JP3135864B2 - Ion plasma type electron gun and its control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an ion plasma type electron gun and a control method thereof, and more particularly to an ion plasma type electron gun capable of performing a stable operation in a wide dose range and a control method thereof.

[0002]

2. Description of the Related Art An ion plasma type electron gun generates a plasma of helium gas in a plasma chamber and accelerates helium ions toward a cathode plate arranged through a honeycomb grid. Secondary electrons generated by the collision are accelerated in a direction opposite to that of the helium ions, and emitted to the atmosphere through a honeycomb grid and further through a window foil supported on a window support.

When an object to be irradiated on a sheet is processed by an electron beam, it is desired that the electron beam has a uniform dose over its entire width. In addition, it is desired that the dose can be stably controlled according to the moving speed of the irradiation object.

Conventionally, direct current (DC) control and plasma pulse voltage / current control have been known as techniques for controlling the dose of an electron beam generated in an ion plasma type electron gun.

[0005] DC control is a method of controlling a plasma current so that an electron beam current is constant in an ion plasma type electron gun that extracts an electron beam using DC plasma. Beam current is usually proportional to the current supplied to the cathode by a high voltage power source. Therefore, this is a method of measuring and controlling the current applied to the cathode so that the irradiation amount is uniform and constant.

However, the current applied to the cathode is generated in the plasma chamber and is formed by the sum of incident helium ions incident on the cathode and secondary electrons emitted from the surface of the cathode. The ratio between the emitted electrons and the incident ions, that is, the secondary emission coefficient, depends on the surface condition of the cathode surface that emits electrons. The surface state of the cathode is extremely variable, and merely monitoring the high voltage current does not necessarily control the secondary electron output generated and keep it constant.

The voltage / current control of the plasma pulse is performed by providing a target capable of generating a voltage when secondary electrons collide, and using this output signal to keep the output of the secondary electrons substantially constant. How to control.

If a lower dose is desired, the plasma current needs to be reduced. However, reducing the plasma current tends to reduce the uniformity of the beam width method. From the viewpoint of the uniformity of the dose in the beam width direction, there is a certain threshold, and it is desired that the plasma current be maintained at the threshold or more.

If a high dose is desired, the plasma current needs to be increased. However, when the current value exceeds a certain value, the plasma chamber is contaminated, and the high-voltage chamber is also contaminated. As a result, a high voltage cannot be applied.

For example, when the moving speed of a workpiece to be irradiated with an electron beam is extremely slow, it is desired to lower the plasma current. For the above-mentioned reason, it is necessary to lower the plasma current to a certain threshold value or less. Can not. Therefore, the moving speed of the object can be reduced only to a certain value. Conversely, when the speed of the object to be processed increases, it is necessary to increase the plasma current.

Further, when the speed of the object to be processed changes suddenly, it is difficult to quickly change the dose of the electron beam to follow the speed.

In the plasma chamber, the generation of the secondary electrons on the inner wall of the plasma chamber maintains the plasma generation. Incidentally, secondary electron emission depends on the surface condition of the inner wall of the plasma chamber. This surface condition is extremely variable, and is particularly remarkable at the start of operation. The plasma density changes even with the same plasma current, and this plasma density is directly related to the beam current of the electron beam.

Therefore, simply controlling the current and voltage of the pulsed plasma based on the current value of the electron beam stably controls the irradiation amount or the secondary electron emission amount of the ion plasma type electron gun and keeps it constant. Is difficult to do.

[0014]

As described above,
In the ion plasma type electron gun, it has been difficult to stably change the dose of the electron beam generated in a wide dose range.

An object of the present invention is to provide an ion plasma type electron gun capable of performing a stable operation in a wide dose range.

Another object of the present invention is to provide an ion plasma type electron gun which can ensure the uniformity of the width of an electron beam in a wide dose range.

Still another object of the present invention is to provide an ion plasma type electron gun capable of obtaining a stable dose from the start of operation.

Still another object of the present invention is to provide a method for controlling an ion plasma type electron gun capable of generating an electron beam having good uniformity in the width direction over a wide dose range.

[0019]

According to one aspect of the present invention, there is provided an ion plasma type electron gun for accelerating positive ions in plasma to collide with a cathode and extracting secondary electrons generated from the cathode to the outside. An ion plasma type electron gun having an arithmetic circuit for determining a repetition frequency of a plasma pulse based on a desired dose, and a plasma power supply for applying an anode voltage pulse having a constant pulse width to the plasma generation anode at the repetition frequency. Is provided.

According to another aspect of the present invention, there is provided a method for controlling an ion plasma type electron gun in which positive ions in a plasma are accelerated and collided with a cathode to extract secondary electrons generated from the cathode to the outside. Determining a repetition frequency of a plasma pulse based on a desired dose; and applying an anode voltage pulse having a constant pulse width to the plasma generating anode at the repetition frequency.
A method for controlling a plasma-type electron gun is provided.

The plasma pulse has a constant pulse width,
By using a plasma pulse whose repetition frequency changes, stable operation and control over a wide dose range become possible.

[0022]

FIG. 1 is a sectional view schematically showing the configuration of an ion plasma type electron gun. FIG. 1A shows a cross-sectional view, and FIG. 1B shows a vertical cross-sectional view. Vacuum chamber 11 houses cathode assembly 12 therein. The cathode assembly 12 includes a cathode plate 21 and a cathode cover 22 formed of Mo or the like, and is supported on a flange by a ceramic bushing 24. A high-voltage cable 25 is introduced into the vacuum chamber 11 through the center of the flange, and applies a high negative voltage to the cathode plate 21.

Below the vacuum chamber 11, a plasma chamber 14 is connected. On the upper surface of the plasma chamber 14, a honeycomb-shaped grid 18 in which a number of openings are disposed is arranged so that gas, ions, electrons, and the like can be moved between the plasma chamber 14 and the vacuum chamber 11. . On the bottom of the plasma chamber 14,
Window support 15 with multiple openings similar to grid 18
Are disposed, and a window foil 16 formed of a foil film of Ti, Al, or the like is hermetically disposed on an outer surface thereof. The hermetic sealing is configured using, for example, an O-ring. On the central axis of the plasma chamber 14, a wire anode 19 is arranged.

By operating a vacuum pump 33 such as a turbo molecular pump coupled to the vacuum chamber 11, the inside of the vacuum chamber 11 and the plasma chamber 14 can be evacuated to a vacuum.

Although not shown, the plasma chamber 14
Is provided with a gas inlet provided with a piezo valve so that high-purity He can be introduced into the plasma chamber and the vacuum chamber. Also, a plasma ignition means such as a tungsten hot filament is
It is provided within.

FIG. 1A shows a lead shield 20 arranged so as to surround the electron beam emitted from the window foil 16, a beam stopper 31 for shielding the electron beam in the traveling direction of the electron beam, and a cover. The irradiation object is conveyed by rollers 30a and 30b and rollers 30a and 30b. The web 32 to be irradiated is also shown.

As shown in FIG. 2, a high-voltage power supply 36 of, for example, -200 KV is connected between the housing of the vacuum chamber 11 and the cathode plate 21, and a plasma power supply 35 of, for example, 300 V is connected to the housing of the vacuum chamber 11 and the wire anode 19. Connect between. The housing is connected to ground 37.

High-purity He gas is introduced into the plasma chamber 14 so as to maintain a pressure of, for example, 17 mTorr, and thermal electrons are supplied from plasma ignition means to generate plasma 4.
Generates 0. The plasma 40 is applied to the wire anode 19
Formed around. The grid 18 is grounded,
Since a high negative voltage of, for example, about −200 KV is applied to the cathode plate 21, a high electric field is formed in the space between the grid 18 and the cathode plate 21.

When the He ions generated by the plasma 40 reach the space between the grid 18 and the cathode plate 21 through the grid 18, they are applied by a high electric field, and
Collide with 1. High acceleration energy He is applied to the cathode plate 21.
When ions collide, the cathode plate 21 generates multiplied secondary electrons. For example, when the cathode plate is made of molybdenum, 14 electrons are generated per colliding He ion.

The generated secondary electrons are accelerated in the direction of the grid 18 by the high electric field. 2 transmitted through grid 18
The secondary electrons pass through the plasma 40 and reach the window support 15. The window support 15 is provided with an opening corresponding to the opening of the grid, and the accelerated electrons reach the window foil 16 through the opening of the window support 15.

The window foil 16 has a thickness of, for example, several μm to 20 μm.
It has a thickness of m and is made of a material that can pass electrons. Therefore, an electron beam can be formed by the secondary electrons that have passed through the window foil 16.

FIG. 3 is a circuit diagram showing a driving circuit of the ion plasma type electron gun shown in FIGS. In the figure, an ion plasma type electron gun having a vacuum chamber 11 and a plasma chamber 14 is shown at the center. A plasma power supply 35 is connected to the wire anode 9 in the plasma chamber 14. Further, a high-voltage power supply 36 is connected to the cathode assembly 12 in the vacuum chamber 11.

In the plasma power supply 35, the AC voltage from the AC power supply is supplied to the rectifier REC1 via the constant current source S1.
And a rectified voltage is supplied to the transistor Tr via a smoothing circuit formed by the inductance L1 and the capacitance C1. The voltage that has passed through the transistor Tr is supplied from the positive electrode P to the wire anode 19.

The ground 37 is connected to the shunt resistor SH from the negative electrode N.
1 is connected to the rectifier circuit REC1. The gate of the transistor Tr is controlled by the output signal of the switching circuit SW. The switching circuit SW has a gate open period determined by a pulse signal from the pulse generator PG. The open level of the gate open period is determined by the output signal from the comparator COMP. Comparator COM
P is the voltage of the shunt resistor SH1 representing the plasma current;
Compare with the voltage supplied from the sequencer SEQ.

The sequencer SEQ has a frequency target value T
Give (f) to the pulse generator PG, giving plasma current target value T of the (I _P) to the comparator COMP. Pulse generator PG, based on the given frequency target value T (f), has a constant pulse width t _P, the pulse interval 1 / f
A pulse voltage of (repetition frequency f) is generated.

The comparator COMP compares the measured plasma current I _P with the target plasma current value T (I _P ), and corrects the deviation if the actually measured plasma current deviates from the target plasma current value. Form the output.

The sequencer SEQ includes high voltage current peaks.
Value I_HVTarget value T (I_HV) And averaged within one cycle
High voltage current I_HVavTarget value T (I_HVav) Is set. Step
Pulse width t of plasma pulse_PIs pre-selected to a predetermined value.
Has been selected. For example, t _P= 500 μsec
You. Average high-voltage current target value T (I_HVav) The desired dose
It is set to a value as given. Eye of peak value of high voltage current
The standard value T (I_HV) Is set to a value that guarantees stable operation.
Is set. For example, T (I_HV) = 50 mA
You. Under this condition, the pulse repetition frequency f is

[0038]

It is set according to f = (1 / t _P ) ｛{T (I _HVav ) / T (I _HVav )}.

Thus, the conditions for exciting the plasma in the plasma chamber 14 are set. Helium ions from the plasma chamber 14 toward the cathode assembly 12 emit secondary electrons. Cathode assembly 1
The current flowing through 2 is the sum of the incoming helium ions and the emitted secondary electrons.

The cathode assembly 12 includes a high-voltage power supply 36
Is connected. The high-voltage power supply 36 has a rectifier circuit REC.
Includes a constant current source S2, a shunt resistor SH2, and the like. The current flowing through the shunt resistor SH2 is the current flowing through the cathode assembly 12. Therefore, the shunt resistance S
By measuring the voltage drop due to the current flowing in H2, a high-voltage current can be measured. This high voltage current is referred to as _IHV . The high voltage current I _HV is integrated by the integration circuit INT,
The averaged high voltage current I _HVav is obtained. This averaged high voltage current I _HVav is different from the high voltage current I _HVav.

[0041]

## _EQU2 ## It has a relationship of I _HVav = I _HVav / t _P · f. The sequencer SEQ calculates the high-voltage current I _HV based on the input averaged high-voltage current I _HVav based on this relational expression. This current I _HV does not always coincide with the set high voltage current target value T (I _HVav ) _depending on the state of the cathode surface. Therefore, based on the difference between the set value and the measured value, the corrected plasma current M (I _P ) is set as follows.

[0042]

M (I _P ) = T (I _P ) + a · {T (I _HV ) −I _HV } Here, the coefficient a is experimentally determined. For example, the initial value of the plasma current I _P is set to about 400～1000MA. This calculation is performed, for example, every 1 to 10 seconds. For example, when T (I _HV ) −I _HV is about 10 mA and a = 1, the setting of the modified plasma current M (I _P ) changes by 10 mA every 1 to 10 seconds.

The modified repetition frequency M of the pulse
(F) is set as follows.

[0044]

M (f) = T (f) + b · _ｆT (I _HVav ) −I
_HVav ｝ / T (f) Here, the coefficient b is also determined experimentally. This frequency correction control is performed, for example, in a cycle of 10 to 100 msec.

The reason why the high-voltage current I _HV is integrated and converted into an averaged high-voltage current to correct the plasma current I _P is that there is a time delay in the change of the plasma intensity and the high-voltage current. For example, for a plasma reaction,
There is a time delay of about ec. By using an integrating circuit to absorb the time delay and slowly apply feedback, good accuracy can be achieved.

FIG. 3B shows an example of the waveform of the high voltage current _IHV obtained in this manner. When feedback is directly applied to the plasma power supply using the high-voltage current I _HV without using the integration circuit, hunting may be caused due to the above-described time delay.

FIG. 3C shows a waveform example of the high voltage current _IHV when the high voltage current _IHV is directly fed back to the plasma power supply.

FIG. 4A is a graph showing the uniformity of the dose in the electron beam irradiation width direction. Beam current 120m
A, shows how uniformity is obtained in the beam width direction when changed to 20 mA. The discharge current was 600 mA and the beam voltage was 250 kV. In addition, the speed of the irradiation object is such that the beam current is 20 m.
A was 10 m / min at A, and 24 m / min at 120 mA beam current. Under any conditions, 65c
Good uniformity is obtained over a beam width of m.

FIG. 4B shows the uniformity in the width direction when the dose is controlled by the conventional method for comparison. The discharge current was changed to 400 mA, 600 mA, and 800 mA. The beam voltage was 200 kV, the beam current was 120 mA, and the speed of the web to be irradiated was 24 m /
Minutes. When the discharge current is between 800 mA and 600 mA, good uniformity is obtained in the beam width direction of 65 cm. However, when the discharge current is reduced to 400 mA, the beam width at which a uniform dose is obtained is reduced to 50 cm. are doing.

By comparing FIGS. 4A and 4B, it can be seen that the control method of this embodiment is superior to the conventional control method.

FIG. 5 is a graph showing the followability when the beam current is changed with time. A trapezoidal control signal was input, and the beam current was changed according to the control signal. The dotted line indicates the control signal, and the solid line indicates the measured beam current value.

As shown in the figure, it can be seen that the beam current changes quickly according to the change in the condition.

The present invention has been described in connection with the preferred embodiments.
The present invention is not limited to these. For example, it will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

[0054]

As described above, according to the present invention,
An ion-plasma electron gun that can perform stable operation over a wide dose range by applying an excitation voltage with a repetition frequency that changes based on the desired dose to the ion-plasma electron gun and repeatedly generating a plasma pulse with a constant pulse width You get a gun.

[Brief description of the drawings]

FIG. 1 is a sectional view schematically showing a configuration of an ion plasma type electron gun according to an embodiment of the present invention.

FIG. 2 is a sectional view schematically showing a configuration of an ion plasma type electron gun according to an embodiment of the present invention.

FIG. 3 is an electric circuit diagram showing a circuit configuration of the ion plasma type electron gun shown in FIGS.

FIG. 4 is a graph showing the uniformity in the beam width direction obtained by this embodiment in comparison with the uniformity in the beam width direction according to the conventional method.

FIG. 5 is a graph showing followability to a beam current change command.

[Explanation of symbols]

 Reference Signs List 11 vacuum chamber 12 cathode assembly 14 plasma chamber 15 window support 15a main body of window support 15b low sputter rate metal layer on window support surface 16 window foil 18 grid 19 wire anode 20 lead shield 21 cathode plate 22 cathode cover 24 Bushing 25 High-voltage cable 30 Roller 31 Beam stopper 32 Web 33 Vacuum pump 35, 36 Power supply 37 Ground 40 Plasma

Claims (6)

(57) [Claims] An ion plasma type electron gun for accelerating positive ions in plasma to collide with a cathode and extracting secondary electrons generated from the cathode to the outside, wherein a repetition frequency of a plasma pulse is determined based on a desired dose. And a plasma power supply for applying an anode voltage pulse having a constant pulse width to the plasma generating anode at the repetition frequency. 2. The plasma processing apparatus according to claim 1, wherein the arithmetic circuit is capable of setting a plasma current target value during plasma excitation, wherein the plasma power supply is means for measuring a plasma current flowing through the plasma circuit; Means for correcting the anode voltage pulse based on the difference. And a means for applying a high negative voltage to the cathode to measure a cathode current flowing through the cathode, and a means for averaging the measured cathode current to generate a signal representing the average cathode current. 3. The ion plasma type electron gun according to claim 2, wherein said arithmetic circuit controls said plasma current target value based on said average cathode current. 4. A method for controlling an ion plasma type electron gun for accelerating positive ions in plasma to collide with a cathode and extracting secondary electrons generated from the cathode to the outside, wherein a plasma pulse is generated based on a desired dose. Determining a repetition frequency of: and applying an anode voltage pulse having a constant pulse width to the plasma generating anode at the repetition frequency. 5. A step of setting a target value of a plasma current during plasma excitation, a step of measuring a plasma current flowing through a plasma circuit, and an anode based on a difference between the measured plasma current and the target value of the plasma current. 5. The method according to claim 4, further comprising the step of correcting the voltage pulse. 6. A step of applying a negative high voltage to the cathode to measure a cathode current flowing through the cathode, averaging the measured cathode currents to generate a signal representing an average cathode current, and 6. The method according to claim 5, further comprising the step of controlling the target value of the plasma current based on a cathode current.