US 3889114 A
In an electron microscope dark-field illumination is obtained by including an annular diaphragm between the electron source and the second condenser lens and by varying the energisation of the condenser lenses whilst retaining imaging of the electron source on the specimen. The specimen is illuminated by a beam in the form of a hollow cone of varying apical angle. By means of a known objective diaphragm those electrons are selected from the scattered beam for the purpose of image formation which leave the specimen within a narrow cone about the optical axis, irrespective of the angle of incidence.
Description (OCR text may contain errors)
United States Patent J an van Oostrum June 10, 1975 ELECTRON MICROSCOPE FOR DARK-FIELD ILLUMINATION  Inventor: Karel Jan van Oostrum,
Emmasingal, Eindhoven, Netherlands  Assignee: U.S. Philips Corporation, New
 Filed: Mar. 11, 1974  Appl. No.: 449,844
 Foreign Application Priority Data Mar. 28, l973 Netherlands 7304298  U.S. Cl. 250/306; 250/311; 250/396  Int. Cl. HOlj 37/26  Field of Search 250/3l0, 311, 306, 307, 250/396, 397
 References Cited UNITED STATES PATENTS 2,464,419 3/1949 Smith 2.50/307 3,795,813 3/1974 Kunath 250/311 Primary ExaminerJame's W. Lawrence Assistant Examiner-C. E. Church Attorney, Agent, or Firm-Frank R. Trifari  ABSTRACT In an electron microscope dark-field illumination is obtained by including an annular diaphragm between the electron source and the second condenser lens and by varying the energisation of the condenser lenses whilst retaining imaging of the electron source on the specimen. The specimen is illuminated by a beam in the form of a hollow cone of varying apical angle. By means of a known objective diaphragm those electrons are selected from the scattered beam for the purpose of image formation which leave the specimen within a narrow cone about the optical axis, irrespective of the angle of incidence.
ELECTRON MICROSIIGPE FOR DARK-FIELD ILLUMINATION The invention relates to an electron microscope hav-' ing an annular diaphragm placed about an optical axis for illuminating an object at an angle to the optical axis.
Such an electron microscope is described, for example, in a paper by Heinemann and Poppa in Applied Physics Letters, Vol. 16 (1970 page 515. In the electron microscope described the annular diaphragm is inserted in the usual manner in or after that condenser lens which, viewed in the direction .of propagation of the electrons beam, is the final condenser lens. Thus what is generally termed conical illumination of a specimen is obtained in a simple manner. A disadvantage of this method is that the angle at which the specimen is illuminated is entirely determined by the position and geometry of theannular diaphragm. Hence each different angle of illumination requires another annular diaphragm to be mounted.
It is also known, see for example the paper by Hashimoto, Kumoa and I-Iino in Japanese Journal of Applied Physics, Vol. (1971), page 1115, to illiminate the specimen in an adjustable direction introduced by a ray deflection device (Wobbler). In this method electrons which have not been scattered when passing the specimen are intercepted by a diaphragm and do not contribute to the image formation. A disadvantage of this dark-field illumination when used with structure (atoms) having a rotation-symmetrical diffraction pattern is the low brightness in the image formed.
It is an object of the present invention to provide an electron microscope in which the said disadvantages are eliminated. For this purpose, according to the invention an electron microscope of the type described at the beginning of this specification is characterized in that it includes, viewed in the direction of propagation of the electron beam, after an electron source a first condenser lens and a second condenser lens for imaging the electron source on the specimen, in that the condenser system has a supply source which is individually adjustable for each of the condenser lenses in a comparatively large range, and in that the annular diaphragm is disposed between the electron source and the second condenser lens.
When in an electron microscope according to the invention the energisation of the first condenser lens is varied and the electron source or the smallest crosssectional area of the illuminating electron beam is always imaged on the specimen by the second. condenser lens, the apical angle of the hollow cone at which the specimen is illuminated varies. A preferred embodiment of the invention is suitable for angular scanning of a specimen. For this purpose, for example, the speciemen is successively illuminated in a plurality of discrete energisation positions of the condenser system. Each of the resulting images is recorded in a store and FIG. 2 shows in block-schematic form an apparatus for television recording of signals obtained by means of an electron microscope according to the invention.
Referring now to FIG. 1, an electron microscope shown schematically comprises an electron source 1, a first condensor lens 2, a second condensor lens 3, a specimen holder 4 having an object point 5, an objective lens 6, an objective diaphragm 7 and a display f screen 8 having an image point 9. So far the electron microscope does not differ in any respect from known electron microscopes and hence need not be described more fully. According to the invention an annular diaphragm 10 is included in the first condenser lens. Within the scope of the invention the annular diaphragm maybe disposed at another position, provided that this lies between the electron source 1 and the second condenser lens 3. Positions comparatively near to the electron source and positions at locations .at which within a desired energisation range of the first condenser lens an image plane 11 may be situated are unpractical although not impossible. If now with a fixed position and geometry of the annular diaphragm the power of the first condenser lens is varied, the imaging plane 11 and hence an imaging point 12 is shifted along an optical axis 13 of the electron microscope. The imaging point 12 acts as an object point for the second condenser lens 3. At each power setting of the first condenser lens the secondcondenser lens is preferably automatically energized so that the point 12 is imaged in the object point 5. Thus the angle at which the object point 5 is illuminated and which is designated by 0 varies with the axial displacement of the point 12. FIG. 1 shows in schematic section two conical illuminating beams 14 and 15 for angles of illumination 0 and 6 respectively. It should be borne in mind that the point 12 is a virtual point for a given energisation range of the condenser system. In the specimen the electrons of the illumi- I nating beam (14 or 15) are at least partly scattered in a direction within a comparatively narrow cone 16 along the optical axis 13. All the electronsscattered in this direction are transmitted by the objective diaphragm 7 and contribute to the image formation in the image plane 8. Consequently these electrons are always scattered at an angle about equal to the angle of illumination.
FIG. 2 shows schematically another preferred embodiment of the invention. A supply source 20 for the condenser system 2, 3 includes a timer 21. The timer couples a sequence of discrete illumination periods illustrated by a sequence 22 along a horizontal time axis with a sequence of discrete angles of illumination. Thus each of the two condenser lenses of the condenser system is energized according to a block sequence 23 and 24 respectively in which the horizontal time axis is given by the time sequence whilst the current strength compared at will with images obtained at another angle of illumination. To achieve optimal information the period of time during which the specimen is illuminated at each angle of illumination may be programmed.
Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:
FIG. 1 is a schematic sectional view of the path taken by the electron beam in an electron microscope according to the invention, and
plotted along the vertical axis determines the angle of illumination. Obviously the currents in the two sequences 23 and 24 are matched to yield optimal imaging of the electron source on the specimen. Thus a selected sequence of illumination angles can be maintained during an illumination period which can be set or programmed at will.
Any discrete illumination angle from a selected sequence of illumination angles can be maintained during an illumination period which can be set or programmed at will. This correlation between a sequence of illumination times and a sequence of illumination angles with reference to a known scattering pattern of a given structure, for example a type of atom, enables the said structures to be selected from a specimen. The correlation preferably consists in that the output signal is equal for all the illumination angles for the selected structure only. For other structures also present in the specimen the said condition is not satisfied; they produce a signal in the image plane which varies with the angle of illumination, and are not displayed An image signal derived from a television camera tube 2 5 at each time sequence may be recorded in a store 26 of a computer. For this purpose such a store may comprise storage elements, permitting information from an image comprising 100 by 100 image points to be located in 100 by 100 image points. Each of the said 10 locating storage elements is associated with a word store of, for example, 16 bits.v If the said time sequence for the illumination comprises four discrete time periods, which is sufficient for satisfactory discrimination, each word of 16 bits can be divided into four parts. Each part can be filled with image information from an image point for a given angle of illumination. 7
.Thus the complete information from an image comprising IOfimage points can be recorded for four different illumination angles. The information can be suppliedfrom the camera tube to the store during a period of time to be s'elcted at will, i.e. during a number of scans to be selected at will. Both during and after recording the information stored in the store can be displayed on a monitor 27 without the information being lost. During display the contrast in the image can be set to'a desired value by additional information from the camera tube. The information may also be displayed on a'colour monitor by interposition of a chromatising device 28 according to known techniques (see for exam ple German Patent Specification No. image elements having the same diffraction pattern being assigned the same colour.
What is claimed is:
1. Electron microscope having an annular diaphragm which is arranged about an optical axis and serves to illuminate a specimen at an angle to the optical axis, characterized in that the electron microscope includes, viewed in the direction of propagation of the electron beam, after an electron source a first condenser lens and a second condenser lens for imaging the electron source on the specimen, in that the condenser system is provided with a supply source which is individually adjustable for each of the condenser lenses, and in that the annular diaphragm is disposed between the electron source and the second condenser lens.
2. Electron microscope as claimed in claim 1, characterized in that in the condenser supply source the energisation for the two condenser lenses is coupled for automatic adjustment for source imaging on the specimen with varying energisation of the first condenser lens.
3. Electron microscope as claimed in claim 1 characterized in that the annular diaphragm is included in the construction of the first condenser lens.
4. Electron microscope as claimed in claim 1, characterized in that the supply source for the condenser system is provided with a programmable timer.
5. Electron microscope as claimed in claim 4, characterized in that it includes a television camera tube coupled to the timer and an image recording device connectd to said tube.
6. Electron microscope as claimed in claim 5, characterized in that a chromatiser and a colour monitor are associated with the recording device.