Patent Number: 062394304
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The beam producer in the form of a thermal field emission source is shown in FIG. 1, denoted overall as (1) and the cathode tip which emits electrons as (1a). Electrons are extracted from the cathode tip (1a) by means of an extraction electrode (2) which is at a positive potential, and are then accelerated to the potential of the anode (3). The anode is electrically connected to a beam guiding tube (4) which passes through the whole apparatus and is of electrically conductive material. The anode potential, and hence the potential of the beam guiding tube (4), is about 10 kV relative to ground. The anode is directly followed by a magnetic condenser lens (5), and the condenser lens (5) is followed by an aperture diaphragm (6). The energy filter (7) is an imaging dispersive electron energy filter which images a so-called input image plane (9) (first input plane) stigmatically and achromatically into an output image plane (10) (first output plane) and simultaneously images an input diffraction plane (second input plane) dispersively and stigmatically into an output diffraction plane (second output plane). The aperture diaphragm (6) is arranged in the input diffraction plane; that is, this plane coincides with the plane of the aperture diaphragm (6). The energy selection diaphragm (8) is arranged in the output diffraction plane, and is constituted as a slit diaphragm. The dispersive filter (7) itself is a purely magnetic filter; the three magnet sectors of the filter are denoted by (7a-7c). The detailed construction of the filter is described in U.S. Pat. No. 4,740,704, which should be consulted for constructional details of the filter. In spite of the construction of the filter (7) which is symmetrical with respect to a plane perpendicular to the plane of the drawing in FIG. 1, the filter (7) has a dispersion, that is, the electrons passing through the filter have beyond the filter a deflection perpendicular to the optical axis and dependent on their energy, so that those electrons are trapped which have an energy deviation from the mean energy which is greater than the energy deviation defined by the dispersion and slit width. In order for high voltage fluctuations to bring about no lateral drift of the successive images of the cathode tip (1a) emitting the electrons, the cathode tip (1a) is imaged by the condenser lens (5) in the input image plane (9) of the filter (7), and in fact is imaged by the filter (7) achromatically in the output image plane (10). A second condenser lens (11) follows the energy selection diaphragm (8), and images the selection diaphragm (8) in the rear focal plane of the objective lens (13). Simultaneously, the condenser lens (11), by imaging the output image plane (10) of the filter (7) produces a further intermediate image of the cathode tip (1a), which is imaged by the succeeding objective lens (13), once again greatly reduced, onto the specimen (15) to be investigated. The objective lens (13) is a combination of a magnetic and electrostatic lens. The specimen (15) and the pole shoes of the objective lens (13) are at ground potential, so that the electrons after leaving the beam guiding tube (4) are substantially braked to the target energy of between 10 eV and 5 keV between the end of the beam guiding tube (4) and the outer pole shoe of the objective lens (13). A further magnetic deflecting system (14) is arranged in the pole shoe gap of the objective lens (13), for scanning a large lateral region of the specimen (15). For the detection of the secondary electrons leaving the specimen (15), a rotationally symmetrical electron detector (12) with a middle bore is arranged between the objective lens (13) and the second condenser lens (11). This detector can be constructed as a scintillation detector, semiconductor detector, or microchannel plate detector. The detailed electron optical beam path is shown in FIG. 2. The cathode tip (1a) which emits electrons is imaged by the first condenser lens (5) into the input image plane (9) of the dispersive filter (7), which is imaged achromatically and stigmatically by the filter into the output image plane (10). The aperture diaphragm (6) is arranged between the first condenser lens (5) and the input image plane (9) of the filter (7), in that plane which is imaged by the filter (7), stigmatically and dispersively, into the output side conjugate plane in which the selection diaphragm (8) is arranged. The imaging of the cathode tip (1a) into the input image plane (9) here takes place with an enlargement of 5-40, and there thus results a corresponding reduction of the effective aperture within the filter (7) and in the plane of the selection diaphragm (8). The energy filtering on the output side of the filter (7) thereby leads to no appreciable cutting down of the aperture of the electron beam bundle. When the energy filter has a dispersion of 10-15 .mu.m/eV at an electron energy of 10 keV, an energy width of 0.1-0.2 eV is set with a selection diaphragm (8) which has a slit width of 2 .mu.m. Since the distance between the input image plane (9) and the input diffraction plane (6) amounts to 40-80 mm in typical dispersive energy filters, an aperture of 1.5.times.10.sup.-5 is transmitted without problems, in spite of the small slit width. With the subsequent two-stage imaging system of the second condenser lens (11) and the objective lens (13), by means of which the probe tip (1a) image present in the output image plane (10) is imaged on the object (15) with a reduction of about 400-700 times, optimum end apertures in the region of 6.times.10.sup.-3 through 1.times.10.sup.-2, and probe sizes between about 1 nm and 3 nm, then result in the specimen plane. The electrons which are back-scattered at the specimen (15) are accelerated back into the beam guiding tube (4) by the deceleration field between the specimen end of the beam guiding tube (4) and the specimen (15), and again have exactly the same energy as the primary electrons and therefore reach the filter system (7) backward. However, because of the opposite direction of motion, these back-scattered electrons are deflected in the opposite direction in the magnet sector (7a) of the filter, and on this path reach a primary electron detector (16). This primary electron detector (16) can be constructed in the usual manner as a scintillation detector, semiconductor detector, or microchannel plate detector. A second slit diaphragm (17) is furthermore arranged between the magnet sector (7a) and the primary electron detector (16), and filters out electrons which have other energies, and which have for example undergone an interaction with the specimen (15) or with the selection diaphragm (8). In the embodiment example of the invention shown in FIGS. 1 and 2, the dispersive filter is constructed according to U.S. Pat. No. 4,740,704. A so-called Omega filter is concerned here, and is also used by the inventors employer in the transmission electron microscope 912 Omega. The filter (7) can also be alternatively constructed as a so-called "alpha filter" corresponding to U.S. Pat. No. 4,760,261 or corresponding to U.S. Pat. No. 5,449,914. In the embodiment example according to FIG. 1, the first condenser lens (5) is constructed as a magnetic lens. In the embodiment example shown in FIG. 3, the condenser lens (24) is an electrostatic lens which is integrated into the beam producer (21) with the cathode tip (21a). This asymmetrical electrostatic immersion lens (24) is arranged between the extraction electrode (21) and the anode (23). It has a considerably greater aperture diameter on the side facing the beam producer (21) than on the side facing the anode (23). The cathode tip (21a) which emits electrons is also directly imaged, magnified by such an electrostatic immersion lens (24), in the input image plane (not further illustrated) of the succeeding dispersive filter. Two alternative objective lenses are shown in FIGS. 4a and 4b, and are preferably used in combination with the invention. The difference between the objective lens (33) in FIG. 4a and the objective lens (13) in FIG. 1 is that in the objective lens (33) the outer pole shoe (33a) is shortened, and ends at the same height as the inner pole shoe (33b). An annular pole shoe gap (33b) results which is aligned perpendicularly to the optical axis (shown dot-dashed). By this construction of the pole shoe gap, the magnet field exits in the direction toward the specimen (35), resulting in a stronger immersion of the specimen (35) and hence a reduction of the aperture aberrations. Electrodes (34) for the superposed electrostatic lens are then constructed as .extensions of the outer pole shoe (33a). The objective lens (36) in FIG. 4b differs from the objective lenses described hereinabove in that the beam guiding tube (4) is extended and ends only at the height of the outer pole shoe (36a) of the magnetic lens (36) or even behind it. The electrostatic lens between the specimen side end of the beam guiding tube (4) and the braking electrode (37) arranged between the specimen,(38) and the objective then first arises beyond the magnetic lens (36). In this embodiment example, the specimen (38) and the braking electrode (37) are at a common potential, which is negative relative to ground. The use of this objective offers advantages particularly at the lowest target energies, since even at the lowest target energies the cathode can be kept at a relatively high potential relative to ground, thus making the negative influence of leakage fields less strongly noticeable.