Ion source and operation method thereof

Details Ion source and operation method thereof Ion source and operation method thereof

2001-11-09

2003-02-25

Vu, David (Department: 2821)

Electric lamp and discharge devices: systems

Discharge device load with fluent material supply to the...

Electron or ion source

C250S427000

Reexamination Certificate

active

06525482

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ion source which has a filament for emitting electrons and a reflector for reflecting the electrons and which applies a magnetic field to the inside of a plasma production vessel, and more particularly to means for improving an ion production efficiency, prolonging the life of the filament, etc.
2. Description of the Related Art
FIG. 12
shows a related art example of the ion source. This ion source is called Bernas-type ion source. An ion source of a similar structure is also disclosed, for example, in Japanese Patent Unexamined Publication No. Hei. 9-63981.
The ion source comprises a plasma production vessel
2
, for example, shaped like a rectangular parallelepiped and also serving as a positive potential. Gas (containing also the case where the gas is vapor) for producing plasma
16
is introduced into the inside of the plasma production vessel
2
. The plasma production vessel
2
is formed on a wall face (long-side wall) with an ion extraction slit
4
for extracting an ion beam
18
. In the example, the ion beam
18
is extracted toward the rear of the plane of the figure.
A filament
6
, for example, shaped like U, for emitting an electron e is placed in one side (one short-side wall side) of the plasma production vessel
2
. The filament
6
and the plasma production vessel
2
are electrically insulated by an insulator
12
.
An opposed reflector
8
for reflecting the electron e is placed facing the filament
6
in an opposite side of the plasma production vessel
2
(namely, the other short-side wall side facing the filament
6
). The opposed reflector
8
and the plasma production vessel
2
are electrically insulated by an insulator
13
. The opposed reflector
8
may be placed in a floating potential without connecting to any point. The opposed reflector
8
may be also connected to one end of the filament
6
(more particularly, the negative potential terminal of a filament power supply
24
) by a conductor
28
for placing the opposed reflector
8
in filament potential as described in the above-mentioned Japanese Patent Unexamined Publication No. Hei 9-63981.
A rear reflector
10
for reflecting the electron e is placed facing the opposed reflector
8
at a place positioned behind the filament
6
in the plasma production vessel
2
. Namely, the rear reflector
10
is placed between the U-shaped portion of the filament
6
and the wall face of the plasma production vessel
2
behind the U-shaped portion. The rear reflector
10
and the plasma production vessel
2
are electrically insulated by insulators
12
and
14
. The rear reflector
10
has been connected to one end of the filament
6
(more particularly, the negative potential terminal of the filament power supply
24
) for placing the rear reflector
10
in filament potential.
In the plasma production vessel
2
, a magnetic field generator
20
placed outside the plasma production vessel
2
applies a magnetic field
22
along the axis connecting the filament
6
and the opposed reflector
8
to produce and confine the plasma
16
. However, the direction of the magnetic field
22
may be opposite to that shown in the figure. The magnetic field generator
20
is, for example, an electromagnet.
DC filament voltage V
B
(for example, about 2 to 4 V) is applied from the DC filament power supply
24
to the filament
6
to heat the filament
6
for emitting an electron (thermoelectron) e.
From a DC arc power supply
26
, arc voltage V
A
(for example, about 40 to 100 V) is applied between one end of the filament
6
and the plasma production vessel
2
with the filament
6
as the negative potential to produce arc discharge between the filament
6
and the plasma production vessel
2
.
FIG. 13
shows an example of potential variation in the ion source according to the elated art. In the example, the opposed reflector
8
is connected to one end of the filament
6
by the conductor
28
. However, if the opposed reflector
8
is not connected to any point for placing the opposed reflector
8
in floating potential, the potential of the opposed reflector
8
becomes the same extent as that in the example, namely, the same extent as the potential of the filament
6
. The reason is that if the opposed reflector a is placed in the floating potential, a far larger number of light and high-mobility electrons in the plasma
16
than the number of ions are incident on the opposed reflector
8
and thus the opposed reflector
8
is charged at negative potential.
The gas introduced into the inside of the plasma production vessel
2
is ionized by the above-mentioned arc discharge to produce the plasma
16
. From the plasma
16
, the ion beam
18
can be extracted by an electric field. Usually, an extraction electrode for extracting the on beam
18
is placed at a point opposed to the ion extraction slit
4
(the rear of the plane of the figure), but is not shown here.
The production process of the plasma
16
will be discussed in detail. The electron e emitted from the filament
6
is accelerated toward the plasma production vessel
2
by the above-mentioned arc voltage V
A
(the filament voltage V
F
is small as mentioned above and therefore is ignored in the description). Then accelerated electron e with the energy corresponding to the voltage V
A
collides with a gas molecule for ionizing the gas molecule, whereby plasma
16
is produced. The ions and electrons (also containing thermoelectrons emitted from the filament
6
) e in the plasma
16
are trapped by the above-mentioned magnetic field
22
and further repeat collision with gas molecules, thereby producing and confining the plasma
16
.
The potential of the plasma
16
becomes a potential between the potential of the plasma production vessel
2
and the potentials of both the reflectors
8
and
10
, as shown in
FIG. 13
, and a potential difference occurs between the plasma
16
and both the reflectors
8
and
10
. The potential difference causes electrons e emitted from the filament
6
or produced in the plasma
16
to be reflected on both the reflectors
8
and
10
and reciprocate between both the reflectors
8
and
10
. Consequently, the collision probability between the electrons e and gas molecules is increased and plasma
16
with a high density can be produced. As a result, the extracted ion beam
18
can be increased.
There is a demand for extracted multiply charged ions of doubly charged or more ions for use as the ions forming the ion beam
18
from the ion source as described above. The reason why there is such a demand is that a multiply charged ion can provide acceleration energy valence times that of a singly charged ion at the same acceleration voltage (for example, a doubly charged ion provides acceleration energy twice that of a singly charged ion) and thus high energy can be easily provided.
However, in the ion source in the related art as described above, production of multiply charged ions is not considered and thus the production amount of the multiply charged ions is small as compared with that of molecular ions or singly charged ions. That is, the ratio of the multiply charged ions in the plasma
16
and thus the ratio of the multiply charged ions contained in the ion beam
18
are not high. Therefore, the multiply charged ions cannot be used effectively.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an ion source and operation method thereof which can improve the production efficiency of multiply charged ions in an ion source for increasing the ratio of multiply charged ions contained in an ion beam. Other objects are described later.
In order to accomplish the object above, the following means are adopted. According to the present invention, there is provided an ion source of a first aspect comprising a rear reflector, an opposed reflector, a filament, a filament power supply, a plasma production vessel, an arc power supply, and a DC bias power supply. The rear reflector is electrically insulated from the filament and

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