JP3118559B2 - Electron microscope apparatus and sample image reproducing method using electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microscope capable of correcting and observing a phase image and an amplitude image of a sample by correcting aberrations.

[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.

[0004]

In the active modulation type imaging method described above, a clear phase image and amplitude image having almost no aberration can be obtained by simply adding and subtracting a plurality of defocused images without taking special measures. Has a specific effect of being able to image the image. However, since the resolution is limited due to the use of the vertical coherent illumination, it is strongly required to further increase the resolution. In particular, if a higher resolution can be obtained, it is expected that an object of about 1 mm such as a single metal atom or a gas molecule can be clearly observed at a relatively low acceleration voltage.

Accordingly, the present invention has a useful feature of the active modulation type imaging method, and it is possible to use a hollow cone illumination together with the method.
It is an object of the present invention to realize an electron microscope which can obtain twice the resolution and can observe a phase image and an amplitude image which have been subjected to aberration correction similar to those formed by an aberration-free lens, separately.

[0006]

A microscope apparatus according to the present invention comprises an electron gun for generating an electron beam, a means for irradiating a sample to be observed with the generated electron beam, and an irradiation beam for irradiating a hollow cone. Means for weighting the amount of irradiated electrons, an objective lens for forming an image of the sample by forming an electron beam scattered by the sample, means for changing the focus of the objective lens, and an imaging device for imaging the sample image. A defocused image is 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 image information obtained is arithmetically processed to form a sample image. Features.

In order to obtain a high-resolution image with an electron microscope, it is necessary to correct the aberration and obtain an image similar to that obtained 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 imaging method, the resolution is further improved by hollow cone irradiation. With this configuration, both the phase image and the amplitude image whose aberration has been corrected can be solved at once.

As a result of various experiments and analysis by the present inventors, a weighting function used for hollow cone irradiation is defined as a general expression. Weighting function W _R for amplitude-related playback
(Δf) is given by the following equation.

(Equation 1)

Here, Δf is the amount of defocus, w
Is the spatial frequency,

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

Also, a weighting function W _I between phase image reproductions
(Δf) 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 defocused images is taken while changing the focus of the objective lens, and the relationship between the amount of defocus and the weighting 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.

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

[0014]

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. An electron beam having passed through the sample 1 is formed 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 the focus of the objective lens is changed 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.

What is important 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, by using a weighting function determined by the system used, a series of both weighted and defocused images is obtained, and by adding these images there is no spatial frequency band comparable to incoherent illumination. Reproduction becomes possible due to spherical 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 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. The hollow cone beam enters the sample 15, and the electron beam scattered by the sample is imaged by the objective lens 16 to form a sample image. The electron beam emitted from the objective lens passes through the intermediate lens 17 and the projection lens 18 and is converted into an optical image by the fluorescent plate 19. The sample image converted into the optical image is captured by the TV camera 20. As the imaging means, various means can be used, and a means such as a two-dimensional CCD camera and a photosensitive film having a charge storage ability like a TV camera can be used.

In this embodiment, the computer 21 controls signal processing such as formation of defocus, control of weighting of an electron beam, and formation of an image signal. A high voltage generator 22 for generating a high voltage of, for example, about 200 kV is connected to the electron gun 11 and a high voltage modulator 23 for generating a voltage of ± 300 V is connected to the electron gun 11. The high voltage modulator 23 is controlled by the computer 21 and generates a time-varying voltage. The voltage from the high voltage modulator 23 is applied to the high voltage generator 22.
And is applied to the electron gun 11 while being superimposed on the high voltage. Therefore, a temporally modulated high voltage is applied to the electron gun 11, and the focal point of the objective lens temporally changes with time. As a result, the sample image formed by the objective lens is given a defocus amount that changes according to the modulated high voltage.

The deflection coil 14 includes a deflection coil drive circuit 24
The deflection coil drive circuit 24 is connected to a computer 21. Objective lens 16
Is connected to a lens control circuit 25, and the drive of the objective lens 16 is controlled by a control signal supplied from the computer 21. The image captured by the TV camera 20 is taken into the image subtractor 23. As described above, since the weighting has a positive or negative sign, the image subtractor 23 has two frame memories and an image subtraction circuit, and one of the frame memories stores the positively weighted image information. The other frame memory fetches image information with negative weighting. 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 24 and can be displayed on the CRT 25 by a control signal from the computer 21.

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 22 and the high voltage modulator 23 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 23. 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 30, and is converted into a convergent beam by a second condenser 31. A condenser annular stop 32 is disposed downstream of the second condenser 31 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. 5B, the electron beam that has passed through the condenser diaphragm 33 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 Deflection coil 15 Sample 16 Objective lens 17 Intermediate lens 18 Projection lens 19 Fluorescent plate 20 TV camera 21 Computer 22 High voltage generator 23 High voltage modulator 24 Deflection coil drive circuit 25 Object Lens control circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-105149 (JP, A) JP-A-7-153408 (JP, A) JP-A-4-233150 (JP, A) JP-A-60-105 105548 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01J 37/153 H01J 37/22

Claims (5)

(57) [Claims] 1. 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 assigning a weight of an irradiation electron amount to the irradiation beam when irradiating the hollow cone. An objective lens that forms an image of the sample by imaging the electron beam scattered by the sample,
A means for changing the focus of the objective lens, and an imaging device for imaging the sample image, which captures a defocused image while weighting the irradiation electron beam according to a weighting function that defines a relationship between the defocusing amount and the weighting amount. An electron microscope apparatus that performs arithmetic processing on the captured image information to form a sample image. 2. The means for weighting the electron beam and the means for changing the focus of the objective lens comprise a variable high voltage generator for applying a high voltage to an electron gun and a control circuit therefor. The electron microscope apparatus according to claim 1, wherein the output voltage of the electron microscope is temporally changed according to the weighting function. 3. A high voltage is applied to an electron gun to generate an electron beam, the generated electron beam is irradiated on a sample by a hollow cone, and the electron beam scattered by the sample is imaged by an objective lens to form a sample image. In imaging the sample image with an imaging device and reproducing a high-resolution electron microscope sample image,
A time-varying voltage is applied to the electron gun to temporally change the focus of the objective lens, and a voltage value for the time applied to the electron gun is weighted to define the relationship between the defocus amount and the weight. A method for reproducing a sample image by an electron microscope, wherein the method is defined based on a function. 4. The method according to claim 3, wherein the imaging device is an imaging device having a charge storage capability. 5. A voltage whose voltage value changes stepwise with respect to time is applied to the electron gun based on one of the positive and negative weighting portions of the weighting function, and the one of the weighting is applied. A second sample image obtained by capturing a first sample image and then applying a voltage whose voltage value changes stepwise with respect to time based on the other weighted portion to the electron gun and applying the other weight to the electron gun 5. The method according to claim 4, wherein the two sample images are subtracted from each other to reproduce the sample image.
3. A sample image reproducing method using an electron microscope according to item 1.