JP2003257355A - Scanning electron microscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning electron microscope with high speed and resolution by upgrading a multi-beam type scanning electron microscope and improving its image forming performance. <P>SOLUTION: Electrons released from FEA1 are accelerated, and then pass through a zoom lens 2 consisting of a plurality of electrodes, and form an image in a beam separator 3 consisting of an electromagnetic field superimposed multipole of a magnetic bipole and an electrostatic hexapole. Electron beams passing through the beam separator 3 pass through a first objective lens 4, and electron beams from each electron source are to be converged on a crossover 5 by the operation of the first objective lens 4. A scan electrostatic deflector 6 is provided at the position of the crossover 5. Electron beams deflected by the scan electrostatic deflector 6 are irradiated to the surface of a sample 8 through a second objective lens 7. Since the crossover 5 is to be positioned at the rear focal point of the second objective lens 7, each electron beam is vertically entered to the surface of the sample 8. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to irradiation electrons emitted from a plurality of electron sources in the same lens barrel at the same time to irradiate different points on a sample and emit the electrons from each irradiation point. The present invention relates to a multi-beam scanning electron microscope that separately detects electrons and acquires a scan image.

[0002]

2. Description of the Related Art Conventionally, there is a contact hole conduction failure,
As a method of inspecting a defect of a semiconductor wafer that cannot be optically inspected, a method of observing a potential contrast image using an SEM and determining the presence or absence of the defect by the observation is the mainstream. However, since the SEM is point scanning, it takes time to obtain a potential contrast image, and there is a problem that inspection throughput is poor. Therefore, in recent years, various methods have been sought for high-speed inspection.

Increasing the speed of inspection by electron beam observation means increasing the amount of current flowing into the detector. The following are typical attempts.

One is simply high-speed observation by increasing the SEM current. It uses the traditional method as is,
The aim is to increase the speed by increasing the current generated by the electron source, but this is a technology that is on an extension line that has been repeatedly improved, and no significant improvement can be expected. A feature of this device is that the device performance largely depends on the resolution of the irradiation optical system.

The other is to irradiate the surface of the sample with electrons,
This is a mapping projection type electron microscope in which secondary electrons generated are imaged on a two-dimensional electron detector by an imaging optical system and a potential contrast image is collectively obtained. Because this is surface irradiation, SE
It is considered that a larger current irradiation than M is possible. However, compared with the SEM, the performance of the image is greatly deteriorated due to the effect of charge-up on the sample surface that occurs during irradiation, and therefore irradiation with a large current is not possible. Further, since it is an imaging type, the resolving power of the mapping projection system itself is much worse than that of the SEM, and there is a problem in that the detection rate of secondary electrons must be greatly reduced in order to achieve high resolution. A feature of this device is that the device performance greatly depends on the resolution of the image projection type detection optical system.

As an attempt to obtain the good aspects of each of the above two types of devices, in an improved version of SEM, a single lens barrel simultaneously scans a plurality of electron beams to generate a current. A profitable, multi-beam scanning electron microscope has also been devised. In this method, the irradiation current amount can be increased by the number of beams as compared with the conventional SEM. Further, since the SEM method is used, the influence of charge-up on the sample surface is at the same level as the SEM. However, since it is a multi-beam type, there is a restriction on the dynamic correction during scanning, and the resolving power is said to be slightly worse than that of a single-beam SEM.

On the other hand, the detection system needs to guide the secondary electrons generated from a plurality of positions on the sample surface to individual detectors, and it is necessary to have one having a mapping projection type characteristic. for that reason,
Although the detection rate of secondary electrons is worse than that of a single-beam SEM, it is not required to be as high as the resolving power of a projection electron microscope. Therefore, the detection rate of secondary electrons is better than that of a projection electron microscope. However, although it is difficult to make a comprehensive evaluation, it is difficult to say that it is currently particularly superior to the above two types of methods.

In any case, high-speed inspection by irradiation with a large current causes a large charge-up on the sample surface. The influence not only deteriorates the resolving power at the time of observation but also causes a discharge or the like, which may damage the sample itself. Nondestructive testing is the most disturbing issue.

As an observing means for avoiding the charge-up itself, there is a mirror type electron microscope for observing a sample surface with irradiation electrons reflected by a mirror. Since the irradiated electrons are reflected by the mirror, the sample surface hardly charges up, but since it is a projection type, it is inferior to the SEM type in terms of resolution.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an improvement of a multi-beam type scanning electron microscope and an improvement in its image forming performance enable high-speed resolution of a resolution. An object is to provide a good scanning electron microscope.

[0011]

A first means for solving the above problems is to irradiate different points on a sample by scanning irradiation electrons emitted from a plurality of electron sources in the same lens barrel. A scanning electron microscope of a multi-beam type that separately detects the electrons emitted from each irradiation point to obtain a scanning image, and in the beam separator that separates the irradiation optical system and the detection optical system, The scanning electron microscope (claim 1) is characterized in that the beam separator itself has a lens action at the same time that the image of the electron source is formed.

Generally, in a multi-beam type scanning electron microscope, irradiation electrons (electron beams) from a plurality of electron sources are used.
To the beam separator through the irradiation optical system, the irradiated electrons passing through the beam separator are irradiated to the sample surface through the objective lens system, and the electrons generated from the points irradiated by each electron beam are again objective lens system. Through the beam separator, the electron beam is guided by the beam separator in a direction different from the irradiation optical system, and an image is formed on the imaging surface via the projection optical system (detection optical system) arranged in that direction. There is.

A Wien filter or the like is used as this beam separator, but the optical characteristics (aberration, distortion, etc.) of these beam separators are not as good as those of general lens systems.
Therefore, in this means, the image of each electron source is formed in the beam separator to prevent the deterioration of the optical characteristics due to the beam separator. At the same time, by giving the beam separator a lens effect,
The resolution characteristics are improved by improving the aberration and distortion and adjusting the optical path of the electron beam.

A second means for solving the above problems is
In the first means, the lens action adjusts the orbit of the irradiation electrons so as to change the magnification of the electron microscope so that the irradiation electrons are substantially perpendicular to the sample surface when the magnification is changed. It is characterized in that it has a lens function of adjusting (claim 2).

Generally, in a multi-beam type scanning electron microscope, irradiation electrons (electron beams) from a plurality of electron sources are used.
Are incident on the sample surface almost vertically. However, when the magnification of the electron microscope changes, the incident angle of the electron beam that enters the beam separator from the irradiation optical system changes, and as a result, the electrons that enter the sample surface do not vertically enter. Therefore, it is necessary to correct the angle of the electron beam with some lens. In this means, since the beam separator itself has such a correcting lens action,
Even if the magnification of the electron microscope is changed, it is possible to always make the irradiated electrons substantially perpendicular to the sample surface without providing a special lens. It should be noted that "substantially vertical" means that the verticality may be such that it does not affect the resolution required for the scanning electron microscope.

A third means for solving the above-mentioned problems is as follows.
In the first means or the second means, the beam separator is composed of a non-magnetic electrostatic multipole having six or more poles arranged in the same cylindrical surface and a dipole having two or more poles arranged outside thereof. An electromagnetic field superposition type multipole having a magnetic multipole is provided (Claim 3).

The beam separator in the present means is a Wien filter composed of an electrostatic multipole and a magnetic multipole, and the electrostatic multipole is non-magnetic and has a hexapole or more arranged in the same cylindrical surface. Thus, it is possible to simultaneously have the action of the Wien filter and the action of the lens with a simple configuration. Considering the correction of the 3θ field of the Wien filter and the adjustment of the astigmatic field unique to the Wien filter, the electrostatic multipole needs to have a hexapole or more. Since the magnetic multipole element only needs to generate a magnetic field, it is sufficient if it has two or more dipoles.

A fourth means for solving the above-mentioned problems is as follows.
In any one of the first to third means, the scanning deflector is of an electrostatic type, is installed at a focal position behind the objective lens, and crosses over irradiation electrons from each electron source. Is formed at the position of the scanning deflector (claim 4).

Generally, a multi-beam type scanning electron microscope has a scanning deflector for scanning irradiation electrons (electron beams) from a plurality of electron sources on a sample surface.
In this means, since the scanning deflector is installed at the focal position behind the objective lens, even if the electron beam is deflected, the electron beam always enters the sample surface almost perpendicularly after passing through the objective lens. Become.

A crossover of irradiation electrons from each electron source is formed at the position of the scanning deflector. Here, the crossover of irradiation electrons from each electron source does not refer to the crossover of each electron beam, but refers to the portion where the electron beams from all electron sources converge, and in the present specification, The word crossover is used for this meaning. The crossover of irradiation electrons from each electron source is formed at the position of the scanning deflector, so that the electron beam from any electron source is equally deflected, and as described above, the deflection is performed. Even if it is received, it is incident on the sample surface almost vertically.

In the present means, the reason why the scanning deflector is limited to the electrostatic type is that the deflection direction of irradiation electrons and the deflection direction of detected electrons are the same. In the magnetic scanning deflector, the deflection direction of the irradiated electrons is opposite to the deflection direction of the detected electrons, and the detected electrons cannot be returned to the beam separator well.

The fifth means for solving the above-mentioned problems is as follows:
The fourth means is characterized in that the aperture stop installed in the detection optical system is installed at a position after the detection optical system is separated from the irradiation optical system by the beam separator. Item 5).

In the conventional multi-beam type scanning electron microscope, an aperture stop for the detection optical system is provided at the focal position behind the objective lens, and a crossover of electron beams from each electron source is formed at this position. I often did. But,
In the fourth means, since the scanning deflector is installed at this position, it is difficult to install the aperture stop. Therefore, it is preferable to provide the aperture stop at a position after the detection optical system is separated from the irradiation optical system by the beam separator. In particular, it is preferable to limit the aperture angle by forming a crossover where electrons emitted from the sample surface converge and providing an aperture stop at that position.

A sixth means for solving the above-mentioned problems is as follows.
In any one of the first to fifth means, each electron source is arranged so that an image formed in a beam separator is arranged in parallel with a deflection component of a magnetic field excited by the beam separator. It is characterized by being arranged in (6).

Generally, the beam separator fulfills its function by the interaction between the magnetic field and the electric field, but the electron beam passing therethrough is not bent in the direction parallel to the deflection component of the magnetic field,
It is bent in a direction perpendicular to it (that is, a direction parallel to the deflection component of the electric field). Therefore, by arranging each electron source so that their images formed in the beam separator are arranged in parallel with the deflection component of the magnetic field excited by the beam separator, The electron beam is subjected to the same bending force by the beam separator, and the change in the optical path length and the bending angle are the same. Therefore, the difference between the imaging point and the aberration or distortion between the electron beam emitted from each electron source and the electron beam generated thereby does not become so large.

The seventh means for solving the above-mentioned problems is as follows.
In any one of the first to sixth means, the electron used for observation is a mirror electron.

Mirror electrons are electrons that, when irradiated to the sample surface, change their direction by the electric field near the sample surface and are specularly reflected without losing kinetic energy. Since the mirror electrons are reflected without reaching the sample surface, the irradiation current amount can be increased without almost charging the sample surface, which is particularly suitable for use in a multi-beam scanning electron microscope. ing.

Further, the secondary electron emission angle is widely distributed up to ± 90 °, whereas the mirror electron reflection angle distribution is
It is almost equal to the distribution of incident angles and narrow. This increases the detection rate of observation electrons even if the electron optical performance of the detection system is the same.

In a multi-beam scanning electron microscope using mirror electrons, when the irradiated electrons are vertically incident on the sample surface, the mirror electrons pass through the incident path in the opposite direction without losing kinetic energy. Reach the beam separator. Therefore, in the beam separator, since it passes through the beam separator in reverse at the same speed as the irradiation electrons, the beam separator can be operated under ideal conditions,
Irradiated electrons and observed electrons can be efficiently separated.

Further, since the electron source and the sample surface are usually conjugated, the observed electrons form an image of the sample surface in the beam separator. Therefore, it is possible to reduce the deterioration of the characteristics of the image on the sample surface due to the optical characteristics of the beam separator.

Further, in the case of the fourth means, since the observation electrons also form a crossover at the focal position behind the objective lens, the same path as the irradiation electrons is caused by the action of the scanning deflector installed at this position. It is possible to go in the opposite direction.

Further, the energy dispersion of the mirror electrons is
Since the energy dispersion of the electron source of the irradiation system is almost determined, if a cold cathode or the like is used as the electron source, observation with much higher monochromaticity than secondary electrons becomes possible and the resolution becomes high.

[0033]

1 is a schematic diagram of an optical system of a scanning electron microscope which is an embodiment of the present invention. (A) is a front view (view seen from a direction perpendicular to the beam scanning direction),
(B) is a side view (view seen from the beam scanning direction). In this embodiment, an electron beam is scanned in a predetermined direction, a stage on which a sample is mounted is scanned in a direction perpendicular thereto, and outputs obtained by these scans are combined to obtain a two-dimensional image. There is.

A field emission array (FEA) 1 is used as a multi-beam electron source, and the respective electron sources are arranged in parallel with the beam scanning direction. After being accelerated, the electrons emitted from the FEA 1 pass through a zoom lens 2 composed of a plurality of electrodes and enter a beam separator 3 composed of a magnetic dipole and an electrostatic hexapole electromagnetic field superposition type multipole. Form an image.

The beam separator 3 constitutes a Wien filter, and the multipole element is installed so that the direction of the magnetic field coincides with the arrangement direction of each electron source in the FEA 1. In the case of FIG. 1, the direction of the magnetic field is set to the left-right direction of the paper surface of (a), and the direction of the electric field is set to a direction perpendicular to the paper surface of (a). Further, the multipole element is set so that the Wien condition is satisfied with respect to the irradiation system, and therefore the irradiation electrons pass through the irradiation system without being deflected.

The electron beam passing through the beam separator 3 is
After passing through the first objective lens 4, the action of the first objective lens 4 causes the electron beam from each electron source to converge at one location (crossover) 5. An electrostatic deflector 6 for scanning is provided at the position of this crossover 5. The electron beam deflected by the scanning electrostatic deflector 6 is applied to the surface of the sample 8 through the second objective lens 7. At this time, since the position of the crossover 5 is set to the rear focus position of the second objective lens 7, each electron beam is vertically incident on the surface of the sample 8. Moreover, since the surface of the sample 8 is placed at a position conjugate with each electron source, an image of the electron source is formed on the surface of the sample 8.

These electron beams are decelerated by the deceleration electric field formed between the second objective lens 7 and the surface of the sample 8,
Immediately before reaching the surface of the sample 8, the direction is changed, and while being accelerated by the deceleration electric field, the incident path is traced in the opposite direction and the light is incident on the second objective lens 7. That is, the irradiated electrons that have entered become mirror electrons and enter the second objective lens 7.

Although the mirror electron passes through the second objective lens 7, since the kinetic energy is the same as that of the irradiation electron, it passes through the same path as the irradiation electron in the opposite direction up to the beam separator 3, and the surface of the sample 8 is reached. Is formed in the beam separator 3. Strictly speaking, however, the voltage applied to the scanning electrostatic deflector 6 when the irradiated electrons pass through the scanning electrostatic deflector 6 and when the mirror electrons pass through the scanning electrostatic deflector 6. However, the difference in deflection is different, but the time difference between them is several nanoseconds, so in practice,
It can be considered that there is no difference in the amount of deflection. Since the Wien condition does not hold for this mirror electron (observation electron), a force is applied in the direction of the electric field of the beam separator 3,
As shown in FIG. 1B, the light is deflected in a direction different from that of the irradiation system. At this time, an image is formed in the beam separator 3 as at the time of incidence.

The mirror electrons that have passed through the beam separator 3 pass through the imaging lens 9 and the contrast aperture 10, and then are passed through the zoom lens 11 to the sample 8 on the image pickup device 12.
Form an image of the surface. In this embodiment, a combination of multi-anode and MCP is used as the imaging device 12. At this time, the optical system is set so that each beam forms an image on each anode. Therefore, the mirror electrons from the points irradiated by each electron beam can be detected separately. On the contrary, if the optical system and the imaging device 12 have such a resolution, it is possible to image the surface of the sample 8. Depending on the shape of the multi-anode, the zoom lens 11 may form an image only in the direction in which the anodes are lined up.

As can be seen from FIG. 1, since each electron source is arranged in parallel with the direction of the magnetic field of the beam separator 3, the sample 8 is irradiated by the electron beam emitted from each electron source.
The mirror electron beam generated from the beam is changed by the beam separator 3 in a direction perpendicular to the arrangement direction of the electron sources. Therefore, the optical path length of each mirror electron beam to the image pickup device 12 is almost the same, and therefore, any mirror electron beam is imaged on the image pickup device 12 with high accuracy.

Further, in this embodiment, the zoom lens 11 is interlocked with the zoom lens 2 so that the magnification ratio is inversely proportional. The position where the contrast aperture 10 is installed is preferably a position where the electron beam of each mirror electron forms a crossover.

The stage on which the sample 8 is placed moves at a constant speed in a direction perpendicular to the scanning direction. The signal from the multi-anode is converted into a composite image of a plurality of scan images by the image processing system according to the scanning speed and the stage speed.

When the magnification of the zoom lens 2 is changed, the incident angle of the electron beam of the irradiating electrons incident on the beam separator 3 from the zoom lens 2 is changed, and the vertical incident condition of each electron beam on the sample 8 is broken as it is. Sometimes. In order to deal with this, a correction lens field is superposed on the beam separator 3 in accordance with the magnification of the zoom lens 2 and the irradiation electrons are adjusted so as to always enter the sample surface vertically.

If the zoom range is not required to be large, the zoom lenses 2 and 11 shown in FIG. 1 may be changed to have a fixed magnification, and the number of electrodes of the objective lens 4 may be increased to allow a slight zoom. Good.

FIG. 2 is a view showing the pole structure of the beam separator used in this embodiment. Two magnetic poles 21 and 22 having the same shape are provided so as to face each other, and coils 23 and 2 are provided.
It is excited by 4 to form a vertical magnetic field in the plane of the drawing. The electrode 25 is provided with six poles of the same shape, and by applying voltages V1 to V6 to each of them,
A horizontal electric field in the plane of the drawing is formed so that a Wien filter is formed, and a voltage forming a lens field is superimposed as necessary. It is also possible to correct the aberration and distortion of the Wien filter by adjusting the voltages V1 to V6.

[0046]

As described above, according to the present invention,
A high-speed and high-resolution multi-beam scanning electron microscope can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic diagram of an optical system of a scanning electron microscope which is an embodiment of the present invention.

FIG. 2 is a diagram showing a pole structure of a beam separator used in the present embodiment.

[Explanation of symbols]

1: Field Emission Array (FEA) 2: Zoom lens 3: Beam separator 4: First objective lens 5: Crossover 6: Electrostatic deflector for scanning 7: Second objective lens 8: Sample 9: Imaging lens 10: Contrast aperture 11: Zoom lens 12: Imaging device 21, 22: magnetic pole 23, 24: coil 25: Electrode

Claims (7)

[Claims] 1. An irradiation electron emitted from a plurality of electron sources is scanned within the same lens barrel to irradiate different points on a sample,
A multi-beam scanning electron microscope for separately detecting electrons emitted from each irradiation point to obtain a scanning image, wherein each electron is included in a beam separator separating an irradiation optical system and a detection optical system. A scanning electron microscope characterized in that the beam separator itself also has a lens effect while forming an image of the source. 2. The scanning electron microscope according to claim 1, wherein the lens action is such that when the magnification of the electron microscope is changed, the irradiated electrons are incident substantially perpendicularly to the sample surface. A scanning electron microscope having a lens function of adjusting an orbit of an irradiation electron according to zooming. 3. The scanning electron microscope according to claim 1, wherein the beam separator is a non-magnetic and multipole electrostatic pole having six or more poles arranged in the same cylindrical surface.
A scanning electron microscope comprising an electromagnetic field superposition type multipole element having a magnetic multipole element having two or more dipoles arranged outside thereof. 4. Any one of claims 1 to 3
The scanning electron microscope according to the item 1, wherein the scanning deflector is of an electrostatic type, is installed at a focal position behind the objective lens, and the crossover of irradiation electrons from each electron source causes the scanning deflection. A scanning electron microscope characterized in that it is formed at the position of the vessel. 5. The scanning according to claim 4, wherein the aperture stop installed in the detection optical system is installed at a position after the detection optical system is separated from the irradiation optical system by the beam separator. Electron microscope. 6. Any one of claims 1 to 5
The scanning electron microscope according to the item 1, wherein each electron source is arranged such that an image formed in the beam separator is arranged in parallel with a deflection component of a magnetic field excited by the beam separator. A scanning electron microscope characterized in that 7. Any one of claims 1 to 6
The scanning electron microscope according to the item 1, wherein the electrons used for observation are mirror electrons.