JPS6240816B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a filament heating control device,
In particular, the present invention relates to a heating control device for a filament of an electron gun.

Conventionally, the optimum filament heating power was set based on changes in the emission current. In other words, the emission current changes when the filament heating power is changed, but as shown in Figure 1, the increase in the emission current is small in areas where the filament heating power is small, and as the filament heating power is changed, the emission current changes. As the emission current increases gradually, it gradually saturates. The optimum state for use of the filament is, for example, the point at which the emission current is saturated, as indicated by Wa in the figure. Therefore, the change in the emission current with respect to the filament heating power is determined in advance as shown in FIG. 1, and the optimum emission current Ia with respect to the filament heating power Wa is obtained. Then, while visually checking the emission current meter, the filament heating power is manually increased, and when the emission current reaches a predetermined value, the increase is stopped.
When trying to automatically supply the filament heating power, semi-automation is possible, but complete automation is difficult for the following reasons. That is, if we simply consider an automation method, we can consider a configuration in which the filament heating power is automatically boosted, the emission current at that time is detected, and the boosting is stopped when the emission current matches a reference value. Now, looking at Figure 1 again, each a,
b and c are the cases where the acceleration voltage changes, and
This shows that the relationship between filament heating power and emission current changes when the attachment position of the filament changes due to filament replacement. Therefore, for each of a, b, and c, the optimal filament heating power Wa, Wb, Wc
exist, and the corresponding emission currents Ia, Ib, Ic
will be different for each. Therefore, when the accelerating voltage changes or the mounting position changes, the reference value for comparison described above must be changed manually. In this respect, it can only be called semi-automatic.

The present invention has been made in view of the above-mentioned problems, and its object is to provide a filament heating control device that can automatically set the optimum filament heating power and has good operability.

As a result of studying the relationship between the filament heating power and the emission current shown in FIG. 1, the inventors of the present invention focused on the amount of change in the emission current that changes as the filament heating power increases, that is, the differential value. FIG. 2 is a plot of the relationship between the filament heating power and the differential value of the emission current. Among these, the maximum value of the differential value of the emission current Ida・max, Idb・
max, Idc・max, and calculate these maximum values and the optimal filament heating power Wa, Wb, Wc.
The differential values of the emission current corresponding to Ida, Idb,
I looked at the correlation between Idc. As a result, it was found that the ratio of both differential values was almost constant.

Ida/Ida・max=Idb/Idb・max
=Idc/Idc·max≒K In the above formula, K is a constant.

The present invention has been made based on the above-mentioned knowledge of the inventor of the present application.

An embodiment of the present invention will be described using FIGS. 3 to 6. Thermionic electrons emitted from the filament 1 are accelerated by an electric field created between the filament 1 and the anode 3 by an accelerating voltage 8. The accelerated electron beam 4 is irradiated onto the sample 5. A bias resistor 7 is connected between the filament 1 and Wehnelt 2, and this bias voltage controls the intensity of the electron current generated from the filament to be kept constant.

The microcomputer 15 inputs the filament heating power data via the interface 12.
It is sent to the A converter 11 and converted into an analog quantity. The analog quantity supplies heating power to the filament 1 by means of a power amplifier 10.

On the other hand, the emission current is detected by a detection resistor 9. The voltage across this detection resistor 9 is A/D
A converter 13 converts the data into digital quantities and transfers the data to a microcomputer 15 through an interface 14 .

The operation will be explained with reference to FIGS. 3, 4, 5, and 6.

When the operator presses the automatic filament heating control start switch 16, the microcomputer 1
5, a command is generated to inject heating power of _W1 shown in FIG. 4 into the filament 1, and the filament is heated. Then, an electron beam 4 is emitted from the filament 1. After the emission current changes, the emission current is A/D converted by the A/D converter 13 and the data is transferred to the microcomputer 13. This measured value is set as the emission current _I1 shown in FIG. 5, and this value is stored.

Operate in the same manner, and place the fourth filament on filament 1.
Figure ₅ shows the measured values of the emission current obtained when the heating power of W ₂ shown in the figure is injected.
shall be. As shown in FIG. 4, the heating power is kept constant and gradually increased in set steps.

Here, in the microcomputer, I ₂ and
The difference Id ₁ from I ₁ is calculated and stored.

Id ₁ = I ₂ − I ₁ …(2) If we assume that the series of operations is executed n times, Id ₂ = I ₃ − _{I 2} …(3) Id ₃ = I ₄ − _{I 3} … …(4) 〓 Id _o =I _o+1 −I _o …(5) Information can be obtained. Since the change in heating power is constant, this information represents the differential value of the emission current I.

Figure 6 is a graph of this information.
Assuming that the maximum value among the data from Id ₁ to Id _o is Idmax, the following relational expression holds between the differential value Ide of the emission current in the optimum usage state.

Ide=K·Idmax (6) K is a constant, which is given by the constant setting switch 17 in FIG.

In actual operation, when the filament heating power is gradually increased, the difference value Id of the emission current also increases from Id ₁ to Idmax. Then, when the filament heating power is increased by one step, Id
decreases and Idmax is determined. From this, the stop level Ide is calculated, and the filament heating power is increased until iId becomes Id≦Ide...(7), and when the formula (7) is satisfied, the increase in the filament heating power is stopped. , the filament can be set to optimal usage conditions.

In this embodiment, the heating power for the filament can be optimally set regardless of changes in the emission current due to the attachment position of the filament, acceleration voltage, etc.

In the above embodiment, the increase step of the filament heating power is shown in FIG. 4 as being large, but more precise control can be achieved by making the increase step smaller.

Also, instead of providing a special constant setting switch 17, standard values are stored in the ROM of the CPU,
If necessary, input may be made using the numeric keypad on the operation panel.

In FIG. 4, it is written that the filament heating power is changed from the smallest, but since it is sufficient to find Idmax, it is also possible to gradually increase the filament heating power from a slightly lower filament heating power than the filament heating power that will give Idmax. This has the effect of shortening the time required to set the filament heating power.

In the operation explanation, it was written to memorize all Id ₁ to _{Id o} , but the necessary data among Id ₁ to _{Id o} is
Since Idmax, it is only necessary to always remember the maximum value of the measured Ids. In this way, the amount of data to be stored can be reduced and the memory capacity can be reduced.

According to the present invention, the optimum filament heating power can be automatically set, improving operability.

[Brief explanation of the drawing]

FIG. 1 is a curve diagram showing the relationship between the emission current and the filament heating power, FIG. 2 is a curve diagram for explaining the principle of the present invention, and FIG. 3 is a curve diagram showing an embodiment of the present invention. FIG. 4 to FIG. 6 are explanatory diagrams of the operation of one embodiment of the present invention. 1...Filament, 9...Emission current detection resistor, 11...D/A converter, 13...
A/D converter, 15... microcomputer,
16...Start switch, 17...Constant setting switch.

Claims (1)

[Claims] 1 a filament, means for supplying electric power to the filament and increasing the electric power sequentially, means for detecting an emission current when electric power is supplied to the filament and increasing the electric power sequentially, a reference value based on this maximum value and a constant value substantially representing the ratio of the differential value of the emission current corresponding to the optimum heating power of the filament to the maximum value; and, after detecting the maximum value, stopping the increase in the filament heating power when the differential value of the emission current reaches the reference value.