JP2008270056A - Scanning transmission electron microscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To acquire a high-resolution image of a confocal scanning transmission microscope, to acquire a cross-sectional image, and to acquire confocal STEM imaging of a dark-field image. <P>SOLUTION: This transmission electron microscope allows a sample stage holding a sample to be adjusted to be movable by a nano-drive mechanism in the Z-axis direction being an optical axis direction of an electron beam and X-axis and Y-axis directions (the X-axis and the Y-axis are orthogonal to each other) orthogonal thereto. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  In the present invention, irradiated electrons emitted from the electron gun are focused on the sample by the focusing lens, and the electrons transmitted through the sample are focused again on the aperture position on the upper surface of the detector below the sample by the imaging lens to converge on the sample. The present invention relates to a transmission electron microscope having a confocal optical system in which only a central part of an electron probe is transmitted through a diaphragm and detected by a detector.

For this type of transmission electron microscope, as shown in Patent Document 1, a converging electron beam scan-descan system as shown in FIG. 1 is adopted in order to realize a confocal scanning transmission electron microscope. However, there is a problem that high-resolution observation cannot be performed due to the synchronization accuracy of scan-descan.
Specifically, an electron gun (including an acceleration tube), one or more converging lenses, a sample, one or more imaging lenses, a diaphragm, a transmission electron detector, and a plurality of electromagnetics for aligning these electron beams It consists of a deflector.
The irradiated electron beam emitted from the electron gun is focused on the sample so as to have a minimum probe diameter by a converging lens. The irradiation electron beam is scanned in the XY directions parallel to the sample surface by the scan coil above the sample. The electrons that have passed through the sample are again focused on the aperture position on the detector under the sample by the imaging lens. The positional deviation of the electron beam from the optical axis due to the XY scan on the sample is returned to the optical axis by a descan coil below the sample, and only a part of the center of the convergent electron probe on the sample is transmitted by the diaphragm. Detect with a detector.
In such a conventional configuration, in order to obtain an image, the beam is scanned by the scan coil, and the deviation of the beam position from the optical axis at that time is turned back by the descan coil so as to be incident on the diaphragm in front of the detector. ing.
When a sub-nanometer high-resolution image is obtained, a lateral displacement occurs due to the accuracy of the return, and in principle, it is difficult to obtain a high-resolution STEM image. Also, changing the focus of the objective lens to obtain a tomographic image raises and lowers the beam convergence point at the sample position, so the convergence point to the diaphragm position before the detector also moves up and down, and the maximum convergence at the diaphragm position. I can't get points.
In addition, if the focal strength of the objective lens is changed, the focal point of incidence on the stop under the imaging lens system also changes, so that the focal position cannot be changed in order to maintain the confocal condition. For this reason, it is difficult to acquire a tomographic image in the depth direction.
In Non-Patent Document 1, a spherical aberration correction device is incorporated in the focusing system and the irradiation system to realize an ideal confocal optical system with almost no image blur, but an image cannot be acquired because there is no scanning system.
In addition, all known materials are aimed at obtaining a bright-field image, and the image is subjected to diffraction effects and phase modulation. As a result, it was not possible to obtain a dark field image having a contrast that depends only on the atomic number and thickness of the sample by focusing high-angle scattered electrons on one point on the image plane.
US Pat. No. 6,548,810 Applied Physics Letter 89, 124105 (2006) p. D. Nellist et al.

  In view of such circumstances, the present invention not only enables high-resolution image acquisition of a confocal scanning transmission electron microscope, but also provides a tomographic image acquisition and further obtains a confocal STEM image of a dark field image. It was.

  The transmission electron microscope according to the first aspect of the present invention is configured such that the sample stage holding the sample is moved by a nano-drive structure in the Z-axis direction, which is the optical axis direction of the electron beam, and the X-axis and Y-axis directions orthogonal thereto It is characterized in that the movement can be adjusted to be perpendicular to each other.

  A second aspect of the invention is a transmission stage electron microscope of the first aspect, wherein the sample stage automatic adjustment mechanism operates the nano-drive structure in a direction in which the out-of-focus blur is eliminated in response to the out-of-focus condition acquired by the detector of the confocal optical system. Is provided.

  Invention 3 is characterized in that in the transmission electron microscope of Invention 1 or 2, the nano-drive structure is constituted by a piezo drive.

  Invention 4 is the transmission electron microscope according to any one of Inventions 1 to 3, wherein a diaphragm that transmits only electrons inclined at a predetermined angle with respect to the optical axis is disposed below the sample, and transmitted through the diaphragm. A focusing lens system in which electrons are focused on a detector is provided, and the detector has a dark field STEM structure for obtaining a dark field STEM image of transmitted electrons by the detector.

  A fifth aspect of the invention is the transmission electron microscope according to the fourth aspect, wherein the spherical aberration is detected by the defocus state of the dark field STEM obtained by the dark field STEM structure, and the sample stage is moved so as to minimize the spherical aberration. It is characterized by having an automatic adjustment mechanism.

  A sixth aspect of the present invention is the transmission electron microscope according to the fourth or fifth aspect, wherein the diaphragm is an annular diaphragm.

Since the confocal optical system can be fixed according to the first aspect of the present invention, it is possible to obtain a high-resolution image of a confocal scanning transmission electron microscope by XY scanning of the sample stage without causing blurring as in the prior art.
In addition, the tomographic image of the confocal scanning transmission electron microscope can be acquired by moving the sample stage in the Z axis.

  According to the invention 4, it is possible to collect a wide range of high-angle scattered electrons on a single image plane and obtain a dark field image having a contrast dependent only on the atomic number and thickness of the sample by confocal STEM imaging. did.

  The electron microscope of the present invention includes an electron gun (including an acceleration tube), one or a plurality of converging lenses, a sample stage capable of three-dimensional movement of XYZ with a piezo drive, a sample supported by the sample stage, and a circle disposed under the sample. An annular diaphragm, one or a plurality of imaging lenses, a spherical aberration correction device incorporated below the sample, a transmission electron detector, a diaphragm arranged on the detector, and a plurality for aligning the electron beams It consists of an electromagnetic deflector.

The irradiated electron beam emitted from the electron gun is focused on the sample so as to have a minimum probe diameter by a converging lens. Of the electrons transmitted through the sample by the annular diaphragm, only the electrons scattered at a high angle are again focused on the diaphragm position on the transmission electron detector by the imaging lens.
The electron beam focused on the aperture position on the transmission electron detector is converged to a single point by the spherical aberration correction device without any focal blur depending on the spherical aberration. Only a part of the center of the converged electron beam is transmitted by the diaphragm and detected by the detector. The sample is scanned in a sample surface parallel direction (XY direction) by a sample stage capable of three-dimensional movement of XYZ. A confocal dark field STEM image is obtained by reading the transmitted electron intensity at each XY point and outputting it as an image from the XY position information and the transmitted electron intensity by a computer.
Further, each time one image is acquired, the sample is moved in the Z direction by a sample stage capable of three-dimensional movement of XYZ, and a scanning image at each Z position is acquired, thereby obtaining a three-dimensional tomographic image.

By using an annular diaphragm disposed on the focal plane of the objective lens of the present invention, the position of the convergent electron beam is fixed, the sample stage is scanned, and high-angle scattered electrons are detected by a transmission electron detector, A high-angle scattering annular dark field STEM (HAADF-STEM) image could be obtained.
In addition, a spherical aberration correction device is incorporated in the imaging lens system, and the transmitted electrons are focused again on the image plane under the condition that there is no blur due to spherical aberration. If imaged, a scanned image can be obtained by scanning the sample stage while maintaining an ideal confocal optical system.

As a result, a bright field STEM image having lateral resolution and depth resolution exceeding the beam diameter of the convergent beam can be obtained, and a tomographic image having a contrast (Z contrast) depending on the atomic number can be observed.
Furthermore, if a spherical aberration correction device is incorporated in the focusing lens system, the focused electron beam is made to have a minimum electron beam diameter that is not affected by spherical aberration, and the confocal optical system as described above is realized, further lateral resolution and depth can be achieved. A confocal bright-field / dark-field STEM image with high resolution is obtained.

In the following embodiment, a spherical aberration correction device is incorporated in the imaging lens system, and transmitted electrons or high-angle scattered electrons are focused again on the image plane under the condition that there is no blur due to spherical aberration, and only a part of the center thereof is converged. And a loss energy spectrum based on element species using an electron energy spectrometer under confocal imaging conditions, a tomographic image of element distribution can be obtained.
An element distribution image can be obtained by measuring the characteristic X-rays of an arbitrary element generated from the beam position while synchronizing with the focused electron beam scanning, and is currently widely used.
Recently, in order to improve the detection efficiency of X-rays, a technique for collecting and measuring generated X-rays with an X-ray lens has been developed. Only the X-rays from the fixed point are detected efficiently, and X-rays from the region deviating from the fixed point cannot be detected.

If the scanning sample stage of the present invention is used, an element distribution image by characteristic X-rays can be obtained by fixing the position irradiated with the electron beam while performing scanning. As a result, it is possible to acquire an X-ray analysis element image that collects characteristic X-rays only from such a fixed point with an X-ray lens and acquires the X-ray analysis elements with high detection efficiency.
A method of collecting the light generated by the irradiation of the focused electron beam outside the electron microscope with a condensing lens and obtaining the emission spectrum is called cathodoluminescence spectroscopy and is currently widely used.

If the electron beam is scanned, a distribution image of the emission spectrum can be obtained. However, the condensing lens can efficiently detect only light from a certain fixed point (that is, the condensing point), and from a region outside the fixed point. Cannot be detected.
By using the scanning sample stage of the present invention, it is possible to obtain an emission spectrum by fixing the position irradiated with the electron beam while performing scanning. As a result, it is possible to perform emission spectrum image acquisition in which light only from such a fixed point is collected by a condenser lens and acquired with high detection efficiency.

A scanning sample stage for a scanning transmission electron microscope (hereinafter referred to as STEM) and its control system were manufactured. Further, in the 200 kV transmission electron microscope, a confocal optical system was realized by attaching a minimum aperture just before the transmission electron detector.
A confocal STEM image acquisition experiment was performed using a scanning sample stage. In addition, an annular diaphragm was mounted at the objective diaphragm position (objective lens focal plane).
FIG. 4 The observation sample is held at the tip of an XYZ-drive piezo element incorporated in a sample goniometer mounted in an electron microscope. The irradiated electrons emitted from the electron gun are focused on the sample so as to have a minimum probe diameter by the converging lens.

This convergent electron probe is scanned within the sample surface (XY) by a deflection coil. The transmitted electrons that have passed through the sample are again focused on the aperture position on the upper surface of the detector under the sample by the imaging lens, and an enlarged projection image of the convergent electron probe on the sample is formed. Only a part of the center of the convergent electron probe is transmitted by the diaphragm, its intensity is measured by the detector, and the intensity is synchronized with the electron beam scanning to change the intensity of the transmitted electron from each point on the sample surface. (Bright-field-confocal STEM image).
The STEM image output PC inputs XY scanning and height (Z position) control signals of the sample to the XYZ-scanning sample stage control module, and supplies the XYZ voltages output from the XYZ-drive piezo elements. XY scanning and height adjustment of the sample are performed. The transmission electron intensity detected by the detector at each XY point of the sample is amplified by the detector control module, and the STEM image is constructed and output by the STEM image output PC. The tomographic observation becomes possible by changing the Z position of the sample every time one image is acquired and obtaining a large number of images.

  FIG. 5 The scanning sample stage is a general term for sample goniometers incorporating an XYZ-drive piezo element. The XYZ piezo drive unit includes a Y piezo element and an XZ piezo element, and the Y piezo element moves the X position of the sample by expansion and contraction due to voltage application. The piezo element for XZ moves the sample in the X direction by swinging left and right by applying a symmetrical voltage in a direction parallel to the sample surface, and moves up and down by applying a symmetrical voltage in a direction perpendicular to the sample surface. The sample is moved in the Z direction by swinging the head. The voltages for driving these XYZ are output from the XYZ-sample scanning control module. The XYZ voltages output from the XYZ-sample scanning control module are set by the STEM image output PC.

This structure will be described in more detail with reference to FIGS. 12 and 6 as follows.
In the sample stage, a sample holding unit (FIG. 6) is fixed to a sample holder for an electron microscope through a connection of two tube piezo elements, and the tube piezo (for X and Z movement) is a neck by applying a drive voltage. The sample holder is moved in the X direction perpendicular to the optical axis of the electron beam and the sample holder axis and in the Z direction parallel to the optical axis of the electron beam by the swinging action.
The tube piezo (for Y movement) moves the sample holder in a direction parallel to the axis of the sample holder by an expansion / contraction operation effect by applying a drive voltage. A position setting parameter for arbitrary XY scanning is input to the XYZ sample stage scanning control power source by the confocal STEM image acquisition system, and the sample holding unit is moved XY two-dimensionally by the control voltage output therefrom.

In this way, as shown in FIG. 13, the irradiated electrons emitted from the electron gun are focused on the sample by the converging lens, and the electrons that have passed through the sample are brought to the aperture position on the detector upper surface under the sample by the imaging lens. In the confocal optical system that focuses again and transmits only a part of the center of the focused electron probe on the sample through the diaphragm, the transmitted electrons are detected by the detector, and the intensity is amplified by the transmission electron detection module. The signal is input to a confocal STEM image acquisition system. A signal at each XY position is imaged in the confocal STEM image acquisition system as an image intensity to obtain a tomographic image by the confocal STEM.
When one tomographic image is acquired, another Z position parameter is input to the XYZ sample stage scanning control power supply by the confocal STEM image acquisition system, and the sample holding unit is moved in an arbitrary Z direction by the control voltage output therefrom. Obtain the next tomogram. By repeating this, a plurality of tomographic images at different Z positions are obtained.

FIG. 7 is an observation image on a fluorescent screen when a diaphragm having a hole diameter of 300 to 50 microns is attached to the upper side of the transmission electron detector, and a shadow image of the diaphragm is observed superimposed on the enlarged sample image. An aperture diameter of 50 microns corresponds to approximately 0.2 nm on the sample surface.
FIG. 8 shows a confocal STEM image obtained by scanning the sample stage, and it can be seen that the sample is graphite and the C-plane lattice (width 0.34 nm) of graphite is resolved.

FIG. 9 is an image obtained by magnifying and observing the position with an imaging lens with an annular diaphragm attached to the objective diaphragm position (focal plane of the objective lens). Only scattered electrons from 40 mrad to 120 mrad pass and are used for imaging.
As shown in FIG. 10, the ring-shaped aperture is a plate-shaped body made of an electron-impermeable material and having an annular electron-transmitting portion. Depending on the diameter of the electron-transmitting portion, the light of the electron beam to be transmitted An angle with respect to the axis can be set.

  The confocal STEM and confocal dark field STEM according to the present invention enable observation of thick semiconductor devices and biological samples with high resolution, device inspection and performance evaluation in the semiconductor industry, and high resolution and high contrast 3D of biological samples. It becomes a breakthrough tool in terms of observation. Further, Z-contrast tomographic image observation using high-angle scattered electrons is equivalent to a confocal fluorescence microscope in terms of an optical microscope, and element distribution information inside the sample can be observed. Currently, in the field of semiconductor device analysis, FIB is used for 3D observation using a method that exposes the cross-section of the sample and tomography technology, but in the former, observation is performed while destroying the sample. And the obtained image is a two-dimensional cross-sectional image. In the latter case, there are disadvantages that the contrast changes in the viewing direction due to the diffraction effect, and that there is only one rotation axis, so information on the rotation axis direction cannot be obtained. The resolution is also about 1 nm, and atomic resolution cannot be obtained. Confocal STEM does not provide complete three-dimensional information, but can obtain tomographic images at atomic resolution. Regarding biological samples, the samples are generally thick and it is difficult to obtain contrast by electron microscope observation, but high-resolution and high-contrast three-dimensional observation of the sample is possible by confocal observation technology. As described above, the confocal scanning electron microscope aimed at development in this research is an apparatus that can be a breakthrough for semiconductor device analysis and biological sample observation, and has a very high ripple effect on various scientific fields and industrial fields.

Principle of scan-descan confocal STEM Principle diagram of stage scan confocal STEM Principle of high-angle annular dark field-confocal STEM Explanatory drawing of preliminary experiment conducted with scanning sample stage and confocal optical system Schematic diagram of scanning sample stage and its control system Photograph of scanning sample stage for STEM Observation image on a fluorescent screen when a minimum aperture is attached just before the transmission electron detector Confocal STEM image (bright field) obtained by experiment Photograph showing an annular aperture mounted at the objective aperture position (objective focal plane) Drawing for clarifying the structure of an annular diaphragm Schematic diagram explaining the function of an annular diaphragm Perspective view showing a piezo-driven XYZ-driven sample stage Conceptual diagram showing a three-dimensional tomographic image acquisition system by confocal STEM using a piezo-driven XYZ-driven sample stage

Claims (6)

  The irradiated electron emitted from the electron gun is focused on the sample by the converging lens, and the electron transmitted through the sample is again focused by the imaging lens on the aperture position on the detector upper surface below the sample, and the center of the converging electron probe on the sample A transmission electron microscope having a confocal optical system that transmits only a part of the sample by a diaphragm and detects the same by a detector, and the sample stage holding the sample is arranged in the optical axis direction of the electron beam by a nano-drive structure A transmission electron microscope characterized by being movable and adjustable in a Z-axis direction and an X-axis direction and a Y-axis direction orthogonal to the Z-axis direction (X-axis and Y-axis are mutually orthogonal).   2. The transmission electron microscope according to claim 1, further comprising: a sample stage automatic adjustment mechanism that operates the nano-drive structure in a direction to eliminate out-of-focus blur in response to the out-of-focus condition acquired by the detector of the confocal optical system. A transmission electron microscope characterized by being provided.   3. The transmission electron microscope according to claim 1, wherein the nano-driving structure is configured by a piezo drive. 4.   4. The transmission electron microscope according to claim 1, wherein a diaphragm that transmits only electrons inclined at a predetermined angle with respect to the optical axis is disposed below the sample, and the electrons transmitted through the diaphragm. A transmission electron microscope having a dark field STEM structure in which a converging lens system focused on a detector is provided and a dark field STEM image of transmitted electrons is obtained by the detector.   5. The transmission electron microscope according to claim 4, wherein spherical aberration is detected by a defocus state of the dark field STEM obtained by the dark field STEM structure, and the sample stage is moved so as to minimize the spherical aberration. A transmission electron microscope having an automatic adjustment mechanism.   6. The transmission electron microscope according to claim 4, wherein the stop is an annular stop.