US3445708A - Electron diffraction unit - Google Patents

Description

H. F; WEBSTER ELECTRON DIFFRACTION UNIT May 20, 1969 Sheet Filed Feb. 6, 1967 Inventor Harold E Webster,

His Attorney- ELECTRON DIFFRACTION UN IT Filed Feb. 6, 1967 Sheet 4" of 2 Fig. 2.

In van/0r ll Ham/a Webs/er His Attorney United States Patent US. Cl. 313-84 4 Claims ABSTRACT OF THE DISCLOSURE To provide an electron beam of minimal cross section for use in the investigation of atomic structures, a series arrangement including a source of electrons, an accelerating electrode, an equipotential tubular member maintaining an electrically field-free region, a display electrode with an aperture therein aligned with the equipotential tubular member, an equipotential concave grid adjacent the display electrode in combination with an electrically connected shield with an aperture therein, and a target maintained at a positive potential with respect to the source. Coils encircle the series arrangement to produce a magnetic field, and a target surface is displaced from the electron source a distance equal to an integral number of cyclotron wavelengths as determined by the magnitude of the magnetic field and the magnitude of the positive potential at the accelerating electrode.

The invention relates to an electron diffraction unit for examining the atomic structure of a surface. More particularly, the invention relates to a means of producing and utilizing an electron beam of appropriate characteristics for probing a surface.

Electron diffraction as used today entails the probing of a target surface by an electron beam to determine the properties of the target surface from the diffraction patterns of bombarded electrons upon leaving the target surface. If one is to proble details of a surface at the atomic level, it becomes necessary to confine the electron beam to an area of uniform atomic arrangement at that surface if any meaningful information is to be obtained. If a polycrystalline structure with nonuniform characteristics is to be probed, the largest beam across section must be less than the smallest individual crystal which is to be investigated.

In prior diffraction units, the target surfaces were probed by beams of undesirably large cross sections, threeby limiting the information obtained. The nature of the beam itself precluded the examination of a fine grained polycrystalline structure. It was necessary when using prior diffraction units to refine the crystalline matter to be examined to assure a uniform workable surface. This, in turn, precluded the examination of many materials as they appear in nature. It is therefore an object of this invention to achieve an electron beam of minimal cross section at the target surface.

The prior diffraction units have also been plagued by the inelastic electron problem. This problem may be described in terms of a pattern of electrons leaving the target surface and obscured by slow-moving inelastic electrons which carry no information with respect to the structure of the target. These slow-moving electrons fall upon a display means and thus cover up the pattern due to the information-bearing dilfractional electrons. Previously, the problem was solved by the addition of a grid which was impermeable to these slow-moving electrons. It is an object of this invention to eliminate the effect of the slow-moving inelastic electrons without additional elements.

Furthermore, the evaporation products at the source 3,445,708 Patented May 20, 1969 "ice of electrons which are carried with the electron beam have also changed the nature of the target crystal in some known devices. In those that sought to avoid such an effect, the expensive additions of electroelastic deflection means have been required. It is an object of this invention to eliminate the effects of the evaporation products and do so at no additional cost.

Briefly stated, and in accordance with one aspect of the invention, there is provided a source of electrons displaced an integral number of cyclotron wavelengths away from a target surface, which wavelengths are determined by the magnitude of a magnetic field producing lines of flux along the path of the electron beam and by the magnitude of an electric field accelerating the electrons from the electron source. The electrons pass from the source through an essentially electrically field-free enclosure, through an aperture in a display electrode, to a target surface from which they are diffracted to the display electrode.

The specification concludes with claims particularly pointing out and distinctly claiming the subject matter which I regard as my invention. The invention may also be understood from the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a cross-sectional view of one embodiment of the invention in an electron diffraction unit; and

FIGURE 2 is an alternative embodiment of the invention in an electron diffraction unit.

Referring first to FIGURE 1, an electron beam 2 as illustrated is focused by being subjected to a combination of diverging influence at an electron source 3 and a converging influence at a target surface 4 maintained at a positive potential with respect to the electron source 3 by a D.C. supply 11. The diverging influence at the electron source 3 has been the source of limitations in previous electron diffraction unit while the converging influence at the target surface 4 is a critical function of the distance between the electron source 3 and target surface 4. It is this distance which is an important element in the success of the electron diffraction unit embodying the invention disclosed herein. In short, the distance between the electron source 3 and the target surface 4 must be maintained at an integral number of cyclotron wavelengths in order to achieve an electron beam of minimal cross section at the target surface 4. While the distance for a given structure may be fixed, the length of a cyclotron wavelength may be varied in accordance with the equation M Q. T B

wherein )\==the cyclotron wavelength, m-=the mass of an electron, e=the charge of an electron, V=The accelerating potential to which electrons are subjected, and

B=the magnetic field to which the electrons are subjected.

FIGURE 1 illustrates but one of many types of conventional electron guns which may be employed in my diffraction unit. In the gun of FIGURE 1, electrons of the electron beam 2 need only to be subjected to an accelerating potential while en route from the electron source 3 to an accelerating electrode 5 maintained at a positive potential with respect to the source 3 by a D.C. supply 19. In actuality, an effort is made to trim the beam to a minimal cross section by an intermediate electrode 7 maintained at a negative potential with respect to the source 3 by a D.C. supply 8. The electrode 7 achieves the reduction in diameter of the electron beam 2 through the utilization of a pinhole aperture 9 aligned with an aperture 10 in the accelerating electrode 5. Still, the electron beam 2 diverges as it leaves the aperture 9 analogous to the divergence of light rays leaving a pinhole aperture at a light source.

However, the resulting divergence created by the abovedescribed conventional beam source or any alternative well known structures may be utilized to achieve convergence at the target surface 4 if the total accelerating potential V of the electron beam 2, which potential is equal to the DC. supply 19 in the particular embodiment shown, is combined with a magnetic field of proper magnitude to satisfy the above equation.

The means for producing the magnetic field of proper magnitude may comprise coils 13 and 14 encircling and evacuated tubular member 15 preferably enclosing the electron beam 2, the electron source 3, and the target surface 4. The magnetic field produced by coils 13 and 14 need not be uniform, nor is it necessary that the coils be arranged as shown. In fact, various alternative well known coil arrangements may be utilized as long as the critical relationship of the beam potential V and the magnetic field B is maintained. If the critical relationship is to be maintained, however, it is necessary to eliminate extraneous electric fields within the members 18, 26 and 27 and apart from the electric field associated with the beam potential V.

To assure the attainment of this critical relationship, the electron source 3 and the intermediate electrode 7 are enclosed and axially aligned within a tubular member 18. The tubular member 18 is connected to the anode of DC. supply 19 which maintains the electron source 3 at a negative potential with respect to ground. Because of the conductivity of the tubular member 18, the electron beam 2 will see equipotential surfaces in all directions except for electrode 7.

Of course, the ultimate goal of the electron diffraction unit shown in FIGURE 1 is the achievement of an electron diffraction pattern bearing information relevant to the structure of the target surface 4. To receive and record this desired diffraction pattern, an ellipsoidal display electrode 23 is provided with an axial aperture 24. This ellipsoid is generated by rotation around the major axis of an ellipse having major and minor axis lengths in the ratio 1r to 2. Since display electrode 23 must be maintained at a positive potential by a DC. supply 25 in order to excite the phosphor which coats display electrode 23, it is also necessary to eliminate the effect of the electric field produced by the electrode 23 upon the beam 2 as it passes to the target 4.

Elimination of this effect is partially achieved by extending a tubular projection 26 of reduced diameter, which forms an extension on tubular conductive member 18, through aperture 24 of display electrode 23. The diameter of tubular projection 26 is sufficiently small to allow an insulating space between its outer surface and the edge of aperture 24.

After electron beam 2 passes through tubular projection 26, it must travel along the axis of ellipsoidal display electrode 23 to reach target surface 4. The illipsoidal display electrode 23 as well as target surface 4 produce electric fields of considerable extent which must be eliminated if the voltage V is to be satisfied by the beam potential V alone.

The effects of an electric field established by target surface 4 and the ellipsoidal display electrode 23 are eliminated by the utilization of an ellipsoidal grid 27 mounted interior to and adjacent the display electrode 23 in combination with a planar target shield 28 electrically connected to grid 27. In order to eliminate field effects between grid 27 and the tubular projection 26, the two are electrically connected at an aperture 29 at the axis of the grid 27 to maintain a completely equipotential enclosure along the path of the electron beam 2. Although, ideally, target shield 28 would cover the target surface 4 to provide a completely equipotential enclosure, the diffraction pattern obtained from such an arrangement would be distorted and, for this reason, an aperture 30 is provided adjacent target surface 4.

As may be seen, paths 2 of the diffracted electrons produce diffraction patterns on display electrode 23 after initial incidence on grid 27 and with subsequent post acceleration to the positive display electrode 23. Grid 27 which is closely spaced from display electrode 23 provides a function in addition to that of producing an equipotential enclosure. This function is creating an electric field of considerable magnitude between grid 27 and display electrode 23, thereby producing considerable electron velocity at display electrode 23 which may then be readily converted to a flourescent record of the diffraction pattern. Without the increased velocity, the flourescent pattern would be too weak to be seen.

As mentioned previously, the invention eliminates the obscuring of the diffraction pattern caused by slow-moving, inelastic electrons. The magnetic field produced by coils 13 and 14 tends to deflect a slow-moving electron to the axis of the ellipsoidal display electrode 23. The net result is a pattern wherein the light produced by the inelastic electrons is limited to a spot on the axis of display electrode 23 while a meaningful pattern may be obtained from the remainder of display electrode 23.

As shown in FIGURE 1, coils 13 and 14 are spaced a considerable distance from the tubular glass member 15, thereby allowing considerable relative movement. Since a uniform magnetic field is not a prerequisite of this invention, the relative movement of the axis of the field to the direct path to the target will not adversely affect the resultant diffraction pattern. In fact, the relative movement may provide additional benefits.

The first type of relative movement, that of inclination of the axis of the coils 13 and 14 with respect to the axis of members 18, 26 and 27 provides a scanning effect. Such inclination results in a tilting of the lines of magnetic flux which scans the convergence point of the electron beam 2 across the surface of the target 4.

A second kind of translation, that of radially displacing the axis of the coils 13 and 14 with respect to the axis members 18, 26 and 27, will produce arc-like lines of flux along the axis of 18, 26, and 27 The arc-like lines of flux result in the bending of the electron beam 2 along a similar arc-like path which will permit electron source 3 and pinhole 9 to be displaced off to the side such that evaporation products from source 3 cannot reach target 4. Since such evaporation products contain no charge and are undeflected by magnetic fields, the arrangement of the target 4, the aperture 29, the displaced aperture 10, and the electron source 3 along a slight arc coinciding with lines of fiux within the members 18, 26 and 27 will result in an electron beam striking the target surface 4 essentially free of any undesirable evaporation products.

The ellipsoidal display electrode 23 is somewhat preferable over a planar display electrode since the radial displacement of a diffraction beam is linearly related to the order of the diffraction spot which of course varies with the material being studied. Yet, the display electrode need not be ellipsoidal or even concave if the linearity of the display is not a particularly critical factor. In fact, the requirement of a concave equipotential surface may render the use of an ellipsoidal display electrode undesirable due to the expense of a similar conforming grid.

Alternatively, a cylindrical grid may be utilized in combination with a planar display electrode. As shown in FIGURE 2, a planar display electrode 23' and a superjacent cylindrical grid 27' will achieve an electron diffraction display similar to that shown in FIGURE 1.

Although specific and identical biasing arrangements have been shown in both FIGURES 1 and 2, it is appreciated that diverse arrangements may be utilized in this invention. For example, it is by no means necessary that the tubular conductive member 18 and the grids 27 or 27' be maintained at ground. Another potential might be used while still attaining the equipotential surface requirement of the enclosure for the electron beam 2.

Furthermore, this invention should not be construed to be limited to any particular means for generating an electron beam, namely, those members including the electron source 3, the intermediate electrode 7, and the accelerating electrode 5. As mentioned previously, wellknown alternative electron beam sources are available for generating an electron beam of initially minimal cross section. It is only necessary that the beam potential accelerating the electrons properly combine with the magnetic field present to satisfy the earlier mentioned equation.

Finally, it is not desired to limit this invention to the display electrodes or the concave grid structures specifically disclosed herein. It is readily appreciated that various geometric figures are available to produce the desired electron diffraction pattern results obtained from the disclosed embodiments of the invention.

Although specific embodiments of the invention have been shown and described, the invention is not limited to the particular forms shown and described and it is intended by the appended claims to cover all modifications within the spirit and scope of the invention.

What I claim as new and desire to secure Patent of the United States is:

1. An electron diffraction unit comprising:

a source of electrons;

an accelerating electrode maintained at a first positive potential with respect to said source and adjacent thereto;

a target surface maintained at a second positive potential with respect to said source and spaced therefrom;

a display electrode with a first aperture therethrough positioned between said source and said target surface such that said electron beam passes through the aperture and strikes said target surface;

a concave grid adjacent said display electrode and opening toward said target surface, said concave by Letters grid producing an essentially electric field-free region between said target surface and said grid, secondary electrons emitted by said target surface passing through said grid to reach said display electrode and being accelerated in the region between said grid and said display electrode;

a target shield with an aperture therein adjacent said target surface, said target shield being electrically connected to said concave grid;

a conductive tubular member joining the aperture of said display electrode and said electron source, said tubular member being electrically connected to said grid and forming an essentially electrically field-free region therein; and

means for producing a magnetic field with lines of flux defining the path of said electron beam such that the magnitude of the magnetic field and of the first potential with respect to said source produces an integral number of electron wavelengths between said target surface and said electron source.

2. The electron diffraction unit of claim 1 wherein said means for producing a magnetic field comprises a plurality of windings encircling the path of the electron beam.

3. The electron diffraction unit of claim 2 wherein said display electrode and said concave grid comprise e1- lipsoidal members.

4. The electron diffraction unit of claim 2 wherein said concave grid comprises a cylindrical surface and said display electrode is planar.

JAMES W. LAWRENCE, Primary Examiner.

R. S. HOSSFELD, Assistant Examiner.

US. Cl. X.R.

Images