JPS5942750A - Focal point detector for charged particle beam scanning type microscope - Google Patents

Abstract

PURPOSE:To permit an AC component to always appear even whenever a sample surface with directivity such as a metal frictional surface is scanned and accurately detect a focal point by forming the trace of a scanning line on the sample surface into a closed curve when the focal point is detected. CONSTITUTION:A picture signal is input to a rectifying and smoothing circuit 9 through a preamplifier 7 and a capacitor 8 for cutting a DC component and the strength of the DC omponent is detected by the same circuit. A lens current and such control circuit 10 controls a scanning signal circuit 11 and adds a sine wave signal to an x-direction deflection coil 4 and a sine wave signal that shifts the phase by 90 deg. to a y-direction deflection coil 5 and then deflects an electron beam 1 on the surface of a sample 2 along the circumference when a focal point is detected. The trace of a scanning line when the focal point is detected can be formed into a closed curve, for example, a circle, an ellipse, or an 8-shaped curve and the AC component can be obtained even for lateral fringes. In addition, the trace becomes useful when scratches, spots, etc. are observed at the center of a screen.

Description

DETAILED DESCRIPTION OF THE INVENTION (() Field of the Invention The present invention relates to a focus detection device for an objective lens in an apparatus such as a scanning electron microscope that obtains information regarding discrimination by scanning a metal surface with a charged particle beam. It is something.

(b) Prior art and its problems In general, focus detection in this type of microscope uses a method of scanning the sample surface and detecting the point where the alternating current component in the image signal obtained is maximum. However, in this conventional method, the sample surface is scanned during focus detection in the same way as during sample observation, so when the condition of the specimen surface has a directionality, such as a polished metal surface, the directionality When the objective lens current passes through the focus value point parallel to the X direction,
There was a problem that the correct focusing objective lens current could not be detected.

(C) Purpose of the Invention The present invention solves the above-mentioned problems by using a scanning signal different from that used in sample observation during focus detection, and by configuring the scanning line during focus detection to have no directionality. The purpose is to solve the following problems.

(d) Description of Embodiment FIG. 1 shows an embodiment of the present invention. 1 is a sample irradiation electron beam, 2 is a sample, 3 is an electromagnetic objective lens, 4 is a deflection coil for scanning in the X direction, 5 is a deflection coil for scanning in the y direction, and 6 detects secondary electrons etc. emitted from the sample 2. The output of the detector is a video signal.

This video signal is input to a rectifying and smoothing circuit 9 via a preamplifier 7 and a DC component cutting capacitor 8.
1 to apply a sine wave signal to the X direction deflection coil 4,
A sine wave signal whose phase is shifted by 900 is applied to the y-direction deflection coil 5, and the electron beam l is swung along the circumference on the surface of the sample 2. At the same time, the control circuit 10 controls the objective lens current power supply 12 to slowly change the objective lens current within a predetermined range.

Figure 2 shows the change in the strength E of the AC component when the excitation current of the objective lens is changed. Set the current to 7° to this value. This completes the focus detection operation, and finally the scanning signal circuit 11 is turned off and the normal
This causes scanning to be performed in the y-direction and the y-direction.

In FIG. 3, arrows indicate the scanning direction, and arrow a shows an example in which alternating current components are likely to appear depending on the directionality of the sample surface, and figure a shows an example in which alternating current components are difficult to appear. Figure 4 shows the locus of the scanning line during focus detection according to the present invention.
It is possible to obtain AC components even for horizontal stripes as shown in Figure 4, and if the scanning line is shaped like a figure 8 as shown in Figure 4, it is advantageous when observing scratches or spots in the center of the screen. It is.

(e) Effects of the Invention As described above, the present invention makes the locus of the scanning line on the sample surface during focus detection a closed curve like a circle or figure 8.
Even when scanning a directional sample surface such as a polished metal surface, there is an advantage that an alternating current component always appears during scanning and focus detection can be performed reliably. In addition, in the case of conventional X-direction scanning, there is a drawback that the focal position is incorrectly detected when the point with the maximum AC component enters the flyback section, but in the present invention, the scanning line is a closed curve. There is no such fear.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the device of the present invention, and FIG.
The figure is a graph showing the relationship between the lens current and the strength of the alternating current component, Figures 3a and b are top views showing the trajectory of the scanning line on the sample surface in the conventional example, and Figures 4a and b are graphs showing the relationship between the lens current and the strength of the AC component. FIG. 3 is a top view showing the trajectory of a scanning line on a sample surface according to the present invention. l... Charged particle beam, 2... Sample, G... ZJ object lens, 4, 5... Deflection coil, 6... Detector, 7
... Preamplifier, 8 ... DC component cutting capacitor, 9 ... Rectifier and smoothing circuit, 10 ... Lens current control circuit, ■] - ... Scanning signal circuit, 12 ... Objective lens current power supply Device. Agent Patent Attorney Kosuke Agata

Claims (1)

[Claims] In a beam scanning microscope that scans the specimen surface with a charged particle beam and obtains an image signal using secondary electrons generated from the specimen 1iri, the objective lens current is changed while scanning the specimen surface to obtain the video signal. A charged particle beam scanning microscope characterized in that the point at which the alternating current component of Focus detection device.