APPLIED ELECTRON BEAM DEVICE AND ELECTRON BEAM GENERATION METHOD

Abstract

To increase the luminance of an electron beam emitted by a photocathode having a photoelectric film, a second pulsed light, whose wavelength is larger than that of the first pulsed light, is additionally shone onto the photocathode.According to one aspect of an applied electron beam device, this device comprises a photocathode 1, a first pulsed light source 7 designed to emit first pulsed light, a second pulsed light source 8 designed to emit second pulsed light with an annular intensity distribution, an optical system 21 for excitation light designed to emit the first pulsed light and the second pulsed light such that the optical path of the first pulsed light and the optical path of the second pulsed light are combined, a converging lens 2 designed to focus on the photocathode the first pulsed light and the second pulsed light emitted by the optical system for excitation light, and an electron-optical system designed to irradiate a sample 20 with the electron beam emitted by the photocathode.

Description

Technical area

[0001] The present invention relates to an applied electron beam apparatus and an electron beam generating method. Technical background

[0002] In a high-resolution electron microscope, an electron source with high luminance and emitting an electron beam with a narrow energy range is essential. Furthermore, in recent years, advances have been made in the technology for pulsing an electron beam as a probe to capture rapid temporal changes, in addition to achieving high spatial resolution. A photocathode that generates pulsed electrons in response to the incidence of a light pulse is very useful for capturing a rapidly changing phenomenon.A photoexcitation electron source utilizing negative electron affinity (NEA) has the feature of high luminance. The work function of a semiconductor surface containing GaAs or the like is reduced by adding Cs, O, or the like, and excited electrons are emitted into a vacuum due to a band-to-band transition in response to light incident into a conduction band. Therefore, the photoexcitation electron source is planar, and the emitted electrons have good linearity, although the focal spot of the excitation light as a light source is large, measuring approximately 1 µm.

[0003] PTL 1 discloses an electron gun using such a photocathode. PTL 1 discloses a structure of the electron gun in which a transparent substrate is used as the photocathode, specifically, a photoelectric film is attached to the transparent substrate, a converging lens with a high numerical aperture (NA) of about 0.5 is arranged near the transparent substrate to focus excitation light up to the diffraction limit onto the photoelectric film to generate a small electron light source, and an electron beam emitted from the electron light source into a vacuum is used.

[0004] PTL 2 discloses that excitation light incident on a photocathode originates from two laser beams, one of the two laser beams being radially polarized light, and an electric field vector generated at a converging position being perpendicular to the surface of the photocathode, thereby reducing the effective work function of the photocathode. Citation listPatent literature PTL 1: JP2001-143648A PTL 2: JP2008-288099A Summary of the inventionTechnical problem

[0005] To further increase the resolution of an electron microscope using a light-excitation electron source, it is desirable to further increase the luminance of the light-excitation electron source. However, the probe current obtained with continuous light does not increase beyond a certain point even if the intensity of the excitation light is increased. This is because the amount of heat generation and the number of charges accumulating on a surface increase as the emitted current increases, thereby limiting the emitted current. One solution is to pulse the excitation light. Accordingly, the accumulation of heat or charges on the surface can be reduced. In the case of a photocathode composed primarily of GaAs, laser light with a wavelength of 760 nm to 800 nm is suitable as the excitation light source.Examples of such a light source include a semiconductor diode and a semiconductor laser using a light-emitting crystal. Such a light source is useful for various measurements because pulsed light can be extracted and the pulse width and interval can be controlled by an electrical or optical signal.

[0006] However, even if the excitation light is pulsed, the luminance is limited. The luminance of an electron source increases as the area of ​​an emission source decreases. However, in the case of a light-excitation electron source, the size of a cathode region for electron emission is the size of a spot formed by focusing excitation light onto a photocathode. This size is determined by the wavelength of the excitation light and is difficult to reduce.

[0007] If a semiconductor with a wide band gap such as GaN or InGaN is used, which enables electrical conduction with a higher current density than a photoelectric film of a photocathode, because the ionization energy of an internally doped acceptor is high and the hole density is low at room temperature, the electrical resistance is high and the emitted current density is limited.

[0008] As disclosed in PTL 2, even when a radially polarized beam is irradiated to generate an electric field vector perpendicular to the surface of the photocathode, the electron emission increases or decreases, thereby limiting the luminance because the electric field generated by light oscillates over time. Solution to the problem

[0009] An applied electron beam device according to an embodiment of the invention comprises a photocathode with a photoelectric film, a first pulsed light source configured to emit first pulsed light, a second pulsed light source configured to emit second pulsed light having a higher wavelength than the first pulsed light and a ring-shaped intensity distribution, an excitation light optical system configured to emit the first pulsed light and the second pulsed light such that the optical path of the first pulsed light and the optical path of the second pulsed light are combined, a converging lens configured to focus on the photocathode the first pulsed light and the second pulsed light emitted by the excitation light optical system, and an electron optical system configuredto irradiate a sample with an electron beam emitted by the photocathode. Advantageous effects of the invention

[0010] It is possible to increase the luminance of the electron beam emitted by the photocathode. Other problems and novel features will become apparent from the description of the present patent specification and the accompanying drawings. Brief description of the drawings

[0011] They show: Fig. 1 is a schematic view of an applied electron beam device, Fig. 2A is a view showing a configuration of an optical system for excitation light, Fig. 2B is a timing diagram showing pulsed light, Fig. 3 an example of an optical modulator (a phase mask), Fig. 4A is a diagram of a light intensity and an electron distribution caused by the first pulsed light, Fig. 4B is a diagram of a light intensity distribution caused by second pulsed light, Fig. 4C is a diagram of an electron distribution caused by the first and second pulsed light, Fig. 5A is a diagram showing an effect of the second pulsed light according to Embodiment 1, Fig. 5B is a diagram showing an effect of the second pulsed light according to Embodiment 1, Fig. 6 is a view showing a configuration of an optical system for excitation light, Fig. 7A an example of a spiral phase plate, Fig. 7B is a diagram of a light intensity distribution of the second pulsed light formed by the spiral phase plate, Fig. 8A is a diagram showing an effect of the second pulsed light according to Embodiment 2, Fig. 8B is a diagram showing an effect of the second pulsed light according to Embodiment 2, Fig. 8C is a diagram showing an effect of the second pulsed light according to Embodiment 2, Fig. 9 is a timing diagram showing pulsed light, Fig. 10 is a diagram of a configuration of an optical system for excitation light, Fig. 11A is a diagram showing an effect of the second pulsed light according to Embodiment 2, Fig. 11B is a diagram showing an effect of the second pulsed light according to Embodiment 2, Fig. 11C is a diagram showing an effect of the second pulsed light according to Embodiment 2, Fig. 12A is a view of a radially polarized beam, Fig. 12B is a view of a radially polarized beam, Fig. 13 is a diagram showing an effect of the second pulsed light according to Embodiment 3, Fig. 14 is a timing diagram showing pulsed light, Fig. 15A is a view showing a configuration of an optical system for focusing the second pulsed light, and Fig. 15B is a view showing a configuration of an optical system for focusing the second pulsed light. Description of embodiments

[0012] Embodiments of the invention will be described below with reference to the drawings. [Embodiment 1]

[0013] Fig. 1 is a schematic view of an applied electron beam device. Pulsed excitation light 12 is incident on a photocathode 1 from an upper surface, and a pulsed electron beam is generated from a lower surface of a portion onto which the pulsed excitation light 12 is incident. Electrons emitted from the photocathode 1 are accelerated by an electric field between opposing extraction electrodes 3 and guided as a pulsed electron beam 13 from an opening 14 of a vacuum chamber 9 to an electron optical system. Here, a scanning electron microscope (SEM) is shown as an example of the electron optical system. The pulsed electron beam 13 extracted from the opening 14 is further reduced by two electron lenses 17 and an objective lens 29 and focused onto a sample 20.An SEM image of a sample surface is obtained by a deflector 18 for scanning a focal point position and an electron detector 19 for measuring signal electrons generated by the sample as a result of the radiation of the pulsed electron beam 13. A diaphragm 38, which limits the aperture angle of the electron beam, is provided on the electron beam path and cooperates with the electron lenses 17 to perform adjustment and create an optimal path shape.

[0014] Photocathode 1 is an electron source that utilizes a phenomenon known as negative electron affinity. A photoelectric film 10 of photocathode 1 is a p-type semiconductor. A typical example of the photoelectric film 10 is GaAs. To reduce the work function, cations such as Cs and oxygen are adsorbed on the surface of the photoelectric film 10.

[0015] Although not shown, a dedicated preparation chamber is provided adjacent to an electron gun 15, and the surface of the photocathode 1 moved to the preparation chamber is adjusted. Adjustment of the cathode surface is performed as follows. An oxide and a carbonized material on the semiconductor surface are removed by heating. It is more effective to generate atomic hydrogen to perform the cleaning process. Next, the work function of the semiconductor surface is reduced by adsorbing Cs evaporated in an oxygen atmosphere onto the semiconductor surface, and a state is obtained in which the vacuum level is lower than that of a conductor within a cathode film.Therefore, in response to the incidence of the excitation light 12, electrons excited from a valence band to a conduction band having sufficient energy for a band-to-band transition in a p-type semiconductor are in a state where the potential energy is higher than that of a vacuum level, and the electrons are emitted into the vacuum by an electric field at the film surface.

[0016] In the case of GaAs, the wavelength of the excitation light must be 850 nm or less, and more preferably between 780 nm and 660 nm. When electrons propagate in the conduction band within the photoelectric film 10, the size of the light source increases, resulting in a decrease in luminance, which is undesirable for an electron gun used in a microscope. Therefore, the thickness of the photoelectric film 10 must be about 1 μm or less, and further, p-type impurities must have a sufficient concentration. To support the photoelectric film 10, it is attached to a transparent substrate 11 that does not absorb excitation light in the photocathode 1. The transparent substrate 11 of the GaAs photoelectric film 10 can be made of glass or a semiconductor with a wider band gap, such as GaP.The photoelectric film 10 can be directly attached to the transparent substrate 11 or attached through an intermediate layer. When a semiconductor is used as the transparent substrate 11, the photoelectric film 10 can be formed by epitaxial growth. In this case, the photoelectric film 10 can be formed on a buffer layer serving as an intermediate layer.

[0017] An accelerating voltage to be applied to an electron source is applied by an accelerating voltage source 5 via a cathode holder 4, which is in electrical contact with the photoelectric film 10, and the pulsed electron beam 13 generated by the photocathode 1 is accelerated between the extraction electrodes 3. The vacuum chamber 9 of the electron gun 15 is evacuated by a vacuum pumping device (not shown), preferably an ion pump and a non-evaporative getter pump.

[0018] A converging lens 2, which focuses the pulsed excitation light 12 onto the photocathode 1, is a lens whose spherical aberration is corrected to minimize the focal spot formed by the transparent substrate 11 on the photoelectric film 10. In the present example, the diameter of the parallel light is 4 mm to 4.2 mm, and the numerical aperture (NA) is 0.5. Thus, in the case of excitation light (laser light) with a wavelength of 660 nm, a focal spot with a diameter of approximately 1 µm (half width) is obtained on the photoelectric film 10, as shown in Fig. 4A is indicated by a solid line A. The parallel pulsed excitation light 12 is emitted from a first pulsed light source 7 outside the vacuum chamber 9 and emitted into the vacuum chamber 9 through an excitation light optical system 21 and a window 6. On the other hand, the excitation light optical system 21 is provided with a second pulsed light source 8 with a long wavelength (for example, 850 nm).

[0019] Fig. 2A shows a configuration of the excitation light optical system 21. The first pulsed light 27 emitted from the first pulsed light source 7 passes through an optical coupler 24 and travels to the photocathode 1 as the pulsed excitation light 12. The second pulsed light 28 emitted from the second pulsed light source 8 also travels along a path of the pulsed excitation light 12 and is irradiated onto the photoelectric film 10 through the condenser lens 2. The second pulsed light source 8 is connected to the excitation light optical system 21 via a fine adjustment mount 51, and the second pulsed light 28 adjusted by the fine adjustment mount 51 is reflected to the photocathode 1 by the optical coupler 24 and adjusted to coincide with the optical path of the pulsed excitation light 12 and travel to the photocathode.The second pulsed light 28 is laser light obtained by an optical modulator 23 which modulates long-wave pulsed light 52 emitted by a long-wave pulsed light source 22 having a wavelength of 850 nm.

[0020] Fig. 3 shows an example in which a phase mask having a phase difference between the center and the outer periphery of laser light is used as an example in the modulation performed by the optical modulator 23. Fig. 3 shows a plan view and a cross-sectional structure of a central portion of the phase mask used as the optical modulator 23. The optical modulator 23 is a transparent plate having a refractive index n corresponding to the wavelength of the second pulsed light 28. A circular phase shift region 40 is located at the center of the optical modulator 23. A step is located between the phase shift region 40 and a peripheral portion. Therefore, the phase of a central portion and the phase of an outer peripheral portion of the obtained second pulsed light 28 are shifted by π, and the intensity distribution of the light on the photoelectric film 10 caused by the converging lens 2 has a ring shape with a dark center, as shown in FIG. Fig. 4B is shown.

[0021] Because the light phases are shifted by π through the phase shift region 40, the height h (see Fig. 3) the stage of the phase shift range 40 satisfy formula 1. h=λ / (2(n−1))

[0022] If the optical modulator 23 consists of a quartz plate (n = 1.46) and the wavelength of the second pulsed light 28 is 850 nm, h = 924 nm. In order to minimize the light intensity in the central section, ie, to make the light intensity distribution ring-shaped, the diameter d (see Fig. 3) of the phase shift range 40 is set to a size determined by (Formula 2). d=D / 2

[0023] Here is D (see Fig. 3) The diameter of the second pulsed light 28. Formula 2 is a condition for the case where the light intensity of the second pulsed light 28 is uniform. For example, if the second pulsed light 28 has an intensity distribution similar to a Gaussian beam, the diameter d must be set accordingly.

[0024] Note that the wavelength of the second pulsed light 28 of 850 nm is used as an example, and it is set to substantially correspond to the band gap energy of a semiconductor GaAs material constituting the photoelectric film 10. Any wavelength that induces stimulated emission for the recombination of excited electrons in the photoelectric film 10 is acceptable. Therefore, when the photoelectric film 10 is made of a crystal other than GaAs, the wavelength is determined by the physical properties of the crystal, and when the photoelectric film 10 has a superlattice structure, the wavelength corresponds to the gap energy of a formed miniband. In this case, one condition is that the wavelength is such that the photoelectric film 10 induces stimulated emission at that wavelength.

[0025] The optical coupler 24, which combines the optical path of the first pulsed light 27 and the optical path of the second pulsed light 28, can be a half mirror, a polarized beam splitter, a dichroic mirror, or the like. When a dichroic mirror is used as the optical coupler 24, the first pulsed light 27 is transmitted, the second pulsed light 28 is reflected, and the first pulsed light 27 and the second pulsed light enter the path of the pulsed excitation light 12. In such a case, a system with low light loss can be achieved.

[0026] An effect of irradiating the photoelectric film 10 with the first pulsed light 27 and the spatially modulated second pulsed light 28 is described. Fig. Figure 5A shows a state in which electrons in a valence band are excited in response to the incidence of the first pulsed light 27 into a conduction band beyond the energy gap Eg in the photoelectric film 10, and energy is lost through phonon scattering or the like, and the electrons temporarily gather at the bottom of the conduction band. The spatial distribution of the electrons in the conduction band at this time is shown in Fig. 4A by a broken line B. Compared to the solid line A in Fig. 4A, the spatial distribution of electrons due to scattering or diffusion of electrons has a spread of approximately 1.5 µm at half the width. Because the electrons are emitted into a vacuum, the size of the electron beam source exceeds the dimension of the intensity distribution of the excitation light and is approximately 1.5 µm at half the width.

[0027] When the second pulsed light 28 is incident while the pulse intensity (height), pulse width and time in Fig. 5A shown state are optimized, as shown in Fig. 5B, in the section onto which the second pulsed light 28 is irradiated, electrons in the conduction band and holes in the valence band immediately recombine, so that light is emitted, resulting in stimulated emission. Because the intensity distribution of the second pulsed light 28, as shown in Fig. 4B, has a ring shape with a weak central section, the distribution of electrons in the conduction band changes from a broken line B to a solid line C, as shown in Fig. 4C, and can be narrowed to 1 µm or less at half its width. Because electrons corresponding to the propagation of the solid line C are emitted into the vacuum, the source size of an electron beam can be narrowed to about 0.8 µm at half its width.

[0028] For an electron source using negative electron affinity, it is quite effective to reduce the area for electron emission to increase the luminance of an electron beam. Even if the area of ​​the emission area is large, the luminance can theoretically be increased if the current density of the emitted electrons is constant. The condition of high luminance requires a high current density of several A / mm2. If the area of ​​the emission area is, for example, 1 mm2 at a certain current density, a high current of several A is required. In this case, side effects such as melting of a GaAs photoelectric film due to heat generation, Cs evaporation, or accumulation of surface charges on the surface of a photoelectric film occur, making it difficult to maintain electron emission.To avoid such side effects, the total current must be reduced by reducing the area of ​​the electron emission region as much as possible.

[0029] In the prior art, by using an aspherical lens as the converging lens 2 and focusing the excitation light to the diffraction limit, an intensity distribution with a diameter of approximately 1.2 µm to 1.5 µm can be obtained. On the other hand, by using the second pulsed light 28, the electron emission range can be reduced by 67% to 53% and the area can be reduced by 44% to 28%, so that the luminance can be increased by two to three times for the same total current.

[0030] In this way, the second pulsed light 28 eliminates a portion of the electrons generated by the first pulsed light 27 in the conduction band of the photoelectric film 10, thereby generating a high-luminance electron beam. The second pulsed light 28 causes excited electrons to recombine with holes generated by stimulated emission and also excites electrons in a valence band into a conduction band. Therefore, the intensity and timing of both pulsed lights must be optimized so that the second pulsed light 28 is used primarily for stimulated emission.

[0031] A setting unit for both pulsed light types is described. In Fig. 2B the vertical axis represents the intensity I and the horizontal axis represents the time. Fig. Figure 2B shows temporal changes in the intensities of both pulsed lights. A pulse of the first pulsed light 27 is indicated by an intensity Ip1 and a pulse width Tp1. The pulse width Tp1 is a time that can be selected according to the purpose of an experiment. A feature value is 1 picosecond or more, and can also be on the order of a nanosecond or microsecond. The intensity Ip1 is set according to the required luminance of an electron gun. On the other hand, a pulse of the second pulsed light 28 is indicated by an intensity Ip2 and a pulse width Tp2. The initial delay time Tr of the second pulsed light 28 from the beginning of the first pulsed light 27 and the final delay time Td of the second pulsed light 28 from the end of the first pulsed light 27 for setting the pulse width Tp2 are optimized through preliminary experiments.

[0032] To achieve this, as in Fig. As shown in Figure 1, a control signal 33 indicating the height and timing of each pulse is sent from a main body control device 32 to an excitation light control device 31. The excitation light control device 31 outputs a laser control signal 37 to both the first pulsed light source 7 and the second pulsed light source 8 to adjust the time and intensity.

[0033] Regarding the values ​​of the initial delay time Tr and the final delay time Td, it should be noted that the electrons generated by the first pulsed light 27 in the conduction band lose energy due to phonon scattering in principle on the order of femtoseconds and accumulate at the bottom of the conduction band. However, the electrons subsequently recombine with holes and disappear in approximately 1 picosecond. Therefore, the initial delay time Tr is selected from the range of 0 picoseconds to 0.1 picoseconds, and the final delay time Td is selected from the range of 0 picoseconds to 1 picosecond. To optimize the initial delay time Tr and the final delay time Td, the luminance of a pulsed electron beam is monitored and adjusted to the highest luminance. The luminance of the pulsed electron beam 13 can be detected from the resolution and brightness of an SEM image obtained by an electron optical system.Furthermore, the luminance of the pulsed electron beam 13 can be more easily detected by inserting a Faraday cup 35 into the central axis 34 of the electron optical system and measuring a probe current with a current detector 36. As described later, the insertion position of the Faraday cup 35 must be closer to the sample 20 than to the aperture 38 because the electron beam, reduced by a plurality of electron lenses 17, passes through the aperture angle limiting aperture 38 with a reduced current. Luminance information obtained in this way is fed back to the main body control device 32 to optimize the conditions of the electron gun 15.

[0034] Higher-resolution observation can be achieved by using the pulsed electron beam 13 with increased luminance in a scanning electron microscope, as described above. Alternatively, high-speed observation can be achieved because the probe current itself can be increased with the same resolution as a prior art device. Specifically, the energy distribution of an electron source according to the present embodiment is between 0.1 eV and 0.2 eV, which is more consistent than 0.3 eV of a cold electric field emission (CFE) electron source using a tungsten filament and 0.8 eV of a Schottky emission (SE) electron source, so resolution can be improved by reducing chromatic aberration, especially at low acceleration.For example, when the accelerating voltage source 5 is set to -2 kV, the electron beam 13 emitted from the electron gun has a radiation angle current density of 39 mA / sr or more.

[0035] To obtain an electron light source diameter of 800 nm, which is smaller than the configuration in Embodiment 1, and enable observation with a resolution of 1 nm or less, when the size of an electron beam probe formed on a sample is set to 0.8 nm or less, the total reduction ratio by the electron lenses 17 and the objective lens 29 in the housing 16 of the electron optical system can be set to about 1 / 1000. For example, if the aperture 38 and an optical condition are set so that the half-aperture angle αo of the objective lens 29 toward the sample 20 is 10 mrad, an electron beam that can pass through the aperture 38 has a beam divergence half-angle αs on the light source side of the electron gun 15 that is in a fairly narrow range of 5 μrad or less.In this case, the probe current of the electrons irradiated onto the sample 20 is approximately 10 pA, allowing observation with high sensitivity and a short time. The depth of a focal spot, expressed as spot diameter / αo, is approximately 100 nm.

[0036] Because the electron beam 13 is pulsed, it can also be used for a time-resolved measurement.

[0037] Fig. 6 shows a modification of the optical system 21 for excitation light. Fig. The excitation light optical system 21 shown in FIG. 6 includes an optical monitoring system 46 that measures light 47 reflected from the photocathode 1 to facilitate optimization of the excitation light optical system 21. A reflected light splitter 25 for separating the reflected light 47 in the lateral direction (in the x direction) is provided in an optical path. Here, for example, a polarized beam splitter (PBS) is used as the reflected light splitter 25, and a 1 / 4-wavelength plate 42 is provided below the reflected light splitter 25. The pulsed excitation light 12 passing through the reflected light splitter 25 is linearly polarized light with an electric field component parallel to the paper surface and becomes circularly polarized light when passing through the 1 / 4-wavelength plate 42.The circularly polarized light reflected by the photocathode 1 and passing through the converging lens 2 is converted into linearly polarized light having an electric field component perpendicular to the paper surface as the circularly polarized light passes through the 1 / 4 wavelength plate from bottom to top. Therefore, the linearly polarized light is reflected by the reflected light separator (PBS) 25, and only the light 47 reflected by the photocathode 1 is introduced into the monitoring optical system 46. The reflected light 47 is imaged onto an imaging element 41 by an imaging optical system including an imaging lens 44, so that the converging position and focal point shape of the excitation light on the photocathode 1 can be observed in an enlarged manner.Based on the observed merging position and the focal point shape of the excitation light, the optical paths of the first pulsed light 27 and the second pulsed light 28 are adjusted so that the focal point positions of the first pulsed light 27 and the second pulsed light 28 coincide with each other. In the present configuration example, the optical path of the second pulsed light 28 is aligned with the optical path of the first pulsed light 27 by adjusting the position and angle of the second pulsed light source 8 through the fine adjustment mount 51. Furthermore, the monitoring optical system 46 is provided with a light attenuation unit such as an ND filter 45 to optimize the intensity of the light observed by the imaging element 41.

[0038] Furthermore, a removable polarizer 43 may be provided in the monitoring optical system 46. By inserting the polarizer 43, the reflected light 47, which is linearly polarized, cannot pass through the polarizer 43. On the other hand, because the light obtained by the incidence of the first pulsed light 27 and the second pulsed light 28 through stimulated emission is circularly polarized, the light obtained through stimulated emission can be observed on the imaging element 41. The incident positions of the first pulsed light 27 and the second pulsed light 28 can be adjusted by the intensity and shape of the light obtained through stimulated emission. For example, a polarizer capable of switching the polarization direction of the light that can pass through the polarizer may be used.

[0039] Details of a modification are described below.

[0040] To adjust the timing, an optical signal can be used as laser control signal 37, which is generated by the Fig. 1 to the first pulsed light source 7 and the second pulsed light source 8. In the case of an optical signal, the timing can be adjusted using a device that sets the delay time of light transmission by adjusting the optical path length, which is referred to as an optical delay, and the optical signal is advantageous for setting a minute time.

[0041] The optical modulator 23 is not limited to the Fig. 3, and a spatial phase modulator such as a liquid crystal element may also be used. Furthermore, a Fig. 7A can be used as a phase modulator, the central portion of which forms a dark circular focal spot on the photocathode 1. A spiral phase plate 70 is circular, and in this example, it is circumferentially divided into eight parts, and its height changes counterclockwise from a divided region 71-0 to a divided region 71-7. The phase changes by π / 4 at each step. The height h of a step between adjacent counterclockwise divided regions is obtained by (Formula 3). h=λ / (8(n−1))

[0042] The second pulsed light 28 passing through the spiral phase plate has a helical polarization and is focused to form a light intensity pattern on the photocathode 1 as shown in Fig. 7B. The width of a central dark section can be made smaller than in a phase plate with the Fig. 3. When the spiral phase plate is used, because an electron beam emission source with a smaller diameter of 0.5 μm can be used, the luminance of the electron beam can be four times or more by adjusting the electron beam with the first pulsed light 27. The spiral phase plate may have a structure in which it is divided into eight or more parts or the height is continuously changed, and the same effect can be obtained when the phase difference over a circumference is 2π.

[0043] The photoelectric film 10 is not limited to GaAs, and other materials that utilize electron emission using negative electron affinity can be used. For example, GaP, mixed crystals such as GaAs-GaP and GaAs-AlAs, or a superlattice can be used. As described above, the wavelength of the second pulsed light 28 must be selected according to the material and structure of the photoelectric film 10.

[0044] Pulse widths of the first pulsed light 27 and the second pulsed light 28 according to the present embodiment are typically selected from a range of 1 picosecond to 1 μs. However, the pulse width used to adjust the light intensity can also be increased. For example, the light can be continuous light with a duration of several milliseconds to several tens of seconds or more, depending on the observation time with an electron microscope.

[0045] Depending on the electron beam device used, the size of the electron beam light source may be reduced during use to increase luminance. In this case, either the first pulsed light 27 or the second pulsed light 28 may be pulsed light, and the other may be continuous light. [Embodiment 2]

[0046] According to the present embodiment, in the Fig. 1, infrared light is used in the second pulsed light source 8. The wavelength of the infrared light does not cause a band-to-band transition in the photoelectric film 10, but the infrared light has energy to excite electrons from a valence band to an acceptor level and to generate holes (positive holes).

[0047] A wide-bandgap semiconductor, such as GaN and diamond, has a much larger dielectric breakdown field than a semiconductor such as Si and GaAs. Therefore, a wide-bandgap semiconductor can transport large amounts of electrons in bulk, making it a highly desirable electron source that requires a high current density. However, when electrons are emitted due to the negative electron affinity of GaN- or GaInN-based superlattices or solid solutions, the current density cannot be increased. This is because p-type doping does not work well.

[0048] For example, when a semiconductor such as Si is doped with an impurity acting as an acceptor, the valence band energy Ea of an acceptor level appearing in a forbidden band is as small as 0.05 eV. Even at room temperature, electrons enter an acceptor due to thermal energy, causing it to become negatively charged. Holes are generated in the valence band as a result of electron loss. The holes can move as charge carriers, thereby increasing electrical conductivity and minimizing the width of a depletion layer that may form near the surface by redistribution, thus weakening an externally applied electric field. On the other hand, in the case of a material with a wide band gap, the energy Ea of the acceptor level is about 0.1 eV for GaN and about 0.5 eV for diamond, and the hole density cannot be increased by thermal excitation at room temperature.Although there is no fixed definition for the band gap energy Eg required to classify a semiconductor as a wide band gap semiconductor, it should be noted that a semiconductor with a band gap energy Eg of 3 eV or more is used here as a wide band gap semiconductor.

[0049] The Fig. 8A to 8C show models of an energy structure of a p-type impurity-doped GaN-based semiconductor with a wide band gap used for the photoelectric film 10. Fig. 8A shows a model in which no light is irradiated in a state where Cs or oxygen is adsorbed on the surface of the photoelectric film 10 and the work function is reduced in vacuum. Because an acceptor is almost uncharged and the hole density is low, there are no charge carriers, and the vicinity of the surface is depleted due to the difference between an internal potential and a surface potential. Even if electrons and holes are generated in response to irradiation with excitation light due to a band-to-band transition, there are almost no charge carriers, and the emission current density does not become high enough due to an attractive force between the electrons and holes and an increase in potential due to the charges accumulated on the surface, so the luminance of an electron source is quite low.

[0050] In this state, the second pulsed light, which is infrared light, is irradiated. The energy hν2 of the second pulsed light equals or exceeds the energy Ea of the acceptor level, and holes are generated as a result of the charging of an acceptor by electrons in the valence band, as shown in Fig. 8B. Note that h is the Planck constant and ν2 is the frequency of the second pulsed light. Therefore, the electric field inside holes with a high density is reduced, and a very narrow depletion layer forms at the surface. In this state, when the first pulsed light 27 with energy hν1 (ν1 is the frequency of the first pulsed light 27) is incident, as a result of a band-to-band transition, as shown in Fig. 8C, electrons excited in a conduction band are emitted into the vacuum at high density.

[0051] The wavelength of the second pulsed light is set so that the second pulsed light has an energy sufficiently lower than the band gap energy and equal to or exceeding the energy Ea of the acceptor level, and it can be set for any band gap energy, for example, a wavelength of 1 µm or larger with a long wavelength limit of 12 µm for GaN and 2.45 µm for diamond.

[0052] Continuous irradiation of the hole-generating infrared light causes an increase in the temperature of the photoelectric film 10, which can lead to Cs evaporation and crystal deterioration, making it impossible to stably emit electrons as an electron source. Therefore, it is effective to irradiate the infrared light synchronously with the first pulsed light 27, and the timing is set so that the infrared light is irradiated simultaneously with or shortly before the irradiation of the first pulsed light 27. This relationship is shown in Fig. 9. The vertical axis represents the intensity I, and the horizontal axis represents time. Fig. Figure 9 shows temporal changes in the intensities of both pulsed light types. A pulse of the first pulsed light 27 is indicated by an intensity Ip1 and a pulse width Tp1, and a pulse of the second pulsed light is indicated by an intensity Ip2 and a pulse width Tp2. A lead time -Tr of a rising edge of the second pulsed light with respect to a rising edge of the first pulsed light 27 and a lead time -Td of a falling edge of the second pulsed light with respect to a falling edge of the first pulsed light 27 are optimized to fall within a range of about 0 picoseconds to several picoseconds, and in order to obtain maximum luminance, the lead time -Tr and the lead time -Td and the intensities Ip1 and Ip2 are determined by feeding back information such as the probe current amount and the resolution of the electron optical system, as in Embodiment 1.

[0053] The wide band gap material of the photoelectric film 10 is not limited to GaN or diamond, and may be solid solutions such as AlGaN and GaInN, or a superlattice of the solid solutions with GaN, or the like.

[0054] Here, light for generating holes may be a combination of light beams having two wavelengths, instead of directly irradiating the merging position of the first pulsed light 27 on the photocathode with infrared light (because in this case, it is not necessary to combine the optical path of the first pulsed light 27 and the optical path of the second pulsed light, the optical system 21 for excitation light can be omitted). Fig. 10 shows a configuration example of the excitation light optical system 21 in this case. An optical coupler 24b causes the optical path of the third pulsed light 54 from a third pulsed light source 53 to coincide with the optical path of the second pulsed light 52 emitted from the second pulsed light source 22, and irradiates the photocathode 1 with the second pulsed light 52 and the third pulsed light 54. The wavelengths of the second pulsed light 52 and the third pulsed light 54 are long wavelengths at which light does not cause band-to-band transition in the photoelectric film 10, and the energy difference between the second pulsed light 52 and the third pulsed light 54 is greater than or equal to the energy Ea of the acceptor level.Because visible light or near-infrared light can be used as the second pulsed light 52 and the third pulsed light 54 in this configuration, the refractive index of the converging lens 2 can be used under nearly ideal conditions. Therefore, the hole density can be increased compared to the case of direct infrared light irradiation, while the range in which the temperature rises due to light irradiation is minimized to only the area surrounding an electron emission source. Because it is possible to resonantly charge an acceptor and generate holes, this provides the advantage of increasing the scattering cross section and efficiency.

[0055] When the second pulsed light 52 and the third pulsed light 54 are irradiated simultaneously, band-to-band transition may occur in two-photon absorption. To prevent band-to-band transition, the energy of the second pulsed light 52 and the third pulsed light 54 is preferably each at most half of the band gap energy Eg. To obtain maximum luminance, the intensities of the second pulsed light 52 and the third pulsed light 54 and the irradiation times of the second pulsed light 52 and the third pulsed light 54 with respect to the irradiation time of the first pulsed light 27 are further determined as in Embodiment 1 by feedback information such as the probe current amount and the resolution of the electron optical system.

[0056] The infrared light radiation causes an increase in luminance not only for the photoelectric film having a semiconductor with a wide band gap, but also for the photoelectric film 10 having a band gap energy Eg of about 1 eV, such as GaAs.

[0057] When GaAs or the photoelectric film 10 consisting mainly of GaAs is used as an electron source using a negative electron affinity, as shown in Fig. As shown in Figure 11A, in an initial stage of electron emission, the intensity of electrons excited by light excitation with energy hν1 in response to the incidence of the first pulsed light 27 in the conduction band decreases to a reduced vacuum level, and the electrons are emitted into the vacuum. However, over time, electrons scattered within the surface are trapped at a level where the electrons are difficult to escape, such as a surface state. Fig. Figure 11B shows a state where electrons are confined in the surface state, thereby satisfying the electron potential energy at the surface of the photoelectric film 10 and reducing the amount of electron emission. This is one reason for limiting the luminance.

[0058] Therefore, by irradiating the second pulsed light, which is infrared light, electrons trapped in the surface state are excited by light into the conduction band or vacuum level, as in Fig. 11C, and they are removed, so that a decrease in luminance can be prevented. The potential energy of the surface state is near the center of the forbidden band, and the potential energy is about 0.7 eV in the case of GaAs, which is half of the band gap energy Eg. Therefore, when the energy hν2 of the second pulsed light is at least 0.7 eV and at most 1.4 eV, electrons trapped in the surface state are excited and removed by drift and diffusion in a solid, or they are emitted into the vacuum, so that electron emission continues and high luminance is achieved.

[0059] The timing of the irradiation of the second pulsed light with respect to the irradiation of the first pulsed light 27 can in this case be set so that the first pulsed light 27 and the second pulsed light are irradiated simultaneously or that there is a slight delay in the irradiation of the second pulsed light, as in Fig. 2B is shown. [Embodiment 3]

[0060] According to the present embodiment, in the Fig. 1, radially polarized long-wavelength light is used as the second pulsed light. Therefore, a light source emitting a long-wavelength radially polarized beam is used as the second pulsed light source 8. For the radially polarized beam, as shown in Fig. 12A, the orientation 55 of the electric field of the light in a cross section of the parallel light is radial and the orientation of the electric field at a focal point formed by the converging lens 2 on the photocathode 1 is perpendicular to the surface of the photoelectric film 10, as shown in Fig. 12B is shown.

[0061] Various techniques have been developed for such a method of generating radially polarized light. For example, linearly polarized light can be converted by using a spatially variable waveplate or a conical Brewster prism. In the second pulsed light source 8, these optical elements can be used instead of the optical modulator 23. Additionally, it is also possible to generate radially polarized light by applying a pulsed voltage to an antenna of a finely structured pattern or the like.

[0062] Fig. Figure 13 shows an energy model for irradiating the photoelectric film 10 with a radially polarized beam in a state where the photoelectric film 10 is irradiated with the first pulsed light 27 to cause electron emission. When the second pulsed light is not irradiated, the vacuum level 60 is smoothly changed depending on the distance z. This is because an electric field is applied between the photocathode 1 and the extraction electrode 3 due to a potential difference (see Fig. 1). In this state, when electrons are emitted from the photoelectric film 10, the speed of the emitted electrons in the immediate vicinity of the surface of the photoelectric film 10, which is Fig. 13, shown as region S, is small. Therefore, when the intensity of the emitted electrons increases, the speed of the electrons becomes low. Therefore, in region S, previously emitted electrons hinder subsequent emission of electrons, and the Coulomb force between previously emitted electrons and later emitted electrons causes the phenomenon of increasing the energy width or broadening the trajectory in the lateral direction. This effect is called the space charge effect, which limits the luminance of an electron source emitting electrons into a vacuum and broadens the energy distribution, leading to the deterioration of the quality of an electron beam used in an electron microscope.

[0063] On the other hand, when the radially polarized second pulsed light is focused and irradiated as in Fig. 13, a state is obtained in which a vacuum level 61 (indicated by a broken line) on which a negative electric field with respect to the surface is superimposed and a vacuum level 62 (indicated by a dotted line) on which a positive electric field with respect to the surface is superimposed oscillate over time. To reduce the space charge effect, electrons in the region S can be accelerated in a shorter time. That is, it is preferable to irradiate the photoelectric film 10 with the first pulsed light 27 at a time when the photoelectric film 10 is in a state in which a positive electric field is superimposed on the vacuum level 62, thereby causing electrons to be emitted from the photoelectric film 10.

[0064] Fig. 14 shows the timing of irradiation of the first pulsed light 27 and the second pulsed light. A waveform 81 represents the intensity I of the first pulsed light 27, and a waveform 82 represents temporal changes in an electric field F on the surface of the photoelectric film 10 caused by the second pulsed light. As described above, by irradiating the photoelectric film 10 with the first pulsed light 27 in a time zone where the electric field F is positive due to the second pulsed light, the space charge effect can be reduced and the luminance of the photoelectric film 10 can be increased. When the pulse width Tp1 of the first pulsed light is 1 picosecond, the time of a section in which an oscillating electric field of the second pulsed light is required must be greater than 1 picosecond.For example, if the oscillation period is 2 picoseconds, the frequency is 0.5 THz, and the wavelength is 600 µm. The pulse width Tp2 of the second pulsed light is a few picoseconds, and the lead time -Tr of a rising edge of the second pulsed light with respect to a rising edge of the first pulsed light 27 is also a few picoseconds. The amount of the lead time -Tr is optimized based on the luminance measurement result in the electron-optical system, as in Embodiment 1.

[0065] If the temporal pulse width of the first pulsed light 27 is reduced, the frequency of the second pulsed light is selected to be on the order of terahertz. For example, if the pulse width of the first pulsed light 27 is 0.1 picoseconds, the wavelength of the second pulsed light is 60 µm or more.

[0066] Because in this way, according to the present embodiment, electrons are emitted at a time when a positive electric field is superimposed as shown in Fig. As shown in Figure 13, electrons are accelerated by the vacuum level 62 superimposed on the positive electric field. Because the energy of electrons is conserved in a vacuum, electrons emitted with the same potential energy reach a fixed speed when they are far enough away from the photoelectric film 10, even if the acceleration depends slightly on a time-varying electric field. This is advantageous in that the energy broadening of an electron beam emitted from an electron source is kept small when the electron beam is guided to an electron optical system, so that the resolution can be improved by reducing chromatic aberration and an experiment using a short pulse is enabled.

[0067] The Fig. 15A and Fig. 15B show examples of a suitable focusing optical system for the second pulsed light according to the present embodiment. The first pulsed light 27 is focused onto the photocathode 1 by the condenser lens 2. On the other hand, because the refractive index is wavelength-dependent, if the wavelength of the second pulsed light is too high for the condenser lens 2, the second pulsed light cannot be sufficiently focused by the condenser lens 2. In such a case, it is effective to use a mirror with a lower light wavelength dependence. For example, as shown in Fig. 15A, a reflective objective lens 56 using a mirror may be used. The second pulsed light passes through a hole formed in the upper portion of a concave mirror 57, is reflected by a convex mirror 58, and is focused onto the photocathode 1 by the concave mirror 57. A dichroic mirror is used as the convex mirror 58, allowing the first pulsed light 27 to pass therethrough.

[0068] Alternatively, as in Fig. 15B, the second pulsed light may be introduced from a direction different from that of the first pulsed light 27 and focused by a parabolic mirror 59 or the like (because in this case, the optical path of the first pulsed light 27 and the optical path of the second pulsed light do not need to be combined, the excitation light optical system 21 may be omitted). As shown in Fig.15B, the parabolic mirror 59 is used when the second pulsed light is a parallel beam, while an ellipsoidal mirror may be used when the second pulsed light is a diverging beam from a small point light source.

[0069] The focusing of the second pulsed light by a mirror can be achieved by placing the mirror on the lower surface of the photocathode 1, i.e., on the side opposite the converging lens 2, with the photocathode 1 in between. Furthermore, the reflective objective lens 56 and the parabolic mirror 59 can be used to focus the first pulsed light 27.

[0070] The photoelectric film 10 of the photocathode 1 is not limited to GaAs and may be made of GaP, AlGaAs, GaN, InGaN, and the like, and a metal film such as gold can also reduce the space charge effect during electron emission.

[0071] Although the invention has been specifically described based on the embodiments, it is not limited to the embodiments, and various modifications can be made without departing from the spirit of the invention. It is also effective to appropriately apply the embodiments in combination rather than individually. Although a scanning electron microscope has been described as an example of the applied electron beam device, the invention can be applied to various applied electron beam devices such as a transmission electron microscope and a scanning transmission electron microscope.The invention is not limited to an applied electron beam device having an electron detector that detects electrons (secondary electrons, reflected electrons, and the like) generated in response to the irradiation of an electron beam, and may include another detector such as a detector that detects characteristic X-rays. In any case, higher resolution of a microscope image, faster measurement, and more advanced functionality can be achieved by obtaining a pulsed electron beam with high luminance. List of reference symbols 1 photocathode 2 converging lens 3 Extraction electrode 4 cathode holders 5 Acceleration voltage source 6 windows 7 first pulsed light source 8 second pulsed light source 9 Vacuum chamber 10 photoelectric film 11 transparent substrate 12 pulsed excitation light 13 pulsed electron beam 14 Opening 15 electron gun 16 Housing of the electron-optical system 17 Electron lens 18 distractors 19 Electron detector 20 samples 21 optical system for excitation light 22 long-wave pulsed light source 23 optical modulator 24 24b: optical coupler 25 Reflected Light Separators 27 first pulsed light 28 second pulsed light 29 objective lens 31 Excitation light control device 32 Main body control device 33 Control signal 34 Central axis 35 Faraday cups 36 Current detector 37 Laser control signal 38 aperture 40 phase shift range 41 Illustration element 42 1 / 4 wavelength plate 43 Polarizer 44 imaging lens 45 ND filters 46 optical monitoring system 47 reflected light 51 Fine adjustment mount 52 long-wave pulsed light 53 third pulsed light source 54 third pulsed light 55 Orientation of the electric field 56 reflective objective lens 57 concave mirror 58 convex mirror 59 parabolic mirrors 60, 61, 62 Vacuum level 70 spiral phase plate 71-0 to 71-7 divided area 81, 82 Waveform QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] JP 2001-143648A

[0004] JP 2008-288099A

[0004]

Claims

[1] Applied electron beam device, comprising: a photocathode with a photoelectric film, a first pulsed light source configured to emit first pulsed light, a second pulsed light source configured to emit second pulsed light having a longer wavelength than the first pulsed light and an annular intensity distribution, an optical system for excitation light configured to emit the first pulsed light and the second pulsed light in such a way that the optical path of the first pulsed light and the optical path of the second pulsed light are combined, a converging lens configured to focus on the photocathode the first pulsed light and the second pulsed light emitted from the excitation light optical system, and an electron optical system designed to irradiate a sample with an electron beam emitted by the photocathode. [2] The applied electron beam device according to claim 1, wherein the wavelength of the second pulsed light is a wavelength at which electrons excited in the photoelectric film are recombined with holes by stimulated emission. [3] An applied electron beam device according to claim 1, wherein the second pulsed light source is provided with an optical modulator which makes the intensity distribution of the second pulsed light annular, and the optical modulator is a phase mask, a spatial phase modulator or a spiral phase plate. [4] An applied electron beam device according to claim 1, wherein the optical system for excitation light comprises an optical monitoring system designed to measure light reflected from the photocathode, and the monitoring optical system comprises a 1 / 4 wavelength plate inserted into the optical path of the first pulsed light and the second pulsed light, a reflected light separator inserted into the optical path of the first pulsed light and the second pulsed light and configured to separate reflected light passing through the 1 / 4 wavelength plate from the optical path of the first pulsed light and the second pulsed light, and an image forming optical system configured to form an image of the reflected light from the reflected light separator on an imaging device. [5] The applied electron beam device according to claim 4, wherein the monitoring optical system is configured to insert a polarizer between the image forming optical system and the reflected light separator, which polarizer prevents light having the same polarization direction as the reflected light from the reflected light separator from passing through the polarizer. [6] An applied electron beam device according to claim 1, further comprising: a control device configured to control the intensities of the first pulsed light and the second pulsed light and the time of emission of the second pulsed light to the photocathode with respect to the time of emission of the first pulsed light to the photocathode, wherein the electron-optical system has an aperture designed to limit the aperture angle of the electron beam emitted to the sample, and the intensity of the second pulsed light and the time of emission of the second pulsed light to the photocathode are optimized with respect to the time of emission of the first pulsed light to the photocathode on the basis of an image of the sample obtained by irradiating the sample with the electron beam through the electron optical system and the current intensity of the electron beam passed through the aperture. [7] Applied electron beam device, comprising: a photocathode with a photoelectric film, a first pulsed light source configured to emit first pulsed light, a second pulsed light source configured to emit second pulsed light, which is infrared light, a converging lens designed to focus the first pulsed light onto the photocathode, and an electron optical system designed to irradiate a sample with an electron beam emitted by the photocathode. [8] Applied electron beam device according to claim 7, wherein the photoelectric film comprises a wide band gap semiconductor with a band gap energy of 3 eV or more, and the energy of the second pulsed light is lower than the band gap energy of the wide band gap semiconductor and equal to or greater than the energy of an acceptor level of the wide band gap semiconductor. [9] The applied electron beam device according to claim 7, wherein the wavelength of the second pulsed light is a wavelength at which electrons confined in a surface state of the photoelectric film are excited into a conduction band or a vacuum level. [10] Applied electron beam device, comprising: a photocathode with a photoelectric film, a first pulsed light source configured to emit first pulsed light, a second pulsed light source configured to emit second pulsed light, a third pulsed light source configured to emit third pulsed light, an optical system for excitation light comprising a first optical coupler configured to combine the optical path of the second pulsed light and the optical path of the third pulsed light, and a second optical coupler configured to combine pulsed light from the first optical coupler with the first pulsed light, a converging lens configured to focus the first pulsed light, the second pulsed light, and the third pulsed light emitted from the excitation light optical system onto the photocathode, and an electron optical system designed to irradiate a sample with an electron beam emitted by the photocathode, wherein the photoelectric film comprises a wide band gap semiconductor with a band gap energy of 3 eV or more, and the difference between the energy of the second pulsed light and the energy of the third pulsed light is equal to or greater than the energy of an acceptor level of the semiconductor with a wide band gap. [11] The applied electron beam device according to claim 10, wherein the energy of the second pulsed light and the energy of the third pulsed light are each half or less of the band gap energy of the semiconductor having a wide band gap. [12] Applied electron beam device, comprising: a photocathode with a photoelectric film, a first pulsed light source configured to emit first pulsed light, a second pulsed light source configured to emit second pulsed light having a longer wavelength than the first pulsed light and being radially polarized, a converging lens designed to focus the first pulsed light onto the photocathode, a focusing optical system configured to focus the second pulsed light onto a merging position of the first pulsed light on the photocathode, a control device configured to control the timing of irradiation of the second pulsed light onto the photocathode with respect to the timing of irradiation of the first pulsed light onto the photocathode, and an electron optical system designed to irradiate a sample with an electron beam emitted by the photocathode, wherein the control device executes control to irradiate the first pulsed light in a time zone in which an electric field at the surface of the photoelectric film is positive due to the irradiation with the second pulsed light. [13] An applied electron beam device according to claim 12, further comprising: an optical system for excitation light configured to emit the first pulsed light and the second pulsed light such that the optical path of the first pulsed light and the optical path of the second pulsed light are combined, wherein the focusing optical system has a reflective objective lens. [14] The applied electron beam device according to claim 12, wherein the second pulsed light is incident from a different direction than the first pulsed light and is focused by a mirror onto the converging position of the first pulsed light on the photocathode. [15] An electron beam generating method for generating an electron beam from a photocathode having a photoelectric film, the electron beam generating method comprising: Focusing the first pulsed light onto the photocathode through a converging lens and Focusing second pulsed light having a longer wavelength than the first pulsed light and an annular intensity distribution onto a converging portion of the first pulsed light on the photocathode.

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