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Improvement of a microwave ion source for surface modification
Materials Science and Engineering, A l l 6 (1989) 221-225
221
Improvement of a Microwave Ion Source for Surface Modification* N. SAKUDO, K. TOKIGUCHI and T. SEKI
Hitachi Research Laboratory, Hitachi Ltd., 4026 Kuji-cho, Hitachi-shi, Ibaraki-ken 319-12 (Japan) H. KOIKE
Naka Works, Hitachi Ltd., Katsuta-shi, Ibaraki-ken 312 (Japan) M. IWAKI
The Institute of Physical and ChemicaI Research (RIKEN), Wako, Saimma 351-01 (Japan) (Received September 16, 1988)
Abstract A microwave ion source for metallic ions was" modified to eliminate the magnetic field from the ion extraction space, thus providingstablebeams, The beam characteristics of the source were measured with both N_, and TiCl4. Ratios of N + to N, + and Ti ÷ to Cl* in the mass spectra varied depending on discharge conditions such as microwave power and gas pressure. Emittance variations were also measured with an apparatus consisting of an emittance measuring device, a mass separator and a beam profile monitor. Conditions for improved matching of the ion source and the mass separator are discussed,
1. Introduction A microwave ion source is well suited for obtaining high-current metal ions by using halides as source materials [1]. Unlike conventional hotfilament-type ion sources which experience quick erosion of the filament by highly reactive halide plasma [2] the microwave ion source does not have any parts which are rapidly consumed. Furthermore, halogen ions and radicals are desirable in the plasma as they keep the metal from depositing on the inner walls of the discharge chamber by bombarding and chemically cleaning the wall. Otherwise, metal deposition might pre-
*Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. 0921-5093/89/$3.50
vent microwaves from entering the discharge chamber. The microwave ion source has been applied to a high-current metal-ion implanter for forming metastable surface alloys [3]. This implanter demonstrated the effectiveness of the ion source for high-dose metal-ion implantation. In general, the dose levels required for surface modification of metals exceed the highest levels used for semiconductor device fabrication (about 2). 10" ions cm Thus more stable ion sources than those of traditional ion implanters for semiconductors are desired. Beam stability is affected by fluctuation of the plasma parameters and sparking between the extraction electrodes in a vacuum. Since the fluctuation of plasma parameters depends mostly on the type of plasma generation, microwave ion sources have an advantage in generating quiescent and uniform plasmas[4-6]. Most ion sources of high-current ion implanters have a magnetic field which is superimposed on the extraction electric field [1, 71. While sparking phenomena between extraction electrodes are not completely understood at present, the magnetic field in the ion extraction space is considered to worsen spark resistivity of the extraction electrodes. This is reasonable in light of plasma generation in an ion source for which a magnetic field is used to trap electrons and ions for effective cascade collisions. From the viewpoint of improving ion source stability a new microwave ion source which eliminates the magnetic field from the ion extraction space has been constructed and its fundamental characteristics tested. © Elsevier Sequoia/Printed in The Netherlands
222
2. A microwave ion source having a closed
3. An apparatus for evaluating beam qualities
magnetic circuit Since a conventional microwave ion source for metal beams has an open magnetic circuit, a magnetic field is superimposed on the ion extraction electric field [1]. A new microwave ion source which has a closed magnetic circuit has been constructed as shown in Fig. 1. A magnetic coil is placed around the discharge chamber at a highvoltage terminal where it is also surrounded by a yoke, a high-voltage flange and an acceleration electrode; these are made of iron or a highpermeability metal. Thus the yoke, the flange and the electrode form a closed magnetic circuit together with two iron blocks to focus the magnetic field into the discharge chamber. Consequently, in the ion extraction space no magnetic field exists and at the high-voltage terminal there is enough space for the external oven. The ion source has three types of inlet system for source materials. Gases or materials of high vapour pressure are introduced into the discharge chamber throughaneedlevalve. Materials vaporized at comparatively low temperature are heated in the external oven up to 300 °C to keep the pressure around 1 0 2 Pa. The conductance of the vapour inlet pipe between the external oven and the discharge chamber is designed so that the pressure in the discharge chamber is 0.1-1 Pa. The temperature of the pipe is kept higher than the oven temperature to prevent the source feed material from condensing on the inner walls of the pipe. Materials vaporized at high temperature are heated in the internal oven. As long as halides are used as source materials, most metals can be vaporized by the external oven.
The apparatus used to study the performance of the microwave ion source is shown in Fig. 2. The apparatus consists mainly of an emittancemeasuring device, a mass separator and a beam profile monitor. The entire system is controlled by a personal computer which also processes the data obtained. The emittance-measuring device is placed at a distance of 450 mm from the extraction slit of the ion source. It consists of a beamsampling aperture (1 mm in diameter) and an ion collector that is placed 60 mm beyond the aperture and which can be moved across the sampled beamlet to measure its angular dispersion. The beam-sampling aperture plate is water-cooled since most of the ions which bombard the aperture plate give up their energy as heat. This apparatus can handle high-power beams of up to 2 kW. The mass separator is a sector magnet type of radius 470 mm and with a 75 ° deflection. The effective field gap is 50 mm. The lens effect of the mass separator field can be changed remotely by varying the angles of the ion beam to the entrance-and exit-edge fields.
vacuum
sealing
4. Beam performance of the ion source
4.1. Fundamental characteristics with nitrogen gas Variations of the mass spectra of N 2 with various microwave powers P, from 200 to 500 W are shown in Fig. 3. The pressure, which is measured by the ionization gauge placed just above the diffusion pump, is 4 x 10 -3 Pa. Since the calculated conductance between the ion extraction slit and the gauge position is 8.1 x 10 -3 m 3 s-1 and the evacuation speed at the gauge is 0.5 m 3 s - 1 , the pressure in the dis-
dlelec~rlc
Waveguide
interna~
oven
,
leratioR electrode (iron)
,
as
lalve
n et
V~lv 6
i e
~
~
ion s o u r c e m
Beam p r o f i l e m o n i t o r
2n:,; .........
"~
i
E t
t
I
t
d
I
b
Fig. 1. A modified microwave ion source. The magnetic field is completely eliminated from the ion extraction space.
Fig. 2. Sketch of apparatus for evaluating beam qualities.
223
N; t
N÷ | t
N2+ N~ N2+ .
•
N~+ . A
~
b) N+ N;
N;
,.
N+
N÷ A
~L
N2+ , I
c~ a~ Fig. 3. Mass spectra of N~ gas for different microwave powerlt, = 400 P~V;(d)Values:l~, = 500(a)~, = 200 W; (b) P~,= 300 W; (c)
charge chamber is estimated to be 0.25 Pa. The ion extraction voltage is 40 kV. The peak for the molecular ion N 2÷ is the highest in the spectrum at low microwave power• With increased microwave power the percentage of the atomic ion N + increases and above 400 W the peak for the atomic ion N + exceeds that for the molecular ion N 2+. According to the measurement of argon plasma parameters in a microwave ion source [8] the electron temperature increases only slightly from 4.5 to 5.0 eV when the microwave power Pu is changed from 200 to 500 W. However, the change of electron density is more distinct• The density, 3.5 x 1012 electrons cm-3 at a microwave power of 500 W, is several times higher than that at 200 W. Although the plasma parameters of the nitrogen plasma are not measured here they are assumed to be similar to those of argon plasma since the plasma generation is based on the same principle• The increase of electron density results in decomposition of N2 + into N + and N by multiple impacts of moderate-energy electrons. An increase of doubly charged nitrogen ion N 2+ with microwave power is observed at the same time. Emittance contours and current density distributions measured at a point 450 mm from the
ion extraction slit are shown in Fig. 4. The emittance contours are shown by dots and the current density distributions of the ion beam by bars• The operational conditions of the ion source are the same as those of Fig. 3. Emittance values Ex which are given by the areas inside the contours, and total extracted ion currents I, are also indicated. From Fig. 4 it is seen that the increase of total current 1, with microwave power P~ results in a widening of the ion current distribution in the X axis. At a microwave power of 200 W the distribution has a single peak. However, above 300 W it splits into two peaks which move to opposite sides depending on the microwave power increase. The peak splitting is assumed to come from distortion of the plasma boundary at the ion exit slit of the ion source. Although the emittance E x also increases a little with the microwave power even the maximum value of 175 mm mrad at 500 W is low enough for the emittance contour to be included by the acceptance contour of the separator, the acceptance value of which is 1200 mm mrad.
4•2• lntroductionofTiCl4 In order to obtain the metal ion Ti ÷, TiC14 is introduced into the ion source• Since TiCI 4 is a high-pressure liquid (103 Pa at 20 °C) it is introduced through a needle valve• The mass spectra of TiC14 at an early stage of the source operation is given in Fig. 5. The microwave power is 500 W and the ion extraction voltage 40 kV. There are mass peaks for HCI + which come from the reaction of TiC14 and H:O according to TiC14 + 2H20 ~ TiO2 + 4HCI (1) Once the gas inlet pipe is exposed to wet air, water molecules stick to the inner wall of the pipe and remain there even after evacuation• A mass spectrum at a vapour flow of 1.5 x 10 -3 Pa m 3 s - 1, which corresponds to pressures of 0.12 Pa in the discharge chamber and 3.0 x 10 3 Pa at the ionization gauge, is given in Fig. 5(a). The peak height of 48Ti + is 75% of that of 35C1 +. This means that a considerable amount of HCI is included. A mass spectrum with the vapour flow increased to 2.5 x 10 -3 Pa m 3 s- 1, which corresponds to pressures of 0.19 Pa in the discharge chamber and 5.0 × 10-3 Pa at the ionization gauge, is given in Fig. 5(b). The peak height of 48Ti+ increases, becoming comparable with that of 35C1+, since the percentage of HCI in the feed material decreases•
224
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Fig. 4. E m i t t a n c e c o n t o u r s in the p h a s e plane (x, x") c o r r e s p o n d i n g to the width direction of the extraction slit. C u r r e n t density distributions at a p o i n t 450 m m f r o m the slit are also shown: (a) P~, = 200 W, E x = 93 m m m r a d , I t = 16 m A ; (b) P~ = 300 W, E x = 146 m m m r a d , I t = 22.5 m A ; (c) P~ = 4 0 0 W, E x = 162 m m m r a d , I, = 26 m A ; (d) Pu = 500 W, E x = 174 n u n m r a d , I t = 30 m A .
c/
(a)
c1+
(b) Ti + m'~
Ti +
m-in
.[J.
J.l
J
_LL__,+,
Fig. 5. M a s s spectra of T i C I 4 at an early stage of the s o u r c e o p e r a t i o n with T i C I 4 v a p o u r flow (a) 1.5 × 10 3 Pa m -+ s - i and (b) 2.5 x 10 -3 Pa m 3 s - i .
225 4
.'-
30
/ "
3
~E 20 v ~ ; -~ ~0 ~o
._g +_ 2 ~~ ~ 7 0
I 20
i 30 Extraction
voltage
~ 40
0
(kV)
Fig. 6. Dependences of the mass-separated Ti + current and the total current on extraction voltage.
T h e d e p e n d e n c e s of the mass-separated Ti + current and the total current on the ion extraction voltage is illustrated in Fig. 6. T h e total current is almost constant in the voltage range 2 0 - 3 5 kV and increases a b o v e 35 kV; the Ti + current increases linearly in the voltage range 2 0 - 4 0 kV. Since in the 2 0 - 3 5 kV range the plasma boundary at the ion exit slit is changing f r o m convex to concave the change of the b o u n d a r y area is small. T h e r e f o r e the total current, which is proportional to the area, is almost constant. However, since the b e a m divergence varies greatly in this region the transmission of the mass separator changes. A b o v e 35 kV the b o u n d a r y area increases with the voltage, resulting in an increased total current.
5.
Conclusions
A new modification of a microwave ion source which has a closed magnetic circuit was proposed. Eliminating the magnetic field f r o m the ion extraction space should provide stable beams. A study of the new ion source showed that the fundamental b e a m characteristics were almost the same as those of the original. T h e long term stability of the new ion source will be verified in the near future by carrying out field tests. Microwave ion sources have the advantage over other hot filament-type ion sources of using halides to obtain metal beams. However, a problem peculiar to using halides as source materials is apparent f r o m the present study; care must be taken with the material inlet system since halides react readily with water. Some method must be used to keep wet air away from the inlet when breaking the vacuum.
References I N. Sakudo, K. Tokiguchi and H. Koike, Vacuum, 34 (1984)245. 2 P. Spinelli, J. Escaron, A. Scoubie and M. Bruel, Nucl. lnstrum. Methods B, 6 (1985) 283. 3 M. lwaki, K. Yoshida, N. Sakudo and S. Satou, Nucl. lnstrum. Methods B, 6(1985) 51. 4 K. Tokiguchi, N. Sakudo, K. Suzuki and 1. Kanomata, J. Va,. Sci. Technol., 17(1980) 1247. 5 N. Sakudo, K. Tokiguchi, H. Koike and I. Kanomata, Rev. sci. lnstrum., 54 (1983) 681. 6 K. Tokiguchi, N. Sakudo and H. Koike, J. Vac. Sci. Technol. A, 2 (1984) 29. 7 J. H. Freeman, Nucl. lnstrum. Methods, 22(1963) 306. 8 N. Sakudo, K. Tokiguchi, H. Koike and I. Kanomata, Rev. Sci. lnstrum., 48 (1977) 762.
221
Improvement of a Microwave Ion Source for Surface Modification* N. SAKUDO, K. TOKIGUCHI and T. SEKI
Hitachi Research Laboratory, Hitachi Ltd., 4026 Kuji-cho, Hitachi-shi, Ibaraki-ken 319-12 (Japan) H. KOIKE
Naka Works, Hitachi Ltd., Katsuta-shi, Ibaraki-ken 312 (Japan) M. IWAKI
The Institute of Physical and ChemicaI Research (RIKEN), Wako, Saimma 351-01 (Japan) (Received September 16, 1988)
Abstract A microwave ion source for metallic ions was" modified to eliminate the magnetic field from the ion extraction space, thus providingstablebeams, The beam characteristics of the source were measured with both N_, and TiCl4. Ratios of N + to N, + and Ti ÷ to Cl* in the mass spectra varied depending on discharge conditions such as microwave power and gas pressure. Emittance variations were also measured with an apparatus consisting of an emittance measuring device, a mass separator and a beam profile monitor. Conditions for improved matching of the ion source and the mass separator are discussed,
1. Introduction A microwave ion source is well suited for obtaining high-current metal ions by using halides as source materials [1]. Unlike conventional hotfilament-type ion sources which experience quick erosion of the filament by highly reactive halide plasma [2] the microwave ion source does not have any parts which are rapidly consumed. Furthermore, halogen ions and radicals are desirable in the plasma as they keep the metal from depositing on the inner walls of the discharge chamber by bombarding and chemically cleaning the wall. Otherwise, metal deposition might pre-
*Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. 0921-5093/89/$3.50
vent microwaves from entering the discharge chamber. The microwave ion source has been applied to a high-current metal-ion implanter for forming metastable surface alloys [3]. This implanter demonstrated the effectiveness of the ion source for high-dose metal-ion implantation. In general, the dose levels required for surface modification of metals exceed the highest levels used for semiconductor device fabrication (about 2). 10" ions cm Thus more stable ion sources than those of traditional ion implanters for semiconductors are desired. Beam stability is affected by fluctuation of the plasma parameters and sparking between the extraction electrodes in a vacuum. Since the fluctuation of plasma parameters depends mostly on the type of plasma generation, microwave ion sources have an advantage in generating quiescent and uniform plasmas[4-6]. Most ion sources of high-current ion implanters have a magnetic field which is superimposed on the extraction electric field [1, 71. While sparking phenomena between extraction electrodes are not completely understood at present, the magnetic field in the ion extraction space is considered to worsen spark resistivity of the extraction electrodes. This is reasonable in light of plasma generation in an ion source for which a magnetic field is used to trap electrons and ions for effective cascade collisions. From the viewpoint of improving ion source stability a new microwave ion source which eliminates the magnetic field from the ion extraction space has been constructed and its fundamental characteristics tested. © Elsevier Sequoia/Printed in The Netherlands
222
2. A microwave ion source having a closed
3. An apparatus for evaluating beam qualities
magnetic circuit Since a conventional microwave ion source for metal beams has an open magnetic circuit, a magnetic field is superimposed on the ion extraction electric field [1]. A new microwave ion source which has a closed magnetic circuit has been constructed as shown in Fig. 1. A magnetic coil is placed around the discharge chamber at a highvoltage terminal where it is also surrounded by a yoke, a high-voltage flange and an acceleration electrode; these are made of iron or a highpermeability metal. Thus the yoke, the flange and the electrode form a closed magnetic circuit together with two iron blocks to focus the magnetic field into the discharge chamber. Consequently, in the ion extraction space no magnetic field exists and at the high-voltage terminal there is enough space for the external oven. The ion source has three types of inlet system for source materials. Gases or materials of high vapour pressure are introduced into the discharge chamber throughaneedlevalve. Materials vaporized at comparatively low temperature are heated in the external oven up to 300 °C to keep the pressure around 1 0 2 Pa. The conductance of the vapour inlet pipe between the external oven and the discharge chamber is designed so that the pressure in the discharge chamber is 0.1-1 Pa. The temperature of the pipe is kept higher than the oven temperature to prevent the source feed material from condensing on the inner walls of the pipe. Materials vaporized at high temperature are heated in the internal oven. As long as halides are used as source materials, most metals can be vaporized by the external oven.
The apparatus used to study the performance of the microwave ion source is shown in Fig. 2. The apparatus consists mainly of an emittancemeasuring device, a mass separator and a beam profile monitor. The entire system is controlled by a personal computer which also processes the data obtained. The emittance-measuring device is placed at a distance of 450 mm from the extraction slit of the ion source. It consists of a beamsampling aperture (1 mm in diameter) and an ion collector that is placed 60 mm beyond the aperture and which can be moved across the sampled beamlet to measure its angular dispersion. The beam-sampling aperture plate is water-cooled since most of the ions which bombard the aperture plate give up their energy as heat. This apparatus can handle high-power beams of up to 2 kW. The mass separator is a sector magnet type of radius 470 mm and with a 75 ° deflection. The effective field gap is 50 mm. The lens effect of the mass separator field can be changed remotely by varying the angles of the ion beam to the entrance-and exit-edge fields.
vacuum
sealing
4. Beam performance of the ion source
4.1. Fundamental characteristics with nitrogen gas Variations of the mass spectra of N 2 with various microwave powers P, from 200 to 500 W are shown in Fig. 3. The pressure, which is measured by the ionization gauge placed just above the diffusion pump, is 4 x 10 -3 Pa. Since the calculated conductance between the ion extraction slit and the gauge position is 8.1 x 10 -3 m 3 s-1 and the evacuation speed at the gauge is 0.5 m 3 s - 1 , the pressure in the dis-
dlelec~rlc
Waveguide
interna~
oven
,
leratioR electrode (iron)
,
as
lalve
n et
V~lv 6
i e
~
~
ion s o u r c e m
Beam p r o f i l e m o n i t o r
2n:,; .........
"~
i
E t
t
I
t
d
I
b
Fig. 1. A modified microwave ion source. The magnetic field is completely eliminated from the ion extraction space.
Fig. 2. Sketch of apparatus for evaluating beam qualities.
223
N; t
N÷ | t
N2+ N~ N2+ .
•
N~+ . A
~
b) N+ N;
N;
,.
N+
N÷ A
~L
N2+ , I
c~ a~ Fig. 3. Mass spectra of N~ gas for different microwave powerlt, = 400 P~V;(d)Values:l~, = 500(a)~, = 200 W; (b) P~,= 300 W; (c)
charge chamber is estimated to be 0.25 Pa. The ion extraction voltage is 40 kV. The peak for the molecular ion N 2÷ is the highest in the spectrum at low microwave power• With increased microwave power the percentage of the atomic ion N + increases and above 400 W the peak for the atomic ion N + exceeds that for the molecular ion N 2+. According to the measurement of argon plasma parameters in a microwave ion source [8] the electron temperature increases only slightly from 4.5 to 5.0 eV when the microwave power Pu is changed from 200 to 500 W. However, the change of electron density is more distinct• The density, 3.5 x 1012 electrons cm-3 at a microwave power of 500 W, is several times higher than that at 200 W. Although the plasma parameters of the nitrogen plasma are not measured here they are assumed to be similar to those of argon plasma since the plasma generation is based on the same principle• The increase of electron density results in decomposition of N2 + into N + and N by multiple impacts of moderate-energy electrons. An increase of doubly charged nitrogen ion N 2+ with microwave power is observed at the same time. Emittance contours and current density distributions measured at a point 450 mm from the
ion extraction slit are shown in Fig. 4. The emittance contours are shown by dots and the current density distributions of the ion beam by bars• The operational conditions of the ion source are the same as those of Fig. 3. Emittance values Ex which are given by the areas inside the contours, and total extracted ion currents I, are also indicated. From Fig. 4 it is seen that the increase of total current 1, with microwave power P~ results in a widening of the ion current distribution in the X axis. At a microwave power of 200 W the distribution has a single peak. However, above 300 W it splits into two peaks which move to opposite sides depending on the microwave power increase. The peak splitting is assumed to come from distortion of the plasma boundary at the ion exit slit of the ion source. Although the emittance E x also increases a little with the microwave power even the maximum value of 175 mm mrad at 500 W is low enough for the emittance contour to be included by the acceptance contour of the separator, the acceptance value of which is 1200 mm mrad.
4•2• lntroductionofTiCl4 In order to obtain the metal ion Ti ÷, TiC14 is introduced into the ion source• Since TiCI 4 is a high-pressure liquid (103 Pa at 20 °C) it is introduced through a needle valve• The mass spectra of TiC14 at an early stage of the source operation is given in Fig. 5. The microwave power is 500 W and the ion extraction voltage 40 kV. There are mass peaks for HCI + which come from the reaction of TiC14 and H:O according to TiC14 + 2H20 ~ TiO2 + 4HCI (1) Once the gas inlet pipe is exposed to wet air, water molecules stick to the inner wall of the pipe and remain there even after evacuation• A mass spectrum at a vapour flow of 1.5 x 10 -3 Pa m 3 s - 1, which corresponds to pressures of 0.12 Pa in the discharge chamber and 3.0 x 10 3 Pa at the ionization gauge, is given in Fig. 5(a). The peak height of 48Ti + is 75% of that of 35C1 +. This means that a considerable amount of HCI is included. A mass spectrum with the vapour flow increased to 2.5 x 10 -3 Pa m 3 s- 1, which corresponds to pressures of 0.19 Pa in the discharge chamber and 5.0 × 10-3 Pa at the ionization gauge, is given in Fig. 5(b). The peak height of 48Ti+ increases, becoming comparable with that of 35C1+, since the percentage of HCI in the feed material decreases•
224
. ~
100
,o ~
eee~eee8 7
,.,
-100 -20
-10
10
Position 1+0
E
J" -
m m
I I°
.
100
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.~
+
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.
.
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.
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.
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,
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o
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Position
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)
,-Fl=1S
[
(mm)
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+
20
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,o
-10+
0
-30
100 , b)
+ .~
+
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14 7
-"
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-50
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loo ~ -30
-I-. -20
-10
0
Position
10
20
o
30
1+o -30
(mm)
-20
-10 Position
0
10
20
30
(ram)
Fig. 4. E m i t t a n c e c o n t o u r s in the p h a s e plane (x, x") c o r r e s p o n d i n g to the width direction of the extraction slit. C u r r e n t density distributions at a p o i n t 450 m m f r o m the slit are also shown: (a) P~, = 200 W, E x = 93 m m m r a d , I t = 16 m A ; (b) P~ = 300 W, E x = 146 m m m r a d , I t = 22.5 m A ; (c) P~ = 4 0 0 W, E x = 162 m m m r a d , I, = 26 m A ; (d) Pu = 500 W, E x = 174 n u n m r a d , I t = 30 m A .
c/
(a)
c1+
(b) Ti + m'~
Ti +
m-in
.[J.
J.l
J
_LL__,+,
Fig. 5. M a s s spectra of T i C I 4 at an early stage of the s o u r c e o p e r a t i o n with T i C I 4 v a p o u r flow (a) 1.5 × 10 3 Pa m -+ s - i and (b) 2.5 x 10 -3 Pa m 3 s - i .
225 4
.'-
30
/ "
3
~E 20 v ~ ; -~ ~0 ~o
._g +_ 2 ~~ ~ 7 0
I 20
i 30 Extraction
voltage
~ 40
0
(kV)
Fig. 6. Dependences of the mass-separated Ti + current and the total current on extraction voltage.
T h e d e p e n d e n c e s of the mass-separated Ti + current and the total current on the ion extraction voltage is illustrated in Fig. 6. T h e total current is almost constant in the voltage range 2 0 - 3 5 kV and increases a b o v e 35 kV; the Ti + current increases linearly in the voltage range 2 0 - 4 0 kV. Since in the 2 0 - 3 5 kV range the plasma boundary at the ion exit slit is changing f r o m convex to concave the change of the b o u n d a r y area is small. T h e r e f o r e the total current, which is proportional to the area, is almost constant. However, since the b e a m divergence varies greatly in this region the transmission of the mass separator changes. A b o v e 35 kV the b o u n d a r y area increases with the voltage, resulting in an increased total current.
5.
Conclusions
A new modification of a microwave ion source which has a closed magnetic circuit was proposed. Eliminating the magnetic field f r o m the ion extraction space should provide stable beams. A study of the new ion source showed that the fundamental b e a m characteristics were almost the same as those of the original. T h e long term stability of the new ion source will be verified in the near future by carrying out field tests. Microwave ion sources have the advantage over other hot filament-type ion sources of using halides to obtain metal beams. However, a problem peculiar to using halides as source materials is apparent f r o m the present study; care must be taken with the material inlet system since halides react readily with water. Some method must be used to keep wet air away from the inlet when breaking the vacuum.
References I N. Sakudo, K. Tokiguchi and H. Koike, Vacuum, 34 (1984)245. 2 P. Spinelli, J. Escaron, A. Scoubie and M. Bruel, Nucl. lnstrum. Methods B, 6 (1985) 283. 3 M. lwaki, K. Yoshida, N. Sakudo and S. Satou, Nucl. lnstrum. Methods B, 6(1985) 51. 4 K. Tokiguchi, N. Sakudo, K. Suzuki and 1. Kanomata, J. Va,. Sci. Technol., 17(1980) 1247. 5 N. Sakudo, K. Tokiguchi, H. Koike and I. Kanomata, Rev. sci. lnstrum., 54 (1983) 681. 6 K. Tokiguchi, N. Sakudo and H. Koike, J. Vac. Sci. Technol. A, 2 (1984) 29. 7 J. H. Freeman, Nucl. lnstrum. Methods, 22(1963) 306. 8 N. Sakudo, K. Tokiguchi, H. Koike and I. Kanomata, Rev. Sci. lnstrum., 48 (1977) 762.