Electron beam application device

Abstract

This electron beam application device comprises: a photo-excited electron source that is provided with a light source 21 and a photocathode 1 and concentrates excitation light 12 from the light source onto the photocathode to generate an electron beam; an electron optical system that focuses the electron beam from the photo-excited electron source and irradiates a sample with the electron beam; a first ammeter 7 that measures an emission current amount, which is a current amount of the electron beam emitted from the photo-excited electron source; a second ammeter 8 that measures, as a monitored current amount, a current amount of the electron beam irradiated onto an aperture 22A that is provided in the electron optical system; and a control unit 26 which, on the basis of the emission current amount and the monitored current amount, controls the light source so that an irradiation current amount, which is a current amount of the electron beam irradiated onto the sample, is set to a prescribed setting value.

Description

Electron beam application equipment

[0001] The present invention relates to an electron beam application device.

[0002] Achieving high spatial resolution in an electron microscope requires a high-brightness electron source. Photoexcitation electron sources utilizing negative electron affinity (NEA) offer brightness comparable to that of conventional high-performance electron sources. Furthermore, photoexcitation electron sources can be excited with short-pulse excitation light to generate short-pulse electron beams. This allows electron microscopes equipped with photoexcitation electron sources to perform time-resolved measurements that combine high spatial and temporal resolution. Furthermore, photoexcitation electron sources emit electron beams with a significantly narrower energy range than conventional high-performance electron sources. This feature allows scanning electron microscopes (SEMs) using photoexcitation electron sources to minimize chromatic aberration, which is advantageous for observing sample surfaces with high spatial resolution using low-energy electron beams. Transmission electron microscopes (TEMs) using photoexcitation electron sources also offer the advantage of high energy resolution in electron energy loss spectroscopy measurements.

[0003] Among photocathodes, which are photoexcited electron sources, those with a semiconductor surface such as GaAs are known as high-brightness electron sources. Photocathodes using GaAs are used by creating a surface with a low work function by adsorbing Cs and oxygen onto the GaAs surface. This surface treatment is called NEA activation (surface activation).

[0004] In actual equipment, gas adsorption onto the photocathode surface during operation can cause oxides and carbides to form on the GaAs surface, or adsorbed Cs atoms can desorb, resulting in reduced quantum efficiency, changes in brightness, and increased noise. This causes fluctuations in the current of the electron beam irradiating the sample. Fluctuations in the current of the electron beam irradiating the sample not only introduce noise into electron microscope images, but also lead to problems such as a loss of quantitativeness and reproducibility in measurements. For this reason, electron beam application equipment must be able to stably maintain the current of the electron beam irradiating the sample, or control it to an arbitrary value.

[0005] Patent Document 1 discloses a method of adjusting the intensity of the electron beam emitted from the photocathode by placing a Faraday cup with an aperture on the path of the electron beam emitted from the photocathode, measuring changes in the intensity of the electron beam using a measuring instrument connected to the Faraday cup, and adjusting the intensity of the excitation light according to the measurement results.

[0006] International Publication No. 2022 / 219814

[0007] The method described in Patent Document 1, in which the intensity of the emitted electron beam is simply adjusted, does not allow for precise control of the electron beam current irradiating the sample. This is because the brightness of the electron beam emitted from the photocathode changes over time due to the aforementioned changes in the photocathode surface, resulting in changes in the spatial and angular profiles. Using Figures 1A to 3, we explain the change in the divergence angle as an example of the change in the angular profile of the electron beam from the photocathode. Figure 1A shows an excerpt from the electron beam application device described in Patent Document 1. The photocathode 1 generates an electron beam 13 when irradiated with excitation light 12 focused by a focusing lens 2. The electrons emitted from the photocathode 1 are accelerated by the electric field between the photocathode 1 and the opposing anode electrode 3. A portion of the electron beam 13 is blocked by a Faraday cup 31 with an aperture placed on the electron beam path and detected by an ammeter 32 connected to the Faraday cup 31. Meanwhile, the electron beam 13 that passes through the aperture in the Faraday cup 31 is irradiated onto the sample.

[0008] 1A and 1B, the aperture of the Faraday cup 31 is defined as a circle with a diameter D1, the irradiation range of the electron beam 13 on the bottom surface of the Faraday cup 31 is defined as a circle with a diameter D2, the divergence angle of the electron beam 13 is defined as θ, and the distance from the surface of the photocathode 1 to the bottom surface of the Faraday cup 31 is defined as h. In a typical electron gun arrangement, the size of the electron emission surface on the photocathode 1 is sufficiently small compared to the distance h, so the electron emission surface is treated as a point. In addition, the electron trajectory is approximated as a straight line connecting the arrival position of the electron on the Faraday cup 31 and the focusing position of the excitation light 12.

[0009] When there is nothing between the photocathode 1 and the Faraday cup 31 blocking the electron beam 13 and the spatial distribution of the electron beam 13 is uniform, the following relationships hold: Ie is the total current (emission current) emitted from the photocathode 1, Ip is the current (probe current) of the electron beam irradiating the sample, and Im is the current (monitor current) measured by the ammeter 32. The symbol ^ denotes the exponentiation symbol, and the xth power of a is represented as a^x. Ip = Ie × D1^2 / D2^2 (Equation 1) Im = Ie × (D2^2 - D1^2) / D2^2 (Equation 2) Furthermore, the diameter D2 has the relationship shown in Equation 3: D2 = 2h × tan θ (Equation 3) From the above relationships, the ratio of the probe current Ip to the monitor current Im can be expressed as Equation 4. Ip / Im = D1^2 / {(2h × tan θ)^2 - D1^2} ... (Equation 4) Figure 2 is a graph showing the dependency of Ip / Im on the divergence angle θ when h = 100 mm and D1 = 0.5 mm. As shown in (Equation 4), the ratio of the probe current Ip to the monitor current Im is a function of θ, so when the divergence angle θ changes over time, it is difficult to control the probe current Ip by monitoring only the monitor current Im.

[0010] In addition, a known method for stabilizing the current value of the ammeter 32 is known for known high-performance electron sources. This method monitors the emission current from the electron source and feeds it back to the voltage applied to the extraction electrode. Even if this method is applied to a photoexcited electron source and the emission current from the photocathode 1 is adjusted by monitoring the emission current Ie, it is difficult to control the electron beam current Ip irradiating the sample if the divergence angle θ changes over time. The ratio of the electron beam probe current Ip to the emission current Ie is expressed as Equation 5, and this ratio is also a function of θ: Ip / Ie = D1^2 / (2h × tan θ)^2 (Equation 5). Figure 3 shows the dependence of Ip / Ie on the divergence angle θ when h = 100 mm and D1 = 0.5 mm.

[0011] When the photocathode 1 is mounted on an electron microscope, the divergence angle θ of electrons after passing through the anode electrode 3 is less than 100 mrad, and as shown in Figures 2 and 3, in this angular region, Ip / Im or Ip / Ie changes sharply with the divergence angle θ. Therefore, even if the electron beam current from the photocathode 1 is monitored at a single point on its path and controlled to remain constant, it is difficult to maintain a constant current Ip of the electron beam irradiating the sample. If the probe current Ip is not controlled to a constant value, the quantitativeness and reproducibility of the measurement will be reduced.

[0012] Although the example described here is one in which only the divergence angle θ of the electron angular profile changes, the same applies to cases in which the angular profile changes in a complex manner, as well as cases in which the spatial profile of the electron beam changes, or cases in which the spatial profile and angular profile change simultaneously and in a complex manner.

[0013] An electron beam application apparatus according to one embodiment of the present invention includes a photoexcited electron source including a light source and a photocathode, and configured to generate an electron beam by focusing excitation light from the light source onto the photocathode; an electron optical system configured to focus the electron beam from the photoexcited electron source and irradiate a sample with the electron beam; a first ammeter configured to measure an emission current, which is the amount of current of the electron beam emitted from the photoexcited electron source; a second ammeter configured to measure, as a monitor current, the amount of current of the electron beam irradiated onto an aperture provided in the electron optical system; and a control unit configured to control the light source based on the emission current and the monitor current so that an irradiation current, which is the amount of current of the electron beam irradiated onto the sample, is set to a predetermined set value.

[0014] According to the present invention, an electron beam application device having a photoexcited electron source capable of adjusting the current of the electron beam irradiated onto a sample can be obtained. Other objects and novel features will become apparent from the description of this specification and the accompanying drawings.

[0015] 1 is a diagram for explaining the problem to be solved by the present invention. FIG. 1 is a diagram for explaining the problem to be solved by the present invention. FIG. 2 is a diagram for explaining the dependency of the ratio of the irradiation current Ip and the monitor current Im on the divergence angle θ of the electron beam. FIG. 3 is a diagram for explaining the dependency of the ratio of the irradiation current Ip and the emission current Ie on the divergence angle θ of the electron beam. FIG. 4 is a schematic diagram of an electron beam apparatus equipped with a photoexcited electron source according to Example 1. FIG. 5 is a diagram for explaining a method of adjusting the time-averaged intensity of a pulsed light source. FIG. 6 is a diagram for explaining a method of adjusting the time-averaged intensity of a pulsed light source. FIG. 7 is a diagram for explaining a method of adjusting the time-averaged intensity of a pulsed light source. FIG. 8 is a flowchart for controlling the irradiation current Ip in Example 1. FIG. 9 is a schematic diagram of the behavior of the emission current Ie and the monitor current Im when the irradiation current Ip is controlled to be constant. FIG. 10 is an example of an area of ​​a sample irradiated with an electron beam. FIG. 11 is a time chart for acquiring an SEM image while controlling the irradiation current Ip for each pixel area. FIG. 12 is a schematic diagram of an electron beam apparatus equipped with a photoexcited electron source according to a modification of Example 1. FIG. 13 is a schematic diagram of an electron beam apparatus equipped with a photoexcited electron source according to a modification of Example 1. FIG. 1 is a schematic diagram of an electron beam apparatus equipped with a photo-excited electron source according to a second embodiment. FIG. 2 is a time chart for acquiring an SEM image while controlling the amount of irradiation current Ip. FIG. 3 is a schematic diagram of an electron beam apparatus equipped with a photo-excited electron source according to a modified example of the second embodiment. FIG. 4 is a schematic diagram of an electron beam apparatus equipped with a photo-excited electron source according to a modified example of the second embodiment. FIG. 5 is a schematic diagram of an electron beam apparatus equipped with a photo-excited electron source according to a third embodiment. FIG. 6 is a diagram for explaining a method for adjusting the excitation light focusing position. FIG. 7 is a diagram for explaining a method for adjusting the excitation light focusing position. FIG. 8 is a diagram for explaining a method for adjusting the electron beam path. FIG. 9 is a diagram for explaining a method for adjusting the electron beam path. An example of a segmented aperture. FIG. 10 is a diagram for explaining a method for adjusting the electron beam path using a segmented aperture.

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same components are designated by the same reference numerals, and detailed description of overlapping parts will be omitted.

[0017] An embodiment of the present invention will be described below with reference to Figures 4 to 8. An SEM equipped with a photoexcited electron source will be shown as an example of an electron beam application device.

[0018] Figure 4 is a schematic diagram of an electron beam application device. The photocathode 1 is formed by forming a photoelectric film 10 on a transparent substrate 11 that does not absorb excitation light 12. The photocathode 1 is held in a cathode holder 4 inside an electron gun 15, which has a window 6 that transmits the excitation light 12. The excitation light 12 is incident on the transparent substrate 11, and an electron beam 13 is generated from the portion of the photoelectric film 10 on which the excitation light 12 is incident. The excitation light 12 is focused by a condenser lens 2 as needed. Electrons emitted from the photocathode 1 are accelerated by the electric field between the photocathode 1 and the opposing anode electrode 3. The electron beam 13 is guided to an electron-optics housing 16 through an aperture in a differential pumping diaphragm 14 in a vacuum chamber 9. An acceleration voltage is applied to the electron source by an acceleration power supply 5 via the cathode holder 4, which is in electrical contact with the photoelectric film 10. Here, the term "electron-optics" refers to an optical system consisting of an electron lens, a deflector, and other components that focus the electron beam from the photoexcited electron source and irradiate the sample.

[0019] The electron beam 13 extracted from the differential pumping aperture 14 is focused onto the sample 20 by two electron lenses 17 and an objective lens 27. A deflector 18A for scanning the electron beam 13 over the sample 20 and an electron detector 19 for measuring signal electrons generated from the sample 20 by irradiation with the electron beam 13 provide an SEM image of the surface of the sample 20, making it possible to observe the microstructure. Note that, instead of the electron detector 19, a photodetector may be used to enable cathodoluminescence measurement, or an ammeter may be connected to the sample 20 to enable electron-beam-induced current measurement.

[0020] An aperture 22A and an electron lens 17 that limit the divergence angle of the electron beam 13 are provided on the path of the electron beam 13. The electron lens 17 adjusts the divergence angle of the electron beam 13 passing through the aperture 22A to an optimum angle.

[0021] A first ammeter 7 is connected between the cathode holder 4, which is electrically connected to the photocathode 1, and the accelerating power supply 5. The first ammeter 7 floats on the potential to which the cathode voltage is applied and measures the current. The first ammeter 7 measures the emission current Ie of the electron beam emitted from the photocathode 1. When electrons are emitted from the photocathode 1, corresponding holes are generated in the photocathode 1, and electrons corresponding to the holes are supplied from the accelerating power supply 5 via the cathode holder 4. The first ammeter 7 measures the emission current Ie by measuring the current required to neutralize the holes generated in the photocathode 1. This emission current Ie primarily reflects changes in the quantum efficiency of the photocathode due to changes in the surface of the photoconductor film 10, as described above. The current measured by the first ammeter 7 is referred to as the first current.

[0022] A second ammeter 8 is connected to the aperture 22A to measure the current due to electrons irradiating the aperture 22A in the portion of the electron beam 13 that diverges from the center of the trajectory. The current irradiating the aperture 22A primarily reflects the changes in the spatial and angular profiles of the electron beam emitted from the photocathode due to changes in the surface of the photoconductor film 10, as described above. In the configuration example shown in FIG. 4, the current irradiating the aperture 22A is measured as the monitor current, but the monitor current is not limited to the current irradiating the aperture 22A as long as it reflects changes in the spatial and angular profiles of the electron beam. Such examples will be described later. The current measured by the second ammeter 8 is referred to as the second current.

[0023] The current signal 24 measured by the first ammeter 7 and the current signal 25 measured by the second ammeter 8 are transmitted to a CPU 26 connected via a signal line or the like. The CPU 26 calculates the electron beam current Ip irradiating the sample 20 from the results of the first and second current measurements and transmits an excitation light intensity control signal 23 to the light source 21 to control the light source 21 so that the irradiation current Ip is the desired value. The emission current Ie of the electron beam 13 emitted from the photocathode 1 changes in response to changes in the intensity of the excitation light 12, and therefore the irradiation current Ip of the sample 20 also changes. This allows the irradiation current Ip of the sample 20 to be arbitrarily controlled by controlling the intensity of the excitation light 12. Furthermore, the calculated irradiation current Ip of the sample 20 may be recorded in association with each pixel of the SEM image and used for image processing to remove brightness variations in the SEM image. Such image processing has the advantage of improving the signal-to-noise ratio of the SEM image. The method by which the CPU 26 calculates the amount of irradiation current to the sample 20 and the method by which the CPU 26 controls the light source 21 will be described in detail later. The CPU 26 may communicate with a memory 28 as needed. The CPU 26 may also be called a control unit.

[0024] The vacuum vessel 9 of the electron gun 15 is evacuated by a vacuum pumping device (not shown), preferably an ion pump and a non-evaporable getter pump. Also, although not shown, a preparation chamber for surface treatment of the photocathode 1 is provided adjacent to the electron gun 15, and the surface of the photocathode 1 moved to the preparation chamber is adjusted. Below, the functions and configurations of some of the components are individually described in detail.

[0025] The photocathode 1 generates an electron beam 13 when irradiated with excitation light 12. One photocathode capable of generating a high-brightness, monochromatic electron beam utilizes a phenomenon known as negative electron affinity (NEA). An example of the photoconductive film 10 of such a photocathode 1 is p-type GaAs. To reduce the work function, Cs and oxygen, for example, are adsorbed on the surface. Note that the photoconductive film 10 is not limited to p-type GaAs. It may be, for example, GaN, GaSb, AlN, Ce2Te, InAs, or a mixed crystal thereof. Furthermore, the material adsorbed to reduce the work function is not limited to Cs and oxygen, but may also be Te, K, Cs2Te, or Rb. Furthermore, the photoconductive film 10 is not limited to those utilizing NEA and may not require surface treatment. Such a photoconductive film 10 may be, for example, a metal material, such as LaB6, Mg, Cu, Nb, or Ag.

[0026] The surface of the photocathode 1, whose photoelectric film 10 is made of p-type GaAs, is prepared as follows: Oxides and carbides are removed from the semiconductor surface using a heater. A cleaning process using atomic hydrogen is even more effective. Then, Cs vaporized in an oxygen atmosphere is adsorbed onto the semiconductor surface. This lowers the work function of the semiconductor surface, resulting in a state where the vacuum level is lower than that of the conductor inside the cathode film. As a result, electrons excited from the valence band to the conduction band by the incidence of excitation light with sufficient energy to cause band-to-band transitions in the p-type semiconductor already have a potential energy higher than the vacuum level, and are emitted into the vacuum by the electric field at the film surface. When GaAs is used for the photoelectric film 10, the wavelength of the excitation light must be 850 nm or less, preferably 780 nm to 660 nm. Glass or a semiconductor with a wider band gap, such as GaP, can be used as the transparent substrate 11 for the GaAs-based photoelectric film 10. The photoelectric film 10 can be formed directly on the transparent substrate 11 or via an intermediate layer.

[0027] The condenser lens 2 focuses the excitation light 12 to a desired size on the photoelectric film 10. It may be an optical lens, such as an aspherical lens, a plano-convex lens, or a cylindrical lens, or it may be configured by combining other optical elements or lenses. Although not shown in the figure, a position adjustment mechanism may be provided between the light source 21 and the condenser lens 2 to adjust the focusing position on the photocathode 1 surface and the focusing diameter of the excitation light 12 on the photoelectric film 10. This allows adjustment of the electron beam generation position, current amount, and electron beam emission area. A small focusing diameter of the excitation light 12 results in a high-brightness electron beam. Therefore, a high-brightness electron beam can be obtained by using an aspherical lens corresponding to the wavelength of the excitation light 12 for the condenser lens 2 and focusing the excitation light 12 to the diffraction limit on the photoelectric film 10.

[0028] The light source 21 supplies excitation light 12 for generating the electron beam 13 from the photocathode 1, and the intensity of the excitation light can be adjusted. Examples of light sources 21 include semiconductor lasers, solid-state lasers, gas lasers, rare gas lamps, and white light sources. An appropriate light source can be used depending on the characteristics of the photocathode 1. To change the intensity of the excitation light, known methods can be used. For example, if the light source 21 is a continuous light source, the output of the light source and / or the amplification factor of an optical amplifier (not shown) can be controlled, or the attenuation factor of an attenuator (not shown) inserted in the optical path of the excitation light 12 can be controlled. For example, if the light source 21 is a pulsed light source, the time-averaged intensity of the excitation light 12 can be adjusted by changing the peak intensity, pulse width, and pulse oscillation interval, as shown in Figures 5A to 5D, in addition to the methods described above for continuous light sources. Figure 5A shows a rectangular pulsed light beam in a reference state. The rectangular pulsed light beam in the reference state has a pulse width Δt, a pulse oscillation interval T, a peak intensity Pmax, and a time-averaged intensity Pavg. The time-average intensity of the pulsed light is controlled to 0.5 times (0.5Pavg) this reference state. Fig. 5B shows an example of controlling the peak intensity Pmax, Fig. 5C shows an example of controlling the pulse width Δt, and Fig. 5D shows an example of controlling the pulse oscillation interval T. Note that the time-average intensity of the pulsed light may be adjusted by a combination of these, i.e., by changing any two or all of the peak intensity, pulse width, and pulse oscillation interval of the pulsed light.

[0029] Next, a method for controlling the irradiation current Ip of the sample 20 in this embodiment will be described. In the electron beam application apparatus shown in FIG. 4 , when there is nothing between the photocathode 1 and the aperture 22A to block the electron beam 13, the current Ip of the electron beam 13 irradiating the sample 20 can be calculated using Equation 6, which uses the emission current Ie measured by the first ammeter 7 and the monitor current Im measured by the second ammeter 8: Ip = Ie - Im (Equation 6) Therefore, when the intensity of the excitation light 12 is changed, the CPU 26 calculates the irradiation current Ip of the sample 20 using Equation 6, and controls the intensity of the excitation light 12, thereby enabling real-time control of the irradiation current Ip. In this embodiment, by calculating the irradiation current Ip of the sample 20 from the measurement results of the first ammeter 7 and the second ammeter 8 and controlling the intensity of the excitation light 12, it is possible to precisely control the current Ip of the electron beam irradiating the sample while suppressing the influence of temporal fluctuations in the spatial or angular profile of the electron beam 13. Furthermore, by repeating the above-described control, it is possible to maintain the probe current Ip at a constant value for a long period of time. The repetition period can be determined based on the properties of the photocathode 1, the object to be measured, and the characteristics of the device. Typically, the period is several milliseconds to several minutes.

[0030] As an example of repetitive control, an example of feedback control executed by the CPU 26 is shown in the flowchart of FIG. 6A. First, the first current Ie and the second current Im are measured (S01), and the electron beam current Ip irradiating the sample is calculated from the first current Ie and the second current Im (S02). Next, it is determined whether the calculated irradiation current Ip is sufficiently close to a set value (S03). The determination method can be set according to the device and the object being measured, such as determining whether the set value and the calculated irradiation current Ip are within a certain range. If it is determined that the calculated irradiation current Ip is sufficiently close to the set value, the CPU 26 maintains the excitation light intensity (S06). If not, the CPU 26 calculates an excitation light intensity that brings the calculated irradiation current Ip closer to the set value, and controls the light source 21 to change the excitation light intensity (S04). The method for calculating the excitation light intensity change can be based on known methods used, for example, in PID control or adaptive controllers. After the excitation light intensity is changed, the excitation light intensity is kept constant (S05), and after a certain time has elapsed, the first current amount Ie and the second current amount Im are measured again (S01). Note that although Fig. 6A shows an example of feedback control, feedforward control, adaptive control, model-based control, etc. may also be used.

[0031] 6B is a schematic diagram showing the behavior of the probe current Ip, emission current Ie, and monitor current Im when the intensity of the excitation light 12 is controlled to maintain the probe current Ip constant as the divergence angle θ (see FIG. 1A) monotonically increases over time. While FIG. 6B shows an example in which the divergence angle θ monotonically increases, the probe current Ip can also be controlled in a similar manner when the divergence angle θ decreases, when it repeatedly increases and decreases, or when it changes irregularly.

[0032] In this embodiment, control may be performed not only to maintain the probe current Ip constant, but also to dynamically change the probe current Ip. For example, an example of acquiring an SEM image while arbitrarily changing the electron beam current irradiating the sample in synchronization with the deflector 18A will be described with reference to FIGS. 7A and 7B. FIG. 7A shows an area (3 × 3 pixel area) 40 on the sample to be irradiated with the electron beam, and each pixel area is identified by a number within the frame. Here, a pixel area refers to an area equivalent to one pixel in the SEM image, and its size varies depending on factors such as the imaging magnification of the SEM image. The electron beam is scanned across area 40 in the order of the numbers within the frame, starting with pixel area 1 and ending with pixel area 9. In this case, if the probe current irradiating pixel areas 1, 2, 4, 6, and 7 is Ia, the electron beam is controlled to irradiate pixel area 3 with a probe current of 2Ia, and pixel areas 5, 8, and 9 with a probe current of Ia / 2.

[0033] Figure 7B shows a time chart for acquiring an SEM image of region 40. The pixel region irradiated with the electron beam 13 moves as the deflector 18A scans the electron beam. In synchronization with the electron beam scan, the CPU 26 changes the set value of the irradiation current, thereby changing the intensity of the excitation light 12. As a result, the irradiation current Ip for each pixel region changes according to the set value. Note that the amount of change in the intensity of the excitation light 12 varies depending on the sample 20 being measured, the state of the photocathode 1, the device configuration, and other factors; the intensity waveform in Figure 7B is merely an example. This type of control is effective when the sample 20 is made up of multiple materials and the current of the electron beam irradiated within a single field of view needs to be adjusted for each region.

[0034] The relational equation for calculating the probe current Ip does not have to be the theoretical equation (Equation 6) itself. Multiple relational equations between the probe current Ip and the emission current Ie measured by the first ammeter 7 and the monitor current Im measured by the second ammeter 8, which are calibrated for the device settings and the object to be measured, may be registered in advance in memory 28, and CPU 26 may select and call one of the relational equations according to the device settings and the object to be measured. Alternatively, the user may individually input a relational equation, or the device may automatically estimate a relational equation.

[0035] As described above, in this embodiment, in an electron beam application device using a photoexcited electron source, the amount of electron beam current irradiated onto a sample can be stably maintained for a long period of time or controlled with high precision, thereby improving the reproducibility and accuracy of measurements. In particular, when the focused diameter of the excitation light is reduced to obtain high brightness with a photoexcited electron source, a structure like this embodiment is advantageous. This is because, since the generation area of ​​the electron beam 13 on the photoelectric film 10 becomes smaller, contamination of the surface of the photoelectric film 10 by carbides and oxides and Cs desorption are more likely to affect the electron emission characteristics.

[0036] Furthermore, in a pulsed SEM that emits a pulsed electron beam using pulsed excitation light as the excitation light 12, it is possible to quantitatively control the peak current and time-averaged current of the electron beam irradiated onto the sample. For example, in measurements that quantitatively estimate the time constant of a sample's charge / discharge process from the contrast of SEM images, quantitatively controlling the peak current and time-averaged current of the electron beam irradiated onto the sample improves the quantitativeness of the time constant measurement. This is also effective in time-resolved pump-probe SEM measurements using pulsed SEM. In time-resolved pump-probe SEM measurements, the instrument is equipped with a light source that irradiates pulsed light (e.g., ultraviolet laser light) to excite the sample 20. The time (delay time) Δτ between the pulsed light (pump) and the pulsed electron beam (probe) reaching the sample 20 is varied to acquire multiple SEM images, and the dependence of signal intensity on the delay time Δτ is measured. To capture changes between SEM images due to differences in the delay time Δτ, the peak current and time-averaged current of the pulsed electron beam must be stable with high precision throughout the period required to acquire the required number of SEM images. The structure and control method of this embodiment are therefore effective.

[0037] A modified example of the device configuration will now be described. In Fig. 4, the differential exhaust diaphragm 14 is arranged closer to the photocathode 1 than the aperture 22A and the electron lens 17, but this arrangement is not limitative. As shown in Fig. 8, the aperture 22A may be arranged closer to the photocathode 1 than the differential exhaust diaphragm 14. In Fig. 8, the aperture 22A is arranged closer to the photocathode 1 than the two electron lenses 17, but this does not pose any problems.

[0038] Furthermore, the differential pumping diaphragm 14 may be positioned at a position offset from the central axis of the photocathode 1 and the sample 20 in order to block gas blown up from the sample 20 during observation of the sample 20. Regardless of the position at which the differential pumping diaphragm 14 is positioned, a deflector or the like for alignment adjustment may be positioned closer to the photocathode 1 than the differential pumping diaphragm 14 so that the electron beam 13 is not blocked by the differential pumping diaphragm 14.

[0039] 9 shows an example of an electron beam application apparatus in which a first ammeter 7 is connected to a Faraday cup 29 placed near the optical axis of the electron beam 13, and the electron beam 13 is guided to the Faraday cup 29 by a deflector 18B to measure the emission current Ie. The Faraday cup 29 is placed upstream of an aperture 22A that narrows the electron beam 13. If there is nothing obstructing the electron beam 13 between the photocathode 1 and the deflector 18B, the Faraday cup 29 can measure the emission current Ie. A second ammeter 8 that measures the monitor current Im is connected to the aperture 22A. In this case, the probe current Ip can also be calculated based on Equation 6. When the Faraday cup 29 is used, the emission current Ie cannot be measured continuously. However, it is possible to measure the emission current Ie frequently at timings such as when the scanning position is switched (for example, when moving from pixel region 3 to pixel region 4 in region 40 (see FIG. 7A )), and for the emission current Ie in a time period when the first current amount is not being measured, use a deemed emission current amount based on the emission current amount Ie measured at the immediately preceding timing, thereby preventing a decrease in control accuracy.

[0040] In Example 1, an example was shown in which the first current amount and the second current amount were continuously measured to control the current amount of the electron beam irradiated to the sample. In Example 2, an example will be described in which the irradiation current amount is controlled with high precision when one of the first current amount and the second current amount, which is measured on the more upstream side of the electron beam, is continuously measured and the other is discontinuously measured.

[0041] 10A, a Faraday cup 29 is installed near the path of the electron beam 13 on the sample 20 side of the aperture 22A, a second ammeter 8 is connected between the Faraday cup 29 and a reference potential point (ground point), and the electron beam 13 is guided to the Faraday cup 29 by a deflector 18C to measure the probe current Ip. As shown in FIG. 10A, if the components are arranged so that nothing obstructs the electron beam 13 between the deflector 18C and the sample 20, the measurement result of the second ammeter 8 will coincide with the probe current Ip. Alternatively, the relationship between the measurement value of the second ammeter 8 and the probe current Ip may be determined based on the configuration of the apparatus or through experiments.

[0042] A first ammeter 7 that continuously measures the emission current Ie is connected to the cathode holder 4. When the Faraday cup 29 is used, the probe current Ip cannot be continuously measured. The deflector 18C is controlled at a timing that does not affect the SEM measurement, such as when switching the scanning position (for example, when moving from pixel area 3 to pixel area 4 in area 40 (see FIG. 7A)), and the probe current Ip is measured discontinuously.

[0043] Figure 10B shows a timing chart for acquiring an SEM image using the electron beam application device shown in Figure 10A. Waveform 51 represents the control waveform for electron beam scanning by deflector 18A. At timing "1," electron beam 13 is irradiated onto a pixel region, and at timing "0," electron beam 13 is not irradiated onto a pixel region (e.g., when moving from pixel region 3 to pixel region 4 in region 40 (see Figure 7A)). Waveform 52 represents the control waveform for current measurement by first ammeter 7. The emitted current Ie is continuously measured at timing "1." Waveform 53 represents the control waveform for electron beam movement by deflector 18C. This control waveform is a combination of control waveform 53a, which moves electron beam 13 toward Faraday cup 29, and control waveform 53b, which cancels the movement caused by control waveform 53a. Waveform 54 indicates the control waveform for current measurement by second ammeter 8, and becomes "1" at the timing sandwiched between a pair of control waveforms 53a and 53b, during which time the probe current Ip is measured by Faraday cup 29. Waveform 55 indicates the arrival position of electron beam 13.

[0044] In the second embodiment, the CPU 26 determines a relational expression between the emission current Ie measured by the first ammeter 7 and the probe current Ip measured by the second ammeter 8 during the period 55a. Thereafter, during the period 55b in which the emission current Ip is not measured, the CPU 26 calculates the probe current Ip using the emission current Ie continuously measured by the first ammeter 7 and the relational expression determined by the above-mentioned measurement. The electron beam 13 is controlled based on the determined probe current Ip in the same manner as in the first embodiment. Each time the Faraday cup 29 measures the probe current Ip, the CPU 26 updates the relational expression.

[0045] A modification of the second embodiment will be described.

[0046] The first modification is an example in which a relational expression for calculating the probe current Ip is obtained using a relational expression other than the emission current Ie. The electron beam application device shown in FIG. 11 includes an aperture 22B in its electron optical system that blocks a portion of the electron beam 13, and the current irradiating the aperture 22B is measured by the first ammeter 7 as a monitor current Im. If the electron optical system has multiple apertures, it is desirable to measure the current irradiating the aperture located most upstream. The time chart in this case is also similar to that shown in FIG. 10B. The relational expression obtained by the CPU 26 is the relational expression between the monitor current Im measured by the first ammeter 7 and the probe current Ip measured by the second ammeter 8.

[0047] The second modification is an example in which the deflector 18A is used to introduce the electron beam into the Faraday cup 29. In the electron beam application apparatus shown in Fig. 12, the Faraday cup 29 is placed on or near the sample 20, and the deflector 18A is given both the function of moving the electron beam 13 between the sample 20 and the Faraday cup 29 and the function of scanning the electron beam 13 over the sample 20.

[0048] The relational equation may be updated each time a measurement is taken by the second ammeter 8, which is performed discontinuously, or the accuracy of the inference based on the relational equation may be monitored and the relational equation may be updated when the accuracy begins to deteriorate.

[0049] In Example 3, an example will be described in which a first ammeter 7 and a second ammeter 8 are used to adjust the electron optical system of an electron beam apparatus using a photoexcited electron source. Compared to the configuration of the electron beam apparatus shown in Figure 4, the electron beam apparatus shown in Figure 13 has the following features: a position adjustment mechanism 30 for the excitation light 12 that can adjust the focusing position of the excitation light 12 on the photocathode 1; and a deflector 18D that is installed between the aperture 22A and the anode electrode 3 of the photoexcited electron source that can adjust the path of the electron beam 13.

[0050] The measurement value of the first ammeter 7 is monitored while the position adjustment mechanism 30 scans the focusing position of the excitation light 12 on the photocathode 1. This operation allows the focusing position of the excitation light at which the maximum quantum efficiency is obtained to be determined. Then, while the deflector 18D scans the aperture 22A, the measurement value of the second ammeter 8 is used to calculate and monitor the current of the electron beam irradiated onto the sample 20. This allows the optimal electron beam path to be determined and the spatial profile of the electron beam to be measured. By performing the above-mentioned measurements while controlling the intensity of the excitation light 12 with the CPU 26, the optimal excitation light intensity, excitation light focusing position, and electron beam path can be set to irradiate the electron beam 13 onto the sample 20 with the desired irradiation current Ip.

[0051] An example of a method for determining the focusing position of the excitation light at which the electron beam is emitted with maximum quantum efficiency will be described using Figures 14A and 14B. Two axes in a plane parallel to the photocathode surface are the x-axis and the y-axis, and the excitation light focusing position at which maximum quantum efficiency is obtained is defined as (x, y) = (0, 0). By acquiring measurements from the first ammeter 7 while scanning the excitation light focusing position within the xy plane, the excitation light focusing position at which maximum quantum efficiency is obtained can be identified, as shown in Figure 14B. Furthermore, while monitoring the measurements from the first ammeter 7, the focusing lens 2 may be moved in the z-axis direction or the aperture angle at which the excitation light 12 is incident on the focusing lens 2 may be adjusted to optimize the focusing diameter.

[0052] Next, an example of a method for adjusting the path of an electron beam will be described using FIGS. 15A and 15B . States a to c shown in FIG. 15A show the electron beam 13 being scanned over the aperture 22A by the deflector 18D in any direction within the xy plane. The position of the electron beam when the center of the electron beam 13 coincides with the center of the aperture 22A is defined as (x, y) = (0, 0). FIG. 15B shows the transition of the magnitude of the probe current Ip calculated when the electron beam 13 is scanned over the aperture 22A as described above. The magnitude of the probe current Ip is calculated by the CPU 26 using the method described in Example 1. As shown in FIG. 15B , the probe current Ip has a maximum value at (x, y) = (0, 0). The maximum value of the irradiation current Ip can be adjusted by adjusting the intensity of the excitation light 12 with the CPU 26, and the amount of deflection of the electron beam 13 by the deflector 18D and the intensity of the excitation light 12 can be controlled to irradiate the sample 20 with the electron beam 13 having the desired irradiation current Ip. In addition, the spatial profile of the electron beam can be obtained by differentiating the current amount of the electron beam irradiated to the sample obtained in Figure 15B with respect to x and y.

[0053] As a modified example, the aperture 22A may be a four-part divided aperture 33 as shown in FIG. 16A, with the second ammeters 8A to 8D connected to the plates 34A to 34D that make up the divided aperture 33, respectively. The plates 34A to 34D have the same shape. This makes it possible to adjust the electron beam path without scanning the electron beam 13 across the aperture using a deflector. FIG. 16B illustrates a case in which the electron beam 13 is irradiated onto the divided aperture 33 in several states. In this example, the spatial profile of the electron beam 13 across the divided aperture 33 is assumed to be uniform. The current values ​​detected by the second ammeters 8A to 8D are designated IA to ID, respectively.

[0054] In state α shown in FIG. 16B , the center of the electron beam 13 coincides with the central axis of the opening of the split aperture 33. Since the same amount of electron beam is irradiated onto plates 34A to 34D, IA = IB = IC = ID holds. On the other hand, when the center of the electron beam 13 is shifted in the positive direction of the y-axis, as in state β, IA = IB > IC = ID holds. The amount of deviation between the center of the electron beam 13 and the central axis of the split aperture 33 can be estimated from the difference between IA (or IB) and IC (or ID). Furthermore, when the center of the electron beam 13 and the center of the split aperture 33 are shifted in the (x, y) = (1, 1) direction, as in state γ, IB > IA = IC > ID holds. Furthermore, the amount of deviation between the center of the electron beam 13 and the central axis of the split aperture 33 can be estimated from the difference between IB and ID. By setting the initial state as state α and monitoring the measured values ​​of the second ammeters 8A to 8D, the deviation of the electron beam 13 can be detected. Based on the detected deviation, the deflector 18D can be controlled so that the electron beam 13 coincides with the center of the split aperture 33, i.e., so that IA = IB = IC = ID, thereby maintaining an optimal electron beam path. The total of the measurements of the second ammeters 8A to 8D (IA + IB + IC + ID) can be used as the monitor current value Im to monitor the amount of electron beam current irradiating the sample. While a split aperture divided into four has been described as an example, the number of divisions in the split aperture is not limited to four, and any other suitable number, such as 2, 8, or 16, may be used.

[0055] Adjustments to the excitation light focusing position and the optimal electron beam path may be performed repeatedly at timings that do not affect the scanning electron microscope measurements. This allows for detection of fluctuations in the excitation light focusing position and electron beam path due to mechanical misalignment of components or fluctuations in the instrument's state, as well as associated fluctuations in the electron beam current irradiated to the sample. When a fluctuation is detected, the position adjustment mechanism 30 and / or the deflector 18D can be controlled to correct the excitation light focusing position and electron beam path so that the electron beam current irradiated to the sample matches the current before the fluctuation, thereby improving measurement accuracy and reproducibility. Furthermore, by adjusting the focusing position of the excitation light 12 within the photoelectric film 10 and the electron beam path based on the monitoring of the electron beam current irradiated to the sample using the above-described method, the electron beam current irradiated to the sample can be changed without changing the settings of the electron optical system by changing the focusing diameter of the excitation light. This allows for observation of changes in the SEM image due to differences in the electron beam current irradiated to the sample without changing the electron beam focusing position on the sample.

[0056] The present invention has been specifically described above based on the embodiments, but is not limited thereto and various modifications are possible without departing from the spirit and scope of the invention. For example, while a scanning electron microscope has been described as an example of an electron beam application device, the present invention can be applied to various electron beam application devices, such as a transmission electron microscope, a scanning electron microscope, and an electron beam lithography device. Furthermore, the present invention is not limited to electron beam application devices equipped with an electron detector that detects electrons (secondary electrons, backscattered electrons, etc.) generated by irradiation with an electron beam, but may also be equipped with other detectors, such as a detector that detects characteristic X-rays, a detector that detects sample emission in the ultraviolet to infrared range, or a detector that measures electron beam-induced current.

[0057] Furthermore, the above-described embodiments and modifications have been described in detail to make the present invention easier to understand, and the present invention is not necessarily limited to those including all of the described configurations. It is possible to replace part of the configuration of one embodiment or modification with the configuration of another embodiment or modification, and it is also possible to add the configuration of another embodiment or modification to the configuration of one embodiment or modification. It is also possible to add, delete, or replace part of the configuration of each embodiment or modification with other configurations.

[0058] 1: photocathode, 2: condenser lens, 3: anode electrode, 4: cathode holder, 5: acceleration power supply, 6: window, 7: first ammeter, 8: second ammeter, 9: vacuum vessel, 10: photoelectric film, 11: transparent substrate, 12: excitation light, 13: electron beam, 14: differential exhaust diaphragm, 15: electron gun, 16: electron optical system housing, 17: electron lens, 18: deflector, 19: electron detector, 20: sample, 21: light source, 22: aperture, 23: excitation light intensity control signal, 24: current amount signal, 25: current amount signal, 26: CPU, 27: objective lens, 28: memory, 29: Faraday cup, 30: position adjustment mechanism, 31: Faraday cup, 32: ammeter, 33: split aperture, 34: plate, 40: area, 51, 52, 53, 54, 55: waveform, 53a, 53b: control waveform, 55a, 55b: period.

Claims

1. An electron beam application apparatus comprising: a photoexcitation electron source including a light source and a photocathode, which condenses excitation light from the light source onto the photocathode to generate an electron beam; an electron optical system that focuses the electron beam from the photoexcitation electron source and irradiates a sample; a first ammeter that measures a emission current amount which is the current amount of the electron beam emitted from the photoexcitation electron source; a second ammeter that measures, as a monitor current amount, the current amount of the electron beam irradiated to an aperture included in the electron optical system; and a control unit that controls the light source so that an irradiation current amount, which is the current amount of the electron beam irradiated to the sample, becomes a predetermined set value based on the emission current amount and the monitor current amount.

2. The electron beam application apparatus according to claim 1, wherein the photoexcitation electron source includes: a cathode holder that holds the photocathode and is in electrical contact with a photoelectric film included in the photocathode; an anode electrode disposed to face the photoelectric film; and a power source connected to the cathode holder and applying an acceleration voltage between the photocathode and the anode electrode, and the first ammeter is connected between the cathode holder and the power source.

3. The electron beam application apparatus according to claim 1, wherein the electron optical system includes an electron lens that adjusts an opening angle of the electron beam passing through the aperture.

4. The electron beam application apparatus according to claim 1, wherein the control unit has a plurality of relational expressions for obtaining the irradiation current amount from the emission current amount and the monitor current amount registered in advance, and selects any one of the plurality of registered relational expressions and uses it for controlling the light source.

5. The electron beam application apparatus according to claim 1, wherein the control unit sets different values for each pixel region as the predetermined set value.

6. In claim 1, the photoexcitation electron source includes a position adjustment mechanism for adjusting the condensing position where the excitation light from the light source is condensed on the photocathode. The electron optical system includes a deflector for deflecting an electron beam between the photoexcitation electron source and the aperture. The position adjustment mechanism is set such that the position where the emitted current amount reaches a maximum value when the condensing position is scanned on the photocathode is the condensing position when irradiating the sample with the electron beam. The deflector is an electron beam application device that gives, as the deflection amount when irradiating the sample with the electron beam, the deflection amount at which the monitor current amount reaches a maximum value when scanning the electron beam on the aperture in an arbitrary one direction.

7. In claim 1, the photoexcitation electron source includes a position adjustment mechanism for adjusting the condensing position where the excitation light from the light source is condensed on the photocathode. The electron optical system includes a deflector for deflecting an electron beam between the photoexcitation electron source and the aperture. The aperture is a split aperture composed of a plurality of plates having the same shape, and each of the plurality of plates is provided with the second ammeter. The position adjustment mechanism is set such that the position where the emitted current amount reaches a maximum value when the condensing position is scanned on the photocathode is the condensing position when irradiating the sample with the electron beam. The deflector is an electron beam application device that gives, as the deflection amount when irradiating the sample with the electron beam, the deflection amount at which the measured values of the plurality of second ammeters become equal.

8. An electron beam application apparatus comprising: a photoexcitation electron source including a light source and a photocathode, which condenses excitation light from the light source onto the photocathode to generate an electron beam; an electron optical system that focuses the electron beam from the photoexcitation electron source and irradiates a sample; a first ammeter that measures, as a monitor current amount, a discharge current amount that is the current amount of the electron beam emitted from the photoexcitation electron source or a current amount of the electron beam irradiated onto an aperture included in the electron optical system; a second ammeter that measures, at a predetermined timing, an irradiation current amount that is the current amount of the electron beam irradiated onto the sample; and a control unit that obtains a relational expression between the discharge current amount or the monitor current amount and the irradiation current amount based on the discharge current amount or the monitor current amount measured by the first ammeter and the irradiation current amount measured by the second ammeter at the predetermined timing. When the relational expression is a relational expression between the discharge current amount and the irradiation current amount, the control unit controls the light source so that the irradiation current amount calculated based on the discharge current amount and the relational expression becomes a predetermined set value. When the relational expression is a relational expression between the monitor current amount and the irradiation current amount, the control unit controls the light source so that the irradiation current amount calculated based on the monitor current amount and the relational expression becomes a predetermined set value.

9. The electron beam application apparatus according to claim 8, further comprising a Faraday cup disposed near an optical axis of the electron optical system, wherein the electron optical system includes a first deflector that deflects the electron beam, the second ammeter is connected between the Faraday cup and a reference electric potential point, and at the predetermined timing, the first deflector guides the electron beam to the Faraday cup, so that the second ammeter measures the irradiation current amount.

10. The electron beam application apparatus according to claim 9, wherein the first deflector moves the electron beam to the Faraday cup at the predetermined timing and scans the electron beam on the sample at times other than the predetermined timing.

11. The electron beam application apparatus according to claim 9, wherein the electron optical system includes a second deflector that scans the electron beam on the sample, and the predetermined timing is set during a period other than when the second deflector scans a pixel region on the sample with the electron beam.

12. In claim 8, the photoexcitation electron source includes a cathode holder that holds the photocathode and is in electrical contact with a photoelectric film provided on the photocathode, an anode electrode disposed to face the photoelectric film, and a power supply connected to the cathode holder for applying an acceleration voltage between the photocathode and the anode electrode, and the first ammeter is an electron beam application device connected between the cathode holder and the power supply.

13. In claim 8, the electron optical system is an electron beam application device including an electron lens that adjusts the opening angle of an electron beam passing through the aperture.

14. In claim 8, the control unit is an electron beam application device that sets different values for each pixel region as the predetermined set value.

Images