JP2000306536A - Magnifying image formation lens system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize out-of-axis aberration without affecting an intermediate image surface position and magnification by providing an angle restriction diaphragm arranged between a first lens and a second lens, and a diaphragm position control lens arranged at the position of an intermediate image formed by the first lens. SOLUTION: The electron beam generated by an electron gun 1 is converged by a convergent lens 2 and irradiates a sample 3. A first lens L1 (objective lens) is arranged in a subsequent stage of the sample 3, and an angle restriction diaphragm 4 is arranged on a focal plane after the first lens L1. Then, a diaphragm position control lens LA is arranged at the position of an intermediate image formed by the first lens L1, and the intensity of the diaphragm position control lens LA is so set that the trajectory of the electron beam in a second lens L2 is formed as low as possible and the trajectory passes the center of the second lens L2. This is equivalent to positioning a diaphragm at the position of the second lens L2. Thereby, the out- of-axis aberration of the second lens L2 can be minimized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a magnifying imaging lens system such as a transmission electron microscope (TEM), a photoelectron microscope (PEEM), and a low energy reflection electron microscope (LEEM).

[0002]

2. Description of the Related Art In a magnifying imaging lens system such as a transmission electron microscope, a photoelectron microscope, and a low-energy reflection electron microscope, under high-magnification conditions, the axial aberration (spherical aberration, chromatic aberration) of the objective lens as a whole increases. The performance is almost determined, but under low magnification conditions, the electron beam passes off the axis of each lens, so the lower the magnification, the more off-axis aberrations (coma, astigmatism, curvature of field, distortion) It is difficult to keep it small.

Conventionally, the following procedure has been used to determine low magnification lens conditions. First, one combination of lens intensities at which a predetermined magnification is obtained is obtained, and off-axis aberrations of the entire system are evaluated. If the off-axis aberration falls within the allowable range, it is adopted; otherwise, a lens that makes a large contribution to off-axis aberration is found. Then, the combination of the strengths of the lenses is changed so that the aberration of the lens becomes small or the electron trajectory cancels out with other lenses.

[0004]

However, this work is extremely complicated due to the high degree of freedom.
It requires experience and intuition. Further, it is not always possible to suppress off-axis aberration to an allowable range only by a combination of lens intensities.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an enlarged imaging lens system capable of minimizing off-axis aberrations.

[0006]

Means for Solving the Problems To achieve this object, a magnifying imaging lens system according to the present invention comprises a first lens and a second lens sequentially arranged along an electron beam path; And a stop position control lens disposed at a position of an intermediate image formed by the first lens, wherein the stop position control lens is The second lens is controlled so that off-axis aberration of the second lens is minimized.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments of the present invention, the concept of the present invention will be described first.

In general, a magnifying imaging lens system requires a stop for limiting the exit angle of electrons emitted from each point of the sample to a certain range. The diameter of the stop is usually such that the spatial resolution is the best. Set to value.

In a transmission electron microscope, this stop is located at the back focal plane of the objective lens and is called an objective stop. In the following description, it is generally called an angle limiting stop. When electrons are emitted from each point of the sample substantially isotropically as in a photoelectron microscope, the angle limiting aperture does not need to be at the focal plane.

On-axis aberrations, ie, spherical aberration and chromatic aberration,
Since the angle is determined by the opening angle of electrons emitted from a point on the optical axis of the sample surface, it is not essential where the angle limiting aperture described above is placed. However, off-axis aberrations are generally a function of this angle-limited aperture position, and some off-axis aberrations may be zero at a particular aperture position, while others may have a minimum aperture position.

Therefore, when all off-axis aberrations are taken into consideration, there is a stop position that minimizes the combined effect thereof. However, this optimum aperture position changes for each magnification. Therefore, a method of moving the diaphragm to an optimum position according to the magnification can be considered first, but this is, of course, not practical.

The present invention has been conceived as being realistic, and the technique of the present invention is to add one lens to the intermediate image plane position of the magnifying imaging lens system. The lens placed at the intermediate image position does not affect the position of the image plane and the overall magnification, and functions to equivalently move the angle limiting aperture position. That is, it is possible to change only the off-axis aberration while keeping the image position and the magnification unchanged. Therefore, by setting the strength of this lens to an optimum value for each magnification, off-axis aberration can always be minimized. Hereinafter, this lens is called an aperture position control lens.

The concept of the present invention has been described above. Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a view showing an example of a magnified imaging lens system according to the present invention, and is a view showing a transmission electron microscope provided with the magnified imaging lens system.

In FIG. 1, reference numeral 1 denotes an electron gun, and an electron beam generated by the electron gun 1 is focused by a focusing lens 2 to form a sample 3.
Is irradiated.

A first lens (objective lens) L ₁ for forming an image of electrons transmitted through the sample is disposed at a stage subsequent to the sample 3, and an angle limiting aperture is provided on a rear focal plane of the first lens L _1. 4
Is arranged. Also, the position of the intermediate image making the first lens L ₁ is the diaphragm position control lens L _A is disposed.

_A second lens L ₂ is disposed downstream of the stop position control lens LA, and a fluorescent plate 5 is disposed on the final image plane of the subsequent stage.

[0018] Having described the structure of the transmission electron microscope of FIG. 1, FIG. 3 is a diagram illustrating a conventional transmission electron microscope non an aperture stop position control lens L _A.
In this conventional electron microscope, as a result of the expansion operation of the first lens L _1, (away from the optical axis) orbit of the electron beam in the second lens L ₂ becomes high, off-axis aberrations L ₂
Are mainly generated.

[0019] Therefore, as shown in FIG. 1 in the present invention, L ₁ of which the position control lens L _A stop at the position of the intermediate image is arranged to make, so that the trajectory of the electron beam in the L ₂ becomes as low as possible in other words, it tracks the strength of L _a so as to pass through the center of L ₂ is set. Consequently, off-axis aberrations of L ₂ is minimized. This equivalently, equivalent to put a stop to a position of L _2.

In practice, the second lens L ₂ is composed of a plurality of lenses including an intermediate lens system and a projection lens. Even in this case, there is an optimum aperture position for each magnification with respect to off-axis aberration. it is the same, may be set the strength of L _a for each magnification as the diaphragm comes equivalently to that position.

[0021] Having described apparatus of Figure 1, in this example, the diaphragm position for the L ₁ is not changed by L _A, only the diaphragm position with respect to L ₂ is changed by the strength of L _A. If even possible to reduce the off-axis aberrations of L ₂ by the use of L _A, there may be a case where the off-axis aberrations of the remaining L ₁ becomes a problem. At this time, as shown in FIG. 2, first, place the actual angle limiting aperture near the main surface of the L _1, when the position conjugate plane of L ₂ of the aperture position (L ₂ comprises a plurality of lenses, the combined their axis aberrations may be set to L _a so that the position) is minimized. This gives L ₁ and L
₂ of both off-axis aberrations are minimized.

Further, as it is the same thing as in Fig. 2, Place the actual angle limiting aperture is aligned with the major surface of the L _2, a conjugate plane of the diaphragm position is matched to the main surface of the L ₁ the L _a may be set. For example, if L ₁ is an electrostatic lens or a field superimposing type lens, it is generally difficult to put the actual angle limiting aperture in the center of the lens, in such cases this method is effective.

FIGS. 4 to 6 are diagrams showing examples of actual aberration calculation corresponding to FIGS. In each figure, the upper part shows the electron trajectory and the axial magnetic field distribution created by each lens, and the lower part shows the aberration pattern on the final image plane. The lenses are all assumed to be standard magnetic lenses.

As shown in FIG. 6, a large distortion is conspicuous in the prior art, and the blur of a beam emitted from one point on the sample increases as the distance from the axis increases.

[0025] In contrast, in the apparatus of FIG. 1, as shown in FIG. 4, the off-axis aberrations (in this example, mainly curvature aberration) still remaining about blur, off-axis aberrations of L ₂ is Has been minimized.

[0026] Then, in the apparatus of Figure 2, as shown in FIG. 5, by being minimized to off-axis aberrations of L _1, the aberration is sized no problem in the apparatus of FIG.

The case where the present invention is applied to a transmission electron microscope has been described above.
M can also be applied.

That is, LEEM which irradiates a sample with an electron beam and forms an image based on reflected electrons generated from the sample on a screen, or irradiates a sample with ultraviolet rays or the like and applies an image based on photoelectrons generated from the sample to a screen. PEE to be imaged
In M, if the present invention is applied to those imaging lens systems, off-axis aberrations can be minimized without affecting the intermediate image plane position and magnification.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an enlarged imaging lens system of the present invention.

FIG. 2 is a diagram showing another example of the present invention.

FIG. 3 is a view shown for explaining a conventional transmission electron microscope.

FIG. 4 is a diagram showing an example of actual aberration calculation in the apparatus of FIG. 1;

FIG. 5 is a diagram showing an example of actual aberration calculation in the apparatus of FIG. 2;

FIG. 6 is a diagram illustrating an example of actual aberration calculation in the apparatus of FIG. 3;

[Explanation of symbols]

 1. Electron gun, 2. Focusing lens, 3. Sample, 4. Angle limitation
Aperture, 5 ... Fluorescent plate, L ₁... First lens, L_Two... the second record
, L_A… Aperture position control lens

Claims (3)

[Claims] 1. A method according to claim 1, further comprising:
And a second lens, an angle-limiting diaphragm disposed between the first lens and the second lens, and a diaphragm position control lens disposed at a position of an intermediate image formed by the first lens. An enlarged imaging lens system, wherein the stop position control lens is controlled such that off-axis aberration of the second lens is minimized. 2. A method according to claim 1, further comprising:
And a second lens; an angle-limiting diaphragm disposed near the main surface of the first lens; and an enlarged imaging system comprising a diaphragm position control lens disposed at a position of an intermediate image formed by the first lens. Lens system, wherein the aperture position control lens,
A magnified imaging lens system, wherein the off-axis aberration of the second lens is controlled to be minimized. 3. A first arrangement sequentially arranged along an electron beam path.
And a second lens, an angle-limiting diaphragm disposed on a main surface of the second lens, and an enlarged image-forming lens comprising a diaphragm position control lens disposed at a position of an intermediate image formed by the first lens. A magnified imaging lens system, which is a lens system, wherein the stop position control lens is controlled such that a conjugate plane of the angle limiting stop position coincides with a main surface of the first lens.