Patent Number: 
Section: description

The preferred embodiments of the present invention are hereinafter described with reference to the accompanying drawings. FIG. 1 shows the structure of a magnetic energy filter in accordance with the present invention. FIGS. 2(a) to 2(f) are diagrams illustrating fundamental trajectories based on the results of a simulation of the magnetic energy filter in accordance with the present invention. FIG. 2(a) illustrates the trajectory of an electron beam forming an electron microscope image projected onto an entrance window plane. The electron beam creating a focused electron microscope image on the entrance window plane forms four crossover points in total and then forms a focused electron microscope image on the exit split plane again. FIG. 2(b) illustrates the trajectory of the electron beam having certain (two) energies within electron beams passing through the entrance window. The electron beams passing through the entrance window are focused three times and undergo a fourth focusing action on the exit slit plane. If electron beams have different energies, they draw different trajectories and are focused at different locations on the exit slit plane. Therefore, only an electron beam having a desired energy can be selected with an exit slit. FIG. 2(c) illustrates the trajectory xxcex1 (x alpha) of the electron beam forming a microscope image, the trajectory being parallel to the magnetic polepiece plane. FIG. 2(d) illustrates the trajectory yxcex2 (y beta) of the electron beam forming a microscope image, the trajectory being in the same direction as the direction of the magnetic field. FIG. 2(e) indicates the trajectory xxcex3 (x gamma) of an electron beam with a desired energy parallel to the magnetic polepiece plane, as well as the magnitude of dispersion x"khgr" (x chi) along the optical axis. FIG. 2(f) indicates the trajectory yxcex4 (y delta) of an electron beam having a desired energy in the direction of the magnetic field. Referring to FIG. 1, there are shown magnets M1-M4, an entrance window I, and an exit slit S. The trajectory of an electron beam in the geometry of FIG. 1 is described below. The electron beam impinges in the direction of the optical axis indicated by the arrow. The beam forms a crossover point (focused point) on the plane of the entrance window I and then hits a filter. The electron beam passed through the entrance window I enters the magnetic field of magnet M1 having a deflection angle of xcfx861. The beam is deflected through xcfx861. In the figure, the beam is shown to be deflected in a clockwise direction. The beam then exits from the field and undergoes a first focusing action from the filter. Then, the beam enters the magnet M2 having a deflection angle of xcfx862 and is deflected through xcfx862 in the reverse direction. In the figure, the beam is shown to be deflected in a counterclockwise direction. The beam then leaves the magnet M2 and passes across a point O at which a straight axis intersects the trajectory of the electron beam. The beam undergoes a second focusing action near this point O. After passing across the point O, the beam enters the magnet M3 that is located on the opposite side of the straight axis and has a deflection angle of xcfx863. In this magnet, the beam is deflected through xcfx863 in a clockwise direction and exits from the magnet, where xcfx863=xe2x88x92xcfx862. Then, the beam undergoes a third focusing action. The beam going out of the magnet M3 passes into the magnet M4 having a deflection angle of xcfx864, where xcfx864=xe2x88x92xcfx861 In this magnet, the beam is deflected through xcfx864 in a counterclockwise direction and exits from the magnet. The electron beam leaving the magnet M4 reaches the exit slit and undergoes a fourth focusing action on the exit slit plane. Under this condition, the electron beam is dispersed sufficiently because of variations in energy. Accordingly, the electron beam having only the desired energy passes through the exit slit and leaves the filter. The point O located midway between the magnets M2 and M3 is referred to as the midpoint. An axis that passes through this point and is vertical to the plane of the paper gives a two-fold rotational symmetry axis or a two-fold rotation axis for the selected electron beam path through the filter. That is, the physical structure of the filter gives a two-fold rotational symmetry around the axis passing through the point O (with the exception of the polarities of the magnets explained hereafter). In the magnetic energy filter in accordance with the present invention, magnets are located on opposite sides of the straight axis as described above. Furthermore, they are arranged with two-fold rotational symmetry. Compared with conventional filters, such as an OMEGA filter and an ALPHA filter where magnets are placed on only one side of a straight axis, the beam path can be elongated and the sum of the absolute values of the beam deflection angles can be increased without increasing the distance between the entrance and the exit of the filter, i.e., the distance D between the entrance window I and the exit slit S. Therefore, the magnetic energy filter designed as described above in accordance with the present invention is more compact than the prior art OMEGA filter. In the geometry of FIG. 1, the beam deflection angles are assumed to be 110xc2x0, xe2x88x92250xc2x0, 250xc2x0, and xe2x88x92110xc2x0 in this order from the entrance side. The sum of the absolute values of the deflection angles is 720xc2x0, which is twice that of the deflection angle of the ALPHA filter. In the case of the prior art OMEGA filter, the limit values of practical deflection angles of the four magnets are assumed to be 125xc2x0, xe2x88x92125xc2x0, xe2x88x92125xc2x0, and 125xc2x0 in this order. The sum of the absolute values of the deflection angles is 500xc2x0, which is 1.4 times as large as the conventional value. Where the energy filter proposed heretofore is made up of four magnetic fields M1, M2, M3, and M4, the magnetic fields M2 and M3 located on the opposite sides of the symmetry plane are identical in polarity. In a magnetic energy filter (hereinafter referred to as the S filter) in accordance with the present invention, the magnetic fields M2 and M3 located on the opposite sides of the rotational symmetry axis are opposite in polarity. In the example of FIGS. 1 and 2, the sum of the absolute values of the deflection angles is 720xc2x0. In the S filter where the magnetic fields 1 and 2 are opposite in polarity, if the sum of the absolute values of the deflection angles is set greater than 540xc2x0, the beam path can be elongated and the sum of the absolute values of the beam deflection angles can be increased without increasing the distance between the entrance and exit of the filter, i.e., the distance D between the entrance window I and the exit slit S. Hence, the purpose of the present invention is achieved. Another important characteristic is the number of times that the trajectory xxcex3 crosses the optical axis. Generally, it can be thought that as the number of times that trajectory xxcex3 crosses the optical axis indicating the center trajectory in the filter increases, greater dispersion takes place. The number of times the trajectory xxcex3 in the prior art OMEGA filter crosses the optical axis indicating the center trajectory in the filter is three, as indicated by the arrows in FIGS. 7(b) and 8(b) for both types A and B of FIGS. 5 and 6, respectively. On the other hand, in the filter in accordance with the present invention, the number of times is four as indicated by the arrows in FIG. 2(e). FIG. 3 is a diagram showing a magnetic energy filter in accordance with another embodiment of the present invention. This filter has magnets M5-M8, an entrance window I, and an exit slit S. In FIG. 3, the magnets M5 and M6 are both on the entrance side, i.e., on the side of the entrance window. The magnetic fields produced by these magnets M5 and M6 produce the same direction of beam deflection. The direction of beam deflection affected by the magnets M7 and M8 on the exit side, i.e., on the side of the exit slit, is opposite to the direction of beam deflection affected by the magnets M5 and M6 on the incident side where the entrance window is present. That is, these magnets are arranged with two-fold rotational symmetry. In particular, the magnets M5 and M6 have beam deflection angles of xcfx865 and xcfx866, respectively, while the magnets M7 and M8 have deflection angles of xcfx867(=xe2x88x92xcfx866) and xcfx868(=xe2x88x92xcfx865), respectively. This filter is referred to as the 8-shaped filter. In this 8-shaped filter, the sum of the absolute values of the beam deflection angles is approximately 720xc2x0. Supplementary, it can be said that the magnetic energy filter shown in FIG. 1 and built in accordance with the present invention is one modified form of an OMEGA filter and that the magnetic energy filter shown in FIG. 3 and built in accordance with the present invention is one modified form of an ALPHA filter. It is to be understood that the present invention is not limited to the embodiments described above. For instance, in the above-described embodiments, OMEGA and ALPHA filters are modified, and magnets are arranged on the opposite sides of a straight axis with two-fold rotational symmetry. Different types of magnets may be placed on the opposite sides of a straight axis. For example, a modification of an OMEGA filter, such as an S-filter, and a modification of an ALPHA filter may be placed on the opposite sides of a straight axis. Furthermore, in the embodiments described above, the filter is made up of four magnets such as M1-M4 or M5-M8. In summary, a magnetic energy filter is made of four magnetic fields. For example, each of some, or all of the magnets M2 and M3 shown in FIG. 1 and the magnets M6 and M7 shown in FIG. 3 may be split into two. For example, the magnet M2 may be split into two parts M2xe2x88x921 and M2xe2x88x922. That is, if one magnet is split into two and a drift space is inserted between them, the structure remains substantially unchanged. As can be understood from the description provided thus far, the present invention provides a magnetic energy filter which has four magnetic fields to deflect the trajectory of an electron beam from the entrance window plane to the exit slit plane, the energy filter having the following features. The number of the magnetic fields is at least four. The magnetic fields located on the opposite sides of a rotational symmetry axis are opposite in polarity, the rotational symmetry axis being located midway between the second and third magnetic fields. Deflecting magnets are mounted on the opposite sides of a straight axis. The sum of the absolute values of beam deflection angles of the magnetic fields is in excess of 540xc2x0 or is about 720xc2x0. Consequently, the beam path length and the sum of the absolute values of the deflection angles can be increased compared with the prior art OMEGA filter and ALPHA filter. In addition, the distance between the entrance window and the exit slit is shortened, thus making the energy filter more compact. Having thus described our invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.