JP7126916B2 - ION GUN CONTROL DEVICE AND ION GUN CONTROL METHOD - Google Patents

Description

The present invention relates to an ion gun control device and an ion gun control method.

An ion gun that irradiates an ion beam to treat the surface of a substrate is known (see, for example, Patent Document 1). The ion gun has a cathode and an anode. The cathode includes a pair of magnetic pole members spaced apart to form a magnetic field in the gap formed by the pair of magnetic pole members. The anode is positioned so that an electric field is formed in a direction perpendicular to the direction in which the pair of magnetic pole members face each other. The ion gun applies a voltage between the electrodes while flowing gas through the gap formed by the cathode, thereby generating a magnetron discharge and irradiating the target with an ion beam.

JP 2012-164677 A

However, the ions generated in the gap sputter each magnetic pole member that constitutes the cathode, thereby reducing the irradiation amount of the ion beam.
SUMMARY OF THE INVENTION It is an object of the present invention to provide an ion gun control device and an ion gun control method capable of suppressing a decrease in the dose.

A control device for an ion gun for solving the above-mentioned problems includes a first electrode composed of a pair of magnetic pole members having opposite magnetism and positioned with a gap therebetween, and a second electrode facing the first electrode. and supplying a gas from between the electrodes to between the magnetic pole members during a constant voltage period in which a predetermined voltage is applied between the first electrode and the second electrode. The ion gun control device for controlling the driving of the ion gun so as to ionize the gas flow rate so that the detected value of the current flowing between the first electrode and the second electrode becomes a target value A gas control unit that repeats control to compensate for the decrease in the detected value by increasing the gas flow rate at the end of the constant voltage period relative to the gas flow rate at the start of the constant voltage period within the constant voltage period. Prepare.

A control method for an ion gun for solving the above-mentioned problems includes a first electrode composed of a pair of magnetic pole members having opposite magnetism and positioned with a gap therebetween, and a second electrode facing the first electrode. and supplying a gas from between the electrodes to between the magnetic pole members during a constant voltage period in which a predetermined voltage is applied between the first electrode and the second electrode. In the ion gun control method for controlling the driving of the ion gun so as to ionize the gas flow rate so that the detected value of the current flowing between the first electrode and the second electrode becomes a target value repeating control to compensate for the decrease in the detected value by increasing the gas flow rate at the end of the constant voltage period relative to the gas flow rate at the start of the constant voltage period within the constant voltage period. .

As the constant voltage period progresses, the gap between the magnetic pole members forming the first electrode widens. The expansion of the gap between the magnetic pole members causes a decrease in ion density between the magnetic pole members, that is, a decrease in the dose of the ion beam. In this regard, according to the above configurations, the decrease in the detected value is compensated for by increasing the gas flow rate so that the detected value reaches the target value, and the gas flow rate is constant within the constant voltage period with respect to the gas flow rate at the start of the constant voltage period. The gas flow rate at the end of the voltage period increases. Therefore, the decrease in the current value is suppressed by the increase in the gas flow rate, thereby suppressing the decrease in the irradiation dose due to the widening of the interval.

The ion gun control device further includes a voltage control unit for increasing the voltage applied between the first electrode and the second electrode immediately after the current constant voltage period ends to start the next constant voltage period. The voltage control unit may include, as a condition for ending the current constant voltage period, estimating that the width of the gap is equal to or greater than a period reference value.

Increasing the width of the gap in the pole members reduces the ion density between the pole members and increases the width that the ion beam has. In this respect, according to the above configuration, the voltage applied between the electrodes increases when the width of the gap is estimated to be equal to or greater than the period reference value. That is, the ion velocity is increased when the width of the gap is estimated to be equal to or greater than the period reference value. As a result, it is possible to suppress the expansion of the width of the ion beam in the irradiation target due to the width of the gap being equal to or greater than the period reference value.

In the above-described ion gun control device, the voltage control unit includes power consumption calculated from the voltage applied between the first electrode and the second electrode, the detected value, and the usage time of the ion gun. The width of the gap may be estimated to be greater than or equal to the period reference value when the amount is greater than or equal to a threshold. According to this ion gun control device, it is also possible to easily estimate whether or not the width of the gap is equal to or greater than the period reference value.

In the ion gun control device described above, the voltage control unit includes a storage unit that stores a plurality of mutually different threshold values and voltage setting values associated with the respective threshold values, and the width of the gap is the period reference. In estimating whether or not it is greater than or equal to the value, a threshold value that is greater than the previous constant voltage period is used in the current constant voltage period, and the set value associated with the threshold value used in the current constant voltage period is used in the next constant voltage period. It may be used for the voltage period. According to this ion gun control device, since the power consumption thresholds are used in ascending order, even if the constant voltage period continues for three or more times, it is possible to obtain the above effect in each constant voltage period. It also becomes.

In the ion gun control device described above, the gas control unit includes a storage unit that separately stores the relationship between the target value and the gas flow rate for each set value, and the voltage control unit stores The gas flow rate may be controlled based on the relationship associated with the set values used in .

The current flowing between the first electrode and the second electrode increases as the voltage increases. In this regard, according to this configuration, the increase in current accompanying the increase in voltage is corrected by supplying the gas flow rate based on the relationship stored in the storage unit. As a result, when the voltage is switched between the previous constant voltage period and the current constant voltage period, it is possible to suppress the deviation of the irradiation amount caused by the voltage increase.

FIG. 1 is a configuration diagram showing the configuration of an embodiment of an ion gun control device together with an ion gun. FIG. 2 is a block diagram showing an electrical configuration of an ion gun control device; FIG. 5 is a diagram showing the relationship between thresholds and set values in voltage setting data; 4 is a graph showing the relationship between gas flow rate and power consumption in gas flow rate setting data; 4 is a flow chart showing a procedure of voltage control included in the ion gun control method.

An embodiment of an ion gun control device and an ion gun control method will be described below.
[ion gun]
As shown in FIG. 1, the ion gun 10 comprises a first electrode 20, a second electrode 30, a magnet 40 and a gas unit 60. As shown in FIG.

The first electrode 20 is composed of a pair of magnetic pole members positioned with a gap 23 therebetween. A pair of magnetic pole members is composed of a first magnetic pole member 21 and a second magnetic pole member 22 . The first magnetic pole member 21 has a rail shape that extends in a direction perpendicular to the plane of the drawing and opens upward. The second magnetic pole member 22 has a plate-like shape that extends in a direction perpendicular to the plane of the drawing and partially fills the opening.

The magnet 40 has a columnar shape extending along the extending direction of the second magnetic pole member 22 . Magnet 40 is magnetically connected to first magnetic pole member 21 and second magnetic pole member 22 . The magnets 40 impart opposite polarities to the separate pole members 21 , 22 and produce a magnetic field in the gap 23 between the first and second pole members 21 , 22 .

The first electrode 20 and the magnet 40 define a discharge space 24 inside the first magnetic pole member 21 . A second electrode 30 is positioned within the discharge space 24 . An insulating material 31 electrically insulates between the first electrode 20 and the second electrode 30 . The second electrode 30 is electrically connected to the power supply unit 50 . The power supply unit 50 and the first electrode 20 are connected to ground potential.

The gas unit 60 communicates with the discharge space 24 through the gas flow path 61 . The gas unit 60 and the power supply unit 50 are electrically connected to an ion gun control device 70 (hereinafter also referred to as the control device 70).

[Ion gun controller]
As shown in FIG. 2 , the power supply unit 50 includes a power supply 51 and a current monitor 52 . The gas unit 60 includes a gas supply source 61S and a mass flow controller (MFC62).

The power supply 51 applies a predetermined DC voltage between the first electrode 20 and the second electrode 30 . The second electrode 30 is an anode to which a positive potential is applied. The first electrode 20 is a cathode connected to ground potential. A period during which the power supply 51 applies a constant voltage is a single constant voltage period. The power supply 51 changes the voltage step by step according to the command from the control device 70 . The control device 70 causes the power supply unit 50 to output a predetermined voltage different from the voltage applied during the current constant voltage period, thereby starting the next constant voltage period.

A current monitor 52 detects the current output by the power supply 51 . The current monitor 52 is connected to a power supply circuit that applies a voltage between the electrodes 20 and 30 and detects current flowing through the power supply circuit as current flowing between the electrodes 20 and 30 .

The gas unit 60 introduces gas into the discharge space 24 through the gas flow path 61 . The introduced gas causes magnetron discharge due to the electric field generated between the first electrode 20 and the second electrode 30 and the magnetic field generated between the magnetic pole members 21 and 22 . The gas ions generated by the magnetron discharge are accelerated by the electric field applied between the first electrode 20 and the second electrode 30 and irradiated as an ion beam from the gap 23 .

The control device 70 includes a gas control section 71 and a voltage control section 72 . The current monitor 52 outputs the detected value Ia of the current monitor 52 to the gas control section 71 and the voltage control section 72 . The current monitor 52 matches the detection value Ia used by the gas control unit 71 for processing with the detection value Ia used by the voltage control unit 72 for processing.

The voltage control unit 72 includes a voltage control processor 72P and a voltage control memory 72M, which is an example of a storage unit.
The voltage control processor 72P is not limited to processing all of the various processes by software. For example, the voltage control processor 72P may include dedicated hardware (application specific integrated circuit: ASIC) that performs at least some of the various types of processing. The voltage control processor 72P is configured as a circuit including one or more dedicated hardware circuits such as ASICs, one or more processors (microcomputers) that operate according to computer programs (software), or combinations thereof. .

The voltage control memory 72M stores the initial value of the set value Vset, the power consumption Pa up to the present, and the voltage setting data D1. The initial value of the set value Vset is used when the ion gun 10 is unused. The power consumption Pa up to the present is the power consumed up to the present by the ion gun 10 used this time. The power consumption Pa up to now is used when the ion gun 10 that has been used once is used continuously.

As shown in FIG. 3, the voltage setting data D1 associates each threshold Pth of the power consumption Pa with the set value Vset. The threshold Pth included in the voltage setting data D1 is a plurality of mutually different thresholds P0, P1, P2, P3, and P4. The set value Vset included in the voltage setting data D1 is a plurality of mutually different set values V1, V2, V3, V4, and V5. Each set value Vset is the value of the voltage applied between the first electrode 20 and the second electrode 30 . Each set value Vset increases as the threshold value Pth corresponding to the set value Vset increases.

For example, the threshold value P0 is the initial value of the threshold value Pth used when the ion gun 10 is started to be used. The set value V1 associated with the threshold value P0 is the set value Vset used in the next constant voltage period. Threshold P1 is greater than threshold P0. A set value V2 associated with the threshold value P1 is greater than the set value V1. Threshold P2 is greater than threshold P1. The set value V3 associated with the threshold P2 is greater than the set value V2.

Prior to starting the current beam irradiation process, the voltage control processor 72P specifies the set value Vset at the start of the process. For example, the voltage control processor 72P reads the initial value of the set value Vset when the ion gun 10 is unused, and specifies the read initial value as the set value Vset at the start of processing. Also, the voltage control processor 72P reads the power consumption Pa when the ion gun 10 that has been used once is used continuously. The voltage control processor 72P refers to the voltage setting data D1, extracts the maximum value from among the thresholds Pth smaller than the read power consumption Pa, and converts the set value Vset associated with the maximum value to the voltage setting data D1. Read from data D1. That is, the voltage control processor 72P identifies the set value Vset that was set in the previous constant voltage period as the Vset at the start of the current process.

Returning to FIG. 2, the gas control unit 71 includes a gas control processor 71P and a gas control memory 71M, which is an example of a storage unit.
The gas control processor 71P is not limited to processing all of the various processes by software. For example, the gas control processor 71P may include dedicated hardware (application specific integrated circuit: ASIC) that performs at least part of the various types of processing. The gas control processor 71P is configured as a circuit including one or more dedicated hardware circuits such as ASICs, one or more processors (microcomputers) that operate according to computer programs (software), or combinations thereof. .

The gas control memory 71M stores the target value Ip and the gas flow rate setting data D2. The target value Ip is the target current value I in the current beam irradiation process. The target value Ip is externally input to the controller 70 prior to the current beam irradiation process. The gas flow rate setting data D2 includes a control gain DP used for PID control of the gas flow rate. The gas flow rate setting data D2 indicates the relationship between the value of the current flowing between the electrodes 20 and 30 (current value I) and the initial value Q of the flow rate. The initial flow rate value Q is the gas flow rate used at the start of the constant voltage period.

As shown in FIG. 4, the relationship indicated by the gas flow rate setting data D2 is such that the current value I increases as the flow rate initial value Q increases. The gas flow rate setting data D2 stores the relationship between the current value I and the flow rate initial value Q for each set value Vset. The gas flow rate setting data D2 separately stores the relationship between the current value I and the flow rate initial value Q for each set value Vset.

For example, the gas flow rate setting data D2 shows the relationship that the higher the flow rate initial value Q, the larger the current value I with respect to the set value V1. The gas flow rate setting data D2 indicates that the higher the flow rate initial value Q, the larger the current value I with respect to the set value V2, and gives a larger gradient than the relationship at the set value V1. Further, the gas flow rate setting data D2 shows the relationship that the higher the flow rate initial value Q, the larger the current value I with respect to the setting value V3, and gives a larger gradient than the relationship at the setting value V2.

Prior to the start of the current constant voltage period, the gas control processor 71P refers to the set value Vset in the current constant voltage period, and reads the gas flow rate setting data D2 associated with the current set value Vset. The gas control processor 71P sets the initial flow rate associated with the current value I corresponding to the target value Ip based on the read gas flow rate setting data D2 so that the target value Ip is obtained in the current constant voltage period. Identify the value Q.

The gas control processor 71P repeats the adjustment of the gas flow rate at a predetermined control cycle throughout the current constant voltage period. In adjusting the gas flow rate, the gas control processor 71P adjusts the gas flow rate in the current control cycle and the detected value Ia in the current control cycle so that the detected value Ia rises toward the target value Ip and converges. , the gas flow rate in the next control cycle is calculated. The gas control processor 71P controls driving of the MFC 62 so that the calculated gas flow rate is supplied. The gas control processor 71P compensates for the decrease in the detected value Ia by increasing the gas flow rate.

The voltage control processor 72P updates the power consumption Pa. In the update process, from the detected value Ia in the current constant voltage period, the voltage set value Vset in the current constant voltage period, and the time the ion gun was used in the current constant voltage period, the current constant voltage period The power consumption Pa of the ion gun at is calculated. The voltage control processor 72P updates the power consumption Pa at the above control cycle.

The voltage control processor 72P monitors whether the updated power consumption Pa is less than the current threshold Pth. Monitoring of the power consumption Pa is estimation of whether or not the width of the gap 23 is equal to or greater than the period reference value. When the voltage control processor 72P determines that the power consumption Pa is equal to or greater than the current threshold Pth, it reads the voltage setting value Vset associated with the current threshold Pth based on the voltage setting data D1. The voltage control processor 72P controls driving of the power supply 51 so that a voltage corresponding to the read set value Vset is output, thereby starting the next constant voltage period.

[Action]
A method of controlling the ion gun 10 by the controller 70 will be described below.
The voltage control unit 72 specifies, for example, the set value Vset corresponding to the current power consumption Pa prior to starting the beam irradiation process. The gas control unit 71 reads gas flow rate setting data D2 corresponding to the setting value Vset specified by the voltage control unit 72 . The gas control unit 71 specifies the flow rate initial value Q corresponding to the current target value Ip based on the read gas flow rate setting data D2. The gas control unit 71 controls driving of the MFC 62 to start gas supply at the specified flow rate initial value Q. FIG. The voltage control unit 72 controls driving of the power supply 51 to start applying voltage at the specified set value Vset. The ion gun 10 generates ions in the gap 23 between the magnetic pole members 21 and 22 and starts irradiating the target with the ion beam.

Continuation of the ion beam irradiation causes the magnetic pole members 21 and 22 to be sputtered by the discharge to widen the width of the gap 23 . As the width of the gap 23 widens, the ion density in the gap 23 decreases, which reduces the current flowing between the electrodes 20 and 30, resulting in a decrease in the amount of ions contained in the ion beam.

In this regard, the control device 70 repeats control to compensate for the decrease in the detected value Ia by increasing the gas flow rate so that the detected value Ia becomes the target value Ip. Then, within the constant voltage period, the gas flow rate at the end of the constant voltage period increases with respect to the gas flow rate at the start of the constant voltage period. That is, the control device 70 increases the flow rate of the gas, thereby performing control to suppress the decrease in the detected value Ia.

For example, the gas control processor 71P calculates the difference between the detected value Ia and the target value Ip for each control cycle. The gas control processor 71P calculates the gas flow rate in the next control cycle so that the difference becomes smaller, and controls the driving of the MFC 62 so that the calculated gas flow rate is supplied. In the control of the gas flow rate such that the detected value Ia becomes the target value Ip, the gas flow rate may repeat an increase and a decrease in each control cycle. However, in the constant voltage period sufficiently longer than the control cycle, the gas flow rate at the end of the constant voltage period increases with respect to the gas flow rate at the start of the constant voltage period.

Further continuation of the ion beam widens the gap 23 and widens the width that the ion beam has. The ion beam with an expanded width reduces the density of the amount of ions reaching the target.

In this regard, the control device 70 regards an increase in the power consumption Pa as an increase in the width of the gap 23, and increases the set value Vset in response to the power consumption Pa becoming equal to or greater than the threshold value Pth, thereby Control to increase the velocity of the ions in the direction.

Specifically, as shown in FIG. 5, the voltage control processor 72P sets the voltage setting value Vset to the setting value V1 at the start of the beam irradiation process, and sets the threshold value P0 associated with the setting value V1 to the threshold value Pth. (step S11). The voltage control processor 72P uses the set value Vset in the current control cycle, the detected value Ia in the current control cycle, and the length of the control cycle to control the power consumption Pa from the start of use of the ion gun to the present. It is updated for each cycle (step S12).

The voltage control processor 72P determines whether or not the updated power consumption Pa is greater than or equal to the threshold value Pth in the current constant voltage period (step S13). The voltage control processor 72P updates the power consumption Pa and controls the relationship between the updated power consumption Pa and the current threshold Pth until the updated power consumption Pa becomes equal to or greater than the threshold Pth in the current constant voltage period. A comparison is made (No in step S13).

When the updated power consumption Pa becomes equal to or greater than the threshold value Pth in the current constant voltage period (Yes in step S13), the voltage control processor 72P refers to the voltage setting data D1 and sets the set value Vset to the current constant voltage period. The set value Vset associated with the threshold value Pth in the period is changed. That is, the voltage control processor 72P starts the next constant voltage period. In addition, the voltage control processor 72P updates the threshold value P1, which is one step higher than the threshold value Pth in the current constant voltage period, as a new threshold value Pth, and repeats the processing of steps S12 and S13 again (step S14). ).

Here, the electrodes 20 and 30 used for discharge also serve as electrodes used for accelerating the ion beam. The voltage increase performed to accelerate the ion beam increases the current flowing through the power supply circuit.

In this regard, the gas control processor 71P, when increasing the voltage from the set value Vset in the current constant voltage period to the set value Vset in the next constant voltage period, corresponds to the set value Vset in the next constant voltage period. Read the gas flow setting data D2. Then, the gas control processor 71P calculates the initial flow rate value Q corresponding to the target value Ip based on the read gas flow rate setting data D2 so that the target value Ip can be obtained in the next constant voltage period. Then, the gas control processor 71P once lowers the flow rate initial value Q at the start of the next constant voltage period.

As described above, according to the above embodiment, the following effects can be obtained.
(1) A decrease in the detected value Ia is compensated for by increasing the gas flow rate so that the detected value Ia becomes the target value Ip. Within the constant voltage period, the gas flow rate at the end of the constant voltage period is increased relative to the gas flow rate at the start of the constant voltage period. As a result, the gas flow rate is increased so that the detected value Ia approaches the target value Ip, so that it is possible to suppress a decrease in the ion beam dose.

(2) When the width of the gap 23 is estimated to be equal to or greater than the period reference value, the voltage applied between the electrodes 20 and 30 is increased to increase the velocity of the ions. As a result, it is possible to suppress the expansion of the width of the ion beam in the irradiation target due to the width of the gap 23 being equal to or greater than the period reference value.
(3) Since the width of the gap 23 is estimated from the power consumption Pa, it can be easily estimated whether the width of the gap 23 is equal to or greater than the period reference value.

(4) Since the threshold value Pth of the power consumption Pa is used in ascending order, even when the constant voltage period continues for three or more times, the same effects as (1) to (3) are obtained in each constant voltage period. is obtained.

(5) When the next constant voltage period starts, the increase in current due to the increase in voltage is corrected by supplying gas flow rate. As a result, when the voltage is switched between the previous constant voltage period and the current constant voltage period, it is possible to suppress the deviation of the irradiation amount caused by the voltage increase.

It should be noted that the above embodiment can also be implemented with the following changes.
- The gas flow rate setting data D2 can be changed to a configuration in which each set value Vset is associated with the flow rate initial value Q one by one. Even if there is one flow initial value Q for each set value Vset, it is possible to obtain the effects according to the above (1) to (4) as long as the target value Ip is constant. be. As described in the above embodiment, if the gas flow rate setting data D2 includes one relationship between the current value I and the flow rate initial value Q for each set value Vset, the target value Ip is mutually It is possible to obtain an appropriate flow rate initial value Q for two or more different types of beam irradiation processing.

The voltage control memory 72M may store a map in which the gas flow rate and the detected value Ia are associated with the width of the gap 23 for each set value Vset. Then, the voltage control processor 72P reads the map corresponding to the set value Vset of the current constant voltage period, applies the gas flow rate and the detected value Ia in the current control cycle to the read map, and can be estimated. That is, the estimation of whether the width of the gap 23 is equal to or greater than the period reference value can be performed based on the gas flow rate, the set value Vset, and the detected value Ia.

The estimation of whether or not the width of the gap 23 is equal to or greater than the period reference value is not limited to the estimation using various parameters, but is an estimation based on image processing using the imaging results of the camera that captures the gap 23. good too.

Ia... detected value, Ip... target value, Pa... power consumption, Pth, P0, P1, P2, P3, P4... threshold value, Vset, V1, V2, V3, V4, V5... set value, 10... ion gun, Reference Signs List 20 First electrode 21 First magnetic pole member 22 Second magnetic pole member 23 Gap 30 Second electrode 40 Magnet 50 Power supply unit 60 Gas unit 70 Control device 71 ... gas control section, 72 ... voltage control section.

Claims (5)

Using an ion gun comprising a first electrode composed of a pair of magnetic pole members having opposite magnetism and positioned with a gap therebetween, and a second electrode facing the first electrode, the first electrode and Ions for controlling driving of the ion gun so as to ionize the gas by supplying gas from between the electrodes to between the magnetic pole members during a constant voltage period in which a predetermined voltage is applied between the second electrode and the second electrode. A control device for a gun,
Repeating control for compensating for a decrease in the detected value by increasing the gas flow rate so that the detected value of the current flowing between the first electrode and the second electrode reaches a target value; A gas control unit that increases the gas flow rate at the end of the constant voltage period relative to the gas flow rate at the start of the voltage period ,
further comprising a voltage control unit that increases the voltage applied between the first electrode and the second electrode immediately after the end of the current constant voltage period to start the next constant voltage period,
The voltage control unit includes, as a condition for ending the current constant voltage period, estimating that the width of the gap is equal to or greater than a period reference value.
Ion gun controller. When the power consumption calculated from the voltage applied between the first electrode and the second electrode, the detected value, and the usage time of the ion gun reaches a threshold value or more, The ion gun control device according to claim 1 , wherein the width of the gap is estimated to be equal to or greater than the period reference value. The voltage control unit includes a storage unit that stores a plurality of mutually different threshold values and voltage setting values associated with the respective threshold values, and determines whether or not the width of the gap is equal to or greater than the period reference value. In the estimation, a threshold larger than the previous constant voltage period is used in the current constant voltage period, and a set value associated with the threshold used in the current constant voltage period is used in the next constant voltage period. A controller for the described ion gun. The gas control unit includes a storage unit that stores the relationship between the target value and the gas flow rate separately for each set value, and is associated with the set value used by the voltage control unit for the current constant voltage period. 4. The ion gun control device according to claim 3, wherein gas flow rate is controlled based on said relationship. Using an ion gun comprising a first electrode composed of a pair of magnetic pole members having opposite magnetism and positioned with a gap therebetween, and a second electrode facing the first electrode, the first electrode and Ions for controlling driving of the ion gun so as to ionize the gas by supplying gas from between the electrodes to between the magnetic pole members during a constant voltage period in which a predetermined voltage is applied between the second electrode and the second electrode. A method of controlling a gun, comprising:
Repeating control for compensating for a decrease in the detected value by increasing the gas flow rate so that the detected value of the current flowing between the first electrode and the second electrode reaches a target value; increasing the gas flow rate at the end of the constant voltage period relative to the gas flow rate at the beginning of the voltage period ;
further comprising increasing the voltage applied between the first electrode and the second electrode immediately after the end of the current constant voltage period to start the next constant voltage period, and ending the current constant voltage period. The condition includes estimating that the width of the gap is greater than or equal to the period reference value
Ion gun control method.

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