Patent Number: 047711787
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION In FIGS. 1 and 2, reference numeral 10 identifies the housing of the objective lens of the electron microscope. With the coil 11 having electric current flowing therethrough, the magnetic field required for forming an image of the specimen 15 is generated between the pole pieces 12 and 13. In terms of the coordinate directions shown on the right-hand side of the figures, the orientation of the optical axis 14 of the objective lens is, as is typically the case, in the Z direction. The goniometer stage is firmly connected to the objective via the bearing block 20, which has a ball-like inner bearing 21. In this bearing, the ball-like end of the bearing sleeve 70 can be moved about the common center point 22. The bearing block 20 of the goniometer stage is firmly connected to the housing 23 on which the bearing disc 30 is attached. The bearing disc 30 can be adjusted in the Y-Z plane by adjusting screws 31 and 32 as well as 33 and 34 and fixed with screws not shown. In FIGS. 1 and 2, the bearing disc 30 has intentionally been shown in an exaggeratedly extreme position to show the location of various axes. At its center, the bearing disc 30 has an accurately ground slide bearing 37. The cylinder 40 is rotatably seated in this slide bearing and is secured against displacement in the X direction by two axial roller bearings (not shown). The cylinder 40 may be rotated about its axis 41, for example, by means of the motor 38 attached to the bearing disc 30 via the gear 39 and the gear rim 44. The cylinder 40 is connected to the outer end of the bearing sleeve 70 (as shown with particular clarity in FIG. 3) via a precise right-angle mechanical-stage guide that assures a displacement of the bearing sleeve 70 in the Y direction and Z direction without reciprocal influence on one another. In the embodiment shown, the mechanical-stage guide comprises two electric motor drives 52 and 62 for the movement of the spindles 51 in the Y and Z directions and resilient restoring means 54 and 64, which act upon the faces 71, 72, 73 and 74 of the bearing sleeve 70, which are disposed precisely at right angles to one another. The drives 52 and 62 are connected directly with position transducers 53 and 63 for transmitting the displacements. By means of the positioning devices 50 and 60, the bearing sleeve 70 is moved about the center point 22 of the ball-like bearing surface 21 in the Y and Z directions. The bearing sleeve 70 is furthermore connected via the disc 75 and the bellows 76 with the cylinder 40. Because of its torsion-resistant nature, this bellows, which is preferably of metal, transmits the rotational movement of the cylinder 40 to the bearing sleeve 70 without slippage and without play; however, for angular and lateral deflections, the bellows 76 is sufficiently flexible to enable the displacement of the bearing sleeve 70 in the Y and Z directions. Since the rotational movement of the cylinder 40 is transmitted directly to the bearing sleeve 70 via the bellows 76, the adjusting devices 50 and 60 are not subjected to load during a rotational movement; these devices would without the bellows have to transmit the rotational movement and their precision and reproducibility would be thereby negatively influenced. The points of application of the positioning devices 50 and 60 on the bearing sleeve 70 are located as close as possible to the bearing disc 30, so that wobble errors of the outer bearing 37 have the least possible influence. The points of application can be placed into the plane of the bearing disc 30 with the tilting levers (not shown). Furthermore, it is advantageous for the positioning devices 50 and 60 to be statically balanced with respect to the axis 41 of the cylinder 40, so that upon a rotational movement of the cylinder 40, no changes occur in the forces in the outer bearing 37. For precise and reproducible displacements it is particularly advantageous that the positioning devices 50 and 60 are equipped with short, thermally insensitive and mechanically stable spindles 51 and 61. Upon a rotation of the cylinder 40, the bearing sleeve 70 and the positioning devices 50 and 60 are rotated about the tilting axis 45, which passes through the center point 22 of the ball-like inner bearing 21 and the point of intersection 43 of the axis 41 of the cylinder 40 with the plane 42 in which the positioning devices 50 and 60 have their points of application upon the bearing sleeve 70. In the bearing sleeve 70, the specimen holder rod 80 is accommodated in an axially movable manner, with the specimen 15 seated on its inner end. For introducing the specimen into the vacuum and for transferring it out of the vacuum, a vacuum transfer lock, configured in a known manner and not shown in the drawings, is accommodated inside the bearing sleeve 70. In the ball-like end of the bearing sleeve 70, the specimen holder rod 80 is pressed by a spring (not shown) against a V bearing (also not shown), so that it has no radial play and therefore its position with respect to the bearing sleeve is not influenced by gravity upon a rotation of the bearing sleeve 70. The displacement of the specimen in the X direction is effected by displacing the specimen holder rod 80 in the bearing sleeve 70 by the positioning device 90, which is located on the side of the goniometer stage opposite the tilting device and which acts counter to the air pressure applied externally upon the specimen holder rod 80. The positioning device 90 comprises a spindle 91, which is moved for instance by an electric motor drive 92 having a position transducer 93, and a pendulum rod 94, which transmits the axial movement of the spindle 91 to the specimen holder rod 80 and which upon deflection of the specimen holder rod 80 out of its central position by means of the Y and Z displacement and upon tilting describes a movement on an imaginary conical surface about the tilting axis 45. The positioning device 90 is adjusted relative to the housing 10 or to the bearing block 20 such that the center of the ball 95 on the tip of the spindle 91 lies precisely on the tilting axis 45 or eucentric axis. For this purpose, the positioning device 90 is adjustable in the vertical direction with the adjusting screws 96 and 97 and in the horizontal direction with the adjusting screws 98 and 99 on a spherical surface 9C the center point of which coincides with the center point 22 of the ball-like bearing surface 21. When correctly adjusted, the center axis 9D of the positioning device 90 therefore is located precisely in the extension of the tilting axis 45 or of the eucentric axis. The adjustment is set, that is, fixed, by means of the screws 9A and 9B. Upon a rotation of the cylinder 40, the specimen 15 is tilted about the tilting axis 45 in the path of the rays of the electron microscope. Since the bearing disc 30 is adjustable in the Z direction and Y direction--as described above--the tilting axis 45 can be adjusted such that it intersects the optical axis 14 of the objective lens in the focusing plane. This tilting axis is known as the eucentric axis. If in this case a specific section of the specimen 15 is moved by means of the positioning devices 50, 60 and 90 into the focusing plane and into the observation field of the electron microscope, then the field is located on the eucentric axis, and if it is tilted the focusing and the selected section of the specimen are maintained. By means of the adjusting device for the bearing disc 30 in the Z direction (with the aid of the screws 31 and 32), it is possible to drive the objective lens of the electron microscope in various operating modes as well, for example, as a single-field condenser-objective lens for optimal STEM operation or as a so-called second-zone lens for optimal TEM operation, and in this case to always adjust the tilting axis 45 to be precisely on the point of intersection of the objective axis with the focusing plane, so that it becomes the eucentric axis. The positioning devices 50, 60 and 90 may also be actuated otherwise than by electric motor drives as described for the embodiment here. Mechanical drives with counter mechanisms as position readout devices are particularly simple. Piezoelectric, pneumatic or hydraulic drives can also be used. It is also possible to connect the positioning devices to devices for open-loop or closed-loop control, so that predetermined sections of the specimen 15 can easily be recalled once again, or to enable systematic scanning of the specimen. It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.