JPH01209642A - Brightness zoom-regulating device for electron microscope - Google Patents

Abstract

PURPOSE:To improve the response by finding a constant from the exciting current of an irradiation lens and the magnification of an expansion lens system at a set brightness, and controlling the exciting current of the irradiation lens responding to the variation of the magnification of the expansion lens system. CONSTITUTION:A constant K is found from the exciting current J of an irradiation lens 1 and the magnification M of an expansion lens system 7 by the first operation device 5, and the exciting current J of the irradiation lens 1 is controlled responding to the variation of the magnification M of the expansion lens system 7 by the second operation device 3. As a result, even though the magnification M of the expansion lens system 7 is converted as desired, the exciting current J of the irradiation lens 1 can be computed from the magnification M, and the exciting current J is controlled to obtain a specific brightness. Since the brightness is zoomed only by the operation-process in such a way, the exciting current J of the irradiation lens 1 can be determined instantly without delay. The response is improved consequently.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an electron microscope that stabilizes the brightness of an image formed on a fluorescent screen by controlling the excitation current of an irradiation lens according to the magnification of a magnifying lens system. The present invention relates to a brightness zoom adjustment device.

[Conventional technology]

FIG. 4 is a diagram showing an example of the configuration of a control system of an electron microscope equipped with a conventional brightness zoom, and 11 is an irradiation lens (CL).
, 12 is an objective lens (OL), and 13 is an intermediate lens (IL).
), 14 is a projection lens (PL), 15 is a fluorescent screen (FS)
, 16 is the magnification setting knob (MS), 17 is the detection circuit (D
T), 18 is a current value measuring circuit, 19 is an initial value storage circuit, and 20 and 21 are amplifiers. 22 is the brightness setting knob (BS
), 23 is a changeover switch (S), and 24 is a sample.

Conventionally, the brightness zoom of an electron microscope has been carried out using a negative detection circuit that connects a detection circuit 17 to a fluorescent screen 15 to detect the brightness on the fluorescent screen 15 as the magnification changes and controls the excitation of the irradiation lens 11, as shown in FIG. A feedback control system is adopted. Detection circuit 17
A signal corresponding to brightness is detected by a current detection element such as CdS provided on or near the fluorescent plate 15. In this example, first, the changeover switch 23
The desired brightness is set by controlling the excitation of the irradiation lens 11 by operating the brightness setting knob 22 while keeping it in the state shown in the figure. Therefore, the detection value of the detection circuit 17 at this set brightness is stored in the initial value storage circuit 19 as an initial value (target value).

If the set brightness is to be maintained, the changeover switch 23 is reversed from the illustrated state, the detection circuit 17 is connected to the current value measurement circuit 18, and the amplifier 20 is connected to the amplifier 21. By switching this circuit connection, the value stored in the initial value storage circuit 19 by the amplifier 20, that is, the set brightness, is compared with the output value of the current value measurement circuit 18, that is, the current brightness detected by the detection circuit 17. . As a result, the comparison output is amplified by the amplifier 21 and the excitation current of the irradiation lens 11 is controlled, so even if the magnification of the magnifying lens system changes, the brightness on the fluorescent screen 15 is stored in the initial value storage circuit 19. The brightness will be controlled according to the set brightness.

[Problem that the invention seeks to solve]

However, in the conventional brightness zoom as described above, the current of the fluorescent screen 15 is as small as 10-h to 10-''A, and in order to detect this current, the detection circuit 17 generally has to have a high impedance. The problem is that there is no.

Therefore, since the detection circuit 17 itself has a relatively long time constant, it takes time to detect brightness and cannot follow rapid changes. Furthermore, since the detection signal is minute, there is a problem in that the detection sensitivity is reduced.

Further, between the sample 24 and the irradiation lens 11, there are conditions for excitation of the irradiation lens with -degree crossover and conditions for excitation of the irradiation lens without crossover. The former is called the over side, and the latter is called the under side. If the excitation is increased on the over side, the beam diameter on the sample will increase, while on the under side it will become smaller. Therefore, even when generating a signal for changing the brightness to a predetermined level from the same dark state, the difference depends on whether the brightness zoom is activated under over or under conditions. In other words, when starting under the over side condition, the irradiation lens 11
The excitation of the irradiation lens 11 must be weakened, whereas the excitation of the irradiation lens 11 must be strengthened when starting under the under condition. Therefore, the variable range of this brightness zoom needed to be limited in that it could only be used within a certain range of conditions.

The present invention solves the above-mentioned problems, and provides a brightness zoom adjustment device for an electron microscope that can instantly set and control the brightness of a fluorescent screen to a predetermined brightness according to the magnification setting. The purpose is to

[Means for solving problems]

To this end, the brightness zoom adjustment device for an electron microscope of the present invention stabilizes the brightness of an image formed on a fluorescent screen by controlling the excitation current of the irradiation lens according to the magnification of the magnifying lens system. The adjustment device includes a first calculation means for calculating a constant by performing a predetermined calculation from the excitation current of the irradiation lens and the magnification of the magnification lens system, and a first calculation means that calculates a constant from the constant and the magnification of the magnification lens system, and a brightness equivalent to that when calculating the constant from the constant and the magnification of the magnification lens system. a second calculation means for calculating the excitation current of the irradiation lens to be obtained; the first calculation means calculates a constant from the excitation current of the irradiation lens at the set brightness and the magnification of the magnifying lens system; The present invention is characterized in that the excitation current of the irradiation lens is controlled in response to changes in the magnification of the magnifying lens system.

[Effect]

In the brightness zoom adjustment device for an electron microscope of the present invention, when the excitation current of the irradiation lens and the magnification of the magnifying lens system are set so as to obtain the desired brightness, and the first calculation means performs a constant calculation, Even if the magnification of the magnifying lens system is arbitrarily changed, the excitation current of the irradiation lens is calculated from the magnification and controlled so as to obtain a predetermined brightness. This is not a negative feedback control system that detects the brightness of the fluorescent screen and feeds it back.
Since brightness zoom is performed using only arithmetic processing, the excitation current of the irradiation lens can be determined immediately without any time delay, improving responsiveness. Furthermore, since the brightness on the fluorescent screen is not directly detected, it can be controlled independently of the brightness detection sensitivity on the fluorescent screen.

〔Example〕

Examples will be described below with reference to the drawings.

FIG. 1 is a diagram showing one embodiment of a brightness zoom adjustment device for an electron microscope according to the present invention, and FIG. 2 is a diagram for explaining the flow of brightness zoom adjustment according to the present invention.

In FIG. 1, 1 is an irradiation lens, 2 is an excitation current setting circuit, 3 is an excitation current calculation control circuit, 4 is a constant storage circuit, and 5
1 is a constant calculation circuit, 6 is a brightness zoom control circuit, 7 is a magnifying lens system, and 8 to 10 are gate circuits. The excitation current setting circuit 2 sets the excitation current of the irradiation lens 1 using, for example, an adjustment knob, and controls the irradiation lens 1 through a gate circuit 8.9. The excitation current calculation control circuit 3 is
The constant k stored in the constant storage circuit 4 is read out, and a predetermined calculation J=g (k,
M) is performed to determine the excitation current J of the irradiation lens 1, and the irradiation lens 1 is controlled through the gate 8.10. The constant calculation circuit 5 calculates a constant k by performing a predetermined calculation =h (J, M) from the excitation current J of the irradiation lens 1 and the magnification M in the magnifying lens system 7, and stores this in the constant storage circuit 4. .

When the brightness zoom control circuit 6 receives a command to turn on the brightness zoom from the operator, for example, it operates the constant calculation circuit 5, controls the gate 9.10, and changes the excitation current of the irradiation lens l to the excitation current setting circuit 2. This is to switch from the excitation current calculation control circuit 3 to the excitation current calculation control circuit 3.

Next, the brightness zoom adjustment according to the present invention will be explained with reference to FIG. First, as shown in FIG. 2(a), the operator sets the magnification lens system to MA, for example, and operates the adjustment knob using the excitation current setting circuit 2 to set the excitation current JA to obtain the desired brightness. do.

Therefore, when a command to turn on the brightness zoom is input, the constant calculation circuit 5 is controlled by the brightness zoom control circuit 6, a constant kA is calculated and stored in the constant storage circuit 4, and the excitation current calculation control is performed from this constant kA and the magnification MA. In circuit 3, excitation current JA is determined. On the other hand, the switching control signal for the gate circuit 9.10 in the zoom control circuit 6 is set to logic "0".
” to logic “1”, the gate circuits 9 and 1O are switched so that the excitation current for the irradiation lens 1 is supplied from the excitation current calculation control circuit 3.

Therefore, if the magnification setting knob MS is used to change the magnification MA to M as shown in FIG. The excitation current of the irradiation lens 1 is controlled to follow the change in magnification.

This excitation current J8 provides the same brightness on the fluorescent screen as shown in FIG. 2 (C1).

In the following, calculation and implementation of the excitation current to obtain the same brightness in response to changes in the magnification M of the magnifying lens system will be described in detail.

FIG. 3 is an irradiation lens ray diagram of sample irradiation before starting the brightness zoom. Here, the distance between the irradiation lens CL and the object surface is a (mu), the distance between the irradiation lens CL and the virtual image plane is b (fin), the distance between the irradiation lens CL and the sample is l (am), and the distance between the irradiation lens CL and the sample is l (am). The magnification of the lens CL is Mc, and the magnification on the sample surface is Ms.

In reality, the image on the sample surface is projected onto a fluorescent screen.

Further, when the focal length of the irradiation lens CL is f (mm), it is expressed as b-f f f (1).

Also, if the magnification of the magnified fluorescent screen FS in the magnifying lens system (OL-PL) is M, then for brightness zoom, Ms-M=k (2). Good. However, k is a value determined by the initial conditions of activation, and becomes a reference value after entering zoom.

On the other hand, the focal length of the irradiation lens CL is approximately expressed as U″ f=25xDx□ (3). However, D (wm) is the Nohole piece constant of the irradiation lens CL, It is expressed by the sum of the gap length of the pole piece and the bore diameter. Also, J (A) is the excitation current of the irradiation lens CL, and U'' (V) is the acceleration voltage relative correction value.

Therefore, by substituting equations (1) and (3) into equation (2), we get k= (--1)M, and the initial conditions include, if the excitation current J and the magnification M of the magnifying lens system are known, p1 It is an arithmetic value that can be simply calculated.

As shown above, the magnification lens system magnification M before starting the brightness zoom
A and the irradiation lens CL excitation current JA, then, the J value after zoom startup (the excitation current value of the irradiation lens CL for zooming), for example, the excitation when the magnification of the magnifying lens system is changed to Ml. The current value J can be given by the zoom target value kA, the magnification M8, and the constant K.

As is clear from the above explanation ■Calculation of kA when brightness zoom is turned on.

■Calculation of J when changing the magnification.

■Excitation by Jl of CL.

If this is satisfied, brightness zoom can be instantaneously performed when changing the magnification. Furthermore, since this calculation takes the sign into account, it is not restricted by the conditions at the time of starting the zoom. That is, this holds true whether the irradiation lens CL is forming a real image or a virtual image.

The problem is that - - is assumed as a light source, but
In reality, it has a Gaussian distribution. However, the condition for starting the brightness zoom is that the image plane of the irradiation lens CL is not on the sample, but is defocused on the sample. Relatively speaking, there is no problem in considering it as a --like light source. Furthermore, (2) the system receives only magnification information from the imaging lens system and processes it through calculations (controlling the irradiation area according to the magnification), so changes in brightness due to absorption within the sample are not taken into account.

However, in -J'IQ, a transmission electron microscope uses a very thin sample, and the ratio of the outgoing electron beam to the incoming electron beam is 95%.
Since the above samples are common, there is no problem in practical use.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, instead of the conventional negative feedback control using a detection circuit with a large time constant and an arithmetic circuit, the magnification can be changed using only the magnification information and the irradiation lens excitation current information at the time of zoom startup. Calculate the appropriate excitation current after
The excitation current can be instantly set to the optimum value.

In particular, since the brightness zoom is performed only by arithmetic processing, rather than by a negative feedback control system that detects brightness in a fluorescent screen with a small detection current, the excitation current of the irradiation lens is controlled with no time delay (
Decisions can be made immediately and responsiveness can be improved.

Furthermore, since the brightness on the fluorescent screen is not directly detected, it can be controlled independently of the brightness detection sensitivity on the fluorescent screen.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the brightness zoom adjustment device for an electron microscope according to the present invention, FIG. 2 is a diagram for explaining the flow of brightness zoom adjustment according to the present invention, and FIG. 3 is a diagram showing the brightness Irradiation lens ray diagram of sample irradiation before starting the zoom, 4th
The figure is a diagram showing an example of a circuit configuration of a conventional electron microscope equipped with a brightness zoom. 1... Irradiation lens, 2... Excitation current setting circuit, 3.
... Excitation current calculation control circuit, 4... Constant storage circuit, 5
... constant calculation circuit, 6 ... brightness zoom control circuit,
7... Magnifying lens system, 8~lO... Gate circuit. d! '+/
++/+-/

Claims (1)

[Claims] (1) A brightness zoom adjustment device for an electron microscope that stabilizes the brightness of an image formed on a fluorescent screen by controlling the excitation current of the irradiation lens according to the magnification of the magnification lens system, which A first calculating means calculates a constant by performing a predetermined calculation from the magnification of the magnifying lens system, and a second calculating means calculates an excitation current of the irradiation lens to obtain the same brightness as when calculating the constant from the constant and the magnification of the magnifying lens system. The first calculating means calculates a constant from the excitation current of the irradiation lens and the magnification of the magnifying lens system at a set brightness, and the second calculating means calculates a constant corresponding to a change in the magnification of the magnifying lens system. A brightness zoom adjustment device for an electron microscope, characterized by controlling an excitation current of an irradiation lens.