US3917946A

US3917946A - Electron-optical device for the recording of selected diffraction patterns

Info

Publication number: US3917946A
Application number: US479468A
Authority: US
Inventors: Oostrum Karel Jan Van
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1972-04-12
Filing date: 1974-06-14
Publication date: 1975-11-04
Anticipated expiration: 1992-11-04

Abstract

The direction of an illumination electron beam in an electron microscope is varied while a portion of the diffracted beam is projected in a predetermined fixed direction on the image plane and the remaining portions of the beam are intercepted.

Description

Ilted States Patent van Dostrum ELECTRON-OPTICAL DEVICE FOR THE RECORDING OF SELECTED DIFFRACTION PATTERNS Karel Jan van Oostrum, Eindhoven, Netherlands Assignee: U.S. Philips Corporation, New

York, NY.

Filed: June 14, 1974 Appl. No.: 479,468

Related US. Application Data Continuation of Ser. No. 348,062, April 5, 1973, abandoned.

Inventor:

Foreign Application Priority Data Apr. 12, 1972 Netherlands 7204859 U.S. Cl. 250/307; 250/310; 250/397 Int. Cl. HOlJ 37/26 Field of Search 250/310, 311, 396, 307,

References Cited UNITED STATES PATENTS 7/1961 Herrmann 250/397 1 Nov. 4, 1975 3,180,986 4/1965 GribSOrl 250/397 3,626,184 12/1971 Crewe 250/311 3,628,014 12/1971 Grubic 250/311 3,737,659 6/1973 Yanaka et a1 250/311 3,833,811 9/1974 Koike 250/397 FOREIGN PATENTS OR APPLICATIONS 1,089,088 9/1960 Germany 250/311 Primary ExaminerJames W. Lawrence Assistant Examiner-43. C. Anderson Attorney, Agent, or Firm-Frank R. Trifari; George B. Berka [57] ABSTRACT The direction of an illumination electron beam in an electron microscope is varied while a portion of the diffracted beam is projected in a predetermined fixed direction on the image plane and the remaining portions of the beam are intercepted.

8 Claims, 5 Drawing Figures VIIIIIIIIIIII U.S. Patent Nov. 4, 1975 Sheet 2 of3 3,917,946

U.S. Patent Nov. 4, 1975 Sheet 3 of3 3,917,946

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ELECTRON-OPTICAL DEVICE FOR THE RECORDING E SELECTED DIFERACTION PATTERNS This is a continuation of application Ser. No. 348,062, filed Apr. 5, 1973, now abandoned.

The invention relates to a method of and a device for the generation and recording of diffraction patterns of a comparatively small part of a specimen to be selected.

In known methods, used in electron microscopes, a so-termed selected area diaphragm is used to select a part of a specimen which is irradiated by an electron beam for this purpose. As will be demonstrated herein after, the transverse dimensions of the part of the specimen contributing to the image formation cannot be chosen to be sufficiently small in all circumstances.

The invention has for its object to provide a method and a device in which a substantial reduction of the transverse dimensions of the part of the specimen to be separately examined is accompanied by increased ease of operation, also in the case of comparatively large diffraction angles.

A method of the kind set forth according to the invention is characterized in that the specimen is illuminated by an electron beam, the direction of incidence of which is varied during the recording of a diffraction pattern, a part of the electron beam which is diffracted in a fixed direction by the specimen being selected for the imaging of the object in an image plane.

In order to form the image, the method according to the invention utilizes only the electrons which move in a fixed direction after diffraction by the specimen. In a preferred embodiment, this direction is made to coincide with an optical axis of an electron-optical imaging device with the result that lens aberrations can exert only a comparatively small effect on the image formation. Using a diaphragm in the electron-optical device, a limited directional region of electrons emerging from the specimen is thus selected. In a further preferred embodiment according to the invention, the variation of the direction of incidence of the illumination electron beam is synchronized with the scanning of a scanning electron beam in a display unit. The output signal of a detector, selecting the part to be studied from the final image, is used to modulate the scanning beam in the display unit so as to record a diffraction pattern.

In the method according to the invention, the diffraction pattern is obtained from the detector as a signal which varies in time, so that digital data processing can be applied. Using a television camera tube for the recording of a final image, the deflection of the illumination electron beam and that of the scanning electron beam are synchronized with respect to each other in the television camera tube in a preferred embodiment according to the invention. Given directional regions can then be readily selected, and the display of an image can be realized in colour by the successive display of sub-images, each of the said colours corre sponding to selected diffraction angles.

Known electron microscopes are often provided with a deflection unit for the illumination electron beam. It is thus possible to use a microscope of this kind for performing the method according to the invention, without essential structural modifications being required.

Some preferred embodiments according to the invention will be described in detail hereinafter with reference to the drawings.

FIG. 1 is a diagrammatic view of a beam path in a known method.

FIG. 2 is a diagrammatic view of a beam path in a method according to the invention,

FIG. 3 is a diagrammatic view of a beam path of an illumination electron beam according to a preferred embodiment of the invention,

FIG. 4 is a diagrammatic view of a beam path of an illumination electron beam for area scanning of the specimen, and

FIG. 5 is a block diagram of a preferred embodiment of a device for performing the method according to the invention.

FIG. 1 shows a specimen plane 1, a diagrammatically represented objective lens 2, a paraxial image plane 3, a zonal image plane 4, and a selected area diaphragm 5 of a known imaging system. An electron beam 6, forming part of the illumination beam, is incident on an object point P in the specimen plane 1. The object in P diffracts part of the incident electron beam in preferred directions which are dependent of the object and of the position of the object with respect to the incident beam 6. For the sake of simplicity, a non-diffracted beam 8, that is to say a beam extending along an optical axis 7 of the system, and an annular beam 9 which is diffracted at an angle 0 and which originates from, for example, a polycrystalline object, will be considered. The objective forms an image P in the paraxial image plane 3 of the object point P in the non-diffracted beam 8. In the diffracted annular beam 9, the objective lens images the point P in a point P of the zonal image plane 4. The zonal image plane does not coincide with the paraxial image plane 3 as a result of spherical aberration of the objective lens which is a function of the angle 6. In the paraxial image plane, the beam 9 forms a ring about the point P. The selected area diaphragm 5, corresponding to a part of the object in the specimen plane, will allow or will not allow passage of this ring, depending on the angle 6.

Electrons which are diffracted in a corresponding manner by an object point Q in the specimen plane, can also arrive at P via a zonal image point Q" as is denoted in the Figure by a broken line 10. The diffraction pattern produced on a target by the electron microscope, consequently, not only originates from a disc about the object point P which is paraxially associated with the selected area diaphragm, but also from an object point O which is situated outside this disc. At a commonly used diffraction angle 0 of 0.1 rad., selection of a part of the specimen which is dimensioned smaller than approximately 1 micron by means of a selected area diaphragm is not possible according to this method.

Corresponding elements in FIG. 2 are denoted by the digits and letters used in FIG. 1. The direction of incidence at the object point P of a part 12 of the illumination electron beam which is directed at P is continuously varied during the recording of a diffraction pattern. Expressed in spherical co-ordinates 0 and (b, 6 being given by the angle between the main beam of the electron beam incident in P and the optical axis 7 of the system, the variation in the'direction of incidence can be realized, for example, in a pattern at which the angle (1) completes a 360-trajectory, each time at a fixed value of 0. A spiral-like movement can also be chosen, the angle 0 then being changed comparatively slightly at each 360-trajectory for d). the illumination beam can also be deflected according to a normal television frame pattern. For the display of sub-images in selected illumination as described in the preamble, the current intensity in the beam can be controlled, for example, such that illumination takes place only at preselected directions of incidence. Therefore, the current intensity control is preferably binary, but selected directions can alternatively be illuminated with a less intensive electron beam than other directions. For the same purpose, discrimination as regards given preferred directions can also take place during recording. A part 13 of the beam 12 is diffracted in a direction along the optical axis 7. Thediffraction angle is again denoted by 6 in FIG. 2. Using electrons moving along the optical axis 7, the objective lens forms an image P of P in the paraxial image plane 3. Non-diffracted electrons which form a beam 14 are intercepted by a diaphragm 15 which is arranged in the image focal plane of the objective.

In fact, this angle selection constitutes dark-field illumination in a direction which varies in time. The diaphragm 15 allows passage only of electrons which are diffracted in the object point P at an angle 6. Electrons from an object point Q cannot reach the image point P. Regardless of the direction of incidence, the point P is reached only by electrons which paraxially pass through the electronoptical system behind the specimen plane. If the direction of incidence of beam 12 is varied, the intensity of the electron beam incident in P will be varied. This is because the diffracted fraction of the beam 12 is a function of the directional coordinates d) and 6. The nautre of this function is dependent of the nature of the object in P. Because only paraxial beams contribute to the image formation, the effect of aberrations ofthe objective on the image formation will be comparatively small. The object disc about P, now corresponding to a sensitive surface of a detector 16 in the image plane, can also be substantially smaller than in the known method. with commonly used objective lenses, the transverse dimensions of the object disc can be less than approximately 10 A. By means of the detector 16, the intensity of the electron beam incident in the image point P is measured as a signal which varies in time.

A preferred embodiment according to the invention for the control of the direction of the illumination beam as shown in FIG. 3 comprises an electron-optical lens system 17 for which any commonly used objective lens can be used. A part AB, the field of vision, of a specimen is imaged, in a sectional view, as A'B in the image plane 3. To this end, a cross-over of the electron beam in a point Q is imaged, by means of a condenser part 19 of the lens system 17, in a virtual point 18 from which the field of vision AB is illuminated. The cross-over in Q is formed in known manner by a preceding condenser system (not shown). By means of a deflection system 20, the point Q is shifted in a horizontal plane, i.e. transverse to the optical axis 7 of the system. By means of the deflection system 20 the illumination electron beam is tilted about a virtual tilting point R, As a result, the angle 6 at which the field of vision is illuminated is varied, without the field of vision itself being varied. To this end, the point R is chosen as the tilting point, this point being virtually associated with a centre point C of the field of vision AB by the condenser lens 19. Consequently, C is the real tilting point. By means of the diaphragm 15, the image-formation by an objective lens 21 is limited to the electrons which are diffract ed by the specimen along the optical axis 7. Electrons originating, for example, from a virtual point 22 and which pass the object without being diffracted, are intercepted by the diaphragm 15. The foregoing is indicated by means of an electron beam 23 which is denoted by a broken line. The point 18 can be displaced in a plane 25, for example, between the

points

22 and 23 by means of the deflection system 20. This movement can be performed, for example, in a pattern of concentrical circles about the axis 7 of the system. The diffraction direction can be readily selected. The illumination of an object at a varying angle of incidence 6 and azimuth d) is thus realized by displacement of a cross-over of the illumination electron beam in a plane transverse to the optical axis.

For the sake of comparison, FIG. 4 shows a beam path for the area scanning of a specimen. An electron beam 26 is focussed in a virtual point 27 by a condenser system not shown, and is deflected by means of a deflection system 28, a point 29 serving as the tilting point. A condenser part 30 of the lens 17, in this case formed by a Ruska condenser objective lens, see Riecke and Ruska, Proceedings VIth, Intern. Congress for Electron Microscopy, Kyoto, Japan 1966, Volume I, page 19, forms, on the basis of the virtual cross-over in 27, a real point 31 in the specimen plane 1 which is situated in the centre in this lens. By moving the electron beam 26 about the tilting point 29, area scanning of a specimen takes place. For the tilting point 29 the focal point of the condenser lens 30 is chosen such that any point of the specimen is illuminated at the same direction of incidence. An angle of incidence of is shown. An objective lens 32 of the Ruska lens 17 forms a diffraction pattern in the image focal plane 33 thereof, the said pattern thus retaining its place when the electron beam 26 is moved in the specimen plane. In the image focal plane or in an enlarged image formed by subsequent lenses which are not shown, the diffraction pattern can be measured, for example, by means of a ring detector.

The change-over from directional scanning according to the invention to area scanning can be realized in one and the same lens system by adaptation of the beam path of the illumination electron beam. It is then necessary to choose a correct position of the tilting point and the cross-over of the beam. This can be realized by means of suitable adjustments of the various electron-optical lenses and deflection systems. In an electron microscope such as, for example, the Philips EM 300, the method according to the invention can be performed without major structural modifications being necessary. This is because an electron microscope of this kind is provided with a wobbler (deflection system) having a range of approximately 0.1 rad. in 6. A range of this kind is more than sufficient for performing the method according to the invention. Besides the possibility of examining a smaller part of the specimen, there is an additional advantage in that the case of operation is enhanced. A final image a diffraction pattern can be studied at the same lens adjustments according to the invention. Using the method according to the invention, a recordable final image is successively obtained for different illumination directions, in which a desired sub-image can be readily selected in the image plane by means of a detector so as to record the diffraction pattern of the corresponding part of the specimen.

Another method of making a diffraction pattern of a small part of a specimen is described in the article Beugungsexperimente mit sehr feinen Elektronen- 5 strahlen" in Zeitschrift Angew. Phys. 27, No. 3, 1969, pages 155-165. Therein, use is made of a condenser objective lens as described with reference to FIG. 4. The selection of the part to be studied is not effected by means of a diaphragm in the paraxial image plane of the objective, but by limitation of the illumination to the selected part of the specimen. Because the condenser part of the lens used for this purpose is very strong and has little spherical aberration, the illumination beam on the specimen can have a small cross-section, so this method also enables selection of a comparatively small part of the specimen. In contract with this method, in the method according to the invention the minimum achievable dimensions of the selected part are not dependent of aberrations in the deflection and condenser system, and hence they are independent of the use of the special lens with a strong condenser field.

A method of imaging a specimen in selected dark field illumination is described in the article selected zone dark-field Electron Microscopy, Appl. Phys. Lett. 20, No. 3, 1-2-1972, pages 122-125. Therein, the diffraction angle 6 is selected by using an annular diaphragm in the image focal plane of the objective lens. According to this method, a final image of an object is produced by means of electrons which are diffracted inside a directional region selected by the diaphragm. The adjustment of such an annular diaphragm is difficult. Moreover, a different diaphragm must be used for each diffraction direction.

The block diagram of FIG. 5 shows an electron gun 40 for generating the illumination electron beam, two sets of deflection coils 41 and 42 for deflecting the electron beam, each set being active in two directions which are perpendicular to each other, with the result that deflection in all directions and d) with respect to an axis 47 is possible, a specimen plane 43, an objective 44, and a target 45. All said elements are components of a commonly used electron microscope such as the said Philips EM 300. Coupled to the target 45 is a detector 46 for detecting the intensity of the electron beam. The diagram also shows an electronic control circuit 48, an excitation unit 49 for the deflection coils, a signal converter 50, and an x-y display unit 51. A signal derived from the detector 46 is applied to the signal converter 50. The signal converter converts the signal which is analogous to the measured intensity into a modulation signal which is applied to the x-y display unit as z-modulation. Like the x-y display unit, the signal converter is coupled to the control circuit 48. Via the excitation unit 49, the control circuit controls the deflection of the illumination beam in synchronism with a scanning beam in the x-y display unit. The diffraction pattern of the part of the specimen which corresponds to the detector surface is displayed on the screen of the display unit. If the electron microscope is provided with a television camera tube for image recording, the part of the video signal which orignates from the selected region in the image plane can be used as the detector signal.

In a preferred embodiment according to the invention, the electron gun 40 is controlled by the control circuit such that an electron beam is produced only during a desired or pre-set sequence of directions of incidence, measured according to coordinates 6 and d). A final image, recorded by a television camera tube 52, then only containing structures having a selected diffraction pattern. The detector is equipped with a luminescent screen to which the television camera tube is connected, preferably by means of a fibre-optical win dow. A similar selection can be performed by recording the video signal, at a continuous variation in the direction of incidence, only when the illumination beam is in a selected directional region. By alternately selecting a number of measuring sequences and by assigning a colour to each of these sequences in accordance with known colour television techniques by means of a chromatizing unit 53, an image can be displayed on a colour monitor 54 in which each colour is characteristic of a specimen structure having its own diffraction angle. So as to achieve proper synchronization, it is ob vious that the chromatizing unit must be coupled to the control circuit. In the case of a scanning pattern given by circles for different values of 0, the zero-order diffraction can be readily avoided in the colour television signal and can, if necessary, be separately displayed on a black-white monitor. It is obvious from the described block diagram that the method can be performed on any electron microscope provided with a Wobbler unit for deflecting the electron beam according to television techniques. A device for performing the method according to the invention can be constructed by addition of known electronic circuits and systems for recording and displaying images which are known from the television technique.

What is claimed is:

l. A method of generating and selectably recording diffraction patterns of a specimen in an electron microscope, comprising the steps of directing an illumination electron beam in a first direction at a substantially fixed angle to said specimen, wherein said specimen is located at the intersection of a specimen plane with the optical axis of the microscope and said illumination beam is inclined at an acute angle to said axis, projecting in a fixed second direction a portion of the beam diffracted from the specimen, on an image plane in which said second direction coincides substantially with said axis, intercepting remaining portions of the beam, and varying said first direction of the illumination beam while recording the projected diffraction pattern in said image plane.

2. A method as claimed in claim 1, comprising the steps of arranging a detector in the image plane for recording a time-dependent signal for a diffraction pattern of a part of the specimen which is selected.

3. A method as claimed in claim 2 wherein said detector is a television camera.

4. A method as claimed in claim 3, wherein said illumination electron beam is deflected in synchronism with the recording signal of said detector.

5. A method as claimed in claim 4, wherein said illumination electron beam is deflected by a control unit by means of an electron-optical deflection system, in accordance with a television frame pattern, and controls in synchronism therewith a recording device, said method further comprising the step of recording a signal only at preset first directions of incidence of said illumination electron beam.

6. A method as claimed in claim 5 wherein said detector is a color television camera.

7. A method as claimed in claim 1, wherein said vari ation in said first direction of incidence is realized in spherical coordinates 0 and d) and over a trajectory of 360 at different, substantially fixed values of the angle 0 and the angle :1).

8. A method as claimed in claim 7, that the angle 0 is varied in steps, the angle (1) varying over 360 between

Claims

1. A method of generating and selectably recording diffraction patterns of a specimen in an electron microscope, comprising the steps of directing an illumination electron beam in a first direction at a substantially fixed angle to said specimen, wherein said specimen is located at the intersection of a specimen plane with the optical axis of the microscope and said illumination beam is inclined at an acute angle to said axis, projecting in a fixed second direction a portion of the beam diffracted from the specimen, on an image plane in which said second direction coincides substantially with said axis, intercepting remaining portions of the beam, and varying said first direction of the illumination beam while recording the projected diffraction pattern in said image plane.

3. A method as claimed in claim 2 wherein said detector is a television camera.

7. A method as claimed in claim 1, wherein said variation in said first direction of incidence is realized in spherical coordinates theta and phi and over a trajectory of 360* at different, substantially fixed values of the angle theta and the angle phi .

8. A method as claimed in claim 7, that the angle theta is varied in steps, the angle phi varying over 360* between successive steps in theta .