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Bucket type ion source using a microwave plasma cathode
Nuclear Instruments and Methods North-Holland, Amsterdam
in Physics
Research
R37/38
BUCKET TYPE ION SOURCE USING A MICROWAVE Yoshimi HAKAMATA,
143
(1989) 143-146
PLASMA
CATHODE
Takashi IGA, Ken’ichi NATSUI and Tadashi SAT0
Hitachi Research Luboratoty, Hitachi Ltd., Hitachi Ibaraki, 319-12, Japan
For plasma processes one wants a large diameter and long life ion source. A filamentless ion source is desired when using reactive gases. We examined microwave plasma regarding its capability as an electron source and succeeded in extracting an electron current in the ampere range from the plasma. Then we developed a bucket type ion source using a microwave plasma cathode which replaces the conventional filament.
1. Introduction A bucket type ion source has been developed as a neutral beam injector in nuclear fusion experiments [1,2]. It has considerable advantages, such as a simple structure, plasma uniformity, and ease of making a large diameter ion beam. Recently, the ion source has been used as a broad beam ion source for plasma processes such as ion milling and reactive ion beam etching [3]. The bucket type ion source uses a filament as a hot cathode and plasma is generated by arc discharge. During this, the filament is consumed. In particular, the consumption increases for reactive ion beam etching using reactive gases such as O,, CF,, etc. Thus a filamentless ion source is desired. To solve this problem without losing the merits of a bucket type ion source, it will be sufficient to exchange the filament for anything supplying electrons. Therefore we examined microwave plasma regarding its capability as an electron source and succeeded in extracting an electron current in the ampere range from the plasma. Then we developed a bucket type ion source using the microwave plasma cathode which replaces the conventional filament.
100 mm diameter circle. The upper electrode and ES chamber are supplied with a negative voltage and the lower electrode is at the earth potential. The electrons are extracted into the vacuum chamber which is evacuated by a turbomol~ular pump (550 I/s). 2.2. Ion source Fig. 2 shows the bucket type ion source using microwave plasma as an electron source. It is set on the same vacuum chamber as in fig. 1. The ion source has two types of plasma generating chambers. One is the microwave plasma generating chamber (MP chamber) and the other one is the arc plasma generating chamber (AP chamber). They are insulated from each other.
microwave electron p2rraanent
source magnet
2. Experimental setup 2.1. Basic experimental
system of electron extraction
Fig. 1 shows a schematic view of the electron extraction system from the microwave plasma. The electron source (ES) chamber is surrounded with permanent magnets which produce a magnetic field in the ES chamber. The magnetic flux density is about 0.07 T at the chamber center and 0.12 T near the wall. Microwave plasma is generated by microwaves introduced through a window and the magnetic field. Two electron extracting electrodes which are attached to the ES chamber have many small holes (each 4 mm diameter) inside a 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Fig.
1. Schematical view of the electron extraction from the microwave plasma.
systems
II. ION SOURCES
144
Y. Hakamata et al. / Bucket type ion source
The MP chamber is the same as the electron source in fig. 1, in principle. But the chamber shape and arrangement of permanent magnets are different. The magnetic flux density in the MP chamber is about 0.2 T near the chamber wall. An electron extracting electrode is attached to the MP chamber. The side wall of the electrode is a mesh grid. Applying a dc positive voltage (arc voltage) to the AP chamber against the microwave plasma, electrons are extracted from the microwave plasma to the AP chamber through the electron extracting electrode, and an arc plasma is generated in the AP chamber. The AP chamber is 180 mm long with an inner diameter of 200 mm. It is surrounded by permanent magnets whose magnetic poles are alternatively faced to the chamber wall. The magnets produce the multipole line cusp magnetic field near the chamber wall. The magnetic field rapidly falls off when going into the chamber. So plasma is confined by the magnetic field and becomes uniform. Ion extracting electrodes are located at the bottom of the AP chamber. Both electrodes have many holes (4 mm diameter) inside a 100 mm diameter circle. The upper electrode is at the ion acceleration voltage and the lower electrode is supplied with a negative voltage called the deceleration voltage which reflects electrons from the plasma made by the ion beam. Then the ion beam is extracted from the arc plasma to the vacuum chamber. 2.3. Microwave source a simple microwave every 20 ms. There-
microwave
c
microwaveplasm
generating chamber
_
permanentmagnet
1
I
Ill
extracting
insulator permanent magnet
E VZX
fore microwave plasma and arc plasma are turned on and off according to the microwave radiation. Fig. 3 is an example oscillogram of current which flows into the vacuum chamber (II) and the lower electrode of the electron extracting electrodes (Iz) in fig. 1, respectively. Clearly the currents pulsate according to the microwave power supply. The current value of the microwave power supplied period is about three times the average value which is measured by a dc-ammeter.
3. Experimental results 3.1. Electron extraction characteristic
In this experiment, we used source which radiates periodically
electron
Fig. 3. Oscillogram of electron current.
plasma generating chanter
Fig. 4 shows the electron extraction characteristic in the microwave plasma generator shown in fig. 1. The current values are the value of the plasma-on period and they are measured from oscillograms. The current flowing into the vacuum chamber increases with increasing extraction voltage which is supplied for the microwave plasma and upper electron extracting electrode. The current is over 1 A at 60 V. This is comparable to the electron current extracted from a filament. The current which flows into the lower electrode Z2 also increases with increasing extraction voltage up to 50 V. But above 50 V the current decreases. This is caused by an electrostatic lens effect of the electrodes. 3.2. Arc plasma in the Al’ chamber
,.arc
insulator
Fig. 2. Bucket type ion source using a microwave plasma cathode.
Fig. 5 shows the arc plasma in the AP chamber described in fig. 2. It is observed under the AP chamber by taking out the ion extracting electrodes. Considering the light radiation region, the plasma is generated in the AP chamber. The feet of the arc plasma correspond to the magnetic poles of the permanent magnets around the AP chamber. The magnetic field pushes the arc plasma from the chamber wall. The plasma confining
145
Y. Hakamata et al. / Bucket type ion source 2.5 ,Gas:arqon
0.13pa
0.13Pa
Gas: argon
1, ( “acLaIm
1 0
20
40
60
A0
100
120
elctron extraction voltage Ve (V)
Fig. 4. Electron extraction characteristic.
0
20
40
60
80
100
arc voltage Varc (V)
effect of the multipole cusp magnetic field has been investigated by the calculation of electron orbits [4]. But it is difficult to observe radiation from the plasma of a usual bucket type ion source because the filament in the ion source radiates intense light. In this experiment, using the microwave plasma in place of the filament, the plasma confining effect of the multipole cups magnetic field is observed. 3.3. V-Z characteristics of arc discharge Fig. 6 shows the V-Z characteristics of arc discharge in the AP chamber. In this experiment, arc current is measured by a dc-ammeter, that is, the current in fig. 6 is an average value. Supplying an arc voltage on the condition that the pressure is 0.13-0.013 Pa, arc discharge starts at 30-70 V, and after that the arc current becomes larger according to the increased arc voltage. This is one reason that the number of electrons which are extracted from microwave plasma increases with arc
Fig. 6. Arc discharge characteristics.
voltage. becomes
As pressure is higher the arc start voltage lower and the arc current becomes larger.
3.4. Zen beam extraction
characteristic
Fig. 7 shows ion beam current characteristics flowing into each electrode. Current flowing into the accelerating electrode means ion beam current passing through the electrode, and current flowing into the decelerating electrode means loss of ion beam current. So the difference of the two currents is the ion beam current extracted into the vacuum chamber. Data are average values measured by dc-ammeters. So the currents of the plasma-on period are about three times the average value. 30
a, ,
Pg:O.O13Pa Varc:85V V&c:-2OOV
25 _ flowing into accelerating electrde
jl:L
flowing into decelerating electrde
0
Fig. 5. Arc plasma confined by a multipole cusp magnetic field.
axI
4xl
Kc
m1ccO
ion acceleration voltage Vacc (V)
Fig. 7. Ion beam extraction
characteristic II. ION SOURCES
Y. Hakamata
et al. / Bucket type ion source
above the ion extracting electrodes in fig. 2. For the microwave plasma, the distribution is measured by removing the AP chamber and the electron extracting electrode. The density is normalized at the center electron densities which are 5 x 10” and 4 x 10” cmp3 for the arc plasma and microwave plasma, respectively. In the microwave plasma, the density decreases monotonously from the center to the chamber wall, while the arc plasma keeps its electron density at nearly the same value inside a 40 mm radius. This is caused by a plasma confining effect of the multipole cusp magnetic field.
5. Conclusion radius R (m) Fig. 8. Electron density distribution.
That is, when the acceleration voltage is 500 V, the ion beam current is 20 mA on average and it means 60 mA in the plasma-on period. The plasma density is regarded uniform inside the ion beam extracting diameter (100 mm) as shown in fig. 8, the average current density is 0.76 mA/cm2 in the plasma-on period. That is a usable current density for ion beam milling.
4. Electron density distribution According to the experimental results mentioned above, the microwave plasma can replace a filament. The merit of the bucket type ion source is the uniformity of the ion beam extracted from the uniform plasma. For this source we examine the uniformity of the plasma. Fig. 8 shows the electron density distributions for the radial direction in the arc plasma and the microwave plasma. They are measured by a Langmuir probe set
The microwave plasma was examined for its capability as an electron source, and an electron current in the ampere range was extracted from the plasma. Then a bucket type ion source’which has a microwave plasma cathode was investigated regarding its discharge characteristics and ion beam extraction characteristics. As a result, it was shown that the microwave plasma could replace the conventional filament and the ion source using the microwave plasma cathode could be used for ion beam milling.
References [l] T.S. Green, 10th Symp. Fusion Tech. 2 (1978) p. 873. [2] T. Obiki, A. Sasaki, F. Sano and K. Uno, Rev. Sci. Instr. 52 (1981) 1445. [3] Y. Ono, T. Kurosawa, T. Sato, Y. Oka and I. Hashimoto J. Vat. Sci. Technol. A4 (1986) 788. (41 Y. Ono, T. Kurosawa, Y. Hakamata and T. Sato, The Institute of Electronics, Information and Communication Engineers (Japan) MR87-41 (1987) p. 9.
in Physics
Research
R37/38
BUCKET TYPE ION SOURCE USING A MICROWAVE Yoshimi HAKAMATA,
143
(1989) 143-146
PLASMA
CATHODE
Takashi IGA, Ken’ichi NATSUI and Tadashi SAT0
Hitachi Research Luboratoty, Hitachi Ltd., Hitachi Ibaraki, 319-12, Japan
For plasma processes one wants a large diameter and long life ion source. A filamentless ion source is desired when using reactive gases. We examined microwave plasma regarding its capability as an electron source and succeeded in extracting an electron current in the ampere range from the plasma. Then we developed a bucket type ion source using a microwave plasma cathode which replaces the conventional filament.
1. Introduction A bucket type ion source has been developed as a neutral beam injector in nuclear fusion experiments [1,2]. It has considerable advantages, such as a simple structure, plasma uniformity, and ease of making a large diameter ion beam. Recently, the ion source has been used as a broad beam ion source for plasma processes such as ion milling and reactive ion beam etching [3]. The bucket type ion source uses a filament as a hot cathode and plasma is generated by arc discharge. During this, the filament is consumed. In particular, the consumption increases for reactive ion beam etching using reactive gases such as O,, CF,, etc. Thus a filamentless ion source is desired. To solve this problem without losing the merits of a bucket type ion source, it will be sufficient to exchange the filament for anything supplying electrons. Therefore we examined microwave plasma regarding its capability as an electron source and succeeded in extracting an electron current in the ampere range from the plasma. Then we developed a bucket type ion source using the microwave plasma cathode which replaces the conventional filament.
100 mm diameter circle. The upper electrode and ES chamber are supplied with a negative voltage and the lower electrode is at the earth potential. The electrons are extracted into the vacuum chamber which is evacuated by a turbomol~ular pump (550 I/s). 2.2. Ion source Fig. 2 shows the bucket type ion source using microwave plasma as an electron source. It is set on the same vacuum chamber as in fig. 1. The ion source has two types of plasma generating chambers. One is the microwave plasma generating chamber (MP chamber) and the other one is the arc plasma generating chamber (AP chamber). They are insulated from each other.
microwave electron p2rraanent
source magnet
2. Experimental setup 2.1. Basic experimental
system of electron extraction
Fig. 1 shows a schematic view of the electron extraction system from the microwave plasma. The electron source (ES) chamber is surrounded with permanent magnets which produce a magnetic field in the ES chamber. The magnetic flux density is about 0.07 T at the chamber center and 0.12 T near the wall. Microwave plasma is generated by microwaves introduced through a window and the magnetic field. Two electron extracting electrodes which are attached to the ES chamber have many small holes (each 4 mm diameter) inside a 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Fig.
1. Schematical view of the electron extraction from the microwave plasma.
systems
II. ION SOURCES
144
Y. Hakamata et al. / Bucket type ion source
The MP chamber is the same as the electron source in fig. 1, in principle. But the chamber shape and arrangement of permanent magnets are different. The magnetic flux density in the MP chamber is about 0.2 T near the chamber wall. An electron extracting electrode is attached to the MP chamber. The side wall of the electrode is a mesh grid. Applying a dc positive voltage (arc voltage) to the AP chamber against the microwave plasma, electrons are extracted from the microwave plasma to the AP chamber through the electron extracting electrode, and an arc plasma is generated in the AP chamber. The AP chamber is 180 mm long with an inner diameter of 200 mm. It is surrounded by permanent magnets whose magnetic poles are alternatively faced to the chamber wall. The magnets produce the multipole line cusp magnetic field near the chamber wall. The magnetic field rapidly falls off when going into the chamber. So plasma is confined by the magnetic field and becomes uniform. Ion extracting electrodes are located at the bottom of the AP chamber. Both electrodes have many holes (4 mm diameter) inside a 100 mm diameter circle. The upper electrode is at the ion acceleration voltage and the lower electrode is supplied with a negative voltage called the deceleration voltage which reflects electrons from the plasma made by the ion beam. Then the ion beam is extracted from the arc plasma to the vacuum chamber. 2.3. Microwave source a simple microwave every 20 ms. There-
microwave
c
microwaveplasm
generating chamber
_
permanentmagnet
1
I
Ill
extracting
insulator permanent magnet
E VZX
fore microwave plasma and arc plasma are turned on and off according to the microwave radiation. Fig. 3 is an example oscillogram of current which flows into the vacuum chamber (II) and the lower electrode of the electron extracting electrodes (Iz) in fig. 1, respectively. Clearly the currents pulsate according to the microwave power supply. The current value of the microwave power supplied period is about three times the average value which is measured by a dc-ammeter.
3. Experimental results 3.1. Electron extraction characteristic
In this experiment, we used source which radiates periodically
electron
Fig. 3. Oscillogram of electron current.
plasma generating chanter
Fig. 4 shows the electron extraction characteristic in the microwave plasma generator shown in fig. 1. The current values are the value of the plasma-on period and they are measured from oscillograms. The current flowing into the vacuum chamber increases with increasing extraction voltage which is supplied for the microwave plasma and upper electron extracting electrode. The current is over 1 A at 60 V. This is comparable to the electron current extracted from a filament. The current which flows into the lower electrode Z2 also increases with increasing extraction voltage up to 50 V. But above 50 V the current decreases. This is caused by an electrostatic lens effect of the electrodes. 3.2. Arc plasma in the Al’ chamber
,.arc
insulator
Fig. 2. Bucket type ion source using a microwave plasma cathode.
Fig. 5 shows the arc plasma in the AP chamber described in fig. 2. It is observed under the AP chamber by taking out the ion extracting electrodes. Considering the light radiation region, the plasma is generated in the AP chamber. The feet of the arc plasma correspond to the magnetic poles of the permanent magnets around the AP chamber. The magnetic field pushes the arc plasma from the chamber wall. The plasma confining
145
Y. Hakamata et al. / Bucket type ion source 2.5 ,Gas:arqon
0.13pa
0.13Pa
Gas: argon
1, ( “acLaIm
1 0
20
40
60
A0
100
120
elctron extraction voltage Ve (V)
Fig. 4. Electron extraction characteristic.
0
20
40
60
80
100
arc voltage Varc (V)
effect of the multipole cusp magnetic field has been investigated by the calculation of electron orbits [4]. But it is difficult to observe radiation from the plasma of a usual bucket type ion source because the filament in the ion source radiates intense light. In this experiment, using the microwave plasma in place of the filament, the plasma confining effect of the multipole cups magnetic field is observed. 3.3. V-Z characteristics of arc discharge Fig. 6 shows the V-Z characteristics of arc discharge in the AP chamber. In this experiment, arc current is measured by a dc-ammeter, that is, the current in fig. 6 is an average value. Supplying an arc voltage on the condition that the pressure is 0.13-0.013 Pa, arc discharge starts at 30-70 V, and after that the arc current becomes larger according to the increased arc voltage. This is one reason that the number of electrons which are extracted from microwave plasma increases with arc
Fig. 6. Arc discharge characteristics.
voltage. becomes
As pressure is higher the arc start voltage lower and the arc current becomes larger.
3.4. Zen beam extraction
characteristic
Fig. 7 shows ion beam current characteristics flowing into each electrode. Current flowing into the accelerating electrode means ion beam current passing through the electrode, and current flowing into the decelerating electrode means loss of ion beam current. So the difference of the two currents is the ion beam current extracted into the vacuum chamber. Data are average values measured by dc-ammeters. So the currents of the plasma-on period are about three times the average value. 30
a, ,
Pg:O.O13Pa Varc:85V V&c:-2OOV
25 _ flowing into accelerating electrde
jl:L
flowing into decelerating electrde
0
Fig. 5. Arc plasma confined by a multipole cusp magnetic field.
axI
4xl
Kc
m1ccO
ion acceleration voltage Vacc (V)
Fig. 7. Ion beam extraction
characteristic II. ION SOURCES
Y. Hakamata
et al. / Bucket type ion source
above the ion extracting electrodes in fig. 2. For the microwave plasma, the distribution is measured by removing the AP chamber and the electron extracting electrode. The density is normalized at the center electron densities which are 5 x 10” and 4 x 10” cmp3 for the arc plasma and microwave plasma, respectively. In the microwave plasma, the density decreases monotonously from the center to the chamber wall, while the arc plasma keeps its electron density at nearly the same value inside a 40 mm radius. This is caused by a plasma confining effect of the multipole cusp magnetic field.
5. Conclusion radius R (m) Fig. 8. Electron density distribution.
That is, when the acceleration voltage is 500 V, the ion beam current is 20 mA on average and it means 60 mA in the plasma-on period. The plasma density is regarded uniform inside the ion beam extracting diameter (100 mm) as shown in fig. 8, the average current density is 0.76 mA/cm2 in the plasma-on period. That is a usable current density for ion beam milling.
4. Electron density distribution According to the experimental results mentioned above, the microwave plasma can replace a filament. The merit of the bucket type ion source is the uniformity of the ion beam extracted from the uniform plasma. For this source we examine the uniformity of the plasma. Fig. 8 shows the electron density distributions for the radial direction in the arc plasma and the microwave plasma. They are measured by a Langmuir probe set
The microwave plasma was examined for its capability as an electron source, and an electron current in the ampere range was extracted from the plasma. Then a bucket type ion source’which has a microwave plasma cathode was investigated regarding its discharge characteristics and ion beam extraction characteristics. As a result, it was shown that the microwave plasma could replace the conventional filament and the ion source using the microwave plasma cathode could be used for ion beam milling.
References [l] T.S. Green, 10th Symp. Fusion Tech. 2 (1978) p. 873. [2] T. Obiki, A. Sasaki, F. Sano and K. Uno, Rev. Sci. Instr. 52 (1981) 1445. [3] Y. Ono, T. Kurosawa, T. Sato, Y. Oka and I. Hashimoto J. Vat. Sci. Technol. A4 (1986) 788. (41 Y. Ono, T. Kurosawa, Y. Hakamata and T. Sato, The Institute of Electronics, Information and Communication Engineers (Japan) MR87-41 (1987) p. 9.