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A new low-energy ion implanter for bombardment of cylindrical surfaces
Vacuum/volume 35/number Printed in Great Britain
0042-207x/8553.00+ .oo
12/pages 577 to 578/l 985
Pergamon Press Ltd
A new low-energy ion implanter for bombardment of cylindrical surfaces M Renier, A A Lucas and S E Donnelly*, Bruxelles, B - 5000 Namur Belgium
lRIS--Fact&es
Universitaires
Notre-Dame
de la Paix 61, Rue de
An ion implanter of cylindrical geometry is described suitable for uniform implantation at a few keV of inert gas ions of narrow energy distribution into conducting targets of axial symmetry. For helium implantation the ion current in the present device can easily be maintained at over 1 mA, allowing high dose implantation of large areas (several tens of cm2) in minutes. For temperature control of the target, a thermal shunt is incorporated. The-sizes and the implantation energy of the device are scalable. Reactive gases can also be implanted with the device when operated at lower pressures and ion currents.
1. Introduction We have constructed an ion implantation system for inert gases, departing from the usual axial geometry’ and allowing for simultaneous implantation of large cylindrical surfaces. The need for such a cylindrical ion implanter arosein the course of our effort to develop a new source of vacuum ultra-violet radiation2*3. However here we envisage, as one possible application of the implanter, the implantation of nitrogen into steel objects such as drill bits, axle rods and other items of axial symmetry with the aim to improve their wear characteristics4-‘. In its present form, the ion implanter can be used to implant gases into the surface of metallic cylinders of any diameter less than 4 cm and any length under 10 cm (objects longer than the implanter can be implanted by smooth translation). The primary ion energy can be varied from below 500 eV up to 5 keV and more. However, the present construction sizes are easily scaled up or down and the maximum ion energy can be increased, for scaled up systems, to 30 keV or more with appropriate insulation care. In a typical implantation run in our prototype, a dose of 2 x 10” He ions cm-’ was achieved within 15 min of steady operation in an Al cylinder 3 cm in diameter and 7 cm in length. A standard, commercially available linear ion gun would have taken at least 100 times longer and required rotation and translation of the sample to produce the same result. The new implanter also has advantages over gas-discharge systems4s5 among which are tunability of the primary ion energy and stability of ion current over a wider range of values.
coaxially mounted as shown in Figure 1. Equality of length of the filaments is crucial in ensuring uniformity of thermionic emission if they are electrically connected in parallel. The metal surface to be bombarded was an Al (circular) cylinder mounted coaxially with the grids. The assembly is enclosed in a vacuum chamber in which He or others gases could be introduced at the desired pressure (2 10m4 mbar) through a leak valve. If monoenergetic ion implantation is required, the gas pressure should be maintained below a maximum value for which the mean free path is sufficiently large (for He, this gas pressure is of order 10e4 mbar in the present system). The purpose of device A was to test for implantation uniformity and temperature, as described in the following section. However, in order to further increase the total ion current, a second version oftheimplanter (device B) was built incorporating a magnetic confinement system in the ionization region. Figure 2
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* Present address: Division Clayton, Victoria, Australia
of Chemical 3168.
Physics,
CSIRO
PO Box 160,
/’
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.’ 8’
IF .N_
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In its initial version (device A) the implanter comprised a set of three stainless steel cylindrical grids and a set oftungsten filaments
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2. Details of construction
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Figure 1. Cross-section of the implanter in configuration A. Bias voltages of the various electrodes are as follows: filaments F, 0 V; grids G 1, G3,O V; grid G2, + 350 V; target, - 5 kV. 577
M Renier,
A A Lucas
and S E Donnelly.
A new
low-energy
Ion lrnplanter
. . . .
.
l
.
. . . . .
A
I
’
A
.
.
I
I
I
20
30
40
TIME Figure 2. Cross-sectton of implnntcr B lncorporntlng ;t set ol’h prrm;~ncn~ magnets for eleclron confinement. The magnets arc 111contact ~lth srbd GZ biased at ~ 350 V. G 1 grounded. Target. ~ 5 kV.
Figure 3. Heating cul-bcr for an Al c!llndcr ,a~-gc, under ~rnplan(at~~~n 1,; de\lcc A. The lW,o upper cur\cs correspond IO 1’1W111 Icmpcrnturc (HI or prc-cooled target (0) at the start of implantation. Curve ( A 1wasobtained with the target thermally shunted to an external heat sink
200
shows. to scale. the optimized arrangement of permanent magnets, grids and filaments. As described in the next section a gain by a factor 5 in total Heion current was obtained in device B as compared to A for the same conditions.
3. Results Using implanter A. the implantation uniformity was measured to be better than 5”,, in both the longitudinal and azimuthal directions. Device A was also investigated for temperature variations in the implanted target. Figure 3 shows the heating curves during operation with or without a cooling shunt of the test target (for some applications’ it is necessary to avoid cxccssive heating of the implanted surface). Regarding total ion currents, system A allowed a maximum Hc ion current of 0.18 mA with a thermoionic current of 150 mA, target voltage of 5 keV and lo-” mbar He gas pressure. Under the same conditions, device B (with magnetic confinement) gave an ion current of 1 mA. In both systems as shown in Figure 4. the ion implantation rate was not very sensitive to the value of the primary voltage down to 800 V. This feature is a further advantage over gas-discharge systems which are very sensitive to discharge voltage.
578
(Mlnutesl
.
III 1 ._
. 150
.
.
.
.
.
.
.
0042-207x/8553.00+ .oo
12/pages 577 to 578/l 985
Pergamon Press Ltd
A new low-energy ion implanter for bombardment of cylindrical surfaces M Renier, A A Lucas and S E Donnelly*, Bruxelles, B - 5000 Namur Belgium
lRIS--Fact&es
Universitaires
Notre-Dame
de la Paix 61, Rue de
An ion implanter of cylindrical geometry is described suitable for uniform implantation at a few keV of inert gas ions of narrow energy distribution into conducting targets of axial symmetry. For helium implantation the ion current in the present device can easily be maintained at over 1 mA, allowing high dose implantation of large areas (several tens of cm2) in minutes. For temperature control of the target, a thermal shunt is incorporated. The-sizes and the implantation energy of the device are scalable. Reactive gases can also be implanted with the device when operated at lower pressures and ion currents.
1. Introduction We have constructed an ion implantation system for inert gases, departing from the usual axial geometry’ and allowing for simultaneous implantation of large cylindrical surfaces. The need for such a cylindrical ion implanter arosein the course of our effort to develop a new source of vacuum ultra-violet radiation2*3. However here we envisage, as one possible application of the implanter, the implantation of nitrogen into steel objects such as drill bits, axle rods and other items of axial symmetry with the aim to improve their wear characteristics4-‘. In its present form, the ion implanter can be used to implant gases into the surface of metallic cylinders of any diameter less than 4 cm and any length under 10 cm (objects longer than the implanter can be implanted by smooth translation). The primary ion energy can be varied from below 500 eV up to 5 keV and more. However, the present construction sizes are easily scaled up or down and the maximum ion energy can be increased, for scaled up systems, to 30 keV or more with appropriate insulation care. In a typical implantation run in our prototype, a dose of 2 x 10” He ions cm-’ was achieved within 15 min of steady operation in an Al cylinder 3 cm in diameter and 7 cm in length. A standard, commercially available linear ion gun would have taken at least 100 times longer and required rotation and translation of the sample to produce the same result. The new implanter also has advantages over gas-discharge systems4s5 among which are tunability of the primary ion energy and stability of ion current over a wider range of values.
coaxially mounted as shown in Figure 1. Equality of length of the filaments is crucial in ensuring uniformity of thermionic emission if they are electrically connected in parallel. The metal surface to be bombarded was an Al (circular) cylinder mounted coaxially with the grids. The assembly is enclosed in a vacuum chamber in which He or others gases could be introduced at the desired pressure (2 10m4 mbar) through a leak valve. If monoenergetic ion implantation is required, the gas pressure should be maintained below a maximum value for which the mean free path is sufficiently large (for He, this gas pressure is of order 10e4 mbar in the present system). The purpose of device A was to test for implantation uniformity and temperature, as described in the following section. However, in order to further increase the total ion current, a second version oftheimplanter (device B) was built incorporating a magnetic confinement system in the ionization region. Figure 2
__----__ _!-!=
/--
I
/’
-.
/’
/’
‘\
‘\\
/’ ‘.
‘\
* Present address: Division Clayton, Victoria, Australia
of Chemical 3168.
Physics,
CSIRO
PO Box 160,
/’
‘\ ‘\
. .
c-
---__--*
‘. -;--___a--
.’ 8’
IF .N_
25 mm
“1
‘, -;- > ‘, \ \
,I’
.’
--__-*
In its initial version (device A) the implanter comprised a set of three stainless steel cylindrical grids and a set oftungsten filaments
‘\
‘\
,flc----N-\
/
‘\
‘\
,__-------_
‘,,_Target
2. Details of construction
-.
1
1’ /
-w
\ \ 1 I /I\-I--\ jG1 \0 /:
,/
_#I
.’
/’
Figure 1. Cross-section of the implanter in configuration A. Bias voltages of the various electrodes are as follows: filaments F, 0 V; grids G 1, G3,O V; grid G2, + 350 V; target, - 5 kV. 577
M Renier,
A A Lucas
and S E Donnelly.
A new
low-energy
Ion lrnplanter
. . . .
.
l
.
. . . . .
A
I
’
A
.
.
I
I
I
20
30
40
TIME Figure 2. Cross-sectton of implnntcr B lncorporntlng ;t set ol’h prrm;~ncn~ magnets for eleclron confinement. The magnets arc 111contact ~lth srbd GZ biased at ~ 350 V. G 1 grounded. Target. ~ 5 kV.
Figure 3. Heating cul-bcr for an Al c!llndcr ,a~-gc, under ~rnplan(at~~~n 1,; de\lcc A. The lW,o upper cur\cs correspond IO 1’1W111 Icmpcrnturc (HI or prc-cooled target (0) at the start of implantation. Curve ( A 1wasobtained with the target thermally shunted to an external heat sink
200
shows. to scale. the optimized arrangement of permanent magnets, grids and filaments. As described in the next section a gain by a factor 5 in total Heion current was obtained in device B as compared to A for the same conditions.
3. Results Using implanter A. the implantation uniformity was measured to be better than 5”,, in both the longitudinal and azimuthal directions. Device A was also investigated for temperature variations in the implanted target. Figure 3 shows the heating curves during operation with or without a cooling shunt of the test target (for some applications’ it is necessary to avoid cxccssive heating of the implanted surface). Regarding total ion currents, system A allowed a maximum Hc ion current of 0.18 mA with a thermoionic current of 150 mA, target voltage of 5 keV and lo-” mbar He gas pressure. Under the same conditions, device B (with magnetic confinement) gave an ion current of 1 mA. In both systems as shown in Figure 4. the ion implantation rate was not very sensitive to the value of the primary voltage down to 800 V. This feature is a further advantage over gas-discharge systems which are very sensitive to discharge voltage.
578
(Mlnutesl
.
III 1 ._
. 150
.
.
.
.
.
.
.