US3864596A

US3864596A - Multiple electron mirror apparatus

Info

Publication number: US3864596A
Application number: US272778A
Authority: US
Inventors: Kent N Maffitt
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1972-07-18
Filing date: 1972-07-18
Publication date: 1975-02-04
Anticipated expiration: 1992-02-04
Also published as: CA993114A; BR7305333D0

Abstract

An electron beam system using an electron beam source, a magnetic prism and a plurality of electron mirrors with associated electron beam focusing or cross-section control lenses and beam deflection units permits an electron beam from the electron beam source to be modified or acted upon as required by the electron mirrors as the electron beam is sequentially presented to the electron mirrors. An electron beam is directed to the first electron mirror where it is reflected and directed to the next electron mirror, etc. The electron beam is acted upon at an electron mirror in accordance with the voltage bias applied to the electron mirror. The voltage bias may be set to passively mirror the entire beam or a portion thereof, modulate the beam in accordance with the surface condition of the mirror or permit the beam to impinge on the mirror surface. The electron beam focusing or cross-section control lenses and deflection units provide for selective direction of the beam and control of the cross-section to determine the portion and size of such portion of an electron mirror surface which will influence or be influenced by the electron beam.

Description

ite Maititt States Patent MULTIPLE ELECTRON MIRROR APPARATUS [75] Inventor: Kent N. Mafiitt, Minneapolis, Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.

[22] Filed: July 18, 1972 [21] Appl. No.: 272,778

Primary Examiner-Leland A. Sebastian Assistant ExaminerP. A. Nelson Attorney, Agent, or Firm-Alexander, Sell, Steldt and DeLaHunt [451 Feb. 4, 1975 ABSTRACT An electron beam system using an electron beam source, a magnetic prism and a plurality of electron mirrors with associated electron beam focusing or cross-section control lenses and beam deflection units permits an electron beam from the electron beam source to be modified or acted upon as required by the elsetron mi ror as he e tr m is qu tially presented to the electron mirrors. An electron beam is directed to the first electron mirror where it is reflected and directed to the next electron mirror, etc. The electron beam is acted upon at an electron mirror in accordance with the voltage bias applied to the electron mirror. The voltage bias may be set to passively mirror the entire beam or a portion thereof, modulate the beam in accordance with the surface condition of the mirror or permit the beam to impinge on the mirror surface. The electron beam focusing or cross-section control lenses and deflection units provide for selective direction of the beam and control of the cross-section to determine the portion and size of such portion of an electron mirror surface which will influence or be influenced by the electron beam.

10 Claims, 5 Drawing Figures sum 10F 2 FIG. 2

PAIENTEDFEB m SHEET 2 OF 2 F IG. 5

1 MULTIPLE ELECTRON MIRROR APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention presented herein relates to electron beam systems and devices and particularly to such systems or devices using electron mirrors.

2. Discussion of the Prior Art The prior art discloses electron beam electron mirror systems and devices which use only a single electron mirror together with various types of deflection units, magnetic prisms and beam focusing or cross-section control lenses for controlling the electron beam. Electron microscopes use a single electron mirror and systems are known using a single electron mirror where an electron beam is modulated in accordance with the mirror surface condition to which it is presented. US. Pat. Nos. 2,901,627 to Wiskott et al., 3,l76,278 to Mayer and 3,278,679 to Newberry are typical of such prior art patents. The prior art does not show or suggest the use of a plurality of electron mirrors.

SUMMARY OF THE INVENTION An electron beam electron mirror system is provided which uses electron mirror techniques and is characterized by the use ofa plurality of electron mirrors; a magnetic prism; and an electron beam source for providing an electron beam which is directed at the magnetic prism which serves to direct the beam toward one of the electron mirrors where it is acted upon in accordance with the bias on the electron mirror and returned to the prism, which then directs the beam to a second one of the plurality of mirrors. This action is repeated as required at the second mirror and at each of the remaining plurality of electron mirrors. Electron beam focusing or cross-section control lenses plus electron beam deflection units are provided for each of the electron mirrors as may be required to permit the electron beam to be controlled as required during travel to the associated mirror. The bias voltage presented at a particular mirror determines the action of the electron beam at the mirror. The beam may be passively mirrored in its entirety or a portion of the beam may be mirrored, or the beam may be modulated in accordance with the condition of the mirror surface or may be permitted to impinge on the mirror surface. The electron beam focusing or cross-section control lenses and deflection units provide for selective direction of the beam to a portion of the mirror surface and control of the cross-section of the beam to establish the size and shape of such portion of an electron mirror which is to influence the beam or be influenced by the electron beam.

One embodiment of the invention uses a single magnetic prism with the electron source and the plurality of electron mirrors positioned about the prism. The deflection unit and cross-section control unit when used with an electron mirror are positioned between the mirror and the magnetic prism.

Another embodiment uses a single magnetic prism formed from two elongated, U-shaped magnets, which can be electromagnets or permanent magnets. The magnets are positioned with a gap between opposite poles of the two magnets. The plurality of electron mirrors are positioned about the prism by locating several of the mirrors on each side of the magnetic prism along the length of the prism with the electron beam source positioned on one side of the magnetic prism. As with the first embodiment, the deflection unit and crosssection control unit when used with an electron mirror are positioned between the mirror and the magnetic prism.

The multiple electron mirror arrangement can be used in a number of ways. A system having a plurality of electron mirrors will permit the construction of systems having higher image contrast and better resolution since the energy spread of the electrons in a beam can be reduced at one or more electron mirrors by allowing the mirror to collect high energy electrons while mirroring the low energy electrons for direction to a second mirror. Energy filtering in this manner can be used to advantage in any device or system where high resolution is needed. A multiple electron mirror arrangement also permits the transfer of a mirror image from one mirror to another mirror which can be useful in an information storage and retrieval system. The latter arrangement is used herein to illustrate a use of the invention.

BRIEF DESCRIPTION OF THE DRAWING The invention will be understood and its various advantages will become apparent from the description to follow given in conjunction with the accompanying drawings wherein:

FIG. I is a schematic showing of the operation of an electron mirror;

FIG. 2 is a schematic showing of one embodiment of the invention;

FIG. 3 is a schematic showing of a second embodiment of the invention;

FIG. 4 is an end view of the magnetic prism 10 used in the embodiment shown in FIG. 3 and is taken along 4-4 of that figure; and

FIG. 5 is a top view of the preferred structure for the electron focusing lenses used in the apparatus of FIG. 3.

DESCRIPTION In order that the invention presented herein can be best understood, the known principles of electron mirror operation will be summarized using FIG. 1 of the drawing. FIG. 1 shows a simple electron mirror arrangement which includes an apertured plate 1 which is positioned parallel to and spaced a few millimeters from the mirror specimen surface 2. The space between the apertured plate I and the mirror specimen surface 2 is referred to as the mirror region. A strong electric field is maintained in the mirror region by biasing the apertured plate a few kilovolts positive and the mirror specimen 2 a few volts negative relative to the electron source. The electric field can be adjusted so the electrons fired into the mirror region do not have sufficient energy to reach the specimen surface 2 and consequently execute the parabolic paths 3 shown in FIG. 1. If these parabolic paths are unperturbed by the specimen 2, the cross-section of the mirrored beam is uniform in density. However, differences in the surface topography, surface potential distribution or magnetization of the specimen cause field gradients which produce perturbations of the parabolic paths. The effect is observed as spatial modulation of the density of the mirrored beam. This modulation can be observed as an electron mirror image of the specimen by allowing the beam to impinge on a phosphorus screen.

The distance from the mirror specimen surface 2 to the point at which the electrons reverse their direction can be increased by making the mirror bias more negative relative to the electron source. This will, of course, cause the spatial modulation to decrease because the intensity of the field gradients caused by the specimen decreases with the increasing distance from the mirror specimen surface 2. The mirror bias therefore provides a means for preventing any spatial modulation of the electron beam. An electron mirror image can therefore be turned on or off as needed by the bias on the mirror specimen relative to the electron source.

Since the mirrored electrons do not touch the specimen 2, it is possible to have non-destructive study of the nature of the specimen surface. The electrons have typically only a few electron volts of energy at the point of turn-around so they are very sensitive to differences in the electric, magnetic and topographic properties of the specimen surface. This makes the electron mirrors more sensitive than other electron beam techniques. This sensitivity and the non-destructive nature of electron mirrors make them very useful as memory devices.

One embodiment of the invention presented herein is disclosed in connection with the multiple electron mirror memory apparatus schematically shown in FIG. 2 and described in greater detail in copending patent application Ser. No. 272,779, filed July 18, 1972, now U.S. Pat. No. 3,789,370, having the same assignee as this application. Spaced about a magnetic prism 10 are a modulating electron mirror 4, two

storage mirrors

6 and 8, an electron gun 11 and a readout array 12. Three element electron focusing lenses 13, l4, l and 16 provided for

mirrors

4, 6 and 8 and the readout array 12, respectively, are also positioned around the prism 10. In addition to the focusing lens 14, the mirror 6 has an objective lens 18 positioned near its storage surface and two

deflection units

19 and 20 positioned between the objective lens 18 and the focusing lens 14. An objective lens 21 and two

deflection units

22 and 23 are similarly provided for mirror 8. The objective lenses 18 and 2] may be three-element multi-apertured immersion type lenses. The components described in connection with FIG. 2 are secured within a housing represented by the dotted line surrounding the components to permit operation of the components at a pressure of approximately Torr. For the sake of brevity and clarity, electrical connections have not been shown in FIG. 2.

A general description will be given of the operation of the apparatus which is sufficient for an understanding of the invention. The apparatus has a write mode and a read mode of operation. The write mode provides for the transfer of an entire page (plurality of bits) of information from mirror 4 to either of

mirrors

6 and 8. This write mode can be referred to as a parallel write mode since all bits of information in the page are transferred simultaneously. A page of information is presented at the surface of the modulating mirror 4 in the form of a pattern of charge determined by the information bits. Each page of information transferred can be stored at a different one of a plurality of storage areas at the mirror surface of

mirrors

6 and 8 or can be applied to a readout array 12. In addition, a transfer made to mirror 6 can be repeated at mirror 8 to provide redundant storage to improve reliability of the system. The read mode of operation provides for the transfer of the entire page of information present at a selected storage area of

mirrors

6 and 8 to readout array 12. This read mode can be called a parallel read since all bits of information in the page are transferred simultaneously. The read mode of operation provides for random access to the stored information in that the electron beam is controlled by the voltages applied to the

deflection units

19, 20 and 22, 23 for address of any storage area to be read.

During the write mode, a well collimated eletron beam generated by the election gun I! is accelerated and directed at the center of the magnetic prism 10 which deflects the beam so as to pass through the focusing lens 13 for modulating mirror 4. In the embodiment shown in FIG. 2, the beam is deflected 30. The focusing lens 13 is constructed with focusing properties to cause the electron beam to present a cross-section large enough to encompass all of the modulation sites making up the page of information at the surface of the mirror 4. During the write mode the pattern of charge presented by the modulation sites on the surface of the mirror 4 spatially modulates the mirrored electron beam into a data or information pattern corresponding to the pattern of charge presented by the modulation sites.

After being mirrored and modulated at the modulating mirror 4, the electron beam passes back through the focusing lens 13 to the magnetic prism 10 where it is again deflected 30 by the prism 10 for passage through the focusing lens 14 for the mirror 6. The lens 14 has focusing properties causing the beam to be collimated and reduced in cross-section. The beam then passes through the two

deflection units

19 and 20 which serve to shape the beam and direct it to the desired location or address in the multi-apertured immersion objective lens 18. Assuming the information contained in the modulated electron beam is to be stored at mirror 6, the bias on mirror 6 is set to allow the beam to impinge on the addressed storage area on the surface of mirror 6 to deposit a pattern of charge corresponding to the beam modulation pattern. If the information is to be stored at mirror 8 instead of mirror 6, the bias on mirror 6 is set so the electron beam is passively mirrored without any spatial modulation by mirror 6. The beam passes back through the objective lens 18, the

deflection units

19 and 20 and the focusing lens 14 to the magnetic prism 10 where it is again deflected 30 by the prism 10 for passage through the focusing lens 15 for mirror 8. The beam then passes through the two

deflection units

22 and 23 where it is directed to the desired location or address in the multi-apertured immersion objective lens 21. Since the information contained in the modulated beam is to be stored at mirror 8, the bias on mirror 8 is set to allow the beam to impinge on the addressed storage area on surface of mirror 8 to create a pattern of charge corresponding to the beam modulation pattern. For redundancy purposes, the page of information presented at the modulation mirror 4 can be stored at

mirrors

6 and 8. It is then possible during the read mode to read a selected page of information from each of the storage mirrors and present it to the readout array 12 to permit a comparison to be made of the information bits obtained from each page of information.

The information contained in the modulated beam can, if desired, be passively mirrored by each of the

mirrors

6 and 8 and applied via the focusing lens 16 to the readout array 12. The focusing lens 16 has focusing properties causing the cross-section of the beam to be increased so that the spacing between the modulation sites within the modulated electron'beam match the spacing between the elements of the readout array 12. The readout array 12 simultaneously senses the electrons in the beam representative all information sites contained in the modulated beam.

When retrieval or reading of the stored information is desired, the modulating mirror 4 does not modulate the electron beam, but simply serves as a passive element to maintain the beam path by merely mirroring the beam for passage to mirror 6. The

deflection units

19 and 20, in accordance with the voltages applied to

units

19 and 20, direct the beam to the desired location or address in the multi-apertured lens 18. Assuming the page of information at the addressed storage area of mirror 6 is to be read or retrieved, the mirror 6 is biased so the beam is spatially modulated by the stored charge pattern at the addressed storage area and mirrored for travel to mirror 8. The beam is directed by the

deflection units

22 and 23 to the addressed location in the multi-apertured lens 21 and is passively mirrored by mirror 8. The beam is then presented to sensing units of the readout array 12 to provide simultaneous readout of all information sites contained in the modulated beam.

In the event the information to be retrieved is located at a storage area of mirror 8, the electron beam is passively mirrored by the electron mirror 6 and then modulated by the pattern of charge at the addressed area of mirror 8 prior to being presented to the readout array 12.

As indicated in connection with the general description regarding the operation of electron mirrors, it is apparent that the voltages presented at the modulating mirror 4 and storage mirrors 6 and 8 determine whether the electron beam will be modulated and mirrored, whether it will impinge on the surface of the mirror or be passively mirrored as is required for the write and read operations that have been described. Details regarding the various voltages used for operation of the apparatus for the write, read and erase operations are set forth in detail in the copending application, now U.S. Pat. No. 3,789,370, referred to earlier as are details for construction of the various components and their assembly to form the apparatus illustrated in FIG. 2.

FIG. 3 is another embodiment of the invention and differs from that shown in FIG. 2 in that a different magnetic prism is used. The prism 10 used in the embodiment of FIG. 3 is formed using two

U-shaped type magnets

61 and 62 which can be of the permanent or electromagnetic type. FIG. 4 is an end view of the prism 10 taken along 44 of FIG. 3. The magnetic polarity shown is that which is required to have the electron beam deflected as indicated by the dotted line path which begins at the electron beam source 11. As is apparent, a prism using two U-shaped magnets as described has a well confined magnetic path which in a multiple electron mirror system reduces the possibility of any interference with the electron beam that may be caused by a magnetic field which is not as well confined.

The reference numerals used in FIG. 2 to identify the various components in that embodiment are applied,

where applicable, to corresponding components used I in the apparatus of FIG. 3. It should be noted, however, I

that the embodiment of FIG. 3 shows the use of more than two

storage mirrors

6 and 8. The additional storage mirrors and their associated deflection units, objective lenses and electron focusing lenses are identified by the addition of a letter to the basic reference numeral. Thus, the additional storage mirrors positioned on the same side of the prism 10 as storage mirror 6 are identified with

reference

6A and 6B. The objective lens for storage mirror 6A is identified by the reference numeral 18A, while

numerals

19A and 20A designate the deflection units for storage mirror 6A. The electron beam focusing lens for storage mirror 6A is identified by the reference 14A. Similar designations are made for storage mirrors 6B. In a similar manner, the additional storage mirrors positioned on the same side of the prism 10 as storage mirror 8 are identified by references 8A and 88.

References

15A, 21A, 22A and 23A identify the electron focusing lens, the objective lens and the two deflection units, respectively, which are used with the storage mirror 8A. The electron focusing lens, objective lens and two deflection units for the storage mirror 88 are identified in a similar manner using references 15B, 21B, 22B and 233, respectively.

Operation of the embodiment of FIG. 3 is substantially the same as that set forth for the embodiment of FIG. 2 except for the number of storage mirrors used and the fact that the degree to which the electron beam is deflected by the prism 10 is substantially less than that required for the FIG. 2 structure. The linear configuration of the apparatus shown in FIG. 3 theoretically permits any number of electron mirrors to be used which is not the case for the circular configuration of FIG. 2.

The

electron focusing lenses

13,14, 15 and 16 for the FIG. 2 embodiment are formed using three concentric rings with holes of appropriate size provided for passage of electron beam with the spacing between the rings, the size of the holes and the voltage applied to the three rings determining the focusing characteristic for the lens. These factors are set forth in detail in the copending application, now U.S. Pat. No. 3,789,370, referred to at the beginning of this description. Threeelement type focusing lenses can also be used for the structure per FIG. 3. The linear configuration, however, permits the use of straight elements rather than ring shaped which greatly simplifies the machining problem. FIG. 5 is a top view of a three element arrangement for the focusing

lenses

14, 14A and 148, for use in the FIG. 3 embodiment. A similar arrangement can be used to provide focusing

lenses

15, 15A and 158.

A multiple electron mirror makes it possible to provide higher image contrast and better resolution, if desired, by reducing the energy spread of the electrons in the beam. This is accomplished by providing an electron mirror which is biased to collect high energy electrons while mirroring the low energy electrons. In the case of the two embodiments shown, the electron beam would be first directed to an electron mirror which is biased to reduce the energy spread. The beam would be mirrored and directed to a modulating electron mirror.

In the light of the above teachings, other alternative arrangements and techniques embodying the invention will be suggested to those skilled in the art. The scope of protection afforded the invention is not intended to be limited to the specific embodiments disclosed, but

is to be determined only in accordance with the appended claims.

What is claimed is: 1. An electron beam system including a magnetic prism; an electron beam source for providing an electron beam and positioned to direct the electron beam to the magnetic prism; a plurality of electrically biased electron mirrors, with one positioned to initiallly receive the electron beam from the magnetic prism, said one electron mirror biased to return the electron beam to the magnetic prism and another of said plurality of electron mirrors positioned to receive the electron beam returned to the magnetic prism by said one electron mirror. 2. An electron beam system is accordance with claim 1 wherein said plurality of electron mirrors are positioned about said magnetic prism.

3. An electron beam system in accordance with claim 1 wherein said magnetic prism is positioned centrally of said plurality of electron mirrors.

4. An electron beam system in accordance with claim 2 wherein said magnetic prism includes a set of Helmholtz coils.

5. An electron beam system in accordance with claim 1 wherein said magnetic prism includes two magnets, each having two spaced poles of opposite magnetic polarity, said magnets positioned with the poles of one magnet opposite and magnetically opposed to the poles of the other magnet.

6. An electron beam system in accordance with claim 5 wherein said two magnets are U-shaped.

7. An electron beam system in accordance with claim 5 wherein said magnets are permanent magnets.

8. An electron beam system in accordance with claim 5 wherein said magnets are electromagnets.

9. An electron beam system in accordance with claim 5 wherein said magnetic prism has one side defined by one set of opposed poles of said two magnets and a second side defined by the other set of opposed poles of said two magnets with at least one of said plurality of electron mirrors positioned on said one side of said magnetic prism and at least another one of plurality of electron mirrors positioned on said second side of said magnetic prism.

10. An electron beam system in accordance with claim 1 wherein at least one of said plurality of electron mirrors is biased to collect high energy electrons and mirror the low energy electrons whereby the energy spread of the electron beam is reduced.

Claims

1. An electron beam system including a magnetic prism; an electron beam source for providing an electron beam and positioned to direct the electron beam to the magnetic prism; a plurality of electrically biased electron mirrors, with one positioned to initiallly receive the electron beam from the magnetic prism, said one electron mirror biased to return the electron beam to the magnetic prism and another of said plurality of electron mirrors positioned to receive the electron beam returned to the magnetic prism by said one electron mirror.

2. An electron beam system is accordance with claim 1 wherein said plurality of electron mirrors are positioned about said magnetic prism.