure and microwave
power. This observation is further confirmed bynumerical simulations, where Poisson's equation wassolved numerically using the successive over-relaxation
method to calculate the distribution of electric potentials
and fields near the PE aperture. The results indicate thatoElectric Field
Probe

1e17X1017

PLE

0.50(b)neni

100 200 300 400 500x ( m)Figure 1. (a) The plasma sheath region and electric fieldprobe. (b) Variation of ion density (ni) and electron
density (ne) with the axial distance (x) in the plasma
sheath region