ELECTRONIC ENERGY FILTER

Abstract: An electron energy filter includes a first pair of magnetic poles (21) for generating a first deflecting magnetic field and a second pair of magnetic poles (22) for generating a second deflecting magnetic field in the same direction as the first deflecting magnetic field. The incident electrons are deflected about 90° with a trace radius of AM1 through the effect of the first deflecting magnetic field, passed through a free space having a distance DL2 that is about a half of the trace radius AM1 and then are incident to the second deflecting magnetic field. The electrons are deflected about 180° with a trace radius AM2 that is about a half of the curvature radius AM1 and are passed through the free space DL2. Then, the electrons are incident to the first deflecting magnetic field again where those electrons are deflected about 90°. The deflected electrons are traveled like a gamma trace so that those electrons outgo in the same direction as the incident one. This filter so designed is made compact and to have a smaller aberration.

Description: TECHNICAL FIELD The present invention relates to an electronic energy filter which is arranged to separate only electrons having specific energy from an electron beam and form an image of those electrons. BACKGROUND ART In a transmission electron microscope, electrons transmitted through a specimen suffer from energy loss that is peculiar to one or more elements composing the specimen. To overcome this shortcoming, the electrons transmitted through the specimen are passed through an energy filter for analyzing energy of those electrons, separating only the electrons suffering from the specific energy loss, and forming an image of the separated electrons. The formed image corresponds to a mapping image of one or more specific elements contained in the specimen. Further, the use of only the electrons having specific energy for forming an image allows the energy loss of the electrons caused by the thickness of the specimen to be restricted to only specific electrons. The resulting image has excellent contrast. For the electron energy filter used for this kind of purpose, there have been known an omega type energy filter (US Patent No. 4740704) and an alpha type energy filter (US Patent No. 4760261). The omega type energy filter is composed of three electromagnets as shown in Fig. 2. The first electromagnet 1 has an opposite deflecting direction to the second and the third electromagnets 2 and 3. Incident electrons 4 are traced like an omega (Ω) and are fired in the same direction as the incident one, for selecting only the electrons having specific energy loss. As shown, a numeral 5 denotes a crossover point. A numeral 6 denotes an inlet image surface. A numeral 7 denotes an outlet image surface. A numeral 8 denotes an energy dispersion surface. The alpha type energy filter is composed of three electromagnets 11, 12 and 13 having the same deflecting direction, as shown in Fig. 3. Incident electrons 4 are traced like an alpha (α) and finally outgoes in the same direction as the incident one, for selecting only the electrons that suffer from specific energy loss. Another kind of alpha type energy filter is shown in Fig. 4 (Perez, J.P., Sirven, J., Sequela, A., and Lacaze, J.C., Journal de Physique, (Paris), 45, Coll. Cs, 171 to 174 (1984)). This energy filter is constructed so that an electromagnet 14 having a deflecting angle of 70° is located as opposed to the other electromagnet 15 having a deflecting angle of 220° with narrow middle space 16, in which those electromagnets yield the corresponding magnetic field intensities. In the construction, an incident electron beam 4 goes around the inside of the filter for analyzing the energy of the electron beam. DISCLOSURE OF INVENTION The electron energy filter is required to form an energy dispersion surface and an image surface with a small aberration. That is, the electrons 4 transmitted through the specimen enter at the crossover point 5 formed on a lens into the electron energy filter, pass through the filter and reach the energy dispersion surface 8 where the electrons having the specific energy are converged in the same direction though the electrons having the other energy are dispersed. On the energy dispersion surface 8, hence, the aberration has to be eliminated in order to restrict a lower resolution of the electrons passing through a slit. Moreover, the image formed on an inlet image surface 6 before the electron energy filter is required to be similarly formed on an outlet image surface point 7. Hence, the aberration has to be reduced to a minimum for suppressing the distortion of the image as much as possible. A convergence condition of the electron optical system provided in the electron energy filter arranged to use a fanlike electromagnet may be calculated by using a calculation program of an ion optical system used for designing a mass spectrometer. The convergence characteristic up to a three degrees, which considers the effect of an end magnetic field, can be precisely calculated by the calculation program TRIO that is completed by Matuo, Matuda, et al. (T. Matsuo, H. Matusda, Y. Fujita and H. Wollnik; Mass Spectroscopy, Vol. 24, No.1, March 1976) The electron optical system of the aforementioned conventional electron energy filter does not make sufficient allowance for the effect of the end magnetic field. The image point formed after the passage of the electrons through the electron energy filter is required to make the energy dispersion disappear, converge the image in two directions, and have the magnifications of the images kept at a value of 1. As mentioned below, the electron optical system does not completely meet these conditions. Assuming that the energy dispersing direction is x and the direction crossed at right angles with the direction x (direction of a magnetic flux) is y, the y-directional convergence is given by the oblique passage of charged particles in the inlet and the outlet end magnetic fields of the fanlike magnetic field. That is, when the angles formed between the vertical plane to the electron beam 30 and the ends of the electromagnets 31 and 32 have the relations of EP11 and EP12 as shown in Fig. 5A (where the angles EP11 and EP12 have positive signs), the electron beam 30 is converted in the y direction as shown in Fig. 5B. On the other hand, the angles formed between the vertical plane to the electron beam 30 and the ends of the electromagnets 31 and 32 have the relations of EP21 and EP22 as shown in Fig. 5C (where the angles EP21 and EP22 have negative signs), the electron beam 30 is dispersed in the y direction as shown in Fig. 5D. By selecting optimal incident and outgoing angles, the distortion and the aberration of the image may be reduced to a minimum. In the TRIO, the secondary aberration may be represented as follows. x 2 = Xx 1 + Aα 1 + Dδ 1 + XXx 1 2 + XAx 1 α 1 + AAα 1 2 + XDx 1 δ 1 + ADα 1 δ 1 + DDδ 1 2 + YYy 1 2 + YBy 1 β 1 + BBβ 1 2 y 2 = Yy 1 + Bβ 1 + YXy 1 x 1 + YAy 1 α 1 + YDy 1 δ 1 + BXβ 1 x 1 + BAβ 1 α 1 + BDβ 1 δ 1 where x is a beam width in the direction of the energy dispersion caused by the fanlike magnetic field, a is a spread angle, δ is a spread of energy, y is a beam width that is vertical to x, and β is a spread angle in the y-direction. A subscript 1 represents an initial condition of the beam. A subscript 2 represents a beam width of a converged image. X, A, D, Y and B represent a first-order aberration coefficient. XX, XA, AA and the like represent a secondary aberration coefficient. For the energy filter, it is necessary to meet the following first-order convergence condition for keeping an image point in focus with no distortion caused in the image. X = ±1, A = 0, D = 0, Y = ±1, B = 0 At the energy dispersion spot, it is necessary to meet the following first-order convergence condition. X = ±1, A = 0, D ≠ 0 Even if the foregoing conditions are met, the resulting energy filter does not offer excellent performance. To keep the distortion of the image minimum, it is necessary to eliminate the secondary aberration. The secondary aberration coefficients XD, AD, DD, YD and BD about the energy width δ may be negligible, because the energy width δ 1 of the electrons are allowed to be restricted to 10 -4 or lower through the slit. Since the other beam spread is as wide as 10 -3 , it is necessary to design the electron optical system so that the secondary aberration coefficient may be made far smaller. The table 1 lists the values of the aberration coefficients calculated by the TRIO in the electron optical systems of the omega type energy filter shown in Fig. 2 and the alpha type energy filter shown in Figs. 3 and 4. The form parameters of each energy filter required for the calculation are derived from Optik 73, No. 3 (1986)99-107. These values are calculated at an MKS unit. Hence, the aberration is a value of µm if the beam spread is in the order of 10 -3 . If the aberration at an image point is 100 µm, the image expanded to be 100 times larger through the projecting lens system of the electron microscope has a distortion of 1 cm, which is not an excellent filtered image. In particular, the alpha type energy filter such as Perez offers an image point where the secondary aberration is 5 mm, the first-order magnification Y in the y direction is 28, and the y-directional aberration coefficient B is 3. Hence, the resulting image is not converged and extremely distorted. The further problems the prior art involves are reduction of magnetic poles in number and compact design, which is due to location of the filter on the way of a mirror body. The conventional systems rather than the alpha type energy filter of Perez, et al. with a large image distortion need four magnetic poles and thus as many end magnetic fields for the incident and outgoing electron beam as eight. The end magnetic fields may make the beam convergence uncertain. Another document which discloses electron energy filters having either two or four magnetic field regions to achieve an Ω-shaped electron beam path is JP-A-61-2 977. The filters with only two magnetic field regions disclosed in this document have these regions arranged closely adjacent each other. There does not exist an entire space between the regions where there would be no field. Other electron energy filters having either one or three magnetic field regions to achieve α- or Ω-shaped electron beam paths are disclosed in JP-A-61-277 141. It is an object of the invention to provide a compact electron energy filter and a method for filtering the energy of an electron beam which have excellent convergence characteristics. This object is solved by the energy filter of claim 1 and the method of claim 17. The subclaims are directed to preferred embodiments of the invention. Claim 16 relates to a transmission electron microscope having an electron energy filter of the invention. To make the incident direction of the electron beam to the filter the same as the outgoing direction, it is necessary to keep the deflecting angles WM1 and WM2 of the first and the second magnetic fields in the following relation. WM1 + WM2/2 = 180° If the deflecting angle WM1 of the first magnetic field is shifted largely from 90°, the area of the magnetic pole is made larger, so the compact design of the filter is not allowed. If WM1 and WM2 are substantially 90° and 180°, respectively, the electron beams in the free space are kept in parallel. BRIEF DESCRIPTION OF DRAWINGS Fig. 1A is a schematic view showing an energy filter according to an embodiment of the present invention; Fig. 1B is a section view showing the same as above; Fig. 2 is a schematic view showing the conventional omega type energy filter; Fig. 3 is a schematic view showing the conventional alpha type energy filter; Fig. 4 is a schematic view showing another alpha type energy filter; Figs. 5A to 5D are explanatory views showing y-directional convergence of an electron beam caused by the oblique incident and outgoing effect; Fig. 6 is a graph showing a relation between an incident angle EP11 and a secondary aberration in a first magnetic field; Fig. 7 is a graph showing a change of a secondary aberration with a ratio of a free space distance to a trace radius of the first magnetic field; Fig. 8 is a graph showing a relation between the incident angle EP21 and the secondary aberration in the second magnetic field; Fig. 9 is a graph showing a change of a secondary aberration with a ratio of a trace radius of the first magnetic field to that of the second magnetic field; Fig. 10 is an explanatory view showing a change of a secondary aberration with a ratio of a trace radius of the first magnetic field to a convex radius of an incident end; Fig. 11 is an explanatory view showing a change of a secondary aberration with a ratio of a track radius of the first magnetic field to a concave radius of an outgoing end; Fig. 12 is a view showing a construction of an electron microscope provided with an energy filter according to the embodiment of the present invention; Fig. 13 is a photograph showing an energy-filtered image formed through an electron microscope provided with an electron filter according to the embodiment of the present invention; Fig. 14 is a graph showing an energy dispersing spectrum caused by the energy filter according to the embodiment of the present invention; Figs. 15A to 15C are photographs taken by an electron microscope for showing the compared images energy-filtered by the energy filter according to the embodiment of the present invention; and Fig. 16 is a schematic view showing an energy filter according to another embodiment of the present invention. EMBODIMENTS (First Embodiment) Fig. 1 is a schematic view showing the magnetic poles of an electron energy filter according to an embodiment of the present invention, in which Fig. 1A is a plane view and Fig. 1B is a central section. An electron beam 4 is deflected through the effect of a first magnetic field generated between a pair of magnetic poles 21 and 21' by virtue of current flowing through coils 25a and 25b and a second magnetic field generated between a pair of magnetic poles 22 and 22' by virtue of current flowing through coils 26a and 26b. After the electron beam 4 is deflected about 90° by virtue of the first magnetic filed yielded by the first pair of magnetic poles 21 and 21', the deflected beam 4 travels straight on a first path and enters into the second magnetic field yielded by the second pair of magnetic poles 22 and 22', in which the beam is deflected 180° in the substantially same direction. Then, the beam travels on the second path in the free space, which is substantially parallel to the first path, and then-is again incident to the first magnetic field in which the beam is deflected 90° with the same radius as the first deflection. Hence, the beam is rotated totally 360° and then outgoes in the same direction as the incident one. The trace radius AM2 of the electron beam in the second magnetic field is about half as long as the track radius AM1 in the first magnetic field, for reducing the magnetic poles 21 and 22 in area. In the filter constructed as above, the electron beam is traced as a gamma (γ). The y-directional convergence effect is brought about by the oblique outgoing and re-incident of the beam from and to the magnetic poles 21 and 21' for generating the first magnetic field. The deflecting angle WM1 and the central trace radius AM1 in the first magnetic field, the deflecting angle VM2 and the central trace radius AM2 in the second magnetic field, and the distance DL2 from the outgoing point of the first magnetic field to the incident point of the second magnetic field are selected to keep the following numerical range, for meeting the foregoing expressions (3) and (4). WM1 is substantially 90° 2 ≤ AM1/AM2 ≤ 3 WM2 is substantially 180° 0.4 AM1 ≤ DL2 ≤ 0.8 AM1, Preferably 0.5 AM1 ≤ DL2 ≤ 0.7 AM1 More preferably, AM1/AM2 is substantially 2. DL2 is substantially equal to AM2. The initial incident angle EP11 and the outgoing angle EP12 of the electron beam in the first magnetic field and the incident and the outgoing angles EP21 of the electron beam in the second magnetic field are selected to keep the following numerical range, for reducing the secondary aberration to a minimum. -12° ≤ EP1 ≤ 0° 30° ≤ EP12 ≤ 40° -3° ≤ EP21 ≤ 1° In another preferable embodiment, EP11 is substantially 42°, EP12 is substantially 11.8°, and EP21 is substantially 30°. The magnetic pole ends of the initial incident point and the outgoing point of the electron beam in the first magnetic pole are worked as a convex and a concave planes in order to reduce the secondary aberration to a minimum. The curvature radii RM1 and RM2 of these planes are selected to keep the following numerical range, where a sign of + indicates the curvature is convex and a sign of - indicates the curvature is concave. Incident Convex Plane: 0.5 ≤ AM1/RM1 ≤ 0.9 Outgoing Concave Plane: -0.5 ≤ AM1/RM2 ≤ 0.05 In another preferred embodiment, AM1/RM1 is substantially 1.0 and AM1/RM2 is substantially 0.7. The representative parameters of the electron optical system used in this embodiment are as follows. EP11 = -6°, RM1 = 0.063 m, AM1 = 0.05 m, WM1 = 90°, EP12 = 33.4°, RM2 = -0.2 m, DL2 = 0.03 m, EP21 = -1.5°, AM2 = 0.022 M, WM2 = 180°, EP22 = EP21 = -1.5° Under these parameters, the first-order aberration coefficients at the image point DL14 and the dispersion point DLD4 are as follows. DL14 = 0.028 m, X = 1.0, A = 0.0, D = 0.0, Y = -1.0, B = 0.0 DLD4 = 0.094 m, X = -1.0, A = 0.0, D = 0.1, Y = -1.6, B = -0.17. These first-order aberration coefficients meet the foregoing expressions (3) and (4). The secondary aberration coefficients of the electron energy filter constructed to use these parameters are listed in the following Table 2. Secondary Aberration Coefficients of the Invention XX XA AA YY YB BB Image Points -8.8 0.6 0.00 1.3 0.3 0.00 Dispersion Points 1.4 0.8 0.03 -40 -4.3 0.02 YX YA BX 14 0.1 0.03 - - - As is obvious from the Table 2, as compared with the conventional omega and alpha types, the secondary aberration of the image point 7 is reduced down to 1/10 or lower. The secondary aberration at the dispersion point 8 is greatly reduced. The optima condition of the electron optical system provided in the energy filter according to this embodiment is determined by varying the parameters. As an example, Figs. 6 to 11 show the variation of the secondary aberration brought about by an incident angle EP11 of the first magnetic field, a ratio (DL2/AM1) of a free space distance between the first and the second magnetic fields to a trace radius of the first magnetic field, a ratio (AM1/RM1) of a trace radius of the first magnetic field to a convex radius of an incident end, and a ratio (AM1/RM2) of a trace radius of the first magnetic field to a concave radius of an outgoing end. The secondary aberration shown in Figs. 6 to 11 are indicated by using the secondary aberration coefficients of the expressions (1) and (2). IN actual, they are represented in µm on the assumption of the beam spread appearing in using this filter for practical purpose. The aberration with a bar on it is a value on the dispersion point. The aberration with no bar on it is a value on the image point. The optimal form is determined on the result of simulating those values. The allowance for the secondary aberration is not unconditionally determined. It depends on the purpose of the device itself. As is apparent from Figs. 6 to 11, the secondary aberration is gradually varied with respect to the abovementoned parameters. It is practically negligible if it stays in the range of ±2 µm. As is understood from Fig. 6, if the incident angle EP11 of the first magnetic field is defined in the range of -18° to 5°, the secondary aberration may be reduced to a value within ±2 µm. If it is defined in the range of -12° to 0°, it may be further reduced to a value within ±1 µm. As is understood from Fig. 7, it is possible to suppress the secondary aberration within ±2 µm if a ratio (DL2/AM1) of a middle free space distance to a trace radius of the first magnetic field is set in the range of 0.4 to 0.8. Further, as is understood from Fig. 8, it is possible to suppress the secondary aberration within ±2 µm if a ratio (AM1/AM2) of a trace radius of the first magnetic field to a trace radius of the second magnetic field is set in the range of 2 to 3. Moreover, as is understood from Fig. 10, it is possible to suppress the secondary aberration within ±2 µm if a ratio (AM1/RM1) of the trace radius of the first magnetic field to a convex radius of the incident end is set in the range of 0.5 to 0.9. As is understood from Fig. 11, it is possible to suppress the secondary aberration within ±2 µm if a ratio (AM1/RM2) of the trace radius of the first magnetic field to the concave radius of the outgoing end is set in the range of -0.5 to 0.05. After the electron energy filter of this embodiment is attached with the known electron microscope (manufactured by Hitach, Ltd., Model: H-8100), the function of the energy filter was checked. Fig. 12 shows the construction of the electron microscope attached with the energy filter. In Fig. 12, this electron energy filter has in-column magnetic poles 21 and 22 located between a middle lens system 53 and a projecting lens system 55. An electron beam 4 is fired by an electron gun 59 and then converged through the converging lens system 50. The converted beam 4 transmits through a specimen 51 and then is converged to a crossover point 5 through the middle lens system 53. The converted beam goes around the inside of the energy filter and then converted again at an energy dispersion point 8. The electrons having respective kinds of energy are dispersed through the effect of the magnetic fields of the magnetic poles 21 and 22 so that those electrons having a different magnitude of energy are transformed into the corresponding line spectrums. A variable slit 54 is located at the energy dispersion point 8 for selectively adjusting the energy width of the line spectrum to a specific one. On the other hand, the incident image surface 6 formed through the middle lens system 53 is formed again on the outlet image surface 7 through the effect of the energy filter. This image is not made vague by the so-called achromatic effect, that is, the effect that the dispersions are offset by the energy filter if the electrons have the corresponding energy width. The electron beam selected as the specific energy through the variable slit 54 passes a projecting lens 55, through which an outlet image 7 is expansively formed on a fluorescent plate 56. A numeral 57 denotes a detector. The electron energy filter provides two magnetic poles 21 and 22 that are independent of each other. Each magnetic pole has a coil wound therearound. The magnetic pole yields a magnetic field for an acceleration voltage of the electrons so that the electron beam travels along a specific trace. Fig. 13 shows an example of a photo representing an energy-filtered image formed by the electron microscope attached with the energy filter according to this embodiment. The used specimen is a carbon grating film about 0.5-µm square. The selected energy is electrons with zero loss. As is obvious from Fig. 13, though the electron beam travels around the inside of the energy filter, the image is represented as a square. When detecting the electron beam transmitting through the slit under the constant magnetic field and the increased voltage for accelerating the electrons, the electron beam is represented as an energy spectrum of the transmitted electrons, which is illustrated in Fig. 14. The conventional electron microscope is served to form all the electrons having any magnitude of energy contained in this energy spectrum as an image and expand it. The electron energy filter enables to select only the electrons having a specific magnitude of energy. Figs. 15A to 15C show electron microscopic photos of a non-dyeing specimen with a thickness of about 70 µm where a cardiac muscle of a mouse is double-fixed by glutaraldehyde and osmium tetroxide. Fig. 15A shows an ordinary image of the electron microscope. Fig. 15A shows an image formed of only the electrons with zero loss selected through the electron energy filter. Fig. 15C is a loss image formed of only the electrons close to -250 eV. As is obvious from the comparison of these photos, the electron microscope attached with the energy filter of this embodiment may offer an image with improved contrast rather than the conventional electron microscope. Further, in the former, by selecting the core-loss electrons, it is possible to form a vivid mapping image of specific elements and improve a function of an analytic electron microscope. As set forth above, the resulting electron energy filter may be compactly designed with a small number of magnetic poles and is low in aberration and excellent in a converging characteristic. (Second Embodiment) In order to meet the expressions (3) and (4), the deflecting angle WM1 and the central trace radius AM1 in the first magnetic field, the deflecting angle WM2 and the central trace radius AM2 in the second magnetic field, the distance DL2 from the outgoing point of the first magnetic field to the incident point of the second magnetic field are selected in the following numerical ranges. WM1 substantially 90° 1.5 AM2 ≤ AM1 ≤ 2.5 AM2, WM2 substantially 180° Am1 ≤ DL2 ≤ 2 AM1 In order to reduce the secondary aberration to a minimum, the first incident angle EP11 and the outgoing angle EP12 of the electron beam in the first magnetic field and the incident and outgoing angle EP21 of the electron beam in the second magnetic field are selected in the following numerical ranges. 20° ≤ EP11 ≤ 30° 20° ≤ EP12 ≤ 30° -3° ≤ EP21 ≤ 1° Further, the magnetic pole end of the first incident point and the outgoing point of the electron beam in the first magnetic field is worked as a convex or a concave. These curvature radii RM1 and RM2 are selected in the following numerical ranges. Incident Plane -1 ≤ AM1/RM1 ≤ 1 Outgoing Plane -1 ≤ AM1/RM2 ≤ 1 where a sign of + represents the convex curvature and a sign of - represents the concave curvature. The energy filter is designed so that the electron beam that outgoes at the crossover point is, again, focused at one point on the energy dispersion surface. This design is based on the following disadvantages appearing if the electron beam is linearly focused without focusing it at one point, (1) It is necessary to locate the energy spectrum and the energy selecting slit strictly in parallel to each other. In order to suppress the adverse effect of the shadow of dust adhering to the slit, it is necessary to make the slit tabular in a wide range. (2) It is likely that an imperfection in an axial condition may slip the electron beam out of the condition of X = Y = 1 on the image surface. (3) To observe a grating pattern, the image surface is formed on the energy dispersion surface. Since the x-axial focal distance is different from the y-axial focal distance, it is difficult to match the focal points of the image surface to each other. The representative parameters of the electron optical system of this embodiment are indicated as follows. EP11 = 22.8°, RM1 = 0 mm, AM1 = 45 mm, WM1 = 90°, EP12 = 27.7°, RM2 = 0 mm, DL2 = 70.4 mm, EP21 = 0°, AM2 = 22 mm, WM2 = 180° In this case, the first-order aberration coefficients at the image point DLI4 and the energy dispersion point DLD4 are indicated as follows, where DLI4 is a distance from the final magnetic pole end to the image point and DLD4 is a distance from the final magnetic pole end to the energy dispersion point. DL14 = 30.4 mm, X = 1.0, A = 0.0, D = 0.0, Y = -1.0, B = 0.0 DLD4 = 102.8 mm, X = -1.0, A = 0.0, D = 0.16, Y = 1.0, B = 0.0 These first-order aberration coefficients meet the expressions (3) and (4). The secondary aberration coefficients of the electron energy filter of this embodiment calculated by the TRIO are illustrated in Table 3. Those secondary aberration coefficients are small enough to inhibit the distorted or vague energy spectrum and image and are maintained in the allowable range. Further, the electron energy filter includes four deflecting magnetic fields, that is, the number of the magnetic poles is as small as four. The electron energy filter is excellent in workability and constructed compactly. Secondary Aberration Coefficient of Electron Energy Filter of the Invention Aberration Coefficient XX XA AA YY YB Image Point -43.4 3.14 0.00 -37.9 2.74 Dispersion Point -105.3 -12.1 -0.44 -138.8 -18.3 BB YX YA BX BA 0.00 41.1 -2.41 -1.48 0.00 -0.67 287.5 18.1 17.9 1.24 (Comparative Example) In order to meet the expressions (3) and (4), the deflecting angle WM1 and the central trace radius AM1 in the first magnetic field, the deflecting angle WM2 and the central trace radius AM2 in the second magnetic field, and a distance from the outgoing point of the first magnetic field to the incident point of the second magnetic field are selected in the following numerical ranges. 75° ≤ WM1 ≤ 85°, preferably, substantially 80° 1.5 AM2 ≤ AM1 ≤ 2.5 AM2 190° ≤ WM2 ≤ 210°, preferably, substantially 200° 0.8 AM1 ≤ DL2 ≤ 2.5 AM1 In order to reduce the secondary aberration to a minimum, the first incident angle EP11 and the outgoing angle EP12 of the electron beam in the first magnetic field and the incident and outgoing angle EP21 of the electron beam in the second magnetic field are selected in the following numerical range. 15° ≤ EP11 ≤ 15° 30° ≤ EP12 ≤ 45° -7° ≤ EP21 ≤ 3° Further, the magnetic pole end of the first incident point and the outgoing point of the electron beam in the first magnetic field is worked as a convex or a concave for reducing the secondary aberration to a minimum. These curvature radii RM11 and RM12 are selected in the following numerical range, where a sign of + represents the convex curvature and a sign of - represents the concave curvature. Incident Plane: 0.5 ≤ AM1/RM11 ≤ 1.5 Outgoing Plane: -1.5 ≤ AM1/RM12 ≤ -0.5 Likewise, the curvature radius RM21 of the magnetic pole end between the incident point and the outgoing point of the electron beam in the second magnetic field is selected in the following numerical range by the trace radius AM2. 0.5 ≤ AM2/RM21 ≤ 1.5 This electron energy filter is designed so that the electron beam that outgoes at the crossover point is, again, focused at one point on the energy dispersion surface. If the electron beam is focused not at one point but linearly, the following disadvantages take place. (1) It is necessary to locate the energy spectrum and the energy selecting slit strictly in parallel to each other. Further, to suppress the adverse effect of the shadow of dust adhering to the slit, it is necessary to make the slit tabular in a wide range. (2) It is likely that an imperfection in an axial condition may slip the electron beam out of the condition of X = Y = 1. (3) To observe the grating pattern, the image surface is formed on the energy dispersion surface. However, since the x-axial focal distance is different from the y-axial focal distance, it is difficult to match the focal points of the image surface to each other. . The representative parameters of the electron optical system of this example are indicated as follows. AM1 = 45 mm, WM1 = 80°, EP11 = 1.18°, EP12 = 36.9°, RM11 = 47.01 mm, RM12 = -47.52 mm, DL2 = 73.65 mm, AM2 = 24 mm, WM2 = 200°, EP21 = 0° In this case, the first-order aberration coefficients at the image point DLI4 and the energy dispersion point DLD4 are indicated as follows, where DLI4 is a distance from the final magnetic pole end to the image point and DLD4 is a distance from the final magnetic pole end to the energy dispersion point. DL14 = 9.83 mm, X = 1.0, A = 0.0, D = 0.0, Y = 1.0, B = 0.0 DLD4 = 92.34 mm, X = -1.0, A = 0.0, D = 0.11, Y = 1.0, B = 0.0 These first-order aberration coefficients meet the expressions (3) and (4). The secondary aberration coefficients of the electron energy filter according to this example, which are calculated by the TRIO, are listed in Table 4. Those secondary aberration coefficients are small enough to inhibit the distorted or vague energy spectrum or image and maintained in the allowable range. This electron energy filter is constructed to have two deflecting magnetic fields for realizing the foregoing optical system, that is, the number of the magnetic poles is as small as four. The electron energy filter is excellent in workability and is constructed compactly. Secondary Aberration Coefficient of Electron Energy Filter Aberration Coefficient XX XA AA YY YB Image Point -43.4 3.14 0.00 -37.9 2.74 Dispersion Point -105.3 -12.1 -0.44 -138.3 -18.5 BB YX YA BX BA 0.00 41.1 -2.41 -1.48 0.00 -0.67 287.5 18.1 17.9 1.24 (Third Embodiment) In order to meet the expressions (3) and (4), the deflecting angle WM1 and the central trace radius AM1 in the first magnetic field, the deflecting angle WM2 and the central trace radius AM2 in the second magnetic field, and the distance DL2 from the outgoing point of the first magnetic field to the incident point of the second magnetic field are selected in the following numerical range. WM1 substantially 90° 1.5 AM2 ≤ AM1 ≤ 2.5 AM2 WM2 substantially 180° AM1 ≤ DL2 ≤ 2 AM1 In order to reduce the secondary aberration to a minimum, the first incident angle EP11 and the outgoing angle Epl2 of the electron beam in the first magnetic field and the incident and outgoing angle EP21 of the electron beam in the second magnetic field are selected in the following numerical ranges. 38° ≤ EP11 ≤ 45° 8° ≤ EP12 ≤ 16° 25° ≤ EP21 ≤ 35° Further, the magnetic pole end between the first incident point and the outgoing point of the electron beam in the first magnetic field is worked as a concave or a convex for reducing the secondary aberration to a minimum. These curvature radii RM11 and RM12 are selected in the following numerical ranges, where a sign of + represents a convex curvature and a sign of - represents a concave curvature. Incident Plane -0.2 ≤ AM1/RM11 ≤ 0 Outgoing Plane 0 ≤ AM1/RM12 ≤ 1 Incident and Outgoing Plane 0.5 ≤ AM1/RM21 ≤ 1.5 The electron energy filter is designed so that the electron beam that outgoes at the crossover point is, again, focused at one point on the energy dispersion surface. If the electron beam is focused not at one point but linearly, the following disadvantages take place. (1) It is necessary to locate the energy spectrum and the energy selecting slit strictly in parallel to each other. Further, to suppress the adverse effect of the shadow of the dust adhering to the slit, it is also necessary to make the slit tabular in a wide range. (2) It is likely that an imperfection in an axial condition may slip the electron beam out of the condition of X = Y = 1 on the image surface. (3) To observe the grating pattern, the image surface is formed on the energy dispersion surface. However, since the x-axial focal distance is different from the y-axial focal distance, it is difficult to match the focal points of the image surface to each other. Next, the representative parameters of the electron optical system of this embodiment are indicated as follows. EP11 = 42.1°, AM1 = 32 mm, AM1/RM11 = -0.1455, WM1 = 90°, EP12 = 11.77°, AM1/RM12 = 0.668, DL2 = 50.92 mm, EP21 = 30°, AM2 = 18 mm, WM2 = 180°, AM2/RM21 = AM2/RM22 = 1.0 Fig. 16 shows a schematic construction of the filter constructed to use these parameters, in which the same members of Fig. 16 as those of Fig. 1A have the same reference numbers and thus are not descriptive herein. In this case, the first-order aberration coefficients at the image point DLI4 and the energy dispersion point DLD4 are indicated as follows, where DLI4 is a distance from the final magnetic pole end to the image point and DLD4 is a distance from the final magnetic pole end to the energy dispersion point. DL14 = 42.1 mm, X = 1.0, A = 0.0, D = 0.0, Y = 1.0, B = 0.0 DLD4 = 111.8 mm, X = -1.0, A = 0.0, D = 0.2, Y = -1.0, B = 0.0 These first-order aberration coefficients meet the expressions (3) and (4). The secondary aberration coefficients of the electron energy filter according to this embodiment, which are calculated by the TRIO, are listed in Table 5. Those secondary aberration coefficients are small enough to inhibit the distorted or vague energy spectrum and image and thus maintained in an allowable range. Further, the electron energy filter is constructed to have two deflecting magnetic fields for realizing the foregoing optical system, that is, the number of the magnetic poles is as small as four. The electron energy filter is excellent in workability and is constructed compactly. Secondary Aberration Coefficient of Electron Energy Filter of the Invention Aberration Coefficient XX XA AA YY YB Image Point -2.22 0.17 0.00 -73.2 5.2 Dispersion Point -1.06 0.026 0.00 -47.9 0.19 BB YX YA BX BA 0.00 6.18 1.19 2.45 0.08 0.00 253 17.7 19.9 1.02

Claims: An electron energy filter for deflecting electrons and filtering only electrons having a specific energy, comprising: means (21, 25) for generating a first deflecting magnetic field, means (22, 26)for generating a second deflecting magnetic field having the same deflection direction as said first deflecting magnetic field, and a free space on which said magnetic fields of said means (21, 25; 22, 26)have no substantial effect; wherein incident electrons are deflected at a first deflecting angle WM1 of substantially 90° with a first trace radius AM1 through the effect of said first deflecting magnetic field, passed through a first path of said free space, and then are incident on said second deflecting magnetic field, after that are deflected at a second deflecting angle WM2 of substantially 180° with a second trace radius AM2, passed through a second path of said free space, said second path being substantially parallel to said first path, and are incident on said first deflecting magnetic field, then are deflected at said first deflecting angle WM1 with said first trace radius AM1 and outgo in substantially the same direction as the electrons incident on said first deflecting magnetic field, the minimum of the distance DL2 of said free space travelled by said electrons between said first and second deflecting magnetic fields being about half the first trace radius AM1. The filter of claim 1, wherein said first path is substantially equal in distance to said second path. The filter of claim 1, wherein said first path is substantially equal in distance to said second path, and said distance DL2 and said first trace radius AM1 meet the following relation: AM1 ≤ DL2 ≤ 2 AM1. The filter of claim 1, wherein said means (21, 25) for generating the first deflecting magnetic field comprises a first pair of magnetic poles (21) and said means (22, 26) for generating the second deflecting magnetic field comprises a second pair of magnetic poles (22) and said first and second deflecting magnetic fields are directed in the same direction; and said first trace radius AM1, said second trace radius AM2 and said free space distance DL2 meet the following relations: 2 AM2 ≤ AM1 ≤ 3AM2; 0.5 AM1 ≤ DL2. The filter of claim 4, wherein a first incident angle EP11 and a first outgoing angle EP12 of a beam of said electrons with respect to said first deflecting magnetic field and an incident and outgoing angle EP21 of said electron beam with respect to said second deflecting magnetic field meet the following relations: -12° ≤ EP11 ≤ 0°; 30° ≤ EP12 ≤ 40°; -3° ≤ EP21 ≤ 1°. The filter of claim 4, wherein a first incident angle EP11 of a beam of said electrons is about 42°, a first outgoing angle EP12 of said beam is about 11.8°, both with respect to said first deflecting magnetic field, and an incident and outgoing angle EP21 of said beam with respect to said second deflecting magnetic field is about 30°. The filter of claim 5, wherein a magnetic pole end at a first incident point of said electron beam with respect to said first deflecting magnetic field has a radius of curvature RM1, a magnetic pole end at a first outgoing point has a radius of curvature RM2, and said curvature radii RM1 and RM2 meet the following relations: 0.5 ≤ AM1/RM1 ≤ 0.9; -0.5 ≤ AM1/RM2 ≤ 0.05. The filter of claim 6, wherein a magnetic pole end at the incident point of said electron beam with respect to said first deflecting magnetic field has a radius of curvature RM1, a magnetic pole end at the outgoing point has a radius of curvature RM2, and said curvature radii RM1 and RM2 meet the following relations: AM1/RM1 = 1.0; AM1/RM2 = 0.7. The filter of claim 4, wherein the distance DL2 of said free space is equal to said first trace radius AM1. The filter of claim 1, wherein said first and second deflecting magnetic field generating means (21, 25; 22, 26) comprise a first pair of magnetic poles (21) for generating said first deflecting magnetic field and a second pair of magnetic poles (22) for generating said second deflecting magnetic field in the same direction as said first deflecting magnetic field; wherein said first trace radius AM1, said second trace radius AM2, and the distance DL2 of said free space meet the following relations: 1.5 AM2 ≤ AM1 ≤ 2.5 AM2; AM1 ≤ DL2 ≤ 2AM1. The filter of claim 7, wherein a first incident angle EP11 and a first outgoing angle EP12 of said electrons with respect to said first deflecting magnetic field and an incident and outgoing angle EP21 of said electrons with respect to said second deflecting magnetic field meet the following relations: 20° ≤ EP11 ≤ 30°; 20° ≤ EP12 ≤ 30°; -3° ≤ EP21 ≤ 1°. The filter of claim 11, wherein a magnetic pole end at a first incident point of said electrons with respect to said first deflecting magnetic field has a radius of curvature RM1, a magnetic pole end at a first outgoing point of said electrons with respect to said first deflecting magnetic field has a radius of curvature RM2, and said curvature radii RM1 and RM2 meet the following relations: -1 ≤ AM1/RM1 ≤ 1; -1 ≤ AM1/RM2 ≤ 1. The filter of claim 10, wherein the distance DL2 of said free space is longer than said first trace radius AM1. The filter of claim 10, wherein a first incident angle EP11 and a first outgoing angle EP12 of said electrons with respect to said first deflecting magnetic field and an incident and outgoing angle EP21 of said electrons with respect to said second deflecting magnetic field meet the following relations: 38° ≤ EP11 ≤ 45°; 8° ≤ EP12 ≤ 16°; 25° ≤ EP21 ≤ 35°. The filter of claim 14, wherein a magnetic pole end at a first incident point of said electrons with respect to said first deflecting magnetic field has a radius of curvature RM11, a magnetic pole end at a first outgoing point has a radius of curvature RM12, a magnetic pole end at an incident point and an outgoing point of said electrons with respect to said second deflecting magnetic field has a radius of curvature RM21, and said curvature radii RM11, RM12 and RM21 meet the following relations: -0.2 ≤ AM1/RM11 ≤ 0; 0 ≤ AM1/RM12 ≤ 1; 0.5 ≤ AM1/RM21 ≤ 1.5. A transmission electron microscope including the electron energy filter of any of claims 1 to 15. A method for filtering energy of an electron beam, comprising the steps of: deflecting incident electrons at a first deflecting angle WM1 of substantially 90° with a first trace radius AM1 by a first deflection magnetic field and letting them pass through a first path of a free space substantially unaffected by a magnetic field; deflecting said electrons passed through said first path at a second deflecting angle WM2 of substantially 180° and of the same sense as said first deflecting angle WM1 and with a second trace radius AM2 by a second deflection magnetic field and letting said deflected electrons pass through a second path of said free space, said second path being substantially parallel to said first path; and deflecting said electrons passed through said second path at said first deflecting angle WM1 with said first trace radius AM1 by the effect of said first deflecting magnetic field and letting said deflected electrons out in substantially the same direction as the incident electrons, wherein the minimum of the distance DL2 of said free space travelled by said electrons between said first and second deflecting magnetic fields is about half the first trace radius AM1.

Inventors: TANIGUCHI YOSHIFUMI, TAYA SHUNROKU, TANIGUCHI, YOSHIFUMI, TAYA, SHUNROKU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1995
Main classification: H01J49
IPC codes: H01J37, H01J49