JP2001084946A - Method for evaluating secondary particle detector system and particle beam device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for quantitatively evaluating detection efficiency of a detector system by obtaining a ratio between counting rate calculated by subtracting a back ground counting rate measured with blocking primary particle beam from the number of output pulse corrected for a counting loss and the number of incident particles per unit time entering into a secondary particle detector calculated from measured probe current of the primary particle beam. SOLUTION: Primary particle beam of which probe current is measured beforehand is made directly enter into a secondary particle detector, and an output pulse counting rate of the secondary particle detector is measured. The output pulse counting rate is corrected for counting loss, and a back ground counting rate measured with blocking the primary particle beam is subtracted from the corrected number of the output pulse to obtain a counting rate to be used. Detection efficiency is expressed by a product of collecting efficiency represented as a ratio of the number of electron entering into the detector to the total number of secondary electrons, backscattered electrons, and sum of the secondary electrons and the backscattered electrons, generated by primary electron beam irradiating a sample, and counting efficiency represented as a ratio of the outputted signal amount on the basis of the number of electrons to the number of electrons coming into the detector.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting, observing, measuring, analyzing, and processing secondary electrons and backscattered electrons generated from a sample such as a scanning electron microscope (SEM) and a focused ion beam apparatus (FIB). The present invention relates to a method for evaluating the detection efficiency of a secondary electron and backscattered electron detection system of an apparatus for performing such operations.

[0002]

2. Description of the Related Art In a device for irradiating a sample with a primary particle beam, such as a scanning electron microscope or a focused ion beam device, to observe a surface, the primary particle beam is scanned on the sample and information on the sample surface and inside is stored. Secondary electrons and backscattered electrons are detected by a detector and converted into a signal. The AD-converted image data is stored in an image memory. Appropriate image processing is performed on the memory to perform digital-to-analog conversion. And CRT
The image is formed by synchronizing the scanning of the image and the surface state is observed. As the detector, a combination of a scintillator and a photomultiplier, an electron multiplier, a microchannel plate (MCP), a semiconductor detector, and the like are used. Means for obtaining an image from the output signal of the detector include, when the signal is a continuous analog signal, an analog detection method in which the voltage value of the analog signal is used as the luminance signal of the image, and a method in which the signal is a discrete pulse signal. In such a case, there is an electronic counting method in which the number of the pulse signals is counted and used as a luminance signal of an image.

In order to improve the image quality of an image obtained by the above-described detector and image acquisition method, it is necessary to efficiently detect secondary electrons and backscattered electrons generated in a sample, that is, to improve the detection efficiency. No. Therefore, it is important to quantitatively evaluate the detection efficiency. As an example of a quantitative evaluation method, a method of evaluating detection efficiency from S / N of a digital image obtained by an analog detection method is a scanning method.
Volume 18 (1996) pp. 533 to 538 (SCANNING Vol 18 (1996) 533-
538).

[0004]

According to the above-mentioned document, the S / N obtained experimentally, the amount of irradiation current of the primary electron beam, the secondary electron emission ratio, the backscattering coefficient, and the primary electron beam are represented by one pixel. , The detection efficiency is defined by the square of the ratio to the theoretical value of S / N, which can be calculated from the number of accumulated images. However, this method does not take into account the noise of the detector and its signal processing circuit, does not describe the counting efficiency of the detector, and cannot accurately evaluate the detection efficiency. In addition, the document does not describe a method for evaluating detection efficiency using an electronic counting method in which noise in a detector and its signal processing circuit can be ignored.

An object of the present invention is to provide a method for quantitatively evaluating the detection efficiency of a detector system from image data obtained by an analog detection method and an output pulse count rate obtained by an electronic counting method, and to realize the method. It is an object of the present invention to provide a particle beam device for performing the above.

[0006]

According to the present invention, in order to achieve the above object, first, a primary particle beam whose probe current is measured in advance is directly incident on a secondary particle detector, and the output of the secondary particle detector is output. When the pulse count rate is measured, the measured output pulse count rate is corrected for the count loss due to the dead time of the secondary particle detector and its signal processing circuit, and the primary particle beam is cut off from the output pulse number corrected for the count loss. The counting efficiency is determined by taking the ratio of the counting rate obtained by subtracting the background counting rate of the above and the number of particles incident on the secondary particle detector per unit time, which can be calculated from the measured probe current of the primary particle beam.

Next, in the analog detection method, a primary particle beam whose probe current is measured in advance is directly incident on the secondary particle detector, and image data obtained when two-dimensional scanning is performed on the incident surface of the secondary particle detector is obtained. Then, a signal intensity distribution for each pixel is created, the S / N is determined from the ratio of the average value of the distribution to the standard deviation of the distribution, and the S / N is determined by the above method by changing the probe current. The relationship between the number of particles incident on the secondary particle detector per unit time and the S / N of the image is determined.
The secondary particle emission rate is known with respect to the incident energy of the primary particle beam, and a flat sample with a flat surface and a mirror surface is irradiated with a pre-measured primary particle beam, and two-dimensional scanning is performed on the standard sample surface. The image data obtained when the secondary particles generated at the time of the detection are detected are obtained, the signal intensity distribution for each pixel is created, and the S / N and the second ratio obtained from the ratio between the average value of the distribution and the standard deviation of the distribution are obtained. From the number of particles incident on the secondary particle detector per unit time and the S / N ratio of the image, the secondary particle flow incident on the secondary particle detector is determined,
The collection efficiency is determined from the ratio of the product of the secondary particles and the product of the probe current.

In the electron counting method, the ratio of secondary particles emitted is known with respect to the incident energy of the primary particle beam.
A primary particle beam whose probe current is measured in advance is applied to a standard sample whose surface is partially flat, and the output pulse count rate of the secondary particle detector when the generated secondary particles are detected is measured. The pulse count rate is corrected for the count loss due to the dead time of the secondary particle detector and its signal processing circuit, and the count rate obtained by subtracting the background count rate when the primary particle beam is cut off from the output pulse number corrected for the count loss. And the product of the secondary particle charge and the secondary particle flow, and the collection efficiency is determined from the ratio of the secondary particle flow and the product of the probe current and the ratio of secondary particle emission.

The detection efficiency of the secondary particle detector system is quantitatively evaluated based on the product of the counting efficiency and the collection efficiency determined above.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS For the purpose of understanding the present invention, the configuration of a scanning electron microscope will be described first with reference to FIG.

The detector 5 comprises a scintillator 10, a light guide 11, and a photomultiplier tube 12. The primary electron beam 2 generated by the electron gun 1 is narrowed down by a converging lens 6 and an objective lens 9 and irradiates a sample 4. The primary electron beam 2 is supplied with a scanning signal from the scanning control unit 17 to the deflecting coils 7 and 8, and is scanned on the sample surface. By scanning the deflection coil 18 of the CRT 19 in synchronization with the scanning of the primary electron beam 2, the position on the sample surface to which the primary electron beam 2 is irradiated can correspond to the pixel of the CRT 19. The secondary electrons 3 emitted from the primary electron beam irradiation position of the sample 4 collide with a scintillator 10 to which a voltage of +10 kV is applied in order to efficiently detect the secondary electrons 3, and are converted into photons. Scintillator 10
Are generated by the photomultiplier tube 12 via the light guide 11 and converted into a current signal. The output current from the photomultiplier tube 12 is subjected to current-voltage conversion by an amplifier, and is further amplified to have an appropriate wave height. In the analog detection method, a voltage signal from an amplifier is converted into a digital signal by an AD converter, then transferred to the image memory 15, and written to an address corresponding to each pixel. In the electronic counting method, a pulse signal from an amplifier is subjected to a pulse height discriminator to remove a pulse having a pulse height serving as a noise component, and only a pulse having a pulse height of a true secondary electron signal is supplied to a counter and counted. The count value counted by the counter is transferred to the image memory 15 and written to an address corresponding to each pixel. In either method, the secondary electron signal stored in the image memory 15 is subjected to appropriate image processing such as image integration on the image memory 15 and is converted into a luminance signal of the CRT 19 by the DA converter 16. A secondary electron image is formed on the CRT 19.

Hereinafter, an example of an embodiment of the present invention will be described. The detection efficiency in the present invention, the secondary electrons generated by irradiating the sample with a primary electron beam, backscattered electrons, or the total number of secondary electrons and backscattered electrons, the total number of incident on the detector And the counting efficiency that indicates how many signal amounts of the electrons incident on the detector are output. Here, a method of evaluating the detection efficiency of the detector system for secondary electrons will be described below as an example.

First, a method for evaluating the counting efficiency is shown in FIG.
This will be described below with reference to the flowchart of FIG.

First, the energy of electrons to be incident on the detector is determined, and an acceleration voltage and a bias voltage on the detector incident surface are set (step 101). Then, the probe current I _PE of the primary electron beam is changed to the Faraday cup and the probe current. The measurement is performed with a meter (step 102).

Next, the primary electron beam is directly incident on the detector (step 103). Thus, the number of electrons incident on the detector per unit time can be expressed by I _PE / e (e: 1.602 × 10 to ¹⁹ C). As a method of causing the primary electron beam to directly enter the detector, a simple method of arranging the detector 5 on the trajectory axis of the primary electron beam 2 in the sample chamber and causing the primary electron beam 2 to enter as shown in FIG. . In the case of using the detector arrangement of the existing device, as shown in FIG. 9, a reflector 20 is installed near the intersection of the trajectory axis of the primary electron beam 2 and the central axis of the detector 5, and the acceleration voltage 2 of the primary electron beam 2 is increased.
A method in which the primary electron beam 2 is bent by applying a retarding voltage 22 of the same voltage as that of 1 to make the primary electron beam 2 incident on the detector 5 disposed on the side of the sample chamber, on the objective lens, or inside the objective lens. is there. As the reflection plate 20, an aluminum disk or the like whose surface is mirror-polished or aluminum-deposited can be used.

Next, the number of pulses n _PEm (count rate) per unit time at the output of the detector is measured by an electronic counting method or observation by an oscilloscope with the output of the amplifier (step 104). The primary electron beam enters the detector at random time intervals, and the detector device (detector and signal processing circuit)
Since there is a time width (dead time) in which two incident electrons cannot be counted separately, a counting loss occurs. For this reason, the count rate is always smaller than the number of incident electrons I _PE / e, but correction can be made using the dead time of the detector device. The dead time can be roughly estimated from the observed pulse width, but can be determined analytically by measuring the count distribution within a certain time interval. A count rate n _PE obtained by correcting the count loss due to the dead time is obtained from the count rate n _PEm (step 105). Here, since there is a detector in which the dead time changes when the number of incident electrons changes, the relationship between the count rate of the output pulse and the correction coefficient of the counting loss due to the dead time is obtained by changing the probe current (step 106). 4). Next, the primary electron beam is cut off, and the background counting rate n
_b is measured to determine n _PE −n _b (step 107).
From the definition of the counting efficiency, the counting efficiency is calculated based on the ratio of n _PE −n _b and the number of electrons I _PE / e per unit time actually incident on the detector (step 108). However, since there are detectors whose counting efficiency is almost constant even if the incident energy changes, and detectors whose counting efficiency changes when the incident energy changes, in the detector where the counting efficiency changes, the incident energy is changed. The relationship between the incident energy and the counting efficiency is determined (step 109, FIG. 5). This counting efficiency is based on SEM and FI
This is an index indicating the performance of the detector of the device B.

Next, a method of evaluating the collection efficiency using the analog detection method will be described below with reference to the flowchart of FIG. An acceleration voltage and a voltage for biasing the detector incident surface are set so that the energy of the electrons incident on the detector becomes equal to the energy of the electrons incident on the detector during actual use (step 201). The scanning speed and the number of integrated images are also set (step 202), and the probe current _IPE is measured (step 203). The primary electron beam is directly incident on the detector as in the measurement of the counting efficiency (step 204). Next, at the input of the AD converter, the DC offset of the signal from the detector is adjusted to zero, and the gain of the detector and the signal processing circuit is adjusted so that the signal falls within the dynamic range of the AD converter (Step 205). . An image obtained under this condition is a gray image having no unevenness information. In this image, the signal intensity distribution (horizontal axis gradient, vertical axis frequency) of each pixel is obtained from the image data stored in the image memory (step 206). The average signal strength S of this frequency distribution and its standard deviation σ are obtained, and S / σ
N is determined (step 207). The S / N for each probe current is obtained in the same manner, and the probe currents I _PE and S
/ N is determined (step 208, FIG. 6).

Next, a standard sample whose surface is flat and mirror surface and whose secondary electron emission ratio δ is known at a certain incident energy is set on the sample stage (step 209). The reason for using a flat or mirror-finished sample is to eliminate the irradiation point dependency of the secondary electron emission ratio.

For example, when the incident energy is 10 keV,
If a silicon wafer is used, δ = 0.215. If the incident energy is 10 keV, the acceleration voltage is set to 10 kV (step 210). Further, the probe current _IPE is set. Setting conditions, .delta.I _PE sets the I _PE to be in the range of I _PE relationship measured in advance I _PE vs. S / N (step 211). The scanning speed and the number of integrated images are adjusted to the conditions described above. The sample is irradiated with a primary electron beam under these conditions (step 212),
As described above, the DC offset at the input of the AD converter is adjusted to zero, and the gain of the detector and the signal processing circuit is adjusted so that the signal falls within the dynamic range of the AD converter (step 213). The image obtained when the generated secondary electrons are collected and used as a signal is also a gray image having no unevenness information because the sample surface is flat or mirror-finished. From this image data, a signal intensity distribution is obtained in the same manner as described above, and (S / N) _SE is obtained (step 2).
14). (S / N) From the relationship between _SE and the determined _IPE to S / N (FIG. 6), the secondary electron current _ISE = incident on the detector =
(I _PE ) _SE is obtained (step 215). From the definition of the collection efficiency described above, the collection efficiency of the secondary electrons can be obtained by I _SE / δI _PE (step 216).

Next, a method for evaluating the collection efficiency using the electronic counting method will be described below with reference to the flowchart of FIG.
A standard sample is set on a sample stage as in the analog detection method (step 301). Set the acceleration voltage (Step 30)
2) The probe current _IPE to which the electron counting method can be applied is set (step 303). In the case of the electron counting method, the primary electron beam does not need to be scanned as in the analog detection method, and spot irradiation may be performed. Therefore, the sample surface does not necessarily have to be a mirror surface. The sample is irradiated with the primary electron beam under these conditions (step 304), and the pulse count rate n _SEm obtained by collecting the generated secondary electrons is measured (step 3).
05). Referring to FIG. 4, a counting rate n _{SE in} which the counting loss due to the dead time of the detector is corrected is obtained (step 306). Next, the primary electron beam is cut off, the background count rate n _b is measured under the same measurement conditions, and n _SE −n _b is determined (step 307). n _SE -n secondary electron current incident on the detector from _{b I SE = e (n SE} -n b) / ε calculating the (step 308). Similarly to the analog detection method, from the definition of the collection efficiency, the collection efficiency of the secondary electrons can be obtained by I _SE / δI _PE (step 309).

Although the detection efficiency of secondary electrons has been described in detail in one example of the above embodiment, the method of evaluating the detection efficiency in the present invention is applied to the evaluation of the detection efficiency of backscattered electrons or secondary electrons + backscattered electrons. Can also be applied.

Further, in the above embodiment, since the standard sample is used, the secondary electron emission ratio at a certain incident energy needs to be known. However, when evaluating the collection efficiency with respect to all the electrons (secondary electrons + backscattered electrons) emitted from the sample, it can also be obtained by using an insulating material as the sample. This will be described below with reference to FIG. In the case of an insulating material, when the incident energy of the primary electron beam to the sample is set in a region (c) where the sum of the secondary electron emission ratio and the backscattering coefficient is 1 or less, the sample is negatively charged. Incident energy decreases.
At the boundary with the region (b), when the energy is lower than the energy, the incident energy is increased because it is positively charged. Therefore, the potential of the insulating material becomes stable near the intersection where the total electron emission ratio becomes 1. In this state, if the probe current I _PE and the electron current I _{SE + BSE} of the secondary electrons and the backscattered electrons are obtained by the method described above, the collection for all the electrons emitted from the sample by I _{SE + BSE} / I _PE Efficiency can be determined. As an insulating material, the incident energy is 1.5 keV
Quartz (SiO ₂ ) or the like having an electron emission ratio of 1 in the vicinity may be used.

An apparatus to which the evaluation method described in detail above is applied,
Alternatively, an example of an apparatus necessary for carrying out the method will be described below.

In an apparatus such as a length measuring SEM which uses a detector continuously, there is a problem that the image quality is deteriorated due to the deterioration of the performance of the detector. Therefore, an example of an apparatus for periodically measuring the collection efficiency under the same measurement conditions and determining the performance degradation of the detector will be described below with reference to FIGS. FIG. 11 shows an example of the operation display screen.
Corresponding to

First, an apparatus using an analog detection method will be described, taking the collection efficiency of secondary electrons as an example. First, a new collection efficiency is measured. When the primary electron beam 2 is directly incident on the detector 5 before the measurement, a function representing the relationship between the S / N obtained from the image obtained from the output signal of the signal processing unit 25 and the incident electron flow (FIG. 6) or The S / N table corresponding to each incident electron flow, the secondary electron emission ratio of the standard sample 4 used for measurement, the scanning speed, and the number of integrated images are stored in the memory 27 in advance.

Next, the standard sample 4 is set on the sample table.
The measurement condition setting button 30b of FIG. 11 is selected by the input unit 27, and the incident energy corresponding to the secondary electron emission ratio stored in the memory 27 is read out to the arithmetic processing / image processing unit 26 to control the acceleration voltage / probe current control. The accelerating voltage is set by the unit 23, and the scanning speed and the number of integrated images are also read out to the arithmetic processing / image processing unit 26 and set by the scanning control unit 17. After the probe current _IPE is set by the input means 28 in the measurement condition setting menu, the measurement button 30a in FIG. 11 is selected by the input means 27 to start the measurement of the collection efficiency. The measurement of the collection efficiency is performed in the following procedure. The DC offset is adjusted, the primary electron beam 2 is irradiated on the sample 4, the gain of the detector is adjusted so that the signal obtained by detecting the secondary electron 3 is not saturated, and an image is obtained. Computes (S / N) _SE from intensity distribution and image processing unit 2
6 is calculated. A function or table representing the relationship between I _{PE and} S / N at the energy at which the generated secondary electrons 3 enter the detector 5 is calculated from the memory 27 by the arithmetic processing / image processing unit 2.
6, the secondary electron current I _SE incident on the detector 5 is obtained by comparing with (S / N) _SE, and the collection efficiency I _SE / δI _PE
Is calculated. I _SE is (I _PE ) _SE obtained from a function or a table. When the counting efficiency changes with the incident energy, the counting efficiency ε (E) at the energy at which the secondary electrons 3 are incident on the detector 5 is calculated and calculated from a function or a table stored in the memory 27. 26 need to be read. The measurement result of the measured collection efficiency is
When the table button 30c in FIG. 11 is selected by the input means 27, the table 30i is displayed, and when the plot button 30d is selected, the graph is displayed in the graph 30h. FIG.
Select the save button 30f of 1 and calculate the collection efficiency,
Measurement date and time, probe current, acceleration voltage, scanning speed, number of images and other measurement conditions and the gain of detector 5 are stored in memory 2.
Save to 7. The stored data can be output by the output unit 29 by selecting the output button 30g by the input unit 27.

For example, two months later, the input means 27
Is selected, and the table 30 in FIG. 11 is selected.
i, the past measurement results are displayed on the graph 30h, and the measurement conditions are read by the arithmetic processing / image processing unit 26. Next, the measurement button 30a in FIG. 11 is selected by the input means 27, and the collection efficiency is measured in the same manner. The measured result is added to the table 30i and the graph 30h in FIG. Save button 30f of FIG.
Is selected, and the measurement results of the collection efficiency are stored in the memory 27 together with the past measurement results.

As described above, if the collection efficiency is periodically measured and the tendency is to decrease, it can be determined that the performance of the detector has deteriorated. This is because the collection efficiency depends only on the arrangement of the detectors when the measurement conditions are the same, and thus does not change with time. For example, if the collection efficiency is reduced by 50% from the initially measured value, it may be a guide to replace the detector and its components.

In the above, the absolute value of the collection efficiency was obtained. However, if the relative evaluation is sufficient, a function representing the relationship between the S / N and the incident electron flow (FIG. 6) or the S corresponding to each incident electron flow is used. It is not necessary to store a table of / N in the memory 27, and the S / S of the secondary electron image measured after a certain time has elapsed.
It can be evaluated by the ratio of N to the S / N of the secondary electron image measured first.

When signal processing is performed by the electronic counting method, instead of a function or a table indicating the relationship between _{IPE and} S / N in the analog detection method, the relationship between the measured pulse count rate and the count loss due to the dead time (see FIG. The function or table indicating 4) is stored in the memory 27. The counting efficiency ε is also stored in the memory 27 in advance.

The measurement of the collection efficiency is performed according to the following procedure. The sample 4 is irradiated with the primary electron beam 2 and the secondary electrons 3
_Is measured, and the count rate n _SEm of the pulse signal obtained by detecting is measured, and the measured value is transmitted to the arithmetic processing / image processing unit 26. A function or a table indicating the relationship between the measured pulse count rate and the count loss due to the dead time is read from the memory 27 to the arithmetic processing / image processing unit 26, and the count rate n _SE obtained by correcting the count loss due to the dead time to n _SEm is calculated. I do. The background count rate n _b when the primary electron beam 2 is cut off is measured, the secondary electron flow I _SE incident on the detector 5 is obtained from n _SE −n _b , and the collection efficiency I _SE / δI _PE is calculated. . I _SE
Can be calculated from e (n _SE −n _b ) / ε. Otherwise, the same operation as in the analog detection method may be used.

If the relative evaluation is sufficient, it is not necessary to store in the memory 27 a function or table indicating the relationship between the measured pulse count rate and the count loss due to the dead time (FIG. 4). It can be evaluated by the ratio of the counting rate measured after the passage of time to the counting rate measured first.

Although the apparatus for measuring the collection efficiency of secondary electrons has been described in detail in the above embodiment, the present invention is also applicable to an apparatus for measuring the collection efficiency of backscattered electrons or secondary electrons + backscattered electrons. Applicable. The backscattering coefficient, or both, may be stored in the memory 27 instead of the secondary electron emission ratio.

When an insulating material is used, the energy E2 shown in FIG.
Is stored in the memory 27, and the acceleration voltage is set slightly higher than E2 by the acceleration voltage / probe current control unit 23.

Further, in an apparatus capable of directly injecting the primary electron beam into the detector, the counting efficiency can be measured and evaluated instead of the collection efficiency.

In recent years, the scanning electron microscope has a tendency to reduce the probe current in order to reduce the contamination. However, the optimum S / N that can be recognized as an image must be maintained. Therefore, an example of an apparatus for controlling the probe current so as to obtain an optimum S / N will be described below with reference to FIG.

First, the optimum S / N is stored in the memory 27. The optimal S / N is the minimum S / N that can be determined as an image, or the S / N that does not affect the resolution evaluation.
N or an S / N ratio that does not degrade the reproducibility of length measurement in an apparatus such as a length measuring SEM may be set according to the purpose. Next, the acceleration voltage and the probe current are set by the acceleration voltage / probe current control unit 23, and the scanning speed and the number of integrated images are set by the scanning control unit 17. When irradiating the sample 4 with the primary electron beam 2, the focus control unit 24 blurs the probe of the primary electron beam 2 in order to eliminate variations in the secondary electron emission ratio due to irregularities on the sample surface. The sample 4 is irradiated with the primary electron beam 2, the generated secondary electrons 3 are detected, and the S / N is determined from an image obtained. The S / N previously stored in the memory 27 is read out to the arithmetic processing / image processing unit 26, and an optimum probe current value is calculated by comparing the S / N with the obtained S / N. Set the probe current. If this method is used, an optimum S / N image can always be obtained no matter how the secondary electron emission ratio changes by changing the acceleration voltage of the primary electron beam 2 or changing the sample 4. .

Although the method for optimizing the probe current has been described above, the S / N also changes depending on the scanning speed and the number of integrated images. Therefore, the optimum S / N can be obtained even if the probe current, the scanning speed, and the number of integrated images are controlled independently or mutually.

In order to determine S / N from image data,
The DC offset of the output signal of the detector 5 must be adjusted to zero at the input of the AD converter 33 so that the maximum value of the output signal does not exceed the dynamic range of the AD converter 33. An example of an apparatus for solving this will be described below with reference to FIG. An offset value / peak value determining unit 32 is provided between an amplifier 31 and an AD converter 33 in a signal processing unit of a conventional scanning electron microscope. First, the DC offset component when the primary electron beam is cut off is determined by the offset value / peak value determination unit 32, and a signal is sent to the offset control unit 34 so that the offset becomes zero. Adjust the offset.
Next, the sample is irradiated with a primary electron beam, and the generated secondary electrons are detected. The maximum value of the signal after the output signal from the photomultiplier tube 12 is amplified by the amplifier 31 is determined by the offset value / peak value determination unit 32, and when the signal is saturated, the photomultiplier is reduced so that the gain is reduced. A signal is sent to the high voltage controller 35 of the multiplier 12 to lower the voltage applied to the photomultiplier 12. This allows accurate S / N of image data
Can be requested.

Although the embodiments of the present invention have been described above, the present invention can be applied to an apparatus using an ion beam instead of an electron beam.

[0041]

As described above in detail, according to the present invention, the counting efficiency including the noise of the secondary particle detector and its signal processing circuit can be obtained by directly irradiating the primary particle beam to the secondary particle detector. Can be evaluated. Therefore, collection efficiency and detection efficiency can be accurately evaluated. Further, by applying the electronic counting method, the noise of the secondary particle detector and its signal processing circuit can be ignored, and the collection efficiency and the detection efficiency can be evaluated more accurately. This provides a method for quantitatively evaluating the detection efficiency of a secondary particle detector system from image data and a pulse count rate obtained by an analog detection method and an electronic counting method, and a particle beam apparatus for realizing the method. be able to.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating a method for evaluating counting efficiency according to an example of an embodiment of the present invention.

FIG. 2 is a flowchart showing a method of evaluating collection efficiency using an analog detection method according to an example of an embodiment of the present invention.

FIG. 3 is a flowchart showing a method for evaluating collection efficiency using an electronic counting method according to an example of an embodiment of the present invention.

FIG. 4 is a diagram illustrating a relationship between a measured pulse count rate and a correction coefficient for counting loss due to dead time in one example of an embodiment of the present invention.

FIG. 5 is a diagram illustrating a relationship between incident energy to a detector and counting efficiency according to an example of an embodiment of the present invention.

FIG. 6 is a diagram showing a relationship between an electron flow incident on a detector and S / N in one example of an embodiment of the present invention.

FIG. 7 is a diagram showing a relationship between incident energy on a sample and an electron emission ratio of the sample in one example of an embodiment of the present invention.

FIG. 8 is a schematic diagram of a conventional electron microscope.

FIG. 9 is a diagram showing a method for directly injecting a primary electron beam to a detector according to an example of an embodiment of the present invention.

FIG. 10 is a schematic diagram of a scanning electron microscope according to an example of an embodiment of the present invention.

FIG. 11 illustrates an example of an operation display screen according to an example of an embodiment of the present invention.

FIG. 12 is a schematic diagram of a signal processing unit that adjusts a DC offset and a gain of a detector according to an example of an embodiment of the present invention.

[Explanation of symbols]

1 ... Electron gun, 2 ... Primary electron beam, 3 ... Secondary electron, 4 ...
Sample, 5: detector, 13: signal processing unit for analog detection method,
14 ... Electronic counting signal processing unit, 15 ... Image memory, 17
... Scan control unit, 20 reflector, 23 acceleration voltage / probe current control unit, 24 focus control unit, 25 signal processing unit, 26 arithmetic processing / image processing unit, 27 memory, 2
8 ... input means, 29 ... output means, 30 ... display means, 31
... Amplifier, 32 ... Offset value / peak value judgment unit, 33
... A / D converter, 34 ... Offset control unit, 35 ... High voltage control unit.

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Claims (23)

[Claims] 1. A primary particle beam whose probe current has been measured in advance is directly incident on a secondary particle detector, an output pulse count rate of the secondary particle detector is measured, and a counting loss correction is made to the measured output pulse count rate. Per unit time to the secondary particle detector that can be calculated from the count rate obtained by subtracting the background count rate when the primary particle beam is cut off from the number of output pulses corrected for the count loss and the probe current of the measured primary particle beam. A method for evaluating a secondary particle detector system, wherein a counting efficiency is determined by taking a ratio with respect to the number of particles incident on a secondary particle detector. 2. A primary particle beam whose probe current is measured in advance is directly incident on a secondary particle detector, and image data obtained by two-dimensional scanning on an incident surface of the secondary particle detector is acquired. The signal intensity distribution is created, the S / N is determined from the ratio of the average value of the distribution and the standard deviation of the distribution, and the S / N is determined by the above method by changing the probe current. The relationship between the number of particles incident per unit time and the S / N ratio of the image is determined. Next, the ratio of secondary particle emission is known with respect to the incident energy of the primary particle beam. The sample is irradiated with a primary particle beam whose probe current has been measured in advance, and image data is obtained when secondary particles generated during two-dimensional scanning on the standard sample surface are detected, and the signal intensity distribution for each pixel is obtained. Create distribution means and distribution markers S / N obtained from the ratio with the quasi-deviation,
The secondary particle flow incident on the secondary particle detector is obtained from the relationship between the number of particles incident on the secondary particle detector per unit time and the S / N ratio of the image, and the secondary particle flow and the secondary particles are emitted. A method for evaluating a secondary particle detector system, wherein the collection efficiency is determined from a ratio of a product ratio of a probe current and a product ratio of a probe current. 3. A secondary particle emission ratio is known with respect to the incident energy of the primary particle beam, and a standard sample having a partially flat surface is irradiated with a primary particle beam whose probe current is measured in advance to generate a secondary particle. The output pulse count rate of the secondary particle detector when secondary particles were detected was measured, the measured output pulse count rate was subjected to count loss correction, and the primary particle beam was cut off from the count loss corrected output pulse number. The secondary particle flow is determined by the ratio of the product of the secondary particle charge and the count rate obtained by subtracting the background count rate, and the counting efficiency, and the secondary particle flow, the secondary particle emission ratio, and the probe A method for evaluating a secondary particle detector system, wherein a collection efficiency is determined from a ratio of a current to a product. 4. The method for evaluating a secondary particle detector system according to claim 2, wherein the primary particles are released such that the ratio of secondary particles emitted by using an insulating material instead of the standard material is 1. A method for evaluating a secondary particle detector system, comprising setting an incident energy of a line and determining a collection efficiency from a ratio between a secondary particle flow and a probe current. 5. The method for evaluating a secondary particle detector system according to claim 4, wherein quartz is used as an insulating material. 6. The product of the counting efficiency according to claim 1 and the collecting efficiency according to claim 2, the product of the counting efficiency according to claim 1 and the collecting efficiency according to claim 3, or the counting efficiency according to claim 1. A method for evaluating a secondary particle detector system, wherein the detection efficiency of the secondary particle detector is determined by the product of the collection efficiencies according to claim 4. 7. The method for evaluating a secondary particle detector system according to claim 1, wherein the counting efficiency is periodically measured under the same measurement conditions, and the performance of the secondary particle detector is compared with the previously measured counting efficiency. A method for evaluating a secondary particle detector system, which comprises determining deterioration. 8. The method for evaluating a secondary particle detector system according to claim 2, wherein the collection efficiency is periodically measured under the same measurement conditions and compared with the previously measured collection efficiency. A method for evaluating a secondary particle detector system, comprising: determining deterioration in performance of a secondary particle detector by using the method. 9. A focusing means for focusing the primary particle beam on the sample, a means for scanning the primary particle beam irradiated on the sample in two dimensions, and an interaction between the primary particle beam and the sample. What is claimed is: 1. A particle beam apparatus provided with a detecting means for detecting secondary particles, comprising: means for directly entering a primary particle beam into a secondary particle detector. 10. The particle beam apparatus according to claim 9, wherein
A reflector is installed at or near the intersection between the orbit axis of the primary particle beam and the center axis of the secondary particle detector, and the primary particle beam is bent by applying a retarding voltage equal to or higher than the acceleration voltage of the primary particle beam. A particle beam apparatus comprising means for directly entering a primary particle beam into a secondary particle detector arranged on a side surface of a sample chamber, above an objective lens, or inside an objective lens. 11. The particle beam apparatus according to claim 10, further comprising an aluminum disk having a mirror-polished or vapor-deposited surface as a reflection plate. 12. The particle beam apparatus according to claim 9, wherein
The primary particle beam whose probe current was previously measured was directly incident on the secondary particle detector, the output pulse count rate of the secondary particle detector was measured, and the measured output pulse count rate was corrected for counting loss, and the counting loss was calculated. The number of particles incident on the secondary particle detector per unit time, which can be calculated from the corrected output pulse number minus the background count rate when the primary particle beam is cut off and the probe current of the measured primary particle beam And a means for determining the counting efficiency by taking the ratio of 13. The particle beam apparatus according to claim 9, wherein
The primary particle beam, whose probe current was measured in advance, was directly incident on the secondary particle detector, image data was obtained when two-dimensional scanning was performed on the incident surface of the secondary particle detector, and the signal intensity distribution for each pixel was obtained. S / N is calculated from the ratio between the average value of the distribution and the standard deviation of the distribution, and the S / N is calculated by the above method by changing the probe current, and the S / N is incident on the secondary particle detector per unit time. The relationship between the number of particles obtained and the S / N ratio of the image is determined. Next, the rate at which the secondary particles are emitted is known with respect to the incident energy of the primary particle beam. Irradiate the primary particle beam measured in advance, acquire image data when detecting secondary particles generated when performing two-dimensional scanning on the standard sample surface, create a signal intensity distribution for each pixel, and The ratio of the mean to the standard deviation of the distribution The secondary particle flow incident on the secondary particle detector is determined from the S / N determined from the above and the relationship between the number of particles incident on the secondary particle detector per unit time and the S / N ratio of the image. A particle beam apparatus comprising: means for determining collection efficiency based on a ratio of a particle flow and a ratio of a product of a secondary particle emission rate and a probe current. 14. The particle beam apparatus according to claim 9, wherein
The secondary particle emission rate is known with respect to the incident energy of the primary particle beam, and the secondary particle generated by irradiating a standard sample with a partially flat surface to the primary particle beam whose probe current is measured in advance is detected. The output pulse count rate of the secondary particle detector at the time of the measurement is measured, the measured output pulse count rate is corrected for the count loss, and the background count when the primary particle beam is cut off from the output pulse number corrected for the count loss is calculated. The secondary particle flow is determined by the ratio of the product of the charge of the secondary particles and the counting efficiency after subtracting the ratio, and the counting efficiency, and the secondary particle flow, the product of the ratio of releasing the secondary particles and the product of the probe current are calculated. A particle beam apparatus comprising means for determining a collection efficiency from a ratio. 15. The particle beam device according to claim 13 or 14, wherein the incident energy of the primary particle beam is set so that the ratio of secondary particle emission using an insulating material instead of the standard material is 1. And a means for determining the collection efficiency from the ratio between the secondary particle flow and the probe current. 16. The particle beam apparatus according to claim 12, wherein quartz is provided as an insulating material. 17. The counting efficiency according to claim 12 and claim 1.
The secondary particle detector is a product of the collection efficiency according to claim 3, a product of the counting efficiency according to claim 12 and the collection efficiency according to claim 14, or a product of the counting efficiency according to claim 12 and the collection efficiency according to claim 15. A particle beam apparatus comprising means for determining the detection efficiency of a particle beam. 18. The particle beam apparatus according to claim 12, wherein a means for periodically measuring the counting efficiency under the same measurement conditions and comparing the counting efficiency with the previously measured counting efficiency to judge the performance deterioration of the secondary particle detector. A particle beam device, comprising: 19. The secondary particle detector according to claim 13, wherein the collection efficiency is periodically measured under the same measurement conditions and compared with the previously measured collection efficiency. A particle beam apparatus comprising means for determining performance degradation of a particle beam. 20. Focusing means for focusing the primary particle beam on the sample, means for scanning the primary particle beam irradiated on the sample in two dimensions, and interaction between the primary particle beam and the sample. In a particle beam apparatus provided with a detecting means for detecting secondary particles and a means for displaying an image or a screen for operating the apparatus, the relationship between the passage of time and the collection efficiency on the operation screen, or the passage of time and S A particle beam apparatus comprising means for displaying a table or a graph showing the relationship between / N or the passage of time and the output pulse count rate. 21. The particle beam apparatus according to claim 9, further comprising means for displaying an image or a screen for operating the apparatus, wherein the relation between the passage of time and the efficiency of counting efficiency or the passage of time on the operation screen. A table showing the relationship of the output pulse count rate,
Alternatively, a particle beam apparatus comprising means for displaying a graph. 22. The particle beam apparatus according to claim 9,
A particle beam apparatus comprising means for adjusting a probe current, a scanning speed, and the number of integrated images of a primary particle beam individually or in combination to always obtain an image having an optimum S / N. 23. The particle processing apparatus according to claim 9, further comprising means for adjusting a DC offset of an output signal of the secondary particle detector and a gain of the secondary particle detector. Means for adjusting the dc offset of the output signal from the secondary particle detector to zero at the input of the analog-to-digital converter so that the output signal falls within the dynamic range of the analog-to-digital converter. A particle beam apparatus comprising means for adjusting a gain of a detector.