Electron microscope, electron-photon correlation measurement device, and electron-photon correlation measurement method

Abstract

An electron microscope of an embodiment according to the present invention is provided with: an electron gun for irradiating electrons onto a sample; an electron detector for detecting electrons irradiated onto the sample; a photon detector for detecting photons emitted from the sample when electrons are irradiated onto the sample; and, a computing unit that, on the basis of the time when the electron detector detects electrons and the time when the photon detector detects photons, computes, for each detected photon, a time difference between the time when electrons are irradiated onto the sample and the time when the photon is emitted from the sample, and computes an electron-photon time correlation indicating the distribution of the time difference.

Description

[Technical Field]

[0001] The present invention relates to an electron microscope, an electron-photon correlation measurement device, and an electron-photon correlation measurement method. [Background technology]

[0002] Fluorescence lifetime microscopes, which measure the fluorescence lifetime of excited substances and living organisms, are capable of microscopically visualizing substances and living organisms, and are therefore an important tool for elucidating the internal structure and function of substances and living organisms. However, due to the diffraction limit of the irradiating light, fluorescence lifetime microscopes cannot obtain information below 100 nm, which is particularly important for elucidating the internal structure and function of substances and living organisms.

[0003] Meanwhile, attempts have been made to improve spatial resolution by using electron microscopes in cathodoluminescence (CL) spectroscopy, which involves irradiating a sample with a pulsed electron beam and measuring the spectral characteristics of the emitted light to identify the sample. For example, Non-Patent Document 1 describes a transmission electron microscope (TEM) that irradiates a sample with a pulsed electron beam using a field emission electron gun (FE electron gun). Also, Non-Patent Document 2 describes a scanning electron microscope (SEM) that irradiates a sample with a pulsed electron beam using an FE electron gun. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Makoto Kuwahara, et al. "The Boersch effect in a picosecond pulsed electron beam emitted from a semiconductor photocathode", Appl. Phys. Lett. 109, 013108 (2016); https: / / doi.org / 10.1063 / 1.4955457. [Non-patent document 2] S. Meuret, et al. "Complementary cathodoluminescence lifetime imaging configurations in a scanning electron microscope", Ultramicroscopy 197 (2019) 28-38; https: / / doi.org / 10.1016 / j.ultramic.2018.11.006. Summary of the Invention [Problem to be solved by the invention]

[0005] The electron microscopes described in Non-Patent Documents 1 and 2 use a pulsed electron gun, and the pulsed electron beam irradiating the sample spreads spatially and energetically due to an effect known as the space charge effect, making it extremely difficult to improve the spatial resolution beyond 10 nm.

[0006] An object of the present invention is to provide an electron microscope having high spatial resolution. [Means for solving the problem]

[0007] An electron microscope according to an embodiment of the present invention comprises an electron gun that irradiates a sample with electrons; an electron detector that detects the electrons irradiated onto the sample; a photon detector that detects photons emitted from the sample when the electrons are irradiated onto the sample; and a calculator that calculates, for each detected photon, the time difference between the time when the electron is irradiated onto the sample and the time when the photon is emitted from the sample based on the time when the electron detector detects the electron and the time when the photon detector detects the photon, and calculates an electron-photon time correlation that indicates the distribution of the time differences.

[0008] In the above electron microscope, the electron detector preferably detects electrons that have been transmitted through or reflected from the sample.

[0009] In the above electron microscope, the computing unit preferably has a discrimination unit that discriminates the specimen based on the electron-photon time correlation.

[0010] In the above electron microscope, it is preferable that the discrimination unit calculates the fluorescence lifetime or luminescence transition probability of the sample from the electron-photon time correlation, and discriminates the sample based on the fluorescence lifetime or luminescence transition probability.

[0011] In the above-described electron microscope, it is preferable that the discrimination unit calculates a curve indicating the frequency at which photons are detected with a time difference when electrons are detected from the electron-photon time correlation, calculates the luminescence transition probability of the sample from the peak value at the origin of the curve, and calculates the time required for the value on the curve to decrease by a factor of 1 / e as the fluorescence lifetime of the sample.

[0012] In the above-mentioned electron microscope, it is preferable that the calculator has a memory unit that stores parameter values ​​that indicate characteristics of the electron-photon time correlation that have been measured in advance for one or more known substances or living organisms, and the discrimination unit calculates the degree of agreement between the calculated parameter value of the electron-photon time correlation of the sample and the parameter value of the electron-photon time correlation of the known substance or living organism, and discriminates the known substance or living organism having the parameter value that has the highest degree of agreement and exceeds a predetermined threshold as being indicative of the substance or living organism of the sample.

[0013] In the above-mentioned electron microscope, it is preferable that the electron detector has a light-emitting body that reacts with electrons irradiated onto the sample to emit photons, and a photon detection unit that detects the photons emitted from the light-emitting body and outputs a detection signal to a calculator.

[0014] In the above-mentioned electron microscope, it is preferable that the photon detector also serves as a photon detection section of the electron detector, detects photons emitted from the sample and photons emitted from the light-emitting body, and outputs a detection signal to a computing unit.

[0015] In the above electron microscope, it is preferable that the electron gun irradiates the sample with electrons while spatially scanning them, and the calculator has an image generating unit that generates an image of the sample based on spatial changes in electron-photon time correlation.

[0016] An electron-photon correlation measurement device according to an embodiment of the present invention includes an electron detector that detects electrons irradiated onto a sample from an electron gun of an electron microscope; a photon detector that detects photons emitted from the sample when the electrons are irradiated onto the sample; and a calculator that calculates, for each detected photon, the time difference between the time when the electron is irradiated onto the sample and the time when the photon is emitted from the sample based on the time when the electron detector detects the electron and the time when the photon detector detects the photon, and calculates an electron-photon time correlation that indicates a distribution of the time differences.

[0017] An electron-photon correlation measurement method according to an embodiment of the present invention includes irradiating a sample with electrons, detecting the electrons irradiated onto the sample, detecting photons emitted from the sample when the electrons are irradiated onto the sample, calculating, for each detected photon, the time difference between the time when the electrons are irradiated onto the sample and the time when the photons are emitted from the sample based on the time when the electrons are detected and the time when the photons are detected, and calculating an electron-photon time correlation that indicates a distribution of the time differences. [Effects of the Invention]

[0018] According to the present invention, an electron microscope having high spatial resolution is provided. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of an electron microscope and an electron-photon correlation measurement device according to the first embodiment. [Figure 2A] FIG. 2A is a diagram schematically showing the electron-photon time correlation calculated by the electron microscope of the first embodiment. [Figure 2B] FIG. 2B is a diagram showing an example of electron-photon time correlation obtained by actual measurement using the electron microscope of the first embodiment. [Figure 3] FIG. 3 is a diagram schematically showing the photon-photon time correlation calculated by the electron microscope of the first embodiment. [Figure 4] FIG. 4 is a flowchart that schematically shows the electron-photon correlation measurement method of the first embodiment. [Figure 5] FIG. 5 is a diagram showing a schematic configuration of an electron microscope and an electron-photon correlation measurement device according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0020] The electron microscope of the embodiment calculates the time difference between the time when an electron passes through a sample and the time when a photon is emitted from the sample for each detected photon, and calculates the electron-photon time correlation, which indicates the distribution of these time differences. The detection system of this electron microscope is composed of a passive detection unit and a calculator that measure the time difference in response to the detection of an electron that has passed through the sample, so that it is not necessary to use a pulsed electron beam, and it is possible to realize an electron microscope with a high spatial resolution of less than 10 nm.

[0021] Furthermore, unlike electron beam excitation emission spectroscopy, which measures spectral characteristics, this electron microscope distinguishes samples based on electron-photon time correlation, which includes information such as the fluorescence lifetime of excited substances and living organisms. This not only allows for the microscopic visualization of samples, but also allows for the acquisition of information such as the temperature of the sample's environment.

[0022] Furthermore, because this electron microscope does not require a large-scale, expensive pulsed electron gun, it can be simplified and reduced in cost, and a luminescence lifetime measurement electron microscope with high spatial resolution can be realized simply by adding a passive detection system to an existing electron microscope body. Therefore, this passive detection system can also be provided as an electron-photon correlation measurement device, which is an attachment to be added to an existing electron microscope.

[0023] Preferred embodiments will be described below with reference to the drawings. Note that the present invention is not limited to the following embodiments and can be modified as appropriate without departing from the spirit of the invention. In addition, in each drawing, parts having the same or equivalent functions are given the same reference numerals, and their description may be omitted or simplified.

[0024] (First embodiment) 1 is a diagram showing a schematic configuration of an electron microscope 1 and an electron-photon correlation measurement device 7 of the first embodiment. The electron microscope 1 comprises an electron gun 2, an electron detector 3, a photon detector 4, and a computing unit 5. The electron microscope 1 can also be realized by adding an electron-photon correlation measurement device 7, which is a passive detection system, to an existing electron microscope main body 8 comprising the electron gun 2 and a chamber.

[0025] In order to improve the linearity of the electrons 20 irradiated from the electron gun 2 toward the sample 6, the electron gun 2 and the sample 6 are placed in an evacuated chamber of the electron microscope main body 8, as shown in FIG. 1. The electron detector 3 is placed on the opposite side of the sample 6 from the electron gun 2 so as to be able to detect electrons 30 that have passed through the sample 6. The photon detector 4 is placed so as to be able to detect photons 40 emitted from the sample 6 through a photon detection window 82 provided in the wall of the chamber. The computing unit 5 is connected to at least the electron detector 3 and the photon detector 4 so as to be able to communicate with them via wire or wirelessly.

[0026] The electron gun 2 irradiates the sample 6 with electrons 20. In order to suppress the space charge effect, the electron gun 2 does not irradiate a pulsed electron beam in which a large number of electrons 20 are compressed in time, but irradiates a continuous electron beam of electrons 20 spontaneously emitted from an electron source at random intervals, one by one, toward the sample 6. As the electron gun 2 that irradiates such a continuous electron beam, any of an FE electron gun, a thermionic electron gun, a Schottky electron gun, etc. can be used, but it is preferable to use an FE electron gun, which has excellent spatial resolution.

[0027] 1, the electron gun 2 may further include an electron focusing lens 21, an electron scanning deflector 22, and a scan control unit 23. The electron focusing lens 21 has a coil, and a magnetic field generated by a current flowing through the coil spatially focuses the quantum mechanical wave packets of the electrons 20 by a lens effect. The electron scanning deflector 22 also has a coil, and the current flowing through the coil is controlled by the scan control unit 23. The magnetic field generated by the current flowing through the coil acts on the electrons 20, thereby spatially scanning the position coordinates of the electrons 20 irradiated onto the sample 6.

[0028] The electron detector 3 detects electrons 30 that have passed through the sample 6 among the electrons 20 emitted from the electron gun 2. The electron detector 3 has a time resolution that allows it to detect each electron 30 that has passed through the sample 6. The electron detector 3 with such high time resolution is configured, for example, as shown in FIG. 1 , using a light emitter 31 and a photon detector 32. The light emitter 31 is a scintillator that reacts with the electrons 30 that have passed through the sample 6 to emit photons 33. The light emitter 31 may be a luminescent semiconductor material such as a metal halide perovskite that has high luminescence efficiency and reacts with the electrons 30 to emit photons 33 in a short time, or a luminescent material of a transition metal complex such as YSO:Ce (cerium-doped yttrium silicate). The photon detector 32 detects the photons 33 emitted from the light emitter 31 through a photon detection window 81 provided in the wall of the chamber and outputs a signal indicating that the electrons 30 have been detected to the calculator 5. As the photon detector 32, an avalanche photodiode or a photomultiplier tube, which can efficiently detect a very small number of photons 33, is used.

[0029] The photon detector 4 detects photons 40 emitted from the sample 6 when the electrons 30 pass through the sample 6. The photon detector 4 has a time resolution capable of detecting each photon 40 emitted from the sample 6. As the photon detector 4 with such high time resolution, an avalanche photodiode or a photomultiplier tube capable of detecting each photon 40 is used, similar to the photon detection unit 32 of the electron detector 3. The photon detector 4 may include multiple avalanche photodiodes or photomultiplier tubes. Each time the photon detector 4 detects each photon 40 emitted from the sample 6, the photon detector 4 outputs a signal indicating the detection of the photon 40 to the calculator 5. The photon detector 4 may have a bandpass filter or polarizer in its upstream stage that passes only photons 40 in a desired wavelength band.

[0030] 1, the photon detector 4 may have a photon reflecting mirror 41 in order to efficiently detect a plurality of photons 40 emitted from the sample 6. The photon reflecting mirror 41 has, for example, a parabolic shape, and reflects, toward the photon detector 4, photons 40 emitted from the sample 6 placed at the focal position that are emitted in the opposite direction to the photon detector 4. The photon detector 4 may further have a photon focusing lens 42. The photon focusing lens 42 spatially focuses the photons 40 emitted from the sample 6 and guides them to the photon detector 4.

[0031] The photon detector 4 may also serve as the photon detection unit 32 of the electron detector 3. In this case, the photon detection window 81 and the photon detection window 82 of the chamber of the electron microscope main body 8 may be provided close to each other or may be shared, and the light emitter 31 may serve as a substrate for mounting the sample 6. The photon detector 4 detects photons 40 emitted from the sample 6 and photons 33 emitted from the light emitter 31 and outputs a detection signal to the computer 5. The photon detector 4 may have a filter that passes only one of the photons 40 and the photons 33 and a filter that passes only the other, in order to separately detect the photons 40 emitted from the sample 6 and the photons 33 emitted from the light emitter 31. In this case, the photon detector 4 may have a first detection unit for detecting the photons 40 emitted from the sample 6 and a second detection unit for detecting the photons 33 emitted from the light emitter 31. This simplifies the electron microscope 1.

[0032] The computing unit 5 includes a processor, memory, a communication I / F, etc. (not shown). A PC (Personal Computer) or the like is used as the computing unit 5. The processor is an example of a discrimination unit and an image generation unit, and includes one or more arithmetic circuits and their peripheral circuits. The memory is an example of a storage unit, and includes a storage medium such as an HDD (Hard Disk Drive), an optical recording medium, a semiconductor memory such as a RAM (Random Access Memory) or a ROM (Read Only Memory), or a combination of these. The communication I / F (Interface) connects the processor to the electron detector 3, the photon detector 4, and the scan control unit 23 of the electron gun 2 via wire or wirelessly, enabling the processor to communicate with the electron detector 3, the photon detector 4, and the scan control unit 23 of the electron gun 2.

[0033] The calculator 5 calculates, for each detected photon 40, the time difference between the time when the electron 30 passes through the sample 6 and the time when the photon 40 is emitted from the sample 6, based on the time when the electron detector 3 detects the electron 30 and the time when the photon 40 is detected by the photon detector 4. Then, the calculator 5 calculates the electron-photon time correlation, which indicates the distribution of the calculated time differences, by the procedure described below.

[0034] FIG. 2A is a diagram schematically showing the electron-photon time correlation calculated by the electron microscope 1 of the first embodiment.

[0035] 2A(a) schematically shows an example of an electron detection signal that the calculator 5 receives from the electron detector 3 via the communication I / F when a first electron passes through the sample 6, and an example of a photon detection signal that the calculator 5 receives from the photon detector 4 via the communication I / F when a second electron passes through the sample 6. Similarly, FIG. 2A(b) schematically shows an example of an electron detection signal that the calculator 5 receives from the electron detector 3 via the communication I / F, and an example of a photon detection signal that the calculator 5 receives from the photon detector 4 when a second electron passes through the sample 6. The horizontal axes of FIGS. 2A(a) and 2A(b) indicate time t, and each pulse signal indicates that one electron 30 or one photon 40 has been detected.

[0036] 2A(a) and (b), when the electrons 30 pass through the sample 6, the computer 5 first receives from the electron detector 3 a signal indicating that the electrons 30 that have passed through the sample 6 have been detected. At the same time, the computer 5 receives from the photon detector 4 signals indicating that photons 40 of the group of photons emitted as a result of the electrons 30 passing through the sample 6 have been detected. Thereafter, the computer 5 receives from the photon detector 4 signals indicating that photons 40 of the group of photons spontaneously emitted from atoms in an excited state in the sample 6, with the frequency of these signals decreasing as time passes since the electrons 30 passed through the sample 6.

[0037] The calculator 5 calculates the time difference τ = t - t0 from the time t0 when the electron detector 3 detects the electron 30 to the time t when the photon detector 4 detects the photon 40 for each detected photon 40. The calculator 5 calculates the time difference τ for each detected photon 40 each time the electron 30 (first electron, second electron, ...) passes through the sample 6, until the number of photons 40 necessary to calculate the electron-photon time correlation satisfying the required accuracy is detected. Here, the time interval from when the electron gun 2 irradiates the first electron to when it irradiates the next second electron is adjusted in advance depending on the expected fluorescence lifetime of the sample 6 to be discriminated, etc.

[0038] The calculator 5 can calculate the time difference τ by digital processing based on the time when the electron detector 3 detects the electron 30 and the time when the photon detector 4 detects the photon 40, but it can also calculate it by analog processing. In this case, the calculator 5 uses, for example, an analog correlator with excellent time resolution, such as a time-to-amplitude converter, to calculate the time when the photon detector 4 detects the photon 40 as the time difference τ, triggered by the detection of the electron 30 by the electron detector 3.

[0039] There is a time difference between the response time from when the electron detector 3 detects the electron 30 until it outputs a signal and the response time from when the photon detector 4 detects the photon 40 until it outputs a signal. Therefore, a delay line may be inserted between the electron detector 3 and the calculator 5 or between the photon detector 4 and the calculator 5 to offset this response time difference.

[0040] The calculator 5 then calculates an electron-photon time correlation curve g showing the distribution of the time difference τ shown in Fig. 2A(c) from the time difference τ between the time when the electron 30 passes through the sample 6 and the time when the photon 40 is emitted from the sample 6. The horizontal axis of Fig. 2A(c) represents the time difference τ, and the vertical axis of Fig. 2A(c) represents the frequency with which the photon 40 is detected at the time difference τ when the electron 30 is detected.

[0041] The curve g of the electron-photon time correlation typically has a waveform that is roughly an exponential function. Therefore, the curve g of the electron-photon time correlation is approximated by the least squares method or the like, for example, using parameter values ​​(γ, τ0) that indicate the characteristics of the curve g, as shown in the following formula (1). Note that the curve g in FIG. 2A shows values ​​obtained by excluding the non-correlated value (=1) in the following formula (1).

number

[0042] Since the electron-photon time correlation calculated in this manner is specific to the material and structure of the sample 6, the discrimination unit of the calculator 5 can compare the calculated electron-photon time correlation of the sample 6 with known electron-photon time correlation samples to discriminate the sample 6. To this end, the memory unit of the calculator 5 stores, for example, parameter values ​​(γ', τ') that indicate the characteristics of the electron-photon time correlation curve g measured in advance for one or more known materials and living organisms. The discrimination unit of the calculator 5 then calculates the degree of agreement d between the parameter value of the electron-photon time correlation and the parameter value of the calculated electron-photon time correlation for each known material or living organism using the following equation (2): where k1 and k2 are weighting coefficients for each parameter and are set appropriately. 1 / d=k1(γ-γ')2 +k2(τ0-τ0') 2 +1 (2)

[0043] For example, when the weighting coefficients k1 and k2 are both 1, the degree of match d has a minimum value of 0 and a maximum value of 1. The discrimination unit of the calculator 5 can discriminate, among known substances, living organisms, etc., those having a parameter value where the degree of match d with the calculated parameter value of the sample 6 exceeds a predetermined threshold and is the maximum, as indicating the substance, living organism, etc. of the sample 6. Here, the predetermined threshold is, for example, determined in advance by actual measurement or the like as the lower limit of the degree of match d at which the substance, living organism, etc. of the sample 6 to be compared can be considered to be the same as the known substance, living organism, etc.

[0044] The calculator 5 can calculate γ as a parameter value indicating the characteristics of the curve g, for example, from the peak value at the origin (τ=0) of the electron-photon time correlation curve g, and can calculate the radiative transition probability of the sample 6 from γ. Furthermore, the calculator 5 can calculate, as another parameter value, the time τ0 required for the value of the electron-photon time correlation curve g to decrease to 1 / e times the baseline 1, as the fluorescence lifetime τ0 of the sample 6. Because the fluorescence lifetime τ0 also includes environmental information such as the temperature of the sample 6, information such as the environmental temperature of the sample 6 can be obtained based on the deviation from the value of the fluorescence lifetime τ0 at a known temperature measured in advance. Furthermore, the radiative transition probability can be calculated from γ, which is a parameter value that cannot be measured using a fluorescence lifetime microscope that irradiates light, and therefore may reveal previously unseen information about substances and living organisms. Note that characteristic values ​​other than the fluorescence lifetime τ0 and γ may also be used as parameter values ​​indicating the characteristics of the curve g.

[0045] 2B shows an example of electron-photon time correlation obtained by actual measurement using the electron microscope of the first embodiment. The electron-photon time correlation shown in FIG. 2B was measured using nanodiamond as the sample 6 and YSO:Ce as the light emitter 31 that reacts with electrons 30 transmitted through the nanodiamond to emit photons 33. This measurement was performed by irradiating the sample 6 with electrons 20 at a current value of about 2.5 pA at room temperature for about 1000 seconds until an electron-photon time correlation curve g with the required accuracy was obtained.

[0046] The horizontal axis of Fig. 2B represents the time difference τ = t - t0 from the reference time t0 when the photon detection unit 32 of the electron detector 3 detects the photon 33 to the time t when the photon detector 4 detects the photon 40. The vertical axis of Fig. 2B represents the measured value of the frequency at which the photon 40 is detected with the time difference τ when the photon 33 is detected, and shows a value that is larger than the vertical axis of the electron-photon time correlation shown in Fig. 2A by the non-correlation value (= 1).

[0047] The electron-photon time correlation curve g shown in FIG. 2B differs from the electron-photon time correlation curve g shown in FIG. 2A in that the curve g is measured even in the negative region of the time difference τ. This is because the time difference τ on the horizontal axis in FIG. 2A uses the time at which the electron 30 is detected as the reference time, whereas the time difference τ on the horizontal axis in FIG. 2B uses the time at which the photon 33 is detected as the reference time. The electron 30 is indirectly detected via the photon 33, as shown in FIG. 1. However, since the luminescent material 31 used as the scintillator, such as YSO:Ce, generally has a fluorescence lifetime, the photon detection unit 32 of the electron detector 3 detects a photon spontaneously emitted from an atom of the luminescent material 31 in an excited state after the photon detector 4 detects the photon 40. In this case, the time difference τ = t - t from the reference time t at which the photon 33 is detected to the time t at which the photon 40 is detected, i.e., the time difference τ = t - t, is negative. That is, the curve g in the region where the time difference τ is negative does not represent the characteristics of the nanodiamond that is the sample 6, but rather the characteristics of the YSO:Ce light emitter 31. The electron-photon time correlation measured using the electron microscope 1 is characterized by the fact that asymmetric data is obtained between the regions where the time difference τ is positive and negative.

[0048] Figure 2B shows a schematic representation of the actual measured values, and although it is not precisely fitted, the electron-photon time correlation curve g shown in Figure 2B gives the following parameter values ​​that indicate the characteristics of nanodiamond, sample 6: fluorescence lifetime τ0 = approximately 20 ns, γ = approximately 0.6. When calculating the fluorescence lifetime τ0 and γ in Figure 2B, the photon detection frequency is calculated by subtracting the uncorrelated value (= 1).

[0049] Furthermore, the scan control unit 23 of the electron gun 2 irradiates the sample 6 with electrons 20 while spatially scanning them, allowing the computer 5 to generate an image of the sample 6 based on the spatial change in the electron-photon time correlation. To this end, when calculating the electron-photon time correlation, the computer 5 receives, for example, from the scan control unit 23 via the communication I / F, the position coordinates on the sample 6 where the electron gun 2 irradiates the electrons 20, and stores the received position coordinates and the calculated electron-photon time correlation data in a memory unit in association with each other. Then, upon completing measurement of the measurement range of the sample 6, the computer 5 can calculate the feature value of the electron-photon time correlation corresponding to the position coordinates on the sample 6 and generate an image of the measurement range of the sample 6. Information such as defect structures of the sample 6 can be obtained from the image of the sample 6 obtained in this manner.

[0050] Furthermore, the electron microscope 1 may distinguish the sample 6 by combining the photon-photon time correlation described below in addition to the electron-photon time correlation shown in FIG. 2A.

[0051] FIG. 3 is a diagram showing a schematic diagram of the photon-photon time correlation calculated by the electron microscope 1 of the first embodiment.

[0052] 3(a) and (b) schematically show examples of photon detection signals that the calculator 5 receives from the photon detector 4 via the communication I / F. The horizontal axis of FIG. 3(a) and (b) indicates time t, and each pulse signal indicates that one photon 40 has been detected.

[0053] The photon-photon time correlation is measured using a photon detector 4 having a detection unit such as at least two avalanche photodiodes or photomultiplier tubes. When the electrons 30 pass through the sample 6, the computer 5 receives from the photon detector 4 a plurality of signals indicating that photons 40 of the photon group emitted from the sample 6 as a result of the electrons 30 passing through the sample 6 have been detected, as shown in (a) and (b) of Figure 3 .

[0054] The calculator 5 calculates the time difference τ = ti - tj between the time ti when the photon detector 4 detects the first photon and the time tj when the photon detector 4 detects the second photon, for each combination of a first photon detected by one detection unit and a second photon detected by another detection unit of the photon detector 4. The calculator 5 calculates the time difference τ for each combination of a pair of a first photon and a second photon in each detected photon group (first photon group, second photon group, ...) until a number of photons 40 necessary for calculating a photon-photon time correlation satisfying the required accuracy have been detected.

[0055] The calculator 5 then calculates the photon-photon time correlation curve g, which indicates the distribution of the time difference τ shown in FIG. 3(c). (2) The horizontal axis of (c) of Fig. 3 indicates the time difference τ, and the vertical axis of (c) of Fig. 3 indicates the frequency with which another photon is detected after the time difference τ when one photon is detected at time t. The photon-photon time correlation curve g (2) is generally calculated using the second-order autocorrelation function shown in the following equation (3), where <> represents the average of the incident light intensity I over time t.

number

[0056] The photon-photon time correlation calculated in this way is also specific to the material and structure of the sample 6. In particular, if the sample 6 is made of a material that simultaneously emits photons 40 when electrons 30 pass through the sample 6, the photon-photon time correlation curve g can be calculated without using the detection of the electrons 30 by the electron detector 3 as a trigger. (2)It can be seen that the position of the peak of the electron-photon time correlation curve g (2) From this, the fluorescence lifetime or luminescence transition probability of the sample 6 can be calculated in the same manner as in the case of the electron-photon time correlation curve g. By combining the electron-photon time correlation curve g and the fluorescence lifetime or luminescence transition probability obtained from the electron-photon time correlation curve, the discrimination unit of the calculator 5 can discriminate the sample 6 with higher accuracy.

[0057] As shown in FIG. 1 , the electron-photon correlation measurement device 7, which is a passive detection system, can also be provided as an attachment to an existing electron microscope main body 8. The electron-photon correlation measurement device 7 includes an electron detector 3, a photon detector 4, and a computing unit 5. The electron detector 3 and the photon detector 4 may be housed in a housing (not shown) and configured to receive external power via a power terminal provided on the housing. In this case, the housing is attached to the existing electron microscope main body 8 using a jig or the like so that the electron detector 3 can detect electrons 30 that have passed through the sample 6 and the photon detector 4 can detect photons 40 emitted from the sample 6 through a photon detection window 82. Furthermore, the photon detection unit 32 of the electron detector 3 may also serve as the photon detector 4. In this case, the photon detection windows 81 and 82 of the chamber of the existing electron microscope main body 8 are provided nearby or are shared.

[0058] The existing electron microscope main body 8 may be either a transmission electron microscope or a scanning electron microscope, and since a large-scale and expensive pulsed electron gun is not required, the electron-photon correlation measurement device 7 can be incorporated as an attachment into most common existing electron microscope main bodies 8. Therefore, a fluorescence lifetime microscope with excellent spatial resolution can be realized simply and at low cost.

[0059] FIG. 4 is a flowchart that schematically shows the electron-photon correlation measurement method of the first embodiment.

[0060] In step S1, the electron gun 2 irradiates the sample 6 with electrons 20.

[0061] In step S2, the electron detector 3 detects the electrons 30 that have irradiated the sample 6.

[0062] In step S3, the photon detector 4 detects photons 40 emitted from the sample 6 when the electrons 30 irradiate the sample 6.

[0063] In step S4, the calculator 5 calculates the time difference between the time when the electron 30 is irradiated onto the sample 6 and the time when the photon 40 is emitted from the sample 6 for each detected photon 40, based on the time when the electron detector 3 detects the electron 30 and the time when the photon detector 4 detects the photon 40.

[0064] In step S5, the calculator 5 determines whether or not a predetermined number of photons 40 necessary for calculating the electron-photon time correlation with the required accuracy have been detected. If the predetermined number of photons 40 has not been detected (No in step S5), the calculator 5 repeats steps S1 to S5.

[0065] If a predetermined number of photons 40 are detected (Yes in step S5), in step S6, the calculator 5 calculates the electron-photon time correlation, which indicates the distribution of time differences.

[0066] (Second embodiment) FIG. 5 is a diagram schematically illustrating the configuration of an electron microscope 1b and an electron-photon correlation measurement device 7b according to a second embodiment. The electron microscope 1 shown in FIG. 1 is a transmission electron microscope that detects electrons 30 that have passed through a sample 6, among the electrons 20 emitted from an electron gun 2. However, the electron microscope 1b shown in FIG. 5 is a scanning electron microscope that detects electrons 30 that have been irradiated onto and reflected by the sample 6, among the electrons 20 emitted from the electron gun 2. The electron microscope 1b can also be realized by adding an electron-photon correlation measurement device 7b, which is a passive detection system, to an existing scanning electron microscope main body 8b that includes an electron gun 2 and a chamber. The differences from the electron microscope 1 shown in FIG. 1 will be described below.

[0067] The electron detector 3 detects, among the electrons 20 emitted from the electron gun 2, electrons 30 that are irradiated onto the sample 6 and reflected by the sample 6, or secondary electrons emitted from the sample 6. For this purpose, the electron detector 3 is arranged on the same side of the sample 6 as the electron gun 2, and is positioned so as to be able to detect the electrons 30 that are irradiated onto the sample 6 and reflected by the sample 6, or secondary electrons emitted from the sample 6. For each detected photon 40, the calculator 5 calculates the time difference between the time when the electron 30 is reflected off the sample 6 or the time when the secondary electron is emitted from the sample 6 and the time when the photon 40 is emitted from the sample 6. Such a configuration of the scanning electron microscope shown in FIG. 5 also makes it possible to realize an electron microscope, an electron-photon correlation measurement device, and an electron-photon correlation measurement method having a high spatial resolution of less than 10 nm.

[0068] As described above, an electron microscope according to an embodiment includes an electron gun that irradiates a sample with electrons, an electron detector that detects the electrons irradiated onto the sample, a photon detector that detects photons emitted from the sample when the electrons irradiate the sample, and a calculator that calculates, for each detected photon, the time difference between the time when the electron irradiated the sample and the time when the photon was emitted from the sample based on the time when the electron detector detected the electron and the time when the photon detector detected the photon, and calculates an electron-photon time correlation that indicates a distribution of the time differences. This provides an electron microscope, an electron-photon correlation measurement device, and an electron-photon correlation measurement method with high spatial resolution of less than 10 nm.

[0069] The above-described embodiments are merely examples of specific implementations of the present invention, and the technical scope of the present invention should not be construed as being limited by these embodiments. In other words, the present invention can be implemented in various forms without departing from its technical concept or main features.

Claims

1. an electron gun for irradiating electrons onto the sample; an electron detector that detects the electrons irradiated onto the sample; a photon detector that detects photons emitted from the sample when the electrons are irradiated onto the sample; a calculator that calculates, for each detected photon, a time difference between the time when the electron is irradiated onto the sample and the time when the photon is emitted from the sample, based on the time when the electron detector detects the electron and the time when the photon detector detects the photon, and calculates an electron-photon time correlation that indicates a distribution of the time differences; An electron microscope comprising:

2. the electron detector detects the electrons transmitted through or reflected by the sample.

2. The electron microscope according to claim 1.

3. the computing unit has a discrimination unit that discriminates the sample based on the electron-photon time correlation; 3. The electron microscope according to claim 1 or 2.

4. the discrimination unit calculates a fluorescence lifetime or a luminescence transition probability of the sample from the electron-photon time correlation, and discriminates the sample based on the fluorescence lifetime or the luminescence transition probability; 4. The electron microscope according to claim 3.

5. the discrimination unit calculates, from the electron-photon time correlation, a curve indicating the frequency at which the photon is detected at the time difference when the electron is detected, calculates a luminescence transition probability of the sample from a peak value at the origin of the curve, and calculates, as the fluorescence lifetime of the sample, a time required for the value on the curve to decrease by 1 / e times.

5. The electron microscope according to claim 4.

6. the computing unit has a storage unit that stores parameter values ​​that indicate characteristics of electron-photon time correlations that have been measured in advance for one or more known substances or living organisms; the discrimination unit calculates a degree of coincidence between the calculated parameter value of the electron-photon time correlation of the sample and a parameter value of the electron-photon time correlation of a known substance or living organism, and discriminates the known substance or living organism having a parameter value for which the degree of coincidence exceeds a predetermined threshold and is maximum, as indicating the substance or living organism of the sample.

6. The electron microscope according to claim 3.

7. The electron detector comprises: a light emitter that reacts with the electrons irradiated onto the sample to emit photons; a photon detection unit that detects photons emitted from the light emitter and outputs a detection signal to the computing unit; The electron microscope according to claim 1 , further comprising:

8. the photon detector also serves as the photon detection unit of the electron detector, detects the photons emitted from the sample and the photons emitted from the light-emitting body, and outputs a detection signal to the calculator.

8. The electron microscope according to claim 7.

9. the electron gun irradiates the sample with the electrons while spatially scanning the electrons; the computing unit has an image generating unit that generates an image of the sample based on a spatial change in the electron-photon time correlation; 9. The electron microscope according to claim 1.

10. an electron detector that detects electrons irradiated onto a sample from an electron gun of the electron microscope; a photon detector that detects photons emitted from the sample when the electrons are irradiated onto the sample; a calculator that calculates, for each detected photon, a time difference between the time when the electron is irradiated onto the sample and the time when the photon is emitted from the sample, based on the time when the electron detector detects the electron and the time when the photon detector detects the photon, and calculates an electron-photon time correlation that indicates a distribution of the time differences; An electron-photon correlation measuring device comprising:

11. The sample is irradiated with electrons, Detecting the electrons irradiated onto the sample; detecting photons emitted from the sample when the electrons are irradiated onto the sample; calculating, for each detected photon, a time difference between the time when the electron is irradiated onto the sample and the time when the photon is emitted from the sample based on the time when the electron is detected and the time when the photon is detected, and calculating an electron-photon time correlation indicating a distribution of the time differences; A method for measuring electron-photon correlation, comprising:

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