JPH0644935A - Deceleration tube - Google Patents

Abstract

PURPOSE:To restrain the divergence of an ion beam near an earth electrode, and stabilize the operation of an ion source by providing a lens electrode between a suppressor electrode and the earth electrode, and applying low voltage of the same polarity as the ion beam to the lens electrode. CONSTITUTION:When ions travelling through a deceleration tube are positive, negative high voltage is applied to the first electrode 1, and higher negative voltage is applied to a suppressor electrode 9 as the second electrode. As a result, secondary electrons generated upon collision of an ion beam with the electrodes and a vessel wall, is prevented from entering an ion source. Also, a lens electrode 10 as the third electrode is provided between the suppressor electrode 9 and an earth electrode 4, and positive low voltage is applied to the electrode 10 for the application of a low repulsive force to the ions. The electrode 10 acts to apply a Coulomb repulsive force to the positive ions from the surrounding thereof and, therefore, causes the positive ions to converge toward a center. Also, cylindrical insulators 11 to 13 are laid among the electrodes for maintaining insulation quality across each electrode and the airtightness of internal space. According to this construction, the divergence of a beam near the earth electrode 4 is restrained, and the operation of an ion source is stabilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a speed reducer for decelerating accelerated ions. The ion source excites the introduced gas or vapor by electric discharge to generate plasma, which is beer
It is to be pulled out. The object may be directly irradiated with this. It may also accelerate further.
It may accelerate and then decelerate. Ion beams of arbitrary energy can be generated by acceleration and deceleration. Since the desired energy of the ion beam differs depending on the treatment, it is necessary to make the ion beam have an appropriate energy before irradiating the object.

A device for accelerating an ion beam is called an accelerating tube. In the case of positive ions, the ion source may be held at a positive high potential and the ion beam may be run toward the ground side. Since the electric field must not be disturbed by the beam and the diameter of the beam must be controlled, several ring-shaped electrodes are arranged along the way. The voltage is appropriately divided and applied to the electrodes. On the other hand, what slows down is called a speed reducer. This has the opposite voltage. In the case of positive ions, the energy of the ions is lowered by traveling from the negative high potential to the ground side. Acceleration and deceleration are in contrast, but not simply the opposite.

[0003]

2. Description of the Related Art FIG. 2 shows a sectional view of a speed reducer according to a conventional example. This is an array of four ring-shaped electrodes.
Reference numeral 1 is an electrode to which a negative high potential is applied. Reference numeral 2 is a suppressor electrode, which is an electrode having a higher negative potential than that of 1. Three
Is an electrode having the same potential as 1. Finally, there is the ground electrode 4. Between these electrodes, ring-shaped insulators 6, 7,
8, there is a seal between these electrodes. The space in which the ion beam 5 travels is kept vacuum. The suppressor electrode has a lower voltage than the electrodes 1 and 3. This is to prevent secondary electrons generated by the collision of the ion beam 5 with the electrodes and the wall of the container from flowing into the moderator tube and entering the ion source.

When the ion beam 5 travels from the left to the right along the center line mn of the electrode or the insulator, it loses energy and is decelerated. For example, in the first negative high voltage electrode 1, -30
It is assumed that the following suppressor electrode 2 has −35 kV, the electrode 3 has −30 kV, and the electrode 4 has 0 V. The kinetic energy of the ions near the electrode 1 is 30.
It is set to 1 keV. The energy after deceleration becomes 100 eV. If such a strong deceleration is attempted, strong convergence and strong divergence occur and the beam transport efficiency decreases.

That is, when passing in the vicinity of the negative high-potential electrodes 1, 2, and 3, the axial velocity is extremely high, the charge density is low, and the electrostatic repulsion force in the lateral direction is small. However, when the velocity is close to 0 near the ground electrode 4, the charges are concentrated in the front-rear direction and the charge density is increased. Therefore, it becomes a space charge limited current. In the precedent example, the initial energy is 30 ke
Since V and the final energy are 0.1 keV, the speed ratio is 1/17. The charge density near the ground electrode increases with this inverse ratio. Therefore, the electrostatic repulsion force in the lateral direction (Kurong repulsion force) becomes extremely dominant. Because of this
It tends to diverge laterally. There may be ions that not only diverge, but also occasionally invert and return to the ion source.

[0006]

If the current value of the ion beam is small and is in the order of several .mu.A to several hundreds .mu.A, the Coulomb repulsion force is small even if the energy of the beam is remarkably reduced, so that the ion beam diverges. The mode is small. However, in recent years, high ion beam density has been required. For example, several mA
A ionic current of several tens of mA is required. When the current value is large as described above, the Coulomb repulsive force between the ions becomes extremely strong. Therefore, the lateral divergence of ion beams becomes remarkable. Due to the divergence of the ion beam, the ion beam easily hits the electrode and the container wall, and the probability of generation of secondary electrons also increases. To suppress the influence of secondary electrons on the ion source, the suppressor electrode is enlarged to enhance the suppression effect. Then the influence of secondary electrons can be suppressed. However, this is not enough. This is because the unexpected phenomenon of backflow of the ion beam itself occurs.

FIG. 3 is a streamline diagram obtained by simulating the progressing state of the ion beam when a suppressor electrode 9 larger than the conventional electrode structure is added. Only the upper half of the centerline is shown here because it is symmetrical about the centerline. Further, there is an electrode 10, which is an electrode to which the present invention newly adds. Here, assuming that this electrode is absent, a potential of 0 V is applied and the movement of the beam is calculated.
Initially, Al ions have an energy of 20.1 keV. -20kV, -25kV, 0 for each electrode
A voltage of kV is applied. Suppressor electrode 9 and the next 0 kV
The four curves between the electrodes are -20kV (f), -15k
It is an equipotential line of V (ki), -10 kV (K), and -5 kV (K).

The group of curves in the axial direction is the streamline of the ion beam. At the beginning, the streamlines are almost parallel because they are accelerated. However, when it passes the suppressor electrode 9, it begins to diverge. This is because the electric charge receives a force in a direction parallel to the equipotential line when going back to the equipotential line, and thus diverges in this way. To put it simply, there are some reasons why equipotential lines have such a gradient with respect to the axis. This is due to the Coulomb's repulsive force, as I mentioned earlier.
However, if we apply the Coulomb repulsive force to each other, it becomes a many-body problem and we cannot solve it. This simulation approximates the Coulomb repulsive force between charged particles to the integral problem and introduces it into the equation. That is, it is assumed that other particles do not exist at a certain point on the streamline, but rather exist along the streamline evenly with a probability inversely proportional to the velocity.

Then, the spatial potential due to other charged particles can be calculated. Assuming that one charged particle moves in this potential, the motion of this charged particle is obtained. With this, one streamline and the velocity along the streamline can be calculated. Do this for all streamlines. All other charged particles are treated as an integral problem on the assumption that they exist on the streamline with equal probability in time. This is a problem that can always be solved. Once such a calculation is performed, the same integral problem is solved again based on this distribution. Repeat this several times to arrive at a consistent result, so this is the solution. Naturally, the electric field distribution and equipotential lines can be obtained at the same time.
This is a heart-hook approximation, but it is considered to be an approximation that holds even if the density of charged particles is considerably high.

As is apparent from FIG. 3, since the velocity in the axial direction decreases near the 0 kV line at the end point, the velocity component in the lateral direction comes to have. Ions located away from the axis have a considerably lateral velocity, and the ions A and A are 0 in particular.
It reverses at the line of kV and returns to the ion source. Since this is simulated by limiting the area up to the position of the ground electrode 4, it is not known that the other beams are reversed, but when reaching 0 kV, more beams are generated. It should be reversed. As described above, in the conventional moderator tube, the ion beam reverses due to the Coulomb repulsion between the high-density space charges. These are undesirable because they destabilize the operation of the ion source when it returns to the ion source.

[0011]

In the speed reducer of the present invention, another lens electrode is provided between the suppressor electrode to which a high voltage is to be applied and the final ground electrode, and the other lens electrode is provided on the opposite side of the high voltage. By applying a voltage with a small polarity, the divergence of the beam in the vicinity of the ground electrode is suppressed.

[0012]

[Action] In other words, the ions passing through the moderator tube are positive ions,
If the first electrode is to apply a high negative voltage, the lens electrode applies a positive voltage. On the contrary, if the ions passing through the moderator tube are negative ions and the first electrode is to apply a positive high voltage, then a negative voltage is applied to the lens electrode. Since a voltage having the same polarity as that of the ions is applied, a repulsive force is exerted on the ion beam. Therefore, the divergence of ion beams can be suppressed.

FIG. 1 shows an example of the arrangement of electrodes of the speed reducer of the present invention. The case of positive ions will be described. The first electrode 1 is applied with a high negative voltage. Even if it says a high voltage, the voltage of the space where the ion beam exists is a negative high voltage. The second electrode 9 is a suppressor electrode. This places an even higher negative high voltage. This is because the ion beam does not enter the ion source when the ion beam collides with the electrode or the wall of the container to emit secondary electrons as described above. The suppressor electrode 9 is wider than that described above in order to solve the problem that the influence of the potential of the external electrode is less likely to occur when the ion beam current becomes larger.

Next to this, the lens electrode 1 is used as a third electrode.
0 is set. This is the feature of the present invention. A voltage that exerts a repulsive force on the ions is applied to this. A positive low voltage is applied to positive ions. For example, a low voltage of about 0.5 to 3 kV may be used. This has the effect of preventing the beam from diverging before reaching the next ground electrode. That is, since the lens electrode exerts a Coulomb repulsive force on the positive ions from the periphery, it has an action of converging the positive ions toward the center. Since it has the same effect as a convex lens, it is referred to as a lens electrode here.
In the case of negative ions, of course, the potential of the lens electrode is a negative low voltage. Between these electrodes is a cylindrical insulator 11, 1
2, 13 are provided. These serve to insulate the electrodes and keep the internal space airtight.

[0015]

EXAMPLE FIG. 4 is a streamline diagram of an ion beam showing an example of the present invention. The conditions are almost the same as those in FIG. The ions are Al ions. Initial energy is 20.1 keV
Is. The first electrode 1 is -20 kV, the second electrode suppressor electrode 9 is -25 kV, and the third electrode lens electrode 10 is 1.
A voltage of kV is applied. The curve drawn between the suppressor electrode 9 and the lens electrode 10 is an equipotential line. Mosquito-
20kV, Ki -15kV, Ku -10kV, Ga -5
Equipotential lines of kV and KO are 0 kV. Lens electrode is 1k
Since it is V, an equipotential line of 0 kV is formed immediately in front of it.

The equipotential line power is deep inside the ion beam because the suppressor electrode 9 is sufficiently wide. Since the suppressor electrode 9 is wide even if the ion beam density is extremely high, the electric field generated by this electrode is not canceled by the beam and enters the inside. The shape and arrangement of the electrodes are almost the same as those shown as the conventional example in FIG.

The only difference is that a lens electrode is added and a voltage of 1 kV is applied to it. The action of the lens electrode is remarkable in the sa, shi, and su areas. I'm pushing the ion beam toward the center line. The streamline is inward.
In FIG. 3, the beams of A and B in the middle are reversed, but this is pressed inward by the action of the lens electrode, and the beams of C and S are
Mu also does not reverse direction. Beams along any streamline are not reversed. This is because the lens electrode is provided between the ground electrode and the suppressor electrode and a positive voltage is applied to the lens electrode.

FIG. 5 shows a streamline of a beam obtained by applying the method of the present invention to a ring-shaped electrode. The suppressor electrode 9 is a simple ring-shaped electrode. This shows that the structure of the suppressor electrode does not have the characteristics of the present invention. There is an equipotential line of 0 kV near the lens electrode 10. The lens electrode exerts a central force on the ion beam. It has a strong effect of pushing the ion beam inward near the center. Naturally, it is more inward due to the action of space charge,
A force inward is also generated near the tool. For this reason, the streamline of the ion beam does not reverse. The present invention can also be applied to such a normal ring-shaped electrode structure.

K, K, K, K and K are equipotential lines. A big difference from the case of FIG. 4 is that the mosquito is eliminated without entering the inside of the ion beam. This is because the range of the electric field is narrow because the electrode is narrow, and when the density of the ion beam is high, the electric field formed due to the space charge is predominant and the electric field of the electrode is eliminated. This explains the reason for widening the suppressor electrode and is not directly related to the present invention.

[0020]

According to the present invention, when the density of ion beams is high and the degree of deceleration is large, the space charge density becomes excessive at the end of the deceleration, which causes a strong divergence of the beam. It tries to overcome the problem. Ions of high energy of several tens of kV are decelerated to low energy of approximately 0.1 kV, and the current density is as high as several mA to several tens mA.

In the present invention, a lens electrode is provided between the suppressor electrode and the ground electrode, and the same voltage as the electric charge of the ion beam is applied to the lens electrode, so that Coulomb repulsive force is applied to the beam that has lost most of its velocity to cause divergence. It is what you want to prevent. As a result, the beam moves in the direction of convergence, and the ion beam does not return. When the ion beam returns to the ion source, the operation of the ion source becomes unstable, and the ion source sometimes fails. The present invention can eliminate such a possibility.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an arrangement example of electrodes of a speed reducer of the present invention.

FIG. 2 is a schematic cross-sectional view showing an arrangement of electrodes of a speed reducer according to a conventional example.

FIG. 3 is a diagram showing a simulation result of streamline distribution of an ion beam in the electrode arrangement according to the conventional example. However, only the upper half of the center line is shown.

FIG. 4 is a diagram showing a simulation result of streamline distribution of ion beams in an electrode and potential arrangement according to an embodiment of the present invention. However, only the upper half of the center line is shown.

FIG. 5 is a diagram showing a simulation result of streamline distribution of ion beams in an electrode and potential arrangement according to another embodiment of the present invention. However, only the upper half of the center line is shown.

[Explanation of symbols]

 1 First Electrode (Negative High Voltage) 2 Suppressor Electrode 3 Third Electrode (Negative High Voltage) 4 Ground Electrode 5 Ion Beam 6, 7, 8 Insulator 9 Suppressor Electrode 10 Lens Electrode

Claims (1)

[Claims] 1. An ion beam generated by an ion source and accelerated.
It is a deceleration tube for decelerating the system, and it is composed of an electrode to which a high voltage is applied, a suppressor electrode that is held at a lower potential to prevent secondary electrons from returning to the ion source, and a voltage that exerts a repulsive force on the ions. A deceleration tube characterized in that an applied lens electrode and a ground electrode held at a ground potential are provided along a streamline of an ion beam.