JP2000011934A - Mapping electron microscope - Google Patents

Abstract

(57) [Summary] [PROBLEMS] Any astigmatic difference and magnification difference can be corrected.
To provide a low-cost, high-resolution, highly durable mapping microscope. Kind Code: A1 Abstract: Irradiation means for irradiating a sample surface with an electron beam for irradiation, and an electron beam for imaging emitted from the sample surface are imaged on an electron beam detection means. In a mapping electron microscope provided with mapping means 4, 6, 8, and 10, the mapping means 4, 6, 8, and 10 have a plurality of stigmeters 16 and 17, and a plurality of stigmeters 16 and 1 are provided.
7, at least two stigmeters 16, 17
Are arranged at different positions from each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mapping electron microscope for observing and inspecting a sample surface using an electron beam.

[0002]

2. Description of the Related Art Conventionally, an electron microscope using an electron beam has been widely used for observing and inspecting miniaturized and highly integrated semiconductor devices. In the electron microscope,
There is a so-called low-energy electron microscope using a mapping optical system (K. Tsuno, Ultramicroscopy 55 (1994) 127-14
0 `` Simulation of a Wien filter as beam separator in
a low energy electron microscope ").

FIG. 3 briefly describes a low energy electron microscope. The irradiation electron beam S emitted from the electron gun 1 is shaped by the illumination lenses 2 and 3,
It is incident on an e-cross bee (E × B) 4. Irradiation electron beam S deflected by ecross bee 4
After passing through the aperture stop 5, the sample 7 passes through the cathode lens 6 and is illuminated with incident light. When the sample 7 is irradiated with the irradiation electron beam S, secondary electrons, reflected electrons,
Backscattered electrons and the like are emitted. At least one of these electrons becomes a mapping electron beam K.

The electron beam K for mapping emitted from the sample 7
Passes through the cathode lens 6 and the aperture stop 5 and
It is incident on the cross bee 4. Then, by satisfying the Vienna conditions, the electron beam K for imaging that has passed straight through the e-cross bee 4 passes through the front group 8, the field stop 9, and the rear group 10 of the imaging lens before the MCP ( Micr
o Channel Plate) and the like. Here, the illumination lenses 2 and 3, the cathode lens 6,
The front lens group 8 and the rear lens group 10 are electrostatic lenses such as an Einzel lens. Electron beam detector 11
When the electron beam K for mapping is incident on the device, the electron beam K for mapping is converted into light. The light emitted from the electron beam detector 11, that is, the optical image of the sample 7 is
And is incident on an image pickup device 13 such as a CCD. The light incident on the image sensor 13 is converted into a photoelectric signal and transmitted to the control unit 14.

Next, the configuration of the e-cross bee 4 will be briefly described with reference to FIG. E-cross bee 4
Mainly, yoke 20, electrodes 21a and 21b, coil 22
a, 22b and the like. The yoke 20 is grounded. Then, a voltage (−V ₂ ) is applied to the electrode 21a, a voltage (+ V ₂ ) is applied to the electrode 21b, and a current flows through the coils 22a and 22b. This allows
At the center of the e-cross bee 4, an electric field is generated in the X direction and a magnetic field is generated in the Y direction. The e-cross bee 4 is arranged so that its central axis coincides with the optical axes of the cathode lens 6, the front lens group 8, and the rear lens group 10 in FIG. Is arranged so as to be included in the incident plane of the irradiation system.

In this way, the irradiation electron beam S obliquely incident on the e-cross bee 4 is deflected and proceeds in the + Z direction. On the other hand, the mapping electron beam K emitted from the sample 7
Goes straight in the −Z direction when the Wien condition for the mapping electron beam K is satisfied. in this way,
The e-cross bee 4 has a function as a so-called beam separator. Here, the Wien condition means that the equation of motion for the mapping electron beam K is F = q · (Ev
B) A straight traveling condition by an electromagnetic field satisfying = 0. In the irradiation electron beam S, the traveling direction in the e-cross beam 4 is opposite to the traveling direction of the mapping electron beam K, so that the force by the electric field and the magnetic field acts in the same direction, and the resultant is combined. Is deflected by the applied force.

[0007]

In the above-mentioned conventional mapping electron microscope, astigmatic difference and magnification difference occur, which hinder high-precision observation and inspection. The cause of these differences is due to the difference between the power in the electric field direction (X direction) and the power in the magnetic field direction (Y direction) for the electron beam of ecross bee. That is,
Cross-B had a convex power in the electric field direction and no power in the magnetic field direction. In order to prevent astigmatic difference and magnification difference from occurring, a method of further superposing a quadrupole component on the electromagnetic field of the above-mentioned conventional e-cross bee has been considered. Specifically, as shown in FIG.
A predetermined voltage (−V ₄ ) is applied to 0, and the electrodes 21a, 2
A voltage (+ V ₄ ) having a polarity opposite to the voltage (−V ₄ ) applied to the yoke 20 is applied to 1 b in addition to the voltages (−V ₂ , + V ₂ ) applied in advance. As a result, electric power is also generated in the Y direction by the electric field, and the astigmatic difference and the magnification difference can be corrected with this power.

According to such a method, e-cross
It is possible to correct the astigmatic difference and the magnification difference generated in the bee. However, the astigmatism difference and magnification difference seen in the whole of the mapping electron microscope include those caused by mechanical tolerance and those caused over time due to contamination of irradiated particles. The direction of these differences is an unspecified direction while the difference generated in e-cross bee is a specific direction. Therefore, in the method of correcting only the difference in the specific direction generated in the e-cross bee, that is, the difference in the unspecified direction cannot be sufficiently corrected by the e-cross bee in which the quadrupole components are superimposed. .

[0009] In addition, the e-cross bee on which quadrupole components are superposed has a problem in terms of durability. That is, as described above, a voltage is applied to the yoke 20 made of a ferromagnetic material wound with the coils 22a and 22b in order to superimpose the quadrupole component on the e-cross bee. However, eCross B is usually installed in a closed vacuum chamber, and is hardly dissipated. In such a state, when a coil current flows through the yoke 20 that is not grounded for a long time, the temperature of the yoke 20 gradually increases. As the temperature rises, the e-cross bee is thermally deformed, causing a new astigmatic difference and a magnification difference. Even if a cooling mechanism for suppressing a rise in temperature is provided to solve such a problem of durability over time, another problem such as an increase in the size of the apparatus and an increase in cost will be caused.

[0010] A method of installing one stigmeter in the mapping optical system has also been considered. What is a stig meter?
It is composed of a plurality of static electrodes or a plurality of electromagnetic poles. In general, those using an octupole in one stage, those in which two-stage quadrupoles are rotated by 45 degrees around a central axis, and the like are used. The stigmometer can generate astigmatism in an arbitrary direction in a plane orthogonal to its central axis. If one such stig meter is added to the mapping optical system, an astigmatism opposite to that generated in e-cross-bee or the like is generated, and the entire astigmatism is corrected. Can be. In fact, a scanning electron microscope (S
In EM) and the like, the astigmatic difference is corrected using one electromagnetic octupole stigmeter. However, in the case of a scanning electron microscope, the condensing point is one point, whereas in the case of a mapping microscope, it is necessary to condense all two-dimensional points without distortion. Cannot sufficiently correct the difference. That is, the astigmatism difference is corrected, and the resolution degradation due to the astigmatism is improved, but a magnification difference occurs in the direction in which the astigmatism is corrected. Then, image distortion occurs due to the magnification error. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a low cost, high resolution, highly durable mapping microscope capable of correcting any astigmatic difference and magnification difference.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. That is, when the reference numerals in the accompanying drawings are added in parentheses, the present invention provides an irradiation electron beam (S). ) On the sample surface (7);
In a mapping type electron microscope provided with a mapping means for forming a mapping electron beam (K) emitted from a sample surface (7) on an electron beam detecting means (11), the mapping means comprises a plurality of stigmeters (16). , 17), and at least two of the plurality of stigmeters (16, 17) are arranged at positions different from and conjugate to each other. is there.

Further, according to the present invention, the irradiation electron beam (S) emitted from the irradiation ray source (1) is made incident on the optical path switching means (4) via the irradiation optical system (2, 3), and the light path switching means (4) is provided. ) Is passed through the objective optical system (6) to the sample surface (7), and the imaging electron beam (K) emitted from the sample surface (7) is passed through the objective optical system. (6), the light is made incident on the optical path switching means (4), and the imaging electron beam (K) is guided by the optical path switching means (4) in a direction different from the direction reaching the irradiation source (1). In an imaging electron microscope in which the imaging electron beam (K) after passing through (4) is incident on the electron beam detecting means (11) via the imaging optical system (8, 10), the imaging optical system (8) is used. , 10) has a plurality of stigmeters (16, 17) and a plurality of stigmeters (1).
At least two of the stigmeters (6, 17) are mapping electron microscopes, which are arranged at positions different from and conjugate to each other.

In the mapping electron microscope having the above configuration,
The astigmatic difference and the magnification difference can be corrected simultaneously by at least two stig meters arranged at positions that are not conjugate to each other. As mentioned above, the stig meter not only contributes to the astigmatism, but also to the magnification difference. The contribution ratio of the stig meter differs depending on the optical position where the stig meter is arranged. In other words, the astigmatism and the magnification difference are generally in a trade-off relationship, and when the stigmator is arranged at a position where the contribution to the astigmatism is large, the contribution to the magnification difference is small, and conversely, the contribution to the magnification difference is small. When it is arranged at a position where is large, the contribution to the astigmatic difference decreases. Therefore, in order to simultaneously correct the astigmatic difference and the magnification difference, at least two stigmeters may be arranged at positions that are not optically conjugate.

[0014] As described above, the ratio of the contribution to the difference between the two distances differs depending on the position of the stig meter due to the following. That is, in the stigmeter, an electron beam that passes through a position distant from the optical axis receives a larger deflection angle from the stigmeter than an electron beam that passes through a position near the optical axis. However, the degree to which the declination affects the image depends on the position of the stig meter,
It becomes smaller as it approaches the imaging plane or its conjugate position.

First, when the stigmeter is arranged at the aperture stop or its conjugate position, the principal ray of the electron beam passes near the optical axis, so that the influence on the magnification difference is small. The effect on the astigmatic difference is greater because it is spread over the entire area. That is, the stigmator arranged near the aperture stop can efficiently correct the astigmatic difference. Next, when the position of the stig meter is moved away from the aperture stop or its conjugate position, the chief ray gradually spreads off-axis, so that the influence on the magnification difference increases. Since it does not spread as much as compared to the aperture surface, the effect on astigmatism is reduced or hardly changed. In other words, the stig meter arranged between the aperture stop and the field stop can efficiently correct the magnification difference.

Further, when the position of the stig meter is brought closer to the image forming position of the field stop or the like or its conjugate position, the influence on the bilateral difference is gradually reduced. When the stig meter is arranged on the image forming position or its conjugate position, there is almost no influence on the bilateral difference. Therefore, by disposing one of the two stigmeters, for example, in the vicinity of the aperture stop and disposing the other in the middle between the aperture stop and the field stop, it is possible to efficiently correct the difference between the two. In other words, a stigmeter arranged near the aperture stop can efficiently correct astigmatism, and a stigmeter arranged between the aperture stop and the field stop can efficiently correct magnification difference.

Further, in the mapping type microscope using the e-cross bee, as described above, since the e-cross bee itself has a factor of causing astigmatic difference, one of the two stigmometers is used. The astigmatism difference can be corrected more efficiently by arranging in the vicinity of the e-cross bee or at the conjugate point of the e-cross bee or in the vicinity thereof. At this time, astigmatism caused by other causes,
That is, as long as the astigmatism due to mechanical tolerance or contamination with time can be ignored, most of the astigmatism can be corrected only by this stig meter. In such a case, since the direction of the astigmatic difference generated in the e-cross bee is clear, a quadrupole can be used as a stigmator. However, this does not apply to the mapping microscope in which the conjugate point fluctuates due to zooming or the like.

[0018]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment of a mapping electron microscope according to the present invention. The irradiation electron beam S emitted from the electron gun 1 is shaped by the illumination lenses 2 and 3, and then enters the e-cross bee 4. Ecross bee 4
The irradiation electron beam S deflected by the aperture stop 5
After passing through, the sample 7 is irradiated through the cathode lens 6. When the sample 7 is irradiated with the irradiation electron beam S, secondary electrons, reflected electrons, backscattered electrons, and the like are emitted from the sample 7. At least one of these electrons becomes a mapping electron beam K. The imaging electron beam K emitted from the sample 7 passes through the cathode lens 6 and the aperture stop 5 and enters the e-cross bee 4. Then, by satisfying the Wien condition, the imaging electron beam K that has passed straight through the e-cross bee 4 passes through the first stigmator 16, the imaging lens front group 8, and the second stigmator 17 in this order. An intermediate image is formed on the field stop 9.
The mapping electron beam K that has passed through the field stop 9 further passes through the rear group 10 of the imaging lens, and forms an enlarged projection image on the electron beam detector 11.

Here, the first and second stigmeters 16 and 17 are, for example, electrostatic octupoles. The first stig meter 16 is arranged near the e-cross bee 4 and efficiently corrects the astigmatic difference. On the other hand, the second stigmator 17 is disposed between the front group 8 of the imaging lens and the field stop 9, and efficiently corrects the magnification difference. The illumination lenses 2 and 3, the cathode lens 6, the front lens group 8, and the rear lens group 10 are electrostatic lenses such as an Einzel lens. When the mapping electron beam K is incident on the electron beam detector 11, the mapping electron beam K is converted into light. The light emitted from the electron beam detector 11, that is, the optical image of the sample 7 is transmitted through the relay lens 12 and is incident on an imaging device 13 such as a CCD. The light incident on the image sensor 13 is converted into a photoelectric signal and transmitted to the control unit 14. In the present embodiment, the electron gun 1, the illumination lenses 2, 3, the e-cross bee 4, and the cathode lens 6 serve as irradiation means, and the cathode lens 6, the e-cross bee 4, and the imaging lens The front group 8 and the rear group 10 of the imaging lens form the imaging means.

FIG. 2 shows an embodiment of the mapping means of the mapping electron microscope according to the present invention. The figure shows the trajectory of the electron beam passing through the mapping means in the X direction (electric field direction) and the Y direction.
FIG. 3 is a diagram divided into directions (magnetic field directions). Further, the mapping electron beam emitted from the sample 7 is represented by being divided into a principal ray K1 shown by a solid line and a peripheral light K2 shown by a broken line. Here, the peripheral light K2 is on-axis peripheral light emitted from the aperture stop 5 in an infinite distance system. Hereinafter, the correction of the astigmatic difference and the magnification difference by the first stig meter 16 and the second stig meter 17 will be described in detail with reference to FIG. The electron beam emitted from the sample 7 passes through the cathode lens 6 and the aperture stop 5 and then enters the e-cross bee 4. As described above, the electron beam in the e-cross bee 4 receives a convex power in the e-cross bee electric field direction 4X and has no power in the e-cross bee magnetic field direction 4Y.
As a result, astigmatic difference and magnification difference occur in the direction of the magnetic field with respect to the direction of the electric field. The electron beam that has passed through the e-cross bee 4 then passes through the first stigmator 16, the front group 8 of the imaging lens, and the second stigmator 17, in that order.
An image is formed on the intermediate imaging plane M. This intermediate imaging plane M is the position of the field stop 9.

Here, the first stig meter 16 is an E.E.
The second stigmator 17 is arranged near the cross bee 4 and between the front group 8 of the imaging lens and the intermediate imaging plane M. Therefore, the distance between the incident height of the principal ray K1 at the position of the first stig meter 16 and the incident height of the peripheral light K2,
The chief ray K1 and the peripheral light K at the position of the second stig meter 17
It is very different from the distance from 2. In this way, of the both differences generated in the e-cross bee 4, the astigmatic difference is efficiently corrected mainly by the first stig meter 16, and the magnification difference is mainly corrected efficiently by the second stig meter 17. be able to.

More specifically, the first stigmeter 16 generates a concave power in the first stigmeter electric field direction 16X so as to correct the astigmatic difference of the ambient light K2.
A voltage that generates a convex power in the stigmator magnetic field direction 16Y is applied. At this time, since the principal ray K1 is overcorrected, a magnification difference that is enlarged in the electric field direction occurs. According to a numerical analysis based on a predetermined condition, the magnification difference is 1.2 in the electric field direction when the magnification in the magnetic field direction is 1. In order to further correct the overcorrected chief ray K1 generated by the first stigmator 16 as described above, the second stigmator 17 generates a convex power in the second stigmator electric field direction 17X, A voltage that generates concave power is applied in the magnetic field direction 17Y.

As described above, in this embodiment, the astigmatic difference and the magnification difference can be efficiently corrected simultaneously by balancing the correction by the first stig meter 16 and the correction by the second stig meter 17. . In the present embodiment, the correction of the difference generated in the e-cross bee 4 has been described.
That is, the first stig meter 16 and the second stig meter 17 are also used for the difference due to mechanical tolerance, contamination with time, and the like.
Can be easily corrected by adjusting the applied voltage.

Further, in the mapping means of the present embodiment, a combined convex power is generated by the e-cross bee 4, the first stig meter 16, and the second stig meter 17, but the focal length of the front lens group 8 of the imaging lens. That is, by adjusting the convex power, it is possible to secure a target magnification and overall length. By using the mapping means having the optimum magnification and overall length, a telecentric optical system with less aberration can be constructed. Further, in the mapping microscope according to the present embodiment, images having the same aspect ratio are formed without causing astigmatism, but images having different predetermined aspect ratios can be formed.

[0025]

As described above, according to the present invention, it is possible to provide a low-cost, high-resolution, high-durability imaging microscope capable of correcting any astigmatic difference and magnification difference.
That is, it is possible to provide a mapping electron microscope which can easily and reliably simultaneously correct astigmatism and magnification difference generated therein even when a low-cost type e-cross bee which is easy to manufacture is used. In addition, since astigmatism and magnification differences caused by mechanical factors such as manufacturing errors can be corrected simultaneously,
It is possible to provide a mapping electron microscope in which the mechanical tolerance of the entire apparatus is loose and which is easy to manufacture. In addition, the astigmatic difference and magnification difference caused by contamination of irradiated particles can be corrected simultaneously,
It is possible to provide a mapping electron microscope with high durability against aging.

[Brief description of the drawings]

FIG. 1 is a diagram showing a mapping electron microscope according to one embodiment of the present invention.

FIG. 2 is a diagram showing a mapping unit according to an embodiment of the present invention.

FIG. 3 is a diagram showing a conventional mapping electron microscope.

FIG. 4 is a diagram showing a conventional e-cross bee.

FIG. 5 is a diagram showing an e-cross bee on which quadrupole components are superimposed.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun 2, 3 ... Illumination lens 4 ... E-cross b 5 ... Aperture stop 6 ... Cathode lens 7 ... Sample 8 ... Front group of imaging lens 9 ... Field stop 10 ... Rear group of imaging lens 11 ... Electron beam Detector 12 ... Relay lens 13 ... Imaging element 14 ... Control part 16 ... First stig meter 17 ... Second stig meter 20 ... Yoke 21a, 21b ... Electrode 22a, 22b ... Coil S ... Electron beam for irradiation K ... Electron for imaging Beam M: Intermediate imaging plane

Claims (6)

[Claims] 1. A mapping electron microscope comprising: an irradiating means for irradiating an electron beam for irradiation onto a sample surface; and an imaging means for forming an electron beam for imaging emitted from the sample surface on an electron beam detecting means. The mapping electron microscope, wherein the mapping means has a plurality of stigmeters, and at least two of the plurality of stigmeters are arranged at positions different from and conjugate to each other. 2. An irradiation electron beam emitted from an irradiation source is made incident on an optical path switching means via an irradiation optical system, and the irradiation electron beam passing through the optical path switching means is projected onto a sample surface via an objective optical system. Incident, the imaging electron beam emitted from the sample surface is incident on the optical path switching means via the objective optical system,
The light path switching means guides the mapping electron beam in a direction different from the direction reaching the irradiation beam source, and detects the mapping electron beam after passing through the light path switching means via an imaging optical system. In the projection electron microscope, the imaging optical system has a plurality of stigmeters, and at least two of the plurality of stigmeters are arranged at positions different from and conjugate to each other. A mapping electron microscope characterized by the following. 3. The apparatus according to claim 2, wherein one of said at least two stigmeters is arranged near said optical path switching means, or at or near a conjugate point of said optical path switching means. The mapping electron microscope of the above. 4. The image forming optical system has at least one intermediate image forming surface, and the at least two stig meters are formed on the sample path side of the optical path switching means and the intermediate image forming surface. The mapping electron microscope according to claim 2, wherein the mapping electron microscope is arranged between the intermediate image forming surface and the intermediate image forming surface. 5. An apparatus according to claim 1, wherein an electrostatic lens is provided between said at least two stig meters.
The mapping electron microscope according to any one of claims 1 to 4. 6. The mapping electron microscope according to claim 1, wherein said stigmator is an electrostatic octupole.