JPS607051A - Method of controlling beam current - Google Patents

Abstract

PURPOSE:To perform working with high accuracy and good reproducibility by controlling a beam current by comparing a set level with difference between a current flowing in a beam-detecting circuit during a period when a beam current after its rising is maintained at a constant state and a current flowing in the detecting circuit during a period when the beam current does not flow. CONSTITUTION:An electron beam 54 from an electron gun consisting of a cathode or filament 51, a grid 52 and an anode 53 is accelerated by an accelerating power source 34 before being irradiated upon a substance 55. The beam 54 is detected by a beam-current-detecting circuit 35. Voltage applied to the grid 52 undergoes feedback control by taking out a d.c. voltage almost proportional to bias voltage by means of a transformer 2 and a rectifying smoothing circuit 3 and comparing the above d.c. voltage with signal wave form 4 sent from a bias- voltage-adjusting circuit 31. The circuit 31 compares a set level (Ibc) with an actual level (Iba) which is obtained from difference between a current flowing in the circuit 35 during a period when a beam current after its rising is maintained at a constant state and a current flowing in the circuit 35 during a period when the beam current does not flow.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a beam current control method for an electron beam apparatus that irradiates a workpiece with a high-speed electron beam to perform processing such as drilling, welding, and heat treatment.

In this type of electron beam apparatus, the beam current is generally controlled to perform desired processing, but in order to uniformly process a large number of workpieces without missing pieces, it is necessary to maintain good accuracy and reproducibility of the beam current. Therefore.

An automatic beam current control circuit that detects the beam current value, compares it with the initial value, and performs feedback control to eliminate the deviation is often used. However, the beam current supply source, the electron beam acceleration power supply, is Since a high voltage of 10 kilovolts or more is generated, a considerable amount of noise usually enters the beam current detection circuit. A low-pass filter circuit is used to completely reduce the influence of noise, but this puts a limit on the responsiveness of the automatic beam current control circuit. Therefore, when it is desired to cause the total beam current to rise or fall sharply, a gap may occur. As a way to improve the above responsiveness.

There is a way to raise the input power frequency of the high voltage circuit of the beam accelerating power supply (below the operating frequency) and raise the fundamental frequency of the noise that enters the beam current detection circuit, but in that case, lI
'i A power frequency converter is required, which is disadvantageous in terms of the cost and reliability of the device. Furthermore, leakage current flows through this beam current detection circuit due to high voltage application even when no beam current flows.

Moreover, the magnitude may change gradually, which may impair the accuracy of beam current detection.This invention allows the beam current to rise or fall rapidly, and is relatively inexpensive and highly reliable. The present invention provides a complete beam current control device for an electron beam device. Another object of the present invention is to provide a beam current control system for an electron beam apparatus that has good beam current accuracy and reproducibility and is suitable for processing many workpieces of the same type.

Therefore, in the invention, after the beam current rises, the current flowing through the beam current detection circuit during a part of the period during which the steady state is maintained and the current flowing through the detection circuit during a part of the period during which the beam current does not flow are different. difference.

That is, the beam current control device for an electron beam device detects the net beam armament and compares the detected value with a beam current setting value.

The content of this invention will be described below using figures.

FIG. 1 is an explanatory diagram of a main part of an embodiment of a beam current control device according to the present invention. To fully control the beam current in an electron beam device, the electron beam source, ie the cathode or filament 51. Grid-cathode voltage (grid bias voltage) of an electron gun consisting of a grid 52° and an anode 53
One way is to. 35 is a beam current detection circuit, 34 is an accelerating power source, 55 is an object to be irradiated, and 54 is an electron beam. This figure is an example of a system in which the magnitude of the beam current is completely controlled by this grid bias voltage.
The voltage waveform similar to that in the autotransformer 1 which provides the input for the grid bias electrolytic erosion transformer 57° and the rectifier and smoothing circuit 58 is made approximately proportional to the grid bias voltage by the rectifier and smoothing circuit 3 in addition to the full feedback transformer 2. The DC voltage is output, compared with the signal waveform 4 from the bias voltage setting circuit 31, and the voltage is controlled to follow the signal waveform 4 from the bias voltage setting circuit 31.The bias voltage setting circuit 31 described here is detailed in FIG. In the figure, 61 is a Kuiasumi crush calculation circuit, which calculates the beam current target value 'bc given from the outside.
and accelerating voltage ■9. Then, based on μ and coefficient K of the electron gun that are preset in advance, the bias voltage is calculated by the formula for the beam current of the triode: Fully transformed formula: The bias voltage setting signal V
get go k. Reference numeral 62 denotes a bias voltage correction circuit that corrects the value of K in equation (1) used in the arithmetic circuit 61. That is, before the beam rises.

The voltage of the generator 65 is set to a preset value KO to the coefficient.

The arithmetic circuit 61 calculates Ko after selecting it with the changeover switch 66, but the value of Ko may change slightly due to changes in the characteristics of the electron gun, especially the filament. Therefore, the voltage Vgo actually outputted to the bias control circuit (deviation amplifier 5 and safety risk circuit 6) as the bias voltage, the actual beam current jba detected by the current detection circuit 35, and the actual beam current jba detected by the current detection circuit 35, The value of K of the latest p 3 from the acceleration voltage Vp at the 9th point Kt k Kt=Iba/(V-go+T)X
From the formula, after the beam current rises, the sample hold signal 59 is output from the beam current command signal generator 56 during a part of the period during which the steady state is maintained - 11.12° 13 in Fig. 2. Therefore, a flip-flop 64 is set (with memory) to hold the KlO value kKt' in the sample and hold circuit 63 and to store whether there is data in the sample and hold circuit.
'fK.

Switch from side to 1' side. As a result, the arithmetic circuit 61
The calculation is redone as K:K,', and the difference between the beam current command value Ibc and the actual value Iba is eliminated. Although not shown, the actual value Lba is calculated by subtracting the entire leakage current during the period (14 in FIG. 2) in which no beam current is applied. The data held by the sample hold is held until the primary sample signal 59 is input or the reset 67 is input.If the beam current has a wide variable range and the electron gun characteristics are nonlinear, , If one Kl data cannot cover the entire beam current range, divide the beam current variable into 7 equal parts for each variable range.

Correction circuits 62 each providing one Kl. Sample hold 63. Changeover switch 66, flip-flop 64.
An equal number of preset value generators 65 are prepared and used selectively (not shown).

For example, if you monitor all K values and issue an alarm when the K value goes out of a certain range, you can take appropriate measures to detect a drop in the filament heating current or a poor electron gun setting. You can also get benefits such as being a member of the metal family.

As described here, the deviation amplifier 5 and the thyristor circuit 6 are in a feedback loop in which the value of is updated every moment, and the thyristor circuit 6 is supplied with power from the commercial frequency direct current power source 7. In this way, the grid bias voltage of the electron gun (51-5, 3) can be controlled to resemble signal waveform 4.

In this case, a beam current waveform that changes over time as shown in FIG. 2 is obtained. As mentioned above, the full value of the beam current is detected during the periods 11 and 12°13 during which the beam current reaches a steady value. or 9: A value sampled in synchronization with the ripple shaving wave number to reduce the influence of flow ripple, or a value sampled multiple times to remove noise due to discharge etc., and a value that is significantly different from the previous and subsequent values. Is the value the value you did on the data processing such as excluding it? Use a combination of extraction methods.

As shown above, the feedback control is performed in the form of sampling only in the steady state, and open-loop control is performed during the beam rise and fall periods, so the feedback signal is Due to the discontinuity, there is no possibility of raw edges, overshoots, and undersheets, and the influence of unpredictable leakage current due to high voltage application is also eliminated, and electron beam processing can be performed repeatedly with good reproducibility. .

Furthermore, since the bias voltage feedback sub-crepe shown in Figure 1 is not affected by the high voltage section, it fully exhibits sufficiently fast response and enables rapid beam current rise without increasing the high voltage circuit input power frequency. As described above, the electron beam device using the beam current control circuit of the present invention was able to raise the beam current sufficiently quickly even when a commercial frequency power source was used as the human power source of the high voltage circuit for beam acceleration. ! It has great practical advantages, such as being able to reduce the amount of heat, making the device simple and having good reliability, relatively low cost, and being able to repeat a large number of workpieces.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of main parts showing one embodiment of a beam current control circuit for an electron beam device according to the present invention, FIG. 2 is a waveform diagram showing constant temporal changes in beam current, and FIG. FIG. 3 is a diagram illustrating in detail the bias voltage setting circuit 31 shown in the figure. 1...Single-turn transformer, 2...Transformer, 3
... Rectifier smoothing circuit, 4...1 waveform,
5... - Deviation amplifier. 6...Syrisk #! J, 8, 9, 10...
...Period when the beam current reaches a steady value, 11, 12, 1
3,8° 9. Part of the period within the period when the steady value is reached as shown in 10. 56... Beam current command signal generator, 57...
... Bias transformer, 58 ... Bias rectifier, 51 ... Filament, 52 ...
Grid, 53... Anode, 54...
Electron beam, 55...Electron beam irradiated object, 34.
... Acceleration voltage source, 35 ... Beam current detection circuit, 31 ... Bias voltage setting device. 59...Sample hold signal, 67...
Sample hold reset signal, 61...Bias voltage calculation circuit, 62...Bias voltage correction circuit, 63...Sample hold circuit, 64...
...Flip 7 key, 65... Generator of the initial value KO for the proportional coefficient of the electron gun, 66... K'' changeover switch.

Claims (1)

[Claims] In an electron beam device that processes a workpiece by irradiating it with an electron beam, a beam detection circuit is activated during a part of the period during which a steady state is maintained after the electron beam irradiation is started and the beam current rises. Find the actual value of the beam current, which is the difference between the current flowing through the detection circuit and the current flowing through the detection circuit during a part of the period when no beam current flows. A method for controlling the beam current of an electron beam device in which the actual value and the corresponding beam current initial value are completely compared.