JPH05114378A - Scanning electron microscope - Google Patents

Abstract

PURPOSE:To provide a scanning electron microscope which can quickly and accurately perform automatic focusing operations without being affected by the state of patterns of samples. CONSTITUTION:A control circuit 12 controls focusing lens setting circuit 19 to change the excitation strength of a focusing lens 3 step by step. It also feeds saw-tooth-waves from a saw-tooth-wave generation circuit 14 to an addition circuit 11. Horizontal scanning signals and vertical scanning signals added with saw-tooth-waves are fed to deflection coils 2, 3 via drive circuits 8, 10. As a result, electron beams on the surface of samples are repetitively scanned by saw-tooth-waves in the vertical direction at short periods for the duration of a single horizontal scanning signal. Secondary electrons generated from samples 5 are detected by a detector 6 and signals for the detected secondary electrons are fed to an absolute value circuit 17 and then integrated in an integration circuit 18. The integrated signals are fed to the control circuit 12 to perform automatic focusing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope having an autofocus function for automatically focusing.

[0002]

2. Description of the Related Art In autofocus of a scanning electron microscope or the like, the excitation of a focusing lens is changed in steps, and a specific region of a sample is scanned with an electron beam in the excitation state of each focusing lens in each step, and each scanning is performed. Second, secondary electrons or reflected electrons from the sample are detected, and when the detected signal intensity has the maximum value of the signal intensity, the focus point is set, and the focusing lens is fixed in the excited state at that time.

[0003]

In the above-mentioned autofocus, the electron beam is scanned linearly in the horizontal direction on the sample. At this time, the SN at the time of data collection is acquired.
In order to improve the ratio, it is necessary to lengthen the scanning time of the electron beam. As a result, it takes a long time to collect the data, and the operation time of the entire autofocus becomes extremely long. Further, in the case of a sample having a pattern in the horizontal direction on the sample, there are few uneven portions of the sample in the scanning direction of the electron beam, the amount of backscattered electron signals and secondary electron signals from the sample is small, and signal processing is performed. It is extremely disadvantageous in doing.

The present invention has been made in view of the above points, and an object thereof is a scanning electron microscope capable of accurately performing an autofocus operation and the like without being influenced by the state of a pattern of a sample in a short time. To realize.

[0005]

A scanning electron microscope according to the present invention comprises a focusing lens for focusing an electron beam on a sample, means for changing the excitation of the focusing lens in a stepwise manner, and each excitation state. In the scanning means for scanning the irradiation position of the electron beam on the sample, a detector for detecting a signal obtained from the sample based on this scanning, and a horizontal scanning signal supplied to the scanning means periodically in the vertical direction. It has a function of superimposing a signal that changes to.

[0006]

In the scanning electron microscope according to the present invention, for example, during an autofocus operation, a signal that periodically changes in the vertical direction is superimposed on a horizontal scanning signal for scanning an electron beam with which a sample is irradiated.

[0007]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows a scanning electron microscope which is an embodiment of the present invention, in which 1 is an accelerated electron beam generated from an electron gun (not shown). Reference numerals 2 and 3 denote two-stage deflection coils. Each of the two-stage deflection coils 2 and 3 includes horizontal and vertical deflection coils. Reference numeral 4 is an objective lens, and the sample 5 is irradiated with the electron beam finely focused by the objective lens 4. Reference numeral 6 is a detector for detecting, for example, secondary electrons generated by irradiation of the sample 5 with an electron beam. For the horizontal deflection coils of the two-stage deflection coils 2 and 3, the horizontal scanning signal generation circuit 7 to the drive circuit 8 are provided.
The horizontal scanning signal is supplied via the drive circuit 10 and the vertical deflection signal is supplied from the vertical scanning signal generation circuit 9 via the drive circuit 10 to the vertical deflection coil. An adder circuit 11 is arranged between the vertical scanning signal generation circuit 9 and the drive circuit 10. The speed (cycle) of the scanning signals from the horizontal scanning signal generating circuit 7 and the vertical scanning signal generating circuit 9 is controlled by a control circuit 12 such as a computer. Adder circuit 1
1, the signal from the switch circuit 13 is also supplied,
The switch circuit 13 switches between the sawtooth wave signal and the zero signal from the sawtooth wave generation circuit 14 under the control of the control circuit 12, and supplies the zero signal to the addition circuit 11. The detector 6
The signal detected by is supplied to the cathode ray tube 16 and the absolute value circuit 17 to which the horizontal and vertical scanning signals are supplied. The output signal of the absolute value circuit 17 is integrated by the integrating circuit 18 and then supplied to the control circuit 12. The control circuit 12 also controls the objective lens value setting circuit 19, and the drive circuit 2 is controlled by a signal from the objective lens value setting circuit 19.
0 supplies the objective lens current to the objective lens 4. The operation of such a configuration is as follows.

When observing a normal secondary electron image, the switch circuit 13 is first switched by the control of the control circuit 12 as shown by the dotted line in the figure. Next, the control circuit 12 controls the horizontal scanning signal generating circuit 7 and the vertical scanning signal generating circuit 9 so as to obtain a desired scanning speed, and as a result, a desired deflection signal is supplied to the deflection coils 2 and 3 and the electron beam is emitted. 1 is
A desired area of the sample 5 is scanned by being deflected by the deflection coil. 2 generated by irradiation of the sample 5 with the electron beam
The secondary electrons are detected by the detector 6, and the detection signal is amplified by the amplifier 15 and then supplied to the cathode ray tube 16 to which the scanning signal is supplied. An electronic image is displayed.

Now, the operation of focusing the objective lens 4 will be described with reference to the signal waveform diagrams of FIGS. First, the control circuit 12 controls the focusing lens value setting circuit 19, and as shown in FIG.
The excitation intensity of is changed stepwise. Further, the control circuit 12 controls the switch circuit 13 to switch it as shown by the solid line in the figure to supply the sawtooth wave from the sawtooth wave generation circuit 14 to the adding circuit 11. The signal waveforms at this time are shown in FIG. FIG. 3A shows a vertical scanning signal supplied from the vertical scanning signal generating circuit 9 to the adding circuit 11. One cycle of the vertical scanning signal which changes stepwise is one of the objectives shown in FIG. It is equal to one step period of the stepwise change of the excitation intensity of the lens 4. FIG. 3B shows a horizontal scanning signal from the horizontal scanning signal generating circuit 7. This horizontal scanning signal is a sawtooth wave, and is 1 for each step of the vertical scanning signal.
A periodic sawtooth wave is generated. FIG. 3C shows a sawtooth wave supplied from the sawtooth wave generation circuit 14 to the adder circuit 11 via the switch circuit 13. The frequency F2 of the sawtooth wave is compared with the frequency F1 of the horizontal scanning signal. Has been remarkably fast. The sawtooth wave from the sawtooth wave generation circuit 14 shown in FIG. 3C and the vertical scanning signal shown in FIG. 3A are added by the adder circuit 11. The horizontal scanning signal and the vertical scanning signal obtained by adding the sawtooth wave are supplied to the deflection coils 2 and 3 via the drive circuits 8 and 10. As a result, the electron beam on the surface of the sample is repeatedly scanned in the vertical direction at a fast cycle by the sawtooth wave for the period of one horizontal scanning signal as shown in FIG. Secondary electrons generated by irradiating the sample 5 with the electron beam are detected by the detector 6, and the detected secondary electron signal is supplied to the amplifier 15
Is supplied to the absolute value circuit 17 via, and then integrated in the integrating circuit 18. The integrated signal is supplied to the control circuit 12. The control circuit 12 performs processing for autofocus, and a functional block diagram of the processing is shown in FIG. The value integrated by the integrating circuit 18 for each step of the excitation intensity of the objective lens shown in FIG. 2A is supplied to the signal intensity distribution creating unit 21. In this unit, as shown in FIG. 2B, the intensity distribution of each integrated signal is created. From this distribution, the signal peak detection unit 22 finds the peak of the distribution. Then, the focus value detection unit 23 obtains the objective lens value at the peak point of the distribution. Focus value detection unit 2
The objective lens value obtained by 3 is set in the objective lens value setting circuit 19, and the excitation intensity of the objective lens is set. In this way, the focusing of the objective lens 4 is executed.

As described above, in this embodiment, when the excitation of the objective lens 4 is changed stepwise and the secondary electron detection signal is obtained from the sample 5 during the stepwise change, the electron beam on the sample 5 is changed. Scanning is not performed in a straight line horizontally, but during scanning of the electron beam in the horizontal direction, the electron beam is also scanned in the vertical direction at a fast cycle, so the shape (pattern) of the sample surface is changed in the horizontal direction. Even when only the electron beam exists, the electron beam is scanned in the direction perpendicular to the edge E (see FIG. 4) of the pattern, so that a signal of sufficient intensity necessary for autofocusing can be obtained. Further, in this case, a sufficient signal can be obtained even if the horizontal scanning is accelerated.

Although one embodiment of the present invention has been described in detail above, the present invention is not limited to this embodiment. For example, although secondary electrons are detected, reflected electrons may be detected. Although the sawtooth wave is superimposed on the vertical scanning signal, a sin wave oscillation circuit may be prepared and the sin wave from the oscillation circuit may be added to the vertical scanning signal. Further, when performing the autofocus operation, the excitation intensity of the objective lens was changed in steps, but an auxiliary lens may be provided close to the objective lens and the intensity of this auxiliary lens may be changed in steps. Since the horizontal scanning signal is a sawtooth wave, a sawtooth wave having a low frequency may be obtained from the sawtooth wave generation circuit 14 and used as the horizontal scanning signal. In this case, the horizontal scanning signal generation circuit 7 becomes unnecessary. And the control circuit 1
The memory in 2 stores the amplitude of the sawtooth wave and the speed of horizontal and vertical scanning signals according to the type of sample to be observed, so that the initial setting of each circuit can be easily performed when exchanging the sample. Is desirable. Furthermore, the method of superimposing a sawtooth wave or a sin wave on the vertical scanning signal can be used not only in the autofocus operation but also in the auto astigmatism operation, autocontrast operation, and autobrightness operation.

[0012]

As described above, in the scanning electron microscope according to the present invention, the signal which periodically changes in the vertical direction is superimposed on the horizontal scanning signal for scanning the electron beam with which the sample is irradiated. Therefore, the autofocus operation and the like can be performed accurately without being affected by the state of the pattern of the sample in a short time.

[Brief description of drawings]

FIG. 1 is a diagram showing a scanning electron microscope which is an embodiment of the present invention.

FIG. 2 is a signal wave system diagram used for explaining an autofocus operation by the scanning electron microscope of FIG.

FIG. 3 is a signal wave system diagram used for explaining an autofocus operation by the scanning electron microscope of FIG.

4 is a functional block diagram of autofocus processing in a control circuit of the scanning electron microscope of FIG.

FIG. 5 is a diagram showing a state of horizontal scanning of an electron beam on a sample.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Electron beam 2, 3 ... Deflection coil 4 ... Objective lens 5 ... Sample 6 ... Detector 7 ... Horizontal scanning signal generation circuit 8, 10, 20 ... Driving circuit 9 ... Vertical scanning signal generation circuit 11 ... Addition circuit 12 ... Control Circuit 14 ... Sawtooth wave generation circuit 17 ... Absolute value circuit 18 ... Integration circuit 21 ... Signal intensity distribution creation unit 22 ... Signal peak detection unit 23 ... Focus value detection unit

Claims (1)

[Claims] 1. A focusing lens for focusing an electron beam on a sample, a means for changing the excitation of the focusing lens in a stepwise manner, and a scan for scanning an irradiation position of the electron beam on the sample in each excitation state. Scanning having means, a detector for detecting a signal obtained from a sample based on this scanning, and a function for superimposing a signal which periodically changes in the vertical direction on a horizontal scanning signal supplied to the scanning means electronic microscope.