JPH0119225B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for controlling brightness in an electron microscope,
In particular, the present invention relates to a brightness control method in an electron microscope that can control the brightness of a final image according to its magnification.

In electron microscopy, the magnification of the final image can be varied over a wide range, but it is desirable that the brightness of the final image remains substantially constant.

The brightness of the final image is proportional to the brightness on the sample, but it is inversely proportional to the square of the magnification of the final image, so
In order to keep the brightness of the final image constant regardless of changes in magnification, it is necessary to change the brightness on the sample in proportion to the square of the magnification of the final image. This is achieved by varying the illumination angle of the electron beam that illuminates the sample in proportion to the final image magnification. This is because the brightness on the sample is proportional to the square of the irradiation angle of the electron beam that irradiates the sample.

On the other hand, if the brightness on the sample is changed in proportion to the square of the magnification of the final image, problems of sample damage and contamination will occur at high magnification.

In order to reduce sample damage and contamination, the filament current of the electron gun is adjusted in conjunction with changing the magnification of the final image so that the electron current density on the sample surface is suppressed within a range that does not damage the sample. The concept of controlling the bias voltage is known. The document that describes this idea is JP-A-53
-62978 publication is mentioned.

This known technique is based on the filament current of an electron gun,
It is based on the idea of controlling the electron flow density on the sample by changing the brightness of the electron source by changing the bias.

However, the brightness of the electron source changes as the filament changes over time. Therefore, in order to obtain the same brightness, the filament current and bias must change over time. Under these conditions, the filament current and bias should be adjusted in conjunction with the magnification to reduce sample damage and contamination. It is practically impossible to change the brightness as desired by changing the bias.

It is an object of the present invention to maintain the brightness of the final image substantially constant in a magnification range below a certain magnification, regardless of the magnification of the image, and to prevent damage to the final image for various specimens in the magnification range above. It is an object of the present invention to provide a brightness control method in an electron microscope that can very easily and by simple means reduce contamination and contamination.

In order to achieve the objective, the brightness control method in an electron microscope of the present invention corresponds to the magnification of the condenser lens necessary to change the brightness on the sample in substantially proportion to the square of the final image magnification. The excitation current data and the excitation current data for determining the upper limit of the brightness of the condenser lens, which determines the upper limit of the brightness on the sample, are stored in a storage device, and an arbitrary magnification of the final image is selected by specifying the magnification. The excitation current data corresponding to the magnification corresponding to the selected magnification is read out from the storage device, and an arbitrary brightness upper limit on the sample is selected according to the brightness upper limit specification, and the brightness upper limit corresponding to the selected brightness upper limit is set. The determined excitation current data is read from the storage device, and the read magnification-corresponding excitation current data and brightness upper limit determined excitation current data are compared with each other to determine a magnification range below the magnification at which the former data matches the latter data. Then, the condenser lens is controlled so that the brightness on the sample changes substantially in proportion to the magnification of the final image using the magnification-corresponding excitation current data read out corresponding to the specified magnification, and the former data is characterized in that in a magnification range equal to or higher than the magnification that matches the latter data, the condenser lens is controlled so that the brightness on the sample is maintained substantially constant based on the excitation current data corresponding to the matched magnification. .

FIG. 1 shows the electron optical system of a conventional electron microscope. In the figure, an accelerated electron beam emitted from an electron gun 1 consisting of a cathode, a grid, and an anode is focused by a condenser lens system consisting of first and second condenser lenses 3 and 4, and irradiates a sample 5. The electron beam that has irradiated the sample 5 is transmitted through the sample 5, and the transmitted electron beam is magnified by a magnifying lens system consisting of an objective lens 6, an intermediate lens 7, and a projection lens 8, and is projected onto a fluorescent screen 9. Therefore, the electron microscope image of the sample 5, that is, the final image, can be observed on the fluorescent screen 9.

FIG. 2 shows an embodiment of a brightness control device in an electron microscope for carrying out the method of the present invention.
A condenser lens system normally consists of a first and a second condenser lens 3 and 4, as shown in FIG. 1, but only the second condenser lens 4 is shown in the figure for simplicity. Furthermore, the magnifying lens system may be a three-stage lens system consisting of an objective, intermediate and projection lenses 6, 7, and 8 as shown in Figure 1, or it may be a four-stage lens system using two lenses as projection lenses. Sometimes it is. In the figure, the magnifying lens 11 is shown as representative of the entire magnifying lens system.

The magnification of the final image is determined by the current flowing through the magnifying lens 11.
That is, it is variable by changing the excitation current of the lens, and the data of the excitation current of the magnifying lens corresponding to the magnification to be set is stored as a digital quantity in a read-only memory, that is, ROM 12. The magnification designator 13 specifies the magnification to be set, and for example, a key switch can be used as this.
By operating this magnification designator 13, the magnification designation signal obtained from the interface 1
4 to a central processing unit, ie, CPU 15, which is driven by a drive power supply 29, and thereby reads data corresponding to the magnification designated by the magnification designator 13 from the ROM 12. This data is transferred via an interface 10 to a digital-to-analog converter, namely a D-A converter 1.
6, where it is converted into an analog signal,
The analog signal is applied to one input terminal of the error amplifier 17.

On the other hand, from the DC power supply 18 to the control transistor 1
A direct current is applied to the magnifying lens 11 through the magnifying lens 11. The current flowing through the magnifying lens 11 is detected as a voltage signal by the detection resistor 20, and this is applied to the other input terminal of the error amplifier 17. The error amplifier 17 compares two input signals thereto and generates a difference signal, and based on this difference signal, controls the control transistor 1 so that the difference becomes zero.
9 controls the current flowing through the magnifying lens 11.

Thus, it is understood that simply by specifying a magnification using the magnification designator 13, the magnifying lens 11 can be excited with a current corresponding to the specified magnification.

As mentioned above, the brightness on the sample 5 is proportional to the square of the irradiation angle β of the electron beam irradiating it, and the brightness B of the final image is proportional to the brightness I on the sample 5, and the brightness of the final image is proportional to the brightness I on the sample 5. It is inversely proportional to the square of the image magnification M. Therefore, if the illumination angle β is changed in proportion to the magnification M so that the brightness I on the sample 5 changes in proportion to the square of the magnification M, the brightness of the final image will change regardless of the magnification M of the final image. B can be kept constant.

The illumination angle β can be changed by changing the excitation current i of the second condenser lens 4. Third
The figure shows the relationship between β and i, and this relationship is generally well known. In FIG. 3, i _C represents the excitation current when the maximum irradiation angle β _nax is obtained. i _C also sends the electron beam to sample 5
It is also the excitation current when _{the electron} beam is converged to They will be converged at each location.

It is now assumed that an excitation current range of i _C or higher is used for brightness control. In this excitation current range, the irradiation angle β decreases as the excitation current i increases. Therefore, in this excitation current range, the small excitation current side corresponds to the high magnification side, and the large excitation current side corresponds to the low magnification side. For ease of understanding, the variable magnification range is M _nio ~ M _nax (M _nio <
M _nax ), the excitation current when the minimum magnification M _nio is i _O , and when the magnification is increased from M _nio and reaches M _C (M _C < M _nax ), the excitation current is i _C , and this I _C
shall remain unchanged in the magnification range from M _C to the highest magnification M _nax .

Under these conditions, the magnification M is changed from M _nio to M _nax
Figure 4 shows the relationship between the brightness I on the sample 5 and the magnification M, which is necessary to keep the brightness B of the final image substantially constant even if it is changed within the range of . Become. According to the figure, the brightness I on the sample changes in proportion to the square of the magnification M in the magnification range below M _C , and reaches its maximum value at the position of M _C.
I reach _nax . Further, in the magnification range from M _C to M _nax , the maximum value I _nax is maintained as it is. This means that in the magnification range from M _C to M _nax , the excitation current i
is maintained at i _C , and therefore the irradiation angle β continues to be maintained at the maximum value β _nax .

In the magnification range from M _C to M _nax , as in the magnification range below M _C , if the excitation current i continues to decrease, the illumination angle β will also decrease accordingly, and therefore the brightness I on the sample 5 will be It's getting darker.

However, the magnification range from M _C to M _nax is generally narrow. Therefore, in this magnification range, as mentioned above, as long as the excitation current i is kept constant at i _C and therefore the brightness I on the sample 5 is kept constant at I _nax , the final image can be obtained to a certain extent that there is no problem in practical use. It can be said that the final image is not darkened and has a substantially constant brightness over the entire magnification range, independent of the magnification M.

The ROM 12 further contains digital data on the excitation current of the second condenser lens 4, that is, the excitation current corresponding to the magnification i _C to i _O necessary to obtain the relationship between the brightness I on the sample and the magnification M shown in FIG. It is stored as a quantity. Therefore, when a magnification is designated by the magnification designator 13, the data of the magnification-corresponding excitation current i corresponding to the designated magnification is read out from the ROM 12 by the CPU 15.
This data is converted into an analog signal via an interface 21 by a digital-to-analog converter, that is, a DA converter 22, and the analog signal is applied to one input terminal of an error amplifier 23.

On the other hand, a DC current is applied to the second condenser lens 4 from the DC power supply 18 via the control transistor 24, and the current flowing through the second condenser lens 4 is detected as a voltage signal by the detection resistor 25. is applied to the other input terminal of the error amplifier 23. The error amplifier 23 generates a signal representing the difference between the two input signals thereto, and the control transistor 24 controls the second condenser lens 4 based on this difference signal so that the difference becomes zero.
control the current flowing through the

Therefore, the magnification M is set by the magnification designator 13.
When sequentially specifying from M _nio to M _nax , the corresponding excitation current is given to the magnifying lens 11, and at the same time, the second condenser lens 4 is supplied from i _O to
An excitation current of up to i _C is applied, and as a result, the brightness I on the sample changes as shown in FIG. That is, the irradiation angle β of the electron beam that irradiates the sample increases as the magnification M increases, and reaches a maximum when the magnification M reaches M _C , that is, when the excitation current i corresponding to the magnification reaches i _C. However, in the magnification range from M _C to _{M nax ,} i _C continues to be applied to the second condenser lens 4, and the illumination angle β continues to take the maximum value β _nax during that time. , the brightness I on the sample changes in proportion to the square of the magnification M in the magnification range of M _nio to M _C and maintains the maximum value I _nax in the magnification range of M _C to M _nax .
This is done by comparing the magnification-corresponding excitation current i corresponding to the specified magnification with the value i _n corresponding to the maximum value I _nax ,
A magnification range in which both of them match or higher is achieved by controlling the condenser lens 4 using the data of _i.sub.n. Therefore, as mentioned above, it can be said that the brightness of the final image remains substantially constant over the entire magnification range M _nio to _{M nax} .

In order to reduce contamination and damage to the sample, the upper limit of brightness on the sample (this is defined as I _L ) must be set.
needs to be made smaller. However, this upper limit _IL often depends on the type of sample. Therefore,
By making it possible to switch _IL in multiple stages within a range below the maximum value I _nax , contamination and damage to various types of samples can be reduced.

To explain this point with reference to FIG. 2, the excitation current of the second condenser lens 4 corresponding to a plurality of I _Ls below I _nax , that is, the brightness upper limit determining excitation current (this is designated as i _L ). The data is stored in the ROM 12 in advance. To select which _iL , operate the brightness upper limit designator 30 consisting of a key switch, and then send the brightness upper limit designation signal to the interface 1.
This is done by providing the CPU 15 via the CPU 4. When the magnification designator 13 is operated to sequentially change the magnification M from M _nio to M _nax , the ROM 12
The data of the original excitation current i corresponding to the magnification of the second condenser lens 4 located at is read out, and the second condenser lens 4 is controlled by this data. On the other hand, the i _L specified by the brightness upper limit designator 30 is read out from the ROM 12 by the CPU 15, and the CPU 15 compares this i _L with the original i read out from the ROM 12, and the latter is compared with the former. In a magnification range equal to or higher than the matched magnification, the second condenser lens 4 is controlled by the excitation current i corresponding to the matched magnification. Therefore, the brightness I on the sample changes in proportion to the square of the magnification in the magnification range until i coincides with i _L , but the brightness I changes in proportion to the square of the magnification in the magnification range beyond which i coincides with i _L. i, i.e., i _L in this case, will be maintained as it is.

To briefly summarize the above explanation, the excitation current data corresponding to the magnification of the condenser lens necessary to change the brightness on the sample in substantially proportion to the square of the final image magnification is stored in the ROM, which is a storage device. The brightness upper limit determination excitation current data of the condenser lens, which determines the upper limit of brightness on the sample, is stored in advance and is also stored in the ROM, which is a storage device.

The selection of the magnification of the final image is achieved by specifying the magnification, and once the magnification is selected, the corresponding excitation current data corresponding to the magnification is stored in the storage device.
Read from ROM. Similarly, selection of the brightness upper limit on the sample is accomplished by brightness upper limit designation, and when the brightness upper limit is thus selected, the corresponding brightness upper limit determination excitation current data is also stored in the storage device. Read from some ROM.

The magnification-corresponding excitation current data and the brightness upper limit determining excitation current data read out in this way are compared with each other, and in the magnification range below the magnification where the former data matches the latter data, the brightness on the sample is equal to that of the final image. The condenser lens is controlled by the magnification-corresponding excitation current data read out corresponding to the specified magnification so as to vary substantially in proportion to the magnification, and the magnification range is greater than or equal to the magnification where the former data matches the latter data. Then, the condenser lens is controlled by the matched magnification-corresponding excitation current data, and the brightness on the sample is maintained substantially constant.

From the above explanation, it can be seen that in the magnification range below a certain magnification of the final image, the brightness of the image remains essentially constant regardless of the magnification of the image, and that in the magnification range above it, the brightness of the image remains virtually constant for various specimens. It will be appreciated that reducing contamination can be achieved very easily and with simple means.

Note that the brightness of the electron source changes due to changes in the filament of the electron gun over time. However, this can be easily corrected by changing the filament current and bias voltage. Therefore, problems like those in the prior art described above do not occur.

Preferably, the brightness level of the final image can be varied according to operator preference. Furthermore, there may be cases where it is desired to change the brightness level of the final image depending on the type of sample. Furthermore, when photographing the final image, it is generally necessary to make the final image darker than when the image was observed.

In this way, it is desirable to be able to change the brightness level of the final image as necessary.

To explain this point with reference to FIG. 2, when the brightness adjustment switch 25 consisting of a key switch is pressed, the future brightness designation signal is given to the CPU 15 via the interface 14, and when the switch is pressed. As a function of time,
The data stored in ROM12 is stored in CPU1.
5 and this incremented data is stored in random access memory, or RAM 26. In this state, magnification M is
When the magnification designator 13 is operated to sequentially change from M _nio to M _nax , the data stored in the RAM 26 and the data of the excitation current i corresponding to the magnification of the second condenser lens 4 stored in the ROM 12 are changed. Addition (or subtraction) is performed within CPU 15. This added (or subtracted) data is sent to the D-A converter 2 via the interface 21.
2, where it is converted into an analog signal,
With this analog signal, the second
condenser lens 4 is controlled. Therefore, the brightness curve on the sample shown in FIG. 4 is translated in the vertical axis direction by the brightness change corresponding to the time the brightness adjustment switch 25 was pressed, and the brightness level is changed. will change.

On the other hand, the data added (or subtracted) above is
The data of the excitation current i _C of the second condenser lens 4 corresponding to I _nax in the ROM 12 and the CPU 15
After the former becomes equal to the latter, the second condenser lens 4 is controlled by the added (or subtracted) data at the time when the two become equal.

Of course, if the upper limit of brightness on the sample I _L is set to a value smaller than I _nax , then
It goes without saying that the added (or subtracted) data needs to be compared with the data of i _L corresponding to _IL , not I _C.

From the above description it will be understood that the brightness level can be changed as desired by easy and simple means.

In the explanation so far, an excitation current range of i _C or more in FIG. 2 has been used to control the brightness of the final image, but an excitation current range of i _C or less may be used instead. However, from the viewpoint of electron optics, it is generally better to use an excitation current range of i _C or more than to use an excitation current range of i _C or less because the variation range of the irradiation angle β of the electron beam that irradiates the sample is wider; Since the above brightness variation range is wide, from this point of view it is desirable to use an excitation current range of i _C or more.

Note that a single condenser lens may be used as the converging lens system, and in this case, this single condenser lens is controlled for brightness control.

As is clear from the above description, according to the present invention, the above-mentioned objects of the present invention are achieved, and therefore the practical effects thereof are extremely large.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an electron optical system of a generally known electron microscope, Fig. 2 is a block diagram of an embodiment of a brightness control device in an electron microscope for carrying out the method of the present invention, and Fig. 3 4 is a diagram showing the relationship between the excitation current of the second condenser lens and the irradiation angle of the electron beam that irradiates the sample, and FIG. 4 is a diagram showing the relationship between the brightness on the sample and the magnification. 1... Electron gun, 3 and 4... Condenser lens, 5... Sample, 6... Objective lens, 7... Intermediate lens, 8... Projection lens, 9... Fluorescent screen, 10, 14 and 21... Interface, 11...Magnifying lens, 12...ROM, 13
...Magnification specifier, 15...CPU, 16 and 2
2...D-A converter, 17 and 23...Error amplifier, 25...Brightness adjustment switch, 26...
ROM, 30...Brightness upper limit designator.

Claims (1)

[Claims] 1. Magnification-corresponding excitation current data of a condenser lens necessary to change the brightness on the sample in substantially proportion to the square of the final image magnification and the upper limit of the brightness on the sample. The brightness upper limit determination excitation current data of the condenser lens to be determined is stored in a storage device, an arbitrary magnification of the final image is selected by specifying the magnification, and the magnification-corresponding excitation current data corresponding to this selected magnification is stored in the storage device. At the same time as reading from the storage device, by selecting an arbitrary brightness upper limit on the sample by specifying the brightness upper limit, brightness upper limit determination excitation current data corresponding to the selected brightness upper limit is read from the storage device, and these data are read from the storage device. The read magnification-corresponding excitation current data and the brightness upper limit determination excitation current data are compared with each other, and in the magnification range below the magnification where the former data matches the latter data, the magnification read out corresponds to the specified magnification. controlling the condenser lens by corresponding excitation current data such that the brightness on the sample varies substantially proportionally to the magnification of the final image, in a magnification range above which the former data coincides with the latter data; A method for controlling brightness in an electron microscope, characterized in that the condenser lens is controlled so that the brightness on the sample is maintained substantially constant based on the matched magnification-corresponding excitation current data. 2. Excitation current data corresponding to the magnification of the condenser lens necessary to change the brightness on the sample in substantially proportion to the square of the magnification of the final image, and the brightness of the condenser lens that determines the upper limit of the brightness on the sample. The upper limit determination excitation current data is stored in a read-only memory, and predetermined data is stored in a random access memory, and an arbitrary magnification of the final image is selected by specifying the magnification. Magnification-corresponding excitation current data corresponding to the magnification is read out from the read-only memory, predetermined data in the random access memory is read out, and the two are added or subtracted. Selecting an arbitrary brightness upper limit, reading the brightness upper limit determination excitation current data corresponding to the selected brightness upper limit from the read-only memory, and adding or subtracting the read brightness upper limit determination excitation current data to the brightness upper limit determination excitation current data. In the magnification range below the magnification where the latter data matches the former data, the magnification-corresponding excitation current data read from the read-only memory corresponding to the specified magnification and the random access The condenser lens is controlled by the data obtained by addition or subtraction with the data read from the memory such that the brightness on the sample varies substantially proportionally to the magnification of the final image, the latter data being In a magnification range equal to or higher than the magnification that matches the former data, the condenser lens is controlled by the matched, added or subtracted data so that the brightness on the sample is maintained substantially constant, and the random access 1. A method for controlling brightness in an electron microscope, characterized in that data stored in a memory can be changed as a function of operating time by operating a brightness adjusting means.