Patent GB-1258453-A (GB)
IPC: H01J37/141
Filed: 1969-03-25
Abstract: 1,258,453. Electron lenses; electron microscopes. SIEMENS A.G. 25 March, 1969 [26 March, 1968], No. 15453/69. Heading H1D. In a corpuscular beam microscope for the arbitrary reproduction of either the image or diffraction diagram of an object to be studied, there is provided within the bore of a magnetic objective lens, and between its lens gap and that of a magnetic projection lens, an electromagnetic diffraction lens which when energized enables the diffraction diagram to be observed, the diffraction lens being in the form of a twin lens having two lens gaps whose distance apart along the beam path is less than the focal length of the lens component formed by the first (in the beam direction) lens gap of the diffraction lens and wherein the axial magnetic flux in each lens gap is in opposite directions to prevent undesired image (or diffraction diagram) rotation, and the diffraction lens is arranged in a magnetically isolated fashion in the bore of the objective lens. Preferably, as shown, the objective lens is electromagnetic and has two pole pieces 2 and 3 with respective energizing coils (not shown), and provision for the introduction of an object (not shown) into the lens gap 1. The diffraction lens 7 is housed in the bore 5 of the pole piece 3 on the output side of the objective lens and has two excitation windings 8, 9 and its magnetic flux is guided via an iron circuit constituted by plates 10, 11 and 12 and a tube 13, and there is no central magnetic core. The coils 8, 9 are oppositely energized as indicated to prevent any image rotation, and when energized the diffraction lens focuses the beam on a selector diaphragm plane 14 which may be coincidental with the object plane of a first intermediate lens 17. Dashed line 15 indicates the beam path when the diffraction lens is energized and line 16 when it is not energized. The diffraction lens design enables the iron circuit to be made of high permeability ferrous alloys of very low coercive force and low remanence. The diffraction lens may be supported by a non-magnetic base-plate so as to leave a gap between it and the surrounding objective lens to magnetically isolate it. Figs. 3 and 4 (not shown) illustrate an embodiment of a beam microscope incorporating the diffraction lens in which the construction, cooling and relative disposition of the various lenses, the vacuum chamber and facilities for introducing the object are detailed. Attached to the opposite side of the base-plate securing the diffraction lens may be a stigmater system 6, Fig. 1. The diffraction lens enables the excitation and any effects therefrom of the intermediate lens to remain constant, and its coreless design reduces to a minimum any influences on the beam when it is de-energized. Moreover, the diffraction lens may be regulated such that when the objective lens is de-energized it reproduces an image of the object in the object plane of the projection lens either slightly enlarged or on a scale of 1 : 1, thus producing a small magnification with adequate resolving power. Similarly, adjustment of the diffraction lens may produce a diffraction diagram of greater magnification than that of the image. Preferably the intermediate lenses, which may be used to control the magnification of the image or diffraction diagram when the objective and projection lenses are permanently adjusted to the selector diaphragm, are designed, like the diffraction lens, to produce no rotation of the image or diffraction diagram. In the bore of the first polepiece 2 there may be housed a beam deflection system. It is necessary only for the diffraction lens to be electromagnetic, the other lenses may be permanent-magnetic.