JPS64780B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for improving the image quality of low-magnification images obtained by a transmission electron microscope.

Observation of biological samples using a transmission electron microscope does not require very high resolution, and is often observed at a low magnification of about x1000 to x10000, but it is necessary to obtain a high-quality image over a wide field of view. is strongly required. In low-magnification images, blurring at the image periphery becomes noticeable, but in order to keep this blurring low, it is necessary to reduce the off-axis chromatic aberration of the imaging lens system. Off-axis chromatic aberration includes off-axis curvature astigmatism and off-axis chromatic aberration, but off-axis curvature astigmatism is hardly a problem in observing biological samples. Off-axis chromatic aberration includes lateral chromatic aberration and rotational chromatic aberration, and the amount of blur Δm based on lateral chromatic aberration on the fluorescent screen and the amount Δφ of blur based on rotational chromatic aberration are
is expressed as follows.

Δm=Cm・ΔV/V・R Δφ=Cφ・ΔV/V・R Where, Cm is the lateral chromatic aberration coefficient, Cφ is the rotational chromatic aberration coefficient, R is the distance from the center of the fluorescent screen (optical axis), and V
is the accelerating voltage of the electron beam irradiated on the sample, and ΔV is the accelerating voltage variation range of the electron beam that passes through the sample and enters the imaging lens system. In the case of biological samples, the energy loss ΔV of the electron beam when passing through the sample is relatively large, and is usually estimated to be 20 to 50 eV when converted to voltage. It needs to be made smaller. In a normal transmission electron microscope, the value of the excitation current supplied to each imaging lens is calculated so that these coefficients Cm and Cφ take the minimum value at each magnification.
The power supply is designed so that a current value according to this calculated value is supplied. However, in an actual device, it is difficult to maintain these coefficients at minimum values over all magnifications due to variations in current control resistors and the like. Therefore, the reality is that many transmission electron microscopes have no choice but to display images in a state where off-axis chromatic aberration is not at its minimum value.

In view of these circumstances, the present invention provides the ability to easily or automatically minimize Cm and Cφ at all observation magnifications.
The purpose is to enable you to obtain
means for minutely varying the accelerating voltage of an electron beam that irradiates a sample in the vicinity of a set value; a detector that detects the intensity of the electron beam provided at the periphery of a fluorescent screen on which a sample image is formed; and a detector for detecting the intensity of the electron beam. The present invention is characterized by comprising means for correcting the excitation intensity of the imaging lens system based on the accelerating voltage value when the output fluctuation of the device is minimized and the set accelerating voltage value.

FIG. 1 is a schematic diagram showing an apparatus according to an embodiment of the present invention. In the figure, an electron beam 2 generated from an electron gun 1 irradiates a sample 4 in a substantially parallel state through a focusing lens 3.
The electron beam transmitted through the sample forms an electron microscope image on a fluorescent screen 8 by an imaging lens system consisting of an objective lens 5, an intermediate lens 6, and a projection lens 7. As shown in FIG. 2, a detector 9 consisting of a minute detection element shaped like a part of a ring is arranged around the periphery of the fluorescent screen 8, so that when the position of the electron beam image on the fluorescent screen changes, is extracted as an electrical signal. The detection circuit 10 is for detecting fluctuations in the output of the detector 9, and its output is applied to the CPU 11. Reference numeral 12 indicates a power supply for the imaging lens system controlled by the output of the CPU 11, and reference numeral 13 indicates a high-voltage power supply that provides voltage for accelerating the electron beam to the electron gun 1.

The procedure for minimizing off-axis chromatic aberration using the apparatus of FIG. 1 is as follows.

First, a control signal is applied from the CPU 11 to the high voltage power supply 13 to give a small fluctuation ΔV to the set acceleration voltage value V. As a result, the electron microscope image on the fluorescent screen shows changes as shown in FIG. For example, if a circular image S is displayed at a distance R from the center O of the fluorescent screen 8, it moves by Δm in the direction R and Δφ in the direction perpendicular to R as the accelerating voltage changes. When the acceleration voltage is continuously varied in small steps, the position of the image changes in a U-shape as shown at 14. If the acceleration voltage is set to correspond to the apex of this U-shape, the off-axis chromatic aberration will be set to a minimum. Strictly speaking, even in this state, a slight rotational chromatic aberration remains, but since the relationship Δm>Δφ generally holds, almost no problem actually occurs. In the apparatus shown in FIG. 1, this change in image position is detected by the detector 9, and its output is applied to the CPU 11. When the fluctuation of the output signal is the smallest, the apex of the U-shape is reached. The acceleration voltage value V ₁ at that time is detected and stored.

Next, the CPU 11 instructs the high voltage power supply 13 to output the previously set acceleration voltage value V, and also outputs a voltage value obtained by relativistically corrected V ₁ and V.
A coefficient C=√ ₁ ^{* *} determined from the values of V ₁ ^* and V ^* is calculated. Further, a control signal is applied to the lens power supply 12 so that a current equal to the excitation current value of each lens multiplied by this coefficient C is supplied to each lens. If the optimum value for off-axis chromatic aberration determined by the accelerating voltage is converted into a lens current value and given in this way, an electron microscope image will be displayed in which off-axis chromatic aberration is kept at a minimum value. . However, since the excitation current of the imaging lens system is shifted from its original value, it is inevitable that the magnification of multiple small images will change.

By the way, although it is theoretically possible to make fine adjustments to such off-axis chromatic aberrations in advance at the stage of manufacturing the electron microscope, it is possible that the magnetic hysteresis of each lens and the sample position may be minutely changed each time the sample is replaced. This cannot be said to be realistic if changes are taken into consideration, so it is necessary to configure the apparatus to be able to perform it every time necessary, like the apparatus of the embodiment.

As described above, according to the present invention, when it is desired to reduce the influence of off-axis chromatic aberration, it is possible to easily correct it, so that a great effect can be obtained especially when observing biological samples using an electron microscope.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the apparatus of the present invention, FIG. 2 is a partially enlarged view of FIG. 1, and FIG. 3 is a schematic diagram for explaining the operation of the apparatus of FIG. 1. 1: Electron gun, 2: Electron beam, 3: Focusing lens,
4: Sample, 5: Objective lens, 6: Intermediate lens,
7: Projection lens, 8: Fluorescent screen, 9: Detector, 1
0: detection circuit, 11: CPU, 12: imaging lens system power supply, 13: high voltage power supply.

Claims (1)

[Claims] 1. A means for minutely varying the accelerating voltage of the electron beam that irradiates the sample in the vicinity of a set value, a detector that detects the intensity of the electron beam provided at the periphery of the fluorescent screen on which the sample image is formed, and A transmission electron microscope characterized by comprising means for correcting the excitation intensity of an imaging lens system based on an accelerating voltage value at which the output fluctuation of the detector is minimized and the set accelerating voltage value.