JPH11242941A - Scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate the detection of and the discrimination between secondary electrons and reflected electrons in a scanning electron microscope having a structure which generates a decelerating electric field for electron beams between an object lens and a sample. SOLUTION: A filter voltage 36 of which potential can arbitrarily be controlled is applied to a channel plate 25. For instance, if the filter voltage 36 is set to a negative voltage approximately 10 volts lower than a superposed voltage (voltage applied to the sample), a reverse electric field to a secondary signal, that is, a decelerating electric field to decelerate the secondary signal is formed between the channel plate 25 and a mesh 34. As a result, secondary electrons having low energy out of secondary electrons and reflected electrons released from the sample are driven back and only the reflected electrons having high energy pass through the decelerating electric field and selectively detected by the channel plate 25.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope which scans an electron beam spot on a sample surface to obtain a scanned image of the sample surface. The present invention relates to a scanning electron microscope that facilitates discrimination detection.

[0002]

2. Description of the Related Art Conventionally, it has been used for observing or measuring a contact hole or a line pattern of a submicron order (1 μm or less) in a semiconductor device sample.
A scanning electron microscope is used.

In a scanning electron microscope, an electron beam emitted from a heating type or field emission type electron source is scanned on a sample, and signals (secondary electrons and reflected electrons) obtained secondarily are detected. By using the secondary signal as the luminance modulation input of the cathode ray tube which is scanned in synchronization with the electron beam scanning, the scanned image (SEM
Image). In a general scanning electron microscope, electrons emitted from an electron source are accelerated between an electron source to which a negative potential is applied and an anode at a ground potential, and are irradiated on a test sample at a ground potential.

In recent years, scanning electron microscopes have been used in semiconductor manufacturing processes or inspection processes after completion (for example, inspection of electrical operation by electron beams). To be able to do so, a high resolution of 10 nm or less has been required at a low acceleration voltage of 1000 V or less.

That is, a sample for a semiconductor device is generally formed by laminating an electrical insulator such as SiO 2 or SiN on a conductor such as Al or Si. When such a semiconductor device sample is irradiated with an electron beam, the surface of the electric insulator becomes negatively charged (hereinafter, sometimes simply referred to as charge-up), and the trajectory of secondary electrons emitted changes, The orbit of the electron beam itself changes. As a result, abnormal contrast occurs in the SEM image or severe distortion occurs.

[0006] The image failure caused by such charge-up seriously hinders observation of contact holes and length measurement of lines and spaces, so that not only is it difficult to evaluate a semiconductor manufacturing process but also the quality of a semiconductor device itself. Is a major obstacle in securing Therefore, conventionally, the energy of the primary electron beam applied to the sample is 1K.
A so-called low acceleration SEM having an eV or less has been used.

However, the above-mentioned prior art has the following problems. That is, when the acceleration voltage is reduced, the resolution is significantly reduced due to the chromatic aberration caused by the energy variation of the electron beam, and observation at high magnification becomes difficult. Also,
When the electron current decreases, the ratio of the secondary signal to the noise (S /
N) is significantly reduced, the contrast as an SEM image is deteriorated, and observation with high magnification and high resolution becomes difficult. In particular, in a semiconductor device or the like manufactured by an ultra-fine processing technique, a signal generated from a concave portion such as a contact hole or a line pattern is weak, which is a major obstacle in performing fine observation and length measurement.

As a method for solving such a problem, for example, I Triple E, ninth Annual Symposium on Electron Proceedings of Ion and Laser Beam Technology, pp. 176 to 186 (IEEE 9th Annual Symposium on Electron, Ion)
and Laser Technology) set the acceleration voltage of the primary electron beam high and apply a negative voltage to the sample to form a deceleration electric field for the primary electron beam, thereby reducing chromatic aberration and preventing charge-up. A scanning electron microscope has been disclosed (hereinafter, referred to as a retarding technique).

[0009]

In a scanning electron microscope employing the above-described retarding technique, a deceleration electric field with respect to a primary electron beam acts as an accelerating voltage on secondary electrons emitted from the surface of a sample. The next electron will be strongly accelerated. Secondary electrons that are strongly accelerated by the deceleration electric field are incident on the channel plate detector similarly to reflected electrons that are originally strongly accelerated. Therefore, in the channel plate detector, reflected electrons and secondary electrons are similarly detected.

However, reflected electrons and secondary electrons have their own characteristics. For example, reflected electrons are more suitable for observation of the bottom of a contact hole or a concave portion of a line pattern in a semiconductor device or the like than secondary electrons, so that discrimination detection from secondary electrons is desired.

An object of the present invention is to provide a scanning electron microscope which employs a retarding technique which solves the technical problems of the prior art described above and which employs a retarding technique capable of reducing chromatic aberration and preventing charge-up. The purpose of this is to make possible a realistic detection.

[0012]

In order to achieve the above object, the present invention provides an electron source, electron optical means for irradiating a sample with a primary electron beam emitted from the electron source, and the primary electron beam. In order to detect a secondary signal generated from the sample by irradiating the sample, secondary signal detection means including a channel plate having a passage through which the primary electron beam passes, and the sample and the secondary signal Means for forming a deceleration electric field for decelerating the primary electron beam between the detection means, and an electrode for generating a deceleration electric field for decelerating the secondary signal between the channel plate and the sample. Features.

According to the above configuration, both the secondary electrons and the reflected electrons are decelerated by the decelerating electric field for decelerating the secondary signal. However, by appropriately adjusting the intensity of the decelerating electric field, the secondary electrons after deceleration are adjusted. The energy of electrons or reflected electrons can be controlled to a desired value. Therefore, the secondary signal to be detected is
If the intensity of the deceleration electric field is controlled so as to have the energy within the detection range of the secondary signal detection means, the secondary electrons and the reflected electrons can be discriminated and detected.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic configuration diagram of a scanning electron microscope system according to an embodiment of the present invention. The system includes a scanning electron microscope main body 100 and a sample exchange mechanism 200. FIG. 2 shows the scanning electron microscope main body 1 of FIG.
FIG. 2 is a diagram showing the configuration of the 00 in detail.

In FIG. 2, a cathode 1, an extraction electrode 2 and an anode 3 constitute a field emission type electron gun. An extraction voltage 4 is applied between the cathode 1 and the extraction electrode 2, and the cathode 1 is accelerated. Voltage 5 is applied. Electron beam 20 emitted from cathode 1
a is further accelerated by the voltage applied between the extraction electrode 2 and the anode 3 at the ground potential. Therefore, the energy (acceleration voltage) of the electron beam passing through the anode 3 matches the acceleration voltage 5. Further, a negative superimposed voltage 6 is applied to the sample 18 via the sample holder 21, and a deceleration electric field is formed between the objective lens 17 and the sample 18, so that the electron beam applied to the sample 18 is accelerated. The voltage is a voltage obtained by subtracting the superimposed voltage 6 from the acceleration voltage 5.

The electron beam 20b accelerated through the anode 3
Is converged on the sample 18 by the condenser lens 7 and the objective lens 17. The electron beam that has passed through the objective lens 17 is decelerated by the deceleration electric field formed between the objective lens 17 and the sample 18, and is substantially equal to the acceleration voltage 5 minus the superimposed voltage 6. To reach.

The divergence angle of the electron beam at the objective lens 17 is determined by the stop 8 arranged below the condenser lens 7. Centering of the aperture 8 is performed by operating the adjustment knob 10.

The accelerated electron beam 20b is applied to the upper scanning coil 1
The convergent electron beam 20c deflected by the first and lower scanning coils 12 and decelerated by the deceleration electric field on the sample 18 is raster-scanned. In this embodiment, the scanning coil has a two-stage configuration so that the scanned electron beam always passes through the center of the objective lens 17.

The sample 18 is fixed by a sample holder 21, and the sample holder 21 is placed via a stand 9 on a sample stage 19 that can be adjusted in position such as horizontal adjustment. A superimposed voltage 6 is applied to the sample holder 21.

The secondary electrons 24 generated from the sample 18 by being irradiated with the decelerated electron beam 20c are accelerated by the deceleration electric field created between the objective lens 17 and the sample 18, and are attracted into the objective lens 17, and Rises while undergoing a spiral motion under the influence of the magnetic field of the objective lens 17.

The secondary electrons 24 passing through the objective lens 17
Is sucked by a suction electrode 13 provided outside the electron beam path between the objective lens 17 and the lower scanning coil 12 to which a positive potential is applied, and is accelerated by a scintillator 14 to which 10 kV (positive potential) is applied. To flash the scintillator.

The emitted light is guided to a photomultiplier tube 16 by a light guide 15 and is converted into an electric signal. The output of the photomultiplier 16 is further amplified and becomes the luminance modulation input of the cathode ray tube, but is not shown here.

According to the scanning electron microscope having such a configuration, the energy of the electron beam (electron beam 20b) passing through the condenser lens 7, the aperture 8, and the objective lens 17 is changed to the last stage electron beam (electron beam 20c). , The chromatic aberration was improved, and high resolution was obtained.

In addition, since the primary electron beam applied to the sample is decelerated to have a low energy, the charge-up of the sample is eliminated.

Specifically, the beam diameter which was 15 nm when only the acceleration voltage (500 V) was applied was improved to 10 nm by adding the acceleration voltage (1000 V) and the superimposed voltage 6 (500 V).

In FIG. 1, a field emission cathode 1, an extraction electrode 2, an anode 3, a condenser lens 7, an objective lens 1
Components such as a sample 7, a sample 18, a sample holder 21, an insulating table 9, and a sample stage 19 are housed in a vacuum housing 61.
The illustration of the vacuum exhaust system is omitted.

Here, when a negative voltage is applied to the sample 18, it is necessary to avoid exchanging the sample by the sample exchange mechanism 77 and exposing the vacuum housing 61 to the atmosphere. In other words, the superimposed voltage 6 may be applied only when the electron beam is being scanned on the sample 18.

Therefore, in the present invention, the first condition that the switch S1 is closed and the accelerating voltage 5 is applied, which is a preparatory operation for mounting and replacing the sample, is provided between the cathode 1 and the sample 18. The second condition that both the valves G1 and G2 are open and the third condition that the valve G3 through which the sample exchange mechanism 77 passes to place the sample 18 on the sample stage 19 are closed are all satisfied. Switch S2 only when
Is closed, and the control for applying the superimposed voltage 6 to the sample 18 is performed.

The sample holder 21 and the sample stage 19
Are electrically connected to each other via a discharge resistor R. When the switch S2 is opened, the electric charge charged in the sample 18 has a constant time constant via the sample holder 21, the discharge resistor R, and the sample stage 19. Then, the discharge is quickly performed, and the potential of the sample 18 decreases.

The accelerating voltage 5 can be applied under the condition that the vacuum around the cathode 1 is equal to or higher than the set value, or the valves G1 and G can be applied only when the vacuum of the vacuum housing 61 is equal to or higher than the set value.
It goes without saying that a normal sequence in which 2 is released is set.

In this embodiment, the superimposition voltage 6 is applied when all of the above three conditions are satisfied. However, when one or two of these conditions are satisfied, The switch S2 may be closed first. With the above-described configuration, the short circuit at the time of sample exchange (the sample 1 when the sample exchange mechanism 77 contacts the sample 18).
8 to the sample exchange mechanism 77) can be prevented from being destroyed. In particular, a semiconductor device provided with a large number of circuit elements that operate with minute and minute currents may be destroyed by a sharp change in potential. The evil can be eliminated.

FIG. 3 is a block diagram of a main part of a scanning electron microscope according to a second embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts.

In the first embodiment described above, the secondary electrons 24 are taken out of the electron path by the suction electrode 13 and detected. However, in the present embodiment, the secondary electrons are detected using the channel plate detector 26. There is a characteristic in that it is done.

In the figure, a disc-shaped channel plate main body 25 having a center hole 33 is provided between the objective lens 17 and the lower scanning coil 12. The diameter of the central hole 33 is set so that the electron beam 20b deflected by the scanning coil 12 does not collide. A mesh electrode 34 is provided below the channel plate body 25.

In such a configuration, the accelerated electron beam 20b passes through the central hole 33 of the channel plate, is converged by the objective lens 17, and is irradiated onto the sample 18. The secondary electrons 24 generated in the sample 18 are
The light passes through the mesh electrode 34 placed on the entire surface while diverging, and enters the channel plate 25. Secondary electrons 2 incident on the channel plate 25
4 is accelerated and amplified by an amplification voltage 28 applied to both ends of the channel plate 25. The amplified electrons 27 are further accelerated by the anode voltage 29 and are captured by the anode 37.

After the captured secondary electrons are amplified by the amplifier 30, they are converted into light 32 by the light conversion circuit 31. Light 32
The reason for this is that the amplifier 30 is floating with the amplified voltage 28 of the channel plate 25 and the like.

The light 32 is converted again into an electric signal by the electric conversion circuit 35 of the ground potential, and is used as a luminance modulation signal of the scanned image. In this method, not only secondary electrons but also reflected electrons can be detected.

As is apparent, the same effects as described above can be achieved by this embodiment.

FIG. 4 is a block diagram of a main part of a scanning electron microscope according to a third embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts. In this embodiment,
It is characterized in that a desired secondary signal can be selectively detected.

In FIG. 3, a filter voltage 36 capable of arbitrarily controlling the potential is applied to the channel plate 25. For example, if the filter voltage 36 is set to a negative voltage of about 10 volts further than the superimposed voltage 6, a reverse electric field to the secondary signal between the channel plate 25 and the mesh electrode 34, that is, the secondary signal is decelerated. A deceleration electric field is formed. The energy of the secondary electrons and the energy of the reflected electrons are both reduced by this deceleration electric field, but the energy of the reflected electrons is larger than that of the secondary electrons. Therefore, if the filter voltage 36 is controlled so that only low energy secondary electrons are repelled by the deceleration electric field and only high energy reflected electrons can pass through the deceleration electric field, the detection of reflected electrons in the channel plate 25 can be discriminated. Will be possible.

By measuring the filter voltage 36 at the limit for repelling secondary electrons, it is possible to know the potential of the sample. By adding such a function, it is possible to perform a function test on a completed semiconductor device. Will be able to

FIG. 5 is a block diagram of a scanning electron microscope according to a fourth embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts. The present embodiment is characterized in that an energy filter (potential barrier) is provided above the objective lens 17. Further, the present embodiment shows an example in which the present invention is applied particularly to an in-lens system in which the sample 18 is disposed inside the objective lens 17.

The energy filter comprises a cylinder 46 for passing primary electrons, a set of shield grids 41, and an energy filter 42. As in the case of the third embodiment described with reference to FIG. 3, in this embodiment, the potential of the sample 18 can be measured by controlling the filter voltage 36 and appropriately adjusting the potential of the energy filter 42. Become.

Further, in the present embodiment, by appropriately selecting the voltage of the suction electrode 13, it becomes possible to selectively detect only secondary electrons without detecting reflected electrons.

In the in-lens system, the sample stage 1
9 is placed inside the objective lens 17, and the sample 18 is fixed via the insulating table 9 and the sample holder 21. The superimposed voltage 6 is applied to the sample holder 21, and a deceleration electric field is generated between the sample 18 and the objective lens 17. Objective lens 1
The excitation coil 45 of 7 is fixed on the upper part of the objective lens 17. The objective lens 17 is large enough to accommodate an 8-inch wafer.

FIG. 6 is a block diagram of a scanning electron microscope according to a fifth embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts. The present embodiment is characterized in that an inconvenient sample can be observed when a strong electric field is applied.

In a semiconductor integrated circuit, an element may be damaged by a strong electric field. In order to solve such problems,
In this embodiment, a control electrode 39 is provided between the objective lens 17 and the sample 18, and several tens of volts are applied to the control electrode 39 from a control voltage 40.

According to the present embodiment, the electric field generated between the objective lens 17 and the sample 18 is alleviated by the control electrode 39, so that damage to the element can be prevented.

FIG. 7 is a block diagram of a scanning electron microscope according to a sixth embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts.

In the above-described first and fourth embodiments, the secondary signal is deflected by using the electric field and detected by the detector. In this embodiment, the secondary signal is deflected by using the magnetic field and the electric field. The feature is that the next signal is deflected.

When the acceleration voltage increases and the deceleration ratio of the electron beam in the final stage due to the deceleration electric field decreases, the energy difference between the primary electron beam 20b irradiated on the sample and the secondary electrons 24 emitted from the sample decreases. So the secondary electrons 24
When a relatively large electric field E is generated by the suction electrode 13 in order to attract the primary electrons 20b by the electric field E,
Is also bent.

The present embodiment is made to solve such a problem, and focuses on the fact that the deflection direction of the electron beam due to the magnetic field differs depending on the traveling direction of the electron beam.
The magnetic field B is generated so as to cancel the deflection of the primary electron beam 20b and to supplement the deflection amount of the secondary electrons 24.

That is, in this embodiment, the primary electron beam 20b
However, the magnetic field B is generated such that the magnetic field B is deflected in the direction opposite to the direction of deflection by the electric field E generated by the suction electrode 13. Accordingly, by appropriately controlling the intensity of the magnetic field B, the deflection of the electron beam 20b due to the electric field E is canceled.

On the other hand, for the secondary electrons 24, the direction of deflection by the magnetic field B and the direction of deflection by the electric field E are in the same direction, so that the amount of deflection of the secondary electrons is increased and the detection of the secondary electrons is easy. become.

FIG. 8 is a block diagram of a scanning electron microscope according to a seventh embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts. The present embodiment is characterized in that a secondary signal is detected using a single crystal scintillator.

In the same figure, single crystal scintillator 55
For example, an opening 5 for obliquely cutting a cylindrical YAG single crystal and allowing the primary electrons 20b to pass through the cut surface.
7, the tip of which is coated with a conductive thin film 56 of metal, carbon, or the like, to which a ground potential is applied.

In this embodiment, the crossover 58 of the primary electron beam 20b formed by the condenser lens 12 is
7 so that the crossover 59 of the secondary electrons 24 by the objective lens 17 comes to a position away from the opening 57. By doing so, the primary electron beam 20
b is not blocked by the opening 57, and the secondary electrons 24 can be detected efficiently.

In the above-described eighth embodiment, both the light-emitting portion of the scintillator and the light guide are described as being composed of a YAG single crystal. However, only the light-emitting portion for detecting secondary electrons is formed of a YAG single crystal. The other parts may be made of a transparent member such as glass or resin.

FIG. 9 is a block diagram of a main part of a scanning electron microscope according to an eighth embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts.

In this embodiment, the anode constituting the electron gun is omitted, and the electron beam is accelerated by the extraction voltage 4 applied between the extraction electrode 2 and the cathode 1 which are grounded. The electrons accelerated by the extraction voltage 4 are converged by the objective lens 17 and travel toward the sample 18. The positive electrode side of the acceleration voltage 5 is connected to the sample 18. At this time, since the extraction voltage 4 is higher than the acceleration voltage 5 in the low acceleration voltage region, the electron beam is decelerated to the acceleration voltage 5 between the objective lens 17 and the sample 18 and reaches the sample.
In a typical example using a field emission cathode, the extraction voltage is 3 k
V, the acceleration voltage is 1 kV. According to such a configuration, since the anode and the superimposed voltage source are omitted, the configuration is simplified.

[0062]

As described in detail above, according to the present invention,
A second deceleration for decelerating a secondary signal such as a secondary electron or a reflected electron generated from a sample in a scanning electron microscope in which a deceleration electric field is generated for a primary electron beam to reduce chromatic aberration and prevent charge-up. Means for forming an electric field are further provided, and the energy of the secondary electrons and the reflected electrons can be arbitrarily controlled by changing the intensity of the second deceleration electric field. Therefore, if a detection means capable of selectively detecting only electrons having a predetermined energy is employed, discrimination detection of secondary electrons and reflected electrons becomes possible.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a scanning electron microscope according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a scanning electron microscope unit of FIG.

FIG. 3 is a configuration diagram of a scanning electron microscope unit according to a second embodiment of the present invention.

FIG. 4 is a configuration diagram of a scanning electron microscope unit according to a third embodiment of the present invention.

FIG. 5 is a configuration diagram of a scanning electron microscope unit according to a fourth embodiment of the present invention.

FIG. 6 is a configuration diagram of a scanning electron microscope unit according to a fifth embodiment of the present invention.

FIG. 7 is a configuration diagram of a scanning electron microscope unit according to a sixth embodiment of the present invention.

FIG. 8 is a configuration diagram of a scanning electron microscope unit according to a seventh embodiment of the present invention.

FIG. 9 is a configuration diagram of a scanning electron microscope unit according to a ninth embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Cathode, 2 ... Extraction electrode, 3 ... Anode, 4 ... Extraction voltage, 5
... acceleration voltage, 6: superimposed voltage, 7: condenser lens, 8
... Aperture, 9 ... Insulation base, 10 ... Adjustment knob, 11 ... Upper scan coil, 12 ... Lower scan coil, 13 ... Suction electrode, 14 ...
Scintillator, 15 light guide, 16 photomultiplier,
17 ... objective lens, 18 ... sample, 19 ... sample stage,
Reference numeral 20: primary electron beam, 21: sample holder, 24: secondary electron, 25: channel plate, 28: amplification voltage, 29
... Anode voltage, 30 ... Amplifier, 31 ... Light conversion circuit, 3
Reference numeral 3: central hole, 34: mesh electrode, 35: electric conversion circuit, 36: filter voltage, 37: anode, 39: control electrode, 40: control voltage, 41: shield grid, 42
... Energy filter, 55 ... Single crystal scintillator, 56
... conductive thin film, 61 ... housing, 77 ... sample exchange mechanism

Claims (10)

[Claims] An electron source for irradiating a sample with a primary electron beam emitted from the electron source; and detecting a secondary signal generated from the sample by irradiating the sample with the primary electron beam. A secondary signal detecting means including a channel plate having a passage through which the primary electron beam passes, and a deceleration electric field for decelerating the primary electron beam between the sample and the secondary signal detecting means. A scanning electron microscope, comprising: means for generating a decelerating electric field for decelerating the secondary signal between the channel plate and the sample. 2. The scanning electron microscope according to claim 1, wherein said electrodes are formed in a mesh shape. 3. The scanning electron microscope according to claim 1, wherein a positive voltage is applied to the electrode with respect to the channel plate. 4. The scanning electron microscope according to claim 1, wherein a negative voltage is applied to the electrode with respect to the channel plate. 5. The scanning electron microscope according to claim 1, wherein the intensity of the deceleration electric field for decelerating the secondary electrons is stronger than the intensity of the deceleration electric field for decelerating the primary electron beam. 6. A scanning electron microscope which scans a primary electron beam emitted from an electron source on a sample and obtains a sample image based on a secondary signal generated from the sample, comprising a passage for the primary electron beam. A secondary signal detector including an annular channel plate, a means for forming a deceleration electric field for reducing the incident energy of the primary electron beam with respect to the sample, and a means arranged between the channel plate and the sample;
A scanning electron microscope comprising: a mesh electrode to which a negative voltage is applied. 7. An electron source, electron optical means for irradiating a sample with a primary electron beam emitted from the electron source, and a secondary signal for detecting a secondary signal generated from the sample irradiated with the primary electron beam. A detector for reducing the incident energy of the primary electron beam on the sample;
A scanning electron microscope, comprising: means for forming a deceleration electric field of the above, and means for forming a second deceleration electric field for reducing incident energy of a secondary signal to the secondary signal detector. 8. The secondary signal detector according to claim 7, wherein the second deceleration electric field is formed between the secondary signal detector and a mesh electrode arranged between the sample and the secondary signal detector. And a voltage is applied to each of the components. 9. The scanning electron microscope according to claim 7, wherein the intensity of the second deceleration electric field is higher than the intensity of the first deceleration electric field. 10. The method according to claim 7, wherein
The scanning electron microscope, wherein the secondary signal detector includes a channel plate.