GB2137802A

GB2137802A - Electron Filter

Info

Publication number: GB2137802A
Application number: GB08302214A
Authority: GB
Inventors: John Benjamin; Andrew J Dixon; Karl A Gehring; Michael Keenlyside
Original assignee: Thor Cryogenics Ltd
Current assignee: Thor Cryogenics Ltd
Priority date: 1983-01-27
Filing date: 1983-01-27
Publication date: 1984-10-10
Also published as: GB8302214D0; GB2137802B

Abstract

An electron filter for electrons moving in helical paths along a liner homogeneous magnetic field, comprises an electron absorbing body 11 formed with parallel linear channels therethrough of predetermined length l and predetermined minimum transverse dimension a. The dimensions are selected such that, with the channels aligned with the magnetic field, electrons having cyclotron orbit diameters r2 greater than said predetermined minimum transverse dimension and helical path pitch lengths not substantially greater than said predetermined length, strike and are absorbed by the walls of the channels. By selecting appropriate transverse dimensions and lengths for the channels and locating the filter between specimen and detector in an emission electron microscope, resolution can be enhanced. In a preferred embodiment the body comprises a glass plate, with the channels through the thickness of the plate, having an electrically conducting surface on both its external faces and over the internal wall surfaces of the channels. The body can also comprise a plate of crystalline semiconductor material having the channels etched through the thickness of the plate. <IMAGE>

Description

SPECIFICATION Electron Filter The present invention is concerned with an electron filter for electrons moving in helical paths along a linear homogeneous magnetic field. It is known that electrons moving in strong magnetic fields are constrained to move in helical paths along the direction of the field. The helical paths are such as to embrace a constant bundle of flux lines of the magnetic field.

Where there is a flux of such electrons moving along a magnetic field in such helical paths, it may be desirable to discriminate between the electrons of different cyclotron orbit diameters.

For example, United Kingdom patent application No. 8226367 describes an emission electron microscope which employs the above mentioned phenomenon to constrain electrons emitted from the surface of a specimen to travel along magnetic flux lines from a region of relatively strong magnetic field to a region of weaker field at which the transverse special distribution of the electron flux can be detected to provide an enlarged image indicative of variations in the flux of electrons emitted from the specimen surface.

The special resolution of the resulting image is dependant on the cyclotron orbit diameters of electrons as they are emitted from the specimen surface. The orbits can be minimised by locating the specimen in the strongest possible magnetic field. However, it will be seen that the cyclotron orbit diameter of electrons emitted from the specimen surface are dependant, inter alia, on the angle of emission of the electron from the specimen surface relative to the magnetic field direction. The maximum cyclotron orbit diameter corresponds to electrons emitted at almost 900 to the magnetic field direction. For a specimen in a magnetic field at 7.7 Tesla, the maximum helical radius for an electron emitted almost at right angles to the field is 1 micron for an electron having an emission energy of 5 electron Volts (eV).Accordingly in such conditions the resolution provided by the enlarged image of the specimen's surface cannot be significantly better than 1 micron. In practice a resolution of about 2 microns has been demonstrated where the emission electrons are photo electrons emitted by helium I (UV) radiation.

Resolution of the microscope can be improved by discriminating against those emission electrons travelling in helical paths along the magnetic field line which have the largest cyclotron orbit diameters. If only electrons with relatively small diameters are permitted to proceed to the detector, improved resolution can be achieved, although with reduced electron flux at the detector.

The present invention provides an electron filter which can be used in such an emission electron microscope to enhance special resolution.

According to one aspect of the present invention, an electron filter for electrons moving in helical paths along a linear homogeneous magnetic field, comprises an electron absorbing body formed with parellel linear channels therethrough of predetermined length and predetermined minimum transverse dimension, selected such that, with the channels aligned with the magnetic field, electrons having cyclotron orbit diameters greater than said predetermined minimum transverse dimension and helical path pitch lengths not substantially greater than said predetermined length strike and are absorbed by the walls of the channels.

It can be seen that such a filter has the effect of preventing passage through the absorbing body of all electrons having cyclotron orbit diameters greater than the predetermined minimum transverse dimension of the channels through the body. Thus by selecting appropriate transverse dimension and length for the channels through the body, and locating the filter to intercept the electron flux between the specimen and detector in an emission electron microscope as described above, the resolution of such a microscope can be enhanced.

Preferably the body is formed as a plate with the channels through the thickness of the plate and having an electrically conducting surface over both the external faces of the plate and over the internal wall surfaces of the channels. The electrically conducting surface enables a static charge accumulated by absorbed electrons to be conducted away.

In one example, the body comprises a glass plate with the channels through the thickness of the plate.

In an especially preferred embodiment, the body comprises a plate of crystalline semiconductor material having said channels etched through the thickness of the plate. The channels may be directionally etched. Directional or anisotropic etching of crystalline semiconductor material is known. For example, the plate may be made of crystalline silicon and have a planar surface in the (110) crystal plane of the silicon, the channels being directionally etched preferentially in the < 1 10 > direction of the silicon. Articles on the anisotropic etching of silicon include; "Anisotropic of Silicon" by Kenneth E. Bean, published in IEEE Transactions on Electron Devices, vol. ed. 25 No. 10, October 1978; and "On Etching Very Narrow Grooves in Silicon" by Don L. Kendall published in Appl.

Phys. Lett. vol. 26, No. 4 1 5th February 1975.

Using the technique of directional etching in crystalline silicon, the channels may be formed as slots through the silicon plate with side walls in (111) planes of the silicon. With this technique very narrow slots can be formed through the silicon. Also the semiconductor plate used for the filter body may have formed therein an integrated electronic circuit and the channels through the plate and the integrated circuit may be arranged to generate electric signals indicative of at least one of the number of electrons passing through the channels of the filter and the number of electrons striking and absorbed by the filter.

Selected surface regions of the external faces and internal channel walls of the silicon plate may be doped to increase electrical conductivity and to enable electric fields to be generated at the surfaces to inhibit production of secondary electrons.

According to a further aspect of the present invention, an emission electron microscope comprises means for generating a magnetic field having a first region at which the field is relatively strong and a second region at which the field is relatively weak and which is interconnected by lines of flux with the first region, means for locating a specimen to be examined in said first region, the specimen being selected or arranged in use such that electrons are emitted by at least a portion of a surface of said specimen extending in said first region at an angle to said field, said magnetic field generating means being arranged so that the field in said first region is sufficiently strong that electrons emitted by the specimen at angles to the magnetic field are constrained by the field to move in helical paths along the lines of flux of the field, and electron detector means including a detector located in said second region, said detector means being arranged to provide an indication of the special distribution transversely of the field in said second region of electrons emitted by the specimen and travelling to said second region, thereby to provide a magnified emission electron image of at least part of said surface portion of the specimen, the magnetic field generating means being arranged to provide a homogenous field region along the lines of flux between the specimen and the detector, where the field is linear and homogenous, and the microscope further comprising an electron filter located in the homogenous field region and comprising an electron absorbing body formed with parallel linear channels therethrough aligned with the magnetic field, the channels being of predetermined length and predetermined minimum transverse dimension, selected such that electrons having cyclotron orbit diameters greater than said predetermined transverse dimension and helical path pitch lengths not substantially greater than said predetermined length strike and are absorbed by the walls of the channels. Preferably the microscope is arranged so that the homogeneous field region in which the filter is located is adjacent to or forms part of said second region at which the field is relatively weak.

As electrons travel in helical paths from the high field region at the specimen to the low field region they orbit around a constant bundle of flux lines. It can be seen, therefore, that the orbit diameters of an electron progressively increases and is at a maximum at the lowest field. Thus, for a microscope having linear magnification at the detector means of x50, the cyclotron orbit diameter of an electron at the detector means is fifty times the diameter of the same electron when it was emitted at the surface of the specimen in the region of high magnetic field. As a result, a filter with channels of minimum transverse dimension 1 2.5 microns, located in the low field region near the detector means, effectively filters electrons having cyclotron diameters at the specimen greater than 0.25 micron, increasing resolution of the magnified image accordingly.

In one example, the filter body is a plate with the channels through the thickness of the plate, the plate having a plurality of areas, each area having channels all of the same said predetermined length and minimum transverse dimension, but the channels in one said area having a different predetermined length and/or minimum transverse dimension from the channels in an adjacent said area, and the microscope includes means for directing the electron flux from the specimen to the detector through a selected one of said areas of the plate. In this way different filtering action can be selected. Convenientiy said means for directing comprises adjustable means locating said plate in said homogeneous field region with a selected said area of the plate intercepting said electron flux.

Alternatively, however, said means for directing may comprise adjustable field deflecting means to deflect the magnetic field in said homogeneous field region transversely of the field direction so that said electron flux is directed at a selected one of said areas of the plate.

It will be appreciated that the filter described above can have the effect of producing graininess in the image received by the detector-means of the microscope. Where the channels through the electron absorbing body have a predetermined spacing in the direction of their minimum transverse dimension, the microscope may conveniently include means to oscillate the body relative to the magnetic field in said transverse direction and with an amplitude at least equal to said spacing. This oscillatory movement between the magnetic field and-the filter body can remove the grainy effect on the detector means.

The oscillation may be provided either by mechanically vibrating the filter body, or by rotating the filter or by producing transverse oscillatory movement of the magnetic field.

Examples of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view through the thickness of a filter plate embodying the present invention; Figure 2 is a plan view of the plate of Figure 1.

Figure 3 is an enlarged partial cross-section view of the filter plate of Figure 1, illustrating the filtering action; Figure 4 is a plan view of another form of filter plate and Figure 5 is a schematic diagram of an emission electron microscope embodying the present invention Referring to Figures 1 and 2 a plate 10 is illustrated of electron absorbing material having through the thickness of the plate a plurality of channels 1 The plate 10 is formed of crystalline silicon and may comprise a chip from a silicon wafer. The plate is shown greatly enlarged in Figures 1 and 2, and the channels 11 are shown in the Figure for clarity to be greatly enlarged in proportion to the dimensions of the plate 10.

The channels 11 are slots, as shown in Figure 2, extending through the thickness of the plate normal to the plane surfaces 12 and 13 of the plate. In operation to filter electrons moving in helical paths along a magnetic field H, the plate 10 is located in the electron flux with the channels 11 aligned accurately parallel to the magnetic field. Then, electrons with cyclotron orbit diameters greater than the width of the slots 11 are blocked by the plate 10.

Figure 3 illustrates the action of the filter plates more clearly. If each of the channels or slots 11 has a width a and a length I (equal to the thickness of the plate 10) it can be seen that an electron travelling in a helical path along magnetic field lines parallel to the length of the slots 11 must strike either the front face 12 or the internal surface af a slot 11 if it has a cyclotron orbit radius r greater than a/2 and a helical pitch length p less than 1. It will be appreciated that if the pitch length of the helical path of an electron is in fact larger than the channel length I, some electronic with orbit radii greater than a/2 can pass through the channels of the filter.

Figure 3 illustrates the path 14 of an electron which can just pass through the channel 11 provided its helical path is centered on the median line 1 6 of the channel. On the other hand an electron having the path 1 5 even though centered on the median line, strikes a wall surface of the channel 11 at point 17.

In the way illustrated in Figure 3, the filter plate 10 discriminates against electrons having helical paths with cyclotron orbits greater than the width of the channels 11. In fact, the transparency of the filter plate 10 is at a maximum (corresponding to the proportionate open area of the plate) for electrons travelling closely parallel to the magnetic field (H) For electrons with greater cyclotron orbit radii, the transparency declines, to a zero level for orbit radii greater than a/2, provided that the channel length is greater than the pitch length of the electrons. The channels 1 are conveniently formed through the plate 10 by anisotropic etching, For this process, the plate 10 is prepared of crystalline silicon with the faces 12 and 13 of the silicon wafer aligned in the (1 10) planes of the silicon crystal.With this alignment, the (111) planes of the crystal are normal to the faces 12 and 13. By pre-etching a small portion of the wafer surface, the direction of the (1 1 1 ) planes across the face of the wafer can be determined. A mask defining the desired slots 11 through the wafer is then formed on the wafer surface in the usual way with the slots accurately aligned along the (111) planes. By then using a highly directional etch which attacks the (110) planes of silicon some six hundred times faster than the (111) planes, slots 11 right through the silicon can be formed. A useful etch solution for this purpose is KO H in water used at 80" centigrade. Areas to be etched are defined by photolithography using a silicon nitride mask.

Another etch is KO H in water at 200C. In this case a 0.6 micrometre thick oxide can be used as a mask.

With this technique, slots through the plate 10 can be formed with widths as small as one micron or less. Furthermore, the proportionate open area of the channel region can be maximised by keeping the spacing between adjacent slits to a minimum. For example, slots 1 5 microns wide arid 4 mm long can be etched through a silicon wafer forming the plate 10 of 300 microns thickness. The spacing between adjacent slits may be only.20 to 25 microns, representing a wall thickness between adjacent slits of only 5 to 10 microns. As a result the maximum transparency of the filter to electrons travelling parallel to the magnetic field may be as much as 75% locally or 50% over a wafer as a whole.

Figure 4 illustrates a modified embodiment of the filter plate illustrated in Figures 1 and 2. In the modified embodiment, there are spaced areas 20, 21, 22 and 23 having slots of different widths.

Thus, the area 20 may be provided with slots 24 of say 50 microns width, area 22 with slots of 10 microns width and area 23 with slots of 5 microns width. Between the different areas, unetched regions are left to ensure sufficient strength and rigidity in the resulting filter plate even areas of the same slot width may include such unetched regions to provide structural strength of the slotted area.

In operation in an emission electron microscope, provision is made to deflect the magnetic field relative to the plate illustrated in Figure 4 so that the electron flux intercepts a selected one of the areas 20 to 33 to provide a desired filtering effect.

Figure 5 illustrates schematically the basic elements of an emission electron microscope of the kind in which the electron filter described hitherto can be especially useful. A specimen 32 to be examined is positioned on a locating member 30 in a region of a high magnetic field produced by a super conducting solenoid 31. In one arrangement, the emission electrons employed are photo electrons produced by irradiation of the specimen 32 with, for example, ultraviolet light from a source 33. The magnetic field produced by the solenoid 31 is sufficiently strong typically as high as 7.7 Tesla that electrons emitted by the irradiated surface of the specimen 32 are constrained to travel along the magnetic field lines in helical paths. For photo-electrons emitted with energies of up to 10 eV, the maximum cyclotron orbit at the specimen 32 in a field of about 7 Tesla for electrons emitted substantially at right angles to the field is of the order of 2 microns in diameter. The microscope is arranged to produce a second region indicated generally at 34 of relatively lower magnetic field under the control of a second solenoid 35. A detector 36 is located in the region 34 of low magnetic field. In the illustrated example, the magnetic fields produced in the instrument are radially symmetrical about a linear axis 37 and lines of magnetic flux close to the axis 37 interlink the detector 36 and the specimen 32. As a result, electrons emitted from the surface of the specimen 32 close to the axis 37 travel in helical paths along the magnetic field lines to the detector 36.As the electrons travel from the region of relatively high magnetic field to the region 34 of lower magnetic field, their kinetic energy in a direction perpendicular to the magnetic field is progessively transferred to kinetic energy parallel to the field so that on reaching the detector 36 substantially all the kinetic energy is parallel to the field. It can also be shown that the electrons travelling in helical paths along a magnetic field always circulate around a constant bundle of magnetic field lines, i.e. the flux linkage of each electron orbit remains constant. As a result, the special distribution of the electrons emitted at the surface of the specimen is maintained and is detected spacially enlarged by the detector 36.The detector 36 may include a phosphor plate providing an optical representation of the special distribution of electron flux transversely of the magnetic field.

Optical means such as a T.V. camera 40 may be provided for viewing or recording the optical image.

It will be appreciated that the resolution of the special image produced at the detector 36 is limited by the orbit diameters of electrons emitted at the specimen surface 32. In order to improve the resolution of the detector 36 an electron filter plate 38 such as described with reference to Figures 1 to 4 above, is included in the low magnetic field region 34. The solenoid 35 is arranged to maintain the magnetic field in the region of the plate 38 linear and homogeneous so that the plate 38 can be closely aligned with the channels through the plate parallel to the magnetic field. It will then be appreciated that electrons with large cyclotron orbits greater than the width of the channels through the plate are absorbed by the plate 38 so that only the electrons with lower orbits reach the detector 36 thereby improving resolution of the image.By locating the filter plate 38 adjacent the detector 36 in the region of low magnetic field, the plate can have relatively larger channel widths corresponding to the larger cyclotron orbits in the low field region.

If the plate 38 is formed with discrete areas of different channel widths, such as illustrated in Figure 4 then mechanical adjustment means 39 may be provided to adjust the position of the plate 38 relative to the magnetic field so that the electron flux from the specimen is directed at the selected one of the areas of the plate.

It will be appreciated that, for a stationary filter plate 38 the resultant image has a graininess corresponding to a shadow of the channels or slits in the filter plate 38. In order to remove this shadow effect, the adjustment means 38 may be arranged to mechanically oscillate the plate 38 transversely of the magnetic field in a direction perpendicular to the lengths of the slits through the plate. The amplitude of the oscillations should be at least equal to the spacing between adjacent slits. Provided the frequency of oscillation is sufficiently high so that every part of the screen has a slit in front of it sufficiently frequently, the complete image is thus built up upon the detector 38 which can be viewed without any appearance of flicker.

In the above example, mechanical adjustment means 39 have been described for physically moving the plate 38 relative to the magnetic field.

Alternatively, the plate 38 may be held stationary in space and magnetic deflection means may be provided (not shown in the drawing) to provide a uniform deflection of the magnetic field. The deflection may be set so as to direct the electron flux from the specimen to a selected area of the filter plate. The deflector means may also be arranged to return the deflected magnetic field to be centered on the axial line 37 after the electrons have passed through the filter plate 38. The deflection means may also be arranged to provide an oscillating deflecting field to remove the shadow image of the filter plate as described above.

It will be appreciated that as electrons are collected by the filter plate 38 the resuitant electric charge must be dissipated to prevent the surfaces of the plate from becoming electrostatically charged. The presence of such an electrostatic charge would otherwise effect the imaging properties of the microscope. With silicon as the material of the plate 38 the bulk resistivity of the plate will normally be low enough for any such electrostatic charging to be conducted away to earth. However, if the electrostatic charging effect should become noticable, the silicon material of the plate may be doped to reduce the bulk resistivity as required by means of diffusion, or may be coated with metal (e.g. aluminium) by means of chemical vapour deposited on both techniques used on the fabrication of integrated circuit structures.

Furthermore, the use of crystalline silicon for the filter plate has the advantage that integrated circuits may be formed on the plate. If regions of the surface of the plate and the wall surfaces of the channels are selectively doped, and provided with conducting inter-connections with a suitable integrated circuit formed on the silicon plate, the arrangement may enable signals to be generated from the plate 38 representative of the quantity of electrons being absorbed by the plate, and possibly also the quantity passing through the filter without being absorbed.

It may be appreciated that a multi-layer structure plate may be provided to increase the thickness of the plate. Furthermore such a multilayer structure may include successive layers having channels of decreasing width which may be interconnected with appropriate integrated circuits so that such a device may be used to indicate the orbit diameter distribution of the electron flux from the specimen. It will be appreciated that the kinetic energy of the electrons passing through the filter plate 38 in the microscope is normally relatively low in the order of some tens of electron volts. It may be desirable to increase the energy of the electrons delivered to the detector 36 to improve their detectability.

This may be achieved by using the filter plate 38 as one electrode to produce an electric field gradient between the plate and the detector 36 thereby to accelerate electrons towards the plate.

In order to avoid an excessive depletion region forming at the surfaces of the silicon material of the plate 38 it may be necessary to produce a metallic conducting electrode on one surface of the plate 38 e.g. by evaporation, or sputtering or chemical vapour deposition techniques.

Claims

1. An electron filter for electrons moving in helical paths along a linear homogeneous magnetic field, comprising an electron absorbing body formed with parallel linear channels therethrough of predetermined length and predetermined minimum transverse dimension, selected such that, with the channels aligned with the magnetic field, electrons having cyclotron orbit diameters greater than said predetermined minimum transverse dimension and helical path pitch lengths not substantially greater than said predetermined length strike and are absorbed by the walls of the channels.

2. An electron filter as claimed in claim 1 wherein the body is a plate with the channels through the thickness of the plate and having an electrically conducting surface over both the external faces of the plate and over the internal wall surfaces of the channels.

3. An electron filter as claimed in claim 1 or claim 2 wherein the body comprises a glass plate with the channels through the thickness of the plate.

4. An electron filter as claimed in claim 1 or claim 2 wherein the body comprises a plate of crystalline semiconductor material having said channels etched through the thickness of the plate.

5. An electron filter as claimed in claim 4 wherein the channels are directionally etched.

6. An electron filter as claimed in claim 5 wherein the plate is of crystalline silicon and has a planar surface in the (1 10) crystal plane of the silicon, with channels being directionally etched preferentially in the < 110 > direction of the silicon.

7. An electron filter as claimed in claim 6 wherein the channels are formed as slots through the plate with side walls in (111) planes of the silicon.

8. An electron filter as claimed in any of claims 4 to 7 wherein the semiconductor plate has formed therein an integrated electronic circuit.

9. An electron filter as claimed in claim 8 wherein the plate, including channels and integrated circuit, is formed to generate electric signals indicative of at least one of the number of electrons passing through the channels of the filter and the number of electrons striking and absorbed by the filter.

10. An electron filter as claimed in any of claims 4 to 9 wherein selected surface regions of the external faces and internal channel walls of the plate are doped to increase electrical conductivity to enable electric fields to be generated at the surfaces to inhibit production of sedondary electrons.

11. An emission-electron microscope comprising means for generating a magnetic field having a first region at which the field is relatively strong and a second region at which the field is relatively weak and which is inter-connected by lines of flux with the first region, means for locating a specimen to be examined in said first region, the specimen being selected or arranged in use such that electrons are emitted by at least a portion of a surface of said specimen extending in said first region at an angle to said field, said magnetic field generating means being arranged so that the field in said first region is sufficiently strong that electrons emitted by the specimen at angles to the magnetic field are constrained by the field to move in helical paths along the lines of flux of the field, and electron detector means including a detector located in said second region, said detector means being arranged to provide an indication of the spatial distribution transversely of the field in said second region of electrons emitted by the specimen and travelling to said second region, thereby to provide a magnified emission-electron image of at least part of said surface portion of the specimen, the magnetic field generating means being arranged to provide a homogeneous field region along the lines of flux between the specimen and the detector, where the field is linear and homogeneous, and the microscope further comprising an electron filter located in the homogeneous field region and comprising an electron absorbing body formed with parallel linear channels therethrdugh aligned with the magnetic field, the channels being of predetermined length and predetermined minimum transverse dimension, selected such that electrons having cyclotron orbit diameters greater than said predetermined minimum transverse dimension and helical path pitch lengths not substantially greater than said predetermined length strike and are absorbed by the walls of the channels.

1 2. An emission-electron microscope as claimed in claim 11 wherein the body is a plate with the channels through the thickness of the plate, the plate having a plurality of areas, each area having channels all of the same said predetermined length and minimum transverse dimension, but the channels in one said area having a different predetermined length and/or minimum transverse dimension from the channels in an adjacent said area, and the microscope includes means for directing the electron flux from the specimen to the detector through a selected one of said areas of the plate.

13. An emission-electron microscope as claimed in claim 12 wherein said means for directing comprises adjustable means locating said plate in said homogeneous field region with a selected said area of the plate intercepting said electron flux.

1 4. An emission-electron microscope as claimed in claim 12 wherein said means for directing comprises means to deflect the magnetic field in said homogeneous field region transversely of the field direction so that said electron flux is directed at a selected one of said areas of the plate.

1 5. An emission-electron microscope as claimed in any of claims 11 to 14 wherein the channels through the electron absorbing body have a predetermined spacing in the direction of their minimum transverse dimension and the microscope includes means to oscillate the body relative to the magnetic field in said transverse direction and with an amplitude at least equal to said spacing.

1 6. An emission-electron microscope as claimed in any of claims 11 to 1 5 wherein the homogeneous field region in which the filter is located is adjacent to or forms part of said second region at which the field is relatively weak.

1 7. An electron filter substantially as hereinbefore described with reference to and asillustrated in Figures 1 to 4 of the accompanying drawings.

1 8. An emission-electron microscope as claimed in any of claims 12 to 17 wherein the electron filter is as claimed in any of claims 2 to 10Or 17.

1 9. An emission-electron microscope substantially as hereinbefore described with reference to and as illustrated in Figure 5 of the accompanying drawings.