JPH05121024A - Anion source - Google Patents

Abstract

PURPOSE:To dislocate the plasma axis and the central axis of a drawer slit hole without providing a mechanical moving mechanism. CONSTITUTION:An anode side magnetic pole is divided into two parts of 30a and 30b, and auxiliary coils A and B are also arranged respectively in return yokes 40a and 40b, and by controlling exciting currents of these auxiliary coils, magnetic flux distribution in a space 60 is changed, and the central axis of a magnetic flux is moved. Thereby, the plasma axis can be moved.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a negative ion source used in a surface analyzer. For example, when analyzing impurities in the field of semiconductors, etc., when the type of element contained in the material is investigated by applying negative ions to the sample and detecting secondary ions emitted from the sample (SIM
S)), or when investigating the type of element contained in the material by detecting X-rays specific to the element emitted from the sample by applying high-energy ions to the sample (analysis by PIXE), etc. Should be negative ions,
The present invention relates to a negative ion source that generates negative ions used to generate high energy ions.

[0002]

2. Description of the Related Art A typical duoplasmatron type negative ion source as a negative ion source generates a desired plasma by two-stage ionization. That is, the first
In the step of ionization, a gas inert to the filament of the cathode is used, and the ions obtained by ionizing the gas are applied to the desired gas to carry out the second step of ionization, whereby the desired plasma is obtained. obtain. Therefore, the duoplasmatron type negative ion source has the advantages that the life of the filament of the cathode can be extended and various gases can be used as the gas for generating the desired plasma.

FIG. 3 shows a schematic structure of this duoplasmatron type negative ion source as a conventional example of the negative ion source.
As shown in FIG. 3, the negative ion source includes a filament 5
1 has a cathode 50, an anode 31, and an intermediate electrode 20 disposed between the cathode and the anode.
0 and the anode 31 also function as magnetic poles (hereinafter, these magnetic poles are referred to as “intermediate electrode side magnetic pole” and “anode side magnetic pole”, respectively). FIG. 2 shows a schematic configuration of this negative ion source from the viewpoint of a magnetic circuit. 2A is a vertical cross-sectional view.
(B) is a bottom view. However, FIG. 2 corresponds to a view obtained by rotating FIG. 3 by 90 degrees around an axis perpendicular to the paper surface. As shown in FIG. 2, the intermediate electrode side magnetic pole 20 → the gap 60 between the intermediate electrode and the anode → the anode side magnetic pole 31 → the return yoke 40.
A symmetrical magnetic circuit having a path of a or 40b → intermediate electrode side magnetic pole 20 is formed, and when an exciting current is passed through the coil 10 wound around the intermediate electrode 20, a magnetic flux passing through the path is generated and the intermediate electrode and A magnetic field is generated in the gap 60 between the anodes. Here, the intermediate electrode side magnetic pole 20 and the anode side magnetic pole 31
Since the shape is axisymmetric, the generated magnetic field is also axisymmetric, and the geometrical central axis coincides with the central axis of the magnetic field.

In the duoplasma type negative ion source having the above-mentioned structure, thermions emitted from the filament 51 of the cathode cause a discharge between the cathode 50 and the intermediate electrode 20 to cause ionization and generate plasma. Further, the plasma causes the gas supplied between the intermediate electrode 20 and the anode 31 to be ionized, and plasma of the desired gas is generated between the intermediate electrode 20 and the anode 31. Generally, plasma is easily diffused, but diffusion is prevented by the above-described magnetic field existing in the gap 60 between the intermediate electrode 20 and the anode 31, and the generated plasma becomes columnar by the magnetic field. In general, the negative ion extraction current extracted through the anode in the negative ion source is the central axis of the columnar plasma (hereinafter referred to as "plasma axis") and the slit hole of the anode for extracting the negative ion current (hereinafter referred to as "extraction slit hole"). There is an amount of deviation that maximizes the negative ion current, that is, an optimum point, as compared with the case where the central axis of 32) coincides with the central axis of 32. For example, the data shown in FIG. 4 is obtained as an example of the measurement result showing the change of the negative ion H ⁻ current with respect to the shift of these central axes (GP Laurence et al., Nucl. Instr. Me.
thod. Phys. Rev. 32 (1965), p.357). In this data, the optimum point is where the plasma axis and the central axis of the extraction slit hole 32 are deviated by about 1 mm (about 40 × 10 ⁻³ inch), and the negative ion H ⁻ current becomes maximum here. Conventionally, in order to efficiently extract negative ions by adjusting to such an optimum point, the central axis of the slit hole 32 is displaced with respect to the plasma axis by mechanically displacing the extraction slit hole 32.

[0005]

However, the extraction slit hole is in a vacuum (a gas necessary for generating plasma exists, but is about 10 ^-1 to 10 ^-2 Torr), and is usually on the ground side. Instead of being placed on the high potential side,
The moving mechanism of the slit hole is complicated and difficult to control.

Therefore, an object of the present invention is to provide a negative ion source capable of displacing the central axis of the slit hole and the plasma axis without providing a mechanical mechanism for moving the slit hole.

[0007]

In order to achieve the above object, the present invention comprises a cathode and an anode and an intermediate electrode arranged between them, and a predetermined electrode is provided between the cathode and the intermediate electrode. Negative ions are taken out from the plasma by the electric field generated by applying a voltage to the anode while preventing the diffusion of the plasma by the magnetic field generated between the intermediate electrode and the anode by turning gas into plasma. In the ion source, by changing the magnetic flux distribution of the magnetic field,
It is configured to have means for moving the central axis of the plasma, which has a columnar shape in the magnetic field.

[0008]

Since the plasma is restricted by the magnetic flux, according to this structure, by changing the magnetic flux distribution in the air gap between the anode and the intermediate electrode, the central axis of the plasma which is columnar in the vicinity of this air gap (Plasma axis) moves. As a result, the plasma axis can be shifted with respect to the central axis of the extraction slit hole to increase the negative ion current.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of an ion source according to an embodiment of the present invention viewed from a magnetic circuit perspective.
1A is a vertical sectional view, and FIG. 1B is a bottom view. This ion source is also a duoplasmatron type negative ion source like the above-mentioned conventional example, and this FIG. 1 corresponds to FIG. 2 of the conventional example. This ion source is different from the above-mentioned conventional example, as shown in FIG. 1B, the magnetic pole on the anode side (the anode also serving as the magnetic pole) is magnetically divided into two parts 30a and 30b, and The auxiliary coil A is connected to the return yoke 40a connected to the anode side magnetic pole 30a, and the auxiliary coil B is connected to the return yoke 40b connected to the anode side magnetic pole 30b.
It is the point that each is provided. Other parts are the same as in the conventional example (FIG. 2), and corresponding parts are designated by the same reference numerals and description thereof is omitted.

The ion source of the present invention is constructed so that the exciting currents of the auxiliary coils A and B can be changed independently, so that it exists in the space 60 between the anodes 30a and 30b and the intermediate electrode 20. Of the magnetic flux, the density of the magnetic flux corresponding to one of the divided anode side magnetic poles 30a can be made higher or lower than the density of the magnetic flux corresponding to the other anode side magnetic pole 30b. That is, in the magnetic circuit of the ion source, one of the divided magnetic poles 30a on the anode side is divided.
Two independent closed paths, a closed path passing through the anode side magnetic pole 30b and the closed path passing through the other anode side magnetic pole 30b, are formed. By changing the number of magnetic fluxes passing through the closed paths by the auxiliary coils A and B independently, In the space 60 between them, the magnetic flux distribution in the plane perpendicular to the central axis of the extraction slit hole 32 can be changed.

As can be seen from the above structure, when the exciting current of one of the auxiliary coils A and B is made larger than that of the other coil or the exciting current is caused to flow in the opposite direction, magnetic flux is generated between the anode and the intermediate electrode. The magnetic flux distribution in the void 60 is asymmetric, and the central axis of the magnetic flux deviates from the geometrical central axis (the central axis of the extraction slit hole 32). By the way, the plasma generated between the cathode and the intermediate electrode has a columnar shape due to the presence of the magnetic field in the gap 60 between the anode and the intermediate electrode. The columnar plasma is restricted by the magnetic flux and its plasma axis is the magnetic flux central axis. It moves with the movement of. This is, if viewed relatively, the drawer slit hole 3
2 is equivalent to moving the central axis of the auxiliary coil A and B with respect to the plasma axis. Can be adjusted. Therefore, by controlling the exciting currents of the auxiliary coils A and B, it is possible to easily adjust the negative ion current to the maximum, that is, the optimum point for extracting the negative ions.

In the above embodiment, the magnetic pole on the anode side is divided into two and divided into three.
0a, 30b and the auxiliary coils A, B are provided on the return yokes 40a, 40b, respectively, but the configuration of the present invention is not limited to this, and even if the magnetic poles on the intermediate electrode side or both magnetic poles are divided. Of course, various modifications in which the number of divided magnetic poles and the arrangement position / number of auxiliary coils are changed are also conceivable. That is, in principle, if the means for changing the magnetic flux distribution on the surface perpendicular to the central axis of the slit hole 32 in the gap 60 between the anode and the intermediate electrode can be provided to move the magnetic flux central axis (or the slit hole). If a magnetic field component perpendicular to the central axis of 32 can be generated to incline the plasma axis), the same effect as that of the above embodiment can be obtained.
From this point of view, the following modified examples can be considered.

In the above embodiment, the magnetic flux distribution is changed by the change of the exciting current of the auxiliary coil, that is, the change of the magnetomotive force to move the magnetic flux central axis (and hence the plasma axis). However, the change of the magnetic resistance causes the plasma axis to move. A configuration is also possible. For example, as shown in FIG. 5, voids 41a and 41b are provided in the return yokes 40a and 40b, and the void distances La and Lb are mechanically changed, so that the magnetic flux distribution in the void 60 between the anode and the intermediate electrode is changed. The plasma axis can be moved by changing it.

Further, in the above-mentioned embodiments and modified examples, either one or both of the anode side magnetic pole and the intermediate electrode side magnetic pole is divided, but instead of dividing the magnetic pole, or with dividing the magnetic pole,
It is also conceivable that a new auxiliary coil is provided between the intermediate electrode and the anode, and an exciting current is passed through the auxiliary coil to generate a magnetic field component perpendicular to the central axis of the extraction slit hole to tilt the plasma axis. ..

In the above embodiment, since the magnetic pole was divided into two, the central axis of the magnetic flux (and therefore the plasma axis) could be moved only in one direction. It can be moved in all directions (in a plane perpendicular to the central axis of 32). Normally, the life of the ion source is determined by the damage of the slit hole. Therefore, if the magnetic pole is divided into three or more, there is an advantage that the damaged portion can be evenly distributed around the slit by changing the moving direction to extend the life. ..

[0016]

As described above, according to the present invention, since the plasma axis can be moved by changing the magnetic flux distribution, the optimum point for extracting negative ions can be adjusted without providing a mechanical moving mechanism. be able to. Therefore, the structure is simpler than that of the conventional one, and the negative ion source can be downsized and reduced in price. Further, since the magnetic flux distribution can be configured to be changed by adjusting the exciting current of the auxiliary coil, in this case, the deviation between the plasma axis and the central axis of the slit hole can be more accurately compared with the mechanical control. Can be controlled.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of an ion source according to an embodiment of the present invention as viewed from a magnetic circuit perspective.

FIG. 2 is a diagram showing a schematic configuration of a conventional ion source viewed from a magnetic circuit perspective.

FIG. 3 is a sectional view showing a schematic configuration of a conventional duoplasmatron type negative ion source.

FIG. 4 is a diagram showing a change in negative ion current with respect to a deviation between a plasma axis and a central axis of an extraction slit hole.

FIG. 5 is a diagram showing a schematic configuration of another embodiment (modification) of the present invention viewed from the viewpoint of a magnetic circuit.

[Explanation of symbols]

 10 ... Main coil 20 ... Intermediate electrode (intermediate electrode side magnetic pole) 30a, 30b ... Anode (anode side magnetic pole) 32 ... Extraction slit holes 40a, 40b ... Return yoke 60 ... Air gap between intermediate electrode and anode A, B ... Auxiliary coil

Claims (1)

[Claims] 1. A cathode and an anode, and an intermediate electrode arranged between them, and a predetermined gas is turned into plasma by electric discharge between the cathode and the intermediate electrode, and between the intermediate electrode and the anode. In a negative ion source for extracting negative ions in the plasma by an electric field generated by applying a voltage to the anode while preventing diffusion of the plasma by the generated magnetic field, by changing the magnetic flux distribution of the magnetic field, A negative ion source comprising means for moving a central axis of the plasma in a columnar shape in a magnetic field.