JP2010113810A - Transmission electron microscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a TEM improved significantly in vibration resistance by weighting an influence of each part of a lens barrel for image migration due to environmental disturbance from an electronic optical standpoint and performing such design that the rigidity of the part having the highest influence on vibration resistance is chiefly is enhanced in consideration of moment by inertial force. <P>SOLUTION: In order to enhance loop rigidity from a pole piece to a sample, two members of a compression flange 32 and a reinforcing ring 33 that join an outer yoke a25 to an inner yoke 27 are placed. Coupling between the two members by bolts 34 is performed, in which the respective whole joining areas of the inner yoke 27 and the outer yoke a25 are fixed by adhesion or the like. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a transmission electron microscope, and more particularly to a technique for improving vibration resistance for obtaining a high-resolution observation image.

  A transmission electron microscope (hereinafter abbreviated as TEM) is an observation apparatus that can obtain atomic resolution. FIG. 5 shows a schematic configuration example of a TEM capable of observing a scanning transmission image (hereinafter, STEM image). In FIG. 5, a sample holder 2 for holding a sample 1 is held so that it can be inserted into and removed from a sample stage 3 for positioning observation. The sample stage 3 is fixed to the lens barrel 4, and linear movement in the X, Y, and Z axis directions in the drawing and tilting around the X axis are possible. The lens barrel 4 is installed on a gantry 7 placed on a floor 6 via a vibration isolator 5, and the vibration of the floor 6 is suppressed from being transmitted by the vibration isolator 5. The natural frequency of the vibration isolator 5 is set to about several Hz and suppresses vibration transmission at higher frequencies.

Although the principle of TEM is well known and will not be described in detail, when observing a transmission electron image (hereinafter referred to as TEM image), an electron beam 10 generated by an emitter 9 of an electron gun 8 is converted into a condenser lens 11 and a condenser mini lens. 12. Using the objective lens, the sample 1 is irradiated as a parallel beam, and the transmitted electron beam 10 is magnified by the objective lens 13, the intermediate lens 14, and the projection lens 15 to form an image on the fluorescent plate 16 or the camera film 17. A TEM image is obtained.

  Further, when observing the STEM image, as indicated by a broken line in FIG. 5, the electron beam is converged on one point of the sample 1 and the amount of transmitted electron beam is measured by the STEM detector 18. An STEM image is obtained by two-dimensionally scanning the beam irradiation position 20 on the XY plane by the irradiation system deflector 19. Note that the electron optical system in this figure is a schematic one for easy understanding and is not strict.

  Now, disturbance vibrations such as floor vibrations transmitted via the vibration isolator 5 vibrate the entire apparatus. At this time, an inertial force accompanying the acceleration of vibration is generated in each part. Each mechanical element is elastically deformed by these inertial forces. At the time of STEM image acquisition, if the relative displacement between the sample 1 and the beam irradiation position 20 due to this elastic deformation exceeds the observation resolution, the influence of vibration becomes a problem.

  On the other hand, at the time of TEM image observation, if the amount of movement of the transmission image magnified on the fluorescent screen 16 or the camera film 17 exceeds the observation resolution, it becomes a problem as image vibration.

  In the conventional design, in order to increase the vibration resistance of the TEM, these elastic deformations are suppressed by increasing the rigidity of each part. At this time, from the model of forced vibration input to the one-degree-of-freedom system, it was considered that increasing the natural frequency of the system was advantageous for improving the vibration resistance.

ここに、λは系の固有振動数ｆ_ｎに対する周波数ｆの比、ξは１自由度系の減衰比である。 FIG. 2 is a diagram illustrating a one-degree-of-freedom vibration model. In FIG. 2, when the base is forced to vibrate at a frequency of fHz and a displacement a ₀ , the mass displacement is a, and the standardized relative displacement | a ₀ −a | / a ₀ between the base and the mass is It is represented by (1).

Here, λ is the ratio of the frequency f to the natural frequency f _n of the system, and ξ is the damping ratio of the one-degree-of-freedom system.

よって、相対変位を小さくしてＴＥＭの耐振性を高めるためには、系の固有振動の周波数ｆ_ｎを高くすることが必要である。そのため、ばね定数を高めるか、あるいは質量を軽減することが重要である。 In the region where λ is sufficiently smaller than 1, the relative displacement is proportional to λ. As described above, a frequency component of several Hz is often a problem in floor vibration. Since the frequency of several Hz is sufficiently lower than the natural frequency of the apparatus, the condition that λ is sufficiently smaller than 1 is satisfied. When the frequency f of the forced vibration is constant, the relative displacement is inversely proportional to the square of frequency f _n of the system. That is, it is expressed as the following formula (2).

Therefore, in order to increase the vibration resistance of the TEM by reducing the relative displacement, it is necessary to increase the frequency f _n of the natural oscillation of the system. Therefore, it is important to increase the spring constant or reduce the mass.

In Japanese Patent Application Laid-Open No. 9-283067 of Patent Document 1, rigidity is improved by integrally coupling the electron gun unit and the body of the body to increase the natural frequency of the electron gun unit. A technique for reducing the fluctuation of the electron source is disclosed.

Japanese Patent Laid-Open No. 9-283067

  The resolution of a TEM with atomic resolution has reached 0.1 nm or less due to the advent of a multipole aberration corrector developed in recent years. An environmental disturbance such as floor vibration and noise existing in the TEM installation room elastically deforms the apparatus, and image vibration caused by the deformation becomes a rate-determining condition for observation resolution. With the increase in resolution of TEM, the upper limit of allowable disturbance in the installation environment has been severely limited. Therefore, as a countermeasure against environmental disturbances, development of a noise-resistant sample holder for TEM, and suppression of floor vibration amplification over several tens to several hundreds of Hz due to the coupling of the natural mode of the gantry and the floor tilt vibration have been performed. .

  However, these measures cannot suppress the influence of low-frequency floor vibration of several Hz, and it is necessary to fundamentally review the vibration-proof design policy of the lens barrel for a TEM having atomic resolution. The structure of a TEM is complex, and not all of its components can be modeled with a one-degree-of-freedom system. Further, the diameter of the electron beam is enlarged and reduced at each part of the optical system. Therefore, when designing the mechanical rigidity, it is necessary to take into account the influence of the scaling of the electron beam.

  Here, the TEM is divided into three parts, that is, an irradiation system, an imaging system, and an objective lens part, and the vibration resistance is examined. Let us consider a case where vibration in the axial direction of the sample holder 2 is inputted in the horizontal direction with respect to the lens barrel 4. At this time, an inertial force is generated in each part, and elastic deformation occurs. In order to make this easy to imagine, as shown in FIG. 3, the lens barrel 4 is fixed at the position of the vibration isolator like a cantilever extending in the horizontal direction, and gravity (acceleration) is indicated in the direction of arrow A in the figure. Consider the case where it takes.

  At this time, the lens barrel 4 is elastically deformed like a cantilever. The sample stage 3 and the sample holder 2 also move downward due to deflection due to their own weight. Now, in this state, it will be examined how the image is affected by the deflection of each part from the viewpoint of electron optics. In general, in the electron optical system, the upper side of the sample 1 is referred to as an irradiation system 21 and the lower side is referred to as an imaging system 22. Here, the objective lens unit 23 positioned in the middle will be described as another part.

  First, in the STEM image observation mode, the relative displacement between the electron beam 10 and the sample 1 is the amount of image movement. In the irradiation system 21, the deflection amount of the emitter section 9 is a mechanical movement amount, but the electron beam is reduced to about 1/1000 in the irradiation system. Therefore, 1/1000 of the mechanical deflection is the amount of movement of the irradiation position of the electron beam 10. When the observation resolution of the STEM image is 0.1 nm, the allowable amount of deflection of the emitter section 9 is 0.1 μm, which is 1000 times the amount of movement of the irradiation position of the electron beam 10. On the other hand, regarding the sample stage 3 and the sample holder 2, the amount of mechanical deflection becomes the amount of image movement as it is. Therefore, the amount of movement of the image is affected by about 1000 times the mechanical deflection of the irradiation system 21.

  Next, examination in the TEM image observation mode is performed. Since the irradiation system 21 irradiates the sample 1 with a beam in parallel, the deflection thereof is not related to the image movement, so that it need not be taken into consideration. In the imaging system 22, the electron beam diameter is enlarged. When the image observed on the fluorescent screen is 1 million times, the magnification of the imaging system is about 10,000 times. For example, if the resolution of a TEM image is 0.1 nm during 1 million magnification observation, the length on the fluorescent screen is 0.1 mm. Therefore, the allowable amount of deflection of the expansion system is estimated to be 10 nm which is 1 / 10,000. This is 10 times the influence of the irradiation system 21 in the STEM image observation mode, but is smaller than the influence of the sample stage 3 and the sample holder 2.

  Finally, attention is paid to the objective lens unit 23. FIG. 4 shows a more detailed structure of the conventional objective lens 113. In FIG. 4, the outer yoke a25 is fastened by an outer yoke b26 and a bolt 24a, and an inner yoke 27 and a bolt 24b. The stage base 3 a is a part of the sample stage 3 and is a member for fixing the sample stage 3 to the lens barrel 4. The stage base 3a is fixed to the upper part of the outer yoke a25 with bolts 24c. A pole piece lower pole 29 and a pole piece upper pole 31 are opposed to the upper surface of the inner yoke 27, and a spacer 31 is provided between them.

  A coil 28 is provided in the gap between the inner yoke 27 and the outer yoke a25, and a magnetic field lens is configured by passing a current. The magnetic circuit of the lens includes a pole piece lower pole 29, an inner yoke 27, an outer yoke a 25, an outer yoke b 26, and a pole piece upper pole 30. Here, the outer yoke b26 and the pole piece upper pole 30 are not in mechanical contact, and a slight gap is provided.

  When the moment of the irradiation system 21 due to gravity (acceleration) is applied to the objective lens part 23, the outer yoke a25 is fixed to the solid line in FIG. 4 (the broken line indicates an unbent state). The stage base 3a, the sample stage 3, and the sample holder 2 are moved together. On the other hand, the pole piece lower pole 29 and the pole piece upper pole 30 arranged on the inner yoke 27 are not subjected to the moment of irradiation system, and therefore no positional fluctuation occurs. In FIG. 4, the outer yoke b <b> 26 is drawn against the pole piece upper pole 30. Therefore, the degree of influence of the deflection of the outer yoke a25 is as great as the influence of the sample stage 3 and the sample holder 2. Further, in the case of the sample stage 3 and the sample holder 2, the deflection is generated only by its own weight, whereas the entire moment of the irradiation system 21 is applied to the outer yoke a25. Therefore, the bending rigidity of the outer yoke a25 needs to be very high. There is. Since the deflection of the outer yoke a25 is equivalent to the position fluctuation of the sample 1, both the TEM image and STEM image observation modes affect the image vibration.

  From the above examination, the lens barrel 4 that requires the highest rigidity is the bending rigidity of the yoke of the objective lens unit 23. In other words, the rigidity of the loop structure from the pole piece of the objective lens to the sample (loop rigidity) ). Here, for example, the loop structure of the pole piece upper electrode 30 and the sample 1 is such that the pole piece upper electrode 30 and the sample 1 face each other with a slight distance, and the pole piece upper electrode 30, the outer yoke b 26, and the stage base. This means the entire series of components when adjacent components are assembled in contact with each other in the order of the stage 3a, the sample stage 3, the sample holder 2, and the sample 1. The loop stiffness refers to the stiffness of the entire series of components described above.

  Furthermore, the mechanical rigidity of the irradiation system 21 may be lower than that of the objective lens unit 23. Therefore, for example, as disclosed in Japanese Patent Application Laid-Open No. 9-283067 of Patent Document 1, a design method that has often been used in the past, that is, to increase the mass of the tip of the lens barrel in order to increase the rigidity of the irradiation system, As a result, since the amount of deflection of the yoke of the objective lens 13 is increased, it can be seen that the vibration resistance may be lowered.

  The present invention has been made to solve the above-mentioned problems, and its purpose is to weight the degree of influence of each part of the lens barrel with respect to image movement due to environmental disturbance from the viewpoint of electro-optics, and to take into account moments due to inertial forces. Thus, it is intended to provide a TEM that significantly improves the vibration resistance by performing a design that mainly increases the rigidity of the portion that most affects the vibration resistance.

To solve the above problem,
The invention according to claim 1 includes a first outer yoke, a second outer yoke and an inner yoke, a pole piece upper pole fixed to the first outer yoke, the second outer yoke and the inner yoke. A pole piece lower pole fixed to the yoke, an objective lens including a coil disposed in a space formed between the second outer yoke and the inner yoke, and a sample stage fixed on the objective lens And a transmission electron microscope comprising a sample holder supported by the sample stage,
By disposing a member for suppressing relative displacement between the sample and the pole piece between the members constituting the loop structure from the sample to the pole piece attached to the sample holder, It is characterized by increased rigidity.

  In the invention according to claim 2, the opening is provided on the side of the objective lens where the second outer yoke and the inner yoke support the sample stage, and the reinforcing member is closed so as to close the opening. Is provided to suppress relative deformation between the second outer yoke and the inner yoke.

  According to a third aspect of the present invention, the reinforcing member comprises a reinforcing flange, a reinforcing ring, and a bolt for fastening between the two members, and the joint between the second outer yoke and the inner yoke is bonded or bonded. It is fixed by either soldering, bolt fastening or brazing.

  The invention according to claim 4 is characterized in that the sample stage and the reinforcing flange are fastened by a bolt.

  The invention according to claim 5 is that the sample stage and the pole piece are fixed by using a spacer disposed between the pole piece upper pole and the pole piece lower pole of the objective lens. Features.

  The invention described in claim 6 is characterized in that the member for suppressing the relative displacement between the sample and the pole piece is a non-magnetic material.

  According to the present invention, the structure has a structure in which the bending stiffness of the yoke of the objective lens, which has the strongest influence on the image movement due to environmental disturbances, is mainly increased, thereby increasing the loop stiffness from the pole piece of the objective lens to the sample. Since the amount of movement of the observation image due to elastic deformation of the lens barrel due to disturbance vibration can be suppressed, high-resolution observation at the atomic level in a transmission electron microscope can be made possible.

  Hereinafter, an embodiment of the present invention will be described with reference to FIG. However, the technical scope of the present invention is not limited by this illustration. 1 that perform the same or similar operations as those in FIG. 4 are denoted by the same reference numerals, and detailed description is not repeated.

  FIG. 1 is a diagram showing the structure of an objective lens 213 for implementing the present invention. In FIG. 1, an outer yoke a25 is fastened by an outer yoke b26 and a bolt 24a, and an inner yoke 27 and a bolt 24b. The stage base 3 a is a part of the sample stage 3 and is a member for fixing the sample stage 3 to the lens barrel 4. The stage base 3a is fixed to the upper part of the outer yoke a25 with bolts 24c. A pole piece lower pole 29 and a pole piece upper pole 31 are opposed to the upper surface of the inner yoke 27, and a spacer 31 is provided between them.

  A coil 28 is provided in the gap between the inner yoke 27 and the outer yoke a25, and a magnetic field lens is configured by passing a current. The magnetic circuit of the lens includes a pole piece lower pole 29, an inner yoke 27, an outer yoke a 25, an outer yoke b 26, and a pole piece upper pole 30. Here, the outer yoke b26 and the pole piece upper pole 30 are not in mechanical contact, and a slight gap is provided.

  In the objective lens embodying the present invention, two members of a reinforcing flange 32 and a reinforcing ring 33 that connect the outer yoke a25 and the inner yoke 27 are arranged in order to increase the loop rigidity from the pole piece to the sample. The two members are fastened with bolts 34. A bolt 34 ′ is a bolt that performs the same function as the bolt 34, and indicates that the bolt 34 ′ is arranged at a different part.

  The entire surfaces of the joint portions between the inner yoke 27 and the outer yoke a25 are fixed by bonding and are joined with high rigidity. These joints may be fixed by soldering or bolt fastening, not necessarily by bonding. The reinforcing flange 32 and the reinforcing ring 33 suppress the relative deformation between the outer yoke and the inner yoke, and produce the effect of increasing the loop rigidity between the sample and the pole piece.

  Next, an example of another objective lens 313 embodying the present invention will be described with reference to FIG. In addition to the reinforcement performed by the objective lens 213, the objective lens 313 further performs reinforcement by fixing the central axis side (side closer to the electron beam axis) of the reinforcement flange 40 with the stage base 3 a and the bolt 35. A bolt 35 ′ is a bolt having the same function as the bolt 35, and indicates that the bolt 35 ′ is arranged at a different part. The reinforcing flange 40 has a slightly different shape from the reinforcing flange 32 of the objective lens 213 in order to strengthen the fastening of the bolt 35 to the stage base 3a, but the reinforcing flange 40 and the stage base 3a have the same shape. A spacer (not shown) may be incorporated between the two. These reinforcements can further increase the loop rigidity between the sample and the pole piece.

  Further, the spacer 37 disposed between the stage base 3a and the pole piece lower pole 29 is the conventional one shown in FIG. 4 so that the stage base 3a and the spacer 37 can be fastened with a bolt 36. The spacer 31 has a different shape. Therefore, it is possible to obtain a higher loop rigidity between the sample and the pole piece.

  In FIG. 7, the bolt fastening between the reinforcing flange 40 and the stage base 3a and the bolt fastening between the stage base 3a and the spacer 37 are performed at the same time, but only one of them is fastened. May be.

  Next, another example of the objective lens 413 embodying the present invention will be described with reference to FIG. In the objective lens 413, a screwed portion 39 a for screwing the reinforcing flange 41 is formed in the inner yoke 39. After screwing the reinforcing flange 41, the screwed portion 39a may be reinforced by soldering. Further, the outer yoke a 38 and the reinforcing flange 41 are fastened with bolts 42. Therefore, the outer yoke a38 and the reinforcing flange 41 have different shapes from the conventional outer yoke a25 and the reinforcing flange 32 shown in FIG. A bolt 42 ′ is a bolt having the same function as the bolt 42, and indicates that the bolt 42 ′ is arranged at a different part. These reinforcements can further increase the loop rigidity between the sample and the pole piece.

  In the description of the objective lenses 213, 313, and 413 described above, a nonmagnetic material is used for fastening members such as the reinforcing flange 32, the reinforcing flange 41, the reinforcing ring 33, and each bolt so as not to affect the magnetic path. Yes.

  Next, verification of the effect of improving the lens barrel rigidity according to the present invention will be described. FIG. 6 is a graph of experimental results obtained by measuring the effect of the lens barrel rigidity improvement shown in FIG. In order to evaluate vibration resistance, it is preferable to mount the entire apparatus on a shaker and measure the vibration amount of the image at the time of vibration. However, the result of the evaluation by this method includes the effect of elastic deformation of other parts such as a goniometer, so that only the effect of improving the lens barrel rigidity cannot be evaluated.

  In view of this, the comparative measurement between the objective lens of the conventional design and the objective lens embodying the present invention was performed for the amount of image movement when a constant load was applied to the side surface of the lens barrel. The measurement was carried out in both the STEM image and TEM image observation modes. Several positions for applying the load were provided in the height direction, and the amount of image movement by each was measured. The load was applied in the sample holder insertion direction and in two directions orthogonal thereto.

  The result in the STEM image observation mode is shown in FIG. A comparison was made by multiplying the coefficient so that the image movement amount before improvement was 1 when the uppermost part of the electron gun was loaded in the sample holder insertion direction. The horizontal axis in the figure is the height from the position of the vibration isolator to the position where the load is applied, and the vertical axis is the image movement amount (comparison value). The effect of the improvement was remarkable. When the load was applied to the top of the electron gun in the direction perpendicular to the sample holder, the direction of image movement was reversed, which significantly suppressed the elements that controlled the image movement and the influence of other elements appeared. Indicates. The relationship between the load height before the improvement and the image movement amount is arranged substantially linearly, and the intersection point with the image movement amount substantially coincides with the lower part of the outer yoke a25 in FIG. It was estimated that this was because the dominant elastic deformation cause of image movement was concentrated on the outer yoke a25 before the improvement, and the elastic deformation amount was proportional to the bending moment due to the load.

  Next, a similar comparison in the TEM image observation mode is shown in FIG. The horizontal axis is the height from the position of the vibration isolator to the position where the load is applied, and the vertical axis is the image movement amount (comparison value) multiplied by the same coefficient as in FIG. The effect of the improvement was also remarkable in the TEM image observation mode. The amount of image movement in the TEM image observation mode before the improvement was almost the same as that in the STEM image observation mode. Further, the relationship between the load height before the improvement and the image moving amount is the same as in the STEM image observation mode, and also substantially coincides with the flange height below the outer yoke a25 in FIG. Therefore, also in the TEM image observation mode, the rigidity of this portion is dominant with respect to image movement. From the above results, the electron optical bending rigidity of the lens barrel was remarkably improved with respect to both the STEM image and TEM image observation modes.

  As described above, according to the present invention, by increasing the mechanical loop rigidity from the objective lens to the sample, the amount of movement of the observation image due to the elastic deformation of the lens barrel due to disturbance vibration is suppressed, and the atomic level High-resolution observation can be made possible.

The figure which shows the structural example of the objective lens which implements this invention. The figure which shows the vibration model of a 1 degree-of-freedom system. FIG. 3 is a model diagram for examining an image moving amount when an inertial force is generated in each part of the lens barrel and elastically deforms. The figure which shows the structural example of the conventional objective lens. The figure which shows the schematic structural example of the transmission electron microscope which can observe a scanning transmission image. The graph of the experimental result which investigated the effect of the lens-barrel rigidity improvement by this invention. The figure which shows the other structural example of the objective lens which implements this invention. The figure which shows another structural example of the objective lens which implements this invention.

Explanation of symbols

(Common reference numerals are used for the same or similar operations.)
DESCRIPTION OF SYMBOLS 1 ... Sample 2 ... Sample holder 3 ... Sample stage 3a ... Stage base 4 ... Lens barrel 5 ... Vibration isolator 6 ... Floor 7 ... Base 8 ... Electron gun 9 ... Emitter 10 ... Electron beam 11 ... Condenser lens 12 ... Condenser mini Lens 13 ... Objective lens 14 ... Intermediate lens 15 ... Projection lens 16 ... Fluorescent screen 17 ... Camera film 18 ... STEM detector 19 ... Irradiation system deflector 20 ... Beam irradiation position 21 ... Irradiation system 22 ... Imaging system 23 ... Objective lens section 24a, 24b, 24c ... bolt 25 ... outer yoke a
26 ... Outer yoke b
27 ... Inner yoke 28 ... Coil 29 ... Pole piece lower pole 30 ... Pole piece upper pole 31 ... Spacer 32 ... Reinforcement flange 33 ... Reinforcement rings 34, 35, 36 ... Bolt 37 ... Spacer 38 ... Outer yoke a
39 ... Inner yoke 39a ... Screwed portions 40, 41 ... Reinforcement flange 42 ... Bolt

Claims (6)

A first outer yoke, a second outer yoke and an inner yoke, a pole piece upper pole fixed to the first outer yoke, and a pole piece lower pole fixed to the second outer yoke and the inner yoke; An objective lens composed of a coil disposed in a space formed between the second outer yoke and the inner yoke, a sample stage fixed on the objective lens, and supported by the sample stage In a transmission electron microscope equipped with a sample holder,
By disposing a member for suppressing relative displacement between the sample and the pole piece between the members constituting the loop structure from the sample to the pole piece attached to the sample holder, A transmission electron microscope characterized by increased rigidity. An opening is provided on the side of the objective lens where the second outer yoke and the inner yoke support the sample stage, and a reinforcing member is disposed so as to close the opening. The transmission electron microscope according to claim 1, wherein relative deformation between the yoke and the inner yoke is suppressed. The reinforcing member includes a reinforcing flange, a reinforcing ring, and a bolt for fastening between the two members, and a joint portion between the second outer yoke and the inner yoke is bonded, soldered, bolted, or brazed. The transmission electron microscope according to claim 2, wherein the transmission electron microscope is fixed. The transmission electron microscope according to claim 3, wherein the sample stage and the reinforcing flange are fastened with bolts. The transmission according to claim 1, wherein the sample stage and the pole piece are fixed using a spacer disposed between the pole piece upper pole and the pole piece lower pole of the objective lens. Type electron microscope. The transmission electron microscope according to claim 1, wherein the member that suppresses relative displacement between the sample and the pole piece is a nonmagnetic material.

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