JP2000149843A - Optical system of charged particle beam map - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce aberration, including aberration with lapse of time, caused by ununiformity of an electromagnetic field among monochrome aberration, by adjusting a shape of the electromagnetic field by providing a movable part to move at least either one of a pair of electrodes or a pair of magnetic poles of a Wien filter. SOLUTION: A charged particle beam for irradiation departed from an irradiated beam source is injected into a Wien filter 1 equipped with a pair of magnetic poles 2 and a pair of magnetic poles 4, and the charged particle beam for irradiation is injected onto the surface of a specimen after they passes the filter. A charged particle beam for observation is led in a different direction from the direction of the irradiated beam source by the Wien filter 1, and the charged particle beam is injected into a means of detecting after it passes through the Wien filter 1. A N magnetic pole 2a and a S magnetic pole 2b are made movable in the direction of a magnetic field B by adjusting devices 6a, 6b, and a positive electrode 4a and a negative electrode 4b are made movable in the direction of an electric field E by adjusting devices 6c, 6d. The adjusting devices 6a, 6b, 6c, 6d are distance adjusting devices by feed screws for example. Thereby, an optical system with high resolution is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a charged particle beam mapping optical system, and more particularly, to a charged particle beam mapping optical system such as a mapping electron microscope using a Wien filter.

[0002]

2. Description of the Related Art An electron beam (electron beam) has been used for observing and inspecting a semiconductor device which has been miniaturized and highly integrated.
Electron microscopes using such as are often used. Among electron microscopes, there is a so-called mapping electron microscope in addition to a scanning electron microscope (SEM). A scanning electron microscope is a microscope that performs so-called point-to-point illumination and image formation, whereas a mapping electron microscope is a microscope that can perform surface-to-surface illumination and image formation. In recent years, development of charged particle beam mapping optical systems of such mapping electron microscopes has been actively carried out.

Generally, a charged particle beam mapping optical system is provided with a Wien filter (E.C.B) as a beam separator for performing epi-illumination.
The Wien filter mainly includes an opposing electrode and an opposing magnetic pole, and forms an electromagnetic field inside. Then, under the Wien condition, the trajectory of the primary electron beam (irradiation electron beam) incident on the Wien filter is deflected, and the trajectory of the secondary electron beam (observation electron beam) emitted from the Wien filter is moved straight. That is, the Wien filter matches the optical axis of the illumination system of the imaging optical system after oblique incidence with the optical axis of the imaging system. Here, the Wien condition means that the electric field E between the opposing electrodes and the magnetic field B between the opposing magnetic poles are: F = e · (E−vB) = 0. Actually, the system is such that the impulse along the trajectory becomes zero.

[0004]

In the above-mentioned conventional mapping optical system using a Wien filter, chromatic aberration has occurred in the imaging system. In order to correct this chromatic aberration, it has been proposed to provide an e-cross bee that generates an electromagnetic field having a direction opposite to the direction of the electromagnetic field of the Wien filter. However, with respect to monochromatic aberration, a number of means for correcting astigmatism derived from variations in electron velocity due to changes in electric field direction potential have been disclosed, but means for correcting other aberrations have not been proposed so much. Was.

Hereinafter, aberrations generated by the Wien filter will be described in detail. Generally, an electric field and a magnetic field orthogonal thereto are superimposed inside the Wien filter. Here, when an electron beam is incident, the electron beam receives a convex power from the electric field direction and does not receive a power from the magnetic field direction. Therefore, an astigmatic difference occurs between the electric field direction and the magnetic field direction. In addition to the astigmatism, it is known that image distortion occurs symmetrically with the axis in the direction of the electric field passing through the center of the Wien filter (K. Tuno, Optik 8
9, No. 1 (1991) 31-40 `` Aberration analysis of a Wien fi
lter for electrons "). This is because even if the Wien condition is satisfied on the central axis of the Wien filter, the off-axis region does not satisfy the Wien condition as the distance from the axis increases. As a result, the electron beam is symmetrically deflected with respect to the central axis of the Wien filter.

Generally, in a scanning electron microscope, a stig meter is provided to correct astigmatism occurring in the entire system. On the other hand, also in the mapping electron microscope using the Wien filter, the astigmatism generated between the electric field direction and the magnetic field direction can be corrected to some extent by installing the stig meter separately from the Wien filter. However, the distortion generated by the above-described Wien filter cannot be corrected by a stig meter. This is because the further away from the central axis of the Wien filter in the direction of the magnetic field, the deviation from the Wien condition results in non-uniform power mainly in the direction of the electric field. Therefore, the distortion generated in the Wien filter appears as an image flow in the electric field direction near the axis depending on the object height in the magnetic field direction. Further, this distortion also causes variation of the astigmatic difference. Failure to correct this distortion does not mean that the astigmatic difference has been completely corrected.

To correct such a distortion, it is conceivable to optimize the shape of the Wien filter. However, even if the optimum shape of the Wien filter is found at the design stage, a difference from the design shape always occurs at the machining stage. Moreover, in reality, the absolute accuracy of the three-dimensional electromagnetic field analysis application used in the design is not so high, and it is difficult to find the optimum shape in the design. Furthermore, the electromagnetic field itself of the Wien filter may change with time due to contamination by scattered electrons. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a charged particle beam mapping optical system that can easily reduce, even over time, aberration caused by non-uniformity of an electromagnetic field among monochromatic aberrations generated by a Wien filter.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. That is, when the reference numerals in the attached drawings are added in parentheses, the present invention provides an irradiation source (21). Of the charged particle beam for irradiation emitted from the device through the irradiation optical system (22) to be incident on the Wien filter (1) provided with the electrode pair (4) and the magnetic pole pair (2) and passed through the Wien filter (1). The charged particle beam for observation is made incident on the sample surface (23) via the objective optical system (24), and the charged particle beam for observation emitted from the sample surface (23) is passed through the objective optical system (24) to the Wien filter ( 1) and incident on the irradiation source (2) by the Wien filter (1).
The charged particle beam for observation is guided in a direction different from the direction leading to 1), and the charged particle beam for observation after passing through the Wien filter (1) is detected via the imaging optical system (28).
In the charged particle beam mapping optical system to be incident on 9), the Wien filter (1) includes an electrode pair (4a, 4b) and a magnetic pole pair (2a, 2b) so that the shape of an electromagnetic field formed inside can be adjusted. And a movable part (6a, 6b, 6c, 6d) for moving at least one of the above. At that time, the movable part (6
a, 6b, 6c, 6d) may be formed such that the electrode pairs (4a, 4b) move in the direction of the electric field (E) and the magnetic pole pairs (2a, 2b) move in the direction of the magnetic field (B). preferable.

In the charged particle beam mapping optical system having the above configuration,
First, in the design stage, instead of designing a Wien filter that suppresses the occurrence of astigmatism, a Wien filter that mainly leaves only a uniform astigmatic difference is designed. Then, at the manufacturing stage or over time, the movable counter electrode and the counter pole are moved to change the ratio of the electrode interval to the pole interval and the relative position, thereby bringing the superposed electromagnetic field closer to the ideal shape (ideal design solution).

Hereinafter, aberrations generated in the Wien filter of the charged particle beam mapping optical system having the above-described configuration will be described in detail. When the shape of the electromagnetic field of the Wien filter deviates from the ideal shape due to design calculation accuracy, manufacturing error, or the like, the largest aberration component generated near the optical axis is low-order aberration. Generally, an electron beam passes through an optical optical element such as a Wien filter or a stig meter only in an optical axis and a region near the optical axis. Therefore, correcting low-order aberrations is synonymous with correcting the overall aberrations and bringing the electromagnetic field closer to the ideal shape.

The Wien filter is arranged so that its central axis coincides with the optical axis of the imaging system. When the optical axis direction is the Z direction, the electric field direction is the X direction, and the magnetic field direction is the Y direction, the equation of motion of electrons in the Wien filter is: Fx / e = Ex−vz · By (1) Fx : X component of the force received by the electron e: electric charge Ex: X component of the electric field vz: Z component of the speed of the electron By: Y component of the magnetic field However, the Z component of the electromagnetic field is ignored.

Here, it is assumed that the center of the Wien filter is the origin, the electric field at the origin is E ₀ , the magnetic field is B ₀ , and the electron velocity is v ₀ . Furthermore, the amount of minute displacement from the origin is
(Δx, δy). At this time, as an approximate model,
Assuming that the opposing electrode and the opposing magnetic pole are an electric dipole and a magnetic dipole centered on the origin, the electromagnetic fields Ex and By near the center can be expressed as follows. l _e : length of electric dipole l _m : length of magnetic dipole

The following equation holds for electrons incident on the approximate model in the Z direction. m: mass of the electron Therefore, the velocity vz of the electron is Becomes

From the above, the equation of motion of electrons in the Wien filter of equation (1) can be expressed as follows. Here, it is assumed that the electromagnetic field in the XY plane of the Wien filter is an ideal orthogonal electromagnetic field, and the electron velocity v ₀ in the effective region of the Wien filter is sufficiently high. In this case, the (2) the first term is zero polynomial of equation, l _e, also third and fourth paragraphs that l _m becomes infinite 0. Therefore, equation (2) is arranged as follows.

The above equation is an equation in which only the second term of the polynomial part of the equation (2) remains. In the Wien filter, the well-known convex power proportional to the ray height in the electric field direction, that is, the astigmatic difference is expressed. It is an expression. This astigmatism is, as described above,
It can be corrected by installing a stig meter. However, in practice, the calculation accuracy in the design, manufacturing error, Z
A low-order aberration component, that is, the third and fourth terms of the polynomial part of equation (2) remain due to the electromagnetic field component in the direction, the passage time of electrons, and the like. The δx ² component in this low-order aberration is a component showing asymmetric convergence with respect to the convex power in the electric field direction, and the δy ² component is a component showing image distortion flowing in the electric field direction. Such low-order aberrations such as the δx ² and δy ² components cannot be completely corrected by a stig meter. Therefore, the electromagnetic pole is adjusted to an optimal position by the electromagnetic pole adjusting device so that such low-order aberration components are minimized.

A specific method of adjusting the electromagnetic pole is to adjust the electrode voltage and the magnetic pole coil so that the predetermined Wien condition is satisfied on the optical axis, that is, so that the first term of equation (2) becomes zero. While changing the current, the distance between the opposing electrode and the opposing magnetic pole is moved so that the third and fourth terms in equation (2) also become zero. In the first place, the design error and manufacturing error with respect to the ideal design solution are actually not so large,
The amount of correction of the electromagnetic pole with respect to the design arrangement is very small.
Finally, all aberrations as a mapping optical system are removed by performing correction with a stig meter so that the second term of equation (2) becomes zero.

[0017]

Embodiments of the present invention will be described with reference to the drawings. 1 and 2 show an embodiment of a charged particle beam mapping optical system according to the present invention. FIG. 1 is a schematic diagram showing a mapping optical system according to one embodiment of the present invention. First, the electron gun 21
The emitted primary electron beam passes through the irradiation lens system 22 and enters the Wien filter 1. here,
The Wien filter 1 includes a counter pole 2, a counter electrode 4, an adjusting device, and the like. The primary electron beam is
The optical path can be bent by the Wien filter 1. The primary electron beam after passing through the Wien filter 1 passes through the objective lens system 24 and illuminates the surface of the sample 23 with incident light. Here, the illumination system includes an irradiation lens system 22, the Wien filter 1, an objective lens system 24, and the like.

When the primary electron beam is irradiated on the sample 23, a reflected electron beam having a relatively high energy reflected on the sample 23 and a low energy secondary electron beam emitted from the surface of the sample 23 are generated. appear. Of these electron beams, a secondary electron beam is usually used for imaging.
The secondary electron beam passes through the objective lens system 24 and enters the Wien filter 1. The secondary electron beam that has passed straight through the Wien filter 1 is
The light passes through the imaging lens system 28 in this order and enters the detector 29. Observation, inspection, and the like of the sample 23 are performed based on the information of the secondary electron beam incident on the detector 29. Here, the imaging system includes the objective lens system 24, the Wien filter 1, the stigmator 13, the imaging lens system 28, and the like.

FIG. 2 is a schematic view showing a Wien filter according to an embodiment of the present invention. Vienna filter 1
Are composed of an N magnetic pole 2a and an S magnetic pole 2b. Each magnetic pole can be moved in the magnetic field direction B by adjusting devices 6a and 6b. Here, the N magnetic pole 2a and the S magnetic pole 2b are
On the opposing surface, the absolute value is a ferromagnetic material having polarities different from each other. The counter electrode is composed of a positive electrode 4a and a negative electrode 4b, and each electrode is controlled by adjusting devices 6c and 6d.
It can move in the electric field direction E. Here, the positive electrode 4
a and the negative electrode 4b are electrodes made of a non-magnetic material. The positive voltage applied to the positive electrode 4a and the negative voltage applied to the negative electrode 4b have the same absolute value. Here, the adjusting devices 6a, 6b, 6c, and 6d are distance adjusting devices using a feed screw, for example.

Next, the effect of aberration correction by the Wien filter according to one embodiment of the present invention will be described in detail with reference to FIGS. FIG. 3 is a diagram showing a model of the imaging system of the mapping optical system. Here, at the main axis z of the imaging system, the electron beam K incident on the Wien filter 1 is
Satisfy the Vienna conditions. In the vicinity of the main axis z, the direction of the electromagnetic field (EB
Consider an electron beam K that is incident on a (plane) at equal intervals. After passing through the Wien filter 1, these electron beams K pass through a stigmator 13. here,
The stig meter 13 can correct only the uniform astigmatic difference component generated in the Wien filter 1. Section M in FIG.
Reference numerals 1, M2, and M3 denote sample surfaces for knowing changes in the trajectory of the electron beam K. That is, the cross section M1 is a cross section of the trajectory of the electron beam K before entering the Wien filter 1, the cross section M2 is a cross section of the trajectory after emitting the Wien filter 1 and before entering the stigmator 13, and the cross section M3 Is a track cross section after the stig meter 13 is emitted.

FIGS. 4 to 6 show the trajectories of the electron beam K in the cross sections M1 to M3 depending on whether or not the electrode interval between the positive electrode 4a and the negative electrode 4b of the Wien filter 1 is corrected. FIG. 4 is a trajectory cross-sectional view of the electron beam K at the cross-section M1. The white circles in the figure represent the electron beam K, and the intersections of the broken lines represent the position of the main axis z. Here, from the above-described condition of incidence on the Wien filter 1, the magnetic field direction B shown on the vertical axis,
Each electron beam K is equally spaced with respect to the electric field direction E shown on the horizontal axis.

FIG. 5 is a cross-sectional view of the track at the cross section M2. FIG. 5A shows the state before the electrode spacing of the Wien filter 1 is corrected, and FIG.
3 shows a state after the correction of the electrode interval is performed. In FIG. 7A, in the magnetic field direction B, the deviation from the Wien condition is deflected depending on the ray height and flows in the electrode direction. Further, in the electric field direction E, the velocity of the electron beam K passing through the region having a higher potential than the main axis becomes lower, and the speed becomes higher in the region having a lower potential than the main axis. An astigmatic difference that becomes power occurs. On the other hand, FIG.
In, the interval between the electrodes is adjusted while maintaining the Wien condition near the main axis. Specifically, in the case of FIG.
The electric field shape approximate to the design solution is formed by adjusting the distance between the electrodes in the direction of narrowing. Therefore, in FIG. 3B, only the uniform astigmatic difference remains among the aberrations. That is, aberrations that cannot be corrected by the stig meter 13 are removed by adjusting the Wien filter 1.

FIG. 6 is an example of a track sectional view at the section M3, and FIG. 6A shows the stig meter 13 after FIG. 5A.
5 shows a state after correcting the astigmatic difference so as to have a shape close to that of FIG. 4, and FIG. 5B shows a state after the same correction is made by the stig meter 13 after FIG. Is shown. In FIG. 5A, the distortion generated in FIG. 5A remains even after the stigmator 13 corrects the astigmatic difference. On the other hand, in FIG. 5B, the stigmator 13 can remove as much of the uniform astigmatic difference remaining in FIG. 5B as much as possible, so that an electron trajectory free from distortion on the map can be obtained. .

As described above, in this embodiment, the distance between the electrodes 4a and 4b of the Wien filter 1 is adjusted by the adjusting device 6.
By adjusting by c and 6d, it is possible to correct monochromatic aberration mainly caused by non-uniformity of the electromagnetic field near the optical axis. In the present embodiment, since the electrode 4 is a non-magnetic material, there is no influence on the magnetic field by adjusting the electrode interval. However, when the electrode 4 is a magnetic material, the interval between the magnetic poles 2 in addition to the electrode 4 is also increased. Need to adjust. Even when the electric field or the magnetic field deviates from the ideal shape over time, the initial electromagnetic field shape can be maintained by adjusting the intervals between the electromagnetic poles by the adjusting devices 6a, 6b, 6c, and 6d, as in the present embodiment. it can.

[0025]

As described above, according to the present invention, among monochromatic aberrations generated in the vicinity of the optical axis of a Wien filter, a high-resolution charged particle beam capable of easily reducing low-order aberration components generated due to inhomogeneity of an electromagnetic field. A mapping optics can be provided. Further, it is possible to provide a relatively inexpensive charged particle beam mapping optical system capable of relatively relaxing design errors and mechanical tolerances.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a charged particle beam mapping optical system according to an embodiment of the present invention.

FIG. 2 is a schematic view illustrating a Wien filter according to an embodiment of the present invention.

FIG. 3 is a schematic diagram showing an imaging system of a mapping optical system according to an embodiment of the present invention.

FIG. 4 is a cross-sectional view illustrating a trajectory of an electron beam before entering a Wien filter according to an embodiment of the present invention.

5A and 5B are a cross-sectional view illustrating a trajectory of an electron beam before adjusting a distance between electrodes after emitting a Wien filter according to an embodiment of the present invention, and FIG. 5B illustrates a trajectory of an electron beam after adjusting a distance between electrodes. It is sectional drawing.

FIG. 6 is a cross-sectional view showing (A) a trajectory of an electron beam before adjusting an electrode interval after emitting a stigmeter according to an embodiment of the present invention, and (B) shows an trajectory of the electron beam after adjusting the electrode interval. It is sectional drawing.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Wien filter 2 ... Counter magnetic pole 2a ... N magnetic pole 2b ... S magnetic pole 4 ... Counter electrode 4a ... Positive electrode 4b ... Negative electrode 6a, 6b, 6c, 6d ... Adjustment device 13 ... Stigmeter

Claims (2)

[Claims] An irradiation charged particle beam emitted from an irradiation source is incident on a Wien filter having an electrode pair and a magnetic pole pair through an irradiation optical system, and the irradiation charged particle beam passing through the Wien filter is irradiated with the irradiation charged particle beam. A direction in which the light is incident on the sample surface via the objective optical system, and the charged particle beam for observation emitted from the sample surface is incident on the Wien filter via the objective optical system, and is directed to the irradiation line source by the Wien filter. In the charged particle beam mapping optical system that guides the charged particle beam for observation in a direction different from that of the observation, and passes the charged particle beam for observation after passing through the Wien filter to a detection unit via an imaging optical system, The Wien filter has a movable portion that moves at least one of the electrode pair and the magnetic pole pair so that the shape of an electromagnetic field formed inside can be adjusted. A charged particle beam mapping optical system. 2. The charged particle beam mapping optical system according to claim 1, wherein said movable portion is formed so as to move said electrode pair in an electric field direction and move said magnetic pole pair in a magnetic field direction. .