JP6994972B2 - electronic microscope - Google Patents

Description

The present invention relates to an electron microscope.

As a detector for detecting secondary electrons and backscattered electrons mounted on an electron microscope, a detector using a micro channel plate is known (see, for example, Patent Document 1). The microchannel plate is a plate-shaped detector in which elements having an electron amplification action are two-dimensionally arranged.

In a detector using a microchannel plate, the contrast of the obtained backscattered electron image or secondary electron image can be adjusted by adjusting the gain of the detector. The gain of the detector can be adjusted by changing the voltage applied to the microchannel plate.

Japanese Unexamined Patent Publication No. 2014-225354

As described above, in the conventional electron microscope, the gain of the microchannel plate is changed by changing the voltage applied to the microchannel plate, and the contrast of the backscattered electron image and the secondary electron image is adjusted.

However, the microchannel plate is placed inside the lens barrel. Therefore, if the voltage applied to the microchannel plate is changed, it affects the electron beam irradiated to the sample, and the axis of the electron beam is deviated, that is, the electron beam deviates from the optical axis of the optical system. was there. As described above, in the conventional electron microscope, in order to obtain a good backscattered electron image or a secondary electron image, it is necessary to align the electron beam every time the contrast of the image is adjusted, and the operability is improved. It was bad.

The present invention has been made in view of the above problems, and one of the objects according to some aspects of the present invention is to provide an electron microscope having excellent operability.

One aspect of the electron microscope according to the present invention is
A microchannel plate that detects electrons and
A conversion circuit that converts the current signal from the microchannel plate into a voltage signal,
An adjustment circuit that amplifies or attenuates the voltage signal,
A detection signal generation circuit that generates a detection signal based on the output signal of the adjustment circuit,
A gain control unit that controls the gain of the adjustment circuit,
Including
The gain control unit
The gain of the adjustment circuit is controlled based on the detection signal before the voltage applied to the microchannel plate changes and the detection signal after the voltage applied to the microchannel plate changes. ..

Since such an electron microscope includes an adjustment circuit, the gain of the entire circuit (electron detection unit) that detects electrons on the microchannel plate and outputs them as a detection signal is controlled by controlling the gain of the adjustment circuit. Can be done. Therefore, by controlling the gain of the adjustment circuit without changing the voltage applied to the microchannel plate, the contrast of the electron microscope image can be adjusted in the same way as when the voltage applied to the microchannel plate is changed. Can be adjusted. Therefore, in such an electron microscope, it is not necessary to align the electron beam every time the voltage applied to the microchannel plate is changed, that is, every time the contrast of the electron microscope image is changed, and the operability is improved. Can be improved.

Further, in such an electronic microscope, the gain control unit is based on the detection signal before the voltage applied to the microchannel plate changes and the detection signal after the voltage applied to the microchannel plate changes. To control the gain of the adjustment circuit. Therefore, it is possible to reduce or eliminate the difference in gain of the electron detection unit before and after the change in the voltage applied to the microchannel plate. As a result, the gain of the electron detection unit changes continuously, and the user can adjust the contrast of the electron microscope image without being aware of the change in the voltage applied to the microchannel plate.

The figure which shows the structure of the electron microscope which concerns on embodiment. The figure which shows the structure of the high voltage application part, the electron detection part, and the control part. The graph which shows the relationship between the voltage applied to a microchannel plate and the gain of a microchannel plate. The graph which shows the relationship between the gain of voltage and DAC applied to a microchannel plate, and the gain of an electron detection part. The figure which shows the structure of the electron detection part. The flowchart which shows an example of the processing of the processing part of the electron microscope which concerns on embodiment. The graph which shows an example of the relationship between the gain of an electron detection part, the voltage applied to a microchannel plate, and the gain of DAC.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The embodiments described below do not unreasonably limit the content of the present invention described in the claims. Moreover, not all of the configurations described below are essential constituent requirements of the present invention.

1. 1. Electron Microscope First, the electron microscope according to this embodiment will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of an electron microscope 100 according to the present embodiment.

As shown in FIG. 1, the electron microscope 100 includes an electron source 2, a condenser lens 3, a deflector 4, a scanning coil 5, an objective lens 6, a microchannel plate 7, a high voltage application unit 8, and the like. It includes a detection circuit 9 and a control unit 10.

The electron source 2 emits an electron beam. The electron source 2 is, for example, a known electron gun.

The condenser lens 3 is a lens for focusing an electron beam and irradiating the sample S.

The deflector 4 two-dimensionally deflects the electron beam emitted from the electron source 2. The deflector 4 is used, for example, to align (match) the electron beam with the optical axis OA of the optical system (for example, the objective lens 6), that is, to align the electron beam.

The scanning coil 5 is a coil for scanning the sample S with an electron beam (electron probe) focused by the condenser lens 3 and the objective lens 6.

The objective lens 6 focuses the electron beam emitted from the electron source 2 and irradiates the sample S. The objective lens 6 is placed immediately in front of the sample S to form an electron probe. The sample S is held by a sample holder (not shown).

The microchannel plate 7 constitutes a backscattered electron detector that detects backscattered electrons generated when the sample S is irradiated with an electron beam. The microchannel plate 7 is arranged in a lens barrel in which an optical system (deflector 4, scanning coil 5, objective lens 6, etc.) constituting the electron microscope 100 is housed.

Although the case where the microchannel plate 7 constitutes a backscattered electron detector that detects backscattered electrons is described here, the microchannel plate 7 may form a secondary electron detector that detects secondary electrons. .. That is, the electron microscope image acquired by the electron microscope 100 may be a backscattered electron image or a secondary electron image.

The high voltage application unit 8 applies a high voltage to the microchannel plate 7. The gain of the microchannel plate 7 can be controlled by controlling the voltage applied to the microchannel plate 7 by the high voltage application unit 8.

The detection circuit 9 generates a detection signal based on the current signal from the microchannel plate 7. The detection signal is a signal containing information on the intensity of backscattered electrons.

The control unit 10 controls each unit constituting the electron microscope 100, that is, a condenser lens 3, a deflector 4, a scanning coil 5, an objective lens 6, a high voltage application unit 8, and a detection circuit 9. Further, the control unit 10 generates a backscattered electron image based on the detection signal generated by the detection circuit 9, and performs a process of displaying it on the display unit 16 (see FIG. 2).

In the electron microscope 100, the electron beam emitted from the electron source 2 is focused by the condenser lens 3 and the objective lens 6 to form an electron probe. The scanning coil 5 scans the sample S with an electron probe, and the reflected electrons emitted from the sample S are detected by the microchannel plate 7. In the detection circuit 9, a detection signal is generated from the output signal (current signal) of the microchannel plate 7, and in the control unit 10, a reflected electron image is generated based on the detection signal.

The electron microscope 100 may include a lens other than the above-mentioned lens and coil, a coil, an aperture, and the like. Further, the electron microscope 100 may include a detector other than the backscattered electron detector. For example, the electron microscope 100 may include a detector that detects an electron beam that has passed through the sample S. By detecting the electron beam transmitted through the sample S by using a detector that detects the electron beam transmitted through the sample S, a scanning transmission electron microscope image (STEM image) can be obtained.

FIG. 2 is a diagram showing the configurations of the high voltage application unit 8, the electron detection unit 70, and the control unit 10.

The high voltage application unit 8 includes a high voltage generation source 80 and a high voltage changeover switch 82.

The high voltage generation source 80 generates a high voltage applied to the microchannel plate 7.

The high voltage changeover switch 82 switches the high voltage applied to the microchannel plate 7. In the illustrated example, the high voltage changeover switch 82 is configured to be able to switch the high voltage applied to the microchannel plate 7 in three ways: high voltage HV1, high voltage HV2, and high voltage HV3.

The high voltage changeover switch 82 is controlled by the high voltage control unit 120 of the control unit 10. Specifically, when the operation unit 14 receives the input of the voltage information applied to the microchannel plate 7, the high voltage control unit 120 controls the high voltage changeover switch 82 based on the voltage information. As a result, a desired high voltage is applied to the microchannel plate 7.

In the above, the case where the high voltage application unit 8 switches the voltage applied to the microchannel plate 7 in three stages has been described, but the high voltage application unit 8 sets the voltage applied to the microchannel plate 7 to N. It may be configured to be switchable in stages (N is an integer of 2 or more).

A high voltage is applied to the microchannel plate 7 by the high voltage application unit 8, and the detected backscattered electrons are amplified and output as a current signal according to the intensity of the backscattered electrons.

The detection circuit 9 includes a current-voltage conversion circuit 90, a digital-to-analog converter (hereinafter also referred to as “DAC”) 92 (an example of an adjustment circuit), and a detection signal generation circuit 94. ing.

The current-voltage conversion circuit 90 converts the current signal from the microchannel plate 7 into a voltage signal.

The DAC 92 amplifies or attenuates the voltage signal from the current-voltage conversion circuit 90. The voltage signal from the current-voltage conversion circuit 90 is input to the DAC 92 as a reference voltage. The output voltage of the DAC 92 can be changed by changing the data of the DAC 92 input from the gain control unit 122. That is, the gain of the DAC 92 can be controlled by changing the data of the DAC 92. The gain of the DAC 92 is the ratio of the voltage signal (input) from the current-voltage conversion circuit 90 to the output signal (output) of the DAC 92.

In the example shown in FIG. 2, the DAC 92 is arranged between the current-voltage conversion circuit 90 and the amplifier circuit 940, but the position of the DAC 92 is not particularly limited as long as it is between the current-voltage conversion circuit 90 and the DAC 946. ..

The detection signal generation circuit 94 generates a detection signal based on the output signal of the DAC 92. The detection signal generation circuit 94 includes an amplifier circuit 940, a level shift circuit 942, an amplifier circuit 944, and a DAC 946.

The amplifier circuit 940 amplifies the output signal of the DAC 92. The level shift circuit 942 level shifts to a low potential so that the DAC 946 can convert it into a digital signal. As the level shift circuit 942, a photocoupler or the like can be used. The amplifier circuit 944 amplifies the output signal of the level shift circuit 942. The DAC 946 converts the output signal (analog signal) of the amplifier circuit 944 into a digital signal to generate a detection signal. The detection signal is sent to the control unit 10. The control unit 10 generates a reflected electron image based on the detection signal, and the reflected electron image is displayed on the display unit 16.

The microchannel plate 7 and the detection circuit 9 form an electron detection unit 70. The electron detection unit 70 outputs the reflected electrons (input) detected by the microchannel plate 7 as a detection signal. The gain of the electron detection unit 70 is controlled by the voltage applied to the microchannel plate 7 and the gain of the DAC 92. The gain of the electron detection unit 70 is the ratio of the reflected electrons (input) detected by the microchannel plate 7 to the detection signal (output).

FIG. 3 is a graph showing the relationship between the voltage applied to the microchannel plate 7 and the gain of the microchannel plate 7. In the graph shown in FIG. 3, the horizontal axis shows the voltage applied to the microchannel plate 7, and the vertical axis shows the gain of the microchannel plate 7. The vertical axis is a relative value with respect to the gain of the microchannel plate 7 (“1”) when the voltage applied to the microchannel plate 7 is the high voltage HV1.

As shown in FIG. 3, the voltage applied to the microchannel plate 7 is switched in three stages (high voltage HV1, HV2, HV3) by the high voltage application unit 8. Therefore, by switching the voltage applied to the microchannel plate 7, the gain of the microchannel plate 7 can be switched in three stages.

FIG. 4 is a graph showing the relationship between the voltage applied to the microchannel plate 7 and the gain of the DAC 92 and the gain of the electron detection unit 70. In the graph shown in FIG. 4, the horizontal axis shows the voltage applied to the microchannel plate 7, and the vertical axis shows the gain of the electron detection unit 70. Further, the size of the rectangular box represents the range in which the gain of the DAC 92 can be adjusted.

As shown in FIGS. 3 and 4, the voltage applied to the microchannel plate 7 is switched in three stages. Therefore, the gain of the electron detection unit 70 can be switched in three stages by switching the voltage applied to the microchannel plate 7.

Further, the gain of the DAC 92 can be changed by inputting the data of the DAC 92 in the gain control unit 122. In the DAC 92, the gain can be controlled according to the resolution of the DAC 92. Therefore, as shown in FIG. 4, by changing the gain of the DAC 92, the gain of the electron detection unit 70 can be changed more finely than when the voltage applied to the microchannel plate 7 is changed.

That is, in the electron microscope 100, the high voltage application unit 8 switches the voltage applied to the microchannel plate 7 in N stages, and the gain control unit 122 switches the gain of the DAC 92 in n stages, and N <n. For example, when the high voltage changeover switch 82 switches the high voltage in three ways and the resolution of the DAC 92 is 8 bits, N = 3 and n = 255.

As described above, in the electron microscope 100, the gain of the electron detection unit 70 is controlled by switching the voltage applied to the microchannel plate 7 (coarse adjustment) and controlling the gain of the DAC 92 (fine adjustment). As a result, in the electron microscope 100, the gain of the electron detection unit 70 can be adjusted with high accuracy over a wide range.

As shown in FIG. 4, for example, when the high voltage HV1 is used and the high voltage HV2 is used, the gain of the DAC 92 is increased when the range in which the gain of the electron detection unit 70 can be adjusted overlaps. Adopt the range that can be raised and used. That is, in the electron microscope 100, the hatched region in FIG. 4 is not used. When the gain of the DAC 92 is small, the SN ratio is lower than when the gain is large. Therefore, the SN ratio can be improved by increasing the gain of the DAC 92 and adopting a usable range (that is, by not using the hatched region in FIG. 4).

In the electron microscope 100, the contrast of the reflected electron image can be adjusted by adjusting the gain of the electron detection unit 70.

The control unit 10 includes a processing unit 12, an operation unit 14, a display unit 16, and a storage unit 18.

The operation unit 14 acquires an operation signal corresponding to the operation by the user and performs a process of sending the operation signal to the processing unit 12. The operation unit 14 is, for example, a button, a key, a touch panel type display, a microphone, or the like. Further, the operation unit 14 functions as an input unit that receives input of voltage information applied to the microchannel plate 7. The operation unit 14 also functions as an input unit that receives input of gain information of the electron detection unit 70. That is, the user can adjust the contrast of the reflected electron image by operating the operation unit 14.

The display unit 16 displays the image generated by the processing unit 12. The display unit 16 can be realized by a display such as an LCD (liquid crystal display) or an organic EL (electroluminescence) display.

The storage unit 18 stores programs, data, and the like for the processing unit 12 to perform various calculation processes and control processes. Further, the storage unit 18 is used as a work area of the processing unit 12, and is also used to temporarily store the calculation results and the like executed by the processing unit 12 according to various programs. The storage unit 18 can be realized by a RAM (random access memory), a ROM (read only memory), a hard disk, or the like.

The storage unit 18 stores information used for axis alignment of electron beams, which will be described later. Specifically, the storage unit 18 contains information on the relationship between the voltage applied to the microchannel plate 7 and the conditions of the optical system (deflector 4, etc.) in which the electron beam is aligned with the optical axis OA. Is remembered. In the electron microscope 100, the voltage applied to the microchannel plate 7 is three types of high voltage HV1, HV2, and HV3. Therefore, the storage unit 18 is provided with information on the conditions of the optical system in which the electron beam is aligned with the optical axis OA when the high voltage HV1 is applied to the microchannel plate 7, and the high voltage HV2 is applied to the microchannel plate 7. Information on the conditions of the optical system in which the electron beam is aligned with the optical axis OA when is applied, and when the high voltage HV3 is applied to the microchannel plate 7, the electron beam is aligned with the optical axis OA. Information on the conditions of the optical system in the state of being in the state is stored.

For information on the conditions of the optical system in which the electron beam is aligned with the optical axis OA when the high voltage HV1 is applied to the microchannel plate 7, for example, the high voltage HV1 is applied to the microchannel plate 7. It can be obtained by aligning the axes using an optical system (for example, a deflector 4) and recording the conditions of the optical system at that time.

Information on the conditions of the optical system in which the electron beam is aligned with the optical axis OA when the high voltage HV2 is applied to the microchannel plate 7, and when the high voltage HV3 is applied to the microchannel plate 7. Information on the conditions of the optical system in which the electron beam is aligned with the optical axis OA can also be obtained as in the case of the high voltage HV1.

The processing unit 12 performs various control processing and calculation processing according to the program stored in the storage unit 18. The function of the processing unit 12 can be realized by executing a program on various processors (CPU (central processing unit) or the like). The function of the processing unit 12 may be realized by FPGA (field-programmable gate array) or the like.

The processing unit 12 includes a high voltage control unit 120, a gain control unit 122, and an optical system control unit 124.

The high voltage control unit 120 controls the high voltage changeover switch 82. The high voltage control unit 120 controls the high voltage changeover switch 82 based on the information of the voltage applied to the microchannel plate 7 input to the operation unit 14. The high voltage control unit 120 switches the high voltage changeover switch 82 to switch the voltage applied to the microchannel plate 7.

The gain control unit 122 inputs predetermined data to the DAC 92 and controls the gain of the DAC 92. The gain control unit 122 obtains a data value input to the DAC 92 based on the gain information of the electron detection unit 70 input to the operation unit 14, that is, the contrast information of the reflected electron image, and controls the gain of the DAC 92.

Further, the gain control unit 122 receives information on the voltage applied to the microchannel plate 7 when the voltage applied to the microchannel plate 7 is switched, that is, when information on the voltage applied to the microchannel plate 7 is input to the operation unit 14. Gain of DAC 92 based on the detection signal before the voltage applied to (before switching) and after the voltage applied to the microchannel plate 7 changes (after switching). To control.

Specifically, when the gain control unit 122 first inputs information on the voltage applied to the microchannel plate 7 to the operation unit 14, the gain control unit 122 takes a predetermined time before the voltage applied to the microchannel plate 7 changes. The average value of the detection signals (hereinafter, also referred to as "the average value of the detection signals before the voltage change") and the average value of the detection signals for a predetermined time after the voltage applied to the microchannel plate 7 changes (hereinafter, the average value). Also referred to as "the average value of the detection signals after the voltage change"), is calculated. At this time, the time for averaging the detected signals before the change (predetermined time) and the time for averaging the detected signals after the change (predetermined time) are, for example, equal.

Next, the gain control unit 122 calculates the difference between the average value of the detection signals before the voltage change and the average value of the detection signals after the voltage change. Then, the gain control unit 122 controls the DAC 92 so that this difference becomes small, more preferably the difference becomes zero.

As a result, the difference in gain of the electron detection unit 70 before and after switching the voltage applied to the microchannel plate 7 can be made small, or the difference in gain can be made zero. As a result, the change in the contrast of the reflected electron image due to the change in the voltage applied to the microchannel plate 7 can be reduced. Therefore, even when the voltage applied to the microchannel plate 7 is switched, the gain of the electron detection unit 70 changes continuously. Therefore, the user can adjust the contrast of the reflected electron image without being aware that the voltage applied to the microchannel plate 7 has changed.

The average value of the detection signal before the voltage change and the average value of the detection signal after the voltage change are obtained by scanning a predetermined region of the sample S with an electron probe and detecting reflected electrons with the microchannel plate 7. At this time, in order to acquire the average value of the detection signal before the voltage change, the scanning area of the sample S when scanning with the electronic probe and the average value of the detection signal after the voltage change are acquired by the electronic probe. It is preferable that the scanning region of the sample S at the time of scanning is the same region. As a result, the gain of the DAC 92 can be set more appropriately, and the change in the contrast of the reflected electron image due to the change in the voltage applied to the microchannel plate 7 can be made smaller.

The optical system control unit 124 controls the optical system (for example, the deflector 4) so that the electron beam is in a state aligned with the optical axis OA according to the voltage applied to the microchannel plate 7. Specifically, when the voltage applied to the microchannel plate 7 is switched, the optical system control unit 124 transfers the voltage applied to the microchannel plate and the electron beam stored in the storage unit 18 to the optical axis OA. The optical system is controlled based on the information of the relationship with the conditions of the optical system in the matched state.

For example, when the voltage applied to the microchannel plate 7 is switched to the high voltage HV1, the electron beam is stored in the storage unit 18 when the high voltage HV1 is applied to the microchannel plate 7. The optical system conditions are determined from the information on the optical system conditions that match the optical axis OA, and the optical system is controlled.

As a result, even when the voltage applied to the microchannel plate 7 is switched, the electron beam can be in a state of being aligned with the optical axis OA. That is, even when the voltage applied to the microchannel plate 7 is switched, the axis deviation of the electron beam can be eliminated or the axis deviation can be reduced.

2. 2. Operation example of DAC Next, an operation example of DAC92 will be described. FIG. 5 is a diagram showing the configuration of the electron detection unit 70. In FIG. 5, the detection signal generation circuit 94 is shown as one amplifier circuit. Here, the operation of the DAC 92 when the voltage applied to the microchannel plate 7 is switched from the high voltage HV1 to the high voltage HV2 will be described with reference to FIG.

In the example shown in FIG. 5, the resolution of the DAC 92 is 8 bits. Further, the gain A1 of the detection signal generation circuit 94 is set to A1 = 16.

The voltage applied to the microchannel plate 7 is the high voltage HV1, and immediately before switching from the high voltage HV1 to the high voltage HV2, the reference voltage V1 is V1 = Vref1 and the data value of the DAC 92 is FF (HEX) (255). (DEC)). In addition, "(HEX)" indicates that it is a hexadecimal number, and "(DEC)" indicates that it is a decimal number.

Immediately after the voltage applied to the microchannel plate 7 is switched from the high voltage HV1 to the high voltage HV2, the reference voltage V1 is V1 = Vref2, and the data value of the DAC 92 is 0F (HEX) (15 (DEC)). ). The data value of the DAC 92 immediately after the high voltage is switched is a value automatically set at the time of high voltage switching so that the gain characteristic of the electron detection unit 70 does not deviate significantly.

When the voltage applied to the microchannel plate 7 is the high voltage HV1, the output voltage V2 of the DAC 92 is V2 = Vref1, and the output voltage V0 of the detection signal generation circuit 94 is V0 = 16 × V2.

Further, when the voltage applied to the microchannel plate 7 is the high voltage HV2, the output voltage V2 of the DAC 92 and the output voltage V0 of the detection signal generation circuit 94 are as follows.

V2 = (0F / FF) × Vref2 = (15/255) × Vref2 ≈ 0.0588 × Vref2 ... (1)
V0 = 16 × V2

From the graph shown in FIG. 3, Vref2 is represented as follows.

Vref1 × 10 = Vref2 ... (2)

Therefore, from the equations (1) and (2), the output voltage V2 of the DAC 92 when the high voltage HV2 is applied to the microchannel plate 7 is expressed as follows.

V2 = 0.0588 x Vref1 x 10 = 0.588 x Vref1

The output voltage V0 of the detection signal generation circuit 94 when the high voltage HV1 is applied to the microchannel plate 7 is as follows.

V0 = 16 × Vref1 ... (3)

Further, the output voltage V0 of the detection signal generation circuit 94 when the high voltage HV2 is applied to the microchannel plate 7 is as follows.

V0 = 16 × 0.588 × Vref1 = 9.408 × Vref1 ... (4)

The difference between the above equation (3) and the above equation (4) is calculated by the control unit 10 (gain control unit 122) and reflected in the data of the DAC 92.

Here, the data of the DAC 92 has the following values.

15 × (16 / 9.408) ≒ 25 (DEC)
DATA: 19 (HEX)

After switching the high voltage applied to the microchannel plate 7 from the high voltage HV1 to the high voltage HV2, by writing 19 (HEX) to the data of the DAC 92, it is close to the gain of the electron detection unit 70 immediately before switching the high voltage. It can be in a (preferably the same) state.

3. 3. Operation Next, the operation of the electron microscope 100 will be described. Hereinafter, the processing of the processing unit 12 when switching the voltage applied to the microchannel plate 7 will be described. FIG. 6 is a flowchart showing an example of processing of the processing unit 12 of the electron microscope 100 according to the present embodiment.

First, the processing unit 12 determines whether or not there is an instruction (voltage switching instruction) for switching the voltage applied to the microchannel plate 7 from the user (step S100), and waits until the voltage switching instruction is given. When the operation unit 14 receives the input of the voltage information applied to the microchannel plate 7, the processing unit 12 determines that the voltage switching instruction has been given.

When it is determined that the voltage switching instruction has been given (Yes in S100), the gain control unit 122 starts the process of controlling the gain of the DAC 92. Specifically, the gain control unit 122 first acquires the detection signal from the electron detection unit 70 for a predetermined time, and calculates the average value A of the detection signals (that is, the average value of the detection signals before the voltage change). (S102).

After the gain control unit 122 acquires the detection signal for a predetermined time, the high voltage control unit 120 switches the voltage applied to the microchannel plate 7 based on the voltage information input to the operation unit 14 (S104).

When the voltage applied to the microchannel plate 7 is switched, the optical system control unit 124 controls the conditions of the optical system so that the electron beam is aligned with the optical axis OA (S106). As a result, it is possible to eliminate or reduce the misalignment of the electron beam due to the change in the voltage applied to the microchannel plate 7.

Next, the gain control unit 122 acquires the detection signal from the electron detection unit 70 for a predetermined time, and calculates the average value B of the detection signals (that is, the average value of the detection signals after the voltage change) (S108).

Next, the gain control unit 122 controls the gain of the DAC 92 based on the difference between the average value A and the average value B (S110). Specifically, the gain control unit 122 controls the gain of the DAC 92 so that the difference between the average value A and the average value B becomes small (preferably zero). As a result, the change in the contrast of the reflected electron image due to the change in the voltage applied to the microchannel plate 7 can be reduced.

After controlling the gain of the DAC 92 (after S110), the processing unit 12 returns to step S100 and repeats the processing of steps S100, S102, S104, S106, S108, and S110.

4. Features The electron microscope 100 has the following features, for example.

The electron microscope 100 amplifies or attenuates a microchannel plate 7 for detecting electrons, a current-voltage conversion circuit 90 for converting a current signal from the microchannel plate 7 into a voltage signal, and a voltage signal from the current-voltage conversion circuit 90. A DAC 92, a detection signal generation circuit 94 that generates a detection signal based on the output signal of the DAC 92, and a gain control unit 122 that controls the gain of the DAC 92 are included, and the gain control unit 122 is applied to the microchannel plate 7. The gain of the DAC 92 is controlled based on the detection signal before the voltage changed and the detection signal after the voltage applied to the microchannel plate 7 changes.

As described above, since the electron microscope 100 includes the DAC 92, the gain of the electron detection unit 70 can be controlled by controlling the gain of the DAC 92. Therefore, the contrast of the backscattered electron image is the same as when the voltage applied to the microchannel plate 7 is changed by controlling the gain of the DAC 92 without changing the voltage applied to the microchannel plate 7. Can be adjusted. Therefore, in the electron microscope 100, it is not necessary to align the electron beam every time the voltage applied to the microchannel plate 7, that is, each time the contrast of the electron microscope image is changed, and the operability is improved. can.

In the electron microscope 100, for the gain adjustment of the electron detection unit 70, that is, the adjustment of the contrast of the reflected electron image, the user first selects the voltage applied to the microchannel plate 7 suitable for observing the sample S (coarse adjustment). ), And then the gain of the DAC 92 can be adjusted (fine adjustment). Therefore, in the electron microscope 100, it is not necessary to align the electron beam every time the contrast of the reflected electron image is adjusted, and the electron microscope 100 has excellent operability.

Further, in the electron microscope 100, the gain control unit 122 sets the detection signal before the voltage applied to the microchannel plate 7 changes and the detection signal after the voltage applied to the microchannel plate 7 changes. Based on this, the gain of the DAC 92 is controlled.

Therefore, in the electron microscope 100, the difference in gain of the electron detection unit 70 before and after switching the voltage applied to the microchannel plate 7 can be made small, or the difference in gain can be made zero. As a result, the change in the contrast of the reflected electron image due to the change in the voltage applied to the microchannel plate 7 can be reduced. Therefore, the change in the gain of the electron detection unit 70 becomes continuous, and the user can adjust the contrast of the backscattered electron image without being aware of the change in the voltage applied to the microchannel plate 7. Therefore, in the electron microscope 100, the operability can be improved.

In the electron microscope 100, the gain control unit 122 has a first average value which is an average value of detection signals for a predetermined time before the voltage applied to the microchannel plate 7 changes, and a voltage applied to the microchannel plate 7. The difference between the second average value, which is the average value of the detection signals for a predetermined time after the change, is calculated, and the gain of the DAC 92 is controlled so that the difference between the first average value and the second average value becomes small. Therefore, in the electron microscope 100, the difference in gain of the electron detection unit 70 before and after switching the voltage applied to the microchannel plate 7 can be made small, or the difference in gain can be made zero.

The electron microscope 100 includes an operation unit 14 as an input unit for receiving input of voltage information applied to the microchannel plate 7, and the gain control unit 122 includes a voltage control unit 12 for which the operation unit 14 applies a voltage to the microchannel plate 7. When the information is received, the process of controlling the gain of the DAC 92 is started. Therefore, in the electron microscope 100, it is possible to adjust the contrast of the reflected electron image without being aware of the change in the voltage applied to the microchannel plate 7.

In the electron microscope 100, the electron beam depends on the optical system (condenser lens 3, deflector 4, scanning coil 5, objective lens 6, etc.) that irradiates the sample S with the electron beam and the voltage applied to the microchannel plate 7. Includes an optical system control unit 124 that controls the optical system so that the lens is aligned with the optical axis OA of the optical system. Therefore, in the electron microscope 100, even if the voltage applied to the microchannel plate 7 changes, the electron beam can be automatically brought into a state of being aligned with the optical axis OA.

The electron microscope 100 includes an optical system control unit 18 that stores information on the relationship between the voltage applied to the microchannel plate 7 and the conditions of the optical system in which the electron beam is in the optical axis OA. The unit 124 controls the optical system based on the above-mentioned related information. Therefore, in the electron microscope 100, even if the voltage applied to the microchannel plate 7 changes, the electron beam can be automatically brought into a state of being aligned with the optical axis OA.

The electron microscope 100 includes a high voltage application unit 8 that switches the voltage applied to the microchannel plate 7. Further, the high voltage application unit 8 of the electron microscope 100 switches the voltage applied to the microchannel plate 7 to N stages, and the gain control unit 122 switches the gain of the DAC 92 to n stages, and N <n. Therefore, in the electron microscope 100, the gain of the electron detection unit 70 can be adjusted with high accuracy in a wide range.

5. Modifications The present invention is not limited to the above-described embodiment, and various modifications can be carried out within the scope of the gist of the present invention.

5.1. First Modification Example FIG. 7 is a graph showing an example of the relationship between the gain of the electron detection unit 70, the voltage applied to the microchannel plate 7, and the gain of the DAC 92.

As shown in FIG. 7, the gain of the electron detection unit 70 may have a one-to-one correspondence with the voltage applied to the microchannel plate 7 and the gain of the DAC 92. That is, the voltage applied to the microchannel plate 7 and the gain of the DAC 92 may be uniquely determined according to the contrast value of the backscattered electron image. This allows the user to adjust the contrast of the backscattered electron image without selecting the voltage applied to the microchannel plate 7.

5.2. Second Modification Example In the above embodiment, as shown in FIG. 2, the gain control unit 122 controls the gain of the DAC 92 based on the detection signal output from the DAC 946. That is, in the above embodiment, the detection signal for generating the backscattered electron image and the detection signal for controlling the gain of the DAC 92 are obtained from the same output signal of the DAC 946.

On the other hand, in this modification, although not shown, a DAC for obtaining a detection signal for controlling the gain of the DAC 92 is separated from the DAC 946 for obtaining a detection signal for generating a backscattered electron image. May be provided. This makes it possible to build a circuit suitable for controlling the gain of the DAC 92.

It should be noted that the above-described embodiments and modifications are merely examples, and the present invention is not limited thereto. For example, each embodiment and each modification can be combined as appropriate.

The present invention includes substantially the same configurations as those described in the embodiments (eg, configurations with the same function, method and result, or configurations with the same purpose and effect). The present invention also includes a configuration in which a non-essential part of the configuration described in the embodiment is replaced. Further, the present invention includes a configuration having the same action and effect as the configuration described in the embodiment or a configuration capable of achieving the same object. Further, the present invention includes a configuration in which a known technique is added to the configuration described in the embodiment.

2 ... electron source, 3 ... condenser lens, 4 ... deflector, 5 ... scanning coil, 6 ... objective lens, 7 ... microchannel plate, 8 ... high voltage application unit, 9 ... detection circuit, 10 ... control unit, 12 ... Processing unit, 14 ... operation unit, 16 ... display unit, 18 ... storage unit, 70 ... electron detection unit, 80 ... high voltage source, 82 ... high voltage changeover switch, 90 ... current voltage conversion circuit, 92 ... DAC, 94 ... Detection signal generation circuit, 100 ... Electronic microscope, 120 ... High voltage control unit, 122 ... Gain control unit, 124 ... Optical system control unit, 940 ... Amplifier circuit, 942 ... Level shift circuit, 944 ... Amplifier circuit, 946 ... DAC

Claims (7)

A microchannel plate that detects electrons and
A conversion circuit that converts the current signal from the microchannel plate into a voltage signal,
An adjustment circuit that amplifies or attenuates the voltage signal,
A detection signal generation circuit that generates a detection signal based on the output signal of the adjustment circuit,
A gain control unit that controls the gain of the adjustment circuit,
Including
The gain control unit
The gain of the adjustment circuit is controlled based on the detection signal before the voltage applied to the microchannel plate changes and the detection signal after the voltage applied to the microchannel plate changes. ,electronic microscope. In claim 1,
The gain control unit
The first average value, which is the average value of the detection signals for a predetermined time before the voltage applied to the microchannel plate changes, and the detection for a predetermined time after the voltage applied to the microchannel plate changes. Calculate the difference between the second average value, which is the average value of the signal, and
An electron microscope that controls the gain of the adjustment circuit so that the difference between the first average value and the second average value becomes small. In claim 1 or 2,
Includes an input unit that receives input of voltage information applied to the microchannel plate.
The gain control unit is an electron microscope that starts a process of controlling the gain of the adjustment circuit when the input unit receives information on the voltage applied to the microchannel plate. In any one of claims 1 to 3,
An optical system that irradiates a sample with an electron beam,
An optical system control unit that controls the optical system so that the electron beam is aligned with the optical axis of the optical system according to the voltage applied to the microchannel plate.
Including an electron microscope. In claim 4,
A storage unit for storing information on the relationship between the voltage applied to the microchannel plate and the conditions of the optical system in which the electron beam is in a state of being aligned with the optical axis is included.
The optical system control unit is an electron microscope that controls the optical system based on the related information. In any one of claims 1 to 5,
An electron microscope including a voltage application unit that switches the voltage applied to the microchannel plate. In claim 6,
The voltage application unit switches the voltage applied to the microchannel plate in N stages.
The gain control unit switches the gain of the adjustment circuit in n stages.
An electron microscope in which N <n.

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