US3150257A - Electron beam aperture plate - Google Patents

Description

P 1964 A. P. WILSKA' 3,150,257

ELECTRON BEAM APERTURE PLATE Filed July 5, 1962 INVENTOR. ALVA F? MLSK4 United States Patent 3,150,257 ELECTRON BEAM APERTURE PLATE Alvar P. Wilska, Tucson, Ariz., assignor to Philips Electronics and Pharmaceutical Industries Corp, New York, N.Y., a corporation of Maryland Filed July 5, 1962, Ser. No. 207,681 3 Claims. (Cl. 250-495) This invention relates to an aperture plate for use in electron beam devices and particularly for a plate having an aperture which forms the object aperture of an electron microscope.

It is customary in electron microscopes and other electron beam devices to have a plate transverse to the beam and provided with an aperture through which the beam, or at least some of the electrons in the beam, can pass. The purpose of such an aperture plate is to confine the beam initially to just the relatively homogeneous central electrons thereof by blocking off the peripheral electrons in order to obtain a uniform beam that will respond uniformly to subsequent electron lenses through which it must pass.

However, even a very thin plate is relatively ponderous in comparison to the dimensions of electron beam crosssection and size of object in an electron microscope, and the aperture plate must be considered as being relatively thick. This means that the aperture is really a tunnel, or channel, through the plate, and it has been found that particles of non-conducting material lodge on the sides of the aperture and become charged by the beam and thus deflect at least some of the electrons thereby adversely affecting the resolution of any image formed by the beam. This is especially true and especially critical in electron microscopes, which by their nature are demountable devices and are therefore subject to having stray particles brought into the interior of the device. Also, the oil used in sealing the openings and in the vacuum pump that evacuates the microscope sometimes volatizes and condenses on the walls of the aperture.

In accordance with the present invention an aperture plate is provided with at least one tapered channel extending through it. The plate is placed transverse to the electron beam with the side of the plate having the smaller end of the channel facing the source of the beam. The angle of taper is greater than the angle of divergence of the beam so that the electrons in the beam can strike only the sharp edge of the constricted end of the channel and cannot strike the interior Walls of the channel. Thus, only contaminants on the very edge of the plate around the constricted end of the channel can be struck and charged by the beam while those contaminants which become attached to the channel walls at the interior of the plate are not struck by the electrons and hence do not become electrically charged.

The invention will be described in greater detail in connection with the drawings in which:

FIG. 1 is a simplified and schematic sketch of an electron beam device showing the use of an aperture plate in accordance with the invention;

FIG. 2 shows a cross-sectional view of an aperture plate constructed in accordance with the invention and suited for use in the device shown in FIG. 1;

FIG. 3 is a cross-sectional view of an improved modification of the aperture plate of FIG. 2; and

3,150,257 Patented Sept. 22, 1964 "ice FIG. 4 is an enlarged cross-sectional view of an electron microscope objective lens; and

FIG. 5 shows a modified aperture plate.

The electron microscope shown in FIG. 1 comprises a filament 11 to emit electrons and a Wehnelt cylinder 12 surrounding the cathode 11 to form the electrons into a rough electron beam and to control the intensity of the beam. The filament 11 and the Wehnelt cylinder 12 are carried by an insulating support member, here indicated by reference character 13. These electrodes are connected, when the microscope is in operation, to a negative terminal of a high-voltage supply which is not shown, being of conventional nature and not related to the invention. The magnitude of the voltage may run as high as 200,000 volts, or even higher, but one of the advantages of the present invention is that it also permits operation at lower (though still high) voltages of 5,000 volts or less, such as in the microscope disclosed in my co-pending application, Ser. No. 207,680, filed concurrently herewith and entitled Column for Electron Microscopes. An anode 14 is connected to the outer causing 16 of the microscope and to the positive terminal of the voltage source to provide the necessary voltage gradient to establish electron beam. The beam itself, or at least the path traversed by certain typical electrons therein, is identified by reference character 17 as emanating from the filament 11 and passing through an aperture in the anode 14. Thereafter the electron beam 17 passes through an electron lens 18 formed within the gap in a soft iron pole structure 19 surrounding a coil 21. This electron lens is commonly referred to as the condenser lens of the electron microscope.

After passing through the condenser lens the electrons in the beam 17 irradiate an object 22. The object is here indicated in the customary way as an arrow, and for the purpose of clarity only those electrons in the beam 17 which pass through the point of the arrow are traced out. After passing through the object 22 the electron beam passes through an objective aperture 23 and an aperture plate 24. This aperture plate is also provided with a second aperture 26, which may be brought, by operation of a knob 27, into the position presently occupied by the aperture 23.

An objective lens 28 is formed in the gap in a soft iron pole structure 20 surrounding a coil 31. In the embodiment of FIG. 1 the objective lens 28 is shown as being located beneath the aperture plate 24. In practice it is desirable in many instances to form the plate 24 so that the apertures 23 and 26 are suspended down in the plane of the objective lens 28.

The electrons in the beam 17 may be further acted upon by another electron lens 32 formed in a gap of the soft iron pole structure 33 surrounding a coil 34. This latter lens is commonly referred to as the projection lens, and it focuses a greatly enlarged image 36 on an image plane 37. In order to render the electron optical image 36 visible, the image plane, which is the upper surface of a supporting member 38, and is normally coated with a fluorescent layer, or screen 37. If the image is to be viewed from below, the supporting member 38 may be glass, as is indicated in FIG. 1. Alternatively, the image may be viewed at an angle from above by means of an optical enlarger 39 in the side wall of the casing 16, in

which case the supporting member 38 need not be transparent.

FIG. 2 shows an enlargement of the objective aperture plate 24 and the aperture 23 therein. The purpose of the objective aperture plate 24 is to render the beam more uniform by cutting off the peripheral electrons making up the outer edge of the beam, leaving only the so-called paraxial, or central, portion. This is shown in FIG. 2 by the fact that the electron beam 17 above the plate 24 is wider and encompasses a greater angle than does the paraxial portion of this beam, which passes through and emerges below the plate 24.

In accordance with the invention the aperture 23 is in the form of a tapered, of frusto-conical, tunnel through the plate 24, and the included angle of this tunnel is greater than the included angle of the electron beam 17. As a result, only the extreme upper edge of the tunnel is struck by the electrons of the beam 17, while the inner wall 27 is not. This prevents drops of oil, pieces of lint, and other material, indicated by reference character 43, from being struck by the electrons of the beam. These contaminants enter the electron microscope when the interior of the casing 16 is at least momentarily open to the outside atmosphere. Other contaminants, particularly drops of oil, may evolve from the vacuum pump or vacuum seals of the electron microscope. Such contaminants are largely made up of non-conductive material which becomes electrically charged when struck by electrons. The electrostatic field produced by the charge accumulating on these randomly distributed particles adversely affects the uniformity of the electron beam passing through the objective aperture plate 24 and therefore distorts, or otherwise adversely affects, the image 36 (FIG. 1). By providing an aperture 23 in the form of a tapered tunnel, most of the contaminants are kept out of the path of the electron beam and only those relatively few contaminants that lodge upon the edge 41 can be struck by the electron beam. This greatly improves the uniformity of the beam.

In addition to contaminants which become lodged on the interior of the passageway through the aperture plate 24, contaminants can become lodged elsewhere on the plate. For the most part, this is of no consequence since the electron beam 17 strikes only a limited area of the plate. However, part of the area struck by the electron beam is in the region immediately surrounding the aperture 23 and on the upper surface of the plate 24. Contaminants becoming attached to this area also form a source of adverse charge field effects. This is prevented by the improvement shown in FIG. 3. Here the plate is represented by reference character 124 to indicate that it is somewhat different, although essentially the same as, plate 24 in FIGS. 1 and 2. The aperture in 123 through the plate 124 is also in the form of a tapered tunnel, but the upper end of the tunnel facing the electron beam source projects up beyond the level of the surrounding part of the plate 124 to form a short lip 141. Contaminating particles 43 attached to the inner wall of the tunnel are shielded from the electrons in the beam 17 as in the case of the aperture plate 24 of FIG. 2, but, in addition, contaminating particles 44 on the upper surface of the plate 24 are also shielded, so that, although they become charged by the electrons of the beam 17, they do not significantly affect the beam because the field produced by the charge on these particles 44 is prevented by the upwardly extending lip 141 from reaching the critical area of the beam. As in the case of the aperture plate 24 shown in FIG. 1, the aperture plate 124 may be provided with a plurality of apertures 123 each of which may be brought into position in turn by manipulation of external controls.

FIG. 4 shows the objective lens portion of an electron microscope with aperture plate 224 located more nearly at the proper plane within the lens. The reminder of the microscope may be of the type shown in FIG. 1, and as has been described previously, the electron beam 17 illuminates a specimen, or object, 22. Rays passing through the specimen 22 are focused by a magnetic electron lens, the general location of which is indicated by reference number 128, which comprises a hollow generally toroidal-shaped, soft iron pole structure having an upper plate 46 with a central aperture 47 and a cylindrical inner section 48 terminating in an upwardly extending frusto-conical portion 49 having a central aperture 51. A coil 131 lies within the hollow toroid surrounding the inner section 48 to provide magnetic flux that is directed by the configuration of the upper plate 46 and a frustoconical section 49 into a proper shape to form an objective magnetic electron lens. An annular brass member 50 physically closes the gap between the upper plate 46 and the inner section 48 but does not affect the magnetic lens field.

The objective aperture plate 224 is located within this gap directly in the path of the electron beam 17. This electron aperture plate 224 contains at least one, and preferably several, of the same type of frusto-conical tunnel 223 shown in greater detail in either FIG. 2 or FIG. 3 with the constricted end of the tunnel facing upward toward the object 22. The plate 224 is supported and positioned by a bar 126 running upward through the opening 47 in the upper plate 46 and then out to the side to a control knob (not shown) similar to the knob 27 of FIG. 1. By virtue of the depending arrangement of the plate 224, the aperture 223 may be located in the center of the objective lens 128, which is the optimum position for an objective aperture. Furthermore, the bar 126 may be moved around in order to bring other apertures (not shown in FIG. 4) into position on the axis of the electron beam 17.

Frequently it is desirable to heat the aperture plate as an additional means of preventing material from accumulating on it. FIG. 5 shows a modification of the aperture plate of FIG. 4 made so as to be heated easily. The aperture plate 324 of FIG. 5 consists of the flattened bight of a suitable wire, such as platinum. After the bight is flattened, it is pierced to form the same type of frusto-conical tunnel 323 as is shown in FIG. 3. The tunnel 323 has the same orientation with respect to the electron beam path 17; that is, the constricted end of the tunnel faces the object 22 (see FIG. 4). By virtue of the fact that the aperture plate 324 is supported by two wires 226a and 226b, an electric current may be directed through these two wires so as to heat up the flattened part forming the plate 324. The flattened part may be caused to heat up more than the wires themselves by reducing the cross-sectional area of the flattened part. Furthermore, since the two wires 226a and 226b are parallel to each other, the magnetic fields produced by the current flowing through them substantially cancels out and therefore has negligible, if any, effect on the focusing magnetic field of the objective lens.

Although the invention has been described in terms of specific embodiments, it will be recognized by those skilled in the art that modifications may -be made therein within the scope of the following claims.

What is claimed is:

1. An electron beam device comprising: a source of an electron beam; an object to be examined; an objective lens to magnify an electron beam image of said object; and an aperture plate substantially transverse to said beam on the side of said object away from said source, said plate having at least one frusto-conical tunnel therethrough, the included conical angle of said tunnel being greater than the angle of divergence of said beam at said plate and said plate being placed with the constricted end of said tunnel facing the source of said beam.

2. An electron beam device according to claim 1 in which said aperture plate is substantially in the center of said objective lens.

3. An electron beam device comprising: a source of around the constricted end of said tunnel extending from all electron beam; an Object to be eXamined; an obiectlve the surface of said plate toward said object and forming lens to magnify an electron beam image of said object; an extension of said tunmL an aperture plate substantially transverse to said beam on the side of said object away from said source, salild plaltle 5 R f r Cited in the fil of this patent having at least one frusto-conical tunnel theret roug the included conical angle of said tunnel being greater UNITED STATES PATENTS than the angle of divergence of said beam at said plate 2,877,353 Newberry Mar. 10, 1959 and said plate being placed with the constricted end of 3 03 ,993 Masuda Jun 12, 1962 said tunnel facing the source of said beam; and a rim 1O

Claims (1)

1. AN ELECTRON BEAM DEVICE COMPRISING: A SOURCE OF AN ELECTRON BEAM; AN OBJECT TO BE EXAMINED; AN OBJECTIVE LENS TO MAGNIFY AN ELECTRON BEAM IMAGE OF SAID OBJECT; AND AN APERTURE PLATE SUBSTANTIALLY TRANSVERSE TO SAID BEAM ON THE SIDE OF SAID OBJECT AWAY FROM SAID SOURCE, SAID PLATE HAVING AT LEAST ONE FRUSTO-CONICAL TUNNEL THERETHROUGH, THE INCLUDED CONICAL ANGLE OF SAID TUNNEL BEING GREATER THAN THE ANGLE OF DIVERGENCE OF SAID BEAM AT SAID PLATE AND SAID PLATE BEING PLACED WITH THE CONSTRICTED END OF SAID TUNNEL FACING THE SOURCE OF SAID BEAM.

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