JPH07335163A - Method and device for generation of ion beam - Google Patents

Abstract

PURPOSE:To establish a method and device for generating an ion beam, with which the current density in the ion beam drawn out of an ion source can be made smaller than the lower limit according to the conventional arrangement. CONSTITUTION:An ion beam generating device is equipped with an ion source 2 emitting an ion beam 40 and a power supply device 30a which impresses a voltage for ion beam extraction. The ion source 2 has a plasma source part 4 to produce plasma 14 and an extracting electrode system 20 which extracts the ion beam 40 from the source part, while the power supply device 30a has the first power supply 31 to impress a positive high voltage with reference to the grounding potential on the first electrode 21 constituting the extracting electrode system 20, the second power supply 32a with variable output voltage to impress a positive voltage with reference to the potential of the first electrode 21 on the second electrode 22, and the third power supply 33 which impresses a negative voltage with reference to the grounding potential on the third electrode 23. This allows suppressing extraction of ions from the plasma through the electrode situated nearest the plasma.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ion beam processing object such as an ion implantation apparatus, an ion doping apparatus (non-mass separation type ion implantation apparatus), and a thin film forming apparatus using both ion irradiation and vacuum deposition. The present invention relates to an ion beam generation method and apparatus used when irradiating an object for processing.

[0002]

2. Description of the Related Art FIG. 9 is a sectional view showing an example of an ion implantation apparatus using a conventional ion beam generator. In this ion implantation apparatus, an ion source 2 of an ion beam generator having an ion source 2 for ejecting an ion beam 40 and a power supply device 30 for applying a voltage for extracting the ion beam thereto is attached to an upper part of a processing chamber 42. It has a different structure.

A holder 44 for holding a substrate 46, which is an example of an object to be processed, is provided in the processing chamber 42. The processing chamber 42 is evacuated by a vacuum exhaust device (not shown).

The ion source 2 shown in the figure is a high frequency ion source, and a plasma source section 4 for generating a plasma 14 by ionizing the gas by a high frequency discharge and a vicinity of an outlet of the plasma source section 4 And an extraction electrode system 20 for extracting the ion beam 40 from the plasma 14 in the plasma source section 4 by the action of an electric field.

The plasma source part 4 is provided with a plasma generating container 6 having a side wall 6a and a back plate 6b attached to the side wall 6a with an insulator 8 interposed therebetween. Gas is introduced from the downstream side. Also, in this example, the side wall 6a and the back plate 6b
Also serve as electrodes (discharge electrodes), and a high frequency power supply 16 is connected between them via a matching circuit 18.
However, the gas may be directly introduced into the plasma generation container 6.

In this example, the extraction electrode system 20 has a first electrode 21, a second electrode 22, a third electrode 23 and a fourth electrode 24 arranged from the most plasma side toward the downstream side. Reference numeral 26 is an insulator. Each of these electrodes 21 to 2
Although 4 is a porous electrode in this example, it may have an ion extraction slit.

The first electrode 21 is used for extracting the ion beam 4
It is an electrode for determining the energy of 0, and a positive high voltage is applied from the first power supply 31 constituting the power supply device 30 with the ground potential as a reference. The first electrode 21 is also called a plasma electrode or an acceleration electrode, and the first power supply 31 is also called an acceleration power supply.

The second electrode 22 is an electrode that produces a potential difference between the second electrode 22 and the first electrode 21 and extracts the ion beam 40 from the plasma 14 by the electric field generated by the potential difference.
A negative voltage based on the potential of the first electrode 21 is applied from the second power source 32 forming 0. The second electrode 22 is also called an extraction electrode, and the second power supply 32 is also called an extraction power supply.

The third electrode 23 is an electrode for suppressing backflow electrons from the downstream side, and a negative voltage is applied from the third power supply 33 constituting the power supply device 30 with reference to the ground potential. The third electrode 23 is also called a suppression electrode, and the third power supply 33 is also called a suppression power supply.

The fourth electrode 24 is grounded and is also called a ground electrode.

An example of the electric potential in the ion source 2 is shown in FIG.
It shows in 0. The potentials of the first electrode 21 and the back plate 6b are the first
It is determined by the output voltage V1 of the power source 31, and the second electrode 2 is placed at a position lower than the output voltage V2 of the second power source 32 by that.
There is a potential of 2, and the potential of the third electrode 23 is determined by the output voltage V3 of the third power supply 33. In addition, the plasma 14
Since the moving speed of the electrons that make up the substrate is much higher than that of the ions, the back plate 6b and the first electrode 21 have a negative potential with respect to the plasma 14 due to the large number of electrons.
From a different point of view, the plasma 14 is generated by the back plate 6b and the first plate.
It has a positive potential with respect to the electrode 21.

Explaining an operation example of the ion implantation apparatus of FIG. 9, while holding a desired substrate 46 on a holder 44 in the processing chamber 42 and evacuating the inside of the processing chamber 42, a plasma source part of the ion source 2 is provided. When a desired gas is introduced into the inside of 4 from the downstream side of the extraction electrode system 20 and a high frequency power of, for example, 13.56 MHz is supplied from the high frequency power supply 16 between the side wall 6a and the back plate 6b, a space between the side wall 6a and the back plate 6b is obtained. High-frequency discharge occurs in the plasma 14
Is made. Ions in the plasma 14 are extracted as an ion beam 40 by the extraction electrode system 20. The extracted ion beam 40 is directly applied to the substrate 46 without mass separation, and ion implantation (ion doping) is performed.

[0013]

For example, in the case of manufacturing a polycrystalline silicon thin film transistor, 10 ¹¹ to 10 10
Implantation with a low dose of ¹² ions / cm ² is required.

In order to use the above ion beam generator for such a low dose implantation, the ion current density of the ion beam 40 extracted from the ion source 2 must be reduced, but in the conventional ion beam generator. There was a limit in reducing the ion current density.

This is because the plasma 14 generated in the plasma source section 4 is inevitably at a potential higher than that of the first electrode 21 due to the above-mentioned action, so that the output voltage V2 of the second power supply 32 is set to 0. Also the plasma 14 and the first electrode 2
Ions are extracted due to the above potential difference existing between 1 and
This is because the ion current density of the ion beam 40 cannot be zero. Therefore, conventionally, the ion beam 40
In order to reduce the density of the plasma 14, the reduction of the density of the plasma 14 was also used. However, in order to reduce the density of the plasma 14, the power supplied to the plasma 14, that is, the high frequency power supply 16 supplies the plasma 14 to the plasma 14. It is necessary to reduce the electric power, and if so, there is a limit in reducing the ion current density of the ion beam 40 because the plasma 14 cannot be maintained or has a lower limit that can be maintained.

FIG. 11 illustrates the above situation. When the power supplied to the plasma 14 is lowered below the lower limit for maintaining the plasma, the plasma disappears and the ion beam 40 cannot be extracted.

Although the power supply device 30 has a configuration as shown in FIG. 12 in the related art, in this case as well, even if the output voltage V2 of the second power supply 32 is set to 0, it is the same as in the case of FIG. For the same reason, the ion current density of the ion beam 40 cannot be below a certain lower limit.

When the ion current density of the ion beam 40 cannot be made too small, when low dose implantation is performed,
The implantation time becomes, for example, several seconds or 1 second or less, and the controllability and reproducibility for performing the prescribed dose implantation deteriorate.

Therefore, it is a main object of the present invention to provide an ion beam generating method and apparatus capable of making the ion current density of an ion beam extracted from an ion source smaller than the conventional lower limit.

[0020]

In order to achieve the above object, the ion beam generating method of the present invention is an electrode located downstream of the most plasma side electrode among the electrodes constituting the extraction electrode system of the ion source. Is set so as to be higher than the potential of the electrode on the most plasma side, and the ion beam is extracted.

Further, in the ion beam generator of the present invention, the power supply unit controls the electric potential of the electrode downstream of the electrode on the most plasma side of the electrodes forming the extraction electrode system of the ion source to the maximum plasma. It is characterized by having a power supply whose output voltage is variable so as to be higher than the potential of the side electrode.

[0022]

According to the above ion beam generating method, the ion beam is extracted by setting the potential of the electrode downstream of the electrode on the most plasma side to be higher than the potential of the electrode on the most plasma side. The electric field due to the potential difference between the two electrodes becomes an electric field in the direction opposite to the ion beam extraction direction, and acts to suppress the extraction of ions from the plasma via the electrode on the most plasma side. As a result, the ion current density of the ion beam extracted from the ion source can be made smaller than the conventional lower limit without lowering the power supplied to the plasma.

Further, according to the above ion beam generator, the power supply device outputs the potential of the electrode on the downstream side of the electrode on the most plasma side of the ion source higher than the potential of the electrode on the most plasma side. Since it has a variable voltage power supply, it is possible to extract the ion beam by setting the potential of the electrode on the downstream side higher than the potential of the electrode on the most plasma side, and at that time, the electric field due to the potential difference between these two electrodes is ,
Since the electric field is in the opposite direction to the extraction direction of the ion beam,
It acts to suppress extraction of ions from the plasma via the electrode on the most plasma side. As a result, the ion current density of the ion beam extracted from the ion source can be made smaller than the conventional lower limit without lowering the power supplied to the plasma.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view showing an example of an ion implantation apparatus using the ion beam generator according to the present invention.
The same or corresponding portions as those of the conventional example in FIG. 9 are denoted by the same reference numerals, and the differences from the conventional example will be mainly described below.

In this embodiment, the conventional power supply device 3 is used.
Which is equivalent to 0, and is the ion source 2 as described above.
A power supply device 30a having the following configuration that constitutes the ion beam generator is also provided.

That is, the power supply device 30a is composed of the ion source 2
A first power source 31 for applying a positive high voltage to the first electrode 21 constituting the extraction electrode system 20 with reference to the ground potential, and a positive voltage to the second electrode 22 with reference to the potential of the first electrode 21. And a third power supply 33 for applying a negative voltage to the third electrode 23 with reference to the ground potential. The second power supply 32a has its negative output terminal connected to the line connected to the first electrode 21, and its positive output terminal connected to the second electrode 22. The output voltage V2a is variable. The first power supply 31 and the third power supply 33 are the same as those of the conventional example, but it is arbitrary whether their output voltages V1 and V3 are variable or fixed.

FIG. 2 shows an example of the potential of the ion source 2 when the power supply device 30a as described above is used. The potentials of the first electrode 21 and the back plate 6b are determined by the output voltage V1 of the first power supply 31, and the potential of the second electrode 22 is higher than that by the output voltage V2a of the second power supply 32a. The potential of 23 is the output voltage V3 of the third power supply 33.
Determined by In addition, the plasma 14 causes the back plate 6b.
As described above, the potential becomes positive with respect to the first electrode 21.

In this embodiment, the ion beam 40 can be extracted by setting the potential of the second electrode 22 on the downstream side of the potential of the first electrode 21 to be higher by the output voltage V2a of the second power source 32a. . Since the output voltage V2a of the second power supply 32a is variable, the potential of the second electrode 22 is set to almost zero by setting the output voltage V2a to almost zero.
Of the output voltage V2
By increasing a, the electric potential of the second electrode 22 can be made equal to or higher than the electric potential of the plasma 14.

The electric field due to the potential difference between the second electrode 22 and the first electrode 21 (which is equal to the output voltage V2a of the second power source 32a in this embodiment) is opposite to the extraction direction of the ion beam 40. Therefore, the first electrode 21
It acts to suppress the extraction of ions from the plasma 14 in the plasma source unit 4 via the. as a result,
Even if the input power to the plasma 14 is not reduced, the ion source 2
It is possible to further reduce the ion current density of the ion beam 40 to be extracted from the conventional lower limit.

The ion current density in that case is determined by the second power source 3
2a depends on the output voltage V2a. A schematic example of the relationship between the two is shown by a solid line in FIG.

As can be seen from FIG. 3, the output voltage V2
By increasing a, more specifically, the output voltage V2a
By increasing the potential of the second electrode 22 higher than that of the plasma 14, the lower limit of the ion current density can be extended to zero. The ion current density J ₀ when the output voltage V2a is 0 is the conventional lower limit, and in this embodiment,
By varying the output voltage V2a, the ion current density can be varied from the conventional lower limit J ₀ to 0. Incidentally, the region on the negative side of the output voltage V2a shown by the broken line in FIG. 3 is the variable range of the ion current density in the conventional ion beam generator.

As described above, the ion current density of the ion beam 40 also depends on the electric power supplied to the plasma 14. The second power supply 32 when this input power is changed
FIG. 4 shows an example of actual measurement results of the relationship between the output voltage V2a of a and the ion current density.

From this figure, the smaller the input power, the more
Since the slope of the curve becomes gentle, it is understood that the controllability of the ion current density by the output voltage V2a is improved.

The second component of the conventional power supply unit 30
The output voltage V2 of the power supply 32 is a high voltage of, for example, several KV, but from FIG. 4 described above, the power supply device 30 of this embodiment is
It can be seen that the output voltage V2a of the second power supply 32a that constitutes a can reduce the ion current density to 0 at a relatively low voltage of several hundreds V or less.

As described above, the ion current density of the ion beam 40 extracted from the ion source 2 can be made smaller than the lower limit of the prior art, and as a result, the implantation time when the substrate 46 is implanted at a low dose amount. Since it can be made longer, the controllability and reproducibility in the case of implanting a predetermined dose amount are improved.

FIG. 5 is a diagram showing another example of the power supply device constituting the ion beam generator according to the present invention. The power supply device 30 a of this example is the extraction electrode system 20 of the ion source 2.
And a first power supply 31 for applying a positive high voltage to the second electrode 22 that constitutes the second electrode 22, and an output voltage for applying a negative voltage to the first electrode 21 with respect to the potential of the second electrode 22. It has a variable second power supply 32a and a third power supply 33 that applies a negative voltage to the third electrode 23 with reference to the ground potential. The second power supply 32a has a negative output terminal connected to the first electrode 21, and a positive output terminal connected to the second electrode 22.
Is connected to the line leading to. The output voltage of each power supply is V1, V2a, and V3, respectively.

FIG. 6 shows an example of the potential of the ion source 2 when the power supply device 30a as described above is used. The potential of the second electrode 22 is determined by the output voltage V1 of the first power source 31, and the potential of the first electrode 21 and the back plate 6b is lower than that by the output voltage V2a of the second power source 32a. The potential of 23 is the output voltage V3 of the third power supply 33.
Determined by

Also in this case, the ion beam 40 can be extracted by setting the potential of the second electrode 22 on the downstream side of the potential of the first electrode 21 to be higher by the output voltage V2a of the second power source 32a. As in the case of the example shown in FIG. 1, the ion current density of the ion beam 40 can be made smaller than the conventional lower limit. The relationship between the output voltage V2a and the ion current density in that case is the same as that shown by the solid line in FIG.

When the power supply device 30a shown in FIG. 5 is used, the potentials of the first electrode 21 and the plasma 14 rise and fall by changing the output voltage V2a of the second power supply 32a. The energy of the ion beam 40 extracted from 2 (this is the first electrode 21
(More precisely, the potential of the plasma 14 is determined). The power supply device 30 shown in FIG.
In the case of “a”, since the potentials of the first electrode 21 and the plasma 14 are determined by the output voltage V1 of the first power supply 31, such a thing does not occur. However, since the output voltage V2a of the second power source 32a is usually much smaller than the output voltage V1 of the first power source 31, the energy change of the ion beam 40 due to the above is usually not a problem.

FIG. 7 is a diagram showing still another example of the power supply device constituting the ion implantation device according to the present invention. The power supply device 30a of this example has a first power supply 31 for applying a positive high voltage to the first electrode 21 constituting the extraction electrode system 20 of the ion source 2 with reference to the ground potential, and a first power supply 31 for the second electrode 22. A variable output voltage second power source 32a that applies a positive voltage based on the potential of the electrode 21, a third power source 33 that applies a negative voltage to the third electrode 23 based on the ground potential, and a second electrode Two
The second power supply 34 applies a negative voltage to the second electrode 21 with reference to the potential of the first electrode 21, and the voltage applied to the second electrode 22 is from the second power supply 32a and the fourth power supply 34. And a switch 36 for selectively switching with.

Both the positive output terminal of the second power source 32a and the negative output terminal of the fourth power source 34 are connected to the second electrode 22, and the negative output terminal of the second power source 32 and the fourth power source. 34
The positive output terminal of the switch 36 is connected to one and the other terminals of the switch 36, respectively, and the common terminal of the switch 36 is connected to the line connected to the first electrode 21. The output voltage of each power supply is V1, V2a, V3 and V4, respectively. The fourth power supply 34 and its output voltage V4 are
It corresponds to the second power supply 32 of the conventional power supply device 30 and its output voltage V2. It is preferable to make the output voltage V4 of the fourth power source 34 variable, because the variable range of the ion current density of the ion beam 40 becomes wider.

FIG. 8 shows an example of the potential of the ion source 2 when using the power supply device 30a. The potentials of the first electrode 21 and the back plate 6b are determined by the output voltage V1 of the first power source 31, and are raised by the output voltage V2a of the second power source 32a, or by switching the switch 36, the fourth power source 34. The potential of the second electrode 22 is at a position where the output voltage V4 of the third power source 33 is reduced by the output voltage V3 of the third power source 33.

By switching the switch 36 to the side of the second power source 32a, the potential of the second electrode 22 on the downstream side of the potential of the first electrode 21 is set higher by the output voltage V2a of the second power source 32a. Since the beam 40 can be extracted, the ion current density of the ion beam 40 can be made smaller than the conventional lower limit, as in the case of the example shown in FIG. The relationship between the output voltage V2a and the ion current density in that case is the same as that shown by the solid line in FIG.

By switching the switch 36 to the side of the fourth power source 34, the potential of the second electrode 22 on the downstream side of the potential of the first electrode 21 is set lower by the output voltage V4 of the fourth power source 34. Since the ion beam 40 can be extracted by using the same method as in the case of the conventional example shown in FIG.
The ion current density of the extracted ion beam 40 can be increased. The relationship between the output voltage V4 and the ion current density in that case is the same as that shown by the broken line in FIG.

By using the power supply device 30a as shown in FIG. 7, it is possible to change the ion current density of the ion beam 40 from a value smaller than the lower limit of the prior art to a large value similar to the conventional value. Therefore, the variable range of the ion current density is widened. As a result, various substrates 46
On the other hand, not only low-dose implantation with good controllability and reproducibility but also high-dose implantation in a short time becomes possible.

Even if a so-called bipolar power supply whose output voltage is continuously variable from positive to negative is used instead of the second power supply 32a, the fourth power supply 34 and the switch 36 of FIG. 7, the same effect can be obtained. .

The plasma source section 4 of the ion source 2
Is not limited to the method of ionizing gas by high frequency discharge to generate plasma 14 as in the above example, but other methods, for example, plasma 14 by ionizing gas by microwave discharge. A method of generating plasma or a method of generating plasma 14 by ionizing gas by DC arc discharge may be used.

Further, the number of electrodes constituting the extraction electrode system 20 of the ion source 2 is not limited to four as in the above example, and a plurality of other electrodes, for example, two or three,
5 or the like may be used.

[0049]

Since the present invention is configured as described above, it has the following effects.

According to the ion beam generating method of the first aspect, the potential of the electrode on the downstream side of the electrode on the most plasma side is set to be higher than the potential of the electrode on the most plasma side. Therefore, the electric field due to the potential difference between these two electrodes acts to suppress the extraction of ions from the plasma through the electrode on the most plasma side, and as a result, it is possible to reduce the input power to the plasma. The ion current density of the ion beam extracted from the ion source can be made smaller than the conventional lower limit.

Therefore, if such an ion beam generating method is used for ion implantation, for example, the implantation time can be lengthened when a low dose implantation is performed on a substrate, so that when a predetermined dose implantation is performed. Controllability and reproducibility are improved.

According to another aspect of the ion beam generator of the present invention, the power supply device makes the potential of the electrode on the downstream side of the electrode on the most plasma side of the ion source higher than the potential of the electrode on the most plasma side. Since it has a variable output voltage power supply, the ion beam can be extracted by setting the potential of the electrode on the downstream side higher than the potential of the electrode on the most plasma side, and at that time, the electric field due to the potential difference between the two electrodes can be extracted. Is
It acts to suppress the extraction of ions from the plasma through the electrode on the most plasma side, and as a result, the ion current density of the ion beam extracted from the ion source can be reduced to the lower limit of the conventional level without lowering the power input to the plasma. Can be made even smaller.

Therefore, if such an ion beam generator is used for ion implantation, for example, the implantation time can be lengthened when a low dose implantation is performed on a substrate, so that when a predetermined dose implantation is performed. Controllability and reproducibility are improved.

According to the ion beam generator of claim 3, in addition to the same effect as the device of claim 2, even if the output voltage of the second power supply is changed, the potentials of the first electrode and the plasma do not change. Therefore, a further effect is obtained that the energy of the ion beam extracted from the ion source does not change.

According to the ion beam generator of the fourth aspect, the same effect as that of the apparatus of the second aspect can be obtained.

According to the ion beam generator of the fifth aspect, the ion current density of the ion beam can be changed from a value smaller than the conventional lower limit to a large value similar to the conventional one. The variable range of the current density is widened.

Therefore, if such an ion beam generator is used for, for example, ion implantation, not only low dose implantation with good controllability and reproducibility but also high dose in a short time can be applied to various substrates. It is also possible to inject a dose.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of an ion implantation apparatus using an ion beam generator according to the present invention.

FIG. 2 is a diagram showing an example of a potential in the ion source in FIG.

3 is a schematic diagram showing an example of a relationship between an output voltage of a second power supply and an ion current density in the ion source in FIG.

FIG. 4 is a diagram showing an example of an actual measurement result of a relationship between an output voltage of a second power source and an ion current density in the ion source in FIG.

FIG. 5 is a diagram showing another example of a power supply device constituting the ion beam generator according to the present invention.

6 is a diagram showing an example of a potential in an ion source when the power supply device of FIG. 5 is used.

FIG. 7 is a diagram showing still another example of the power supply device constituting the ion beam generator according to the present invention.

8 is a diagram showing an example of a potential in an ion source when the power supply device of FIG. 7 is used.

FIG. 9 is a cross-sectional view showing an example of an ion implanter using a conventional ion beam generator.

10 is a diagram showing an example of a potential in the ion source in FIG.

11 is a schematic diagram showing an example of the relationship between the input power and the ion current density in the ion source in FIG.

FIG. 12 is a diagram showing another example of a power supply device constituting a conventional ion beam generator.

[Explanation of symbols]

 2 Ion Source 4 Plasma Source Part 14 Plasma 16 High Frequency Power Supply 20 Extraction Electrode System 21 First Electrode 22 Second Electrode 23 Third Electrode 24 Fourth Electrode 30a Power Supply Device 31 First Power Supply 32a Second Power Supply 33 Third Power Supply 34 Fourth Power supply 36 Switch 40 Ion beam 42 Processing chamber 46 Substrate

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H01L 21/268 Z

Claims (5)

[Claims] 1. A plasma source part which introduces a gas and ionizes it by discharge to generate plasma, and an ion beam which is extracted from the plasma in the plasma source part by the action of an electric field and has a plurality of electrodes. By using an ion source having an extraction electrode system, the potential of the electrode on the downstream side of the electrode on the most plasma side of the electrodes forming the extraction electrode system is set higher than the potential of the electrode on the most plasma side. A method for generating an ion beam, characterized in that the ion beam is extracted by setting as follows. 2. A plasma source part which introduces gas and ionizes it by discharge to generate plasma, and an ion beam which is extracted from the plasma in the plasma source part by the action of an electric field and has a plurality of electrodes. An ion beam generator comprising: an ion source having an electrode system; and a power supply device for applying a voltage for extracting an ion beam to an electrode forming an extraction electrode system of the ion source, wherein the power supply device is an extraction electrode for the ion source. Ion beam generation characterized by having a variable output voltage power supply that makes the potential of the electrode on the downstream side of the electrode on the most plasma side of the electrodes constituting the system higher than the potential of the electrode on the most plasma side apparatus. 3. The extraction electrode system has a first electrode, a second electrode, a third electrode and a fourth electrode arranged from the most plasma side toward the downstream side, and the fourth electrode is grounded. The power supply device applies a positive voltage to the first electrode with reference to the ground potential, and an output voltage to apply a positive voltage to the second electrode with reference to the potential of the first electrode. The ion beam generator according to claim 2, further comprising a variable second power source and a third power source for applying a negative voltage to the third electrode with reference to the ground potential. 4. The extraction electrode system has a first electrode, a second electrode, a third electrode and a fourth electrode arranged from the most plasma side toward the downstream side, and the fourth electrode is grounded. The power supply device applies a positive voltage to the second electrode with reference to the ground potential, and an output voltage to apply a negative voltage to the first electrode with reference to the potential of the second electrode. The ion beam generator according to claim 2, further comprising a variable second power source and a third power source for applying a negative voltage to the third electrode with reference to the ground potential. 5. The extraction electrode system has a first electrode, a second electrode, a third electrode and a fourth electrode arranged from the most plasma side toward the downstream side, and the fourth electrode is grounded. The power supply device applies a positive voltage to the first electrode with reference to the ground potential, and an output voltage to apply a positive voltage to the second electrode with reference to the potential of the first electrode. A variable second power source, a third power source that applies a negative voltage to the third electrode with reference to the ground potential, and a fourth power source that applies a negative voltage to the second electrode with respect to the potential of the first electrode. 3. The ion beam generator according to claim 2, further comprising: a switch for selectively switching the voltage applied to the second electrode between the one from the second power source and the one from the fourth power source.