JP2013026152A - Electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron microscope which has a relatively large use range of acceleration voltage without increasing sample damage and contamination.SOLUTION: An electron microscope comprises: an electron beam optical system that irradiates a sample with an electron beam from an electron gun; and an electron detector that detects an electron from the sample. The electron detector includes a ceramic scintillator and a photoelectric conversion element that converts light from the ceramic scintillator into current. The ceramic scintillator includes a ceramic fluorescence substance that is formed by sinter a fluorescence substance, and the thickness of the ceramic fluorescence substance is 200 to 300 μm.

Description

  The present invention relates to an electron microscope, and more particularly to an electron detector provided in an electron microscope.

A scintillator is used in an electron detector used in an electron microscope. The scintillator is a glass plate coated with a phosphor. The thickness of the fluorescent film is usually about 30 μm. The phosphor emits light when irradiated with an electron beam. This light is guided to a photomultiplier tube by a light guide or the like, and the light is converted into an electric current. Known phosphors used in scintillators for electron detection include p47: Y ₂ SiO ₅ ; Ce, p43: Gd ₂ O ₂ S; Tb, and the like having a short afterimage time.

  Patent Document 1 describes an example of a scintillator type electron detector used in a scanning transmission electron microscope. Patent Document 2 describes an example of a scintillator type backscattered electron detector used in a scanning electron microscope.

  A scintillator is also used to detect X-rays and γ-rays. Patent Documents 3 and 4 describe examples in which X-rays are measured using a ceramic scintillator in an X-ray CT apparatus. X-rays are more transmissive than electron beams. Therefore, the ceramic scintillator for X-ray measurement differs from the scintillator of the electron detector in the fluorescent material, shape, dimensions, and the like. For example, a ceramic scintillator for X-ray measurement uses a phosphor containing Pr, Gd, etc., which has high emission efficiency for X-rays, and is composed of a laminated sintered body having a thickness of about 1 to 2 mm.

Japanese Patent Laid-Open No. 09-167591 JP-A-11-273608 JP 2000-178547 A JP 2004-101367 A

  In order to obtain a high-quality electron microscope image, it is better that the amount of signal output from the electron detector is large. If the acceleration voltage is increased to increase the signal amount, the sample damage increases and contamination occurs. Accordingly, there is a trade-off relationship between increasing the detection signal amount and suppressing sample damage and contamination.

  Then, as a result of earnest consideration by the inventors of the present application, the following knowledge could be obtained. In order to increase the detection signal amount without increasing sample damage and contamination, the sensitivity of the scintillator used for the electron detector may be increased. That is, the conversion efficiency from electron beam to light in the scintillator may be increased.

  Therefore, first, it is conceivable to increase the thickness of the fluorescent film. Increasing the thickness of the phosphor film increases the number of electrons that collide with the phosphor. However, the amount of light emitted from the fluorescent film to the outside does not necessarily increase. On the other hand, when the thickness of the fluorescent film is reduced, the number of electrons transmitted without colliding with the phosphor increases. Therefore, the amount of light emission decreases. Therefore, in the conventional electron detection device, the thickness of the fluorescent film is substantially determined, and is usually about 30 μm and is white. Therefore, there is a limit to the use range of the acceleration voltage.

  An object of the present invention is to provide an electron microscope in which the use range of the acceleration voltage is relatively large without increasing sample damage and contamination.

  According to the present invention, an electron microscope includes an electron beam optical system that irradiates a sample with an electron beam from an electron gun, and an electron detector that detects electrons from the sample. The electron detector includes a ceramic scintillator and a photoelectric conversion element that converts light from the ceramic scintillator into an electric current. The ceramic scintillator includes a ceramic phosphor formed by sintering a phosphor. The thickness of the phosphor is 200 to 300 μm.

  According to the present invention, it is possible to provide an electron microscope in which the use range of the acceleration voltage is relatively large without increasing sample damage and contamination.

It is a figure which shows the structural example of the transmission scanning electron microscope of this invention. It is a figure which shows the structure of the principal part of the transmission scanning electron microscope of this invention. It is a figure which shows the example of the structure of the conventional scintillator. It is a figure which shows the example of the structure of the ceramic fluorescent substance of the ceramic scintillator of this invention. It is a figure which shows the relationship between the acceleration voltage of the ceramic scintillator of this invention, and an electron beam penetration depth. It is a figure which shows the example of the structure of the ceramic scintillator of this invention. It is a figure which shows the example of the structure of the ceramic scintillator of this invention. It is a figure which shows the structure of the bright field image detector of this invention. It is a figure which shows the structure of the dark field image detector of this invention. It is a figure which shows the structure of the secondary electron detector of this invention.

  An example of a transmission scanning electron microscope according to the present invention will be described with reference to FIG. The transmission scanning electron microscope apparatus of this example includes an electron gun 1, first and second irradiation lens coils 2 and 3, first and second deflection coils 4 and 5, objective lens coil 6, first and second. Electromagnetic sample image moving coils 7 and 8, first and second intermediate lens coils 9 and 10, first and second projection lens coils 11 and 12, excitation power supplies 13 to 23, digital analog converter (DAC) 24 to 34, a microprocessor 35, a storage device 36, a computing device 37, a CRT controller 38, a monitor (CRT) 39, an interface (I / F) 40 to 41, a magnification switching rotary encoder 42, and an input rotary encoder 43 A keyboard 44, a mouse 45, a RAM 46, a ROM 47, and an image capturing interface 48. Since the objective lens coil 6 is a strong excitation lens, lenses are formed on the upper and lower sides of the sample. A sample stage 53 is provided on the optical axis.

  The transmission scanning electron microscope apparatus of this example further includes a secondary electron detector 61, a dark field image detector 64, and a bright field image detector 65. A scanning image is obtained by the secondary electron signal detected by the secondary electron detector 61. A scanning transmission image is obtained by the electronic signals detected by the dark field image detector 64 and the bright field image detector 65.

  The secondary electron detector 61, the dark field image detector 64, and the bright field image detector 65 of this example have a ceramic scintillator and a photoelectric conversion element. A photomultiplier tube may be used as the photoelectric conversion element. The structure of these detectors will be described later with reference to FIGS.

  Here, although an electron microscope will be described, the present invention is applicable not only to an electron microscope but also to a charged particle beam apparatus.

  FIG. 2 shows a transmission scanning electron microscope unit used for obtaining a transmission scanning image in the electron microscope apparatus of this example. The transmission scanning electron microscope section includes an electron gun 1, a converging electron lens, deflection coils 4 and 5, a secondary electron detector 61, a dark field image detector 64, and a bright field image detector 65.

  A procedure for obtaining a scanning image and a transmission scanning image will be briefly described. The observer uses the keyboard 44 and the mouse 45 to search for an imaging target from the field of view. The scanning image or transmission scanning image lens data stored in the ROM 47 is read out and supplied to the digital-analog converters (DACs) 24-34. Digital-analog converters (DACs) 24 to 34 convert lens system data into analog signals and supply them to excitation power supplies 13 to 23. The excitation power supplies 13 to 23 output current to the lens coils 2, 3, 6, 9 to 12 of each lens system.

  The electron beam generated by the electron gun 1 is converged by the first and second irradiation lens coils 2 and 3, scanned by the first and second deflection coils 4 and 5, and imaged by the objective lens coil 6. The sample on the sample stage 53 is irradiated. Secondary electrons generated from the sample are detected by the secondary electron detector 61, and a scanned image is obtained. The scanned image is displayed by a monitor (CRT) 39.

  Transmitted electrons from the sample are detected by the bright field image detector 65, and scattered electrons from the sample are detected by the dark field image detector 64. The bright field image obtained by the bright field image detector 65 and the dark field image obtained by the dark field image detector 64 are displayed by a monitor (CRT) 39.

  FIG. 3 shows a cross-sectional structure of a conventional scintillator. The scintillator includes a phosphor layer composed of a particulate phosphor p47 and a binder filled therebetween, and exhibits a white color. The binder is a dried collodion film. The phosphor layer is supported on a glass plate. The thickness of the phosphor layer is usually about 30 μm so that the electron beam can be sufficiently transmitted. An aluminum coating having a thickness of about 20 nm is formed on the surface of the phosphor layer. As shown in the figure, when an electron beam enters the phosphor layer, fluorescence is generated and spreads in a triangular pyramid shape inside the phosphor layer.

FIG. 4 shows a cross-sectional structure of the ceramic phosphor of the ceramic scintillator of the present invention. The ceramic scintillator of the present invention includes a ceramic phosphor. The ceramic phosphor is a sintered body obtained by heating the phosphor p47 to several thousand ° C., for example, about 5000 ° C. to vitrify it. By heating and pressurizing, the binder is completely burned out, and a translucent sintered body made only of the phosphor is formed. As a phosphor used for the ceramic scintillator, p47: Y ₂ SiO ₅ ; Ce having an afterimage time of ₂₀₀ nsec or less is preferable. However, other phosphors having a short afterimage time, such as p43: Gd ₂ O ₂ S; Tb, may be used.

  FIG. 5 shows the results of the inventors of the present invention manufacturing ceramic phosphors under various conditions and using them to conduct electron detection experiments. The horizontal axis represents the acceleration voltage (kV) of the electron beam, and the vertical axis represents the penetration depth (μm) of the electron beam into the ceramic phosphor. The inventor of the present application has obtained the following knowledge from this result.

(1) When the acceleration voltage is 300 kV, the electron beam enters up to a depth of 200 μm of the ceramic phosphor. Therefore, in the present invention, the thickness of the ceramic phosphor can be 200 μm or more. The phosphor film of the conventional scintillator is white, but the ceramic phosphor of the present invention is translucent. Therefore, in the ceramic phosphor of the present invention, it is possible to generate fluorescence as much as the electron beam enters and take it out to the outside.

(2) It is considered that when the thickness of the ceramic phosphor is increased, the light emission amount is increased in proportion thereto. However, it has been found that when the thickness of the ceramic phosphor is about 200 to 300 μm, the amount of fluorescence extracted is not increased even if the thickness of the ceramic phosphor is further increased. This is probably because the ceramic phosphor is not transparent but translucent. Therefore, according to the present invention, the thickness of the ceramic phosphor is preferably about 200 to 300 μm.

(3) In the ceramic phosphor of the present invention, it was observed that the fluorescence generated by electron beam irradiation spreads three-dimensionally inside the phosphor layer. In the conventional scintillator shown in FIG. 3, the fluorescence spreads in a triangular pyramid shape, but in the ceramic phosphor of the present invention, the fluorescence spreads in the lateral direction and propagates three-dimensionally. Therefore, fluorescence is generated from the entire ceramic phosphor. This is presumably because the fluorescent substance formed into a glassy sintered body functions as an optical waveguide like an optical fiber.

  The thickness of the fluorescent film of the conventional scintillator is about 30 μm. Therefore, the thickness of the ceramic phosphor of the present invention is about 10 times the thickness of the phosphor film of the conventional scintillator. However, the sensitivity of the ceramic phosphor of the present invention was 1000 times or more that of the conventional scintillator. This is considered to be due to the fact that the fluorescence spreads three-dimensionally in the ceramic phosphor of the present invention.

(4) It was found that the ceramic phosphor of the present invention can extract a sufficient amount of fluorescence even when a low voltage of several kV to 100 kV is used as the acceleration voltage. This is because the sensitivity of the ceramic phosphor of the present invention is extremely high. Of course, a sufficient amount of fluorescence can be obtained even when an intermediate voltage of several hundred kV to 200 kV is used as the acceleration voltage. In the conventional scintillator, the range of the acceleration voltage was narrow, but in the present invention, the range of the acceleration voltage can be widened. That is, according to the present invention, by setting the thickness of the ceramic phosphor to about 200 to 300 μm, an acceleration voltage in a wide range from a low voltage of several kV to 100 kV to a high voltage of 200 to 350 kV can be used. .
Therefore, the use range of the acceleration voltage of the electron beam in the ceramic scintillator of the present invention is 3 to 5 times the use range of the acceleration voltage of the electron beam in the conventional scintillator.

(5) According to the present invention, since a high voltage of 200 to 350 kV can be used as the acceleration voltage, a high-quality image can be obtained. For example, if the sample damage and the occurrence of contamination are not taken into consideration, the acceleration voltage can be increased in order to obtain high image quality.

(6) In the ceramic phosphor of the present invention, the fluorescence propagates three-dimensionally, spreads in the lateral direction, and propagates to the end face of the ceramic phosphor. In order to efficiently extract fluorescence, it is necessary to prevent leakage of fluorescence at the end face of the ceramic phosphor. Therefore, the end face of the ceramic phosphor of the present invention is provided with a fluorescent leakage preventing material for preventing fluorescent leakage. The fluorescent leakage prevention material may be an aluminum ring.

  The surface of the ceramic phosphor of the present invention is covered with an aluminum coating. The aluminum coating is formed by aluminum vapor deposition. The thickness of the aluminum coating is 10 to 30 nm, in particular about 20 nm. Instead of providing an aluminum ring, an aluminum coating may be formed as a fluorescent leakage preventing material provided on the end face of the ceramic phosphor. However, the thickness of the aluminum coating is sufficiently large so as to prevent the leakage of fluorescence, for example, several hundred μm to several mm.

  FIG. 6A shows the structure of the first example of the ceramic scintillator of the present invention. The ceramic scintillator of this example includes a ceramic phosphor 54a, an aluminum coating 54b formed on the main surface (electron beam incident surface), and a fluorescent leakage prevention material 54c formed on the peripheral end surface thereof. The ceramic phosphor 54a may be supported by a transparent glass plate 58 or a quartz plate. The glass plate 58 may be a glass plate provided on the light incident surface of the photomultiplier tube.

  The thickness t1 of the aluminum coating 54b on the main surface of the ceramic phosphor 54a is t1 = 10 to 30 nm, preferably about 20 nm. The aluminum coating 54b is formed by vapor deposition of aluminum. The fluorescent light leakage preventing material 54c is constituted by an aluminum ring or an aluminum coating. The thickness t2 of the fluorescent light leakage preventing material 54c is, for example, t2 = several hundred μm to several mm.

  FIG. 6B shows the structure of the second example of the ceramic scintillator of the present invention. In the ceramic scintillator of this example, the main surface (electron beam incident surface) of the ceramic phosphor 54a is a curved surface like a concave lens. The ceramic phosphor 54a of this example has a small thickness at the central portion and a large thickness at the peripheral portion. In the ceramic scintillator of this example, the main surface (electron beam incident surface) of the ceramic phosphor 54a is a curved concave surface, so that an electron beam can be captured efficiently.

  FIG. 7 shows an example of the bright field image detector of the present invention. The bright field image detector 65 detects transmitted electrons from the sample. The bright field image detector 65 includes a ceramic scintillator 54 and a photomultiplier tube 55. A transparent glass plate or quartz plate is usually provided at the upper end of the photomultiplier tube 55. A ceramic scintillator 54 is disposed on the glass plate or quartz plate. As shown in FIGS. 6A and 6B, the ceramic scintillator 54 includes a ceramic phosphor 54a, an aluminum coating 54b formed on its main surface (electron beam incident surface), and a fluorescent leakage formed on its peripheral end surface. It has the prevention material 54c.

  FIG. 8 shows an example of a dark field image detector of the present invention. The dark field image detector 64 detects scattered electrons from the sample. The dark field image detector 64 includes a ceramic scintillator 54, a photomultiplier tube 55, and a light guide 57. A transparent glass plate or quartz plate is usually provided at the upper end of the photomultiplier tube 55. A ceramic scintillator 54 is disposed on the glass plate or quartz plate. As shown in FIGS. 6A and 6B, the ceramic scintillator 54 includes a ceramic phosphor 54a, an aluminum coating 54b formed on its main surface (electron beam incident surface), and a fluorescent leakage formed on its peripheral end surface. It has the prevention material 54c.

  FIG. 9 shows an example of the secondary electron detector of the present invention. The secondary electron detector 61 includes a ceramic scintillator 54, a photomultiplier tube 55, and a light guide 57. The structure of the ceramic scintillator 54 is shown in FIGS. 6A and 6B.

  Although the examples of the present invention have been described above, the present invention is not limited to the above-described examples, and it is easy for those skilled in the art to make various modifications within the scope of the invention described in the claims. Will be understood.

1: electron gun, 2: first irradiation lens coil, 3: second irradiation lens coil, 4: first deflection coil, 5: second deflection coil, 6: objective lens coil, 7: first electromagnetic sample image movement Coil: 8: second electromagnetic sample image moving coil, 9: first intermediate lens coil, 10: second intermediate lens coil, 11: first projection lens coil, 12: second projection lens coil, 13-23 : Excitation power source, 24-34: Digital-analog converter (DAC), 35: Microprocessor, 36: Storage device, 37: Arithmetic device, 38: CRT controller, 39: Monitor (CRT), 40-41: I / F, 42: Rotary encoder for switching magnification, 43: Rotary encoder for input, 44: Keyboard, 45: Mouse, 46: RAM, 47: ROM, 48: Image capture interface, 53 Sample stage 54: ceramic scintillator, 55: photomultiplier, 57: light guide 61: secondary electron detector, 64: dark field image detector 65: bright field image detector

Claims (20)

In an electron microscope having an electron beam optical system that irradiates a sample with an electron beam from an electron gun, and an electron detector that detects electrons from the sample,
The electron detector includes a ceramic scintillator and a photoelectric conversion element that converts light from the ceramic scintillator into an electric current,
The ceramic scintillator includes a ceramic phosphor formed by sintering phosphor p47: Y ₂ SiO ₅ ; Ce, and the thickness of the ceramic phosphor is 200 to 300 μm. The electron microscope according to claim 1,
An electron microscope characterized in that a fluorescent leakage preventing material for preventing fluorescent leakage is provided on an end face around the ceramic phosphor. The electron microscope according to claim 1,
The surface of the ceramic phosphor is covered with an aluminum coating, and the thickness of the aluminum coating is 10 to 30 nm. The electron microscope according to claim 1,
The surface of the said ceramic fluorescent substance has a concave surface, The thickness of the said ceramic fluorescent substance is small in a center part, and is large at a peripheral part, The electron microscope characterized by the above-mentioned. The electron microscope according to claim 1,
The electron detector includes a secondary electron detector that detects secondary electrons from a sample to obtain a secondary electron image, a bright field image detector that detects transmitted electrons from the sample to obtain a bright field image, An electron microscope comprising: at least one of a dark field image detector that detects scattered electrons from a sample to obtain a dark field image. An electron beam optical system for irradiating the sample with an electron beam from an electron gun, a secondary electron detector for detecting secondary electrons from the sample, a bright-field image detector for detecting transmitted electrons from the sample, and a sample In a transmission scanning electron microscope having a dark field image detector for detecting scattered electrons of
At least one of the secondary electron detector, the bright field image detector, and the dark field image detector has a ceramic scintillator and a photoelectric conversion element that converts light from the ceramic scintillator into a current,
The transmission scintillation electron microscope, wherein the ceramic scintillator includes a ceramic phosphor formed by sintering a phosphor, and the thickness of the ceramic phosphor is 200 to 300 μm. The transmission scanning electron microscope according to claim 6,
A transmission scanning electron microscope, wherein a fluorescent leakage preventing material for preventing leakage of fluorescence is provided on an end face around the ceramic phosphor. The transmission scanning electron microscope according to claim 6,
The surface of the ceramic phosphor is covered with an aluminum coating, and the thickness of the aluminum coating is 10 to 30 nm. The transmission scanning electron microscope according to claim 6,
A transmission scanning electron microscope characterized in that the surface of the ceramic phosphor has a concave surface, and the thickness of the ceramic phosphor is small at the center and large at the periphery. The transmission scanning electron microscope according to claim 6,
The transmission scanning electron microscope, wherein the phosphor is p47: Y ₂ SiO ₅ ; Ce. In an electron detector used for an electron microscope to detect electrons from a sample,
A ceramic scintillator and a photoelectric conversion element that converts light from the ceramic scintillator into an electric current;
The ceramic scintillator includes a ceramic phosphor formed by sintering a phosphor, and the ceramic phosphor has a thickness of 200 to 300 μm. The electron detector according to claim 11.
The surface of the ceramic phosphor is covered with an aluminum coating, and the thickness of the aluminum coating is 10 to 30 nm.   12. The electron detector according to claim 11, wherein a fluorescence leakage preventing material for preventing leakage of fluorescence is provided on an end face of the ceramic phosphor.   The electron detector according to claim 13, wherein the fluorescent light leakage preventing material is formed of an aluminum ring.   14. The electron detector according to claim 13, wherein the fluorescent leakage preventing material is made of an aluminum coating, and the thickness of the aluminum coating is several hundreds μm to several mm. The electron detector according to claim 11.
The phosphor is p47: Y ₂ SiO ₅ ; Ce, wherein the phosphor is an electron detector. The electron detector according to claim 11.
The surface of the said ceramic fluorescent substance has a concave surface, The thickness of the said ceramic fluorescent substance is small at a center part, and is large at a peripheral part, The electron detector characterized by the above-mentioned. The electron detector according to claim 11.
The ceramic scintillator is configured to receive secondary electrons from a sample in order to obtain a secondary electron image. The electron detector according to claim 11.
The ceramic scintillator is configured to accept a transmission electron from a sample in order to obtain a bright field image. The electron detector according to claim 11.
The ceramic scintillator is configured to accept scattered electrons from a sample in order to obtain a dark field image.

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