JPH04237939A - Automatic focusing method - Google Patents

Abstract

PURPOSE:To realize the automatic focusing method correctly performing an automatic focusing action without being affected by noise. CONSTITUTION:Secondary electrons generated by the face scanning of an electron beam are detected by a detector 6, the detection signal is separated into two signal components according to the frequency by a signal separating circuit 8. The separated signal component and the noise component are fed to a computer 15 to obtain their ratio, and this ratio corresponds to the S/N ratio of the detection signal. When the detected S/N ratio is the stored S/N ratio or above, the computer 15 performs an automatic focusing action by face scanning. When the detected S/N ratio is the stored S/N ratio or below, the computer 15 switches the scanning of the electron beam to line scanning from face scanning to perform the automatic focusing action.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus method for automatically adjusting focus in a scanning electron microscope or the like.

0002

[Prior Art] In an autofocus system such as a scanning electron microscope, the excitation of a focusing lens is changed in steps, and a specific region of a sample is scanned with an electron beam in each step of the excitation state of the focusing lens. Secondary electrons or reflected electrons from the sample are detected, and the time of the maximum value of the detected signal intensity is set as the focus point, and the focusing lens is fixed in the excitation state at that time.

0003

[Problem to be Solved by the Invention] In the above-mentioned autofocus, if the S/N ratio of the detection signal of secondary electrons or reflected electrons is poor, the signal strength difference will vary due to the influence of noise, making it difficult to find the peak value of the signal. . Currently, the scanning of the electron beam on the sample when performing autofocus has two stages: plane scanning, which scans a specific two-dimensional area, and line scanning, which scans the electron beam on a straight line. Two types of scanning are used by switching between them. Since the sampling time is the same for area scanning and line scanning, the amount of signal during line scanning is greater than that for area scanning, and the S/N ratio of the detection signal is improved. However, this SN ratio is also affected by the condition of the sample surface, and even if the probe current value is large, the SN ratio may be poor.

The present invention has been made in view of the above-mentioned problems, and its object is to realize an autofocus method that can accurately perform autofocus operations without being affected by noise.

[0005]

[Means for Solving the Problem] The autofocus method based on the present invention changes the excitation of a focusing lens for focusing an electron beam on a sample in steps, and irradiates the electron beam on the sample in each excitation state. In an autofocus method that scans a position, detects a signal obtained from the sample based on this scan, and determines the excitation intensity of the focusing lens based on the detection signal intensity corresponding to each scan, the detection It is characterized in that the signal is separated into two components depending on the frequency, and the scanning speed of the electron beam on the sample is changed based on the intensity ratio of the two separated components.

[0006]

[Operation] The autofocus method based on the present invention scans an electron beam over a sample for autofocus, and separates the resulting detection signal into two components, a signal component and a noise component, according to the frequency. The scanning speed of the electron beam on the sample was changed based on the intensity ratio of the two separated components.

[0007]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an example of a scanning electron microscope for implementing the autofocus method of the present invention.
is an electron gun. The electron beam generated from the electron gun 1 is
The light is narrowly focused onto the sample 4 by the focusing lenses 2 and 3, and scanned by the deflection coil 5. 6 is a secondary electron detector, and the output signal of the detector 6 is amplified by an amplifier 7 and then supplied to a signal separation circuit 8 and an absolute value circuit 9. The two types of signals separated by the signal separation circuit 8 are supplied to absolute value circuits 10 and 11, respectively, and then integrated by integration circuits 12 and 13. The output signal of the absolute value circuit 9 is also supplied to the integration circuit 14 and integrated, and the output signal of each integration circuit 12, 13, 14 is sent to the computer 15.
supplied to The computer 15 controls a switching circuit 18 for switching the scanning speed in the excitation power source 16 of the focusing lens 3 and the deflection circuit 17 that supplies scanning signals to the deflection coil 5.

The autofocus operation in the scanning electron microscope configured as described above will be explained with reference to the flowchart in FIG. 2 and the signal wave diagram in FIG. 3. First, when the autofocus operation is turned on, the computer 15 controls the excitation power source 16 of the focusing lens 3, and changes the excitation intensity of the focusing lens 3 in steps, as shown in FIG. 3(a). Furthermore, the switching circuit 18 is controlled from the computer 15, and the deflection circuit 17 is controlled for each excitation intensity in the focusing lens 3.
A surface scanning signal is supplied from the deflection coil 5 to the deflection coil 5. As a result, a specific two-dimensional area of the sample 4 is scanned by the electron beam. Secondary electrons generated by irradiating the sample 4 with the electron beam are detected by a secondary electron detector 6, and the detection signal is amplified by an amplifier 7 and then sent to a signal separation circuit 8 and an absolute value circuit 9. Supplied. The signal separation circuit 8 is composed of a low-pass filter 19 and a high-pass filter 20, as shown in FIG. As a result, the input signal is separated into two types of signal components according to their frequencies. Here, if we pay attention to the frequency of the signal component and the frequency of the noise component included in the detection signal, the frequency of the signal component is a relatively low frequency that depends almost on the scanning speed of the electron beam, whereas the frequency of the noise component is , is based on shot noise of a detector such as a photomultiplier tube, and is usually a high frequency component. Therefore, noise components and signal components can be separated based on frequency, with a specific frequency as a boundary.

Based on the above-mentioned principle, the signal component and noise component separated by the signal separation circuit 8 are supplied to integration circuits 12 and 13 via absolute value circuits 10 and 11, respectively, and the respective excitation intensities of the focusing lens are It is integrated for each scan of the electron beam. From these integrating circuits 12 and 13, (
The signals shown in b) and (c) are obtained. This figure (b), (
The signal shown in c) is supplied to the computer 15 and its ratio is determined, and this ratio corresponds to the SN ratio of the detection signal. The computer 15 uses the determined SN
The ratio is compared with an SN ratio stored in advance corresponding to the scanning speed of surface scanning. As a result of this comparison, the detected SN
If the ratio is equal to or higher than the stored SN ratio, the SN ratio is good, and in this case, autofocus operation is performed based on all detection signals supplied via the absolute value circuit 9 and the integration circuit 14. conduct. That is, the excitation intensity of the focusing lens 3 is changed as shown in FIG. 3(a), and the integration circuit 1 is
Integrate the entire detected signal by 4. As a result, Figure 3 (d
) is obtained, and the peak of this signal waveform indicates that the electron beam is in focus. The computer 15 controls the excitation power supply 16 and sets the excitation intensity of the focusing lens 3 to the value at the peak of the signal shown in FIG. 3(d). After such an operation is completed, normal electron beam scanning is started, and a scanning electron microscope image of a desired area of the sample 4 is obtained.

[0010] Next, when the detected SN ratio is in a bad state below the pre-stored SN ratio, the computer 15 controls the switching circuit 18 to switch the scanning of the electron beam from surface scanning to line scanning. Therefore, the electron beam is line-scanned for each excitation intensity of the focusing lens 3, and the secondary electrons generated by each line-scanning are detected by the detector 6. The S/N ratio of this detection signal is improved if the time for surface scanning and line scanning is kept constant because the scanning speed for line scanning becomes slower. This detection signal is supplied to the computer 15 via the absolute value circuit 9 and the integration circuit 14, and an autofocus operation is performed in the same manner as in the above-mentioned area scanning.

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 were detected, reflected electrons may also be detected. Further, although the scanning speed is switched between two stages, surface scanning and line scanning, the scanning speed may be arbitrarily changed for each scanning. In that case, it is desirable to store a standard SN ratio corresponding to each scanning speed in the computer 15 for comparison with the detected SN ratio. Furthermore, the signal separated by the signal separation circuit 8 is used only for detecting the S/N ratio, and all detection signals for autofocus are sent to the computer 1 via another route.
5, however, a signal obtained by adding the separated signal component and the noise component may be used for autofocus.

0012

[Effects of the Invention] As explained above, the autofocus method based on the present invention scans an electron beam over a sample for autofocus, and the detection signal obtained as a result is divided into signal components and noise according to the frequency. The electron beam is separated into two components, and the scanning speed of the electron beam on the sample is changed based on the intensity ratio of the two separated components, so the optimum scanning speed of the electron beam can always be determined based on the actual S/N ratio. It is possible to set and perform accurate autofocus.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a scanning electron microscope for implementing the autofocus method of the present invention.

FIG. 2 is a diagram showing a flowchart of an autofocus operation according to the present invention.

FIG. 3 is a signal waveform diagram used to explain one embodiment of the present invention.

[Explanation of symbols]

1... Electron gun 2, 3... Focusing lens 4... Sample 5...
Deflection coil 6...Detector 7
…Amplifier 8…Signal separation circuit 9, 10, 11…Absolute value circuit 12, 13, 14…Integrator circuit

Claims (1)

[Claims] Claim 1: The excitation of a focusing lens for focusing an electron beam on a sample is changed stepwise, the irradiation position of the electron beam on the sample is scanned in each excitation state, and the irradiation position of the electron beam on the sample is scanned based on this scanning. In an autofocus method, the detection signal is detected and the excitation intensity of the focusing lens is determined based on the detection signal intensity corresponding to each scan. An autofocus method that changes the scanning speed of the electron beam on the sample based on the intensity ratio of both components.