JP2000173523A - Electron beam optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron beam optical system with high resolution and little effect of electric noises by reducing chromatic aberration, astigmatic difference and magnification difference generated by E×B. SOLUTION: In this electron beam optical system, an electron beam S for irradiation emitted from an radiation source 10 goes into an optical path switching means 3 via an irradiation optical system 11, the electron beam S for irradiation having passed the optical path switch means 3 goes into a sample surface W via an objective optical system l, an electron beam K for observation emitted from the sample surface W goes into the optical path switch means 3 via the objective optical system 1. And the electron beam K for observation is guided in the direction different from the direction to the radiation source 10 by the optical path switch means 3, the electron beam K for observation after passing the optical path switch means 3 goes into an electron detecting means 7 via imaging optical systems 4a-4b. The objective optical system 1 is constituted to form an intermediate image of the sample surface W by the electron beam K for observation in the optical path switch means 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron optical system in an 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 observation and inspection of miniaturized and highly integrated semiconductor devices. In the electron microscope,
There is a so-called scanning electron microscope (SEM) and a so-called mapping electron microscope. A scanning electron microscope is a microscope that performs so-called point-to-point illumination and imaging, whereas a mapping electron microscope is a microscope that can perform surface-to-surface illumination and imaging. In recent years, in such an electron microscope, particularly in the field of a mapping electron microscope, an E.C.B (E.times.B) is used as a beam separator to switch between an optical path of an irradiation system and an optical path of an imaging system. , That research is being actively conducted.

The configuration of the electron beam optical system of the mapping electron microscope will be briefly described below. First, an irradiation electron beam emitted from an electron gun passes through an irradiation optical system and enters an e-cross bee. The electron beam for irradiation after passing through the e-cross bee becomes an electron beam having a rectangular cross section, passes through a cathode lens (objective optical system), and illuminates the sample surface with incident light. When the sample surface is irradiated with the irradiation electron beam, a reflected electron beam having relatively high energy reflected on the sample surface and a low-energy secondary electron beam emitted from the sample surface are generated. Of these electron beams, a secondary electron beam is usually used for imaging. The secondary electron beam (observation electron beam) passes through the cathode lens and enters the e-cross bee. The observation electron beam that has passed through E.C.B passes through the imaging optical system and enters the two-dimensional detector. Observation, inspection, and the like of the sample surface are performed based on information of the observation electron beam incident on the two-dimensional detector.

[0004]

As in the above-mentioned conventional electron beam optical system, an e-cross beam beam separator is used.
When the bee is installed in the imaging system, the occurrence of chromatic aberration, astigmatism, and magnification difference at the e-cross bee,
Due to the electric noise of the applied voltage or applied current to eCross Bee, it was not possible to obtain a good resolution for observation on a two-dimensional detector.

[0005] Here, e-cross bee will be briefly described. E-cross B generates electric and magnetic fields that are orthogonal to each other. Then, the electrons passing through the inside receive different refractive powers in the electric field direction and the magnetic field direction. For this reason, in the e-cross bee, astigmatic difference and magnification difference occur. Also,
Cross-B is also called a Wien filter, and was originally used as a means for separating electronic chromatic dispersion. From this, it can be said that E.CROSS.B is an element in which chromatic aberration occurs. Furthermore, from the power supply
The applied voltage or current supplied to the bee typically contains noise. Therefore, the refracting power of the e-cross bee changes intermittently. Here, the noise refers to both ripple noise that depends on the power supply itself and system noise that does not depend on the power supply itself.

Here, chromatic aberration and astigmatism generated in the conventional electron beam optical system will be described in detail with reference to FIGS. FIG. 6 is a schematic diagram showing the imaging system of the electron beam optical system from the direction of the electric field, and shows the influence of chromatic aberration on the resolution. Here, the image forming system includes a cathode lens 1, an e-cross bee 3, and an image forming lens 4 (transfer lens). Then, the image of the sample surface W is detected by the two-dimensional detector 7 by the cathode lens 1 and the imaging lens 4.
Telecentric imaging on the surface.

The observation electron beam K emitted from the sample surface W
After passing through the cathode lens 1, the principal ray and the ambient light enter the ecross bee 3 in parallel. here,
The observation electron beam K is, for example, a secondary electron beam having a dispersion of 1 to 2 eV. This observation electron beam K is
After passing through the cathode lens 1, for example,
It is accelerated to 4002 eV. The observation electron beam emitted from the e-cross bee 3 receives a refractive power according to the electron energy. That is, the observation electron beam K1 of 4001 eV proceeds in the direction shown by the solid line in FIG.
The eV observation electron beam K2 travels in the direction indicated by the broken line in FIG. This is because, as described above, the e-cross bee 3 has a property of emitting electrons having different energies in different directions.

In such a telecentric imaging system,
If there is a difference in the emission direction of the observation electron beam K, a blur D of an image as shown in FIG. 1 occurs on the two-dimensional detector 7, that is, the resolution is deteriorated. The same phenomenon as degradation of resolution due to chromatic aberration as described above
This can also occur when noise occurs in the voltage supplied to the cross bee 3. In other words, when noise occurs,
The electric field and the magnetic field of the e-cross bee 3 change at high speed, the deflection direction of the observation electron beam K emitted from the e-cross bee 3 changes, and the resolution on the two-dimensional detector 7 is increased. make worse.

Next, the effect of the astigmatism on the resolution will be described with reference to FIG. FIG. 7 is a schematic diagram in which the electric field direction and the magnetic field direction of the imaging system are superimposed and shown in the same plane. The optical path KE in the electric field direction is indicated by a broken line, and the optical path KB in the magnetic field direction is indicated by a solid line. As shown in the figure, an astigmatic difference is generated in the observation electron beam K that exits the e-cross bee 3.
This is because the refracting power of the e-cross bee 3 is different between the electric field direction and the magnetic field direction. That is, it has a convex power in the electric field direction, but has no power in the magnetic field direction.

In FIG. 1, a two-dimensional detector 7 is arranged so as to be focused in the direction of the magnetic field. At this time, while the resolution in the magnetic field direction is good, the resolution in the electric field direction is not good. Assuming that the two-dimensional detector 7 is
Even if they are arranged so as to match the average image planes in the magnetic field direction and the electric field direction, the resolution is only average in both directions, and the problem is not fundamentally solved. Furthermore,
For the same reason as the generation of the astigmatic difference, a magnification difference also occurs, and the accuracy of observation on the two-dimensional detector 7 is further deteriorated. Accordingly, it is an object of the present invention to provide a high-resolution electron beam optical system which reduces chromatic aberration, astigmatism, and magnification difference caused by e-cross b, and is less affected by electric noise.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. That is, the present invention is intended to provide an irradiation source (10) The irradiation electron beam (S) emitted from the optical path switching means (3) is made incident on the light path switching means (3) via the irradiation optical system (11), and the irradiation electron beam (S) passing through the light path switching means (3) is passed through the objective optical system ( 1) is incident on the sample surface (W), and the observation electron beam (K) emitted from the sample surface (W) is incident on the optical path switching means (3) via the objective optical system (1). The observation electron beam (K) is guided by the light path switching means (3) in a direction different from the direction reaching the irradiation source (10), and the observation electron beam (K) after passing through the light path switching means (3). An electron beam optical system which enters the electron beam detecting means (7) via the imaging optical system (4a-4b) The objective optical system (1) is configured to form an intermediate image of the sample surface (W) by the observation electron beam (K) inside the optical path switching means (3). System. At this time, the imaging optical system (4a
4b) preferably includes an aperture stop (2).
If the effective area of the electron beam detecting means (7) is rectangular, the longitudinal direction of the effective area and the electric field direction of the optical path switching means (3) are arranged to be substantially the same direction (X). Is preferred.

Further, according to the present invention, an irradiation electron beam (S) emitted from an irradiation source (10) is incident on an optical path switching means (3) via an irradiation optical system (11), and the optical path switching means (3) is provided. The passing irradiation electron beam (S) is made incident on the sample surface (W) via the objective optical system (1), and the observation electron beam (K) emitted from the sample surface (W) is emitted from the objective optical system (1). ) Through the optical path switching means (3), and guides the observation electron beam (K) in a direction different from the direction reaching the irradiation source (10) by the optical path switching means (3). ), The observation electron beam (K) after passing through the imaging optical system (4a-4b) is incident on the electron beam detection means (7). The effective area is rectangular, and the longitudinal direction of the effective area is substantially the same as the electric field direction of the optical path switching means (3). An electron beam optical system, characterized in that arranged so that the (X).

In the electron beam optical system having the above-described structure, an intermediate image of the sample surface is formed inside the E.C.B by the cathode lens, and the intermediate image is re-imaged on the detector by the imaging lens. For this reason, chromatic aberration, astigmatism, and magnification difference, which occur in e-cross-b, can be reduced. 4 and 5, the operation of the present invention will be described in detail. FIG. 4 is a schematic diagram showing the image forming system of the electron beam optical system from the direction of the electric field. The principle of reducing chromatic aberration will be described with reference to FIG. After passing through the cathode lens 1, the observation electron beam K emitted from the sample surface W
An intermediate image is formed near the center.

The observation electron beam emitted from the e-cross bee 3 receives a refracting power corresponding to the electron energy. That is, for example, the observation electron beam K1 of 4001 eV advances in the direction shown by the solid line in the figure, and the electron beam K2 for observation of 4002 eV advances in the direction shown by the broken line in the figure. Thereafter, each of the observation electron beams K1, K2
Are re-imaged on the two-dimensional detector 7 by the imaging lens 4. At this time, as shown in FIG.
No image bleeding occurs on the dimension detector 7. This is because the effective surface in which the angle changes, that is, the intermediate image forming surface inside the E-cross bee 3 and the two-dimensional detector 7 are in an image forming relationship.

Next, the principle of reducing the astigmatic difference will be described with reference to FIG. In FIG. 5, it is assumed that there is an object height in the direction of the electric field (X direction) in the imaging system, and a ray diverging from the object point in the direction of the electric field and a ray diverging in the direction of the magnetic field are overlapped on the same plane. Shown in In FIG. 5, the optical path KE in the electric field direction is indicated by a broken line, and the optical path KB in the magnetic field direction is indicated by a solid line. Here, the reason why the optical path KB in the magnetic field direction is displayed on the axis is that the object height in the electric field direction is assumed.

As shown in FIG.
There is no refraction of the light beam in the direction of the magnetic field, but refraction of the light beam occurs because the refracting power acts in the direction of the electric field. However, on the two-dimensional detector 7, there is no light beam position difference in each direction. This is because, similarly to the chromatic aberration described above, the effective surface in which an angle difference occurs, that is, the intermediate image forming surface inside the E-cross bee 3 and the two-dimensional detector 7 are in an image forming relationship. For the same reason, the magnification difference is also reduced.

In FIGS. 4 and 5, for the sake of simplicity, the thickness of the cathode lens 1, the e-cross bee 3, and the imaging lens 4 is neglected, but they are actually thin lenses. It is a thick element. For example, FIG. 8 is a diagram in which the e-cross bee 3 is cut along the XZ plane. At both ends of the e-cross bee 3, shielding plates 18 for an electric field or an electromagnetic field of zero potential are arranged. This shielding plate 18
Is a zero potential, so a part of the imaging system barrel may be used as it is. What is the thickness of ecross bee 3?
Usually, it indicates the width d in FIG. The inside of the ecross bee 3 indicates this range.
The imaging lens 4 has a similar thickness. The actual optimization of such a thick element is performed by numerical simulation such as the finite element method.

As described above, the intermediate image is formed inside the E-cross bee by the cathode lens, and the intermediate image is re-imaged on the two-dimensional detector by the imaging lens, so that the E.B.
It is possible to reduce chromatic aberration, astigmatism, and magnification difference that occur in cross-bees. Also, the change in the refractive power due to the electrical noise of the power supply of e-cross bee results in the change in the angle of the entire light beam contributing to the image formation. , The change in the position of the light beam can be reduced.

[0019]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a first example of the electron beam optical system according to the present invention.
An example will be described. The irradiation electron beam S emitted from the electron gun 10 passes through the irradiation lens 11 and is
It is incident on bee 3. The irradiation electron beam S incident on the e-cross bee 3 is deflected by the e-cross bee 3 so that the principal ray substantially coincides with the optical axis of the imaging system. The electron beam S for irradiation after passing through the e-cross bee 3 passes through the aperture stop 2 and the cathode lens 1 in this order, and approximately illuminates the sample surface W with Koehler illumination. Here, the aperture stop 2 is disposed at the rear focal position of the cathode lens 1 as viewed from the imaging system.

When the sample surface W is irradiated with the irradiation electron beam S, a low energy secondary electron beam, that is, an observation electron beam K is generated from the sample surface W. The observation electron beam K passes through the cathode lens 1 and the aperture stop 2, and
The light is incident on ecross bee 3. At this time, the observation electron beam K is caused by the cathode lens 1 to have the first intermediate image near the power center of the e-cross bee 3, that is, near the center of the e-cross bee 3 in the optical axis direction (Z direction). To form The adjustment of the position of the first intermediate image plane in the optical axis direction can be performed by adjusting the position of the sample plane W in the optical axis direction or the voltage application condition between the sample plane W and the cathode lens 1.
Observation electron beam K that has passed through ecross bee 3
Passes through the front group 4a of the imaging lens, the field stop 5, and the rear group 4b of the imaging lens in this order, and enters the two-dimensional detector 7. Based on the information of the observation electron beam K incident on the two-dimensional detector 7, observation and inspection of the sample surface W are performed.

The structure of the two-dimensional detector 7 is such that a CC is provided after the micro channel plate (MCP).
One in which a D sensor is arranged is conceivable. That is, in this configuration, the observation electron beam K is converted into an optical signal by the micro channel plate, and the optical signal is converted into an electric signal by the CCD sensor. here,
Micro channel plate and CCD sensor
The optical fiber bundle and the optical lens are arranged so as to have an optical mapping relationship. With this arrangement, the two-dimensional electronic signal generated from the sample surface W is
Finally, the image is mapped on a CCD sensor and converted into an electronic signal.

Here, the first intermediate image formed inside the e-cross bee 3 is converted into a second intermediate image by the front lens group 4a. The field stop 5 is arranged at the position of the second intermediate image. Further, the second intermediate image is enlarged by the imaging lens rear group 4b and formed on the two-dimensional detector 7. As described above, in the first embodiment, the sample surface W, the central portion of the e-cross-B 3 that forms the first intermediate image, and the two-dimensional detector 7 have a conjugate relationship. Therefore, it is possible to reduce chromatic aberration, astigmatism, and magnification difference, which occur in the e-cross bee 3, and also to significantly reduce resolution degradation due to electric noise.

In the first embodiment, a two-stage imaging lens of an imaging lens front group 4a and an imaging lens rear group 4b is used in the imaging system, and the voltages applied to these lenses are adjusted. By doing so, the magnification on the two-dimensional detector 7 can be adjusted. On the other hand, when an even larger magnification is required, two or more stages of imaging lenses can be arranged behind the second intermediate image position instead of the imaging lens rear group 4b. . In the first embodiment, it is assumed that the occurrence of aberrations such as chromatic aberration and curvature of field in the cathode lens 1 can be neglected. However, if these aberrations cannot be neglected, numerical simulation such as the finite element method is used. It is necessary to find the position of the intermediate image plane inside the e-cross bee 3 where the influence of aberration is minimized. Furthermore,
By optimizing the voltage applied to the cathode lens 1, the front group 4a of the imaging lens, and the rear group 4b of the imaging lens, a better resolution can be obtained in the two-dimensional detector 7.

Next, FIG. 2 shows a second embodiment of the electron beam optical system according to the present invention. The second embodiment differs from the first embodiment only in the installation position of the aperture stop 2. That is, in the first embodiment, the aperture stop 2 is disposed at the rear focal position of the cathode lens 1 viewed from the image forming system. In the second embodiment, the aperture stop 2 is located behind the imaging lens rear group 4b. It is located at the side focal position. Thus, in the second embodiment, the chromatic dispersion of the observation electron beam K can be blocked by the aperture stop 2. The chromatic aberration generated in the cathode lens 1 can be reduced by such a so-called color filter effect of the aperture stop 2.

The second embodiment is particularly effective when the chromatic aberration generated in the cathode lens 1 increases, for example, when the acceleration electric field formed between the sample surface W and the cathode lens 1 cannot be increased. In the second embodiment, the aperture stop 2 is arranged at the rear focal position of the imaging lens rear group 4b. However, even if the aperture stop 2 is arranged at the rear focal position of the imaging lens front group 4a, Similar effects can be obtained.

Next, FIG. 3 shows a third embodiment of the electron beam optical system according to the present invention. In the third embodiment, a TDI sensor (time delay and integration sensor) is particularly used as a CCD sensor constituting the two-dimensional detector 7. Here, the TDI sensor is an element in which a plurality of pixels 7a are arranged in a rectangle as shown in FIG. Here, as described in the first embodiment, the TDI sensor and the micro channel
Since the plate and the plate have a mapping relationship, the rectangular area in FIG. 7B is considered to be an effective area of the two-dimensional detector 7 after all.

When the TDI sensor is used, it has a function of integrating the image intensity in synchronization with scanning the image in the short direction, so that the TDI sensor is used when the irradiation amount of the electron beam is relatively small. However, there is a feature that the observation electron beam can be reliably detected. FIG.
The configuration of ecross bee 3 of the third embodiment is shown. The e-cross bee 3 mainly consists of a yoke 15, a coil 16
a, 16b, electrodes 17a, 17b and the like. Then, at the center of the e-cross bee 3, an electric field is generated in the X direction and a magnetic field is generated in the Y direction.

In the third embodiment, as shown in FIGS. 3A and 3B, the observation electron beam
In consideration of the difference between the electric field direction and the power of the magnetic field received in the above, the electric field direction of the e-cross bee 3 and the longitudinal direction of the effective area of the two-dimensional detector 7 are in the same direction (X direction). Are located. The effect of this arrangement will be described with reference to FIG. FIG. 9 is a broken line for the principal ray of the optical path KE of the electron emitted from the maximum object height in the electric field direction of the e-cross bee 3, and the principal ray of the optical path KB for the electron emitted from the maximum object height in the magnetic field direction. Is indicated by a solid line. That is,
FIG. 9 is a schematic diagram in which the XZ plane and the YZ plane are displayed in an overlapping manner.

In the third embodiment, the longitudinal direction and the lateral direction of the effective area of the two-dimensional detector 7 are aligned in the direction of the electric field and the direction of the magnetic field, respectively, and the effective area and the sample surface W are mapped. Because of the relationship, the object height has a different value in each direction as shown in FIG. The third embodiment is also similar to the first embodiment in that the first intermediate image formed by the cathode lens 1 is formed in the vicinity of the center of the e-cross bee 3. Now, from FIG. 9, the optical path KE in the direction of the electric field where the object height is large
It can be seen that is refracted toward the center of the imaging lens 4 when passing through the e-cross bee 3. Since the aberration generated in the imaging lens 4 decreases as the height of the light beam passing through the lens decreases, this arrangement has an effect of relatively reducing off-axis aberration in the electric field direction.
Further, regarding the optical path KB in the magnetic field direction, e-cross
The above effect cannot be obtained because it is not refracted by bee 3,
In the first place, since the alignment is performed in the direction in which the object height is low, the aberration is essentially hardly generated. Therefore, by adopting the arrangement of the third embodiment, the e-cross
It is possible to optimize the aberration generated in the element after the bee 3.

As described above, in the third embodiment, it is possible to provide an electron beam optical system with low aberration and less damage to the sample. Although the configuration of the electron beam optical system of the mapping electron microscope has been described in the above embodiments, the present invention can be applied to the electron beam optical system of a scanning electron microscope. At that time, out-of-focus occurring on the detector can be effectively removed without installing a stig meter.

[0031]

As described above, according to the present invention, it is possible to provide an electron beam optical system having a high resolution which is reduced in chromatic aberration, astigmatism, and magnification difference caused by e-cross bee and is less affected by electric noise. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram showing an electron beam optical system according to a first embodiment of the present invention.

FIG. 2 is a view showing an electron beam optical system according to a second embodiment of the present invention.

FIGS. 3A and 3B are diagrams showing (A) an e-cross bee and (B) a two-dimensional detector in an electron beam optical system according to a third embodiment of the present invention.

FIG. 4 is a diagram illustrating the principle of reducing chromatic aberration.

FIG. 5 is a diagram showing a principle of reducing astigmatic difference.

FIG. 6 is a diagram showing the influence of chromatic aberration on resolution in a conventional electron beam optical system.

FIG. 7 is a diagram showing the effect of astigmatism on resolution in a conventional electron beam optical system.

FIG. 8 is a cross-sectional view showing an e-cross bee taken along the XZ plane.

FIG. 9 is a diagram showing an optical path of electrons emitted from a maximum object height in the electron beam optical system.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Cathode lens 2 ... Aperture stop 3 ... E-cross b 4 ... Imaging lens 4a ... Front group of imaging lens 4b ... Rear group of imaging lens 5 ... Field stop 7 ... 2D detector 10 ... Electron gun 11 ... Irradiation lens 15 ... Yoke 16a, 16b ... Coil 17a, 17b ... Electrode 18 ... Shielding plate W ... Sample surface S ... Electron beam for irradiation K ... Electron beam for observation

Claims (4)

[Claims] 1. 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 observation electron beam emitted from the sample surface is incident on the optical path switching means via the objective optical system,
The observation electron beam is guided by the light path switching means in a direction different from the direction reaching the irradiation beam source, and the observation electron beam after passing through the light path switching means is detected by an electron beam through an imaging optical system. In the electron beam optical system, the objective optical system is configured to form an intermediate image of the sample surface by the observation electron beam inside the optical path switching unit. system. 2. An electron beam optical system according to claim 1, wherein said imaging optical system includes an aperture stop. 3. An effective area of said electron beam detecting means is rectangular, and a longitudinal direction of said effective area and an electric field direction of said optical path switching means are arranged substantially in the same direction. Item 3. The electron beam optical system according to item 1 or 2. 4. An irradiation electron beam emitted from an irradiation beam source is 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 observation electron beam emitted from the sample surface is incident on the optical path switching means via the objective optical system,
The observation electron beam is guided by the light path switching means in a direction different from the direction reaching the irradiation beam source, and the observation electron beam after passing through the light path switching means is detected by an electron beam through an imaging optical system. In the electron beam optical system to be incident on the means, the effective area of the electron beam detecting means is rectangular, and the longitudinal direction of the effective area and the electric field direction of the optical path switching means are arranged to be substantially the same direction. An electron beam optical system characterized by the above-mentioned.