Patent Number: 043307094
Section: description

FIG. 1 is a sectional view of the electronic objective according to the invention. This objective is of revolution about its axis z z'. It comprises a core in two parts 1 and 2, one part surrounding the other. The inner part 2 is of ferrite and has a cylindrical portion 21 which is connected to two frustoconical portions 22 and 23 forming a divergent part in the vicinity of its output pupil 3. The outer part 1 of soft iron marries up with the shape of the part 2. It is separated therefrom by an airgap 4 and closes onto the output pupil. The output pupil is carried by support 5 of duralumin which is itself carried by the lower part of the polepiece. According to the invention, two magnetic deflectors 6 and 7 are provided, the deflector 6 being placed in the upstream part in the portion 21 of the lens and the other deflector 7 being placed in the region of the frustoconical portion 22. A diaphragm 8 located at the input of the objective limits the aperture of the electron beam. The two deflectors are each formed by an assembly of coils created a magnetic field perpendicular to the axis z z'. A coil 9 is wound around the parts 1 and 2. FIG. 2 represents, in a non-limitative example, the variation of the intensity B of the magnetic field created by the lens along the axis of the later, the origin z=0 being in the plane of the output pupil 3, the abscissae being in centimeters and the ordinates in Gauss. It can be seen that the two deflectors are placed so that the deflector 6 is upstream in the region where the field does not exceed 10 Gauss, and the other deflector 7 is downstream in a region where the field varies between 100 and 200 Gauss. The maximum value of the field after a relatively small increasing slope reaches its maximum value at 300 Gauss in the vicinity of z=-1 cm and drops to zero where Z=0. This being so, FIG. 3 is a top plan view of one of the deflectors, the other deflector being identical and FIG. 4 is a perspective view of the two deflectors. For reasons of clarity, only a part of each deflector has been shown in FIG. 4. Each deflector comprises (FIG. 3) two parts 61 and 62, 71 and 72, one of which is for scanning in x and the other for scanning in y. Each part is divided into two coils which are symmetrical relative to the axis z z' and disposed in series. Each of these coils is wound as shown in FIG. 4 around a portion of a cylinder of revolution subtending an angle of 120.degree. C. The two parts are disposed in such manner that their transverse axes of symmetry are perpendicular to each other. In one embodiment, the height of the cylinders is the same, namely 3 cm, the diameter of the inner cylinder is of the order of 2.4 cm and that of the outer cylinder is of the order of 5 cm. Each semi-deflector 61 or 62, 71 or 72 carries a variable current. The currents in the coils 61-71, 62-72 (not shown in FIG. 4) are respectively proportional to the amplitudes of deflection of the beam along the two axes Ox and Oy which are perpendicular to each other, the assembly forming with the axis z z' a reference trirectangular trihedral and the axes Ox and Oy being located in the plane of the object to be scanned. FIG. 4 shows that the two deflectors are offset by an angle .theta.=180.degree.+.alpha., .alpha. being between 35.degree. and 45.degree.. The ratio of the intensities in the two deflectors is constant and substantially equal to 2. Tests and calculation have shown that the objective according to the invention has the following advantages: (a) The spacing of the ferrite part 2 from the metal part 1 avoids eddy currents which retard the establishment of the magnetic field in the middle of each deflector in the case where a current pulse is applied to the two coils. On the other hand, the presence of the polepiece of soft iron prevents any saturation of the ferrite piece in the case where the lens is highly convergent. (b) The deflection by two successive deflecting stages and their respective angular offset minimizes aberrations, particularly in the case where the upper deflector is in the region where the axial field is weak, the second being in a region where the axial field is strong. The latter must be sufficiently remote from the output pupil and from the iron polepieces so that the transverse field it creates does not close onto the lower polepiece.