JP4135221B2 - Mapping electron microscope - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a mapping electron microscope for observing, inspecting, and the like of a sample surface using an electron beam.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, an electron microscope using an electron beam is often used to observe and inspect semiconductor elements and the like that are miniaturized and highly integrated. Some electron microscopes are called low-energy electron microscopes using a mapping optical system (K. Tsuno, Ultramicroscopy 55 (1994) 127-140 “Simulation of a Wien filter as beam separator in a low energy electron microscope. ").
[0003]
The low energy electron microscope will be briefly described with reference to FIG. 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 (E × B) 4. The irradiation electron beam S deflected by the e-cross bee 4 passes through the aperture stop 5 and then passes through the cathode lens 6 to be incident on the sample 7.
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.
[0004]
The mapping 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. When the Wien condition is satisfied, the mapping electron beam K that has passed straight through the e-cross bee 4 passes through the imaging lens front group 8, the field stop 9, and the imaging lens rear group 10, and then MCP ( An image is formed on an electron beam detector 11 such as a micro channel plate. Here, the illumination lenses 2 and 3, the cathode lens 6, the imaging lens front group 8, and the imaging lens rear 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 the image sensor 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.
[0005]
Next, the configuration of the e-cross bee 4 will be briefly described with reference to FIG. The e-cross bee 4 is mainly composed of a yoke 20, electrodes 21a and 21b, coils 22a and 22b, and the like. The yoke 20 is grounded. 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. As a result, an electric field is generated in the X direction and a magnetic field is generated in the Y direction at the center of the e-cross bee 4.
The e-cross bee 4 is arranged so that its central axis coincides with the optical axes of the cathode lens 6, the imaging lens front group 8, and the imaging lens rear group 10 in FIG. 3, and the electric field vector described above. Is included in the irradiation system incident surface.
[0006]
Thus, 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 travels straight in the −Z direction when the Wien condition for the mapping electron beam K is satisfied. Thus, the e-cross bee 4 has a function as a so-called beam separator.
Here, the Wien condition means a straight traveling condition by an electromagnetic field in which the equation of motion for the mapping electron beam K satisfies F = q · (E−vB) = 0. In the irradiation electron beam S, the traveling direction in the e-cross bee 4 is opposite to the traveling direction of the mapping electron beam K. Therefore, the force due to the electric field and the magnetic field acts in the same direction and is synthesized. It is deflected by the applied force.
[0007]
[Problems to be solved by the invention]
The conventional mapping electron microscope generates astigmatic difference and magnification difference, which has been a major obstacle to highly accurate observation and inspection. The cause of the difference is that the power in the electric field direction (X direction) and the power in the magnetic field direction (Y direction) with respect to the electron beam of E. cross bee are different. That is, e-Cross Bee had a convex power in the electric field direction and no power in the magnetic field direction.
In order to prevent the astigmatic difference and the magnification difference from occurring, a method of further superimposing a quadrupole component on the electromagnetic field of the conventional e-cross bee has been considered. Specifically, as shown in FIG. 5, a predetermined voltage (−V ₄ ) is applied to the yoke 20, and in addition to the voltages (−V ₂ , + V ₂ ) applied in advance to the electrodes 21 a and 21 b. Thus, a voltage (+ V ₄ ) having a polarity opposite to the voltage (−V ₄ ) applied to the yoke 20 is applied. As a result, power due to the electric field is also generated in the Y direction, and astigmatism difference and magnification difference can be corrected by this power.
[0008]
According to such a method, the astigmatic difference and the magnification difference generated in the e-cross bee can be corrected. However, the astigmatism difference and the magnification difference as seen in the entire mapping electron microscope include those caused by mechanical tolerances and those caused by the contamination of irradiated particles over time. The direction of these gaps is an unspecified direction, whereas the gap generated in e-cross bee is a specific direction. Therefore, the method of correcting only the specific direction difference generated in the e-cross bee, that is, the e-cross bee in which the quadrupole component is superimposed, cannot sufficiently correct the non-specific direction difference. .
[0009]
In addition to this, the e-cross bee in which the quadrupole component is superimposed has a problem in terms of durability. That is, as described above, in order to superimpose the quadrupole component on the e-cross bee, a voltage is applied to the yoke 20 made of a ferromagnetic material around which the coils 22a and 22b are wound. However, e-Cross Bee is usually installed in a sealed vacuum chamber and is not easily radiated. In this state, when a coil current is passed 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 new astigmatic differences and magnification differences. In order to solve such a problem of durability over time, even if a cooling mechanism for suppressing a temperature rise is provided, another problem such as expansion of the apparatus and cost increase is caused.
[0010]
A method of installing one stigmator in the mapping optical system is also considered. The stigmator is a unit composed of a plurality of electrostatic electrodes or a plurality of electromagnetic poles. In general, an octupole is used in one stage, or a two-stage quadrupole is rotated 45 degrees around the central axis. The stigmator can generate an astigmatic difference in an arbitrary direction within a plane orthogonal to its central axis. If one such stigmator is added to the mapping optical system, an astigmatic difference opposite to that generated by e-cross bee or the like is generated, and the entire astigmatic difference is corrected. Can do.
In fact, in a scanning electron microscope (SEM) or the like, the astigmatic difference is corrected by using one electromagnetic octupole stigmator. However, in the case of a scanning electron microscope, the number of condensing points is one, whereas in the case of a mapping type microscope, it is necessary to condense all the two-dimensional points without distortion, so one stigmator. However, sufficient gap correction is not possible. That is, although the astigmatic difference is corrected and the resolution degradation caused thereby is improved, a magnification difference is generated in the direction in which the astigmatic difference is corrected. Then, image distortion due to this magnification error occurs.
Accordingly, an object of the present invention is to provide a mapping microscope that can correct any astigmatism difference and magnification difference, is low-cost, and has high resolution and excellent durability.
[0011]
[Means for Solving the Problems]
The present invention has been made to solve the above-described problems. That is, when the reference numerals in the attached drawings are added in parentheses, the present invention irradiates the sample surface (7) with the irradiation electron beam (S). In the mapping type electron microscope comprising: an irradiating means that causes the imaging electron beam (K) emitted from the sample surface (7) to form an image on the electron beam detecting means (11); A plurality of stigmeters (16, 17), and at least two of the plurality of stigmeters (16, 17) are arranged at positions different from conjugate to each other. This is a mapping electron microscope.
[0012]
In the present invention, the irradiation electron beam (S) emitted from the irradiation source (1) is incident on the optical path switching means (4) through the irradiation optical system (2, 3), and passes through the optical path switching means (4). The irradiated electron beam (S) is incident on the sample surface (7) via the objective optical system (6), and the mapping electron beam (K) emitted from the sample surface (7) is incident on the objective optical system (6). Is incident on the optical path switching means (4), and the optical path switching means (4) guides the mapping electron beam (K) in a direction different from the direction reaching the irradiation source (1), and the optical path switching means (4). In the mapping electron microscope in which the mapping electron beam (K) after passing through the beam enters the electron beam detecting means (11) via the imaging optical system (8, 10), the imaging optical system (8, 10) Has a plurality of stigmeters (16, 17), and at least one of the plurality of stigmeters (16, 17). One stigmator (16, 17) is a mapping electron microscope, characterized in that disposed at a conjugate position different from each other.
[0013]
In the mapping electron microscope having the above configuration, the astigmatic difference and the magnification difference can be simultaneously corrected by at least two stigmeters arranged at positions that are not conjugated with each other.
As described above, the stigmator not only contributes to the astigmatic difference but also contributes to the magnification difference. The contribution ratio varies depending on the optical position where the stigmator is arranged. In other words, the astigmatic difference and the magnification difference are generally in a trade-off relationship, and if the stigmator is placed at a position where the contribution to the astigmatic difference is large, the contribution to the magnification difference becomes small, and conversely, the contribution to the magnification difference. If the position is large, the contribution to the astigmatic difference becomes small. 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 contribution to the difference between the stigmeters varies depending on the following points. That is, in the stigmator, an electron beam passing through a position away from the optical axis has a larger deflection angle from the stigmator than an electron beam passing through a position close to the optical axis. However, the degree of influence of the declination on the image becomes smaller as the arrangement position of the stigmator approaches the imaging plane or its conjugate position.
[0015]
First, when the stigmator is arranged at the aperture stop or its conjugate position, the principal ray of the electron beam passes near the optical axis, so the influence on the magnification difference is small, but the ambient light spreads over the entire aperture surface of the aperture stop. Therefore, the influence on the astigmatic difference becomes large. That is, the stigmometer disposed near the aperture stop can efficiently correct the astigmatic difference.
Next, when the position of the stigmator is moved away from the aperture stop or its conjugate position, the chief ray gradually spreads off-axis, so the influence on the magnification difference increases. Since it does not spread as much as the opening surface, the effect on the astigmatic difference is reduced or almost unchanged. That is, the stigmometer disposed between the aperture stop and the field stop can efficiently correct the magnification difference.
[0016]
Furthermore, when the arrangement position of the stigmator is brought close to the imaging position such as the field stop or its conjugate position, the influence on the difference between the two is gradually reduced. When the stigmator is disposed on the image forming position or its conjugate position, the influence on the difference between the two is almost eliminated.
Accordingly, by arranging one of the two stigmeters in the vicinity of the aperture stop and arranging the other between the aperture stop and the field stop, for example, the difference between the two can be efficiently corrected. That is, the stigmometer disposed near the aperture stop can efficiently correct the astigmatic difference, and the stigmometer disposed between the aperture stop and the field stop can efficiently correct the magnification difference.
[0017]
In addition, in the mapping microscope using the e-cross bee, as described above, the e-cross bee itself has a cause of astigmatism, so one of the two stigmeters is connected to the e-cross bee. Astigmatism difference can be corrected more efficiently by arranging it near the bee, or at or near the conjugate point of the e-cross bee. At this time, if the astigmatism caused by other causes, that is, the astigmatism due to mechanical tolerance, contamination with time, etc. can be ignored, most of the astigmatism can be corrected only by this stigmator. . In such a case, since the direction of the astigmatism generated in the e-cross bee is clear, a quadrupole can be used as a stigmator. However, this is not the case for a mapping type microscope whose conjugate point fluctuates due to zooming or the like.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
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. The irradiation electron beam S deflected by the e-cross bee 4 passes through the aperture stop 5 and then passes through the cathode lens 6 and is irradiated onto the sample 7.
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 mapping 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 Vienna condition, the mapping 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 imaging lens rear group 10 to form an enlarged projected image on the electron beam detector 11.
[0019]
Here, the first stigmator 16 and the second stigmator 17 are, for example, electrostatic octupoles. The first stigmator 16 is disposed in the vicinity of the e-cross bee 4 and efficiently corrects the astigmatic difference. On the other hand, the second stigmator 17 is disposed between the imaging lens front group 8 and the field stop 9, and efficiently corrects the magnification difference.
The illumination lenses 2 and 3, the cathode lens 6, the imaging lens front group 8, and the imaging lens rear 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 the image sensor 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 this embodiment, the electron gun 1, the illumination lenses 2, 3, the e-cross bee 4, and the cathode lens 6 are irradiation means, and the cathode lens 6, the e-cross bee 4, the imaging lens. The front group 8 and the imaging lens rear group 10 are mapping means.
[0020]
Next, 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 divided into the X direction (electric field direction) and the Y direction (magnetic field direction). Further, the mapping electron beam emitted from the sample 7 is divided into a principal ray K1 indicated by a solid line and an ambient light K2 indicated by a broken line. Here, the ambient light K2 is on-axis ambient light emitted from the aperture stop 5 in an infinite system. Hereinafter, correction of astigmatism difference and magnification difference by the first stigmator 16 and the second stigmator 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. Thereby, an astigmatic difference and a magnification difference are generated in the magnetic field direction with respect to the electric field direction. The electron beam that has passed through the e-cross beam 4 subsequently passes through the first stigmator 16, the imaging lens front group 8, and the second stigmator 17 in this order, and then forms an image on the intermediate imaging plane M. This intermediate image plane M is the position of the field stop 9.
[0021]
Here, the first stigmator 16 is disposed in the vicinity of the e-cross bee 4, and the second stigmator 17 is disposed between the imaging lens front group 8 and the intermediate imaging plane M. Therefore, the difference between the incident height of the principal ray K1 at the position of the first stigmator 16 and the incident height of the ambient light K2 and the separation between the principal ray K1 and the ambient light K2 at the position of the second stigmeter 17 are greatly different. . Thus, out of the two differences generated in the e-cross bee 4, the astigmatic difference is mainly corrected efficiently by the first stigmator 16, and the magnification difference is mainly corrected efficiently by the second stigmator 17. be able to.
[0022]
Specifically, in the first stigmator 16, a concave power is generated in the first stigmator electric field direction 16X and a convex power is generated in the first stigmometer magnetic field direction 16Y so as to correct the astigmatic difference of the ambient light K2. Apply a voltage that generates. At this time, since the principal ray K1 is overcorrected, a magnification difference enlarged in the electric field direction is generated. According to 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 this way, in order to further correct the overcorrected principal ray K1 generated in the first stigmator 16, the second stigmator 17 generates a convex power in the second stigmator electric field direction 17X, and the second stigmator 17 A voltage is applied so that concave power is generated in the magnetic field direction 17Y.
[0023]
As described above, in this embodiment, the astigmatic difference and the magnification difference can be efficiently and efficiently corrected by balancing the correction by the first stigmator 16 and the correction by the second stigmator 17.
In this embodiment, the correction of the gap generated in the e-cross bee 4 has been described. However, the gap generated due to other causes, that is, the gap due to mechanical tolerance, contamination with time, etc. is also described in the first stigma. Correction can be made easily by adjusting the voltage applied to the meter 16 and the second stigmator 17.
[0024]
Further, in the mapping means of the present embodiment, a combined convex power is generated by the e-cross bee 4, the first stigmator 16, and the second stigmator 17, but the focal length of the imaging lens front group 8, that is, By adjusting the convex power, the target magnification and overall length can be secured. If a mapping means having an optimum magnification and overall length is used in this way, a telecentric optical system with little aberration can be constructed.
In the mapping microscope according to the present embodiment, images having the same aspect ratio are formed without generating astigmatism, but images having different predetermined aspect ratios can also be formed.
[0025]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a mapping microscope that can correct any astigmatism difference and magnification difference, is low-cost, and has high-resolution durability.
That is, it is possible to provide a mapping electron microscope that can easily and reliably simultaneously correct an astigmatic difference and a magnification difference even when a low-cost e-cross bee that is easy to manufacture is used. In addition, since the astigmatic difference and the magnification difference generated due to mechanical factors such as manufacturing errors can be corrected simultaneously, a mapping type electron microscope can be provided which is easy to manufacture because the mechanical tolerance of the entire apparatus is loose. In addition, since the astigmatism difference and the magnification difference caused by contamination of irradiated particles or the like can be corrected simultaneously, a mapping electron microscope having high durability against changes with time can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram showing a mapping electron microscope according to an embodiment of the present invention.
FIG. 2 is a diagram showing mapping means 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 in which quadrupole components are superimposed.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Electron gun 2, 3 ... Illumination lens 4 ... E cross bee 5 ... Aperture stop 6 ... Cathode lens 7 ... Sample 8 ... Imaging lens front group 9 ... Field stop 10 ... Imaging lens rear group 11 ... Electron beam Detector 12 ... Relay lens 13 ... Image sensor 14 ... Control unit 16 ... First stigmator 17 ... Second stigmator 20 ... Yoke 21a, 21b ... Electrodes 22a, 22b ... Coil S ... Electron beam for irradiation K ... Electron for mapping Beam M ... Intermediate imaging plane

Claims (5)

An irradiation means for irradiating the sample surface with an electron beam for irradiation;
An optical path switching means for switching an optical path of the irradiation electron beam;
An aperture stop through which the irradiation electron beam and the mapping electron beam emitted from the sample surface pass;
A field stop in which the mapping electron beam forms an intermediate image;
In a mapping type electron microscope comprising mapping means for forming an image on the mapping electron beam on an electron beam detection means,
The mapping means has a plurality of stigmators,
At least two of the plurality of stigmeters include the aperture stop, the field stop, the optical path switching unit or a conjugate position thereof, and the aperture stop, the field stop, the optical path switching unit, or a conjugate thereof. mapping electron microscope, characterized in that disposed respectively to the position different from the position. An irradiation electron beam emitted from the irradiation source is made incident on the optical path switching means through the irradiation optical system, and the irradiation electron beam that has passed through the optical path switching means and the aperture stop is made incident on the sample surface through the objective optical system. The electron beam for mapping emitted from the sample surface is incident on the aperture stop and the optical path switching means via the objective optical system, and the optical path switching means causes the electron beam to travel in a direction different from the direction reaching the irradiation source. A mapping electron beam is guided, an intermediate image is formed on the mapping electron beam after passing through the optical path switching means by a field stop, and the mapping electron beam is detected through an imaging optical system. In a mapping electron microscope that is incident on the means,
The imaging optical system has a plurality of stigmators,
At least two of the plurality of stigmeters include the aperture stop, the field stop, the optical path switching unit or a conjugate position thereof, and the aperture stop, the field stop, the optical path switching unit, or a conjugate thereof. mapping electron microscope, characterized in that disposed respectively to the position different from the position. The mapping type electron microscope according to claim 1 or 2, wherein the at least two stigmeters are arranged between the optical path switching means and the field stop . The mapping electron microscope according to claim 1 , wherein an electrostatic lens is provided between the at least two stigmeters. The mapping electron microscope according to claim 1 , wherein the stigmator is an electrostatic octupole.

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