JP2003092078A - Spherical aberration correction device for electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable to obtain magnification change in a spherical aberration correction optical system, and enable to correct rotation system within the plane vertical to the light axis between multiple elements without changing phase angles of the multipole elements. SOLUTION: With the spherical aberration correction device of the electron microscope with two axial symmetry lenses (10, 11) between two multipole elements (8, 9), a rotation correction lens (12) giving electrons rotation action within the plane vertical to the light axis is arranged in a converging face of an electron orbit generated between the axial symmetry lenses.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spherical aberration correction device for an electron microscope.

[0002]

2. Description of the Related Art As a device for correcting the spherical aberration of an axisymmetric lens of an electron microscope, there is known a device in which two axisymmetric lenses are arranged between two multipole elements which generate a hexapole field. FIG. 3 is a diagram showing a schematic configuration of an irradiation system of an electron microscope equipped with a conventional spherical aberration corrector. It should be noted that the deflection system and a part of the focusing system are omitted in the figure. An electron beam 2 from the light source 1 passes through a focusing lens 4 having a diaphragm 3, an electron beam parallel to the optical axis enters a spherical aberration correction optical system 5, and an electron parallel to the optical axis emitted from the correction optical system 5. The line is illuminated on the sample 7 through the objective lens 6.

The spherical aberration correction optical system 5 comprises axisymmetric lenses 10 and 11 arranged between multipoles 8 and 9 which generate a hexapole field. The respective poles of the multipole elements 8 and 9 are arranged so that their phases coincide with each other with respect to the optical axis and do not have a rotational relationship about the optical axis in a plane perpendicular to the optical axis. The axially symmetric lenses 10 and 11 have the same focal length f, and the distance between the multipole element 8 and the axially symmetric lens 10 is f.
When the distance between 11 is 2f, the distance between the axisymmetric lens 11 and the multipole element f is f, the excitation intensity K of the multipole elements 8 and 9 and the width (size) Z in the optical axis direction are the same, Aberration is corrected.

[0004]

However, in the conventional spherical aberration correction optical system, since it is necessary to use the axially symmetric lens having the same focal length and specific arrangement conditions, it is possible to obtain a change in magnification by the correction optical system. Can not. Therefore, the minimum electron probe that can be obtained is limited by spherical aberration, and a sufficiently small electron probe having a sufficient amount of current cannot be obtained, and the reducing action of the electron probe must be handled by another lens.

Further, the multipoles 8 and 9 must be arranged so as not to have a rotational relationship about the optical axis in a plane perpendicular to the optical axis, but in reality, to some extent within the manufacturing and assembling accuracy. It is unavoidable that the rotation relation of is mixed,
Further, the electrons passing through the axially symmetric lenses 10 and 11 are subjected to a rotating action in a plane perpendicular to the optical axis, and even if the coil polarities are reversed, it is unavoidable that a certain rotational relationship is mixed. Therefore, it becomes necessary to correct the mixed rotational relationship by controlling the excitation of the multipole element and rotating the phase angle of the action field.However, when the multipole element is used in this way, higher order aberrations of the multipole element Is likely to occur.

[0006]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems by making it possible to obtain a change in magnification in a spherical aberration correction optical system, and in a plane perpendicular to the optical axis between multipoles. It is intended to make it possible to correct the rotational relation of (1) without changing the phase angle of the multipole element. Therefore, in the present invention, in a spherical aberration correction device for an electron microscope in which two axisymmetric lenses are arranged between two multipoles, a plane perpendicular to the optical axis is formed in a converging surface of an electron orbit formed between the axisymmetric lenses. It is characterized in that a rotation correction lens that gives a rotation effect to electrons is arranged inside. Further, according to the present invention, in a spherical aberration correction device for an electron microscope in which two axisymmetric lenses are arranged between two multipoles, the focal lengths of the front and rear axisymmetric lenses are set to f.
1, f2 (f1 ≠ f2), the distance between the front-stage multipole element and the front-stage axisymmetric lens is f1, and the distance between the axisymmetric lens is f1 + f
2, the distance between the rear-stage axisymmetric lens and the rear-stage multipole is f2, the excitation intensities of the front-stage and rear-stage multipoles are K1, K2, the lengths of the front-stage and rear-stage multipoles along the optical axis are Z1 and Z2, and a = f
When 2 / f1, Z2 = a ² Z1 K2 = K1 / a ⁵ is characterized.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 1 is a diagram for explaining a first embodiment of a spherical aberration correction device for an electron microscope according to the present invention, and the same numbers as in FIG. 3 indicate the same contents. In this embodiment, the multipole element 8,
As a solution to the problem that 9 has a rotational relationship with respect to the electron orbit, a rotation correction lens 12 is newly arranged in the converging surface of the electron orbit formed between the axisymmetric lenses 10 and 11. is there.

An electron beam 2 from a light source 1 passes through a focusing lens 4 having a diaphragm 3, becomes parallel to the optical axis, enters a spherical aberration correction optical system 5 ', and exits from the correction optical system 5'. An electron beam parallel to is irradiated onto the sample 7 through the objective lens 6, the focal lengths of the axisymmetric lenses 10 and 11 of the correction optical system 5 are the same f, and the distance between the multipole element 8 and the axisymmetric lens 10 is f. , The distance between the axisymmetric lenses 10 and 11 is 2f, the distance between the axisymmetric lens 11 and the multipole element 9 is f, and the multipole elements 8 and 9 are
Have the same excitation intensity K and width (size) Z in the optical axis direction, and are arranged so that the multipoles 8 and 9 do not have a rotational relationship about the optical axis in a plane perpendicular to the optical axis. At this time, the spherical aberration is corrected as in the case of FIG.

The spherical aberration correction optical system 5'includes an axisymmetric lens 10 arranged between multipoles 8 and 9 for generating a hexapole field.
The rotation correction lens 12 is arranged in the light condensing surface of the electron trajectory formed between 11. By arranging the rotation correction lens 12 on the light collecting surface, the electron beam passes through the optical axis. Therefore, the rotation correction lens 12 can prevent the electron beam from focusing. On the other hand, the rotation correction lens 12 can apply a rotating action to the electron beam by adopting a magnetic field type lens. As described above, the main function of the rotation correction lens 12 on electrons does not have a focusing function unlike an ordinary lens, but only a rotation function in a plane perpendicular to the optical axis. The rotation angle at this time is proportional to the exciting current of the magnetic field type lens.

Therefore, even if the multipoles 8 and 9 have a certain rotational relationship with respect to the electron orbit due to manufacturing and assembling accuracy, etc., the rotation is not corrected by controlling the phase angle of the multipoles as in the prior art, but the rotation is performed. By changing the lens current of the correction lens 12, the mixed rotational relationship can be corrected. Therefore, it is possible to prevent harmful high-order aberrations associated with the phase control of the multipole element from occurring.

FIG. 2 is a diagram for explaining the second embodiment of the present invention, in which the same numbers as in FIGS. 1 and 3 indicate the same contents.
The aberration correction optical system 5 ″ of this embodiment is the axially symmetric lens 1
The focal lengths of 0 ′ and 11 ′ are f1 and f2, the distance between the multipole element 8 ′ and the axially symmetric lens 10 ′ is f1, the distance between the axially symmetric lenses 10 ′ and 11 ′ is f1 + f2, and the axially symmetric lens 11 is used. ′ And the multipole element 9 ′ are f2, the excitation intensities of the multipole elements 8 and 9 are K1 and K2, and the multipole elements 8 and 9 are respectively.
Assuming that the widths (sizes) in the direction of the optical axis are Z1 and Z2, respectively, the results of the analytical calculation when the multipole is made to act as a hexapole field as in the conventional case show that the light of the electron orbit incident parallel to the optical axis. Let r be the distance from the axis, and let R be the inclination of the electron beam trajectory given by passing through the correction optical system 5 ″ at the time when this electron exits the correction optical system 5 ″, R = r ² (cos3θ ) (K1Z1 (f1 / f2) −K2Z2 (f2 / f1) ² ) + r ³ (K1 ² Z1 ³ (1/3) (f1 / f2) + K2 ² Z2 ³ (1/3) (f1 / f2) ³ ) −K1K2Z2Z1 (f2 / f1) ² r ³ (cos3θ) ² (Z2 (f1 / f2) ² −Z1) (1)

FIG. 4 is a diagram for explaining r and θ when a hexapole is used as the multipole element. The figure is a sectional view through a multipole element 8'perpendicular to the optical axis. O is the optical axis, A is the position when the electrons are incident on the multipole element 8 ', and g is the reference direction when the rotation about the optical axis is considered. r is the optical axis O
Is a distance from the reference g for indicating the direction of the position A when the electron is incident on the multipole element 8 '.

Now, the first term of the above equation (1) is an aberration of second-order and third-fold symmetry, and it is a term which should be set to 0 in order to form a minute electron probe. The second term of the equation (1) is the third-order axially symmetric aberration (−δ) formed by the aberration correction optical system 5 ″,
This is a term for canceling the spherical aberration (δ) of the irradiation system of the electron microscope by using this aberration (−δ). The third of the formula (1)
The term is the aberration of the third-order six-fold symmetry, which is a term that should be set to 0 in order to form the minute electron probe. Therefore, the conditions for spherical correction are as follows.

K1Z1 (f1 / f2) −K2Z2 (f2 / f1) ² = 0 (2) r ³ (K1 ² Z1 ³ (1/3) (f1 / f2) + K2 ² Z2 ³ (1/3) (f1 / f2) ³ ) = − δ (3) Z2 (f1 / f2) ² −Z1 = 0 (4) where δ is the change in the inclination of the trajectory due to spherical aberration (r ³
Is proportional to). K1 and K2, which represent the intensity of the hexapole field, are proportional to the current flowing through the multipole. Here, if a = f2 / f1 (5), the equations (2) and (4) are respectively Z2 = a ² Z1 (6) K2 = K1 / a ⁵ (7).

In the conventional spherical aberration corrector, a = 1 and f
Since 1 = f2, Z2 = Z1, and K1 = K2,
The expressions (4) and (2) naturally become 0, and the spherical aberration is corrected by K1 (= K2) determined by the expression (3).

On the other hand, in the present invention, first, the focal lengths f1 and f2 of the axially symmetric lenses 10 'and 11' are set to different values, and Z2, which is determined by the request from the equation (4) for different values of f1 and f2. The relationship of Z1 is inevitably determined, and f1, f
From the relationship of 2 and the relationship of Z1 and Z2 and the relationship of the equation (2), K
The relationship of 1 and K2 is inevitably determined, and finally the values of k1 and k2 are determined from the request of the equation (3) to correct spherical aberration. In the present invention, since the ratio of f1 and f2 can be arbitrarily designed, it is possible to give a magnification change of the electron orbit by the correction system, and when the correction optical system is formed at infinity, it has a magnification of 1 / a. It can have the function of a lens.

Needless to say, the rotation correction lens 12 in the first embodiment can be applied to the case of the second embodiment as well, as shown in FIG.
By arranging the rotation correction lens 12 in the electron orbit focusing surface formed between 0'and 11 ', it is possible to correct even if the multipole elements 8'and 9'have a rotational relationship with respect to the electron orbit. Is.

Further, in the above description with reference to FIGS. 1 to 3 and 5, 1 is a light source, 4 is a focusing lens, 6 is an objective lens, and 7 is a sample, which is a spherical aberration correction device of an irradiation system of an electron microscope. However, the spherical aberration corrector is also effective as a spherical aberration corrector for an image forming system of an electron microscope. That is, in FIGS. 1 to 3 and FIG.
Let S be the objective lens of the image forming system, 6 be the first intermediate lens of the image forming system, and 7 be the image plane formed by the first intermediate lens 6, and operate similarly as a spherical aberration correction device of the image forming system. Can be explained. An example of application of the spherical aberration corrector of the present invention to an electron microscope will be described below with reference to FIG.

FIG. 6 is a diagram for explaining the case where the spherical aberration corrector of the present invention is used in an electron microscope. 21
Is an electron gun for generating an electron beam to give desired energy, 22 is a focusing lens composed of a plurality of lenses for focusing the electron beam, 23 is a deflector for two-dimensionally deflecting and scanning the electron beam, and 24 is an electron It is an objective lens for irradiating the sample 25 with the beam. An electron optical system composed of these 21 to 24 is called an irradiation system.

In this irradiation system, an electron beam is applied to the sample 2
There are several uses for the form of irradiating No. 5. The first is a method of finely focusing an electron beam to irradiate a desired position on the sample 25, and the second is a method of deflecting a finely focused electron beam to a desired area on the sample 25 two-dimensionally by using a deflector 23. In the third method of irradiating while scanning, a uniform electron beam (electron beam having the same thickness as the desired region) is applied to a desired region on the sample 25 without focusing or scanning the electron beam finely. For example, the irradiation method.

Further, in FIG. 6, reference numeral 26 is an objective lens for irradiating the sample 25 with the electron beam, for example, by the third method, and enlarging the transmission image of the electron beam transmitted through the sample 25, and 27 is the objective lens 26. An intermediate lens composed of a plurality of lenses for further enlarging the transmission image magnified in (2), and 28 is a projection lens for projecting the transmission image magnified on the fluorescent screen 29. An electron optical system composed of these 26 to 29 is called an image forming system. All of the electron guns 21 and below are arranged in a vacuum atmosphere. In addition,
Although the above description is made for convenience of description, the objective lens is described as if it is composed of two lenses, 24 and 26, but normally one lens can perform two functions of the objective lens 24 and the objective lens 26. .

Further, in FIG. 6, reference numeral 30 is a spherical aberration corrector when any one of the spherical aberration correctors 5 ', 5 "and 5'" of the present invention is applied to the irradiation system, and 40 is the present invention. It is a spherical aberration corrector when any one of the spherical aberration correctors 5 ′, 5 ″ and 5 ′ ″ is applied to the image forming system. The spherical aberration correction device 30 is a device that obtains a finer electron probe by correcting the aberration of the focused electron beam in the first and second irradiation methods of the irradiation system. In the third irradiation method (3), the aberration of the objective lens 26 of the imaging system is corrected to obtain a higher resolution magnified image.

[0023]

As described above, according to the present invention, first of all,
The following effects can be achieved. Since the rotational relationship in the plane perpendicular to the optical axis between the two multipoles constituting the spherical aberration corrector can be corrected without changing the phase angle of the multipole, higher-order aberrations accompanying the phase change of the multipole Can be prevented.

Further, when the present invention is applied to the irradiation system of an electron microscope, the following effects can be achieved. Since the spherical aberration of the irradiation system can be corrected, a micro electron probe can be obtained, which enables characteristic X-ray analysis of a micro area and enables high-resolution observation. A part of the original role of the irradiation system designed as a multi-stage reduction system can be shared by the spherical aberration corrector.

Similarly, if the present invention is applied to an image forming system of an electron microscope, the following effects can be achieved. Since the spherical aberration of the imaging system can be corrected, a high resolution transmission electron microscope image can be observed. A part of the original role of the imaging system designed as a multi-stage magnifying system can be shared by the spherical aberration corrector.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a spherical aberration correction device for an electron microscope according to the present invention.

FIG. 2 is a diagram illustrating another example of the spherical aberration correction device for an electron microscope according to the present invention.

FIG. 3 is a diagram showing a schematic configuration of an irradiation system of an electron microscope for explaining a conventional spherical aberration correction method.

FIG. 4 is a diagram for explaining parameters r and θ in the aberration correction device.

FIG. 5 is a diagram illustrating still another example of the spherical aberration correction device for an electron microscope according to the present invention.

FIG. 6 is a diagram illustrating an example in which the spherical aberration corrector according to the present invention is applied to an electron microscope.

[Explanation of symbols]

1 ... Light source, 2 ... Electron beam, 3 ... Aperture, 4 ... Focusing lens,
5 '... Spherical aberration correction optical system, 6 ... Objective lens, 7 ... Sample, 8, 9 ... Multipole, 10, 11 ... Axisymmetric lens, 12
… Rotation correction lens.

Claims (3)

[Claims] 1. A spherical aberration correction device for an electron microscope, wherein two axially symmetric lenses are arranged between two multipole elements, wherein an electron trajectory condensing surface formed between the axially symmetric lenses is in a plane perpendicular to the optical axis. A spherical aberration correction device for an electron microscope, in which a rotation correction lens that gives a rotation effect to electrons is arranged. 2. In a spherical aberration correction device for an electron microscope in which two axisymmetric lenses are arranged between two multipoles, the focal lengths of the front and rear axisymmetric lenses are f1, f2 (f
1 ≠ f2), the distance between the front-stage multipole and the front-stage axisymmetric lens is f1, the distance between the axi-symmetric lens is f1 + f2, the distance between the rear-stage axisymmetric lens and the rear-stage multipole is f2, and the excitation intensity of the front-stage and rear-stage multipoles Where K1 and K2, and the lengths of the front and rear multipoles along the optical axis are Z1 and Z2, and a = f2 / f1, Z2 = a ² Z1 K2 = K1 / a ⁵ Spherical aberration corrector for electron microscope. 3. A rotation correction lens that gives a rotation action to electrons in a plane perpendicular to the optical axis is arranged in a converging surface of an electron orbit formed between the two axially symmetric lenses. 2. The spherical aberration correction device for an electron microscope according to 2.