JP2014017170A - Electron microscope and method for forming image by electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron microscope and a method for forming an image by the electron microscope, which can obtain an SEM image having high picture quality even by using a small amount of electron irradiation.SOLUTION: An electron microscope comprises an irradiation part for irradiating a sample with an electron beam, a control part for controlling irradiation of the electron beam, a first detection part for detecting emission electrons emitted from the sample at an irradiation position of the electron beam, a sampling part for sampling a detection signal of the emission electrons detected in the first detection part, on the basis of the average number of irradiated electrons per a unit time of the electron beam, a processing part for calculating an average value or an integrated value of a plurality of detection signals sampled by the sampling part, and an image formation part for forming an image on the basis of the average value or the integrated value of the plurality of detection signals.

Description

  The present invention relates to a microscope technique for observing a sample form using an electron beam, and more particularly to detection control of emitted electrons and a signal processing technique.

  There is an electron microscope using an electron beam as a microscope capable of magnifying and observing a sample. In particular, a scanning electron microscope (hereinafter abbreviated as SEM) that scans and images an electron beam on a sample is used for sample analysis such as observation of a fine surface shape and local composition analysis. In the SEM, an electron beam accelerated by a voltage applied to an electron source (hereinafter referred to as primary electrons) is focused by an electron lens and irradiated onto a sample. The focused electron beam is scanned on the sample by a deflector. Then, electrons emitted from the sample due to electron beam irradiation (Auger electrons, secondary electrons, reflected electrons, or secondary electrons and reflected electrons) are detected by a detector. Furthermore, the detection signal of the emitted electrons detected by the detector is sampled at a constant period. The emission electron signal is sampled in synchronization with the scanning signal, and an extraction signal corresponding to the pixel of the two-dimensional image is obtained. The intensity of the extracted signal is converted into the brightness of the image. Since the emission rate of emitted electrons from the sample differs depending on the shape of the sample surface, a difference occurs in the extraction signal, and a contrast reflecting the shape is obtained.

  In SEM, observation of a fine surface shape is the most common application. In recent years, there are an increasing number of cases in which a sample containing a soft material such as an organic material or a biomaterial is an observation target of SEM. When the object includes a soft material, the sample is charged by electron beam irradiation, and image disturbance such as image drift during observation and loss of shape contrast becomes a problem. Furthermore, since the sample is damaged by the electron beam irradiation, a change in the shape of the sample such as shrink becomes a problem. Therefore, a low acceleration SEM using an electron beam of several hundred volts has been put into practical use. Even in the low-acceleration SEM, in high-quality observation with a slow scanning speed of electron beams and high-magnification observation with nano-resolution, the amount of electron irradiation increases, and the influence of charging and damage due to electron beam irradiation becomes obvious. Therefore, in addition to lowering the electron beam, observation with a small amount of electron irradiation is required.

  Usually, in SEM, the same location is scanned a plurality of times, and the image quality is improved by integrating the extracted signals obtained in each scan. As a method other than integration of extraction signals, Patent Document 1 discloses a method of acquiring a high-resolution SEM image by controlling the number of times the detection signal of the emitted electrons is sampled according to the scanning speed and the size of the observation region. It is disclosed.

JP 2006-139965 A

  The number of electrons irradiated on the sample depends on the observation field of view, the scanning speed of the electron beam, the irradiation current, and the integrated number of signals. For example, when the number of integrated signals is increased or the scanning speed is decreased to observe with high image quality, 100 or more electrons are irradiated in the same pixel. The detection signal of emitted electrons obtained by 100 or more irradiations is usually sampled about 10 times. When the electron irradiation amount is 100 or more, the number of irradiated electrons is larger than the number of samplings, and an image having a high SN ratio (Signal to noise ratio) corresponding to the integrated number is obtained.

  However, in order to observe a specimen that is markedly charged or damaged by an electron beam, if the number of electrons irradiated in the same pixel is reduced from 1 to several tens, the number of samplings is equal to the number of irradiated electrons, The number is smaller. Therefore, the detection signal is missed or a signal when electrons are not irradiated is extracted, so that the SN ratio is lowered and the image quality is deteriorated. Further, when the number of electrons irradiated in the same pixel is small, even if the integration number is increased, unnecessary signals that are not irradiated with electrons are integrated, and the image quality is not improved.

  In order to solve the above-described problems, the present invention provides an electron microscope and an image forming method for an electron microscope that obtain a high-quality SEM image with a small amount of electron irradiation.

  In order to solve the above problems, it is important to control the sampling of detection while suppressing the detection of the detection signal of the emitted electrons and unnecessary signal detection. In order to solve the above problems, an electron microscope according to the present invention emits an electron beam to a sample, a control unit for controlling the electron beam irradiation, and emits the sample from the sample at the electron beam irradiation position. Based on the first detection unit for detecting the emitted electrons and the average number of irradiated electrons per unit time of the electron beam, the detection signal of the emitted electrons detected by the first detection unit is sampled. A sampling unit; a processing unit that calculates an average value or an integrated value of a plurality of detection signals sampled by the sampling unit; and an image forming unit that forms an image based on the average value or the integrated value of the plurality of detection signals; . According to the electron microscope of the present invention, since the detection signal can be sampled efficiently, it is possible to suppress the detection of the emitted electron detection signal and the integration of unnecessary signals even with a small amount of electron irradiation, and to obtain a high-quality SEM image. Can be provided.

  In addition, according to the present invention, an image forming method for an electron microscope is provided. An image forming method for an electron microscope according to the present invention includes a step of irradiating a sample with an electron beam, a step of controlling irradiation of the electron beam, and a step of detecting emitted electrons emitted from the sample at the irradiation position of the electron beam. Sampling the emission electron detection signal based on the average number of electrons irradiated per unit time of the electron beam, calculating an average value or an integrated value of the plurality of sampled detection signals; Forming an image based on an average value or an integrated value of the plurality of detection signals.

  The present invention can be applied to any apparatus of an electron microscope that continuously irradiates the sample with the electron beam and an electron microscope that irradiates the sample with the electron beam intermittently.

  In the present invention, the period and timing for sampling the detection signal of the emitted electrons are determined by either a time chart of the detection signal of the emitted electrons or a time chart of detecting the electron beam irradiated on the sample. be able to. In the case of the configuration using the time chart of the emission electron detection signal, the electron microscope of the present invention includes a sampling condition analysis unit for analyzing the sampling condition of the emission electron detection signal and the emission electron detection signal based on the sampling condition. A sampling control unit for sampling. This makes it possible to analytically obtain the period based on the average number of irradiated electrons per unit time of the electron beam from the time chart of the emission electron detection signal.

  Further, in the case of the configuration using the time chart for detecting the electron beam, the electron microscope of the present invention includes a second detection unit for detecting the electron beam and the detection signal detected by the second detection unit. A signal processing unit for inputting a sampling trigger signal to the sampling unit. As a result, a signal for sampling the emitted electrons is generated at a cycle based on the average number of irradiated electrons per unit time of the electron beam using the detection signal for the electron beam as a trigger, and the detected signal for the emitted electrons is sampled. can do.

  In the present invention, the average number of electrons irradiated per unit time of the electron beam can be determined by the electron beam irradiation current.

In addition, since the average number of irradiated electrons per unit time usually used in SEM is in the range of 10 ⁹ to 10 ⁶ / s, the sampling period of the detection signal of the emitted electrons is 1 GHz to 10 MHz. It is better to set within the range.

  According to the present invention, it is possible to provide an electron microscope capable of imaging with high image quality even under observation conditions with a small electron irradiation amount.

  Further features related to the present invention will become apparent from the description of the present specification and the accompanying drawings. Further, problems, configurations and effects other than those described above will be clarified by the description of the following examples.

It is a block diagram which shows an example of the electron microscope by 1st Example. It is explanatory drawing which shows an example of the method of processing the detection signal of the emitted electron of 1st Example. It is explanatory drawing which shows an example of the method of processing the detection signal of the emitted electron of 1st Example. It is explanatory drawing which shows an example of the setting flow of the sampling conditions of 1st Example. It is an example of the image acquired on the observation conditions of 1st Example. It is an example of the image acquired on the observation conditions set arbitrarily. It is explanatory drawing which shows an example of the method of processing the detection signal of the emitted electron of 2nd Example. It is explanatory drawing which shows an example of the setting flow of the image formation conditions of 2nd Example. It is a block diagram which shows an example of the electron microscope of 2nd Example. It is explanatory drawing which shows an example of the setting flow of the image formation conditions of 3rd Example. FIG. 10 is a diagram illustrating an example of an image forming condition setting operation GUI according to a third embodiment. It is a block diagram which shows an example of the electron microscope of 4th Example. It is explanatory drawing which shows an example of the setting flow of the sampling conditions of 4th Example.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. The accompanying drawings show specific embodiments in accordance with the principle of the present invention, but these are for the understanding of the present invention, and are never used to interpret the present invention in a limited manner. is not.

<First embodiment>
In this embodiment, an apparatus and a method for forming an image by controlling a sampling period of a detection signal of emitted electrons will be described. FIG. 1 shows a configuration diagram of an electron microscope according to a first embodiment of the present invention. In this embodiment, a scanning electron microscope is used as an example, but an electron microscope using an intermittent electron beam may be used.

  The scanning electron microscope includes an electron optical system, a stage mechanism system, an SEM control system, a signal processing system, and an SEM operation system. The electron optical system includes an electron gun 1, a deflector 2, an objective lens 3, and a detector 4. The stage mechanism system includes a sample holder 6 on which the sample 5 is installed, and a stage 7 that can be tilt-controlled and moved in the XYZ axial directions.

  The SEM control system includes an electron gun control unit 8, a deflection scanning signal control unit 9, an objective lens coil control unit 10, and a stage control unit 11. The signal processing system also includes a detection signal extraction unit 12, an extraction control unit 13, an extraction signal processing unit 14, and an image forming unit 15. In this embodiment, the sampling unit that samples the emitted electrons detected by the detector 4 includes a detection signal extraction unit 12 and an extraction control unit 13, and the extraction control unit 13 uses the sampling conditions for the detection signal of the emitted electrons. A sampling condition analysis unit for analyzing the emission, and a sampling control unit for sampling a detection signal of the emitted electrons based on the sampling condition. Further, the operation system includes an image display unit 16 and an operation interface 17.

  In the scanning electron microscope, the electron beam from the electron gun 1 is focused by the objective lens 3 and irradiated onto the sample 5 on the sample holder 6. The electron beam scans the sample 5 by the deflector 2. Then, the emitted electrons from the sample 5 due to the electron beam irradiation are detected by the detector 4. The detection signal extraction unit 12 extracts a detection signal from the analog detection signal obtained by the detector 4 at a certain sampling period. Here, the sampling period for extracting the detection signal is controlled by the extraction control unit 13. Further, the extracted signal processing unit 14 integrates the signals extracted by the detection signal extracting unit 12. Note that the extracted signal processing unit 14 may calculate an average value of the signals extracted by the detection signal extracting unit 12. The image forming unit 15 forms an image based on the integrated value or average value of the extracted signals, and the image display unit 16 displays the SEM image.

In this embodiment, the electron irradiation amount is controlled by the irradiation current Ip. The electron gun control unit 8 controls the emission current emitted from the electron gun 1 by controlling the irradiation current Ip. That is, the electron gun control unit 8 controls the irradiation current Ip, whereby the average number of irradiated electrons per unit time is controlled.
2A and 2B are conceptual diagrams of a method for processing a detection signal of emitted electrons according to the present embodiment. FIG. 2A shows a time chart 18 in which time is plotted on the horizontal axis and sampling trigger signal is plotted on the vertical axis. FIG. 2B shows a time chart 20 with the horizontal axis representing time and the vertical axis representing extracted signals.

In this embodiment, the period of the sampling trigger signal is determined by the equation (1) based on the average number of irradiated electrons per unit time.
f = I _p / e (1)
Here, f is the sampling period 19 shown in FIG. 2A, and e is an elementary charge. Thus, in the configuration in which electrons from the electron gun 1 is controlled by the emission current I _p, using the emission current I _p of the electron beam can be used to determine the sampling period.

  When the sampling period f is obtained based on the expression (1), the detection signal is extracted at the sampling period 19 as shown in the time chart 18 of FIG. 2A. Further, as shown in the extracted signal time chart 20 of FIG. 2B, the extracted signals included in the integration section 21 are integrated to improve the image quality. Here, the integration section 21 is determined by the integration number of signals set as observation conditions.

FIG. 3 shows a sampling condition setting flow in this embodiment. In step S100, observation conditions are set in advance. These observation conditions are set in the electron microscope via the operation interface 17. Note that the observation conditions in this example were acceleration voltage V _a , observation field of view M, irradiation current Ip, and integration number Na. In this embodiment, V _a = 300 eV, M = 80 nm, Ip = 5.3 pA, and the integrated number Na = 60.

  Next, in step S101, the sampling period is set in the electron microscope (that is, the extraction control unit 13). The sampling period f is obtained by equation (1). For Ip = 5.3 pA, f is 33 MHz. Here, the sampling period f is preferably equal to or less than the band of the signal detection system for emitted electrons.

Further, in the present invention, it is necessary to integrate the detection signal with a designated integration number for each pixel, and the scanning speed of the electron beam depends on the sampling period f and the integration number Na. Therefore, in step S102, the scanning speed of the electron beam is set according to the observation conditions and sampling conditions. The scanning speed V of the electron beam is obtained by the following equation (2).
V = S _p / t _p (2)
Here, S _p is the pixel size, the t _p is the acquisition time per pixel. Pixel size S _p can be calculated by dividing the observation field by the number of pixels. 1 acquisition time t _p per pixel can be calculated from the following equation (3).
t _p = 1 / (f × Na) (3)
Here, f is the sampling period 19 and Na is the cumulative number. With the above processing, the sampling condition setting is completed (S103). Thereafter, the detection signal extraction unit 12 extracts the detection signal at the sampling period f under the control of the extraction control unit 13. Thereafter, the extracted signal processing unit 14 integrates the signals extracted by the detection signal extracting unit 12, and the image forming unit 15 forms an image based on the integrated value of the extracted signals.

FIG. 4A is an image acquired at the sampling period 19 (f = 33 MHz) of the emission electron detection signal set in the flow of FIG. FIG. 4B is an image at a sampling period (f = 3.3 MHz) of an emission electron detection signal set arbitrarily. 4B has many noise components and poor image quality, while FIG. 4A has few noise components and improved image quality.
As described above, according to the present embodiment, since the signal of the emitted electrons can be efficiently sampled based on the average number of irradiated electrons per unit time, a high-quality SEM image can be acquired even with a small amount of electron irradiation. it can.

In addition, since the average number of irradiated electrons per unit time normally used in SEM is in the range of 10 ⁹ to 10 ⁶ / s, the sampling period of the emission electron detection signal is in the range of 1 GHz to 10 MHz. It is preferable to set within.

<Second embodiment>
In this embodiment, an apparatus and a method for forming an image by controlling a sampling period of a detection signal of emitted electrons will be described. In addition, since the structure of the electron microscope in a present Example uses the structure similar to 1st Example, description is abbreviate | omitted. FIG. 5 shows a conceptual diagram of the method for processing the detection signal of the emitted electrons according to the present embodiment. The top diagram of FIG. 5 shows a time chart 18 in which time is plotted on the horizontal axis and sampling trigger signal is plotted on the vertical axis. Further, the remaining three diagrams in FIG. 5 show time charts 23, 24, and 25 in which time is plotted on the horizontal axis and extracted signals are plotted on the vertical axis.

In this embodiment, the sampling period 19 is set based on the following equation (4).
f = n × Ip / e (4)
Here, n is a sampling coefficient and is an integer. In this example, n = 3 was used. The detection signal of emitted electrons is sampled at a frequency higher than the average number of irradiation of electrons. Here, the time chart 18 of FIG. 5 shows a sampling period 19 obtained using a coefficient n = 3.

  In this embodiment, the detection signal extracted at the sampling period 19 is extracted again from the time chart of the detection signal at the period of the expression (1). At this time, three different time charts 23, 24, and 25 that are offset by the sampling timing 22 are obtained depending on the sampling coefficient. In the time charts 23, 24, and 25, solid line portions indicate timings for extracting signals. The sampling timing 22 in this embodiment is defined by the delay time from the pulsed detection signal. The timing at which this delay time is minimized is the optimum sampling timing.

  In the present embodiment, in order to determine the optimum sampling timing, the frequency at which the extraction signal becomes the threshold value 26 or more in the time charts 23, 24, and 25 is analyzed. Here, it is desirable that the threshold value 26 be equal to or higher than the noise generated when the electron beam is cut off and equal to or lower than the signal voltage when one electron is detected. Here, the signal voltage when one electron is detected is obtained from the detection voltage pitch of the extracted signal of the quantized detection signal. In this example, among the time charts 23, 24, and 25 of the extraction signals of the detection signals having different timings, the timing at which the frequency exceeding the threshold value 26 is the highest is the timing at which the delay time from the pulsed detection signal is minimized. .

  In the example of FIG. 5, the timing of the detection signal extraction signal in the time chart 24 has the highest frequency of exceeding the threshold 26, and thus is the optimum sampling timing. Here, since the timing adjustment amount depends on the sampling coefficient n, the adjustment accuracy improves as n increases. According to this embodiment, the sampling timing can be corrected.

FIG. 6 shows a flow of setting the image forming conditions of this embodiment. Similar to the first embodiment, before obtaining an image, the observation conditions (acceleration voltage V _a , observation field of view M, irradiation current Ip, integration number Na) are set in step S110. Next, in step S111, an analysis condition for the sampling condition (sampling coefficient n, detection signal extraction value threshold value Vth) is set. Here, the threshold value Vth corresponds to the threshold value 26 in FIG. These conditions are set in the extraction control unit 13 via the operation interface 17, for example.

  Next, in step S112, based on the set irradiation current Ip and the sampling coefficient n, the sampling period f is set in the electron microscope (that is, the extraction control unit 13) based on the equation (4). Next, in step S113, the electron beam irradiation position corresponding to the pixel is moved. In this embodiment, the irradiation position of the electron beam is controlled by scanning.

  Next, in step S <b> 114, the detection signal extraction unit 12 performs sampling at a sampling period f (= n × fe) under the control of the extraction control unit 13. Here, fe = Ip / e. Further, in step S115, the extraction control unit 13 analyzes the timing with the highest frequency at which the extracted signal is equal to or higher than the threshold value Vth in the manner described with reference to FIG. 5, and determines the optimal timing. Next, in step 116, the detection signal extraction unit 12 extracts only the extraction signals at the optimal timing, and the extraction signal processing unit 14 integrates these extraction signals.

Thereafter, the process returns to step S113 again and moves to the electron beam irradiation position corresponding to the pixel. Then, Steps S114 to S116 are executed, and determination of optimal timing and integration of detection signals are repeatedly executed. Finally, in step S117, the image forming unit 15 forms an image based on signals accumulated for each pixel.
Thus, according to the present embodiment, the timing with the sampling period can be adjusted with high accuracy, and a high-quality SEM image can be acquired even with a small electron irradiation amount.

<Third embodiment>
In this embodiment, a method for analyzing a sampling period and timing in advance and an apparatus having a function for analyzing the sampling period and timing will be described. FIG. 7 shows a configuration diagram of an electron microscope in the present embodiment.

  The configuration of FIG. 7 is a configuration in which a beam blanker 27, a diaphragm mechanism 28, and a beam blanking control unit 29 are added to the configuration of FIG. 1 shown in the first embodiment. The beam blanker 27 is, for example, a deflector that deflects an electron beam, and the aperture mechanism 28 has a path (hole or the like) for allowing the electron beam to pass therethrough. Therefore, by controlling the beam blanker 27 by the beam blanking control unit 29, the electron beam from the electron gun 1 can be passed through the path of the aperture mechanism 28 or not. Thereby, as shown in FIG. 7, the irradiation of an intermittent electron beam is attained.

  Electron beam intermittent conditions (irradiation time, number of irradiations, and interval time between irradiations) can be controlled by a beam bunking voltage signal applied to the beam blanker 27. In the present embodiment, the beam blanking control unit 29 controls the beam blanking voltage signal in synchronization with a clock signal that is a reference signal for managing time.

  The feature of this embodiment is that the sampling period and timing using intermittent electron beam irradiation can be analyzed. A method for analyzing the sampling period and timing will be described below. FIG. 8 is a flow for analyzing the sampling period and timing.

  First, in step S120, the electron microscope is switched to an analysis mode for sampling conditions. At this time, an operation GUI for setting the sampling period and timing is activated, and the operation GUI is displayed on the image display unit 16. Details of this operation GUI will be described later.

Next, in step S121, observation conditions (acceleration voltage V _a , observation visual field M, irradiation current Ip, integration number Na) are input on the operation GUI. In step S122, the sampling condition analysis condition (sampling coefficient n, detection signal extraction value threshold value Vth) is input.

  Next, in step S123, an analysis sampling period f is set from the set irradiation current Ip and sampling coefficient n based on the equation (4). The analysis sampling period f is set in the extraction control unit 13, for example. Next, in step S124, an analysis position for irradiating an electron beam is set in order to analyze the sampling condition. The analysis position may be in the observation region or outside the observation region. Irradiate a fixed analysis position with an intermittent electron beam.

  Next, in step S <b> 125, the detection signal extraction unit 12 extracts an emission electron detection signal in the analysis sampling period f under the control of the extraction control unit 13. Next, in step S126, as in the second embodiment, the sampling timing is analyzed by analyzing the frequency of the extraction signal threshold (Vth) or more from a plurality of time charts of the extraction signal having different timings. . Here, first, as in the second embodiment described above, the timing with the highest frequency that is equal to or higher than the threshold value (Vth) of the extracted signal is selected. Here, the sampling timing in this embodiment is based on the rise of the beam blanking voltage signal. Sampling timing was defined as a delay time from the rising edge of the beam blanking voltage signal to the first detected extraction signal during intermittent irradiation.

  In the present embodiment, after the delay time from the rise of the beam blanking voltage signal to the first detected extraction signal is analyzed, sampling after the delay time may be performed at the sampling period f of the equation (1). . That is, in this embodiment, it is not necessary to analyze the sampling timing for each pixel. Therefore, in step S127, the sampling timing obtained as described above is set in the extraction control unit 13, and the sampling cycle f of the expression (1) is set in the extraction control unit 13 as a sampling cycle used for observation. To do.

  In step S128, under these sampling conditions (sampling period f and timing), the electron microscope is switched to the observation mode to generate a sampling trigger signal. This sampling trigger signal can be used until the irradiation current of the electron beam is changed. In addition, the sampling conditions analyzed in this embodiment can be applied to image formation of a scanning electron microscope or an electron microscope using intermittent electron beams.

  Next, an operation GUI (Graphical User Interface) for setting the sampling period and timing will be described. FIG. 9 shows an operation GUI displayed on the image display unit 16. The operation GUI includes an observation condition input window 30, an analysis condition input window 31, an analysis time chart window 32, and a sampling analysis result window 33.

The analysis conditions of the observation condition and the sampling condition can be input to the observation condition input window 30 and the analysis condition input window 31 of the sampling condition. For example, in the observation condition input window 30, the acceleration voltage V _a , the observation field M, the irradiation current Ip, and the integration number Na are set. In the analysis condition input window 31, a sampling coefficient n and a detection signal threshold value Vth are set.

In the analysis time chart window 32, a time chart 32a having time on the horizontal axis and a blanking voltage signal on the vertical axis, a time chart 32b having time on the horizontal axis and an extraction signal on the vertical axis are displayed. As shown in FIG. 9, the sampling timing 32c can be analyzed based on the time difference between the rising edge of the beam blanking voltage signal and the extraction signal detected first in the intermittent irradiation. The sampling analysis result window 33 displays sampling conditions. In this example, the frequency exceeding the threshold value Vth is displayed for each timing (2.5 ns, 5 ns, 7.5 ns). The user can set the timing with the highest frequency of exceeding the threshold as the optimum timing. According to this configuration, the user can set an optimum timing while confirming the frequency at which the threshold value is exceeded for each timing. For example, when two or more timings with close frequency values exceeding the threshold value are displayed, re-analysis can be performed to select a more optimal timing. In this case, the user can perform reanalysis by resetting the threshold value Vth of the analysis condition input window 31 and pressing the “reanalyze” button.
Thus, according to the present embodiment, the timing with the sampling period can be adjusted with high accuracy, and a high-quality SEM image can be obtained even with a small electron irradiation amount.

<Fourth embodiment>
In this embodiment, a method and apparatus for forming an image by controlling the sampling period of a detection signal of emitted electrons will be described. FIG. 10 shows a configuration example of an electron microscope in the present embodiment.

  The configuration of FIG. 10 is a configuration in which a primary electron detector 34 as a second detection unit and a primary electron detection signal processing unit 35 are added to the configuration of FIG. 1 shown in the first embodiment. It has become. The primary electron detection signal processing unit 35 is electrically connected to the extraction control unit 13.

  In the present embodiment, an electron beam is directly applied to the primary electron detector 34, and an electron irradiation time chart is acquired. The primary electron detection signal processing unit 35 analyzes the average number of irradiated electrons per unit time from the time chart of the detection signal of the primary electrons detected by the primary electron detector 34. Then, the primary electron detection signal processing unit 35 determines the sampling period f from the average number of electron irradiations per unit time, and generates a sampling trigger signal. The sampling trigger signal is directly input from the primary electron detection signal processing unit 35 to the extraction control unit 13. The detection signal extraction unit 12 extracts a detection signal based on the sampling trigger signal input to the extraction control unit 13.

  In the first to third embodiments, the method of analytically setting the sampling period by the expression (1) has been described. On the other hand, according to the present embodiment, the primary electron detector 34 directly measures the period and timing of primary electron irradiation, and the period and timing can be directly used for detection signal extraction. Thereby, the sampling period and timing can be controlled with high accuracy.

  FIG. 11 shows a sampling condition setting flow of the fourth embodiment. First, in step S130, the electron microscope is switched to an analysis mode for sampling conditions. Next, in step S131, the irradiation position of the electron beam is moved to the primary electron detector 34, which is the primary electron analysis position. Next, in step S132, the primary electron detection signal processing unit 35 acquires a time chart of the primary electron detection signal.

Further, in step S133, the primary electron detection signal processing unit 35 analyzes the average number of irradiations of primary electrons per unit time from the primary electron detection signal time chart, and analyzes the primary electrons. A sampling period f based on the average number of electron irradiations per unit time is determined. Next, in step S134, the primary electron detection signal processing unit 35 generates a sampling trigger signal with the detection signal of the primary electrons as a trigger at the sampling period f. The sampling trigger signal is input from the primary electron detection signal processing unit 35 to the extraction control unit 13. In step S135, the extraction control unit 13 changes the electron microscope to the observation mode while holding the sampling trigger signal. Thereby, the detection signal extraction part 12 can extract a detection signal at the timing which the extraction control part 13 hold | maintained.
Thus, according to the present embodiment, primary electrons can be directly detected to set the sampling period, and a high-quality SEM image can be acquired even with a small electron irradiation amount.

<Summary>
According to the first embodiment, the period of the sampling trigger signal is determined by the expression (1) based on the average number of irradiated electrons per unit time. According to the first embodiment, the detection signal is extracted at the sampling period f of the equation (1), and the extraction signal included in the integration section 21 is integrated. As described above, according to the first embodiment, it is possible to efficiently sample the emitted electron signal based on the average number of irradiated electrons per unit time, and the detection of the emitted electron detection signal and the integration of unnecessary signals can be performed. Can be suppressed. Thereby, a high-quality SEM image can be obtained even with a small electron irradiation amount.

According to the second embodiment, first, the sampling period is set based on the equation (4). That is, the sampling period is set to an integer multiple of the period based on the average number of electron irradiations per unit time of the electron beam. Then, the timing at which the detection signal exceeds the predetermined threshold value 26 with the highest frequency is selected as the sampling timing from a plurality of different time charts 23, 24, and 25 offset by the timing 22. Then, the detection signal of the emitted electrons is sampled at the selected timing and at a cycle based on the average number of irradiated electrons per unit time of the electron beam (that is, the sampling cycle f in the equation (1)).
According to the second embodiment, sampling can be performed at finer time intervals using the sampling coefficient n in the equation (4), and an optimum timing can be selected from the sampling. Therefore, the timing with the sampling period can be adjusted with high accuracy, and a high-quality SEM image can be acquired with a small amount of electron irradiation. In addition, by setting the coefficient n large, it is possible to further improve the timing adjustment accuracy with respect to the sampling period.

According to the third embodiment, in the electron microscope using intermittent electron beams, the sampling period and timing can be analyzed in advance. Specifically, in the third embodiment, the sampling timing of the detection signal is determined based on the time difference between the rising edge of the beam blanking voltage signal and the extracted signal detected first from the rising edge. Then, the detection signal is sampled at the determined timing and at a period based on the average number of irradiated electrons per unit time of the electron beam (that is, the sampling period f in the equation (1)).
According to the third embodiment, in an electron microscope using intermittent electron beams, the timing with the sampling period can be adjusted with high accuracy, and a high-quality SEM image can be obtained even with a small amount of electron irradiation. . Furthermore, this timing and sampling period can be used until the irradiation current of the electron beam is changed.

According to the fourth embodiment, an electron beam is directly applied to the primary electron detector 34, and a time chart of electron irradiation is acquired. The primary electron detection signal processing unit 35 analyzes the average number of electron irradiations per unit time from the time chart of the detection signal of the primary electrons detected by the primary electron detector 34, and A sampling period f based on the average number of electron irradiations per unit time is determined. The primary electron detection signal processing unit 35 generates a sampling trigger signal with the detection signal of the primary electrons as a trigger at the sampling period f.
According to the fourth embodiment, there is no need to analytically obtain the sampling period from the detection signal of the emitted electrons, and the sampling period can be set by directly detecting the primary electrons. Thereby, a high-quality SEM image can be acquired even with a small electron irradiation amount.

  In addition, each control unit, each processing unit, and the like of the embodiment of the above-described electron microscope may be realized by hardware by designing a part or all of them with, for example, an integrated circuit. The present invention can also be realized by a program code of software that realizes the functions of each control unit and each processing unit. In this case, a storage medium in which the program code is recorded is provided to an information processing apparatus such as a computer, and the computer (or CPU or MPU) reads the program code stored in the storage medium. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the program code itself and the storage medium storing it constitute the present invention.

  In addition, this invention is not limited to the Example mentioned above, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. In addition, a part of the configuration of one embodiment may be replaced with the configuration of another embodiment, and the configuration of another embodiment may be added to the configuration of one embodiment. Further, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

  In the second embodiment, the timing with the highest frequency at which the extracted signal is equal to or higher than the threshold value Vth is set as the optimum timing, but the present invention is not limited to this configuration. For example, the optimum timing may be set by other methods such as obtaining and comparing the average values of the extracted signals at each of a plurality of timings.

  As shown in FIG. 9, on the operation GUI, the timing with the highest frequency exceeding the threshold value can be set as the optimum timing. For example, the apparatus may automatically set the timing at which the frequency of exceeding the threshold is the highest.

  Further, the control lines and information lines in the drawings are those that are considered necessary for the explanation, and not all the control lines and information lines on the product are necessarily shown. All the components may be connected to each other.

DESCRIPTION OF SYMBOLS 1 Electron gun 2 Deflector 3 Objective lens 4 Detector 5 Sample 6 Sample holder 7 Stage 8 Electron gun control part 9 Deflection scanning signal control part 10 Objective lens coil control part 11 Stage control part 12 Detection signal extraction part 13 Extraction control part 14 Extraction signal processing unit 15 Image forming unit 16 Image display unit 17 Operation interface 27 Beam blanker 28 Aperture mechanism 29 Beam blanking control unit 34 Primary electron detector 35 Primary electron detection signal processing unit

Claims (14)

An irradiation unit for irradiating the sample with an electron beam;
A control unit for controlling the irradiation of the electron beam;
A first detection unit for detecting emitted electrons emitted from the sample at the irradiation position of the electron beam;
A sampling unit that samples a detection signal of the emitted electrons detected by the first detection unit based on an average number of irradiated electrons per unit time of the electron beam;
A processing unit for calculating an average value or an integrated value of a plurality of detection signals sampled by the sampling unit;
An image forming unit that forms an image based on an average value or an integrated value of the plurality of detection signals;
An electron microscope comprising: The electron microscope according to claim 1,
The sampling unit
A sampling condition analysis unit for analyzing a sampling condition of the detection signal of the emitted electrons;
A sampling control unit that samples the detection signal based on the sampling condition;
An electron microscope comprising: The electron microscope according to claim 2,
The sampling condition analyzer is configured to determine a sampling period of the detection signal as the sampling condition based on an irradiation current of the electron beam. The electron microscope according to claim 2,
The sampling condition analysis unit
The sampling period of the detection signal is set to an integer multiple of the period based on the average number of irradiated electrons per unit time of the electron beam,
From a plurality of different timings, the timing at which the detection signal exceeds the predetermined threshold with the highest frequency is selected as the sampling timing of the detection signal,
The sampling control unit samples the detection signal at the selected timing and at a cycle based on an average number of irradiated electrons per unit time of the electron beam. The electron microscope according to claim 2,
The irradiation unit irradiates the sample with an intermittent electron beam,
The sampling condition analysis unit is configured to detect the detection based on a time difference between a rise time of a signal for controlling the intermittent electron beam irradiation and the detection signal first exceeding a predetermined threshold after the rise time. Determine the timing of signal sampling,
The sampling control unit samples the detection signal at the determined timing and in a cycle based on an average number of irradiated electrons per unit time of the electron beam. The electron microscope according to claim 1,
A second detection unit for detecting the electron beam;
A signal processing unit that inputs a sampling trigger signal to the sampling unit based on a detection signal detected by the second detection unit;
Further comprising
The electron microscope characterized in that the signal processing unit inputs a sampling trigger signal to the sampling unit at a sampling period based on an average number of electron irradiations per unit time of the electron beam. The electron microscope according to claim 1,
The electron microscope, wherein a sampling period of the detection signal is in a range from 1 GHz to 10 MHz. Irradiating the sample with an electron beam;
Controlling the irradiation of the electron beam;
Detecting emitted electrons emitted from the sample at the irradiation position of the electron beam;
Sampling the emission electron detection signal based on an average number of irradiated electrons per unit time of the electron beam;
Calculating an average value or an integrated value of a plurality of sampled detection signals;
Forming an image based on an average value or an integrated value of the plurality of detection signals;
An image forming method for an electron microscope, comprising: The image forming method of an electron microscope according to claim 8,
The sampling step includes:
Analyzing a sampling condition of a detection signal of the emitted electrons;
Sampling the detection signal based on the sampling condition;
An image forming method for an electron microscope, comprising: The image forming method of an electron microscope according to claim 9,
The analyzing step is characterized in that the sampling period of the detection signal is determined by an irradiation current of the electron beam. The image forming method of an electron microscope according to claim 9,
The analyzing step includes:
Setting the sampling period of the detection signal to an integral multiple of the period based on the average number of irradiated electrons per unit time of the electron beam;
Selecting a timing at which the detection signal exceeds a predetermined threshold from a plurality of different timings as a sampling timing of the detection signal;
Including
The sampling step includes sampling the detection signal at the selected timing and at a period based on an average number of irradiated electrons per unit time of the electron beam. Image forming method. The image forming method of an electron microscope according to claim 9,
The irradiating step irradiates the sample with an intermittent electron beam,
The analyzing step is based on a time difference between a rise time of the signal for controlling the intermittent electron beam irradiation and the detection signal first exceeding a predetermined threshold after the rise time. Determining the sampling timing of
The sampling step includes sampling the detection signal at the determined timing and at a period based on an average number of irradiated electrons per unit time of the electron beam. Image forming method. The image forming method of an electron microscope according to claim 8,
Detecting the electron beam;
Generating a sampling trigger signal based on the detected electron beam detection signal;
Further including
The step of generating the sampling trigger signal generates the sampling trigger signal at a sampling period based on an average number of electron irradiations per unit time of the electron beam,
The sampling step comprises sampling the emission electron detection signal based on the sampling trigger signal. The image forming method of an electron microscope according to claim 8,
An image forming method for an electron microscope, wherein a sampling cycle of the detection signal is in a range from 1 GHz to 10 MHz.

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