Method and apparatus for focusing the objective lens of a scanning transmission-type corpuscular-beam microscope
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates generally to corpuscular-beam microscopes, and in particular to a method and apparatus for focusing the objective lens of such a microscope. 
     Description of the Prior Art 
     Scanning transmission-type corpuscular-beam microscopes in which the beam is deflected by a deflection system excited in sawtooth fashion so that it generates, on the specimen to be examined, a raster consisting of parallel lines, and which include a beam radiation detector disposed behind the specimen along the beam path which generates an output signal which controls the brightness of a picture tube monitor operated synchronously with the raster, are known in the art. See, for example, the Journal of Applied Physics, Vol. 39, No. 13 (1968), pages 5861 ff. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to image the reduced image of the beam source of the microscope projected by the objective lens thereof in the plane in which the specimen to be examined is disposed with the highest possible accuracy by adjusting the lens current of the microscope. 
     This and other objects of the invention are achieved by measuring, during the exposure of a specimen point, partial radiation intensities in that part of the cone of the microscope beam which has passed through the specimen at two points disposed symmetrically with respect to the longitudinal axis of the beam cone by means of a radiation detector having an effective input area which is smaller than the cross-sectional area of the cone at the same height, and then adjusting the lens current so that output signals generated by the detector are equal for the measurements at both points. 
     The invention is based on the discovery that the part of the ray cone of the beam which has passed through the specimen to be examined projects a shadow image of a larger or smaller specimen area in the plane of the detector when the focal length relative to the specimen is either too short (overfocus) or too long (underfocus). This shadow image is structured, i.e.. partial beam radiation intensities which are measured in the ray cone symmetrically with respect to both sides of the cone axis are, as a rule, different. If, however, the beam is focused on the specimen, the radiation intensity in the cone is unstructured, so that the intensities measured at symmetrical points of the cross-sectional area of the cone are equal. This is strictly accurate, however, only for non-crystalline specimens. Where crystalline specimens are examined, it is preferable to use the carrier foil, which usually consists of amorphous carbon, as a test specimen. 
     A pair of radiation detectors may be used to measure the radiation intensities and are disposed symmetrically with respect to the optical axis of the microscope. Each detector has an input area which is small compared to the cross-sectional area of the cone and generates output signals which can be read, after amplification, directly on measuring instruments. The lens current in the objective lens is adjusted in this case until the measured difference between the two instruments is minimized. 
     The instrumentation required is generally less when one detector disposed on the optical axis is used to measure the radiation intensities and the beam cone is alternately deflected by a deflection system excited in square-wave fashion through two equal and opposite angles with respect to the optical axis of the microscope. In both cases, the radiation intensities may be measured while the deflection system which generates the specimen raster is not excited, i.e., a specimen point is illuminated by a fixed beam. In the second case, the intensities can be measured while a specimen raster is generated, so that the frequency of the deflection of the ray cone below the specimen is large compared to the line frequency of the specimen raster. This causes the time between two deflections of the ray cone to be about as long as the dot time of the specimen raster so that the output signals generated by the detector during the irradiation of a specimen point can be compared. 
     In the latter embodiment of the invention (i.e., that described with reference to the axially-disposed detector), measurement of the detector output signals can be reduced to an a-c measurement by amplifying the detector output signals by means of a narrow-band amplifier which is tuned to the frequency of the deflection system deflecting the bean ray cone below the specimen. An a-c current having a frequency which is low compared to the deflection frequency can be superimposed upon the lens current. The phase difference of this a-c current and the resulting modulation of the detector output signals can then be compared and can be used as the criterion for the direction of the required lens current change. 
     The present invention also relates to an apparatus for automatically carrying out the above-described method of the invention. In the apparatus, a radiation detector is disposed in the optical axis of the microscope behind the specimen along the beam path. Deflection means is disposed between the detector and the specimen. A deflection wobble generator which generates a square-wave output signal controls the deflection means and deflects the ray cone of the beam in two directions symmetrical with respect to the optical axis of the microscope. The frequency of the deflection wobble generator is high compared to the line frequency of the specimen raster. 
     The apparatus also includes a lens current regulating means which is controlled by a lens current wobble generator which superimposes an a-c current on the lens current. Finally, a phase discriminator is provided for comparing the phase between the lens current wobble generator and the output signals generated by the detector. An output signal generated by the discriminator controls the lens current regulating means. 
     These and other objects of the invention will be described in greater detail in the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, wherein similar reference numerals denote similar elements throughout the several views thereof: 
     FIG. 1 is a schematic diagram of an apparatus for automatically focusing the objective lens of a scanning transmission-type corpuscular-beam microscope constructed according to the present invention; 
     FIG. 2 is a graphical illustration of a signal generated by a narrow-band amplifier of an apparatus for focusing an objective lens of a scanning transmission-type corpuscular-beam microscope constructed according to the present invention; and 
     FIG. 3 is a graphical illustration of a signal generated by a lens wobble generator in an apparatus for automatically focusing the objective lens of a scanning transmission-type corpuscular-beam microscope constructed according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, there is shown in FIG. 1, a beam source of a scanning transmission-type microscope, which may comprise, for example, a field emission cathode. A deflection system 3 including deflection stages 3a and 3b, each stage consisting of two pairs of electrostatic deflection plates or, alternatively, magnetic deflection coils, is provided for deflecting the beam of the microscope in two mutually perpendicular directions x and y in raster fashion on the specimen examined. For the purpose of clarity, only the pairs which deflect the beam in the x-direction are shown in the drawings. Deflection system 3 is excited by a raster generator RG. Stage 3a of the system deflects beam 2 out of the optical axis A of the microscope and stage 3b deflects the beam back toward the microscope axis. (The path of the deflected beam is identified by reference numeral 2a.) 
     The beam of the microscope is focused by a magnetic objective lens 4 on a specimen 5 and is tilted by deflection system 3 about a point P disposed in the focal plane of lens 4. Another deflection system 6 and a detector D are disposed underneath specimen 5. Deflection system 6 consists of only one pair of electrostatic deflection plates or, alternatively, magnetic deflection coils. Detector D is coupled by an amplifier V1 to the brightness control of a picture tube monitor 10, the deflection system of which is controlled by raster generator RG. 
     FIG. 1 illustrates a condition of the microscope in which the focus F of the beam, as a result of excessive excitation of objective lens 4, is not disposed on specimen 5, as it should be, but instead is disposed in front of the specimen along the beam path. Consequently, the beam generates a shadow projection of a specimen area 5a with its downwardly directed cone 2a. In other words, a shadow image of area 5a is produced in the plane 8 in which the input area d of detector D is disposed. It is important that this input area be smaller than the cross-sectional area of the ray cone in plane 8, i.e., the detector aperture α must be smaller than the illumination aperture α. 
     The shadow image of specimen area 5a generated in plane 8 is structured as the area itself. The same is true when focus F is disposed below specimen 5 if lens 4 is insufficiently excited. In this latter case, a structured image is also produced in plane 8. If, however, the beam focus is disposed on specimen 5, then the ray cone below the specimen is not structured. 
     Assume first that deflection system 3 is not excited, and that beam 2 therefore always illuminates, when its position is fixed, the same point or area of specimen 5. Deflection system 6 is then excited in two discrete states so that cone 2b is deflected successively to opposite sides of optical axis A, more specifically, so that the axis of the cone forms equal and opposite angles with optical axis A. In the two deflection states, parts of the cone which are disposed symmetrically with respect to the axis of the cone therefore strike the relatively small input area d of detector D. The radiation intensities which are measured by detector D are different when the beam is defocused because of the irregular structure of the shadow image. However, the intensities are equal if focus F is disposed on specimen 5 and cone 2b has no structure. It is thus possible to directly determine by measuring the output signals generated by detector D in the two deflection states whether or not focus F is disposed on specimen 5. If necessary, the excitation of objective lens 4 can be adjusted on the basis of this measurement so that the output signals generated by detector D in the two states are equal. The focus F will then be disposed on the specimen. 
     In a similar manner, but omitting deflection system 6, two detectors arranged symmetrically with respect to axis A are provided in plane 8. The input areas of these detectors are also smaller than the cross-sectional area of the cone and are identified in FIG. 1 by the dashed lines d&#39;. Also in this arrangement, comparison of the output signals generated by the two detectors furnishes a criterion for determining the position of focus F. 
     That part of the apparatus which automatically focuses the beam includes lens current regulator LR which supplies current to objective lens 4. The regulator consists of a regulating portion L which generates a control signal which is constant in time, and a regulating portion L&#39; which generates a control signal which is additive in the positive or negative sense, but in its final state, is also constant in time. Both signals control an amplifier V2 which transmits the current to objective lens 4. A lens wobble generator LWG is also provided which permits a sinusoidal a-c current to be superimposed upon the line current. Regulating portion L can be set manually by an operating element 11. 
     Deflection system 6 is excited by a deflection wobble generator AWG which generates a square-wave a-c current output signal. This signal causes cone 2b to be deflected alternately into two positions disposed symmetrical with respect to axis A. The frequency of deflection generator AWG is high compared to the line frequency of raster generator RG at which the x-axis deflection direction of deflection system 3 is operated. As a result, two successive deflection states of cone 2b are associated with the same point or area of specimen 5, i.e., successive deflections take place during one image dot period. The frequency of deflection wobble generator AWG is also high compared to the frequency of lens wobble generator LWG. Disregarding any lens wobbling, signal S1, in the case of either over or underfocusing, comprises a voltage, the magnitude of which corresponds to the absolute difference of the detector output signals in both positions of the beam cone, identified by the reference character a in FIG. 2. If low-frequency wobble is now applied to the lens current (see FIG. 3), then signal S1 is modulated by the shape of the lens wobble. The phases of this modulation are different from the lens wobble, depending upon whether or not either overfocusing (2a of FIG. 2) or underfocusing (2c of FIG. 2) is present. If the lens d-c current is adjusted correctly, the maximum distances of focus F from specimen 5 are equal during the lens wobble. Consequently, the modulation of the signal S1 described with reference to FIG. 2b comprises two half-waves, one of which has the same phase and the other of which the opposite phase as the lens wobble. Phase discriminator PD thus generates opposite signals in response to the signals identified by the reference numerals 2a and 2c the polarity of which corresponds to the direction of the focus deviation, which are transmitted to the lens current regulator. In response to signal 2b, the discriminator generates no output signal. After the correct focus is obtained, lens wobble generator LWG and deflection wobble generator AWG are switched off for normal operation of the microscope. 
     It should be noted that the automatic focusing device described is operative during normal excitation of deflection system 3, i.e., during the usual generation of a raster on specimen 5. An image of the scanned specimen area can thus be viewed on the screen of picture tube monitor 10. As long as focus F is not in the plane of specimen 5, this image is, as shown in FIG. 1, a double image which merges into one image when the focus is correct. It is thus possible to visually check the sate of the focusing on the monitor screen. 
     Some of the operating parameters for the apparatus illustrated in FIG. 1 are as follows: 
     
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Exposure aperture:                                                        
                ∝B ≈ 2 × 10.sup.-2 rad               
Detector aperture:                                                        
                ∝D ≈ 5 × 10.sup.-3 rad               
Raster generator RG:                                                      
 Frame time     T.sub.frame = 4 sec                                       
 Line time      t.sub.line = 20 msec                                      
 Image dot time τ.sub.BP = 100 usec                                   
Deflection wobble                                                         
generator AWG:                                                            
 Period         τ   = 50 usec                                         
Lens wobble                                                               
generator LWG:                                                            
 Period         τ   = 40 msec.                                        
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     The method as well as the apparatus of the invention may be used to correct astigmatism of the objective lens, since this in principle involves a focus correction in two different planes containing the optical axis of the microscope. To achieve this, deflection system 6, for example, can be rotated effectively in its azimuth by mechanical or electrical means to that the lens current set for correct focusing is the same for all azimuth directions. An additional pair of deflection coils would be required for electrically rotating deflection system 6. 
     It should be noted that although the invention has been described with reference to scanning transmission-type electron microscopes, it is also applicable to ion microscopes of that type. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.