- Email: [email protected]
Improvements in the angular current density of inductively coupled plasma ion source for focused heavy ion beams
Accepted Manuscript Improvements in the angular current density of inductive coupled plasma ion source for focused heavy ion beams Ranjini Menon, P.Y. Nabhiraj, R.K. Bhandari PII:
S0042-207X(13)00122-X
DOI:
10.1016/j.vacuum.2013.04.008
Reference:
VAC 5993
To appear in:
Vacuum
Received Date: 6 March 2013 Revised Date:
30 March 2013
Accepted Date: 10 April 2013
Please cite this article as: Menon R, NPY, Bhandari R, Improvements in the angular current density of inductive coupled plasma ion source for focused heavy ion beams, Vaccum (2013), doi: 10.1016/ j.vacuum.2013.04.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Improvements in the angular current density of inductive
RI PT
coupled plasma ion source for focused heavy ion beams. Ranjini Menona, Nabhiraj P Ya,* and R K Bhandarib
Variable Energy Cyclotron Centre, Sector-1, Block-AF, Bidhan Nagar, Kolkata – 700064 b
Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
SC
a
*Corresponding Author, e-mail : [email protected], Phone: +913323183201,
M AN U
+913323183287
Abstract
A compact inductively coupled plasma ion source (ICPIS) is developed for producing high current micron size beams for high speed micromachining applications.
TE D
Angular current density ( J Ω ) of the beam extracted from ICPIS is measured and found to be three orders higher than that of the conventional Liquid metal ion sources. An
EP
improvement in J Ω by >30% is achieved through the increase of RF power density in the plasma by reducing the plasma volume instead of operating ion source at high RF power.
AC C
Studies on J Ω show that heavier ions have maximum J Ω at lower power and vice versa for the lighter ions. Ion beams of Neon, Argon, Krypton and Xenon extracted at 5kV, have J Ω of 57, 51, 37 and 30mA/Sr respectively at RF power in the range of 75W to 200W. Measurements on proton beam which is very important for imaging applications show J Ω of 45mA/Sr at 200W.
1
ACCEPTED MANUSCRIPT
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It is very often required to mill various types of materials in applications such as sample preparation for transmission electron microscopy (TEM) , cross sectioning of
integrated circuits (ICs) for failure analysis, creating vias in ICs, high speed secondary ion mass spectroscopy etc where removal of volumes of the order of 106 µm3 are involved.
SC
Conventional Gallium Liquid metal ion source based focused ion beam (LMIS-FIB)
M AN U
systems would take prohibitively long time to mill the materials of volumes of these orders. In order to facilitate rapid removal of large volume of material, it is essential to have high currents focused to as small spot size as possible. This capability strongly depends on the brightness of the ion source which is defined by equation (1).
Is J = Ω2 , 2 (πr )(πα o ) πrs 2 s
(1)
TE D
βs =
where, rs is the radius of emission area, I s is the current passing through an aperture,
EP
α o is the angle subtended by the aperture at the source, and J Ω is the angular current density. In conventional FIB system, LMIS exhibits high brightness of the order of 106 AmSr-1V-1 which is mainly due to extremely small emission area. However, its performance is
AC C
2
limited by spherical aberrations and hence high brightness does not necessarily increase the current in a given focused spot. To form an ion probe, the virtual source of radius rs is imaged onto the sample with total magnification M of the ion optical column. Since reduced brightness is a conserved quantity throughout the ion optical column, the current
I s in the probe is determined by equation (2). 2
ACCEPTED MANUSCRIPT
where
Vi , V0
(2)
RI PT
I s = β sπ 2 ( Mrs ) 2 α i2
α i is the image side half angle, Vo and Vi are the beam voltages on object and
SC
image side respectively. In reality, however, this current is distributed over a blurred image due to lens aberrations. In order to obtain high I s using LMIS which has very low J Ω
( α o ). This large
M AN U
(~20µA/Sr), equation (1) suggests the use of focusing column with large acceptance angle
α o introduces large spherical aberrations and contributes to drastic
reduction in the current density at the image plane. This shows the high brightness which is only due to small rs , does not contribute in achieving high I s and high current density at
TE D
sample. In order to achieve high currents with least spherical aberrations, it is essential to have higher brightness due to higher J Ω rather than smaller rs . A typical LMIS-FIB system
EP
at ~100 nA focused to ~2 µm diameter would have a reduced image side brightness β ri of about 500 A m-2 Sr-1 V-1 [1]. Brightness of this order can easily be achieved by other ion
AC C
sources such as penning ion source and inductively couple plasma ion source (ICPIS) with distinct advantages over LMIS. Recently, a few works on the plasma based ion sources that are being used in
producing the micron and submicron size heavy ion beams, demonstrating the β ri >600 A m-2 Sr-1 V-1 have been reported [1,2]. Although the plasma based ion sources have low brightness, they offer large J Ω of the order of ~20 mA/Sr. Hence plasma ion source based
3
ACCEPTED MANUSCRIPT
FIB system with small beam limiting apertures, can produce ion probes of three order higher intensity as compared to the LMIS-FIB. However, plasma ion sources have large
RI PT
source size ( rs ) and hence their performance in FIB system is limited by magnification instead of lens aberrations. For currents higher than about 20 nA, the performance of
plasma ion source based FIB system is superior as shown by authors [2] and Smith et al [1].
SC
Among the plasma ion sources, penning ion source developed by Guharay et al [3] is
reported to produce Ar ion beam of 5 mA/Sr. The J Ω of Ar ion beam of 20 mA/Sr have
M AN U
been reported by Smith et al [1] from magnetically enhanced ICPIS operating at RF power of 300 W. We also have recently developed an ICPIS for high current FIB system and focused > 500nA of 7 keV Ar beam to a spot size less than 9 µm. This source is made of a compact quartz tube plasma chamber operating at 13.56 MHz frequency and uses no
TE D
magnetic field for confinement of the plasma. The operating RF power is also relatively low as compared to other ICP ion sources reported that are designed for FIB applications. The ion source considered in the experiments presented in this article has extraction system
EP
consisting of two thin electrodes with 2 mm apertures with a gap of 2.2 mm between them. Unless otherwise mentioned, all the results are obtained with the use of small plasma
AC C
volume of 25 cm3 and with a pressure of about 0.02 mbar inside the palsma chamber. Detailed description of ion source and characterization and optimization of the ion source are reported by authors recently [2,4,5]. To the best of our knowledge, characterization of plasma based ion source for enhancement of angular current density at low RF power is not reported so far. Also the dependency of angular current density on elemental mass of the ion beam and RF power for plasma ion sources are rarely investigated. Objective of the
4
ACCEPTED MANUSCRIPT
present rapid communication is to report the novel method to improve J Ω while operating the ion source at low RF power, and to present the systematic study investigating the J Ω of
RI PT
ion beams of various gaseous elements. As described in earlier section, it is important to increase the brightness by
increasing the J Ω , in other words, increasing the ion beam intensity with low divergence.
SC
Extracted current can easily be increased by using plasma confining magnetic field.
However, it is known that the presence of magnetic field in the extraction region intorduces
M AN U
beam rotation which ultimately deteriorates the beam quality [6]. Hence in our design, the ion source has no plasma confining magnetic field of any kind. Under these conditions, the only way to improve the intensity of extracted ion beam is to use higher RF power. However, under high RF power operation, the high voltages appearing across the antenna
TE D
couple to the plasma and result into large energy spread of ions (∆E ) . To overcome this limitations and at the same time to achieve high plasma density, we employed a simple and novel technique. Here, the large plasma volume (LPV) of 50 cm3 that was initially used in
EP
our design was reduced to small plasma volume (SPV) of 25 cm3. In this new configuration, the RF power coupled to the plasma is now doubled to 8 W/cm3 at 200 W
AC C
which is a reasonably high power density. This eliminated the requirement of high RF power to obtain high plasma density and extracted current. Having achieved high extracted current, it is essential to have a proper extraction system to achieve lowest possible divergence to obtain high J Ω . In our design, we adapted the guidelines suggested by Coupland et al [7] in designing the extraction system to achieve high current beam with low divergence.
5
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To evaluate the angular current density of the ion beam, an aperture and knife edge sweep scanning method is used. A thin steel foil with an aperture of 500 µm diameter to
RI PT
choose the bright central core of the beam is placed 25 mm behind the ion beam extaction system. This apertured electrode is also made movable to allow the measurement of total current. At 65 mm downstream of the apertured electrode, a sharp steel knife is placed on
SC
the in-vacuum precision motor (M/S Physik Instrumente make) assembly for scanning across the ion beam. A Faraday cup (FC) of 5 cm diameter is used to measure the current.
M AN U
A pair of permanent magnet is assembled on to the FC for suppression of secondary electrons. FC current measured by M/S Keithley make KE 6487 picoammeter as a function of position of the knife edge is recorded to obtain the beamlet profile. By simple Geometry, the linear position of the knife edge is translated into the angle and J Ω is calculated from it’s
TE D
definition as given in equation (1). A typical profile of the central core of the beam is shown in the figure 1. In this article we use 20% - 80% rise distance of the profile as the measure of the beam width. When the knife edge is not obstructing the beam, FC reads the
EP
whole current passing through the aperture (Iaperture) and with the aperture electrode completely removed from the beam path, FC reads the total current extracted from the ion
AC C
source (Itotal). Current measurement accuracies are in the range of ±0.1 % of the reading while position measurement accuracies are in the range of 0.5 micron.
The comparison of the ion source performance with 50 cm3 and 25 cm3 plasma
volumes, in terms of production of total extracted current (Itotal) and J Ω for different RF power is presented in figure 2. Studies on 5 keV Argon ion beam suggests that the
6
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performance of the ion source with small plasma volume is superior. There is an increase in total current by a factor of 1.6 at 75 W of RF power and by a factor of 2 at 200 W. In
RI PT
other words the performance of the ion source with LPV at 200 W could be obtained at 115 W of RF power with SPV. It is worth noting that the total extracted current linearly
increases with larger slope for the smaller plasma volume. Under the same operating
SC
conditions, J Ω for Argon ion beam extracted from LPV was measured and found to be 26 mA/Sr at a very low RF power of 75 W, while it is 46 mA/Sr for 200 W. Although J Ω of
M AN U
ion beam from LPV is quite sufficient to realise high current FIBs, our efforts to achieve better performance of the ion source have resulted in further improvements in J Ω . There is a significant enhancement of J Ω of beam extracted from SPV which is 12% to 50% more than that of the beam extracted from LPV.
TE D
The effect of RF power on J Ω of ion beam of noble gases such as Xenon, Krypton, Argon, and Neon are studied. Due to low ionization potential and the high ionization cross
EP
section of heavier elements, their plasma density and thus the total extracted current are higher than that of the lighter gases at same RF power and pressure. Figure 3 shows the
AC C
variation of Itotal and Iaperture for Neon and Xenon ion beam as a function of RF power. It is seen in both the cases that Itotal linearly increases with RF power but with larger slopes for heavier gases. However, the Iaperture does not follow the trend followed by the total current. For Xenon, Iaperture decreases with RF power while for Neon it increases at a rate more than that of the total current. In case of the heavier ions, this is mainly due to the larger rate of increase in the plasma density causing an increase in the convexity of plasma meniscus and
7
ACCEPTED MANUSCRIPT
also due to the higher coulomb repulsion. Experiments have shown that at higher extraction potential, still higher Itotal and higher Iaperture for all the gases are obtained. However, we
RI PT
wish to keep the extraction potential as low as possible and subsequently accelerate to higher energies to take advantage of the additional magnification due to ratio of energies as given by law of Helmholtz and Lagrange [8,9]. In our experiments, at 5kV extraction
SC
potential and 200W of RF power, we obtained a maximum Itotal of 1450 µA which is
highest for Xenon and then followed by 1318 µA for Krypton, 900 µA for Argon and 450
M AN U
µA for Neon under almost same experimental conditions.
Figure 4 shows the variation of J Ω with RF power for different gases. With increase in the RF power, J Ω increases for ions of lighter gases while it decreases for heavier gases showing negative slope for heavier ions and positive slopes for lighter ions. It
TE D
is worth noting that the performance of the ion source in terms of J Ω of Xenon beam is best at lower RF power while it is best at higher RF power for lighter gases. Due to higher ionization cross section of xenon, higher plasma density can be obtained at lower RF power
EP
and hence matched extraction conditions can be met at lower RF power and vice versa for the lighter ions. For Xe ions, there is a maximum J Ω of 30 mA/Sr at 75W of RF power
AC C
while it is 37 mA/Sr for Kr ions at 125 W of RF power. The Ar and Ne ions have J Ω of 51 and 57 mA/Sr respectively at 200 W of RF power. The trend of J Ω for Argon and Neon ions in figure 4 shows that ion source has capability to produce still higher J Ω at higher RF power. The J Ω of proton ions also have been measured under same experimental conditions and found to be 45 mA/Sr. For every 10W of RF power, J Ω for ions of Ar, Ne and H
8
ACCEPTED MANUSCRIPT
increase by 1.2, 2.8 and 2.5 mA/Sr respectively. This large increase in J Ω with RF power can potentially eliminate the need for the aperture strip used in FIB system to vary the
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current over wide range in the focused spot. By using this newly designed ICPIS, about a 500 nA of 20 keV Xe ions can be focused to a micron diameter facilitating at least 30-40
SC
times faster milling speed than 20 nA 30 keV Ga-LMIS FIB system.
In conclusion, there is an improvement of more than ~30% in J Ω and 100% in total
M AN U
extracted current by reducing the plasma volume without using high RF power. In case of heavier ion beams the ion source can be operated with low RF power and hence it is feasible to achieve very low ∆E contributing to higher figure of merit β r
(∆E ) 2
which can
be about an order less than that of LMIS based FIB. Operation of ion source at low RF
TE D
power enhances the life time of the ion source and long term stability of the FIB system. Since this ion source can produce ions of all gases with high J Ω , it is a versatile system that can be used to achieve high speed milling with heavy ions like Xe and high speed imaging
AC C
EP
with large signal to noise ratio by proton.
References
9
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[1]
N. S. Smith, W. P. Skoczylas, S. M. Kellogg, D. E. Kinion, P. P. Tesch, O. Sutherland, A. Aanesland, and R. W. Boswell, J Vac Sci Technol B 24 (2006), pp.
[2]
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2902-2906.
P. Y. Nabhiraj, R. Menon, G. Mohan Rao, S. Mohan, and R. K. Bhandari, Nucl Instr
[3]
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Meth B, 621 (2010), pp. 57–61.
S. K. Guharay, E. Sokolovsky, and J. Orloff, J Vac Sci Technol B 17 (1999), pp.
[4]
P. Y. Nabhiraj, R. Menon, G. Mohan Rao, S. Mohan, and R. K. Bhandari, Vacuum, 85 (2010), pp. 344–348.
[5]
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2779-2782
P. Y. Nabhiraj, R. Menon, and R. K. Bhandari, J. Vac. Sci. Tehcnol. B, 29(2011), pp.
[6]
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051604–1– 051604–6.
Ian G Brown, The Physics and Technology of Ion Sources Second , Revised and
J. R. Coupland, T. S. Green, D. P. Hammond, and A. C. Riviere, Rev Sci Instrum, 44
AC C
[7]
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Extended Edition, Second ed WILEY-VCH Verlag GmgH &Co, Weinheim, (2004)
(1973), pp.1258–1270.
[8]
[9]
Y. Ishii and A. Isoya, Nucl Instrum Meth B, 181 (2001), pp. 71–77.
D. W. O. Heddle., Electrostatic Lens Systems, Second ed., Institute of Physics Publishing, Bristol, 2000.
10
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Figure 1. Knife edge profile of 5keV Ar ion beam showing angular current density of 48
SC
mA/Sr corresponding to 20%-80% rise distance .
Figure 2. Comparison of the performance of the ion source operated with 50cm3 and 25 cm3
M AN U
plasma volumes.
Figure 3. Variation of the total extracted current and the current through the aperture with RF Power for Xenon and Neon ions.
AC C
EP
with SPV
TE D
Figure 4. Comparison of angular current density of various gases at different RF power
11
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JΩ of ion beam from ICP ion source for high current FIB system is investigated.
•
Achieved high JΩ at low RF powers.
•
JΩ of this source is three orders higher than that of liquid metal ion source.
•
Significant enhancement in JΩ achieved by modifying the plasma chamber.
•
Heavier ions show better performance in terms of JΩ at lower RF power.
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•
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EP
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S0042-207X(13)00122-X
DOI:
10.1016/j.vacuum.2013.04.008
Reference:
VAC 5993
To appear in:
Vacuum
Received Date: 6 March 2013 Revised Date:
30 March 2013
Accepted Date: 10 April 2013
Please cite this article as: Menon R, NPY, Bhandari R, Improvements in the angular current density of inductive coupled plasma ion source for focused heavy ion beams, Vaccum (2013), doi: 10.1016/ j.vacuum.2013.04.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Improvements in the angular current density of inductive
RI PT
coupled plasma ion source for focused heavy ion beams. Ranjini Menona, Nabhiraj P Ya,* and R K Bhandarib
Variable Energy Cyclotron Centre, Sector-1, Block-AF, Bidhan Nagar, Kolkata – 700064 b
Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
SC
a
*Corresponding Author, e-mail : [email protected], Phone: +913323183201,
M AN U
+913323183287
Abstract
A compact inductively coupled plasma ion source (ICPIS) is developed for producing high current micron size beams for high speed micromachining applications.
TE D
Angular current density ( J Ω ) of the beam extracted from ICPIS is measured and found to be three orders higher than that of the conventional Liquid metal ion sources. An
EP
improvement in J Ω by >30% is achieved through the increase of RF power density in the plasma by reducing the plasma volume instead of operating ion source at high RF power.
AC C
Studies on J Ω show that heavier ions have maximum J Ω at lower power and vice versa for the lighter ions. Ion beams of Neon, Argon, Krypton and Xenon extracted at 5kV, have J Ω of 57, 51, 37 and 30mA/Sr respectively at RF power in the range of 75W to 200W. Measurements on proton beam which is very important for imaging applications show J Ω of 45mA/Sr at 200W.
1
ACCEPTED MANUSCRIPT
RI PT
It is very often required to mill various types of materials in applications such as sample preparation for transmission electron microscopy (TEM) , cross sectioning of
integrated circuits (ICs) for failure analysis, creating vias in ICs, high speed secondary ion mass spectroscopy etc where removal of volumes of the order of 106 µm3 are involved.
SC
Conventional Gallium Liquid metal ion source based focused ion beam (LMIS-FIB)
M AN U
systems would take prohibitively long time to mill the materials of volumes of these orders. In order to facilitate rapid removal of large volume of material, it is essential to have high currents focused to as small spot size as possible. This capability strongly depends on the brightness of the ion source which is defined by equation (1).
Is J = Ω2 , 2 (πr )(πα o ) πrs 2 s
(1)
TE D
βs =
where, rs is the radius of emission area, I s is the current passing through an aperture,
EP
α o is the angle subtended by the aperture at the source, and J Ω is the angular current density. In conventional FIB system, LMIS exhibits high brightness of the order of 106 AmSr-1V-1 which is mainly due to extremely small emission area. However, its performance is
AC C
2
limited by spherical aberrations and hence high brightness does not necessarily increase the current in a given focused spot. To form an ion probe, the virtual source of radius rs is imaged onto the sample with total magnification M of the ion optical column. Since reduced brightness is a conserved quantity throughout the ion optical column, the current
I s in the probe is determined by equation (2). 2
ACCEPTED MANUSCRIPT
where
Vi , V0
(2)
RI PT
I s = β sπ 2 ( Mrs ) 2 α i2
α i is the image side half angle, Vo and Vi are the beam voltages on object and
SC
image side respectively. In reality, however, this current is distributed over a blurred image due to lens aberrations. In order to obtain high I s using LMIS which has very low J Ω
( α o ). This large
M AN U
(~20µA/Sr), equation (1) suggests the use of focusing column with large acceptance angle
α o introduces large spherical aberrations and contributes to drastic
reduction in the current density at the image plane. This shows the high brightness which is only due to small rs , does not contribute in achieving high I s and high current density at
TE D
sample. In order to achieve high currents with least spherical aberrations, it is essential to have higher brightness due to higher J Ω rather than smaller rs . A typical LMIS-FIB system
EP
at ~100 nA focused to ~2 µm diameter would have a reduced image side brightness β ri of about 500 A m-2 Sr-1 V-1 [1]. Brightness of this order can easily be achieved by other ion
AC C
sources such as penning ion source and inductively couple plasma ion source (ICPIS) with distinct advantages over LMIS. Recently, a few works on the plasma based ion sources that are being used in
producing the micron and submicron size heavy ion beams, demonstrating the β ri >600 A m-2 Sr-1 V-1 have been reported [1,2]. Although the plasma based ion sources have low brightness, they offer large J Ω of the order of ~20 mA/Sr. Hence plasma ion source based
3
ACCEPTED MANUSCRIPT
FIB system with small beam limiting apertures, can produce ion probes of three order higher intensity as compared to the LMIS-FIB. However, plasma ion sources have large
RI PT
source size ( rs ) and hence their performance in FIB system is limited by magnification instead of lens aberrations. For currents higher than about 20 nA, the performance of
plasma ion source based FIB system is superior as shown by authors [2] and Smith et al [1].
SC
Among the plasma ion sources, penning ion source developed by Guharay et al [3] is
reported to produce Ar ion beam of 5 mA/Sr. The J Ω of Ar ion beam of 20 mA/Sr have
M AN U
been reported by Smith et al [1] from magnetically enhanced ICPIS operating at RF power of 300 W. We also have recently developed an ICPIS for high current FIB system and focused > 500nA of 7 keV Ar beam to a spot size less than 9 µm. This source is made of a compact quartz tube plasma chamber operating at 13.56 MHz frequency and uses no
TE D
magnetic field for confinement of the plasma. The operating RF power is also relatively low as compared to other ICP ion sources reported that are designed for FIB applications. The ion source considered in the experiments presented in this article has extraction system
EP
consisting of two thin electrodes with 2 mm apertures with a gap of 2.2 mm between them. Unless otherwise mentioned, all the results are obtained with the use of small plasma
AC C
volume of 25 cm3 and with a pressure of about 0.02 mbar inside the palsma chamber. Detailed description of ion source and characterization and optimization of the ion source are reported by authors recently [2,4,5]. To the best of our knowledge, characterization of plasma based ion source for enhancement of angular current density at low RF power is not reported so far. Also the dependency of angular current density on elemental mass of the ion beam and RF power for plasma ion sources are rarely investigated. Objective of the
4
ACCEPTED MANUSCRIPT
present rapid communication is to report the novel method to improve J Ω while operating the ion source at low RF power, and to present the systematic study investigating the J Ω of
RI PT
ion beams of various gaseous elements. As described in earlier section, it is important to increase the brightness by
increasing the J Ω , in other words, increasing the ion beam intensity with low divergence.
SC
Extracted current can easily be increased by using plasma confining magnetic field.
However, it is known that the presence of magnetic field in the extraction region intorduces
M AN U
beam rotation which ultimately deteriorates the beam quality [6]. Hence in our design, the ion source has no plasma confining magnetic field of any kind. Under these conditions, the only way to improve the intensity of extracted ion beam is to use higher RF power. However, under high RF power operation, the high voltages appearing across the antenna
TE D
couple to the plasma and result into large energy spread of ions (∆E ) . To overcome this limitations and at the same time to achieve high plasma density, we employed a simple and novel technique. Here, the large plasma volume (LPV) of 50 cm3 that was initially used in
EP
our design was reduced to small plasma volume (SPV) of 25 cm3. In this new configuration, the RF power coupled to the plasma is now doubled to 8 W/cm3 at 200 W
AC C
which is a reasonably high power density. This eliminated the requirement of high RF power to obtain high plasma density and extracted current. Having achieved high extracted current, it is essential to have a proper extraction system to achieve lowest possible divergence to obtain high J Ω . In our design, we adapted the guidelines suggested by Coupland et al [7] in designing the extraction system to achieve high current beam with low divergence.
5
ACCEPTED MANUSCRIPT
To evaluate the angular current density of the ion beam, an aperture and knife edge sweep scanning method is used. A thin steel foil with an aperture of 500 µm diameter to
RI PT
choose the bright central core of the beam is placed 25 mm behind the ion beam extaction system. This apertured electrode is also made movable to allow the measurement of total current. At 65 mm downstream of the apertured electrode, a sharp steel knife is placed on
SC
the in-vacuum precision motor (M/S Physik Instrumente make) assembly for scanning across the ion beam. A Faraday cup (FC) of 5 cm diameter is used to measure the current.
M AN U
A pair of permanent magnet is assembled on to the FC for suppression of secondary electrons. FC current measured by M/S Keithley make KE 6487 picoammeter as a function of position of the knife edge is recorded to obtain the beamlet profile. By simple Geometry, the linear position of the knife edge is translated into the angle and J Ω is calculated from it’s
TE D
definition as given in equation (1). A typical profile of the central core of the beam is shown in the figure 1. In this article we use 20% - 80% rise distance of the profile as the measure of the beam width. When the knife edge is not obstructing the beam, FC reads the
EP
whole current passing through the aperture (Iaperture) and with the aperture electrode completely removed from the beam path, FC reads the total current extracted from the ion
AC C
source (Itotal). Current measurement accuracies are in the range of ±0.1 % of the reading while position measurement accuracies are in the range of 0.5 micron.
The comparison of the ion source performance with 50 cm3 and 25 cm3 plasma
volumes, in terms of production of total extracted current (Itotal) and J Ω for different RF power is presented in figure 2. Studies on 5 keV Argon ion beam suggests that the
6
ACCEPTED MANUSCRIPT
performance of the ion source with small plasma volume is superior. There is an increase in total current by a factor of 1.6 at 75 W of RF power and by a factor of 2 at 200 W. In
RI PT
other words the performance of the ion source with LPV at 200 W could be obtained at 115 W of RF power with SPV. It is worth noting that the total extracted current linearly
increases with larger slope for the smaller plasma volume. Under the same operating
SC
conditions, J Ω for Argon ion beam extracted from LPV was measured and found to be 26 mA/Sr at a very low RF power of 75 W, while it is 46 mA/Sr for 200 W. Although J Ω of
M AN U
ion beam from LPV is quite sufficient to realise high current FIBs, our efforts to achieve better performance of the ion source have resulted in further improvements in J Ω . There is a significant enhancement of J Ω of beam extracted from SPV which is 12% to 50% more than that of the beam extracted from LPV.
TE D
The effect of RF power on J Ω of ion beam of noble gases such as Xenon, Krypton, Argon, and Neon are studied. Due to low ionization potential and the high ionization cross
EP
section of heavier elements, their plasma density and thus the total extracted current are higher than that of the lighter gases at same RF power and pressure. Figure 3 shows the
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variation of Itotal and Iaperture for Neon and Xenon ion beam as a function of RF power. It is seen in both the cases that Itotal linearly increases with RF power but with larger slopes for heavier gases. However, the Iaperture does not follow the trend followed by the total current. For Xenon, Iaperture decreases with RF power while for Neon it increases at a rate more than that of the total current. In case of the heavier ions, this is mainly due to the larger rate of increase in the plasma density causing an increase in the convexity of plasma meniscus and
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also due to the higher coulomb repulsion. Experiments have shown that at higher extraction potential, still higher Itotal and higher Iaperture for all the gases are obtained. However, we
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wish to keep the extraction potential as low as possible and subsequently accelerate to higher energies to take advantage of the additional magnification due to ratio of energies as given by law of Helmholtz and Lagrange [8,9]. In our experiments, at 5kV extraction
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potential and 200W of RF power, we obtained a maximum Itotal of 1450 µA which is
highest for Xenon and then followed by 1318 µA for Krypton, 900 µA for Argon and 450
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µA for Neon under almost same experimental conditions.
Figure 4 shows the variation of J Ω with RF power for different gases. With increase in the RF power, J Ω increases for ions of lighter gases while it decreases for heavier gases showing negative slope for heavier ions and positive slopes for lighter ions. It
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is worth noting that the performance of the ion source in terms of J Ω of Xenon beam is best at lower RF power while it is best at higher RF power for lighter gases. Due to higher ionization cross section of xenon, higher plasma density can be obtained at lower RF power
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and hence matched extraction conditions can be met at lower RF power and vice versa for the lighter ions. For Xe ions, there is a maximum J Ω of 30 mA/Sr at 75W of RF power
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while it is 37 mA/Sr for Kr ions at 125 W of RF power. The Ar and Ne ions have J Ω of 51 and 57 mA/Sr respectively at 200 W of RF power. The trend of J Ω for Argon and Neon ions in figure 4 shows that ion source has capability to produce still higher J Ω at higher RF power. The J Ω of proton ions also have been measured under same experimental conditions and found to be 45 mA/Sr. For every 10W of RF power, J Ω for ions of Ar, Ne and H
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increase by 1.2, 2.8 and 2.5 mA/Sr respectively. This large increase in J Ω with RF power can potentially eliminate the need for the aperture strip used in FIB system to vary the
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current over wide range in the focused spot. By using this newly designed ICPIS, about a 500 nA of 20 keV Xe ions can be focused to a micron diameter facilitating at least 30-40
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times faster milling speed than 20 nA 30 keV Ga-LMIS FIB system.
In conclusion, there is an improvement of more than ~30% in J Ω and 100% in total
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extracted current by reducing the plasma volume without using high RF power. In case of heavier ion beams the ion source can be operated with low RF power and hence it is feasible to achieve very low ∆E contributing to higher figure of merit β r
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which can
be about an order less than that of LMIS based FIB. Operation of ion source at low RF
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power enhances the life time of the ion source and long term stability of the FIB system. Since this ion source can produce ions of all gases with high J Ω , it is a versatile system that can be used to achieve high speed milling with heavy ions like Xe and high speed imaging
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with large signal to noise ratio by proton.
References
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[1]
N. S. Smith, W. P. Skoczylas, S. M. Kellogg, D. E. Kinion, P. P. Tesch, O. Sutherland, A. Aanesland, and R. W. Boswell, J Vac Sci Technol B 24 (2006), pp.
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2902-2906.
P. Y. Nabhiraj, R. Menon, G. Mohan Rao, S. Mohan, and R. K. Bhandari, Nucl Instr
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Meth B, 621 (2010), pp. 57–61.
S. K. Guharay, E. Sokolovsky, and J. Orloff, J Vac Sci Technol B 17 (1999), pp.
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P. Y. Nabhiraj, R. Menon, G. Mohan Rao, S. Mohan, and R. K. Bhandari, Vacuum, 85 (2010), pp. 344–348.
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2779-2782
P. Y. Nabhiraj, R. Menon, and R. K. Bhandari, J. Vac. Sci. Tehcnol. B, 29(2011), pp.
[6]
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051604–1– 051604–6.
Ian G Brown, The Physics and Technology of Ion Sources Second , Revised and
J. R. Coupland, T. S. Green, D. P. Hammond, and A. C. Riviere, Rev Sci Instrum, 44
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[7]
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Extended Edition, Second ed WILEY-VCH Verlag GmgH &Co, Weinheim, (2004)
(1973), pp.1258–1270.
[8]
[9]
Y. Ishii and A. Isoya, Nucl Instrum Meth B, 181 (2001), pp. 71–77.
D. W. O. Heddle., Electrostatic Lens Systems, Second ed., Institute of Physics Publishing, Bristol, 2000.
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Figure 1. Knife edge profile of 5keV Ar ion beam showing angular current density of 48
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mA/Sr corresponding to 20%-80% rise distance .
Figure 2. Comparison of the performance of the ion source operated with 50cm3 and 25 cm3
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plasma volumes.
Figure 3. Variation of the total extracted current and the current through the aperture with RF Power for Xenon and Neon ions.
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Figure 4. Comparison of angular current density of various gases at different RF power
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JΩ of ion beam from ICP ion source for high current FIB system is investigated.
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Achieved high JΩ at low RF powers.
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JΩ of this source is three orders higher than that of liquid metal ion source.
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Significant enhancement in JΩ achieved by modifying the plasma chamber.
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Heavier ions show better performance in terms of JΩ at lower RF power.
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