Patent Number: 
Section: description

Referring to FIG. 5, there is shown a transmission electron microscope embodying the concept of the present invention. This instrument has an objective lens 2 forming a back-focal plane 3. A specimen 1 is placed as shown. The microscope further includes phase lenses 4 and 5. Deflection coils 6 and 8 are mounted above and below, respectively, a phase plate 7. It is assumed that the phase lenses 4 and 5 have the same focal length of ƒ. Each of the deflection coils 6 and 8 is made of a double deflection coil. These components are positioned as follows. The phase lens 4 is placed at a distance of ƒ from the back-focal plane 3 as measured along the optical axis. The phase lens 5 is placed at a distance of 2ƒ from the phase lens 4. The phase plate 7 is placed at a distance of ƒ from the phase lens 5. The deflection coils 6 and 8 are placed immediately above and immediately below, respectively, the phase plate 7. In practice, an electron gun for illuminating the specimen 1 with an electron beam, a system of condenser lenses placed above the specimen 1, an intermediate lens for forming an image of the specimen 1, and a projector lens and a fluorescent screen under the deflection coils 6 and 8 are mounted, but these components are omitted from FIG. 5. FIG. 5 illustrates the manner in which the lenses according to the present invention act on four electron trajectories going out of the specimen 1. The trajectories are indicated by solid lines from the specimen to the back-focal plane 3. These trajectory portions are the same as those in normal TEM not associated with the present invention. The trajectories are indicated by broken lines from the back-focal plane 3 to the phase plate 7. The lens system according to the present invention creates a conjugate plane to the back-focal plane 3 on the phase plate 7. If a phase plate cannot be normally placed in the back-focal plane within the TEM due to spatial restrictions, the phase plate can still be used according to this invention. The illustrated electron trajectories have the feature that the distances of the trajectories from the optical axis are reproduced on the phase plate 7. In addition, the tilts of the trajectories relative to the optical axis on the back-focal plane 3 are reproduced on the phase plate 7. Therefore, the electron trajectory portions behind the phase plate 7 are identical with the trajectory portions behind the back-focal plane of the objective lens in a normal TEM not according to the present invention. Normal imaging can be performed downstream of the phase plate 7 in the instrument according to the present invention by installing an ordinary TEM imaging system after the phase plate 7. Furthermore, alignment of the electron beam to the phase plate 7 and alignment of the imaging lenses following the phase plate 7 can be performed by the deflection coils 6 and 8 located above and below, respectively, the phase plate 7. Where the phase plate is placed in the back-focal plane, it normally would not be possible to install such alignment coils due to spatial restrictions. The phase lenses according to the present invention creates a conjugate plane to the back-focal plane in a spatially unrestricted location, thus permitting the installation of the coils. As described thus far, the present invention can yield the following advantages. (1) A conjugate plane to the objective lens is created in a spatially unrestricted location by a phase plate lens system. A phase plate is placed in the conjugate plane. Consequently, the phase plate can be used even in a TEM where the phase plate could not otherwise be placed in the back-focal plane due to spatial restrictions imposed by the magnetic polepieces of the objective lens. (2) A conjugate plane to the back-focal plane of the objective lens is created in a spatially unrestricted location by the phase plate lens system. A phase plate is placed in the conjugate plane. Alignment coils are mounted above and below, respectively, the phase plate. Accurate alignment of the electron beam to the phase plate and accurate alignment of the beam to the imaging lenses located after the phase plate can be accomplished. (3) The lenses included in the lens system for use with the phase plate are designed to perform imaging such that tilts are made conjugate, as well as positions of the electron beam. Therefore, optics placed after the back-focal plane of the conventional TEM objective lens can be positioned after the lens system for the phase plate. Hence, TEM imaging can be performed without sacrificing the high-magnification and low-aberration characteristics of the normal objective lens. 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.