JP2000294184A - Electron microscope device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron microscope capable of minimizing image deviation caused by defocusing in an electron microscope utilizing an active modulation image formation method and hollow cone irradiation. SOLUTION: This device is provided with the following: means 23, 24 for sequentially varying the focus of an object lens 16 when an electron beam is applied in the form of a hollow cone to a sample to be observed; photographing devices 19, 20 for sequentially photographing a sample image according to the variation; a means 17 arranged between the object lens and the photographing devices for displacing the sample image in a plane perpendicular to an optical axis depending on the defocus amount of the object lens; means 28, 29 for multiplying weighting factors according to a weighting function in relation to the sample image in a series of focus conditions by the use of the weighting function specifying the relationship between the defocus amount and a weighting quantity; and means 28, 29 for forming the sample image weighted and accumulated by adding the weighted multiple images.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron microscope using an active modulation type imaging method.

[0002]

2. Description of the Related Art Conventionally, the resolution of a microscope is determined by the aperture diameter of an objective lens, and is limited by the spherical aberration when the objective lens has a spherical aberration. In order to separately observe the phase image and the amplitude image of the sample, it is necessary to install a special phase plate. For example, in the case of an electron microscope, the resolution is limited by spherical aberration, and an effective phase plate has not been considered. Under such circumstances, there is a strong demand for the development of an electron microscope that can obtain a high resolution by correcting spherical aberration and has an effective phase plate.

Several methods have been proposed for aberration correction. The method proposed this time is called "Image Restration i".
n Coherent Imaging System Involving Spherical Aber
ration ”J. Electron Microscope 38 (1989) 412-422
And the literature “Mathmatical Backgromd of defocus-modulat
ior image processing "is an extension of the known active modulation imaging method described in Optik 96 (1994) 129-135. The principle of this known imaging method is schematically illustrated in FIG. Vertical coherent illumination is performed on the sample to be observed, and at the time of capturing the sample image, a plurality of defocused images are captured while the focus is slightly shifted, and each defocused image is simultaneously weighted according to a predetermined function. And
By adding a plurality of weighted defocus images, a final phase image and amplitude image of the sample are separately observed.

The above-described active modulation type imaging method has a unique effect that a clear phase image and an amplitude image having almost no aberration can be captured by simply adding and subtracting a plurality of defocused images without taking special measures. have. However, since the resolution is limited due to the use of the vertical coherent illumination, it is strongly required to further increase the resolution.

As such a high-resolution electron microscope apparatus, there is known an electron microscope apparatus disclosed in Japanese Patent Laid-Open No. Hei 10-275580 proposed by the present applicant. This electron microscope is a microscope that combines active modulation imaging and hollow cone irradiation, and provides higher resolution and an aberration-corrected phase image and amplitude similar to those formed by an aberration-free lens. It has a very beneficial performance in which the images can be viewed separately.

[0006]

If the electron microscope apparatus proposed by the present applicant described above is ideally realized, an object of about 1 mm, such as a single metal atom or a gas molecule, can be sharpened at a lower accelerating voltage. We can expect that we can observe it. However, when the hollow cone irradiation and the active modulation type imaging method are used in combination, the present applicant has experimentally found that an image shift depending on the defocus amount and / or the inclination angle of the hollow cone irradiation occurs. This image shift reduces the resolution of the reproduced image by the active modulation type imaging method.

Accordingly, an object of the present invention is to provide an electron microscope using the hollow cone irradiation and the active modulation type imaging method proposed by the present applicant, which is a defocus amount and / or a hollow cone that becomes a barrier to high resolution observation. An object of the present invention is to correct an image shift depending on a tilt angle of a projection.

An electron microscope apparatus according to the present invention comprises: an electron gun for generating an electron beam; means for irradiating the sample to be observed with a hollow cone; and charged particles scattered by the sample. An objective lens that forms a sample image by forming an image, means for sequentially changing the focus of the objective lens when irradiating the hollow cone, an imaging device that sequentially captures the sample image according to the change in the focus of the objective lens, and Disposed between the objective lens and the imaging device,
Using a means for displacing the sample image in a plane perpendicular to the optical axis in accordance with the defocus amount of the objective lens, and a weighting function that defines the relationship between the defocus amount and the weight amount, a series of focused sample images is used. It is characterized by comprising means for multiplying a weighting factor according to a weighting function, and means for adding a plurality of weighted images to form a weighted and integrated sample image.

In order to obtain a high-resolution image with an electron microscope, it is necessary to perform aberration correction, obtain an image similar to an image picked up by an aberration-free lens, and to be able to separately observe a phase image and an amplitude image. Therefore, in the present invention, aberration correction is performed using the active modulation imaging method. However, as mentioned above, known active modulation imaging methods have limitations in increasing resolution due to vertical coherent illumination. On the other hand, as a result of performing various experiments and analyzes on electron beam irradiation, the present inventors have found that hollow cone irradiation can provide a spatial frequency transfer characteristic comparable to incoherent irradiation. In addition, it has been clarified that the weighting function required for the spherical aberration correction processing for the inclination angle of the incident electron beam of the hollow cone illumination can be uniquely determined according to the system used.
Based on such recognition, in the present invention, in addition to aberration correction by the active modulation type imaging method, the resolution is further improved by hollow cone irradiation.

As a result of various experiments and analysis by the inventor, the weighting function used for hollow cone irradiation is defined as a general expression. Weighting function for amplitude system reproduction

(Equation 1)

Where Δf is the defocus amount, w
Is the spatial frequency,

[Outside 1] Is a function that determines the degree of reproduction at each spatial frequency,
γ _s (v) is a wavefront aberration surface.

A weighting function W for phase image reproduction
_I (△ f) is given by the following equation.

(Equation 2) Therefore, the resolution can be further improved while maintaining the advantage of the active modulation type imaging method by defining and weighing the amount of irradiation electrons with respect to the amount of defocus based on these weight functions.

As a method of performing weighting in accordance with the amount of defocus, a series of defocus images are taken while changing the focus of the objective lens, and the relationship between the amount of defocus and the weight of the series of images is defined by a computer. There is a method of performing weighting by multiplying by a weighting factor based on the above function, and then adding the weighting factors. Also,
As another method, while changing the focus of the objective lens and changing the amount of irradiation electrons, two sample images weighted corresponding to the defocus are taken, and these sample images are subtracted to obtain a final sample. There is a method of forming an image. With this method, it is possible to reproduce a sample image in real time.

According to the microscope apparatus of the present invention, there is provided an electron gun for generating an electron beam, means for irradiating a sample to be observed with the generated electron beam with a hollow cone, and when irradiating a hollow cone, the irradiation beam is weighted by the amount of irradiated electrons Means for imaging the charged particles scattered by the sample to form a sample image; means for sequentially changing the focus of the objective lens; an imaging device for imaging the sample image; Means for displacing the sample image in a plane perpendicular to the optical axis in accordance with the defocus amount of the objective lens, the weighting function defining the relationship between the defocus amount and the weight amount. Therefore, a defocused image is sequentially captured while giving weight to the irradiation electron beam, and the captured image information is arithmetically processed to form a sample image. To.

An embodiment of the electron microscope apparatus of the present invention is as follows.
The means for weighting the electron beam and the means for changing the focus of the objective lens are constituted by a variable high voltage generator for applying a high voltage to the electron gun and its control circuit, and the output voltage from the variable high voltage generator is And time-dependently according to the weighting function. By applying a time-varying high voltage to the electron gun, the focus of the objective lens can be changed. at the same time,
Due to the temporal change of the high voltage applied to the electron gun, the irradiation time of the electron beam on the sample changes over time, and as a result, the irradiation amount of the electron beam changes according to the defocus amount.

Therefore, by changing the voltage value applied to the electron gun with time based on the weighting function,
Defocus formation and weighting can be performed simultaneously.

An embodiment of the electron microscope apparatus according to the present invention is as follows.
The means for displacing the sample image is constituted by a deflection coil and a coil drive circuit, and the drive current to the deflection coil is defined based on the defocus amount and / or the inclination angle of hollow cone irradiation. The present inventor has analyzed the shift amount of the image captured in the defocus state, and found that the shift amount of the image depends on the defocus amount and the inclination angle of hollow cone irradiation. Therefore, the image displacement means for correcting the image shift is constituted by the deflection coil and its drive circuit, and the drive current to the deflection coil is defined based on the defocus amount and the inclination angle of the hollow cone irradiation,
Image shift can be minimized.

[0017]

FIG. 2 is a diagram schematically showing the basic concept of an electron microscope according to the present invention. The description will be made based on an electron microscope for convenience of description, but the present invention can be similarly applied to any microscope. The electron beam generated from the electron gun is converted into a hollow cone-shaped electron beam having a predetermined inclination angle using a deflection lens system. As a method of forming a hollow cone-shaped electron beam, as described later, the electron beam may be rotated by a deflection lens system, or an expanded electron beam is formed and a hollow cone-shaped electron beam is formed by using an annular diaphragm. You can also. The sample 1 is irradiated with the hollow cone-shaped electron beam. Electrons passing through the sample 1 and scattered by the sample 1 are imaged by the objective lens 2 to form a series of sample images.

When a series of sample images are picked up, an image is picked up while changing the focus of the objective lens to form a series of defocused images. The final sample image is obtained by taking in a series of these sample images and a computer, performing weighting, and performing reprocessing. Also, by changing the irradiation time of the electron beam while giving defocus, a weighted defocus image can be obtained.

An important matter at this time is that the weighting function required for aberration correction is uniquely determined with respect to the tilt angle of the incident electron beam, and is based on the analysis result that it does not depend on the tilt azimuth. Thus, a series of images, both weighted and defocused, is obtained by using a weighting or weighting function determined by the system used, and by adding these images a spatial frequency comparable to incoherent illumination is obtained. The band can be reproduced with aspheric aberration. At this time, only addition and subtraction are required, so that real-time processing can be performed.

FIG. 3 is a diagram showing a specific example of an electron microscope according to the present invention. The electron microscope main body 10 has an electron gun 11 for generating an electron beam. The electron beam generated from the electron gun 11 enters the first deflection coil 14 via the acceleration electrode 12 and the condenser lens 13 and is converted into a hollow cone beam. The method of forming the hollow cone beam will be described later. Hollow cone beam is sample 1
The electron beam incident on the sample 5 and scattered by the sample is
6 forms an image of the sample. The electron beam emitted from the objective lens is corrected for the displacement of the sample image in a plane orthogonal to the optical axis by a second deflection coil 17 functioning as an image displacement correction means, and is passed through a projection lens 18 onto a fluorescent plate 19 on a fluorescent plate 19. Is imaged. The sample image by the electron beam incident on the fluorescent plate is converted into an optical image, and the sample image converted into the optical image is captured by the TV camera 20. Alternatively, a TV having a highly sensitive electronic image / light image conversion function without using a fluorescent plate
It is also possible to form an image directly on the photocathode of the camera 20 and take an image. As the imaging means, various means can be used, and means such as a two-dimensional camera and a photosensitive film having a charge storage ability like the TV camera can be used.

A high voltage generator 23 for generating a high voltage of about 200 kV is connected to the electron gun 11 via a voltage adder 22. A high voltage modulator 24 that generates a variable voltage in the range of ± 300 V is connected to the voltage addition 22. The high-voltage modulator 24 is controlled by the control circuit 21 and generates a voltage that changes with time according to weighting described later. The output voltage from the high voltage modulator 24 is superimposed on the high voltage from the high voltage generator 23 by the voltage addition 22 and applied to the electron gun 11. Therefore, a high voltage modulated according to the weighting function is applied to the electron gun 11, and the objective lens 16
Will change with time. As a result,
The sample image formed by the objective lens 16 is given a defocus amount that changes according to the modulated high voltage.

A deflection coil driving circuit 25 connected to a control circuit 21 is connected to the first deflection coil 14 for performing hollow cone irradiation, and a coil driving current is supplied. Then, the hollow cone angle and the beam rotation frequency of the hollow cone irradiation are controlled based on the supplied coil drive current.

Second deflection coil 17 for correcting image displacement
Is connected to a second coil drive circuit 26, which is connected to the control circuit 21. Then, the coil current supplied to the second deflection coil is controlled in accordance with the control signal supplied from the control circuit 21 to minimize the amount of image displacement caused by the defocus amount. In this case, data representing the relationship between the inclination angle of the hollow cone irradiation and the image displacement amount and data representing the relationship between the defocus amount and the image displacement amount at a series of inclination angles are measured in advance and stored in the memory of the control circuit 21. deep. Then, based on these data, by defining the coil current to be supplied to the second deflection coil in accordance with the inclination angle of the hollow cone irradiation and the defocus amount, it is possible to correct the image displacement amount to be minimum. . In the control circuit 21, the first deflection coil 14,
The function of storing and generating signals for controlling the objective lens 16 and the second deflection coil 17 may be executed by the deflection coil drive circuit 24, the objective lens drive circuit 25, and the second coil drive circuit 26, respectively. It is possible. In this case, the control circuit outputs a signal synchronized with the TV frame or a signal obtained by dividing the TV frame cycle to each of the driving circuits 24, 2
It only needs to be sent to 5, 26.

The objective lens 16 is connected to a lens drive circuit 27 connected to the control circuit 21 and controls the focal length of the objective lens 16 according to control from the control circuit.
At this time, the defocus amount can be given by changing the focal length of the objective lens by changing the lens current supplied to the objective lens with time. Furthermore, by changing the lens current supplied to the objective lens in addition to the high voltage supplied to the electron gun 11, both the defocus amount and the weight can be controlled.

An image picked up by the TV camera 20 is taken into an image subtractor 28. As described above, since the weighting has a positive or negative sign, the image subtracter 28 has two frame memories and an image subtraction circuit, and one frame memory stores the positively weighted image information. ,
Negatively weighted image information is taken into the other frame memory. Then, the subtraction circuit subtracts the positively weighted image information and the negatively weighted image information, and outputs the result as a final image. The obtained image is stored in the video frame memory 29 and can be displayed on the CRT 30 by a control signal from the control circuit 21. Alternatively, the output image of the subtraction circuit 28 can be directly displayed continuously on the CTR 30 without storing it in the video frame memory 29.

Next, the weighting of the electron beam will be described. In this example, a method of changing the irradiation time of the electron beam with respect to defocus is used as a weighting method. FIG. 4 is a diagram for explaining a defocusing and weighting method. 4A shows a load function for phase component reproduction, FIG. 4B shows a change in the defocus amount with respect to time for the load function shown in FIG. 4A, and FIG. 4C shows a time change in a voltage value applied to the electron gun. The load function shown in FIG. 4A is based on the above-described equation, where the horizontal axis represents the weighting amount and the vertical axis represents the defocus amount. In FIG. 4B, the horizontal axis represents time, and the vertical axis represents the defocus amount. Further, in FIG. 4C, the horizontal axis represents time, and the vertical axis represents the voltage value applied to the electron gun.

The high voltage from the high voltage generator 23 and the high voltage modulator 24 is applied to the electron gun 11. The change of the applied voltage at this time is set as shown in FIG. 4C. When the voltage applied to the electron gun 11 changes, the focus of the objective lens changes. At this time, the sample is continuously irradiated with the electron beam over time. Therefore, the change of the applied voltage with respect to time corresponds to the irradiation time of the electron beam to the sample.
Therefore, by applying a time-varying voltage to the electron gun as shown in FIG. 4C, the sample is irradiated with an electron beam weighted according to the defocus amount.
Note that the weighting amount takes both positive and negative values.
Using the charge accumulation capability of the V-camera, for example, for the positive weighting, the applied voltage is changed to create a first sample image weighted according to the defocus, and then the negative weighting is performed in the same manner. Then, a subtraction process is performed in the image subtraction circuit 28. This allows
A sample image weighted with respect to the defocus amount based on the load function can be obtained. Note that the image subtraction circuit can perform the above processing by having two frame memories and an image subtractor. With this configuration, it is possible to reproduce a sample image in real time.

FIG. 5 shows the configuration of the hollow cone beam forming apparatus. As shown in FIG. 5A, an electron beam generated from an electron gun is converted into a divergent beam by a first condenser 40, and is converted into a convergent beam by a second condenser 41. A condenser annular diaphragm 42 is arranged at the subsequent stage of the second condenser 41 to convert the converging electron beam into a hollow cone beam. Then, the hollow cone beam is directly applied to the sample. In the configuration of FIG. 4B, the electron beam passing through the condenser diaphragm 43 is converted into a rotating hollow cone beam by a two-stage deflection coil.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing an active modulation type imaging method.

FIG. 2 is a schematic diagram for explaining an imaging method of the electron microscope device according to the present invention.

FIG. 3 is a diagram showing a specific configuration of an electron microscope apparatus according to the present invention.

FIG. 4 is a diagram for explaining a method of giving a defocus amount and a weight according to a load function.

FIG. 5 is a diagram showing a configuration for forming a hollow cone beam.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sample 2 Objective lens 11 Electron gun 12 Acceleration electrode 13 Condenser lens 14 First deflection coil 15 Sample 16 Objective lens 17 Second deflection coil 18 Projection lens 19 Fluorescent plate 20 TV camera 21 Control circuit 22 High voltage generator 23 High voltage modulator 24 Deflection coil drive circuit 25 Objective lens drive circuit 26 Second coil drive circuit 27 Lens drive circuit 28 Image subtractor 29 Frame memory 30 CRT

Claims (3)

[Claims] 1. An electron gun for generating an electron beam, means for irradiating a sample to be observed with a hollow cone, and an objective lens for forming an image of charged particles scattered by the sample to form a sample image. And when irradiating the hollow cone,
Means for sequentially changing the focus of the objective lens, an imaging device for sequentially capturing a sample image in accordance with the change in the focus of the objective lens, and an image pickup device disposed between the objective lens and the imaging device. Means for displacing the sample image in a plane orthogonal to the optical axis in accordance with the weighting function that defines the relationship between the defocus amount and the weighting amount,
Means for multiplying a series of focus state sample images by a weighting factor according to a weighting function, and means for adding a plurality of weighted images to form a load-integrated sample image. Electron microscope equipment. 2. An electron gun for generating an electron beam, means for irradiating a sample to be observed with the generated electron beam with a hollow cone, and means for weighting an irradiation beam when irradiating the hollow cone with an irradiation electron quantity; An objective lens that forms a sample image by forming an image of charged particles scattered by the sample, a unit that sequentially changes the focus of the objective lens, an imaging device that captures an image of the sample, and Arranged, means for displacing the sample image in a plane orthogonal to the optical axis according to the defocus amount of the objective lens,
The method is characterized in that a defocused image is sequentially captured while weighting the irradiation electron beam according to a weighting function that defines a relationship between the defocus amount and the weighting amount, and the captured image information is arithmetically processed to form a sample image. Electron microscope device. 3. A means for displacing the sample image, comprising a deflecting coil and a coil driving circuit, wherein a drive current to the deflecting coil is defined based on a defocus amount and / or an inclination angle of hollow cone irradiation. The electron microscope apparatus according to claim 1, wherein: