JPS5953660B2 - scanning electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a scanning electron microscope having a single-stage deflection scanning system.

Since the single-stage deflection scanning system has a simpler structure than the two-stage deflection scanning system, it is often used in relatively simple scanning electron microscopes.

FIG. 1 shows the outline of the system. An electron beam from an electron gun (not shown) is focused by a first focusing lens 1, focused on a point P, and projected onto an aperture plate 2 in a diverging state. .

The electron beam that passed through the aperture of this aperture plate is the only one in the X and Y stages.
The light is two-dimensionally deflected and scanned by the deflection coil 3, and the light that passes through the ridge 5 of the final stage focusing lens (objective lens) 4 is focused and irradiated onto the sample 6.

(Refer to Japanese Patent Publication No. 48-14383.) In such a scanning system, when the deflection force and deflection direction by the deflection coil 3 are changed, different parts of the electron beam diverged from the point P are moved to the aperture 5.
Since the electron beam is projected onto the sample 6 through the electron beam, the intensity distribution of the electron beam must be uniform.

In fact, when observing at medium and high magnifications of several hundred times or more, only the central part of the electron beam is used, so the intensity distribution of the electrons irradiated onto the sample becomes uniform.

However, in extremely low magnification observation of several tens of times, it is necessary to utilize the peripheral part of the electron beam as shown in Figure 2, so it is necessary to reduce the intensity as shown by ΔI with respect to the central part. arise.

When a sample image is obtained by electron beam scanning with such a decrease in intensity in the periphery, the brightness and contrast decrease in the periphery of the cathode ray tube screen, and deterioration in image quality is unavoidable.

In actual photographic judgment, at a viewing magnification of 35 times, the brightness is about 15% and the contrast is 20 to 25% lower than the center.

The present invention solves the above-mentioned drawbacks, and is intended to improve the brightness of the cathode ray tube screen so as to increase the brightness of the peripheral area of the cathode ray tube screen in synchronization with the scanning of the electron beam in the X and Y directions. The feature is that it controls brightness.

The present invention will be explained below based on the embodiment shown in FIG.

In the figure, the same symbols as in Figure 1 indicate the same parts, and the deflection coil is
The display is divided into 3X for the direction and 3Y for the Y direction.

7X is an X-direction (horizontal) scanning signal generation circuit, 7Y is a Y-direction (vertical) scanning signal generation circuit, and their output signals are sent to each deflection coil 3X, 3Y via a magnification adjustment circuit 8.

Further, the output signal of the scanning signal generation circuit is transmitted through an amplifier 9X,
Deflection coils 11X and 11Y of the cathode ray tube 10 via 9Y
is being sent to.

12 is a detector that detects information from the sample such as secondary electrons and reflected electrons, and its output signal is sent to the brightness modulation grid of the cathode ray tube 10 via an image amplifier 13 and a multiplier (or adder) 14. applied.

The X and Y output signals from the magnification adjustment circuit 8 are as follows:
After the negative signal is inverted to positive through the absolute value circuits 15X and 15Y, it is supplied to the adder circuit 16.

17 is a variable DC voltage source, and the output voltage of this power source is also supplied to the adder circuit 16.

The output of the adder circuit 16 is added or multiplied with the video signal from the detector 12 and then supplied to the grid of the cathode ray tube 10.

Now, from the magnification adjustment circuit 8 to the absolute value circuit 15X.

The negative voltage of the sawtooth scanning signal sent to 15Y is inverted, resulting in a signal as shown in FIG. 4a.

At this time, it goes without saying that the period of the 15Y output is significantly longer than that of the 15X output, and is usually several hundred to several thousand times as long.

In the adder circuit 16, the output signals of the absolute value circuits 15X, 15Y and the DC voltage source 17 are added, and a luminance signal as shown in FIG. 4 is output.

In the figure, τ is the period of scanning in the X direction (horizontal), T is the period of scanning in the Y direction (vertical), and vo is the DC voltage source 17.
is the output voltage of

When such a signal is multiplied or added to a video signal and applied to the cathode ray tube 10, the brightness and/or contrast at the periphery of the screen of the cathode ray tube is increased to 1, and a uniform image on the entire screen is obtained.

Since the absolute value circuits 15X and 15Y receive scanning signals having different amplitudes depending on the magnification, as the magnification decreases, the fourth
The signal value in the figure increases, and the brightness and contrast of the peripheral area can be automatically corrected in response to changes in magnification.

In the case of an actual device, the above-mentioned brightness and contrast corrections are not necessary for intermediate observation or high magnification observation, so the magnification adjustment circuit 8 and the absolute value circuit 15X are used.

It is recommended to provide a switch between the terminal and 15Y.

At this time, link the magnification adjustment knob and switch,
It is convenient to automatically start the correction operation when the magnification falls below a certain level.

As detailed above, in the device based on the present invention,
The brightness of the cathode ray tube is controlled so that the level of the brightness modulation signal is substantially increased from the center to the periphery of the cathode ray tube screen with a gradient corresponding to the amplitude of the scanning signal adjusted by the magnification adjustment circuit. Therefore, even when the observation magnification is changed, optimal peripheral brightness correction is performed according to the observation magnification, and a homogeneous image can always be observed.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a conventional single-stage deflection scanning system, Fig. 2 is a diagram showing its electron beam intensity distribution, Fig. 3 is a block diagram showing an embodiment of the present invention, and Fig. 4 is its main part. It is an operation explanatory diagram. 1: Focusing lens, 2: Diaphragm plate, 3X, 3Y: Deflection coil, 4: Final focusing lens, 5: Fringe, 6: Sample, 7X, 7
Y: Scanning signal generation circuit, 8: Magnification adjustment circuit, 9X, 9Y
: Amplifier, 10: Cathode ray tube, IIX, IIY: Deflection coil, 12: Detector, 13: Video amplifier, 141 Multiplying circuit, 15X, 15Y: Absolute value circuit, 16: Adding circuit, 17
:DC voltage source.

Claims (1)

[Claims] 1. A deflection device consisting of a single stage deflector placed above the final stage focusing lens, and for two-dimensionally scanning the electron beam in a diverging state on the aperture plane placed at the center of the final stage focusing lens. a scanning signal generation circuit for supplying a scanning signal of 1 to the deflector; a magnification adjustment circuit for adjusting the amplitude of the scanning signal supplied from the scanning signal generation circuit to the deflector; and a sample of the electron beam. In an apparatus equipped with a cathode ray tube for displaying a sample image by scanning in synchronization with the above scanning and using an information signal from a sample as a luminance modulation signal, the amplitude is adjusted by the magnification adjustment circuit. Scanning characterized by comprising means for controlling the brightness of the cathode ray tube so that the level of the brightness modulation signal is substantially increased from the center to the periphery of the cathode ray tube screen with a corresponding gradient. electronic microscope.