Electron microscope and imaging method thereof

Abstract

The problem of an axisymmetric electron gun structure is that some of gas molecules flowing in from a relatively low-vacuum chamber reach a photoelectric film, thereby causing deterioration of an NEA surface, instability of an emission current, and shortening of the life of the photoelectric film. This electron microscope is provided with: an excitation light source, a photoelectric film formed on a transparent substrate; a condensing lens for condensing excitation light onto the photoelectric film; an anode electrode for accelerating an electron beam generated by the excitation light being condensed and projected onto the photoelectric film; a first differential pumping diaphragm having a passage hole in an off-axis location and disposed on the photoelectric film side; and a second differential pumping diaphragm having a passage hole on the optical axis and disposed on the sample side. The electron microscope is provided with a deflector for controlling the trajectory of the electron beam between the first differential pumping diaphragm and the second differential pumping diaphragm.

Description

[Technical Field]

[0001] The present invention relates to an electron microscope as an electron beam application device equipped with an electron gun using a photoelectric film, and to an image capturing method thereof. [Background technology]

[0002] A scanning electron microscope (SEM) irradiates a sample with a focused electron beam (electron beam can also be called an electron beam), scans the sample to detect the emitted electrons, and displays the signal intensity at each irradiation point. This allows the surface morphology and composition distribution of the sample to be measured. To achieve high spatial resolution with an SEM, a high-brightness electron source is required. Field emission (FE) electron sources, which utilize electrons emitted by applying a strong electric field to the tip of a needle-shaped electrode, are widely used as high-brightness electron sources for SEMs. However, in recent years, electron sources using photoelectric films with a negative electron affinity (NEA) on the surface have been developed, which have a brightness (up to 1×10) comparable to that of a Schottky-type FE electron source. 7 A / m 2 / sr / V) has been reported (Patent Document 1).

[0003] The energy spread of the electron beam emitted by this electron source using a photoelectric film with a high-brightness NEA surface is smaller than that of a cold-cathode FE electron source, and so it offers the advantage of providing high observation performance under irradiation conditions with energies of approximately 1 keV or less, which are particularly advantageous for observing the extreme surfaces of specimens. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 192070 Summary of the Invention [Problem to be solved by the invention]

[0005] By adsorbing cesium or oxygen onto the surface of a highly doped p-type gallium arsenide (GaAs) photoconductor, a work function can be reduced by adsorbing cesium or oxygen to create an NEA surface with a vacuum level lower than the energy level of the conduction band minimum of the photoconductor. Irradiating this surface with light efficiently emits electrons excited within the photoconductor. In particular, the angular range of electrons emitted from the NEA surface of a p-type GaAs photoconductor is small (approximately 10° or less), depending on the effective mass of the electrons in the GaAs. Therefore, by focusing the excitation light and reducing the electron emission area to approximately 1 μm in diameter to create a point source, high brightness characteristics comparable to those of a Schottky-type FE electron source can be achieved. To reliably utilize the electron beam emitted from the NEA surface, an ultra-high vacuum (typically below 10-8 Pa) must be maintained around the photoconductor. For this reason, electron guns using the NEA surface of a p-type GaAs photoconductor consist of a multi-stage differential pumping structure, and the trajectory of the emitted electron beam is adjusted so that it passes through an aperture located between the differential pumping chambers. This allows the electron beam emitted from the NEA surface to be used as a probe for an electron microscope.

[0006] When the electron microscope housing has an axisymmetric structure, the electron beam passage holes in the differential pumping diaphragm, which are positioned along the path of the probe electron beam, are all positioned axisymmetrically, providing a path through which gas molecules can pass unobstructed between the sample and the electron source. In this case, gas molecules flow into the electron gun chamber from the relatively low vacuum of the sample chamber. When gas molecules reach the electron-emitting area of ​​the photoelectric film, they adsorb to the photoelectric film, degrading the NEA surface. Furthermore, particularly under high emission current conditions, electrons emitted from the photoelectric film collide with gas molecules, ionizing and accelerating the gas molecules, which then collide with the photoelectric film, degrading the NEA surface through a mechanism known as ion feedback. Thus, electron sources using photoelectric films with an active layer made of p-type GaAs face challenges such as reduced brightness, unstable emission current, and a short NEA surface life due to gas molecules. [Means for solving the problem]

[0007] An electron microscope according to one embodiment of the present invention comprises: the excitation light source that generates excitation light, a photocathode having a transparent substrate and a photoelectric film, a condensing lens that condenses the excitation light toward the photocathode, an anode electrode that is arranged opposite the photocathode and that accelerates an electron beam when the excitation light condensed by the condensing lens passes through the transparent substrate of the photocathode and is incident thereon, generating an electron beam from an excitation point on the photoelectric film of the photocathode, a first differential pumping diaphragm that is arranged on the side of the photocathode and has a first passage hole that is arranged non-axially symmetrically with respect to the electron optical system, a second differential pumping diaphragm that is arranged on the sample side of the first differential pumping diaphragm and has a second passage hole that is arranged axially symmetrically with respect to the electron optical system, and a deflector that is arranged between the first differential pumping diaphragm and the second differential pumping diaphragm and adjusts the trajectory of the electron beam, and a control device that varies a cathode voltage applied to the photocathode with time and controls a deflection signal of the electron beam by the deflector in order to adjust image fluctuation that accompanies the time variation.

[0008] The electron beam generated by irradiating excitation light off-axis on the anode electrode facing the photoconductive film is converged by the electrostatic lens action formed in the gap between the photoconductive film and the anode electrode, and then deflected away from the axis of the anode electrode. This deflected electron beam passes through a first passage hole in a first differential exhaust diaphragm arranged asymmetrically off the optical axis, and then deflected back through a second passage hole in a second differential exhaust diaphragm arranged symmetrically to the optical axis, and is used as a probe for the electron microscope. [Effects of the Invention]

[0009] The above electron gun structure prevents gas molecules flowing into the electron gun chamber from a relatively low-vacuum chamber from reaching the electron-emitting area of ​​the photoelectric film, without compromising the high brightness and monochromaticity inherent in electron sources using photoelectric films made of p-type GaAs with an NEA surface. This reduces the decrease in brightness due to gas molecule adsorption on the photoelectric film surface and ion feedback, stabilizing the emission current and extending the life of the NEA surface. Furthermore, it is possible to mount and use an electron source using a photoelectric film with an NEA surface in an electron microscope used in a low-vacuum environment where the pressure around the sample is approximately 100 Pa. Details of this are described in the following examples. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an example of the schematic configuration of an electron gun according to a first embodiment. [Figure 2] FIG. 1 is a diagram showing a schematic configuration example of an electron gun of a comparative example. [Figure 3] 1A and 1B are diagrams showing an example of a schematic configuration of an electron gun according to a first embodiment and a potential distribution; [Figure 4] FIG. 1 is a diagram showing a schematic configuration of a scanning electron microscope according to a first embodiment. [Figure 5] FIG. 4 is a diagram showing temporal fluctuations of a cathode voltage according to the first embodiment. [Figure 6]FIG. 2 is a diagram illustrating an outline of a method for controlling an electron beam according to the first embodiment. [Figure 7A] FIG. 2 is a diagram showing a first configuration example of a first differential exhaust diaphragm according to the first embodiment. [Figure 7B] FIG. 4 is a diagram showing a second configuration example of the first differential exhaust diaphragm according to the first embodiment. [Figure 7C] FIG. 4 is a diagram showing a third configuration example of the first differential exhaust diaphragm according to the first embodiment. [Figure 8] 4 is a graph showing the relationship between the amount of off-axis of the excitation point and the deflection angle of the electron beam according to the first embodiment. [Figure 9A] FIG. 10 is a diagram showing a fourth configuration example of the first differential exhaust diaphragm according to the first embodiment. [Figure 9B] FIG. 10 is a diagram showing a fifth configuration example of the first differential exhaust diaphragm according to the first embodiment. [Figure 9C] FIG. 10 is a diagram showing a sixth configuration example of the first differential exhaust diaphragm according to the first embodiment. [Figure 10] FIG. 10 is a diagram showing an outline of an excitation optical system and an electron gun according to a second embodiment. [Figure 11] FIG. 10 is a diagram showing an outline of an excitation optical system according to a third embodiment. [Figure 12] FIG. 10 is a diagram showing an outline of an excitation optical system according to a fourth embodiment. [Figure 13] 4 is a flowchart showing an adjustment procedure for the electron gun according to the first embodiment. [Figure 14] 4 is a flowchart showing an adjustment procedure for the electron gun according to the first embodiment. [Figure 15] 3 is a diagram illustrating the passage holes of the differential exhaust diaphragm of the electron gun according to the first embodiment and the passage holes of the differential exhaust diaphragm of the electron gun of the comparative example. FIG. [Figure 16] 3 is a flowchart showing an image capturing method for an electron microscope according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. [Example]

[0012] Fig. 1 is a diagram showing a schematic configuration example of an electron gun according to a first embodiment. Fig. 2 is a diagram showing a schematic configuration example of an electron gun of a comparative example. Fig. 3 is a diagram showing a schematic configuration example and potential distribution of an electron gun according to the first embodiment. Fig. 4 is a diagram showing a schematic configuration example of a scanning electron microscope according to the first embodiment. Fig. 15 is a diagram explaining the passage holes of the differential exhaust diaphragm of the electron gun according to the embodiment and the passage holes of the differential exhaust diaphragm of the electron gun of the comparative example.

[0013] FIG. 1 shows an example of the structure of an electron gun according to an embodiment of the present invention. In FIG. 1, the configuration when an electron gun 10 according to this embodiment is mounted on a scanning electron microscope 100 will be described. In this embodiment, a method for controlling the trajectory of an electron beam 5 emitted from a photoelectric film 1 will be mainly described, and details of the configuration and mechanism of the excitation optical system will be described in embodiment 2 and onwards. The electron beam 5 can also be called an electron beam. The photoelectric film 1 is a photoelectric film whose surface has a negative electron affinity (NEA).

[0014] The photocathode used in this embodiment is configured such that a semiconductor photoelectric film (hereinafter referred to as photoelectric film) 1 that emits electrons when irradiated with excitation light is formed on a transparent substrate 2, and hereinafter the photocathode will be referred to as photoelectric film 1. The electron gun 10 of this embodiment is composed of a photoelectric film 1 formed on a transparent substrate 2, a condenser lens 3, an excitation optical system 4 that condenses and irradiates the photoelectric film 1 with excitation light 15 from an excitation light source 14 (the excitation optical system 4 includes the excitation light source 14, a viewing port 16, the condenser lens 3, the transparent substrate 2, and the photoelectric film 1), an anode electrode 6 that is arranged opposite the photocathode and that accelerates the electron beam 5 generated from the photoelectric film 1, a first differential exhaust diaphragm 7 that is provided to maintain an extremely high vacuum around the photoelectric film 1 and has a passage hole (first passage hole) 7A for the electron beam 5 in a non-axially symmetrical position, a second differential exhaust diaphragm 8 that has a passage hole (second passage hole) 8A for the electron beam 5 in an axially symmetrical position, a deflector 9, and a controller 24 as a control device.

[0015] The deflector 9 can be configured in multiple stages and is disposed between the first differential pumping diaphragm 7 and the second differential pumping diaphragm 8. The deflector 9 is controlled by the controller 24 to deflect the electron beam 5, which has passed through the passage hole 7A of the first differential pumping diaphragm 7, back onto the optical axis of the electron optical system before passing through the second differential pumping diaphragm 8, so that the electron beam 5 passes through the passage hole 8A of the second differential pumping diaphragm 8.

[0016] The above-mentioned axes or optical axes are expressions based on the assumption that the electron optical system inside the electron microscope 100, such as the electrodes for extracting the electron beam 5 from the photoelectric film 1 and the lens 3 for focusing the electron beam 5, has an ideal axisymmetric structure. In the actual configuration of the electron microscope 100, due to the machining accuracy and assembly accuracy of parts and components, even if each component is axisymmetric, the axes do not necessarily all overlap on the same straight line. For this reason, various alignment means such as the deflector 9 must be used to appropriately control the trajectory of the irradiating electron beam 5 and adjust the axis for a specific component. The axes that should pass through each component will be explained as appropriate in the following explanation of the embodiments.

[0017] FIG. 1 illustrates an electron gun 10 in which a photoelectric film 1 is placed. The electron gun 10 is installed in an electron gun chamber (also called a vacuum chamber) 11, the interior of which is maintained at an extremely high vacuum by a vacuum pumping system 13. The vacuum pumping system 13, which maintains the extremely high vacuum around the photoelectric film 1, utilizes an ion pump or a non-evaporable getter pump (NEG). The vacuum pumping system 13 is composed of a first vacuum pumping system 13a and a second vacuum pumping system 13b. The photoelectric film 1 is placed in the vacuum chamber 11 together with a condenser lens 3. Excitation light 15 emitted from an excitation light source 14 placed outside the vacuum chamber 11 passes through a viewing port 16 and is focused on the photoelectric film 1 by the condenser lens 3 placed behind the photoelectric film 1. This focused position is the excitation point 17 of the photoelectric film 1, and the emitted electron beam 5 is used as a probe for a scanning electron microscope 100. At this time, when continuous light is irradiated onto the photoelectric film 1 as excitation light 15, a continuous electron beam (called a continuous electron beam) 5 is emitted, and when pulsed light is irradiated, a pulsed electron beam (called a pulsed electron beam) 5 is emitted with a pulse width and pulse period similar to those of the excitation light 15. The structure of the electron gun 10 of this embodiment is effective under both operating conditions, for continuous electron beams and pulsed electron beams.

[0018] As shown in Figure 1, when a focusing lens 3 is placed near the back surface of the photoelectric film 1, which serves as the electron emission surface, excitation light 15 transmitted through the transparent substrate 2 can be focused with a large numerical aperture (NA) of 0.5 or greater. The focused diameter of excitation light 15 with wavelength λ focused onto the photoelectric film 1 by the focusing lens 3 with a numerical aperture NA is approximately equal to λ / NA. In this case, the size of the electron emission region of the photoelectric film 1 (virtual light source diameter) is approximately equal to the focused diameter of the excitation light 15. By activating the surface of the photoelectric film 1 with NEA, the lower end of the conduction band rises to an energy level higher than the vacuum level. Electrons 5 excited from the valence band to the conduction band upon irradiation with excitation light 15 are efficiently emitted from the inside of the photoelectric film 1 into the vacuum region. In particular, when the active layer of the photoelectric film 1 is p-type GaAs, the effective mass of electrons excited by light irradiation is small, only 0.067 times that of electrons in a vacuum, resulting in a small electron emission angle of approximately 10 degrees or less when emitted from the NEA surface into the vacuum region. These factors result in high brightness characteristics.

[0019] While high brightness can be obtained with an electron source using a photoelectric film 1 made of p-type GaAs with a high impurity concentration, the electron emission characteristics depend on the state of the NEA surface and are susceptible to adverse effects from gas molecules. To alleviate this problem, an exhaust system 13 is connected to each vacuum chamber, and multiple differential exhaust structures are installed between the electron gun chamber 11 and the sample chamber 18, each with apertures (passage holes 7A, 8A) of approximately 1 mm or less in diameter in the partition wall of each vacuum chamber to allow the electron beam 5 to pass through. The configuration of the sample chamber 18 can be seen in Figure 4.

[0020] However, in an axially symmetric electron microscope 100r structure in which the electron gun chamber 11 and the sample chamber 18 are aligned in a straight line, such as the structure of the electron gun 10r of the comparative example shown in Figure 2, gas flows from the vacuum chamber on the sample chamber 18 side, which has a relatively low degree of vacuum, to the vacuum chamber on the electron gun 10r side (e.g., electron gun chamber 11), which has a higher degree of vacuum, and some of the gas molecules that pass through the differential pumping diaphragms 8, 7r reach the surface of the photoelectric film 1, which is the electron source. Although the amount of gas molecules that reach the surface can be reduced by reducing the diameter (e.g., diameter) of the apertures of the differential pumping diaphragms 8, 7r, it is difficult to completely eliminate the adverse effects associated with the flow of gas molecules into the electron gun chamber 11 with the configuration of Figure 2.

[0021] In other words, the through hole 7Ar provided in the first differential exhaust diaphragm 7r and the through hole 8A provided in the second differential exhaust diaphragm 8 are arranged in a straight line, and both the through hole 7Ar and the through hole 8A are arranged in axially symmetrical positions.

[0022] 15, in this embodiment, the through hole 7A provided in the first differential pumping diaphragm 7 of the scanning electron microscope 100 is provided at a position that is asymmetrical with respect to the central axis 7ac of the first differential pumping diaphragm 7. On the other hand, the through hole 8A provided in the second differential pumping diaphragm 8 is provided at a position that is symmetrical with respect to the central axis 8ac of the second differential pumping diaphragm 8.

[0023] In contrast to this, in the comparative example, the through hole 7Ar provided in the first differential exhaust diaphragm 7r of the scanning electron microscope 100r is provided at a position symmetrical with respect to the central axis 7ac of the first differential exhaust diaphragm 7. Also, the through hole 8A provided in the second differential exhaust diaphragm 8 is provided at a position symmetrical with respect to the central axis 8ac of the second differential exhaust diaphragm 8.

[0024] The above-mentioned problem can be avoided by controlling the trajectory of the electron beam 5 emitted from the photoelectric film 1 to be non-axially symmetric. In this embodiment, the voltage 19 applied to the photoelectric film 1 is V0 (<0 V), the voltage 20 applied to the anode electrode 6 is set to ground potential (0 V), and the energy of the electron beam 5 passing through the anode electrode 6 is |eV0|, where e is the elementary charge. However, the voltage values ​​of the voltages 19 and 20 applied to each electrode are not limited to the above values. Furthermore, the anode electrode 6 may be configured to have multiple anode electrodes, such as a first anode electrode for controlling the electric field strength near the photoelectric film 1 and a second anode electrode for accelerating the electron beam after passing through the first anode electrode, so that different voltages can be applied to them.

[0025] When the applied voltages 19 and 20 to the photoelectric film 1 and the anode electrode 6 are V0 (<0 V) and 0 V, respectively, a lens field is generated near the photoelectric film 1, resulting in a convex lens effect 21, and near the anode electrode 6 in a concave lens effect 22, as shown in Figure 3. Because the photoelectric film 1 is a planar electron source, any point on the photoelectric film 1 can be used as the excitation point 17. When excitation light 15 is focused and irradiated so that the excitation point 17 on the photoelectric film 1 is off-axis with the anode electrode 6, the electron beam 5 emitted from the photoelectric film 1 is focused by the convex lens effect 21 and then deflected away from the axis (center axis) 12 of the anode electrode 6 by the concave lens effect 22, as shown in Figures 1 and 3. The position of the excitation point 17 on the photoelectric film 1 is adjusted so that the electron beam 5 passes through the first differential exhaust diaphragm 7, which has a non-axially symmetrical aperture 7A. The deflection angle of the electron beam 5 due to the concave lens effect 22 formed near the anode electrode 6 is θ0. When the anode electrode 6 is at ground potential (0V), if the excitation point 17 is fixed, the deflection angle θ0 will not change even if the applied voltage 19 (V0) to the photoelectric film 1 is changed. This is because, according to the laws of electron optics, the central trajectory of the electron beam 5 is preserved when each electrode voltage is changed to be n times the original. The off-axis amount (di) of the excitation point 17 is defined as the distance between the position of the excitation point 17 on the photoelectric film 1 and the position of the axis 12 of the anode electrode 6.

[0026] The electron beam 5 deflected by the concave lens action 22 formed near the anode electrode 6 is made to pass through a first differential exhaust diaphragm 7 having a passage hole 7A at a non-axially symmetrical position. A second differential exhaust diaphragm 8 disposed directly below the first differential exhaust diaphragm 7 has a passage hole 8A at an axially symmetrical position. In the sample chamber 18, an exhaust device (exhaust pump) is configured so that the pressure around the sample 23 can be set to, for example, a range of several tens of Pa to several hundreds of Pa. Gas flows from the vacuum chamber side of the sample chamber 18, which has a relatively low degree of vacuum, into the vacuum chamber (e.g., electron gun chamber 11) on the electron gun 10 side, which has a higher degree of vacuum.

[0027] As shown in Figures 1 and 4, a deflector 9 is disposed between the first differential pumping diaphragm 7 and the second differential pumping diaphragm 8. The deflector 9 (9A, 9B: see Figure 4) deflects the electron beam 5 that passes through the non-axially symmetrically positioned passage hole 7A and passes through the passage hole 8A of the second differential pumping diaphragm 8. Because the electron gun 10 is configured in this way, that is, the passage hole 7A is positioned non-axially symmetrically and the passage holes 7A and 8A are not aligned in a straight line, gas molecules that fly linearly upward from the sample chamber 18 through the passage hole 8A are blocked by the first differential pumping diaphragm 7 and do not reach the NEA surface of the photoelectric film 1. Meanwhile, the deflection control of the deflector 9 allows the electron beam 5 to be transported to the sample 23 without being blocked in the central portion where the current density is high. Therefore, the electron beam 5 emitted from the photoelectric film 1 can be used as a probe electron beam for the electron microscope 100 without impairing the high brightness characteristics that are a feature of the NEA surface. The probe electron beam 5 that can be used in this way has higher current stability than the configuration of the comparative example (see FIG. 2), and gas molecules do not reach the NEA surface of the photoelectric film 1, thereby achieving a longer lifespan of the NEA surface of the photoelectric film 1. Furthermore, as the lifespan of the NEA surface of the photoelectric film 1 increases, the frequency of surface activation treatment to regenerate the NEA surface of the photoelectric film 1 can be reduced, thereby reducing downtime of the electron microscope 100.

[0028] The multi-stage deflector 9 (9A, 9B: see Figure 4) used for trajectory control to deflect back the electron beam 5 that has passed through the first differential exhaust diaphragm 7, which has a passage hole 7A in an asymmetrical position, may be either an electrostatic (electric field) type or an electromagnetic (magnetic field) type. In particular, when using a photoelectric film 1 whose active layer is p-type GaAs, the electron gun 10 must be baked out at a high temperature of 200°C or higher during vacuum startup in order to create an ultra-high vacuum around the photoelectric film 1. For this reason, it is preferable that the components mounted in the electron gun chamber 11 be heat-resistant to temperatures of 200°C or higher and be made of materials that emit little gas in an ultra-high vacuum environment.

[0029] However, when controlling the deflection of the electron beam 5, deflection chromatic aberration, in which the amount of deflection depends on the energy of the electron beam 5, becomes a problem. The adverse effects of this deflection chromatic aberration are particularly likely to become apparent under irradiation conditions where the irradiation energy of the electron beam 5 is low. To take advantage of the narrow energy spread of the electron beam 5 emitted from the NEA surface, it is preferable to control the deflection so that the chromatic aberration caused by the deflection control does not become apparent. To minimize the adverse effects caused by the asymmetry caused by deflection control, an effective method is to control the alignment of the electron beam 5 by varying the cathode voltage 19 (V0) applied to the photoelectric film 1 over time (see Figure 5).

[0030] Next, an adjustment method for obtaining optimal alignment conditions will be described below based on the configuration of the electron microscope shown in Fig. 4. Fig. 5 is a diagram showing the time variation of the cathode voltage according to the first embodiment.

[0031] Consider minimizing deflection chromatic aberration at the focal point 33 of the electron lens 32 closest to the photoelectric film 1. In this example, we describe a case where the electron lens 32 closest to the photoelectric film 1 is an electrostatic Einzel lens. The electron beam 5 is focused by the final objective lens 34 and scans the sample 23. Signal electrons 35 generated at each point are detected by the detector 36, resulting in an SEM image. Under these SEM observation conditions, if the deflector 9 alignment conditions are optimal and the voltage amplitude ΔV is varied over time around the cathode voltage 19 (V0) (see Figure 5), the observed SEM image will exhibit periodic changes in image blur, repeating in-focus and out-of-focus states. On the other hand, if the deflector 9 alignment conditions are not optimal, in addition to the time variation in image blur, unidirectional image fluctuation will be observed. This image fluctuation occurs because the electron beam 5 with different irradiation energies reaches different locations on the sample 23 when the cathode voltage 19 (V0) is changed. The amplitude of the image fluctuation depends on the deflection conditions of the electron gun 10, and the conditions under which the amplitude of the image fluctuation is minimum correspond to the optimum alignment conditions of the deflector 9. The voltage amplitude ΔV of the cathode voltage 19 (V0) when it fluctuates over time is determined so that the image fluctuation can be identified with an appropriate amount of blur on the SEM image when the above alignment adjustment is performed, and an appropriate voltage amplitude ΔV is set within a range of 10% or less of the absolute value |V0| of the cathode voltage 19 (V0).

[0032] FIG. 6 is a diagram showing an outline of a method for controlling an electron beam according to the first embodiment.

[0033] As shown in Figure 6, the deflection angle of the electron beam 5 caused by the concave lens action 22 formed between the photoelectric film 1 and the anode electrode 6 is θ0, the deflection angle of the electron beam 5 caused by the deflector 9A on the electron source (photoelectric film 1) side mounted between the first differential exhaust diaphragm 7 having a passage hole 7A in a non-axially symmetric position and the second differential exhaust diaphragm 8 having a passage hole 8A in an axially symmetric position is θ1, and the deflection angle of the electron beam 5 caused by the deflector 9B on the sample 23 side is θ2. Using the virtual source position 37 of the electron beam 5 as a reference, let L0 be the distance to the deflection fulcrum caused by the concave lens action 22 formed between the photoelectric film 1 and the anode electrode 6, L1 be the distance to the deflection fulcrum of the electron beam 5 caused by the deflector 9A on the electron source (photoelectric film 1) side mounted between the first differential pumping diaphragm 7 with a non-axially symmetrical aperture 7A and the second differential pumping diaphragm 8 with a axially symmetrical aperture 8A, and L2 be the distance to the deflection fulcrum of the electron beam 5 caused by the deflector 9B on the sample 23 side. Here, the condition for minimizing deflection-related aberration corresponds to the condition for minimizing image fluctuation in the SEM image when the cathode voltage 19 is varied over time. When electrostatic deflectors are used for the deflectors 9A and 9B, the following relational expression (Equation 1) holds true for the condition for minimizing image fluctuation in the SEM image.

[0034] L0θ0+L1θ1+L2θ2= 0 (Equation 1) If the above alignment adjustment is performed on the electron lens 32 closest to the photoelectric film 1, the subsequent alignment adjustment can be performed in the same manner as in conventional electron microscopes to obtain the desired irradiation performance.

[0035] These controls are performed by a controller 24, which is a control device. The control device 24 varies the cathode voltage applied to the photoelectric film 1 over time and controls the deflection signal of the electron beam 5 from the deflector 9 to adjust for image fluctuations associated with the time variation. Note that the alignment adjustment of the electron beam 5 by varying the cathode voltage 19 (V0) over time described above is intended to minimize the adverse effects of chromatic aberration caused by the deflection system and is not an essential control. This control is not necessary, particularly when the energy of the electron beam 5 passing through the deflectors 9A and 9B is high; the adverse effects on the sample 23 can be sufficiently reduced by controlling the magnification of the electron optical system. Furthermore, even when the energy of the electron beam 5 passing through the deflectors 9A and 9B is low, this control is not necessary when used under observation conditions where adverse effects caused by the deflection system are not apparent, such as low-magnification observation.

[0036] In an ideal electron gun 10 configuration, the electron beam 5 needs to be deflected in a direction that creates an electron beam passage hole 7A that is asymmetrical with respect to the optical axis. Therefore, multiple stages of deflectors 9 capable of generating a dipole field would be required, with a minimum configuration consisting of two stages (9A and 9B). However, in reality, the path of the electron beam 5 may be adversely affected by factors such as fringe fields, leakage magnetic fields, and geomagnetism, making it impossible to achieve the desired alignment conditions with a two-stage dipole field. In particular, when the applied voltage 19 (V0) to the photosensitive film 1 is changed, it is expected that the adverse effects of asymmetrical electron beams will differ depending on whether |V0| is small (low electron beam irradiation energy) or large (high electron beam irradiation energy). In order to obtain optimal alignment conditions taking this situation into consideration, the deflector 9 is constructed with electromagnetic poles of multipole elements with good symmetry so that it can generate multipole fields such as quadrupole fields, hexapole fields, and octopole fields, thereby providing sufficient freedom to correct the adverse effects caused by asymmetry.

[0037] Next, the detailed structure of the first differential pumping diaphragm 7, which is placed directly below the photoelectric film 1 and has an electron beam passage hole 7A in a non-axially symmetrical position, will be described below. FIG. 7A is a diagram showing a first configuration example of the first differential pumping diaphragm according to the first embodiment. FIG. 7B is a diagram showing a second configuration example of the first differential pumping diaphragm according to the first embodiment. FIG. 7C is a diagram showing a third configuration example of the first differential pumping diaphragm according to the first embodiment. FIG. 8 is a graph showing the relationship between the amount of off-axis of the excitation point and the deflection angle of the electron beam according to the first embodiment.

[0038] The simplest configuration of the first differential pumping diaphragm 7 is a structure in which a single aperture (passing hole) 7A is arranged non-axially symmetrically, with the center of the central axis 7ac of the first differential pumping diaphragm 7 blocked (Figure 7A). The off-axis eccentricity Lec of the aperture 7A depends on the electrode distance between the photoelectric film 1 and the anode electrode 6, the aperture diameter of the anode electrode 6, the off-axis amount di of the excitation point 17, and the distance Lapt from the anode electrode 6 to the mounting position of the off-axis aperture 7A. Therefore, the trajectory of the electron beam 5 is calculated in advance, taking into account the electrode structure inside the electron gun 10, and the mounting position, aperture diameter, off-axis eccentricity Lec, and off-axis amount di of the excitation point on the photoelectric film of the first differential pumping diaphragm 7 having the passing hole 7A in a non-axially symmetrical position can be determined. Note that multiple first differential pumping diaphragms 7 having passing holes 7A in a non-axially symmetrical position may be arranged on the diaphragm surface.

[0039] Fig. 7B shows an example of a configuration in which two off-axis throttling holes (passing holes) 7A are arranged in a non-axisymmetrical arrangement, and Fig. 7C shows an example of a configuration in which three off-axis throttling holes (passing holes) 7A are arranged in a non-axisymmetrical arrangement. In Figs. 7A to 7C, the non-axisymmetrical passing holes 7A are circular holes, but the shape of the throttling holes is not limited to circular as long as the required differential pumping performance can be obtained, and they may be configured as rectangular or elliptical passing holes.

[0040] As an example, for the electrode structure shown in Figure 1, Figure 8 shows the calculated dependence of the deflection angle θ0 due to the concave lens action 22 on the off-axis amount (di) of the excitation point 17 when the distance between the photoelectric film 1 and the anode electrode 6 is 1 mm, 1.5 mm, 2 mm, and 2.5 mm. Increasing the distance Lapt between the anode electrode 6 and the first differential pumping diaphragm 7, which has a non-axisymmetrically arranged passage hole 7A, increases the off-axis amount (di) of the electron beam 5 on the surface of the first differential pumping diaphragm 7, which has a non-axisymmetrically arranged passage hole 7A. However, because the electron beam 5 spreads laterally, it must be noted that if the aperture diameter of the passage hole 7A (the diameter of the circular hole of the passage hole 7A in a plan view) is fixed, the amount of current of the electron beam 5 that can pass through the aperture is limited. Typically, when the gap distance between the photoelectric film 1 and the anode electrode 6 is 1 mm, if the distance Lapt between the anode electrode 6 and the mounting position of the first differential pumping diaphragm 7 having passage holes 7A arranged non-axisymmetrically is 100 mm, the eccentricity Lec of the off-axis diaphragm hole (passing hole) 7A can be set to about 0.45 mm, and the diameter of the diaphragm of the passage hole 7A can be set to a maximum of φ0.6 mm. This allows the first differential pumping diaphragm 7 having diaphragm holes (passing holes) 7A arranged non-axisymmetrically with their centers blocked to be used as a differential pumping diaphragm.

[0041] Next, a method for adjusting an electron microscope 100 having a first differential pumping diaphragm 7 with diaphragm holes (passage holes) 7A arranged non-axially symmetrically will be described. Fig. 9A is a diagram showing a fourth configuration example of the first differential pumping diaphragm according to the first embodiment. Fig. 9B is a diagram showing a fifth configuration example of the first differential pumping diaphragm according to the first embodiment. Fig. 9C is a diagram showing a sixth configuration example of the first differential pumping diaphragm according to the first embodiment.

[0042] When using the electron beam 5 generated from the NEA surface of the photoelectric film 1 in the electron gun 10 of this embodiment as a probe electron beam for the electron microscope 100, it is practically easier to use a configuration in which, as in the conventional case, a first adjustment step is performed to adjust the first differential pumping diaphragm 7 to an axisymmetric configuration, followed by a second adjustment step to deflect and control the electron beam 5 toward the non-axisymmetrically arranged electron beam passing aperture (7A) of the first differential pumping diaphragm 7. From this perspective, a configuration in which the first differential pumping diaphragm has a passing aperture (third through-hole, third passing aperture) 7C for the electron beam 5 emitted under axisymmetric conditions at the center (central axis 7ac) is considered, in addition to the non-axisymmetrically arranged electron beam passing aperture (7A), as shown in Figures 9A to 9C. When using this structure, a linear feedthrough is used to insert and remove a shielding member (also called a shielding means) such as a shielding plate for shielding the electron beam in a linear direction from the atmospheric region above or below the first differential pumping diaphragm 7. In the initial adjustment, the electron beam 5 is transported to the sample 23 under axially symmetric conditions, and the centering adjustment of the electron lens and the alignment adjustment of the electron beam 5 are completed. After that, a shielding plate is used to block the electron beam 5 from passing through the central aperture hole 7C of the axially symmetric arrangement. Note that a current measuring means for the electron beam 5 that passes through the central aperture hole 7C may be mounted on the tip of the linear feedthrough in order to adjust the position of the focusing lens 3 so that it is near the center of the anode electrode 6. In this case, the current measuring means can also be used as a shielding member for blocking the central aperture hole 7C.

[0043] Next, the adjustment procedure for the electron gun 10 having the first differential exhaust diaphragm 7 (FIGS. 9A to 9C) with the diaphragm holes (passage holes) 7A and passage holes 7C arranged non-axially asymmetrically will be described with reference to FIG. 13. FIG. 13 is a flowchart showing the adjustment procedure for the electron gun according to the first embodiment. FIG. 13 shows a flowchart of the control procedure for the electron beam 5 during initial adjustment for the diaphragm holes (passage holes) 7A arranged non-axially asymmetrically, in which a passage hole 7C for the electron beam 5 arranged axially symmetrically at the center is provided in addition to the diaphragm arranged non-axially asymmetrically. Each step (S10-S17) of FIG. 13 will be described below.

[0044] (S10): The initial adjustment of the electron gun 10 is started.

[0045] (S11): A current measuring means is placed directly below the central aperture 7C.

[0046] (S12): The photoelectric film 1 is irradiated with the excitation light 15, and the current is measured by the current measuring means.

[0047] (S13): The excitation position on the photoelectric film 1 and the focusing of the excitation light 15 are adjusted so that the measurement current is maximized.

[0048] (S14): Next, the excitation optical system is adjusted and controlled so that the excitation point 17 on the photoelectric film 1 is off-axis with respect to the anode electrode 6. The adjustment of the position of the excitation point 17 on the photoelectric film 1 is performed by applying the methods described in Examples 2 to 4 described below. Based on the case where the electron beam 5 passes through the central axisymmetrically arranged passage hole 7C, the optical path of the excitation optical system is adjusted so that the electron beam 5 reaches an appropriate position at the position of the aperture hole (passage hole) 7A arranged asymmetrically.

[0049] (S15): Following the above procedure, the deflector 9 is adjusted so that the electron beam 5, which has passed through the aperture (passage hole) 7A arranged asymmetrically, reaches the sample 23 placed in the sample chamber 18 of the electron microscope 100.

[0050] (S16): Under the condition that the electron beam passes through the differential pumping aperture installed between the electron gun and the sample chamber, it is possible to confirm that the electron beam reaches the sample chamber by observing an image of the electron microscope using a detector installed in the sample chamber. The detection signal is checked, and if the observed image is good (Yes), proceed to S17. If the detection signal is checked and the observed image is not good (No), proceed to S11, and repeat and execute S11-S15.

[0051] (S17): The initial adjustment of the electron gun 10 is completed.

[0052] Next, the adjustment procedure for the electron gun 10 having the first differential exhaust diaphragm 7 (FIGS. 7A to 7C) with the diaphragm holes (passage holes) 7A arranged non-axially symmetrically (without the passage hole 7C) will be described with reference to FIG. 14. FIG. 14 is a flowchart showing the adjustment procedure for the electron gun according to the first embodiment. FIG. 14 shows a flowchart of the electron beam control procedure during initial adjustment when the diaphragm 7A arranged non-axially symmetrically does not have a central diaphragm hole. Each step (S20-S27) of FIG. 14 will be described below.

[0053] (S20): The initial adjustment of the electron gun 10 is started.

[0054] (S21): If the aperture 7A arranged non-axially symmetrically does not have a central aperture hole, the case where the electron beam 5 passes through the central axially symmetrically arranged passage hole 7C cannot be used as a reference, so a current measuring means is placed directly below the aperture hole (passage hole) 7A arranged non-axially symmetrically.

[0055] (S22): The photoelectric film 1 is irradiated with the excitation light 15, and the current is measured by the current measuring means.

[0056] (S23): The excitation position on the photoelectric film 1 is adjusted so that the measured current is maximized.

[0057] (S24): The current measuring means is moved from below the aperture hole 7A arranged asymmetrically.

[0058] (S25): The deflector 9 is adjusted so that the electron beam 5 reaches the sample 23 placed in the sample chamber 18 of the electron microscope 100.

[0059] (S26): Under the condition that the electron beam passes through the differential pumping aperture installed between the electron gun and the sample chamber, it is possible to confirm that the electron beam reaches the sample chamber by observing an image of the electron microscope using a detector installed in the sample chamber. The detection signal is checked, and if the observed image is good (Yes), proceed to S27. If the detection signal is checked and the observed image is not good (No), proceed to S21, and repeat and execute S21-S25.

[0060] (S27): The initial adjustment of the electron gun 10 is completed.

[0061] As a result, the electron beam 5 can be used as a probe electron beam for the electron microscope 100 under the same conditions as when the first differential exhaust aperture 7 for the electron beam 5, which is arranged substantially axially symmetrically at the center, has a passage hole 7C.

[0062] 16 is a flowchart showing the image capturing method for an electron microscope according to the first embodiment. Therefore, as shown in FIG. 16, the image capturing method for an electron microscope is as follows: 0) Initial setting process (see Figures 13 and 14); 1) a first step of generating an electron beam 5 from an excitation point 17 of a photoelectric film 1; 2) a second step of accelerating the electron beam 5 by an anode electrode 6; 3) a third step of passing the accelerated electron beam 5 through a first passage hole 7A provided at a non-axially symmetric position in the first differential exhaust diaphragm 7; 4) a fourth step of adjusting the trajectory of the electron beam 5 having passed through the first passage hole 7A by a deflector 9, and causing the electron beam 5 to pass through a second passage hole 8A provided at an axially symmetrical position in a second differential exhaust diaphragm 8; 5) A fifth step of irradiating the sample 23 with the electron beam 5 that has passed through the second passage hole 8A to obtain an observation image.

[0063] The initial setting process of the electron microscope image capturing method (see FIGS. 13 and 14) is as follows: 6) a sixth step of arranging a current measuring means below the first passage hole 7A of the first differential exhaust diaphragm 7 and measuring the emission current of the photoelectric film 1; 7) a seventh step of adjusting the position of the excitation point 17 of the excitation light 15 on the photoelectric film 1 so that the measured emission current is maximized; 8) an eighth step of adjusting the deflector 9 so that the electron beam 5 that has passed through the first passage hole 7A reaches the sample 23.

[0064] The electron gun 10 described above can reduce the adverse effects of gas molecules, making it possible to apply an electron source using a photoelectric film 1 with an NEA surface to samples that require low vacuum conditions, such as biological samples such as cells, and to electron microscopes equipped with a sample chamber 18 for environmentally controlled measurement of solid-gas reactions in situ.

[0065] In this embodiment, the electron gun of the present invention is mounted on a scanning electron microscope, but a similar electron gun structure can also be applied to electron beam application devices such as transmission electron microscopes and scanning transmission electron microscopes. [Example]

[0066] Next, a second embodiment will be described with reference to Fig. 10. Fig. 10 is a diagram showing a schematic configuration example of an excitation optical system and an electron gun according to the second embodiment.

[0067] In this embodiment, a configuration is shown in which the structure of the electron gun 10 described in Example 1 is combined with an excitation optical system 4 (see Figure 1) for focusing and irradiating excitation light 15 onto the active layer of the photoelectric film 1 so that the excitation point 17 on the photoelectric film 1 is off-axis with the anode electrode 6.

[0068] The photoelectric film 1 is placed in a vacuum chamber (electron gun chamber) 11 together with a condenser lens 3, and excitation light 15 emitted from an excitation light source 14 placed outside the vacuum chamber 11 is shaped into parallel light by a collimator lens 51, and after passing through a viewing port 16 is condensed onto the active layer of the photoelectric film 1 by the condenser lens 3. In order to monitor the condensing state of the excitation light 15 on the photoelectric film 1, the excitation optical system 4 may be configured so that the reflected light from the photoelectric film 1 is reflected off the optical axis of the excitation optical system 4 and condensed onto an imaging element by a projection lens. 50 denotes the optical axis of the excitation optical system 4.

[0069] When using a photoelectric film 1 made of p-type GaAs, the excitation wavelength is preferably 760 to 800 nm. When a focusing lens 3 is placed near the back surface of the photoelectric film 1, which serves as the electron emission surface, the excitation light 15 transmitted through the transparent substrate 2 can be focused with a large numerical aperture of 0.5 or greater. The focused diameter of excitation light of wavelength λ focused on the photoelectric film by a focusing lens with a numerical aperture NA is approximately equal to λ / NA, and the optimal spot diameter is approximately φ1 μm in FWHM. In this case, the electron emission region is used as a point source with a diameter of approximately φ1 μm. When the probe electron beam of the electron microscope 100 is used as a continuous electron beam, continuous light is irradiated; when it is used as a pulsed electron beam, pulsed light is irradiated. Any light source can be used as the excitation light source, such as a spatial light output or an optical fiber output, as long as it can output the intensity required to emit electrons from the photoelectric film 1.

[0070] Figure 10 shows an example of the configuration of the excitation optical system 4 for setting the excitation point 17 off-axis from the anode electrode 6 on the photoelectric film 1. The excitation optical system 4 consists of a light source 14, a collimator lens 51, a condenser lens 3, a transparent electrode 2, and the photoelectric film 1. The excitation light 15 emitted by the light source 14 is collimated by the collimator lens 51 and then focused by the condenser lens 3 onto the photoelectric film 1. The condenser lens 3 is fixed inside the vacuum chamber of the electron gun 10. The light source 14 and collimator lens 51 of the excitation optical system 4 are fixed to the flange of the viewing port 16 using a dedicated holder for fixing optical elements. The entire excitation optical system 4 can be adjusted horizontally by adjusting a four-way set screw 52 installed at the top of the vacuum chamber 11 of the electron gun 10. The distance between the condenser lens 3 and the photoelectric film 1 is adjusted to minimize the diameter of the focused spot on the photoelectric film 1 by adjusting the amount of rotation of a screw part 53, which serves as a focal length adjustment mechanism. For details of the mechanism for adjusting the position of the optical system 4 in the electron gun 10 using the photoelectric film 1, please refer to Fig. 3 in Journal of Applied Physics 103, 064905 (208).

[0071] 10 shows a configuration in which the horizontal position of the excitation optical system 4 is adjusted so that the excitation point 17 is located at a position off-axis by a distance (off-axis amount) di from the central axis of the anode electrode 6. By using the above adjustment mechanism, the position of the condenser lens 3 is adjusted to set the optimal excitation point 17 so that the electron beam 5 passes through the single-aperture diaphragm 7A arranged asymmetrically as shown in FIG. 7A and FIG. 9A. In this way, the electron beam 5 emitted from the NEA surface of the photoelectric film 1 is used as the probe electron beam of the electron microscope. [Example]

[0072] Next, a third embodiment will be described with reference to Fig. 11. Fig. 11 is a diagram showing an outline of an excitation optical system according to the third embodiment.

[0073] In this embodiment, the electron gun structure described in the first embodiment is configured such that the excitation optical system 4 (see FIG. 1) is fixed and excitation light 15 can be focused and irradiated onto multiple points on the photoelectric film 1, and multiple excitation points 17 on the photoelectric film 1 can be used without adjusting the optical path of the excitation optical system 4.

[0074] The photoelectric film 1 is placed inside a vacuum chamber 11 together with a condenser lens 3. Excitation light 15 emitted from an excitation light source 14 located outside the vacuum chamber 11 is collimated by a collimator lens 51 and focused onto the active layer of the photoelectric film 1 by the condenser lens 3 after passing through a viewing port 16. To monitor the focusing state of the excitation light 15 on the photoelectric film 1, the optical system 4 may be configured to reflect the light reflected from the photoelectric film 1 off the optical axis of the excitation optical system 4 and focus it onto an image sensor using a projection lens. When using a photoelectric film 1 made of p-type GaAs, the excitation wavelength is preferably 760 to 800 nm. When the condenser lens 3 is positioned near the back surface of the photoelectric film 1, which serves as the electron emission surface, the excitation light 15 transmitted through the transparent substrate 2 can be focused with a large numerical aperture of 0.5 or greater. The focused diameter of the excitation light 15 of wavelength λ focused onto the photoelectric film 1 by the condenser lens 3 with a numerical aperture NA is approximately equal to λ / NA, with an optimal spot diameter of approximately φ1 μm in FWHM. At this time, the electron emission area is used as a point source with a size of about φ1 μm. When the probe electron beam of the electron microscope 100 is used as a continuous electron beam, continuous light is irradiated, and when it is used as a pulsed electron beam, pulsed light is irradiated.

[0075] An example configuration of the excitation optical system 4 for setting the excitation point 17 off-axis of the anode electrode 6 on the photoelectric film 1 is shown in Figure 11. The excitation optical system 4 is composed of a light source 14, a collimator lens 51, a condenser lens 3, a transparent electrode 2, and a photoelectric film 1. We will explain the case where this configuration is combined with a first differential exhaust diaphragm 7 having multiple passage holes 7A in a non-axially symmetrical arrangement, such as in Figures 7B, 7C, 9B, and 9C.

[0076] The excitation light source 14 is composed of a multi-core fiber 55, and the excitation light source 14 is connected to the multi-core fiber 55. The condenser lens 3 is fixed inside the vacuum chamber 11 using a mechanism similar to that shown in FIG. 10 , and is positioned and fixed near the center of the anode electrode 6 according to the procedure described in Example 2. In this system, the optical path of the excitation light 15 corresponding to the non-axisymmetrically arranged aperture (passing hole) 7A is non-axisymmetric with respect to the excitation optical system 4. The excitation light 15 corresponding to the electron beam 5 passing through the non-axisymmetrically arranged aperture (passing hole) 7A passes off-axis through the collimator lens 51 and the condenser lens 3 and is condensed and irradiated onto the active layer of the photoelectric film 1. However, as long as the off-axis amount (di) of the condensing position of the excitation light 15 is within the maximum angle of view that can be condensed by the condenser lens 3, it is possible to obtain focusing characteristics comparable to those obtained when the optical system 4 is configured with an axially symmetric optical path.

[0077] With the above configuration, by switching the output of the excitation light source 14, it is possible to switch the path of the electron beam 5 for use with multiple apertures 7A arranged asymmetrically with respect to axis by switching the output of the excitation light source 14. Using the distance do between the fiber ends of the multi-core fiber 55, the focal length fo of the collimator lens 51, and the focal length fi of the condenser lens 3, the distance di between the two excitation points 17 focused on the photoelectric film 1 satisfies the following relation (Equation 2):

[0078] di = do × fi / fo (Equation 2) The eccentricity Lec of the aperture hole (passing hole) 7A arranged non-axially symmetrically is calculated based on electron trajectory calculation so that the electron beam emitted from the position of the excitation point at the off-axis amount di passes through the aperture hole (passing hole) 7A arranged non-axially symmetrically.

[0079] At this time, the control system controller 24 controls the deflector 9 in conjunction with the electron beam 5 after passing through the non-axially symmetrically arranged aperture (passing hole) 7A in accordance with the output of the excitation light source 14. By periodically switching the output of the excitation light source 14 and the deflection direction, the NEA surface of the photoelectric film 1 can be extended in life, enabling it to be used as a stable electron source over a long period of time. A pulsed light source with a pulse width and pulse interval of at least 1 nanosecond can be used as the excitation light source 14, allowing the excitation points 17 to be switched at time intervals of approximately 1 nanosecond. If the emission intensity of the electron beam 5 varies depending on the excitation point 17 on the photoelectric film 1, the emission current of the photoelectric film 1 can be stabilized and used by changing the irradiation intensity of the excitation light for each excitation point 17. [Example]

[0080] Next, a fourth embodiment will be described with reference to Fig. 12. Fig. 12 is a diagram showing an outline of an excitation optical system according to the fourth embodiment.

[0081] In this embodiment, the electron gun structure described in the first embodiment is configured to focus and irradiate excitation light 15 onto multiple points on the photoelectric film 1 by utilizing optical elements arranged on the optical path of the excitation optical system 4 (see FIG. 1).

[0082] The photoelectric film 1 is placed inside a vacuum chamber 11 together with a condenser lens 3. Excitation light 15 emitted from an excitation light source 14 located outside the vacuum chamber 11 is collimated by a collimator lens 51 and focused onto the active layer of the photoelectric film 1 by the condenser lens 3 after passing through a viewing port 16. To monitor the focusing state of the excitation light 15 on the photoelectric film 1, the optical system 4 may be configured to reflect the light reflected from the photoelectric film 1 off the optical axis of the excitation optical system 4 and focus it onto an image sensor using a projection lens. When using a photoelectric film 1 made of p-type GaAs, the excitation wavelength is preferably 760 to 800 nm. By placing the condenser lens 3 near the back surface of the photoelectric film 1, which serves as the electron emission surface, the excitation light transmitted through the transparent substrate 2 can be focused with a large numerical aperture of 0.5 or greater. The focused diameter of the excitation light 15 of wavelength λ focused onto the photoelectric film 1 by the condenser lens 3 with a numerical aperture NA is approximately equal to λ / NA, with an optimal spot diameter of approximately φ1 μm in FWHM. In this case, the electron emission area is used as a point source with a diameter of about 1 μm. When the probe electron beam of the electron microscope 100 is used as a continuous electron beam, continuous light is irradiated, and when it is used as a pulsed electron beam, pulsed light is irradiated. The excitation light source 14 can be any light source that can output the intensity required to emit electrons from the photoelectric film 1, such as a spatial light output or an optical fiber output.

[0083] An example configuration of the excitation optical system 4 for setting the excitation point 17 off-axis of the anode electrode 6 on the photoelectric film 1 is shown in Figure 12. The excitation optical system 4 is composed of a light source 14, a collimator lens 51, a condenser lens 3, a transparent electrode 2, and a photoelectric film 1. We will explain the case where this configuration is combined with a first differential exhaust diaphragm 7 having multiple passage holes 7A in a non-axially symmetrical arrangement, such as in Figures 7B, 7C, 9B, and 9C.

[0084] In this embodiment, an optical element (wedge prism) 56 with a wedge-shaped cross section is placed in the region between the collimator lens 51 and the condenser lens 3 to bend the excitation light 15 off the optical axis of the excitation optical system 14. The parallel light incident on the wedge prism 56 is refracted according to Snell's law, as shown in FIG. 12 . The angle at which the parallel light is refracted depends on the material (refractive index) of the wedge prism 56 and the inclination angle of the inclined surface with respect to the optical axis. Therefore, the refraction angle by the wedge prism 56 is determined based on the off-axis amount di of the excitation point at which the electron beam 5 generated on the NEA surface of the photoelectric film 1 can pass through the non-axisymmetrically arranged aperture holes (passing holes) 7A of the first differential exhaust aperture 7.

[0085] As in Example 3, the refracted excitation light 15 passes off-axis through the focusing lens 3 and is focused and irradiated onto the active layer of the photoelectric film 1, but if the off-axis amount di of the focusing position of the excitation light 15 is within the maximum angle of view that can be focused by the focusing lens 3, the configuration of the excitation optical system 4 can obtain focusing characteristics similar to those when the optical path is axially symmetric.

[0086] As in Example 3, it is preferable that the horizontal position of the condenser lens 3 is adjusted and fixed near the center of the anode electrode 6. Therefore, the optical path is first adjusted without the wedge prism 56, and the position of the condenser lens 3 is adjusted so that it is near the center of the anode electrode 6.

[0087] The excitation light can be bent in any direction by rotating the wedge prism 56 around the optical axis of the excitation optical system 4. For this reason, by mounting the wedge prism 56 on a rotation mechanism and controlling the rotation angle of the wedge prism 56 so that the electron beam 5 emitted from an excitation point off the axis of the anode electrode can pass through the aperture hole (passage hole) 7A arranged asymmetrically in the first differential exhaust aperture 7, it is possible to switch the excitation point 17 on the photoelectric film 1 and utilize the emitted electron beam 5.

[0088] At this time, the control system controller 24 controls the deflector 9 in conjunction with the electron beam 5 after passing through the non-axially symmetrically arranged aperture (passage hole) 7A of the first differential exhaust aperture 7, in accordance with the rotation angle of the wedge prism 56. By periodically switching the rotation angle of the wedge prism 56 and the corresponding deflection direction, the NEA surface of the photoelectric film 1 can be extended in life, enabling it to be used as a stable electron source over a long period of time. A pulsed light source with a pulse width and pulse interval of at least 1 nanosecond can be used as the excitation light source 14, allowing the excitation points 17 to be switched at time intervals of approximately 1 nanosecond. If the emission intensity of the electron beam 5 varies depending on the excitation point 17 on the photoelectric film 1, the emission current of the photoelectric film 1 can be stabilized and used by changing the irradiation intensity of the excitation light for each excitation point 17.

[0089] The invention made by the inventor has been specifically described above based on examples, but it goes without saying that the present invention is not limited to the above-described embodiments and examples, and various modifications are possible. [Explanation of symbols]

[0090] 1...photoelectric film, 2...transparent substrate, 3...condenser lens, 4...optical system, excitation optical system, 5...electrons, electron beam, 6...anode electrode, 7...first differential exhaust diaphragm, 7A...electron beam passage hole (first passage hole) of diaphragm arranged asymmetrically, 7B...center of diaphragm plate, 7C...electron beam passage hole (third passage hole) of center of diaphragm plate, 8...second differential exhaust diaphragm, 8A...electron beam passage hole (second passage hole) of diaphragm arranged asymmetrically, 9...multiple stages of deflector, 9A...upper deflector, 9B...lower deflector, 10...electron gun, 11...electron gun chamber, 12...axis of anode electrode, 13...exhaust system (ion pump, NEG pump), 14...light source, excitation light source, 15...excitation Light, 16...viewing port, 17...excitation point, 18...sample chamber, 19...applied voltage to photoelectric film (cathode voltage), 20...applied voltage to anode electrode, 21...convex lens action of electrostatic lens, 22...concave lens action of electrostatic lens, 23...sample, 24...control system (controller), 32...electron lens, 33...focusing point of electron lens, 34...objective lens, 35...signal electron, 36...detector, 37...virtual light source, 50...optical axis of excitation optical system, 51...collimator lens, 52...set screw, 53...screw part, focal length adjustment mechanism, 55...multi-core fiber, 56...wedge prism (optical element with a wedge-shaped cross section)

Claims

1. An excitation light source that generates excitation light; a photocathode having a transparent substrate and a photoelectric film; a condenser lens that condenses the excitation light toward the photocathode; an anode electrode disposed opposite the photocathode, the anode electrode accelerating electron beams generated from excitation points on the photoelectric film of the photocathode when excitation light focused by the focusing lens is incident on the anode electrode after passing through a transparent substrate of the photocathode; a first differential exhaust diaphragm disposed on the side of the photocathode and having a first passage hole disposed non-axially symmetrically with respect to the electron optical system; a second differential exhaust diaphragm which is arranged closer to the sample than the first differential exhaust diaphragm and has second passage holes which are arranged axially symmetrically with respect to the electron optical system; a deflector disposed between the first differential exhaust aperture and the second differential exhaust aperture, for adjusting a trajectory of the electron beam; An electron microscope characterized by comprising a control device that controls a deflection signal of the electron beam by the deflector in order to vary the cathode voltage applied to the photocathode over time and adjust image fluctuations that accompany the time variation.

2. 2. An electron microscope according to claim 1, wherein said photoelectric film is a semiconductor having a surface with a negative electron affinity.

3. 3. The electron microscope according to claim 1, further comprising an adjustment mechanism for adjusting the horizontal position of an excitation optical system including said excitation light source, said condenser lens, and said photoelectric film.

4. It has a multi-core fiber, 3. The electron microscope according to claim 1, wherein the excitation point on the photoelectric film is switched by the output of the excitation light source connected to the multi-core fiber.

5. 5. The electron microscope according to claim 4, wherein said control device controls the output of said excitation light source and the direction of deflection of said electron beam by said deflector in conjunction with each other.

6. 3. The electron microscope according to claim 1, wherein said first differential exhaust diaphragm has a third passage hole arranged axially symmetrically with respect to said electron optical system.

7. 7. The electron microscope according to claim 6, further comprising a shielding means for shielding said third passage hole of said first differential exhaust diaphragm.

8. 8. The electron microscope according to claim 7, wherein said shielding means has a current measuring means.

9. a sample chamber in which the sample is placed; 3. The electron microscope according to claim 1, wherein the pressure around the sample can be set in a range of several tens of Pa to several hundreds of Pa.

10. a first step of generating an electron beam from an excitation point of a photoelectric film; a second step of accelerating the electron beam by an anode electrode; a third step of passing the accelerated electron beam through a first passage hole provided at a non-axially symmetric position in a first differential exhaust diaphragm; a fourth step of adjusting the trajectory of the electron beam having passed through the first passage hole by a deflector to pass the electron beam through a second passage hole provided at an axially symmetrical position in a second differential exhaust diaphragm; a fifth step of irradiating the electron beam onto the sample after the electron beam has passed through the second passage hole to obtain an observation image, moreover, a sixth step of disposing a current measuring means under the first through hole of the first differential exhaust diaphragm and measuring an emission current of the photoelectric film; a seventh step of adjusting the position of the excitation point on the photosensitive film so that the measured emission current is maximized; and an eighth step of adjusting the deflector so that the electron beam that has passed through the first passage hole reaches the sample.

11. an excitation light source that generates excitation light; a photocathode having a transparent substrate and a photoelectric film; a condenser lens that condenses the excitation light toward the photocathode; an anode electrode disposed opposite the photocathode, the anode electrode accelerating electron beams generated from excitation points on the photoelectric film of the photocathode when excitation light focused by the focusing lens is incident on the anode electrode after passing through a transparent substrate of the photocathode; a first differential exhaust diaphragm disposed on the side of the photocathode and having a first passage hole disposed non-axially symmetrically with respect to the electron optical system; a second differential exhaust diaphragm which is arranged closer to the sample than the first differential exhaust diaphragm and has second passage holes which are arranged axially symmetrically with respect to the electron optical system; a deflector disposed between the first differential exhaust aperture and the second differential exhaust aperture, for adjusting a trajectory of the electron beam; a predetermined distance is provided between the excitation point and the optical axis of the electron optical system, the emission direction of the electron beam from the surface of the photocathode is set to a direction substantially parallel to the optical axis of the electron optical system, and the deflector is used to control the traveling direction of the electron beam to a direction substantially parallel to the optical axis; a voltage applied to the photoelectric film is set to a negative voltage, a voltage applied to the anode electrode is set to a ground potential, and the electron beam from the surface of the photocathode is deflected so as to move away from the optical axis of the electron optical system and pass through the first passage hole; an electron microscope, wherein the electron beam having passed through the first passage hole is deflected by the deflector and passes through the second passage hole;

12. An electron microscope as described in claim 11, characterized in that it is equipped with a control device that controls the deflection signal of the electron beam by the deflector in order to vary the cathode voltage applied to the photocathode over time and adjust the image shaking associated with the time variation.

13. An electron microscope as described in claim 11, wherein the photoelectric film is a semiconductor having a negative surface electron affinity.

14. The electron microscope of claim 13, wherein the photoelectric film is made of p-type GaAs.

15. An electron microscope as described in claim 11, characterized in that it is provided with an adjustment mechanism for adjusting the horizontal position of an excitation optical system including the excitation light source, the focusing lens, and the photoelectric film.

16. Having a multi-core fiber, 12. The electron microscope according to claim 11, wherein the excitation point on the photoelectric film is switched by the output of the excitation light source connected to the multi-core fiber.

Images