JPH01134845A - Automatic astigmatism correcting device - Google Patents

Abstract

PURPOSE:To perform correction of astigmatism precisely and automatically by sensing information obtained from circular scanning on a specimen, correcting the astigmatism one after another by a correcting means, and a specified time after correction, cumulating the detection signals over a period integer times as long as the circular scanning, and by sensing the max. value by a sensing means. CONSTITUTION:To a specimen 1 at an electron microscope an electron beam from an electron gun is focused by a focusing lens 2 finely, and scanning of the specimen 1 is made with the electron beam by X-direction and Y-direction deflecting coils 3X, 3Y, and deflecting coil driver circuits 11X, 11Y are connected with these coils 3X, 3Y. X-direction and Y-direction astigmatism correcting coils 4X, 4Y are connected with an astigmatism correction control circuit 14 through astigmatism correcting coil driver circuits 14X, 14Y, respectively. The information obtained by circular scanning over the specimen 1 is sensed by a secondary electron sensor 5, and the astigmatism is corrected through control of a sequence circuit 13, and after passage of a specified time, cumulation is made by a cumulating circuit 15 in a period integer times as great as the circular scanning.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an electron beam apparatus such as a scanning electron microscope, and more particularly to a device for automatically correcting astigmatism.

(Prior art) In a conventional device, as described in Japanese Patent Publication No. 61-34222, for example, an electron beam is A circular scan is performed, the excitation current of the astigmatism correction coil is changed in synchronization with this circular scan, information generated from the sample is detected along with the circular scan, and the magnitude of the change in the detected signal is calculated based on the circular Integrate over one round of scanning. Then, the integrated values corresponding to the excitation current sets of each astigmatism correction coil are compared, and the excitation current when the integrated value is the maximum is determined as the optimum excitation current for correcting astigmatism. It will be done.

(Problem that the invention attempts to solve) The above-mentioned conventional technology had the following problems.

That is, since the detection period of the integrated value is synchronized with the circular scanning of the electron beam and is limited to - times, the response of the astigmatism correction means may be slow, or the accelerating voltage or beam current of the electron beam may When the amount of information detected is small due to conditions such as the type of sample, it is difficult to accurately detect the integrated value, and therefore it is difficult to accurately correct astigmatism.

Furthermore, in the above conventional technology, electron beam irradiation,
Since the excitation current control of the astigmatism correction coil and the detection of information from the sample must be performed synchronously, the circuit configuration is complicated.

An object of the present invention is to solve the above-mentioned problems, and to be able to automatically and accurately correct astigmatism regardless of the response speed of the astigmatism correction means, the electron beam current, etc.
An object of the present invention is to provide an automatic astigmatism correcting device having a simple configuration.

(Means for Solving the Problems) The above object is to provide a means for detecting information obtained from a sample as an electron beam circularly scans the sample, a means for sequentially correcting astigmatism, This is achieved by comprising means for integrating the detection signal from the detection means over a period that is an integral multiple of the circular scan after a predetermined time has elapsed after the correction, and detecting the maximum value.

(Function) As a result, the detection of information from the sample obtained by circular scanning of the electron beam over the sample starts after the scheduled time necessary for the operation of astigmatism correction to stabilize, and , the detection information is an integrated value obtained over a period that is an integral multiple of the circular scan.

Therefore, after the accelerating voltage, beam current, etc. of the electron beam are stabilized, information from the sample can be accurately detected using a simple circuit configuration, and as a result, astigmatism can be accurately corrected. It is possible to produce the effect that it becomes like this.

(Example) An example of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 is a timing chart showing the operation timing of an embodiment of astigmatism correction in a scanning electron microscope to which the present invention is applied. FIG. 2 is a block diagram showing the configuration of a scanning electron microscope to which an embodiment of the present invention is applied.

In FIG. 1, Isx and lsy are current value signals of the X-direction astigmatism correction coil and the Y-direction astigmatism correction coil, respectively;
Σ is a signal instructing integration of secondary electron signals. In addition,
Σ is shown corresponding to the partially enlarged view of Isx.

In FIG. 2, a sample 1 is irradiated with an electron beam generated from an electron gun (not shown) or narrowly focused by a focusing lens 2. 3X and 3Y are deflection coils for scanning the electron beam on the sample in the X direction and the Y direction, respectively, and are connected to deflection coil drive circuits 11X and IIY, respectively.

4X and 4Y are astigmatism correction coils having quadrupole for astigmatism correction in the X direction and Y direction, respectively, and are connected to astigmatism correction coil drive circuits 14X and 14Y, respectively. 5 is a secondary electron detector that detects secondary electrons generated when the sample 1 is irradiated with an electron beam;
is an amplifier that amplifies the detection signal.

7 is a sample image display device for displaying a sample image, and 8 is a filter that differentiates the output signal from the amplifier and generates an absolute value signal of the differential signal.

9 is a rask scanning circuit that generates X and Y deflection signals having sawtooth waveforms.

12 is a circular scanning circuit that generates a sine signal and a cosine signal for circularly scanning the electron beam. Reference numeral 10 denotes a switching circuit for selecting either the output signal from the circular scanning circuit 12 or the output signal from the rask scanning circuit 9 as the signal to be supplied to the deflection coil drive circuit 11X111Y.

13 is the astigmatism correction coil drive circuit 14X.

14Y, integration command Σ output to the integration circuit 15, which will be described later;
and an astigmatism correction control device that controls the switching circuit 10. The integration circuit 15 integrates the absolute value signal output from the filter 8 during the period when the integration command Σ is at "H" level.

In the scanning electron microscope having the above configuration to which the present invention is applied, when the astigmatism correction is not adjusted, the switching circuit 10 selects the Rask scanning circuit 9, and the output signal is transmitted to the deflection coil 3X. , 3Y, the electron beam scans the sample over the sample, and outputs the sample image to the sample image display device 7. Note that the X direction and Y direction of the scanning direction and the X direction and Y direction of astigmatism correction can be determined independently of each other.

When the start of automatic astigmatism correction is instructed, the astigmatism correction control device 13 switches the switching circuit 10 to select the circular scanning circuit 10, so that a circular scanning signal is output to the deflection coils 3X and 3Y. and the scanning beam is (repeatedly) scanned circularly over the sample 1. At this time, the secondary electrons generated from the sample 1 are detected by the detector 5, and the detection signal is outputted to the sample image display device 7 and filter 8 via the amplifier 6.

Next, the astigmatism correction control device 13 gradually changes the astigmatism correction signals 1sx and Isy in stages as shown in FIG. After a certain period of time Tw has passed since changing 1sx,
A constant time TΣ-n equal to an integral multiple of the circular scanning period Tc
*During Tc, the accumulation command Σ is set to "H" level.

The integration circuit 15 converts the absolute value signal output from the filter 8 into
Integrate the circumferential scan and record the integrated value.

At this time, since the integration of information generated from the sample starts after the scheduled time has elapsed after changing the astigmatism correction signal, the irradiation position on the circular trajectory when the "H" level section starts is may be different from each other, but as described above, since the "H" level section T is an integral multiple of the circular scanning period, there is no problem in comparing each. Further, the circular scanning of the electron beam may be performed all the time.

By operating as described above, step 11-1 is performed after the acceleration voltage, beam current, etc. of the electron beam are stabilized, so that it is possible to accurately detect the integrated value.

Furthermore, in the present invention, since the integrated value for one round of beam irradiation is the integrated value for n rounds of beam irradiation, even if the data of one round of scanning is defective due to sudden noise etc.
The influence of the bad data can be alleviated.

Of course, the values of Tw and n are set in advance by the operator.

After the measurement using the combination of Isx and Isy is completed, the astigmatism correction control device 13 performs astigmatism correction to maximize the integrated value from the series of integrated values recorded in the integration circuit 15. Signal 1sxo.

15yo is selected, and the astigmatism correction signal 1sxo is selected.

l5yo is outputted to the astigmatism correction coil drive circuit as Isx and Isy for astigmatism correction, and then the switching circuit 10 is switched so that the rask scanning signal is selected.

As a result, astigmatism of the electron beam is corrected, and the cross section of the electron beam on the sample surface becomes circular. If the detected secondary electron signal is weak due to a small beam current, etc., automatic astigmatism correction can be performed by setting the value of n to a large value.

In the above explanation, the information generated from the sample by electron beam irradiation has been explained as secondary electrons, but the present invention is not limited to this only, and the information can be reflected electrons, absorbed electrons, or X-rays. It may be.

(Effects of the Invention) According to the present invention, even when the response speed of the astigmatism correction means is slow, information such as secondary electrons detected from the sample can be detected for a sufficiently long time under certain correction conditions. , since astigmatism correction can be performed based on the detection signal,
Without being affected by the response speed of the astigmatism correction means, accurate automatic astigmatism correction is possible even under conditions where the amount of detection signals detected per unit time is small.

[Brief explanation of the drawing]

FIG. 1 is a timing chart showing the operation timing of an embodiment of a scanning electron microscope to which the present invention is applied, and FIG. 2 is a block diagram showing the configuration of a scanning electron microscope to which an embodiment of the present invention is applied. be. Isx...X direction astigmatism correction signal, Isy...Y
Directional astigmatism correction signal, Σ...integration command, 1...sample, 2...focusing lens, 3X...X direction deflection coil, 3Y...Y direction deflection coil, 4X...X Directional astigmatism correction coil, 4Y...Y direction astigmatism correction coil, 5...Secondary electron detector, 6...Amplifier, 7...
Sample image display device, 8... Filter, 9... Rask scanning circuit, 10... Switching circuit, IIX, IIY... Deflection coil drive circuit, 12... Circular scanning circuit, 13...
・Astigmatism correction control circuit, 14X, 14Y... Astigmatism correction coil drive circuit, 15... Integration circuit agent
Patent Attorney Michihito Hiraki Figure 1 x Figure 2

Claims (6)

[Claims] (1) A scanning electron microscope that narrowly converges an electron beam and scans it over a sample, comprising: means for correcting astigmatism of the electron beam; means for circularly scanning the electron beam over the sample; means for detecting information generated from the sample by irradiation; means for changing the correction signal intensity supplied to the astigmatism correction means; and each time the correction signal intensity is changed, the detection signal from the detection means is An automatic astigmatism correcting device comprising means for integrating over a period that is an integral multiple of the circular scan and detecting the maximum value. (2) The automatic astigmatism correction device according to claim 1, wherein the information generated from the sample by irradiation with the electron beam is secondary electrons. (3) The automatic astigmatism correction device according to claim 1, wherein the information generated from the sample by irradiation with the electron beam is reflected electrons. (4) The automatic astigmatism correction device according to claim 1, wherein the information generated from the sample by irradiation with the electron beam is absorbed electrons. (5) The automatic astigmatism correction device according to claim 1, wherein the information generated from the sample by irradiation with the electron beam is X-rays. (6) The integration performed over a period that is an integral multiple of the circular scan is started after a scheduled time has elapsed after changing the intensity of the correction signal supplied to the astigmatism correction means. An automatic astigmatism correction device according to any one of claims 1 to 5.