WO1988002180A1

WO1988002180A1 - Differential pressure electron beam system, method and gun

Info

Publication number: WO1988002180A1
Application number: PCT/US1987/002318
Authority: WO
Inventors: Albert V. Crewe
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-09-18
Filing date: 1987-09-10
Publication date: 1988-03-24
Anticipated expiration: 1989-03-18

Abstract

A differential pressure electron beam system comprising means defining at least a first vacuum enclosure (12) adapted to be maintained at a first vacuum level and a second vacuum enclosure (18) adapted to be maintained at a second vacuum level higher than the first vacuum level. An electron gun generates an electron beam within said first and second enclosures; the electron gun means includes the second vacuum enclosure. Gun mounting means mounts the electron gun for movement within said first vacuum enclosure. Valve means (98) between the first and second vacuum enclosures permits communication between the first and second enclosures when the first enclosure is being pumped, and isolates the second vacuum enclosure from the first when the second enclosure is at a higher vacuum than the first enclosure. Active pumping means pumps the first and second vacuum enclosures to the first vacuum level. Ultra-high vacuum pumping means (94) within or in communication with the second vacuum enclosure pumps only the second vacuum enclosure to said second vacuum level.

Description

DIFFERE_NTIAL PRESSURE ELECTRON BEAM SYSTEM, METHOD AND GUN

Cross Reference to Related Applications

This invention is related to, but in no way dependent upon, my copending applications Serial No. 825,219 . filed F -πary ".. iQftfgerial No.895,199 filed ug. n . 1986 , Serial No. 895,200 , filedAug. π , 1986 _; and Serial No. 895,202 , filed Aug 11, 986. Background of the Invention

It is a primary object of the invention described and claimed in my copending application Serial No. (EBM-1 and EBM-4) to provide an electron beam system having an electron gun capable of developing high enough electron probe current densities to permit high rate, no-develop recording and small enough probe diameters ^"to permit ultra-high density recording, and yet of such low mass and compactness as to make feasible rapid random accessing of any area on the system's recording medium. The gun has use also in electron microscopy and other electron beam systems. In order to achieve such high electron probe current density and high rate no-develop recording in a gun of ultra-low mass and compactness, there exists a requirement for an extremely small and bright point source of electrons. At the present it appears that only field emission cathodes can provide the needed source brightness and small source size. Field emission sources, however, require an ultra- high vacuum ("UHV") environment for stable long-term

_ Q operation — for examp l e in the range of 1 0 to 1 0"^{1 0} torr.

I n an e l ectron beam system of the type described in my above-identified copending applications, the electron beam head traverses over the recording medium. The evacuated volume thus must be quite large. This places a great burden on i> the construction and operation of such an electron beam system to develop vacuum levels of 10 —9 to 10—1 π torr throughout the system enclosure. In applications where changing of the recording medium is frequent, the need to re-evacuate the entire 0 system enclosure to 10~⁹ to 10^-1 torr after devacuation would constitute a serious limitation on the usefulness of such a system, as well as greatly increasing its capital cost and cost of^"operation. Prior Art 5 ϋ. S. Patent No. 4,074,313 discloses an electron beam recording system in which a monolithic recording column is trundled on a carriage over a rotated disc recording medium. The entire recording chamber must be devacuated. This approach is alleged 0 in the patent, however, as representing a significant improvement over prior systems in which the electron beam column is stationary and the turntable is moved under the electron beam. The '313 system would appear to be completely impractical for use with a 5 field emission source because of the huge volume within the recording chamber that would have to be evacuated to ultra-high vacuum levels.

Patent '313 states, in column 5, "A diffusion pump 140 is connected to the recording 0 chamber 74 for evacuation thereof. Another diffusion pump 142 (FIG. 2) is coupled to the recorder column 40 for its evacuation. The diffusion pumps 140 and 142 provide a vacuum environment for the electron beam recording operation on the electron-responsive 5 disc master 32." Differential pressure systems in electron microscopy are well known, as field emission systems have required development of techniques for developing the ultra-high vacuum (10 —9 to 10—10 torr) required for long term stable operation of field emission sources. In that connection see: USP 4,020,353 USP 4,066,905

The Development of a Field Emission Scanning Electron Microscope, Swann et al, Scanning Electron Microscopy/1973 (Part I), Proc. of the Sixth Annual Scanning Electron Microscope Symposium, 57-63.

Field Emission Scanning Electron Microscope With Parallel Plate Electrodes; Shimizu et al, Scanning Electron Microscopy/1973 (Part I), Proc. of the Sixth Annual Scanning Electron Microscope Symposium, pp. 57-63. ϋ. S. Patent 3,678,333 concerns a field emission electron gun for electron microscopes and discloses the use of a getter for pumping an ultra- high vacuum region within the gun.

The getter is of the sublimator type and would be completely unsatisfactory in the electron gun with which this invention is concerned due to its contamination of the containing vacuum chamber with conductive material with the inevitably attendant electrical discharge problems. Further, the getter- pumped volume in the gun of this patent is unnecessarily large. See also related patents Nos. 3,766,427 and 3,784,815. Other Prior Art

USP 3,350,503 RE: 30,974

USP 3,750,117 USP 4,001,493

USP 4,306,375 USP 4,534,016 USP 3,705,262 USP 4,010,318 The Current State of High Resolution Scanning

Electron Microscopes, A. V. Crewe, Quart. Rev. of

Biophysics, 3 (1970), pp. 1-54.

Objects of the Invention It is an object of this invention to provide an electron beam system having an ultra compact electron gun which utilizes an ultra-high vacuum cathode to attain high electron probe current densities and very small probe sizes, and yet which does not require evacuation of the entire system enclosure to ultra-high vacuum levels.

It is an object of the invention to provide an electron beam system and method providing allocated zones of different pressure to reduce the system's vacuum pumping requirements.

It is an object to provide for such a system an electron gun having a UHV (ultra high vacuum) cathode, the UHV volume containing which is. extremely small, with attendant savings in the weight, size and cost of the UHV pump and associated gun parts.

It is another object to provide a differential pressure electron beam system and gun utilizing a non-evaporable getter for at least assisting pumping an enclosure containing the gun's cathode to a higher vacuum than the ambient vacuum of the gun.

It is another object to provide a method and apparatus for quick-pumping such a system and electron gun to appropriate differential vacuum levels.

Brief Description of the Drawings

FIGURE 1 is a highly schematic illustration of an electron beam memory system constructed according to the teachings of the present invention; FIGURE 2 is an enlarged perspective view of a portion of FIGURE 1;

FIGURE 3 is a perspective, partly sectioned view of a head constituting part of the FIGURES 1 and 2 system and containing a writing or reading electron gun implementing an aspect of the present invention;

FIGURE 4 is a fragmentary sectioned side elevation view of the electron beam memory system head and electron gun shown in FIGURES 1-3; FIGURE 5 is a diagram showing the manner in which an electron beam is deflected using integrated deflection coils constituting part of the FIGURES 1-4 electron gun assembly;

FIGURE 6 is a side elevational view of a valve shown in FIGURE 4; and

FIGURE 7 is a plan view of a getter device shown in FIGURE 6. ^'Description of the Preferred Embodiment

FIGURE 1 is a schematic view of an electron beam memory system 10 embodying the present invention. The FIGURE 1 system 10 is shown as including a high vacuum enclosure 12. Within the enclosure 12 is a storage medium 14 supported on a rotatable disc 16. The electron beam memory system 10 includes an electron beam head 18. The head 18 could be used for recording information on medium 14. Alternatively, it could be used to read information previously stored on medium 14. My invention would be equally useful in a system containing a plurality of electron beam heads adapted for simultaneous operation. For example, three heads could be employed -- a writing head containing an electron gun for recording information, a verification head containing an electron gun for verifying the fact and integrity of the stored information, and a reading head containing an electron gun for retrieving the stored information.

Means mounting the head 18 for movement across the disc 16 include a- radially arranged linear track 20 and a head carriage 22 which moves along the track 20 in response to carriage drive signals received through a bundle of conductors 24.

Auxiliary electronic and electrical apparatus, shown schematically at 26, provides the necessary carriage drive signals through conductors 24. As will be explained, apparatus 26 also supplies the necessary heater current for field emission source heater and energization potentials for the gun electrodes through the bundle of conductors 24.

FIGURES 2-4 illustrate an electron gun 28 constituting part of head 18 and described and claimed in my referent copending application Serial No. (EBM-4). The FIGURES 2-4 gun is capable of developing a finely focused electron beam probe at high beam current densitites, yet is ultra-compact and of extremely low mass. The electron gun of the aforesaid invention makes possible a truly random accessed electron beam memory system for high rate, ultra-high density electron beam data recording, and yet with recording power making possible no-develop recording, i.e., recording without the need for developing the recording medium after exposure. Electron gun assembly 28 develops an extremely fine electron beam probe 30 of sufficiently great current density to write (record) tracks 32 on the storage medium 14.

The electron gun assembly 28 comprises an extremely axially compact structure, all components of the gun being optimized for axial compactness and minimization of mass. As will be described, the series .of structures making up the electron gun assembly 28 are all essentially ring-like elements of metal or ceramic adapted to be brazed together by well-known techniques to make an extremely rugged and hermetically sealed structure.

The electron gun with which this invention is used preferably utilizes a field emission cathode. The field emission cathode assembly shown in FIGURES 3-4 is described and claimed in my copending application Serial No. (EBM-6). The field emission cathode assembly comprises two metal rings 44, 46 between which is sandwiched a ceramic ring 48. The metal rings 44, 46 and ceramic ring 48 define circular apertures 52, 54, and 56, respectively. The rings 44, 46 are brazed to the ceramic ring; the metal ring 44 is welded to head housing 57 to form a rigid hermetically sealed assembly.

A filament 58 extends substantially diagonally across the apertures 52, 54, 56 formed in the rings 44, 46, 48 and has welded medial ly thereof a field emission tip 60. The tip 60 is aligned on the electron optical axis of the gun. When excited by an appropriate electrical potential, the tip 60 produces an extremely small and intensely bright source of electrons.

The metal rings 44, 46 serve as expedient terminals for the application of filament heating currents for use in applications wherein the field emitter is heated. The ends 62, 64 of the filament

58 are welded to the rings 44, 46, respectively, from which rings the filaments may derive a source of appropriate electrical potential for extracting electrons from the tip 60. The use of rings 44, 46 as terminals for the application of heate'r currents has a decided advantage over bringing wire leads in through the enclosure for the field emitter. They provide a mechanically rugged, hermetically sealed assembly and, of equal importance, large area 5 terminals which serve to dissipate heater current heat over a large terminal area, and thus reduce the possibility of overheating. The use of a filament extending diagonally across the openings, rather than the common "hairpin" filament, has the further

10 advantage of being less susceptible to vibration. The electron gun assembly 28 further includes an electrostatic focus lens for forming a real image of the electron source produced at the field emission source in the vicinity of the

-*-5 recording medium 14. An electrostatic lens is illustrated as comprising a first electrode 66 , a second electrode 68 and a third electrode 70. The first and second ^'electrodes are dished disc ^■ . electrodes. The third electrode is a flat-disc

20 electrode. The electrodes are spaced from each other by ceramic insulators 72, 74. The electrodes are spaced from the field emission cathode assembly by a ceramic insulator 76. The electrodes 66, 68 and the insulators 72, 74 and 76 are brazed together to

25 define a mechanically sound, hermetically sealed assembly.

The field emitter is adapted to receive a predetermined first electrical potential effective to- form a high brightness electron source at tip 60. The

30 first electrode 66 is adapted to receive a predetermined second electrical potential which is positive relative to the potential on the tip 60 and has a value effective to extract electrons from the tip 60. The first electrode 60 has a very small

35 aperture 78 for determining the diameter of the electron beam which is formed.

The second electrode 68 is adapted to receive an adjustable third, focusing electrical potential which is negative relative to the aforesaid second potential applied to the first electrode 66. The second electrode 68 defines an aperture 80 which is much larger than the aperture 78 in electrode 66.

The third electrode 70 is adapted to receive a fourth, accelerating electrical potential which is positive relative to the potential applied to the second electrode 68. The third electrode 70 defines an aperture 82 which is substantially larger than the aperture 78 in the first electrode 66, but smaller than the aperture 80 in the second electrode 68.

The second, third and fourth electrical potentials applied to the electrodes 66, 68 and 70 are selected to establish beam-focusing fields between the first and second electrodes 66, 68 and between second and third electrodes 68, 70.

The ultra-high vacuum enclosure defined by the first electrode 66, the rings 44, 46 and 48 and the head housing 57 comprise an ultra-high vacuum volume within which the field emitter is maintained at an appropriate vacuum level -- typically 10 to 10 torr. The restricted size of the aperture 78 in the first electrode is selected, in part, with consideration for maintaining the high vacuum in the zone containing the field emitter. in accordance with an aspect of this invention, the UHV volume containing the cathode is extremely small, thus minimizing the size, mass and cost of the UHV pump (described below) and the associated gun parts. Because of the necessity of minimizing the magnification of the electron source formed by the field emitting tip 60, the magnification of the source at the storage medium 14 is preferably about .5-2.0. The objectives of system compactness and small probe sizes implies the use of relatively short object and image distances. As will become evident when the dimensions of the system are described below, these objectives leave insufficient room at the exit of the gun to use conventional beam deflection and stigmatizing systems.

The electron gun assembly 28 includes an integrated beam deflection and stigmatizing system following the principles^'of an invention described and claimed in my copending application Serial No. (EBM-5). The beam deflection/stigmatizing system is illustrated as comprising magnetic field generating means for establishing plural fields of magnetic flux through the electrostatic lens for modifying the position and cross-sectional shape of the beam. As shown in FIGURE 3, the magnetic field generating means comprises a system of magnetic windings 84 configured on an electrically insulative cylindrical sleeve-like mandril 86 surrounding the lens assembly. The windings 84 are configured to provide both X and Y beam deflection as well as quadrupolar beam stigmatizing. The system of windings 84 is also configured to create stigmatizing fields extending through the electrostatic lens and so defined as to correct cross-sectional asymmetries of the beam as it passes through the lens.

FIGURE 5 depicts schematically the manner in which electron beam probe 30 is deflected within the electrostatic lens system itself to effect movement of the probe across the storage medium 14. - It will be understood that in operation of the electron beam memory system, the head 18 is moved across the disc 16 to attain the gross positioning of electron beam during reading or writing operations. Fine positioning of the beam is achieved by use of the system of windings 84.

In order to better understand the manner in which deflection takes place in accordance with the teachings of this invention, it should be understood that the aperture 78 in the first electrode 66 is very small compared to apertures 80 and 82 in the second and third electrodes 68 and 70. By way of example, the aperture 78 may be only 6-20 microns radius compared with 1.2 millimeters for the aperture 80 and .4 millimeters for the aperture 82.

It can be seen, then, that the aperture 78 causes the beam to have a relatively small diameter compared to the diameter of the apertures 80 and 82.

It should further be understood that in the particular application illustrated, it is only necessary to have a deflection capability of perhaps 10-20 microns — a distance sufficient to span perhaps 100 tracks on the storage medium 14 — a deflection completely adequate for recording or for locating and/or following a particular track on the storage medium 14. It can readily be seen that movement of a beam having a radius of 6-20 microns in apertures having radii of 1200 and 400 microns will create no problem with interference between the beam and the electrodes 68, 70.

To establish and maintain a high vacuum within the system enclosure 12, there is provided a high vacuum shown schematically at 88 which evacuates the enclosure 12 through an opening 90 in the enclosure 12. Pump 88 will be discussed in more detail below.

It is an object of this invention to provide an electron beam system and method providing allocated zones of differential pressure to reduce the system's vacuum requirements. As described briefly above, in accordance with this invention there is provided within the electron gun 28 an ultra-high vacuum source enclosure 29. In the illustrated embodiment, the enclosure 29 is defined by the first electrode 66, the rings 44, 46 and 48 and the head housing 57. In accordance with an aspect of this invention, a different pressure or vacuum level is established within the enclosure 29 within the gun to accommodate higher vacuum requirements which may be imposed by the type of cathode employed. As noted above, the cathode employed is a field 'emission cathode. A field emission ^'cathode, however, requires" an ultra-high vacuum for long-term stable operation — typically in the order of 10~⁹ to 10~¹⁰ torr.

In accordance with an aspect of this invention, vacuum pumping means within or in communication with the vacuum source enclosure 29 within the gun is provided for at least assisting in pumping the source enclosure to a predetermined vacuum level higher than the vacuum level within the system enclosure.

In the illustrated preferred embodiment, the vacuum source enclosure 29 establishes a substantially closed volume wholly within the gun and the vacuum pumping means is a getter located within the vacuum source enclosure. In the illustrated preferred embodiment, the getter 92 is of the non- evaporated porous type (described in more detail below). A getter of this type is capable of pumping the relatively small volume ultra-high vacuum source enclosure defined within the gun to a predetermined ultra-high vacuum level in the range of 10^-9 to 10~¹⁰ torr. The restricted size of the aperture 78 in the first electrode is selected, in part, with consideration for maintaining the ultra-high vacuum in the volume 29 containing the field emitter tip 60. The aperture 78 is of such small radius as to make possible the maintaining of a significant differential vacuum across the electrode 66 — specifically, a higher vacuum in the source enclosure 29 than in the larger system enclosure — 10 to 1000 times higher, for example.

By way of example, getter 92 may be a non- evaporable porous getter such as manufactured by SAES Getters S.p.A. of Milano, Italy. Such non-evaporable porous getters may use zirconium powder as active material, sintered at high temperature with graphite powder, conferring the required characteristics of high porosity and the large surface area as well as good mechanical strength. In the use of such getters at high temperatures (for example about 300 degrees C), gas sorption is not limited to the surface of the zirconium grains but includes diffusion into the bulk, resulting in superior gettering action.

As shown in FIGURES 4 and 7, the getter 92 may take a cylindrical form; other shapes are available. In order to activate the material, the getter material must be heated under vacuum for a time sufficient to remove from the surface of the getter material the thin protective layer formed at room temperature during the first exposure to air at the end of the manufacturing process. Ful 1 activation of the getter material is obtained by heating the getter material under a vacuum better than 10 torr at, for example, 900 degrees C for 10 minutes. Other combinations of temperature and time are possible to produce complete activation. When only a limited activation time or a low heating temperature are possible, reasonable sorption characteristics can be achieved by a partial activation of the getter. Activation can also be achieved by an intermittent heating of the gettering material provided the cumulative heating time is the same as that of a continuation activation. In this way localized over-heating of nearby parts can be minimized.

The pumping speed of non-evaporable porous getter of the type described tends to decrease with the progressive sorption of gaseous species until it eventually can become too low to cope with the degassing rate of the device in which the getter is mounted. By reheating the getter material, a process called reactivation, it is possible to restore the^* pumping efficiency of the getter. The reactivation is performed by heating the material at a temperature slightly below or equal to the activation temperature (for example 800 degrees-900 degrees C). The time necessary for reactivation is usually shorter than for activation. Such non-evaporable porous getters can be reactivated several times. It is believed that such non-evaporable porous getters as described are capable of pumping an ultra-high vacuum source enclosure in the volume and out-gassing characteristics described above to ultra-high vacuum

_q —in levels in the range of 10 to 10 torr and maintaining that vacuum for hundreds of hours without reactivation.

As shown in the illustrated preferred embodiment, getter 92 comprises a mass 94 of getter material supported by a heater element 96. To activate or reactivate the getter mass 94, it is only necessary to pass sufficient heater current through the heater element 96. The manner in which this may be accomplished will be described in some detail below.

The capacity of the UHV vacuum (getter 92) pump and the HV pump 88 depends on a number of factors — the outgassing rate of the surface of the enclosure materials is an important factor. By way of example, steel has an outgassing rate of 10~H torr, stainless 10^~12 torr, viton 10~1°, quartz

10~¹⁴, copper 10^"11 and Teflon 10~⁹ (values given are those after extensive baking of the materials). A very particular outgassing problem results from gas driven off the first electrode 66 by the electrons emitted by the tip 60. This can be of concern when the anode is first used. One can then expect one microamp of emitted electrons to produce 10 torr liters per second of gas. When the anode is clean this can be reduced to 10 —9 torr liter per second or perhaps lower.

In the differential pumping system disclosed and claimed herein, by way of example, the high vacuum pump 88 establishes a vacuum in the

6 ^Q system enclosure in the range of 10- to 10- torr. The ultra-high vacuum pump (getter 92) establishes an

Q ultra-high vacuum in the source enclosure of 10-^ or 10- , for example. Assuming those vacuum levels, the use of materials such as molybdenum for the lens electrodes, gun parts, and head housing 57, and assuming an aperture 78 in the first electrode 66 of 12-40 microns diameter, a high vacuum pump 88 having a capacity in the range of .01 liter per second would be acceptable; the ultra-high vacuum pump (here getter 92) could have a capacity of about 2 liters per second.

It is an aspect of this invention to provide apparatus and method for quick pumping the aforedescribed differential pressure electron beam system. This aspect of the invention includes providing at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure adapted to be maintained at a second vacuum level higher than the first vacuum level. An electron beam is generated within the first and second enclosures. The first and second vacuum enclosures are pumped to the first vacuum level while causing the first and second enclosures to be in communication. The second vacuum enclosure (very, very small in volume compared with the first enclosure) is then isolated from the first enclosure and it exclusively is pumped to the said second (higher) vacuum level. This quick pumping of the system obviates the UHV pump having to pump the ultra-high vacuum source enclosure from atmosphere to the full 10~⁹ to 10~¹⁰ torr vacuum level after the system is devacuated (for example to change recording media). Using the larger capacity high vacuum pump to pump both the HV system enclosure and the UHV source enclosure to a predetermined high vacuum level

(for example 10 — fι to 10—7 torr), necessitates then that the ultra-high vacuum pump need only pump from that level to the ultra-high vacuum level required for the cathode, when such an ultra-high vacuum cathode is utilized.

To the end of implementing such a quick pumping system and method, valve means are provided. in the illustrated preferred embodiment, the valve means take the form of a spring biased leaf valve 98. The leaf valve 98 is affixed to end plate 100 on head housing 57. FIGURE 4 shows the leaf valve 98 in its open position; it would not normal ly be in the open position, being biased closed against the end plate 100.

The leaf valve 98 permits communication between the system enclosure 12 and the ultra-high vacuum source enclosure 29 within the gun when the enclosure 12 is being pumped. It isolates (when in the closed position) the ultra-high vacuum source enclosure 29 from the system enclosure 12 when the source enclosure 29 is at a higher vacuum than the system enclosure 12. Means responsive to the movement of the gun opens the leaf valve 98 to permit evacuation of the ultra-high-vacuum source enclosure by the high vacuum pump 88. Means are also provided which are responsive to subsequent movement of the gun for subsequently closing the leaf valve 98 to isolate the system and source enclosures in preparation for activating the getter 92.

In operation, when it is desired to devacuate the system and source enclosures to high vacuum and ultra-high vacuum levels, respectively, the head 18 is moved as part of the carriage 22 to its limit position against the wall of enclosure 12. Specifically, in accordance with this invention, the aforesaid means responsive to the movement of the gun for opening the valve means comprise projections 102, 104 which pass through opening 106 in end plate 100 and deflect open the leaf valve 98, bringing the ultra-high vacuum source enclosure into communication with the system enclosure. The high vacuum pump 88 is then energized to evacuate the system enclosure 12 and the source enclosure within the gun 28.

When the source and system enclosures have been pumped to a predetermined high vacuum level, which may be in the range of 10 —6 to 10—7 torr, the head 18 is backed away from the wal 1 of the enclosure

12 to permit the projections 100, 102 to disengage the leaf valve 98, causing the valve to close. The getter 92 will then automatically pump the source enclosure to a predetermined ultra-high vacuum level. It wil 1 be necessary rom time to time to reactivate the getter mass 94. Means responsive to movement of the gun are provided for connecting the getter to a source of energy to activate the getter mass 94. In the illustrated preferred embodiment, this means takes the form of terminals 108, 110 mounted in the wall of enclosure 12 which engage female terminals 112, 114 on the ends of the heater element 96. With the terminals 108, 110 in engagement with terminals 112, 114 on the heater element 96, a source of heater current may be applied to the getter 92 to heat the getter mass 94 to an appropriate activation temperature.

Thus by this method, the system and source enclosures can be quickly pumped to high vacuum levels by the high capacity high vacuum pump 88. Further evacuation of the source enclosure 29 will then be automatically accomplished by the getter 92. Activation of the getter is not necessary each time the system is pumped down. The pumping capacity of the getter 92 is such that the system may be operated for hundreds of hours and devacuated and re-evacuated many times without having to reactivate the getter.

To reiterate, it should be understood that the head is moved to its limit position on two different occasions, not necessarily related to each other. On one occasion the head is moved to its limit position to effect a quick pump down of the system and source enclosures. On this occasion the movement to the limit position is for the purpose of 5 having the projections 102, 104 open the leaf valve 98 to permit the high vacuum pump 88 to quick pump the system and source enclosures to a high vacuum level. The head is then moved away from the enclosure 12, permitting tht leaf valve 98 to close 0 and the getter 92 to automatically begin to pump the ultra-high vacuum source enclosure to a level of ultra-high vacuum. So long as the leaf valve is in the open position, the source and system enclosures are in communication and the getter 92 is unable to 5 pump the source enclosure to ultra-high vacuum levels. With the leaf valve 98 closed, however, the source enclosure is isolated from the system enclosure, and

- the relatively small ultra-high vacuum volume within the gun will be pumped to 10 —9 to 10—10 torr. The second occasion for moving the head to its limit position against the wall of enclosure 12 is for the purposes of activating or reactivating the getter. The fact that at this time the projections 102, 104 open the leaf valve 98 is incidental. On this occasion the movement of the head to the limit position is only for the purpose of engaging the terminals 108, 110 with female terminals 112, 114 to permit activation or reactivation of the getter mass 94. The getter mass retains its pumping power long after the source of heater current is removed.

It wil 1 be understood that an aspect of this invention is a novel electron gun which includes means defining a vacuum enclosure within the gun, cathode means within the vacuum enclosure for forming a source of electrons and getter means within the vacuum enclosure for at least assisting in pumping the enclosure to a predetermined vacuum level. Such a gun will have applications other than to electron beam recording systems. The above embodiments are included as being illustrative and it is contemplated that other structures and methods may be devised to practice the teachings of the invention. For example, electron guns of many different types may be adapted to utilize the differential pressure aspect of the invention in the method and system of quick pumping the system and source enclosures. Many valving structures other than the leaf valve 98 may be employed. Further, means other than the arrangement shown may be employed for selectively connecting the getter to a source of activation energy. For example, the getter may be energized through permanently connected conductors forming part of the bundle of conductors 24 through which the current for the cathode heater, excitation potentials for the electrodes, signals for the carriage drive means, etc., are supplied.

Means other than a getter may be employed for pumping the source enclosure to ultra-high vacuum levels. For example, an ultra-high vacuum pump located within or outside the system enclosure which is in communication with the source enclosure may be employed for the purpose of pumping the source enclosure to predetermined higher levels of vacuum than surrounds the gun within the system enclosure. The aforedescribed principles of this invention may be employed in systems other than electron beam recording systems — for example, electron beam lithography systems and certain electron microscopes. The following claims are intended to cover not only the illustrated structures but other structures which utilize my teachings.

Claims

1. A differential pressure electron beam system comprising: means defining at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure disposed within said first vacuum enclosure and very small in volume relative thereto and adapted to be maintained at a second vacuum level higher than said first vacuum level; means for generating an electron beam within said first and second enclosures; vacuum pumping means for pumping said first and second vacuum enclosures concurrently to said first vacuum level; and second pumping means within or in communication with said second vacuum enclosure for pumping only said second vacuum enclosure to said second vacuum level.

2. A differential pressure electron beam system comprising: means defining at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure adapted to be maintained at a second vacuum level higher than said first vacuum level; means for generating an electron beam within said first and second enclosures; active pumping means for pumping at least said first vacuum enclosure to said first vacuum level; and non-evaporable porous getter means within or in communication with said second vacuum enclosure for passively pumping said second vacuum enclosure to said second vacuum level .

3. A differential pressure electron beam system comprising: means defining at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure within said first enclosure adapted to be maintained at a second vacuum level higher than said first vacuum level; means for generating an electron beam within said first and second enclosures; valve means between said first and second vacuum enclosures for permitting communication between said first and second enclosures when said first enclosure is being pumped, and for isolating said second vacuum enclosure from said first when said second enclosure is at a higher vacuum than said first enclosure; first, relatively high capacity vacuum pumping means for pumping said first and second vacuum enclosures concurrently to said first vacuum level; and second pumping means within or in communication with said second vacuum enclosure for pumping only said second vacuum enclosure to said second vacuum level .

4. A differential pressure electron beam recording system comprising: means defining at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure disposed within said first enclosure and adapted to be maintained at a second vacuum level higher than said first vacuum level ; means for generating an electron beam within said first and second enclosures; valve means between said first and second vacuum enclosures for permitting communication between said first and second enclosures when said first enclosure is being pumped, and for isolating said second vacuum enclosure from said first when said second enclosure is at a higher vacuum than said first enclosure; active pumping means for pumping said first and second vacuum enclosures to said first vacuum level; and non-evaporable getter means within or in communication with said second vacuum enclosure for passively pumping only said second vacuum enclosure to said second vacuum level .

5. A differential pressure electron beam system comprising: means defining at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure adapted to be maintained at a second vacuum level higher than said first vacuum level; electron gun means for generating an electron beam within said first and second enclosures, said electron gun means including said second vacuum enclosure; gun mounting means for mounting said electron gun means for movement within said first vacuum enclosure; valve means between said first and second vacuum enclosures for permitting communication between said first and second enclosures when said first enclosure is being pumped, and for isolating said second vacuum enclosure from said first when said second enclosure is at a higher vacuum than said first enclosure; active pumping means for pumping said first and second vacuum enclosures to said first vacuum level; means responsive to movement of said gun means by said gun mounting means for opening said valve means to permit evacuation of said second vacuum enclosure by said active pumping means to said first vacuum level and responsive to subsequent movement of said gun means by said gun mounting means for subsequently closing said valve means; and ultra-high vacuum pumping means within or in communication with said second vacuum enclosure for pumping only said second vacuum enclosure to said second vacuum level.

6. A differential pressure electron beam system comprising: means defining at least a first vacuum enclosure adapted to be maintained at a first vacuum level' and a second vacuum enclosure adapted to be maintained at a second vacuum level higher than said first vacuum level; electron gun means for generating an electron beam within said first and second enclosures, said electron gun means including said second vacuum enclosure; gun mounting means for mounting said electron gun means for movement within said first vacuum enclosure; active pumping means for pumping said first and second vacuum enclosures to said first vacuum level ; getter means within or in communication with said second vacuum enclosure for passively pumping only said second vacuum enclosure to said second ^'vacuum level; and means responsive to movement of said gun means by said gun mounting means for connecting said getter means to a source of energy to activate said getter means.

7. A differential pressure electron beam system comprising: means defining at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure adapted to be maintained at a second vacuum level higher than said first vacuum level; electron gun means for generating an electron beam within said first and second enclosures, said electron gun means including said second vacuum enclosure; gun mounting means for mounting said electron gun means for movement within said first vacuum enclosure; valve means between said first and second vacuum enclosures for permitting communication between said first and second enclosures, when said first enclosure is being pumped, and for isolating said second vacuum enclosure from said first when said second enclosure is at a higher vacuum than said first enclosure; active pumping means for pumping said first and second vacuum enclosures to said first vacuum level; getter means within or in communication with said second vacuum enclosure for passively pumping only said second vacuum enclosure to said second vacuum level; first means responsive to movement of said gun means by said gun mounting means for connecting said getter means to a source of energy to activate said getter means; and second means responsive to movement of said gun means by said gun mounting means for opening said valve means to permit evacuation of said second vacuum enclosure by said active pumping means to said first vacuum level and responsive to subsequent movement of said gun means by said gun mounting means for subsequently closing said valve means.

8. The combination defined by claim 7 wherein said getter means is a non-evaporable getter having heater terminals and wherein said first means responsive to movement of said gun means comprises mating terminals mounted on said first enclosure which make electrical connection with said heater terminals when said gun means is moved to one limit of its movement .by said gun mounting means.

9. The combination defined by claim 7 wherein said valve means.is a spring-biased leaf valve and wherein said second means responsive to movement of said gun means is a projection from a wal 1 of said first enclosure means, and wherein said leaf valve is actuated by said projection when said gun means is moved by said gun mounting means to one limit of its movement within said first enclosure.

10. For use with a differential pressure electron beam system, the method comprising: providing at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure disposed within said first vacuum enclosure and of very small volume relative thereto and adapted to be maintained at a second vacuum level higher than said first vacuum level; generating an electron beam within said first and second enclosures; concurrently pumping said first and second vacuum enclosures to said first vacuum level while causing said first and second enclosures to be in communication; and isolating said second enclosure from said first enclosure and pumping only said second vacuum enclosure to said second vacuum level.

11. For use with a differential pressure electron beam system, the method comprising: providing at least a first vacuum enclosure adapted to be maintained at a first vacuum level and a second vacuum enclosure located within said first enclosure adapted to be maintained at a second vacuum level higher than said first vacuum level; generating an electron beam within said first and second enclosures; pumping said first and second vacuum enclosures to said first vacuum le_/vel while causing said first and second enclosures to be in ' communication; isolating said second enclosure from said first enclosure; and passively pumping only said second vacuum enclosure to said second vacuum level with non- evaporable getter means located within said second enclosure.

12. For use with a differential pressure electron beam recording system, the method comprising: providing at least a first system vacuum enclosure adapted to be maintained at a first vacuum level; providing within said system enclosure a recording medium and an electron gun for writing or reading information on said medium; - providing wholly within said gun a second vacuum enclosure adapted to be maintained at a second vacuum level higher than said first vacuum level; concurrently pumping said first and second vacuum enclosures to said first vacuum level while causing said first and second enclosures to be in communication; and isolating said second enclosure from said first enclosure and pumping only said second vacuum enclosure to said vacuum level.

13. In a rapid random accessed electron beam recording system, the combination comprising: means supporting an information storage medium; an electron beam head containing an electron gun; carriage means for supporting said head for controlled movement across said medium; a high vacuum system enclosure enclosing the aforesaid combination and high vacuum pump means for pumping said system enclosure to a predetermined high vacuum level; said electron gun including: means defining a vacuum source enclosure within said gun; cathode means within said source enclosure for forming a source of electrons; and vacuum pumping means within or in communication with said vacuum source enclosure for at least assisting in pumping said source enclosure to a predetermined vacuum level higher than said predetermined high vacuum level within said system enclosure.

14. In a rapid random accessed electron beam recording system, the combination comprising: means supporting an information storage disc; an electron beam head containing an electron gun; carriage means for supporting said head for controlled movement across said disc; a high vacuum system enclosure enclosing the aforesaid combination and high vacuum pump means for pumping said system enclosure to a predetermined high vacuum level; said electron gun including: means defining a substantially closed ultra-high vacuum source enclosure wholly within said gun; field emission cathode means located within said source enclosure for forming an extremely bright point source of electrons; and vacuum pumping means within or in communication with said vacuum source enclosure for at least assisting in pumping said source enclosure to a predetermined ultra-high vacuum level higher than said high vacuum level within said system enclosure.

15. In an electron beam recording system, the combination comprising: means supporting an information storage medium; an electron beam head containing an electron gun; carriage means for supporting said head for controlled movement across said disc; carriage driving means for moving said head across said disc in response to applied control signals; a high vacuum system enclosure enclosing the aforesaid combination, and high vacuum pump means for evacuating said system enclosure to a predetermined high vacuum level; a cathode for forming an extremely bright point source of electrons, said cathode requiring an ultra-high vacuum environment, ultra-high vacuum source enclosure including septum means on the electron beam axis of the gun for defining an ultra-high vacuum volume within said gun which contains said source, said septum means including a beam-defining aperture of such small radius as to make possible the maintaining of a significantly higher vacuum in said source enclosure than in said system enclosure, and means for imaging said point source in the vicinity of said medium for recording or reading information thereon; and ultra-high vacuum pump means within or in co'mmunication with said ultra-high vacuum source enclosure within said gun for pumping said source enclosure to a predetermined ultra-high vacuum level .

16. The combination defined by claim 15 wherein said ultra-high vacuum pump means comprises getter means located within said ultra-high vacuum source enclosure.

17. The combination defined by claim 16 wherein said getter means is a non-evaporable porous getter.

18. In an electron beam recording system, the combination comprising: means supporting an information storage medium; an electron beam head containing an electron gun; a high vacuum system enclosure enclosing the aforesaid combination; said electron gun including: a cathode for forming an extremely bright point source of electrons, said cathode 5 requiring an ultra-high vacuum environment; ultra-high vacuum source enclosure means including septum means on the electron beam axis of the gun for defining an ultra-high vacuum volume within said gun which contains said source, • 10 said septum means including a beam-defining aperture of such small radius as to make possible the maintaining of a significantly higher vacuum in said source enclosure means than in said system enclosure, and 15 means for imaging said point source in the vicinity of said medium for recording or reading information, thereon;

_* valve means between said system enclosure and said source enclosure means for permitting

20 communication between said system enclosure and said source enclosure means when said system enclosure is being pumped, but for isolating said source enclosure means from said system enclosure when said source enclosure means is at a higher vacuum than said

25 system enclosure; high vacuum pump means for evacuating said system enclosure means and said source enclosure means to a high vacuum level; and non-evaporable porous getter means within

30 or in communication with said source enclosure means for passively pumping only said source enclosure means to an ultra-high vacuum level higher than said high vacuum level .

19. In a rapid random accessed electron

35 beam recording system, the combination comprising: disc means mounted for rotation and supporting an information storage medium; an electron beam head containing an electron gun; carriage means for supporting said head for controlled movement across said disc; carriage driving means for moving said head across said disc in response to applied control signals; a high vacuum system enclosure enclosing the aforesaid combination, and high vacuum pump means for evacuating said system enclosure to a predetermined high vacuum level; said electron gun being characterized by having ultra-low mass and compactness and including: a field emission cathode having an emitting tip for forming an extremely bright point source of electrons, said tip requiring an ultra-high vacuum environment, ultra-high vacuum source enclosure including first electrode means on the electron beam axis of the gun for defining an ultra-high vacuum volume within said gun which contains said tip, said first electrode means including a beam-defining aperture of such small radius as to make possible the maintaining of a significantly higher vacuum in said source enclosure than in said system enclosure, and electrostatic lens means for imaging said point source in the vicinity of said medium for recording or reading information thereon, said lens means including said first electrode means; getter means within or in communication with said ultra-high vacuum source enclosure within said gun for evacuating said source enclosure to a predetermined ultra-high vacuum level and for maintaining said ultra-high vacuum level therein.

20. The combination defined by claim 19 wherein said getter means is a non-evaporable porous getter.

21. In a rapid random accessed electron beam recording system, the combination comprising: disc means mounted for rotation and supporting an information storage medium; an electron beam head containing an electron gun; carriage means for supporting said head for controlled movement across said disc; carriage driving means for moving said head across said disc in response to applied control signals; a high vacuum system enclosure enclosing the aforesaid combination, and high vacuum pump means for evacuating said system enclosure to a predetermined vacuum level in the range of 10 to 10^"7 torr; said electron gun being characterized by having ultra-low mass and compactness and including: a field emission cathode having an emitting tip for forming an extremely bright point source of electrons, said tip requiring an ultra-high vacuum environment, ultra-high vacuum source enclosure including first electrode means on the electron beam axis of the gun for defining an ultra-high vacuum volume within said gun which contains said tip, said first electrode means including a beam-defining aperture of such small radius as to make possible the maintaining of a significantly higher vacuum in said source enclosure than in said system enclosure, and electrostatic lens means including for imaging said point source in the vicinity of said medium for recording or reading information thereon, said lens means including said first electrode means; and a non-evaporable porous getter within or in communication with said source enclosure for pumping said source enclosure to a predetermined ultra-high vacuum level in the range of

or 10—1 π^υ torr.

22. An electron gun comprising: means defining a vacuum enclosure within said gun; cathode means within said vacuum enclosure for forming a source of electrons; and non-evaporable porous getter means within said vacuum enclosure for at least assisting in pumping said enclosure to a predetermined vacuum level .

23. An electron gun comprising: means defining a substantially closed vacuum enclosure wholly within said gun; a cathode located within said vacuum enclosure for forming a point source of electrons; and non-evaporable porous getter means within or in communication with said vacuum enclosure for at least assisting in pumping said enclosure to a predetermined vacuum level higher than the level of the vacuum environment of the gun.

24. An electron gun comprising: means defining within said gun an ultra- high vacuum source enclosure including as one wall portion of said enclosure septum means, said septum means including a beam-defining aperture of such small radius as to make possible the maintaining of a significantly higher vacuum in said source enclosure than in the environment surrounding said gun; a cathode located within said source enclosure for forming an extremely bright point source of electrons, said cathode requiring an ultra- high vacuum environment; and non-evaporable porous getter means within or in communication with said ultra-high vacuum source enclosure for at least assisting in pumping said enclosure to a predetermined ultra-high vacuum level.

25. An electron gun comprising: means defining a vacuum enclosure within said gun; a field emission cathode within said vacuum enclosure for forming a source of electrons; and a non-evaporable porous getter within said vacuum enclosure for at least assisting in pumping said enclosure to a predetermined vacuum level.

26. An electron gun comprising: means defining a substantially closed ultra-high vacuum source enclosure wholly within said gun; a field emission cathode located within said source enclosure for forming an extremely bright point source of electrons, said cathode requiring an ultra-high vacuum environment; and a non-evaporable porous getter within or in communication with said ultra-high vacuum source enclosure for at least assisting in pumping said source enclosure to a predetermined ultra-high vacuum level.

27. An electron gun comprising: means defining within said gun a closed ultra-high vacuum source enclosure wholly within said gun, including as one wall portion of said enclosure septum means, said septum means including a beam- defining aperture of such small radius as to make possible the maintaining of a significantly higher vacuum in said source enclosure than in the environment surrounding said gun; a field emission cathode located within said source enclosure for forming an extremely bright point source of electrons, said cathode requiring an ultra-high vacuum environment; and a non-evaporable porous getter within said ultra-high vacuum source enclosure for at least assisting in pumping said enclosure to a predetermined ultra-high vacuum level significantly higher than the level of the vacuum environment of the gun.