Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters
In the present paper we compare the positive and negative ion flows created using a recently developed electrostatic grid-type filter with the flows formed using a magnetic filter. Langmuir probe measurements show electron cooling with both filters, allowing effective formation of negative ions vi...
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irk-123456789-989242016-09-14T10:38:18Z Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters Dudin, S. Rafalskyi, D. Popelier, L. Aanesland, A. In the present paper we compare the positive and negative ion flows created using a recently developed electrostatic grid-type filter with the flows formed using a magnetic filter. Langmuir probe measurements show electron cooling with both filters, allowing effective formation of negative ions via electron dissociative attachment in the region of low electron temperature. The energy distribution functions of positive and negative ions extracted from the filtered plasmas are measured in both systems showing an almost monoenergetic nature of the ions with the energy corresponding to the imposed extraction potential. It is shown that in both cases strongly electronegative plasmas where the negative ion density is much larger than the electron density can be formed downstream of the filter. Biasing an internal electrode or the electrostatic filter grid allows control of the plasma potential. In the case of the electrostatic filter the plasma could be biased negatively compared to ground and effective extraction of negative ion was achieved. В данной работе проведено сравнение потоков положительных и отрицательных ионов, генерируемых с использованием недавно разработанного электростатического сеточного фильтра с потоками, формируемыми с использованием магнитного фильтра. Измерения ленгмюровскими зондами показали эффективное “охлаждение” электронов при использовании обоих фильтров, обеспечивающее условия для эффективного образования отрицательных ионов в области с низкой электронной температурой в результате диссоциативного прилипания. Функции распределения по энергии положительных и отрицательных ионов, извлекаемых из вторичной плазмы, измеренные в обеих системах, показали моноэнергетичность генерируемых потоков ионов с энергией, соответствующей приложенному извлекающему потенциалу. Показано, что в обоих случаях возможно формирование сильно электроотрицательной плазмы на выходе фильтра с плотностью отрицательных ионов значительно превышающей плотность электронов. Смещение внутреннего электрода либо сетки фильтра позволило достичь управления потенциалом плазмы. В случае электростатического фильтра потенциал плазмы может принимать отрицательные значения по отношению к заземленному электроду, благодаря чему было достигнуто эффективное извлечение отрицательных ионов. У даній роботі проведено порівняння потоків позитивних і негативних іонів, що генеруються з використанням недавно розробленого електростатичного сіткового фільтру з потоками, що формуються з використанням магнітного фільтру. Вимірювання ленгмюрівськими зондами показали ефективне “охолоджування” електронів при використанні обох фільтрів, що забезпечує умови для ефективного утворення негативних іонів в області з низькою електронною температурою в результаті диссоціативного прилипання. Функції розподілу по енергії позитивних і негативних іонів, витягуваних з вторинної плазми, виміряні в обох системах, показали моноенергетичність потоків іонів, що генеруються, з енергією, відповідною до прикладеного витягуючого потенціалу. Показано, що в обох випадках можливе формування сильно електронегативної плазми на виході фільтру з щільністю негативних іонів що значно перевищує щільність електронів. Зсув потенціалу внутрішнього електроду або сітки фільтр дозволив досягти керування потенціалом плазми. У разі електростатичного фільтру потенціал плазми може приймати негативні значення по відношенню до заземленого електроду, завдяки чому було досягнуте ефективне витягання негативних іонів. 2012 Article Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters / S. Dudin, D. Rafalskyi, L. Popelier, A. Aanesland // Физическая инженерия поверхности. — 2012. — Т. 10, № 1. — С. 22–28. — Бібліогр.: 26 назв. — англ. 1999-8074 http://dspace.nbuv.gov.ua/handle/123456789/98924 537.5; PACS 52.27; 52.50; 52.59 en Физическая инженерия поверхности Науковий фізико-технологічний центр МОН та НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
In the present paper we compare the positive and negative ion flows created using a recently developed
electrostatic grid-type filter with the flows formed using a magnetic filter. Langmuir probe measurements
show electron cooling with both filters, allowing effective formation of negative ions via electron
dissociative attachment in the region of low electron temperature. The energy distribution functions
of positive and negative ions extracted from the filtered plasmas are measured in both systems showing
an almost monoenergetic nature of the ions with the energy corresponding to the imposed extraction
potential. It is shown that in both cases strongly electronegative plasmas where the negative ion
density is much larger than the electron density can be formed downstream of the filter. Biasing an
internal electrode or the electrostatic filter grid allows control of the plasma potential. In the case of
the electrostatic filter the plasma could be biased negatively compared to ground and effective extraction
of negative ion was achieved. |
| format |
Article |
| author |
Dudin, S. Rafalskyi, D. Popelier, L. Aanesland, A. |
| spellingShingle |
Dudin, S. Rafalskyi, D. Popelier, L. Aanesland, A. Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters Физическая инженерия поверхности |
| author_facet |
Dudin, S. Rafalskyi, D. Popelier, L. Aanesland, A. |
| author_sort |
Dudin, S. |
| title |
Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters |
| title_short |
Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters |
| title_full |
Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters |
| title_fullStr |
Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters |
| title_full_unstemmed |
Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters |
| title_sort |
comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters |
| publisher |
Науковий фізико-технологічний центр МОН та НАН України |
| publishDate |
2012 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/98924 |
| citation_txt |
Comparative study of positive and negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic electron filters / S. Dudin, D. Rafalskyi, L. Popelier, A. Aanesland // Физическая инженерия поверхности. — 2012. — Т. 10, № 1. — С. 22–28. — Бібліогр.: 26 назв. — англ. |
| series |
Физическая инженерия поверхности |
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2025-07-07T07:14:28Z |
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22
UDC 537.5; PACS 52.27; 52.50; 52.59
COMPARATIVE STUDY OF POSITIVE AND NEGATIVE ION FLOWS EXTRACTED
FROM DOWNSTREAM PLASMAS BEYOND MAGNETIC AND ELECTROSTATIC
ELECTRON FILTERS
S. Dudin1, D. Rafalskyi2, L. Popelier3, A. Aanesland3
1Department of Physics and Technology, V.N. Karazin Kharkiv National University
Ukraine
2Scientific Center of Physical Technologies of Ukrainian Academy of Science and Ministry of
Education and Science of Ukraine (Kharkiv)
Ukraine
3Laboratoire de Physique des Plasmas, CNRS-Ecole Polytechnique (Palaiseau)
France
Received 28.02.2012
In the present paper we compare the positive and negative ion flows created using a recently developed
electrostatic grid-type filter with the flows formed using a magnetic filter. Langmuir probe measurements
show electron cooling with both filters, allowing effective formation of negative ions via electron
dissociative attachment in the region of low electron temperature. The energy distribution functions
of positive and negative ions extracted from the filtered plasmas are measured in both systems showing
an almost monoenergetic nature of the ions with the energy corresponding to the imposed extraction
potential. It is shown that in both cases strongly electronegative plasmas where the negative ion
density is much larger than the electron density can be formed downstream of the filter. Biasing an
internal electrode or the electrostatic filter grid allows control of the plasma potential. In the case of
the electrostatic filter the plasma could be biased negatively compared to ground and effective extraction
of negative ion was achieved.
Keywords: negative ion source, electron filtering, ion-ion plasma, electronegative plasma, ICP.
В данной работе проведено сравнение потоков положительных и отрицательных ионов, гене-
рируемых с использованием недавно разработанного электростатического сеточного фильтра
с потоками, формируемыми с использованием магнитного фильтра. Измерения ленгмюро-
вскими зондами показали эффективное “охлаждение” электронов при использовании обоих
фильтров, обеспечивающее условия для эффективного образования отрицательных ионов в
области с низкой электронной температурой в результате диссоциативного прилипания. Функции
распределения по энергии положительных и отрицательных ионов, извлекаемых из вторичной
плазмы, измеренные в обеих системах, показали моноэнергетичность генерируемых потоков
ионов с энергией, соответствующей приложенному извлекающему потенциалу. Показано, что
в обоих случаях возможно формирование сильно электроотрицательной плазмы на выходе
фильтра с плотностью отрицательных ионов значительно превышающей плотность электронов.
Смещение внутреннего электрода либо сетки фильтра позволило достичь управления потен-
циалом плазмы. В случае электростатического фильтра потенциал плазмы может принимать
отрицательные значения по отношению к заземленному электроду, благодаря чему было до-
стигнуто эффективное извлечение отрицательных ионов.
Ключевые слова: источник отрицательных ионов, фильтр электронов, ион-ионная плазма,
электроотрицательная плазма, индукционный разряд.
У даній роботі проведено порівняння потоків позитивних і негативних іонів, що генеруються з
використанням недавно розробленого електростатичного сіткового фільтру з потоками, що
формуються з використанням магнітного фільтру. Вимірювання ленгмюрівськими зондами по-
казали ефективне “охолоджування” електронів при використанні обох фільтрів, що забезпечує
умови для ефективного утворення негативних іонів в області з низькою електронною темпе-
ратурою в результаті диссоціативного прилипання. Функції розподілу по енергії позитивних і
негативних іонів, витягуваних з вторинної плазми, виміряні в обох системах, показали моно-
енергетичність потоків іонів, що генеруються, з енергією, відповідною до прикладеного ви-
S. Dudin, D. Rafalskyi, L. Popelier, A. Aanesland, 2012
23ФІП ФИП PSE, 2012, т. 10, № 1, vol. 10, No. 1
INTRODUCTION
There are commonly two ways to reduce the elec-
tron temperature in low temperature plasmas, either
by pulsing the plasma where the electron tempe-
rature decrease rapidly in the afterglow [1] or by
electron filters or barriers [2 − 7]. Plasmas with low
electron temperature are desirable in electronegative
plasmas where the aim is to effectively create ne-
gative ions by electron dissociative attachment.
Strongly electronegative plasmas, where the negative
ion density is much higher than the electron density
are used in a variety of applications from very high
selectivity etching [8], negative ion sources [9] and
recently in electric propulsion [10, 11]. These low
temperature plasmas without energetic electrons
might also be beneficial for low-damage surface
treatments [12].
One of the promising directions of the broad-
beam ion source evolution is the PEGASES concept
[10,11] where simultaneous extraction and ac-
celeration of both positive and negative ions ensure
the space charge and current neutralization of the
beam [10]. The choice of propellant for such sources
is restricted within the group of electronegative gases
among which halogen-containing compounds are
most attractive. Besides, beams of negative halogen
ions are also promising in reactive ion-beam etching
[13].
The most known type of the electron filters is the
magnetic filter [9]. Despite that such filters or mag-
netic barriers are known for decades and its ef-
ficiency is proven, the physics of the particle transport
across these filters are not yet fully understood [14].
The presence of strong magnetic field complicates
both the plasma diagnostic and the numerical mo-
deling. The plasma downstream of a magnetic filter
might be highly inhomogeneous due to ExB drifts
and other anomalous cross-field diffusion or
transport [15, 16].
A new promising type of nonmagnetic electron
filter, namely a grid-type electrostatic filter, has been
recently discovered and characterized [17]. The
main features of this filter is reported in [17] showing
that the electron temperature decreases downstream
of the grid.
The prospective for modern applications using
the electrostatic filter technique is not clearly un-
derstood. The present paper is therefore devoted
to a comparative study of the electronegative plas-
ma and the extraction of positive and negative ions
using the magnetic and electrostatic filter technique.
EXPERIMENTAL SETUPS
The paper is built around the comparison of two
devices with significant level of similarity. One of
them is the system with the electrostatic grid-type
filter recently developed at the LDPTP (Laboratory
for Diagnostics of Plasma Technology Processes of
V.N. Karazin Kharkiv National University and
Scientific Center of Physical Technologies, Kharkiv,
Ukraine), the other is the PEGASES first prototype
using a magnetic filter [10] developed at the LPP
(Laboratoire de Physique des Plasmas, Ecole Po-
lytechnique, France).
Both systems are intended to create strongly
electronegative plasmas where the negative ion
density is much higher than the electron density. The
plasmas are divided by an electron filter that seg-
regate the plasmas and forms two regions: a primary
inductively coupled plasma (ICP) and a secondary
plasma with low electron temperature. In both de-
vices the secondary plasma is terminated by an
energy analyzer serving as ion extraction system.
The experimental setups are schematically shown
in Fig. 1a) and b).
The experimental setup 1 (ES 1) is illustrated
in Fig. 1a). The vacuum chamber is divided by a
grid into two regions: the primary inductively coupled
plasma (ICP) and the downstream cold plasma
(CP). The ICP is generated by a 2-turns shielded
inductive coil placed inside the metallic vacuum
chamber with 250 mm inner diameter and 80 mm
length. RF power (13.56 MHz) is applied to the in-
ductor through a matching network. The RF power
тягуючого потенціалу. Показано, що в обох випадках можливе формування сильно елект-
ронегативної плазми на виході фільтру з щільністю негативних іонів що значно перевищує
щільність електронів. Зсув потенціалу внутрішнього електроду або сітки фільтр дозволив до-
сягти керування потенціалом плазми. У разі електростатичного фільтру потенціал плазми може
приймати негативні значення по відношенню до заземленого електроду, завдяки чому було
досягнуте ефективне витягання негативних іонів.
Ключові слова: джерело негативних іонів, фільтр електронів, іон-іонна плазма, електро-
негативна плазма, індукційний розряд.
S. DUDIN, D. RAFALSKYI, L. POPELIER, A. AANESLAND
24
absorbed by the ICP is 300 W in all the experiments
presented here. The ICP chamber is connected to a
flange holding the electrostatic grid. This stainless
steel grid is 0.12 mm thick with 0.24 mm apertures.
The total area of the grid is 450 cm2 and the optical
transparency is 40 %. The flange holding the grid is
connected to the metallic chamber with 350 mm
inner diameter where the CP is created. At the
opposite side of the grid, the CP is terminated by a
250 mm diameter grounded extraction electrode
placed 100 mm from the grid. The ICP chamber,
the filter grid and the CP chamber are electrically
connected and have the same potential which is
controlled by an external DC power supply in the
range from –50 V to +50 V. All potentials in this
work are measured with respect to the potential of
the grounded extraction electrode.
In order to analyze the flux of charged particles
to the extraction electrode a 20 mm diameter single-
grid magnetically filtered retarding-field energy
analyzer (MRFEA) is used and illustrated in the in-
sert in Fig. 1a). In contrast to our previously
described analyzer [18-20], the MRFEA used in
the present work is modified by a magnetic filter
placed at the MRFEA inlet in order to suppress the
electron flow while keeping the flow of negative ions
unchanged. Similar technique has been described in
detail previously [21].
For measurement of the plasma parameters two
Langmuir probes are inserted in the ICP and CP
regions. The probe tips, 5 mm long and 0.09 mm
diameter, are made of tungsten. The probe mea-
surements were conducted using the Plasma Meter
device [22]. Preliminary experiments revealed
problems of fast (within 1 sec) contamination of the
probe surface by insulating films in SF6 plasma
especially at high electron current to the probe. The
probe measurements were therefore conducted in
pulsed regime with the probe voltage ramp time
about 10 ms. Between the pulse intervals (>1 sec)
the probe potential was highly negative for cleaning
by positive ion bombardment. Besides, sufficient
decrease of the film growth rate was achieved using
addition of the oxygen into the gas mixture. Since
the insulating films are deposited throughout the
chamber mechanical cleaning of electrodes was
performed every hour of operation.
The filling gas mixture was fed into the ICP
chamber and pumped from the CP chamber. Since
ICP discharge operated in SF6 is often complicated
by instabilities [23]. Argon was added to the mixture
for discharge stabilization. Two standard gas mixtures
were used in the experiments. Mixture 1 consisted
of Ar and O2 at partial pressures of 1 mTorr each
(total gas pressure 2 mTorr), while in mixture 2 SF6
was added to mixture 1 at a partial pressure of
2 mTorr (total gas pressure 4 mTorr). All pressu-
res were measured by a hot-filament ionization gauge
with the plasma switched on. The chamber was pum-
ped by a turbo pump with 700 l/s throughput. The
residual pressure in the system was better then
10–5 Torr.
The experimental setup 2 (ES 2) is illustrated
in Fig. 1b) and known as the first PEGASES thruster
prototype. This experiment has been detailed
previously [10, 24] and for simplicity Fig. 1b)
illustrates only half of the PEGASES body with a
symmetry plane indicated on the figure. The thruster
body consists of a quartz cylinder 35 cm long and 6
cm diameter with two rectangular extractor tubes
Fig. 1. Scheme of the experimental setups with electrosta-
tic (a) and magnetic (b) filter, ES1 and ES2, respectively.
COMPARATIVE STUDY OF POSITIVE AND NEGATIVE ION FLOWS EXTRACTED FROM DOWNSTREAM PLASMAS BEYOND MAGNETIC AND ...
ФІП ФИП PSE, 2012, т. 10, № 1, vol. 10, No. 1
25ФІП ФИП PSE, 2012, т. 10, № 1, vol. 10, No. 1
(4 by 4 cm and 6 cm long) attached perpendicular
to the cylinder axis. The gas is introduced in the
centre of the cylinder and since the system is placed
inside a large vacuum chamber the system is pumped
through the extractor opening. The plasma is created
with a three-turn loop antenna wrapped around the
middle of the quartz tube and continuously excited
at 13.56 MHz through a matching network using a
π-circuit; the power supplied to the matching box is
300 W with less than 10W reflected. Four soleno-
ids are placed symmetrically around the cylinder to
create a 110 G magnetic field with field lines parallel
to the cylinder axis. A set of two neodymium magnets
20 mm wide, 40 mm long and 10 mm thick are pla-
ced on each side of one extractor between x = 45
and 65 mm (x is measured from plasma core tube
axis) to enhance the magnetic filtering in the ex-
traction zone. The maximum magnetic field is about
800 G located in the extractor.
Aluminum plates, with a surface of 16 cm2 each,
terminate the cylinder ends and are perpendicular
to the magnetic field lines. These endplates are in
direct contact with the plasma and are either
grounded or dc biased. The endplates bias,
addressed as Vep, is set between –100 and +100 V.
Note that in Fig. 1b) only one of the endplates are
shown, and the figure is symmetrical around the RF
antenna.
A Langmuir probe is used to measure current–
voltage characteristics and thus provides informa-
tion on whether an ion-ion plasma is formed in the
extractor. The probe tip, 6 mm long and 0.25 mm
diameter, is made of platinum to avoid etching by
SF6. It is RF-compensated for the driving frequen-
cy 13.56 MHz and its harmonic 27 MHz with RF
chokes [25].
A retarding field energy analyzer (RFEA) is used
to obtain the ion energy distribution functions
(IEDFs), and has been described previously [26].
For the experiments carried out here, it is important
to note that the RFEA housing is grounded and the
size (external dimensions) is in the order of the
extraction surface. Voltage distribution across the
RFEA is described in [24].
EXPERIMENTAL RESULTS AND
DISCUSSION
The measured current-voltage traces of the
Langmuir probes placed in the CP region of ES 1
and in the extraction region of ES 2 are shown in
Fig. 2a) and b), respectively. Comparison of the
plasma parameters derived from the I – V curves
shows strong influence of both the magnetic and
electrostatic grid-type filter. The electron tempe-
ratures in the plasma cores of both systems are about
5 − 6 eV [17, 24], while the temperature down-
stream of the filters are 0.7 eV for Ar and 1.5 eV
for SF6-containing plasma in ES 1 and 1 − 2 eV for
Ar and SF6 plasmas in ES 2.
In ES1 the addition of SF6 leads to significant
decrease in the negative particles current to the
probe. Since the saturation current of positive ions
remains constant this indicate a strong increase in
the negative ion density. A similar situation occurs in
ES 2 where the I − V curve obtained in both O2
and SF6 are more or less symmetrical. However, in
this case the probe trace measured in Ar demo-
nstrates clearly the effect of the magnetic barrier,
where the strongly reduced electron mobility
Fig. 2. Current-voltage traces of the Langmuir probes pla-
ced in the CP region of ES 1 (a) and in the extraction region
of ES 2 (b).
S. DUDIN, D. RAFALSKYI, L. POPELIER, A. AANESLAND
a)
b)
26
prevents the plasma from entering the extractor in
order to obey quasi-neutrality.
Comparison of the results obtained in ES 1 and
ES 2 shows that both systems effectively create
negative ions and formation of highly electronegative
plasmas is occur. However, it seems like the negative
ions are created in the CP region of ES 1 while in
ES 2 the electrons are completely blocked by the
magnetic barrier and the negative ions are probably
created before or in the magnetic filter in this case.
The symmetry in the I – V curve indicates that the
electronegative plasma in ES 2 is a so-called ion-
ion plasma while there are still some remaining
electrons in ES 1. Interestingly, the current saturation
density (Isat/Aprobe) for the positive ions is the same
in the two systems when operating in SF6, showing
that the electrostatic grid-filter and the magnetic filter
has very similar effects on the downstream plasma
density. It should be noted that absorbed RF power
was 300 W in both systems.
The obtained results show that the grid-type filter
provides effective electron cooling comparable to
the magnetic filter with equivalent density of positive
and negative ions downstream of the filter. However,
real applications where positive and negative ions
are extracted from the plasma also require the
possibility of plasma potential control in order to
create controlled fluxes of positive or negative ions
to the processed surface or to the ion extraction
system.
In order to test the possibility of plasma poten-
tial control in both systems we have measured I −
V traces of the probes at different potentials of the
filter grid in the ES 1 and the endplates in the ES 2.
The results of these measurements are shown in
Fig. 3a) and b), respectively, showing that all the
curves follow the applied potential with almost un-
changed shape. Fig. 4a) and b) shows the measured
dependences of the downstream plasma potentials
on the grid bias in ES 1 and the endplate bias in ES
2, respectively. It is seen that the plasma potential is
in nonlinear dependence on the electrodes (grid or
endplates) potential. For positive grid/endplates bias
the plasma potential increases linearly with the grid/
endplate bias, while for negative bias on the grid/
endplates the downstream plasma potential tends
to saturation. In the case of ES 1 using an
electrostatic filter this saturation depends on the gas
mixture, i.e. the electropositive or electronegative
nature of the plasma. In the Argon case it is seen
that the plasma potential cannot become more
negative than the large extraction electrode, while in
the electronegative plasma the plasma potential is
controlled and is more negative than the grounded
extraction electrode. This result indicates that a
negative space charge sheath might exist in front of
the extraction electrode and the plasma potential is
not (as in typical electropositive plasmas) the most
positive potential.
In the ES 2 case for positive biasing of the
endplate, the plasma potential with respect to the
endplate vary slightly with the type of gas, but
saturates at a positive potential corresponding to a
positive space charge sheath with about 5 eV
electrons.
The demonstrated control of plasma potential
allows to control the energy and polarity of the
extracted ions. The measured ion energy distribution
Fig. 3. I – V traces of the probes at different potentials of
the filter grid in the ES 1 (a) and endplates in the ES 2 (b).
Filling gas is the mixture 2 for ES 1 and SF6 for ES 2.
COMPARATIVE STUDY OF POSITIVE AND NEGATIVE ION FLOWS EXTRACTED FROM DOWNSTREAM PLASMAS BEYOND MAGNETIC AND ...
ФІП ФИП PSE, 2012, т. 10, № 1, vol. 10, No. 1
a)
b)
27ФІП ФИП PSE, 2012, т. 10, № 1, vol. 10, No. 1
functions (IEDFs) of the positive Ar ions in ES2
are shown in the Fig. 5. All the measured IEDFs
show that the ion flow is close to mono-energetic
with mean ion energy corresponding to the plasma
potential (see Fig. 4b)).
It should be noted that in the ES 1 the CP
potential becomes negative with SF6 addition at grid
potentials lower than −7 V (see Fig. 4a)). Therefore,
the flow of negative ions can be extracted from the
plasma towards the extraction electrode.
Measurements of the energy distribution of the
extracted negative ion flow is usually complicated
due to presence of significant electron current from
the plasma which can’t be separated from the
negative ion current. This problem can be solved
using the magnetically filtered energy analyzer
(MRFEA) developed at the LDPTP [17]. The
additional advantage of this analyzer is the capability
of simultaneous analysis of both positive and
negative ions. In Fig. 6 the IEDFs of positive and
negative ion flows from the plasma are shown for
different plasma biasing. One can see that at negative
plasma potentials negative ion flow is extracted from
the plasma while positive ions when the plasma
potential is positive.
Summarizing the presented results we can draw
a conclusion that the system with the grid-type filter
can be a good basis of wide aperture negative ion
source with efficiency similar to systems with mag-
netic filter. Naturally, the development of a source
based on the electrostatic filter requires further
extensive investigations. Nevertheless, the obtained
results show possibility of practical application of
the system with the grid-type electron filter. The first
possibility is reactive negative ion etching of samples
placed on the extraction electrode. Replacement of
this electrode by the gridded extraction system tran-
sforms the system to the source of broad accele-
rated negative ion beam which can be used both in
reactive ion etching technology and in space thruster
applications. Another promising application is the
Fig. 4. The dependences of the downstream plasmas po-
tentials on the grid (ES 1) (a) and endplate potential (ES 2)
(b).
Fig. 5. IEDFs of the positive Ar ions measured in ES 2 at
different endplate bias.
Fig.6. IEDFs of the positive and negative ions measured
in ES 1 at different plasma potentials. The filling gas is
mixture 2.
S. DUDIN, D. RAFALSKYI, L. POPELIER, A. AANESLAND
a)
b)
×10−9
28
surface modification of polymers (activation or
passivation) where low energy beams are crucial.
SUMMARY
In summary, in the present paper two types of
electron filters for the production of electronegative
plasmas with high negative ion densities are
compared: the recently developed grid-type filter
and the more known magnetic filter. The results of
probe measurements show the effective electron
“cooling” with both filters. The control of the plasma
potential via the bias on internal electrode/grid is also
investigated and compared. It is observed that the
plasma potential follows the electrode potential with
nonlinear dependence. The EDFs of the positi-ve
and negative ion flows from the filtered plasmas are
measured in both systems showing almost
monoenergetic nature of the flows. The energy of
the IEDF peaks corresponds to the measured plasma
potential in all the investigated cases. It is shown
that the application of the grid-type filter provides
formation of highly electronegative plasma negatively
biased with respect to an extraction electrode that
allows to extract negative ion flow from the plasma.
The main conclusion of the present research is that
grid-type electron filter demonstrates similar func-
tionality as the magnetic filter and may be very pro-
mising for modern applications.
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