High homogeneity 25 cm low-energy rf ion source with inherent electron
compensation
S. V. Dudin, D. V. Rafalskyi, and A. V. Zykov
Department of Physics and Technology, V.N. Karazin Kharkiv National University, Kharkiv 61108, Ukraine
͑Received 12 April 2010; accepted 17 July 2010; published online 31 August 2010͒
A 25 cm single-grid low-energy rf ion source with inherent electron compensation is described and
characterized. Measurements were carried out using Ar and CF
4
filling gas. The dependence of the
ion beam current to the target as well as the current partition between the beam fast and slow ions
on the rf discharge pressure for both filling gases is discussed. The unique ability of generation of
coinciding ion and electron flows is demonstrated and the measured ion and electron energy
distribution functions are presented as well. The developed broad ion beam source is able of
providing 0.5–5 mA/cm
2
current density in the low ion energy range of 50–250 eV, with
possibility of independent current density and energy control. It is shown that complementing the rf
plasma source with a profiling electrode allows for CF
4
ion source operation attaining Ϯ5% ion
beam uniformity over 250 mm in diameter. The presented CF
4
etching test results exhibit the
possibility of highly directional anisotropic Si and SiO
2
etching utilizing the developed single grid
rf ion source. © 2010 American Institute of Physics. ͓doi:10.1063/1.3477999͔
I. INTRODUCTION
At the present time, broad-beam low-energy ion sources
͑ISs͒ are widely used in different technologies, particularly
in the rare gas and reactive ion beam etching of GaAs opto-
electronic devices and microwave integrated circuits, thin-
film magnetic heads and nanostructure fabrication, and
micromashining.
1–3
Among different types of IS, the devices
using inductively coupled plasma ͑ICP͒ for working gas ion-
ization have great prospective in this applications due to ex-
cellent reactive gas compatibility. By know, numerous ICP-
based IS with beam diameter up to 500 mm have been
developed and are routinely fabricated.
4,5
Typically, in mate-
rial processing applications such sources operate over the ion
energy range 50–1000 eV. Another class of the ICP-based IS
covers the sources for plasma heating in thermonuclear de-
vices that operate using the same principles but are charac-
terized by much higher energy deposition.
6,7
In all the mentioned ion sources rf ICP is used for the
gas ionization, while ion acceleration is provided by a mul-
tigrid ͑usually three-grid͒ ion-optical system ͑IOS͒. Such
system consists of relatively thin ͑ϳ1 mm or less͒ perfo-
rated electrodes ͑grids͒ spaced by several millimeters with
high voltages applied between the grids. Considering signifi-
cant transversal sizes of IOS and essential plasma thermal
load, the multigrid assembly manufacturing, adjustment, and
performance maintenance during operation becomes rather
challenging so the IOS is one of the most demanding and
expensive parts of the IS. Besides, using a three-grid IOS
requires application of an additional high-voltage power sup-
ply ͑even at low ion energies͒ with necessity of careful volt-
age optimization. Moreover, some specific problems such as
the grid assembly insulator coating by conductive films and
intergrid breakdowns are probable.
It is obvious that single-grid ion sources are free of these
limitations because the ions are accelerated in the sheath be-
tween the plasma and the extracting grid.
1,8
Application of
such sources is usually expedient in reactive ion-beam etch-
ing with low energy ions ͑Ͻ300 eV͒. Operation with higher
energies is also possible, but in this case, intense sputtering
of the grid occurs that reduces its lifetime and contaminates
the beam by the metal ions.
Another important problem of the ion-beam material
processing is associated with the necessity of neutralization
of both the space charge of the beam and the ion current to
the surface, especially in the case of insulator processing.
9
Usually for this purpose electron injection from a neutralizer
is used. The neutralizer is a dedicated device based on ther-
moelectron emission or different kinds of gas discharges,
which needs additional power supplies and has limited life-
time.
On the other hand, the ICP-based single-grid ion source
with rf biasing of the plasma interfacing electrode ͑potential
electrode͒ is known
10
as possessing the unique ability of qua-
sisimultaneous generation of coinciding flows of positive
ions and electrons in contrast to the more common two- or
three-grid sources
11
injecting only positive ions. Using the
ICP for ionization ensures its durability to reactive gases, and
due to the single-grid IOS, it is free from the noted above
disadvantages of multigrid systems. In the paper,
11
we have
shown that the ion source rf biasing mode of operation is
superior since the full ion beam current neutralization is pro-
vided for the entire range of ion energy of interest and the
extracted ion current is higher than in the dc biasing mode. It
should be mentioned that the rf bias applied to the potential
electrode affects not only the ion acceleration but also the
plasma as a whole. In fact there is the combined inductive-
capacitive discharge studied in the papers.
12,13
Particularly, in
this work,
12
the significant reduction of the ion energy cost as
a result of applying the rf bias was discovered.
REVIEW OF SCIENTIFIC INSTRUMENTS 81, 083302 ͑2010͒
0034-6748/2010/81͑8͒/083302/6/$30.00 © 2010 American Institute of Physics81, 083302-1