JPH03250538A - Control method for ion beam device - Google Patents

Abstract

PURPOSE:To construct an accelerating and decelerating power supply in light weight and small size by boosting the accelerative voltage and arc voltage to the specified level while keeping the relationship that the current of the decelerating power supply minimizes after the two voltages are dropped to zero when electric discharge has occurred between drawout electrodes. CONSTITUTION:When electric discharge occurs between a pos. electrode 2 and a neg. electrode 3 of the drawout electrode line to cause flowing of a short- circuiting current, a VARC-VACC control device 10 nullifies the arc discharge VARC as well as the accelerative voltage VACC. After a certain while, the voltage VACC is raised again, when the voltage VARC is raised simultaneously. At this time, the control device 10 makes raising while keeping the relationship that IDEC minimizes. This causes drop of the plasma density to lessen secondary electrodes generated when pos. ions run against the decelerative electrode, and no excessive current flows through the accelerating and decelerating power supplies 8, 9 to permit both power supplies to manage with a minor output capacity. Thus the device as object can be embodied in small size and at a low cost.

Description

[Detailed description of the invention] [Industrial application field]

This invention is designed to prevent excessive current from flowing into the extraction power supply when restarting the extraction voltage after the extraction voltage drops due to discharge between the extraction electrodes in an ion source that generates plasma by arc discharge and extracts ions using extraction electrodes. This invention relates to an ion beam control method.

[Conventional technology]

Figure 3 shows a schematic structure of an ion source that uses arc discharge. The arc chamber is a cylindrical vacuum container. The gas to be ionized is flowed into it. Arc chamber 1
A cathode filament 5 is provided on the closed end plate of. This is heated by a filament power source (Vr++) 6 and generates thermoelectrons. This becomes a cathode, so it is called a cathode filament. It is the wall of the arc chamber 1 itself that becomes the anode. An arc discharge is created and maintained between the chamber wall and the filament. Cathode filament 5 to cause arc discharge
There is an arc power supply (VARC) between the and the arc chamber 7
is installed. An extraction electrode system is provided on the opening side of the arc chamber 1. This is composed of three electrode plates with different potentials, which are electrodes made of porous metal plates, and is collectively called an extraction electrode system. From the side closest to the arc chamber 1, a positive electrode 2, a negative electrode 3,
This is the ground electrode 4. Positive electrode 2 is an acceleration power source (VACC)
A positive voltage is applied by 8. Subsequent negative electrode 3
The third electrode to which a negative voltage (Vot.) is applied by the deceleration power supply 9 is the ground electrode 4, which is O
It is V. Furthermore, a number of permanent magnets are often provided along the outer circumferential wall of the arc chamber 1 so that the magnetization directions coincide in the radial direction, so that a cusp magnetic field is created within the chamber. The Cusb magnetic field confines the plasma to the center so that it does not contact the chamber walls. The plasma is confined within the chamber by the force of the magnetic field and the electrostatic force of the electrodes. Some ions are extracted by the action of the protruding electrode system. VACC+v between the positive electrode 2 and negative electrode 3 of the extraction electrode system
This means that the voltage of DEo is applied. Positive ions gain energy of Q (VACC+VDEC) between these electrodes and are accelerated. A voltage of -VDEC is applied between the negative electrode 3 and the ground electrode 4. The positive ions are decelerated by (IVDEC) and have energy of qVACo beyond the ground electrode 4. The positive and negative electrodes 2.3 have the function of accelerating the ions in this way.The ions are decelerated by some kind of tarp ( (not shown), where secondary electrons are generated.Since the secondary electrons should not enter the ion source, the ground electrode 4 and the negative electrode 3 push back the secondary electrons. .

[Problem to be solved by the invention]

When the ion source is operating normally, arc chamber 1
Arc discharge occurs only between the cathode filament 5 and the cathode filament 5, and a certain amount of ion beam is stably extracted. Arc voltage VARC1 acceleration voltage vA. . is constant. The current flowing through these power supplies 7.8.9 is stable. The voltage (VACC+vDEC) between the positive electrode 2 and the negative electrode 3 in the three-lead electrode system is quite large. For this reason,
For some reason, discharge may occur between the positive and negative electrodes 2 and 3. Electrons flow from the negative electrode 3 to the positive electrode 2. This is an abnormal condition, and if it continues for a long time, it will place an excessive burden on the power supply 8.9. This is because a short circuit current flows. FIG. 4(a) shows the voltage waveform of the arc voltage VARCN, and FIG. 4(b) shows the voltage waveform of the accelerating voltage vAcc. Arc voltage VARC
remains constant regardless of such abnormalities. This is because this is the voltage that determines arc discharge and is the voltage between the arc chamber 1 and the filament 5, so it is considered to be unrelated to the extraction electrode system. Accelerating voltage VACC takes a constant value (between A and B) under normal conditions, but when a discharge occurs between the electrodes and a short circuit current flows (point 0), VACC is reduced to 0 by the action of the control system (point C). This is the same as a normal circuit protection method. t ” j o
Then, VACC=0, and the interelectrode discharge disappears. Since the interelectrode discharge has stopped, after a certain period of time (for example, 1 second) (t =
tI), the acceleration voltage VACC is gradually raised again. VACC rises linearly with time (from 2 to HO to VACC) and returns to its original value at t=t2. After this point, stable operation continues again. There is a problem when the voltage palindrome starts up from point 2 to point. At this time, arc voltage VARC, filament voltage V□, ion bleaching gas pressure P, etc. are constant. Since the arc voltage VARC is flat as it is, the point where high-density plasma continues to be generated in the arc chamber 1 is where the accelerating voltage VACC is raised from 0, so VACC
is low, and the positive ion confinement force by the positive electrode 2 is reduced. Positive ions move from the high-density plasma toward the positive electrode 2 and collide with the negative electrode 3 and the positive electrode 2. Especially negative electrode 3
A large amount of strong collision occurs. Therefore, a large amount of secondary electrons are generated. Secondary electrons generated at the negative electrode 3 move toward the positive electrode 2 and are absorbed by it. An excess current flows through the acceleration power source 8 and the deceleration power source 9 due to the current of the secondary electrons. The power supply current at this time is approximately twice that of normal operation. In order to cope with such a situation, the output capacities of the acceleration power source 8 and the deceleration power source 9 are determined so as to be stable even when the power source is twice as large as that during normal motion. In other words, a power source with a capacity approximately twice that required for normal operation is required. Since power supplies are heavy and expensive, it would be convenient if it could be done with a small capacity one. The current flowing through the deceleration power source 9 is written as the deceleration current IDEe (this is almost the same as the current flowing through the acceleration power source 8, so we will discuss the current flowing through the power source based on this. When the aforementioned interelectrode discharge occurs, secondary electrons flows from the negative electrode 3 to the positive electrode 2, so IDEC increases by the amount of secondary electron current.Although discharge between the electrodes does not occur frequently, a large-capacity power source must be used in preparation for this. Even if a discharge occurs between the extraction electrodes, it is desirable that the power supply current IDEC does not increase so much, so that a power supply with a smaller capacity can be used.An object of the present invention is to An object of the present invention is to provide a control method for an ion source that prevents the power supply current from increasing too much even if discharge occurs between the two.

[Means for Solving the Problems] The control method of the present invention provides that when a discharge occurs between the extraction electrodes,
Not only acceleration voltage VACC but also arc voltage V□. is dropped to O, and then the acceleration voltage VACC and arc voltage VAR are reduced while maintaining the relationship in which the deceleration power supply current IDEC is minimized.
C is increased from 0 to a specified value. VACo and v that minimize the deceleration power supply current IDEC
A. The relationship depends on the structure and dimensions of the electrodes, the structure and dimensions of the arc chamber, the filament current, the type of gas to be ionized, the pressure, etc., but many conditions are device specific. If you decide the filament current, pressure, etc.
The relationship between VACC and vARc that minimizes IDEC can be determined almost uniquely. That is, the deceleration electrode current IDEC is IDEC= f (VACCI VARCI P+ V
rz) (1). P is the gas pressure within the chamber. Let P and vrz be parameters. Condition that IDEC is minimum f (VAtCI VARCF P) Vex) = t
ln1mum ■ to VAI? c= g (VACC, P, Vr++)
The relationship between VACC and VARC is determined as shown in (3). In reality, parameters such as PlVrz are fixed and VARC and V
Find the ACC relationship. During normal operation, the optimum ion optical system is selected, so the VARC in FIGS. 4(a) and 4(b) and the VACC between iro and heto are ■. is in a relationship that minimizes Suppose that VARC is increased slightly from this state. Arc discharge becomes more active and plasma density increases. Then, the plasma cannot be confined unless the voltage of the positive electrode 2 is increased to increase the electrostatic repulsion. In other words, to minimize IDEC, VACC must be increased. In other words, it can be seen that VACC is a monotonically increasing function with respect to VARC within a certain range. Now consider the limit where NVACC becomes 0. At this time, the electrostatic repulsion against positive ions by the positive electrode 2 becomes zero. To minimize IDFIC, it is necessary to set VARe to 0 and stop arc discharge. From this thought experiment, V
It turns out that if ACC=O, then VARC=O. Then, the relationship between VACC and VARC becomes as shown in FIG. VACC= O1VARC= VA from point O (point N)
As you increase CC, you will reach the point of emphasis through the points ru, wo, and wa. VACCv VARC at this point are the respective values during normal operation. [Operation] If a discharge occurs between the positive and negative electrodes of the extraction electrode system,
While accelerating voltage VACC is set to O, arc voltage VAR is
Set C to O as well. Since the arc discharge stops, the plasma density decreases. The density of positive ions entering between the positive electrode 2 and negative electrode 3 of the extraction electrode system is also low. Therefore, it is less likely that secondary electrons will be generated by colliding with the negative electrode 3. After a certain period of time, the acceleration voltage vAcc is raised again,
vARc is also started from O. Moreover, while maintaining the relationship that minimizes IDEC, is vAl? . and launched VACo. Since IDEC is minimized, the current flowing through the acceleration power source 8 and the deceleration power source 8 is small. It is not twice as much as normal operation, but is about the same as normal operation. Therefore, it is possible to use a power source with a small output capacity as the acceleration power source 8 and the deceleration power source 9. Since a small-capacity power supply can be used, it is advantageous in terms of space and weight, and can also reduce costs. Particularly, in the present invention, the acceleration voltage VACC% acceleration power supply current IA. . is effective when is large. For example, this is effective when vAcc ≧ 5kV 1^cc i:a 100tsA.

【Example】

An ion beam apparatus according to an embodiment of the present invention will be explained with reference to FIG. Arc chamber 1 is a cylindrical vacuum vessel. The gas to be ionized is introduced into this. One side of the arc chamber 1 is open and provided with an extraction electrode system. The extraction electrode system consists of three electrodes made of porous or single-hole metal plates.
It is a stack of sheets. Positive electrode 2, negative electrode 3, ground electrode 4
It's getting better. At the other end of the arc chamber 1 is a cathode filament 5.
is provided. This is heated by the filament power supply 6. An arc voltage VARC is applied by an arc power supply 7 so that the cathode filament 5 serves as a cathode and the arc chamber 1 serves as an anode. The positive electrode 2 and the arc chamber 1 are at the same potential, and an accelerating voltage VAC6 is applied by an accelerating power source 8. A deceleration voltage VDEC is applied to the negative electrode 3 by a deceleration power supply 9. The ground electrode 4 is Ov because it is grounded. The above configuration is the same as that in FIG. In the present invention, a VARCvAcc controller 10 is further provided to increase or decrease VARe together with VACo. This is a case where discharge occurs between the positive electrode 2 and the negative electrode 3 of the extraction electrode system and a short circuit current flows. VARC''-VACC controller 10 sets the accelerating voltage VACC to 0 when discharge occurs, but also sets the arc discharge VARC to O.After a certain period of time, the accelerating voltage VACC is raised again, but at the same time the arc voltage V
ARC will also be launched. At this time, the VARC-VACC controller 10 maintains the relationship of minimizing IDEC while
We will launch ACOlVARC. Nu in Figure 5,
Raise these voltages according to the relationships of LE, WO, WA, and WAR. Figure 2 shows the temporal changes in VAR (!s VACC!).VACC in Figure 2 (b) is the same as Figure 4 (b). Assuming that a discharge occurs between the electrode system, VACo is set to 0. At the same time, VARC is also set to 0 (so → tsu) as shown in Figure 2 (a). This is the protective operation at t = to. For a certain period of time. Later, at 1=11, vACC is started (2 → E), but VARC is also started at the same time (N → S). Furthermore, VACC is started up and returned to the normal operating value (E → H). In parallel with that, VARC is started. is also returned to the normal operating value (S → U). After that, it will operate normally (He~t, U~M). In the transition process from 2 to 2, VACC and vA3
゜ should satisfy the relationship shown in Fig. 5. The relationship shown in FIG. 5 can be obtained by determining the value of VARC relative to VACC in advance so that IDEC is minimized. l1jEC
The relationship between VACC and VARC that minimizes is determined in advance and stored in the YARcVACC controller 10.

【Effect of the invention】

Even if a discharge occurs between the positive electrode and the negative electrode of the extraction electrode system, according to the method of the present invention, the accelerating voltage vA. . Excessive current does not flow to the acceleration power supply and deceleration power supply when starting up again. This is because the plasma density is lower and positive ions hit the deceleration electrode, resulting in fewer secondary electrons being generated. Since no excessive current flows, a power source with a small output capacity can be used as an acceleration power source or a deceleration power source. The power source can be made lighter, smaller, and cheaper.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an ion beam apparatus according to an embodiment of the present invention. FIG. 2 is a waveform diagram of the arc voltage VARC1 acceleration voltage vACc according to the control method of the present invention. FIG. 3 is a configuration diagram of a conventional ion beam device. Figure 4 shows the arc voltage VARC according to the conventional control method.
s Waveform diagram of acceleration voltage VACC. FIG. 5 is a graph illustrating the relationship between acceleration voltage VACC and arc voltage VARC that minimizes current IDEC flowing through the deceleration power source. 111. arc chamber, 2. ,, positive electrode 310. negative electrode, 4. , ground electrode 501. Cathode filament, e, filament power supply, 7. ,, arc power supply, 8° acceleration power supply, 9
01. Deceleration power supply, 10. ,,V□. - VACC controller') IC', - Figure (a) Time t

Claims (1)

[Claims] An arc chamber that can be evacuated and provides a space to turn introduced gas into plasma, a cathode filament that is provided in the arc chamber and is energized to generate thermoelectrons, a filament power source that supplies power to the cathode filament, and a cathode. An arc voltage V_A_R_ is applied between the filament as a cathode and the arc chamber as an anode.
An arc power source that applies C to cause arc discharge, a positive electrode, a negative electrode, and a ground electrode that are provided on the open end side of the arc chamber and have a hole through which the ion beam passes, and a positive acceleration that is applied to the positive electrode and the arc chamber. In an ion beam device consisting of an acceleration power source that applies voltage V_A_C_C and a deceleration power source that applies a negative voltage to the negative electrode, the relationship between acceleration voltage V_A_C_C and arc voltage V_A_R_C that minimizes the current I_D_E_C flowing through the deceleration power source is determined in advance. When discharge occurs between the positive electrode and the negative electrode, the accelerating voltage V
Both _A_C_C and arc voltage V_A_R_C are 0.
1. A control method for an ion beam apparatus, characterized in that after a certain period of time, V_A_C_C and V_A_R_C are returned to normal operating values while maintaining a relationship in which I_D_E_C is minimized.