Patent Number: 048719110
Section: description

A scanning electron microscope as shown in FIG. 1, comprises an electron source 2, an anode 4, a control electrode 6, a condensor lens system 8, a beam scanning coil system 10, an objective lens system 12, and a specimen table 14. An electron beam 16 generated in the electron source is incident on a specimen 18 arranged on the specimen table. Detectors 20, 22 and 24 are capable of detecting different types of radiation as products of the interaction between the electron beam and the specimen. By a signal selector 25 and a signal processing device 26, the signals are applied to a monitor 28. The monitor in a scanning electron microscope is synchronized by signals from the beam scanning system 10 and is also connected, for example to a control device 30 for beam blanking. Even though the invention is described herein with reference to the scanning electron microscope shown it will be apparent that the invention is by no means restricted thereto; and the invention can also be very well used, for example in electron beam writers, (scanning) transmission electron microscopes and similar apparatus. A column of a scanning electron microscope in adapted form can serve, for example as a column for a beam writer as described, for example, in U.S. Pat No. 3,491,236. An electron source as shown in FIG. 2 comprises a semiconductor element 40 which is accomdated in a housing 42 with a carrier table 44 and a diaphragm aperture 46 for the passage of the electron beam 16. The base plate or carrier table 44 comprises passages 48 for supply leads 50 of the electron emitter. The semiconductor element 40 contains a crystal semiconductor material 52 which consists, for example of Si, GaAs, SiC etc. At a small distance from a free surface 54 a p-n junction 56 is provide in the crystal, with transverse dimensions of the junction, that is to say the dimensions in its plane, being well defined. As is customary, a part of the crystal is covered with, for example, an oxide layer 58 on which there may be provided a conductive layer 60. The conductor 60 can act as a gate electrode. For a more detailed description of the geometry and the construction of such an element reference is made to U.S. Pat. No. 4,303,930. Because the p-n junction is connected in the reverse direction, electrons will be emitted from and emissive surface 62 which is situated opposite the p-n junction, with the electrons reaching the electron-optical system under the influence of a positive potential which acts through the aperture 46. On the free surface 54 of the semiconductor element there may be provided a preferably approximately monomolecular layer of a material which reduces the electron exit potential, such as for example, Cs or Ba, so that the efficiency of the source can be increased, if necessary. Such a layer can be provided, for example, by depositing the desired material in a space 66 within the housing 42 from the gaseous phase. FIG. 3 diagrammatically shows some preferred embodiments of emissive surfaces. For the sake of clarity it is to be again noted that the geometry of the emissive surface is in this case determined by the geometry of the p-n junction. All surfaces are chosen so that optimum adaptation to the electron-optical properties of the apparatus is obtained. Depending on the relevant application, a round emissive surface 70 has a diameter of, for example from approximately 0.5 to 5 .mu.m and thus has optimum dimensions as an object for further imaging in the apparatus. Due to this choice of the dimensions, the electrons which escape in the transverse direction, i.e. electrons which are not deliberately emitted, do not cause a disturbing field or distrubing heating of the semiconductor element at this area. A round emissive surface is suitable, for example for imaging apparatus such as an electron microscope. For applications requiring an electron spot which is sharply defined mainly in one direction, use can be made of an emissive surface 72. The advantages of such a geometry are described in U.S. Pat No. 3,881,136; it can be used extremely well, for example, for the formation of images in which a high resolution is desired mainly in one direction. The dimensions may then be, for example 1.times.5 .mu.m.sup.2. Notably for use in electron beam writers in which an electron spot is required which is sharply defined on two sides there is a square emissive surface 74 with sides of, for example from 0.5 to 5 .mu.m. Such a geometry is also very suitable for beam shaping as well as for splitting the electron spot into geometrically defined parts. As has already been stated, use can also be made of a regular polygon. The emissive surfaces 76 and 78 are composite emissive surfaces. The emissive surface 76 comprises a linear array of, for example 10 emissive sub-surfaces which are identical in this case. An apparatus can then operate with a multiple beam as described in U.S. Pat. Nos. 4,524,278 and 4,568,833, it being possible to control each of the sub-beams individually. The same consideration hold good for the emissive surface 78 in the form of a matrix of emissive sub-surfaces. Composite emissive surfaces are particularly suitable for electron beam writers, notably when such apparatus are used for the direct manufacture of integrated circuits, i.e. without the assistance of masks. A composite emissive surface 78 comprises a central sub-surface 80 and an annular sub-surface 82 as the emissive surfaces and a ring 84 as a non-emissive surface. Such a surface is useful, for example for measurement methods related to dark-field illumination ect. Such a composite surface may alternatively have a rectangular, square or other shape. The references U.S. Pat. Nos. 4,524,278 and 4,568,833 cited in this specification have been published as EP pat. appln. No. 87196 and EP pat. appln. No. 92873 respectively. The article "An efficient silicon cold cathode for high current densities" is published by two of the inventors in Philips J. Res. 39, October 1984, pp 51-60 , describes some properties of the semi-conductor device per se