US3480817A - Aperture assembly for electron beam device - Google Patents

Description

Nov. 25, 1969 K. H. LOEFF'LER 3,

APERTURE ASSEMBLY FOR ELECTRON BEAM DEVICE Filed June so, 1967 2 Sheets-Sheet 1 HEATING CURRENT TURNED 0N SPUT SIZE -HEATING CURRENT TURNED OFF T [ME (MINUTES) FIG. 5

INVENTOR KARL H. LOEFFLER ATTORNEY F IG.2

Nov. 25, 1969 K. H. LOEFFLER 3,480,817

APERTURE ASSEMBLY FOR ELECTRON BEAM DEVICE Filed June 30, 1967 2 Sheets-Sheet 2 United States Patent 3,480,817 APERTURE ASSEMBLY FOR ELECTRON BEAM DEVICE Karl H. Loefiler, San Jose, Calif., assignor to International Business Machines Corporation, Armonk, N.Y.,

a corporation of New York Filed June 30, 1967, Ser. No. 650,397 Int. Cl. H01j 29/46 US. Cl. 313-82 7 Claims ABSTRACT OF THE DISCLOSURE An electron beam generating device utilizing heated aperture plates in a removable heated aperture assembly and incorporating a beam-sensing system for detecting the beam condition 'by sensing beam current flow from each of the electrically isolated aperture plates.

CROSS-REFERENCES TO RELATED APPLICATIONS This invention realtes to an Electron Optical Unit similar to that described in the patent application Ser. No. 575,731, filed Aug. 29, 1966 with Karl Loeflle-r et al. as inventors.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to electron beam devices and more particularly to such devices especially adapted for data recording.

Description of the prior art In electron beam generating devices, commonly called columns, one of the primary maintenance problems has been caused by contamination buildup on the beam-exposed parts. While the contamination buildup occurs on all beam-exposed parts, it is greatest on those parts which directly intercept portions of the beam. The one such part where contamination is the greatest problem is the aperture-forming plate. Since the buildup is usually non-conductive, a charge will collect in the contaminant as the beam strikes thereafter to set up random electric fields which deflect, distort or, otherwise adversely affect the beam. Since the aperture plates are directly exposed to and are struck repeatedly by the beam, the charge is built up faster. Because of the location of the plate in the immediate proximity of the beam, the resulting electric field has a very adverse affect on the beam operation. Additionally, the buildup may become so pronounced as actually to reduce the cross-section of the plate aperture opening.

In the usual case, the contamination is caused by organic matter being present within the vacuum cavity of the electron beam device. The general source for this organic material are hydrocarbons used in the pump and on the seals of the vacuum system. To a lesser degree, contaminants can originate from fingerprints or the presence of dust, etc. being present on the column parts before assembly. In the instance of the organic substances, molecules thereof may come to rest on the aperture plates and, in the absense of the electron beam, will build up only to a thickness of a single layer due to the thermal equilibrium condition within the column. However, if the beam strikes the layer, the matter can become chemically bound to the plate to permit other overlayers to form. These polymerized layers of hydrocarbons and other matter substantially are non-conductive and serve to retain the electrical charge resulting from being struck by the beam thereafter to create electric fields which unpredictably defleet the beam. Because of these uncontrolled deflections of the beam or parts of it, the column ceases to function in the manner desired.

It has been known in the past that the formation rate of this organic buildup on the beam-exposed parts can be reduced in order of magnitude by heating the parts. The resulting higher thermal agitation of the molecules lowers the time during which they rest on the plate. In this manner, the possibility of the molecules being struck by the beam and being caused to adhere to the part on which they temporarily come to rest is reduced. For example, it has been found that by heating the aperture plates to a temperature of 250 C. or higher greatly extends the operating life of such columns by slowing the contamination buildup by several orders of magnitude.

However, the fact must be faced that regardless of the precautions taken in designing, assemhlying and operating the column, there usually will be some contamination buildup. Thus, after steps are taken to heat the beam-exposed parts, the life of the column can be extended more by reducing the effects of contaminants on the operation of the column. An electric charge will collect on a con taminant buildup only if the buildup is non-conductive. By rendering the buildup conductive in some manner, the charge will be dissipated substantially as fast as it is formed. For this purpose, the plates are heated to not only slow the buildup rate, but also to lower the resistance to current flow for hastening the dissipation of the charge collected by the plate.

Additionally, where hydrocarbons from the lubricants of the vacuum system form the contaminants, the beam in striking the buildup eventually will carbonize the layers to render them electrically conductive. Because of this desirable possibility, lubricants other than hydrocarbons usually are not used in electron beam columns. With efficient heating of the plates, it is possible to slow the buildup to a rate nearer that of the carbonization process to further extend the useful life of the column. Naturally, with the carbonization process, the layers become electrically conductive to carry away immediately any local electrical charge formed thereon.

Additionally, it is desirable to sense not only whether conditions much as those heretofore described exist preventing the beam from striking the target, but if not, where the beam is being blocked or otherwise cease to exist. If such information is known, repair of the beamgenerating device can be effected more efficiently. Otherwise when the beam is used to record on such materials as light sensitive film, it cannot be determined until after the film is developed whether the actual data recording is being achieved. By this time, much data can be lost if not recorded by the beam. There can be many reasons for beam failure, many of which are caused by devices located external of the column in addition to the contamination of the internally-positioned aperture plates as previously described. Thus, it is desirable immediately upon failure to determine whether the problem can be corrected by checking the external controls, etc. of the device, or whether the column must be disassembled entirely.

One element normally struck constantly by portions of the beam, and therefore useful in detecting the beam presence, is the aperture plate of the column. However, the need to heat the plates to prevent contamination greatly complicates the using of the plates as sensing points for deflecting the beam current to signal the proper operation of the beam. For instance, leakage current from the heating means can be many times the magnitude of the beam current being sensed. In the past, the plates have been heated by various means to include movement of the aperture plate into proximity with a radiant heating element positioned to one side of the beam axis such as described in the US. Patent 3,038,993, Masuda, entitled Aperture System for Electron Optical Instrument. Another method for heating the aperture plates is described in US. Patent 2,898,467, Von Ardenne, entitled Electron Oscillograph. In this patent, electric current is passed directly through each aperture plate with the heating thereof being accomplished by the heat generated because of the resistance to current flow of the aperture plate itself. Obviously, it is diflicult if not impossible to detect the transmission of a small portion of the beam current if a relatively large heating current is being passed through the plate. It is also difficult to provide an aperture plate mounted for repeated movement in such a precision instrument.

The primary object of this invention is to sense accurately the operating condition of an electron beam device.

A further object of this invention is to sense at the various aperture plates positioned along a column the operating condition of the beam while heating the aperture plates to a suflicient temperature to limit the contamination thereof.

A further object of this invention is to provide an improved electrically-isolated aperture plate assembly for an electron beam device.

SUMMARY OF THE INVENTION A beam sensing system for an electron beam device for detecting the operating condition of the beam at varying sensing points along the beam to include sensing the presence of the beam at an improved electricallyisolated aperture-forming plate which cooperates with the lens assembly for forming the beam, which apertureforming plate additionally is heated to limit and render harmless the contamination thereof.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view of an electron beam column incorporating the subject invention with the beam-sensing system being illustrated in schematic form;

FIGURE 2 is a partial cross-sectional view of the non-magnetic sleeve and contact assembly of the column;

FIGURE 3 is an enlarged cross-sectional view of one of the aperture plate assemblies;

FIGURE 4 is an enlarged end view partially cut away of the aperture plate assembly taken along the lines 44 of FIGURE 3; and

FIGURE 5 shows graphically the variations in spot size of the column as the aperture plate is heated and not heated.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION In FIGURE 1 is shown an electron beam column 8 representing one type of beam generating device in which the subject invention can be employed. In the present instance, the column is adapted for use in data recording wherein the beam is directed onto a target or memory element 9 for recording an image. The column comprises an elongated tubular housing 10 with an associated cathode assembly 11 supported at one of the column and serving as an electron sourcef or emitting a beam of electrons of a preselected magnitude or intensity for passage along a column axis 13.

The electron beam is focused to a small spot size by being passed through the magnetic fields of axiallyspaced electromagnetic lenses 14, 15 and 16 positioned in spaced relationship along the axis 13 and within the housing 10. Each of these lenses includes a pair of polepieces 17 and 18 which transmit the magnetic flux generated in the respective electrical coils 15a, 16a and 17a to a point closely adjacent the beam axis 13. A nonmagnetic spacer 19 in each lens maintains the ends of the polepieces adjacent the axis in axially-spaced relationship. The lenses 15 and 16 also include polepiece extensions 20 and 21 separated by a non-magnetic spacer 22 and held in a non-magnetic cylinder 23, which extensions receive the magnetic flux of the respective lenses and cooperate to form the lens magnetic gap at a position closely adjacent to the beam axis 13.

By a proper energization of the coils of each of the lenses 14, 15 and 16, a magnetic field is formed which deflects the electrons of the beam back towards the axis 13 for focusing the beam to a small spot size. The beam thereafter is passed through aperture assemblies 24a, 24b and 240 positioned downstream of the respective lenses. Each aperture assembly includes an aperture plate 27a, 27b and 27c, respectively, in which is formed a small aperture 28a, 28b and 280, at a position coinciding with the beam axis. By first passing the beam through the magnetic field of each lens, and subsequently through the cooperating aperture assembly, the fringe electrons of the beam are intercepted and the beam is formed to a very small cross-sectional diameter or spot size suitable for writing data onto the target 9 at a very high density. The beam is focused in the plane of the target 9 by energizing a focusing coil 30 positioned adjacent the lens 16. By properly adjusting the magnitude of electric current supplied to this focusing coil (from a controllable current source not shown), the focal power of lens 17 is varied for adjusting the focal point of the beam to coincide with the plane of the target. Additionally, an annular-shaped deflection coil 31 is provided which, when energized, serves to deflect and scan the beam across the memory element for recording the data thereon.

To modulate the beam in response to the data being recorded, a pair of electrostatic deflecting plates 32 and 33 are positioned one to each side of the axis 13. By energizing these plates to opposing potentials, the beam is deflected sufficiently to become misaligned with the aperture 270 of the downstream-positioned aperture assembly 240 thereby effectively shutting off the beam. A more complete description of this column can be obtained by reference to the US. patent application Ser. No. 575,731 heretofore identified.

From the foregoing it can be understood that proper functioning of the electron beam column depends upon close control of the beam size and position as it is formed within the column. It is not unusual to form the beam to a spot size of a few microns thereby requiring that the apertures 28 in the aperture plates 27 also be of only a few microns in diameter. Thus, any contamination buildup on the aperture plate in the manner previously described can result in complete or partial failure of the column to function properly. Furthermore, at the high rate at which the column can record data, it is imperative that immediate warning be given in the event of a malfunction of the device so that the recording process can be stopped.

In accordance with the present invention, beam-sensing elements are positioned along the beam axis which serve to generate an immediate signal indicative of the passage of the beam to the location of the sensor thereby indicating the occurrence of and the approximate position of the malfunction if such occurs. The signal from each sensor is fed to a control which can be programmed either manually or automatically to indicate continuously the passage of the beam to each sensing point thereby giving a constant monitoring of the proper operation of the beam and, in the alternative, indicating between which points the beam ceases to pass thereby giving an immediate indication of the type of malfunction existing in the column.

Accordingly, as shown in FIGURE 1, the beam-sensing system comprises a series of electrical conductors 33 extending from a beam control 34 to a series of resistors 35 connected at one end to ground and at the other end to various sensing elements along the electron beam column by conductors 36. Thus, the elements are positioned to intercept portions of the beam such that any beam current striking the elements is transmitted through the conductor 36 and a resistor 35 to ground. By use of the conductors 33 extending to the beam current control which is itself grounded, the voltage drop across each of the resistors 35 is detected to indicate the bleeding off of the fringe portions of the beam current thereby signalling the passage of the electron beam past that sensing point. The control 34 can take many forms such as a meter indicator or a computer programmed control operative to print out the results of a checking sequence followed automatically in the event of a beam failure.

To detect the passage of the electron beam to the target 9, one sensing element includes a diode 37 positioned behind the target which generates a current flow passing through the connected resistor 37a indicative of the presence of the beam striking the diode. Another type of sensing element positioned along the beam column axis is comprised of the aperture plates 27 of each aperture plate assembly. Shown in FIGURES 3 and 4 is the aperture assembly 2411 on which is mounted the aperture plate 271:. To detect the passage of beam current through the aperture 28a, a conductor 38 connects with the aperture plate and extends to one of three spring contacts 39 held by the screws 40 threaded into an insulating plate 41 mounted on a body 42 of the aperture assembly. The contacts 39 are spaced radially about the body 42. The plate 27a is electrically insulated from the body of the assembly in being removably mounted on a disk 44 mounted on a sleeve 45 which, in turn, is removably held in a rigid position by interfitting within a center opening 46 of an electrical insulating ceramic disk 47 and having a nut 48 threaded thereon. Thus, any beam current striking the plate 27a will be transmitted directly to the contact 39.

As the aperture assembly is placed into the non-magnetic cylinder 23 during assembly of the column, the spring contact 39 comes into abutting relationship with a stationary conductor 49 (FIGURE 2) which is held within an insulating sleeve 50 in an opening 51 in the side of the cylinder 23. Thus, contact is made between each aperture plate and the exterior of the cylinder by the conduc; tor 49 which, in turn, is connected with the conductor 36 leading to the beam check control.

In operation, the beam passes along the beam axis and a fringe portion of the beam strikes the aperture plate 27a which plate is electrically isolated from the rest of the system in being held by the insulating disk 47. Therefore, the only exit for the beam current intercepted by the plate is through the conductor 38, the abutting contacts 39 and 49 and the conductor 36 through the resistor 35 to ground. The flow of current causes a voltage drop across the resistor 35, which voltage drop is detected through the conductor 33 thereby providing a signal to the beam control 34 that the electron beam has reached that sense point formed by the aperture plate 27a. The same structure is used in the other aperture assemblies to provide identical sensing points for the beam.

In accordance with another feature of the invention, each electrically-isolated aperture plate is heated to prevent contamination buildup thereon which is one of the primary causes for failure of the electron beam column. To accomplish this, the aperture plate 27a (FIGURE 3) is mounted on the disk 44 held by the sleeve 45 which preferably is made of a heat conducting material such as molybdenum. The plate is held on the disk by screws 52. The sleeve is mounted by passage of the elongated portion 45a thereof through the opening 46 in the heat insulating' ceramic disk 47 mounted to the body 42 by screws 54. In this manner, the aperture plate and heat conducting sleeve are both heat and electrically insulated from the body of the assembly.

In heat conducting relationship with the sleeve 45 are mounted a pair of heating coils 55 within an annular heat conducting form 56. These coils are supplied with electric current through a conductor 57 leading to a second contact 39a (FIGURE 4) similar in structure to the first contact 39 and mounted at an angularly displaced position on another insulating block 41a. By supplying electric current to the heating coils through another conductor 49 mounted to abut the second contact, the heat conducting form 56 is heated which, in turn, conducts heat onto the sleeve 45. The heated sleeve 45 then radiates heat to the disk 44 and the aperture plate 27a by means of the large surface areas positioned opposite each other. In this manner, the aperture plates of each aperture assembly are heated to approximately 300 C. to limit the contamination thereof. Additionally, conductive heating of the adjacent parts of the column is prevented to a large degree, which heating might otherwise adversely effect such parts of the column as the energizing coils and spacers of the lens assemblies.

In FIGURE 5 is illustrated the eifects on the beam of the contamination buildup on the aperture plate. As heretofore described, the contaminants if non-conductive, can assume an electrical charge to carry stray electric fields which, in turn, randomly deflect the electron beam. The curves illustrate four sets of measurements taken of the spot size change vs. time as the heating current supplied to the heating coils was changed. Curves 58 and 59 illustrate at time zero the measured spot size of the electron beam at the target with the aperture plate cold. At this time, the effects of the stray electric fields in diff-using the beam and thereby increasing the spot size are seen. As the temperature of the aperture plate increases with time measured from the moment of first energizing the heating coils, the spot size diminishes as these stray fields are decreased in magnitude with the heating and subsequent carbonization of the contaminants on the aperture plate.

Curves 60 and 61 begin with the aperture plates heated to a standard operating temperature and at time zero the heating current supplied to the coils is interrupted. Note, that the beam spot thereafter becomes enlarged. Such enlargement is due to the diffusion of the beam because of the stray electric fields created with the formation and subsequent electrical charging of contaminants on the aperture-forming plates 27.

As another feature of the invention, the aperture-forming plates are electrically isolated from the heating coils to prevent leakage current from reaching the aperture plates and causing an erroneous indication of the beam current detected by the plate. For this purpose, insulating plugs 62 are extended between the sleeve 45 and the disk 44 on which the aperture-forming plates are mounted. These plugs preferably are made of a dielectric material such as alumina and are brazed into openings 44a in the disk 44 and openings 45b in the sleeve respectively. Thus, any leakage current transmitted through the sleeve 45 must pass through these high resistance plugs to reach the aperture-forming plates. To bleed olf any such leakage current, a conductor 63 is connected to the sleeve 45 and extended to a third contact 3912 cooperating with a third conductor 49 connected to ground and not shown. Thus, any leakage current in the sleeve is transmitted directly to ground and prevented from reaching the aperture plate.

Experience has also shown that as electrons strike the aperture plate 27, some will pass on through and strike the disk 44 and the insulating plug 62. Because of the conductive qualities of the disk, the electrons will be transmitted directly to the conductor 38. However, the electrons striking the insulator 62 will not be conducted away because of the insulating properties of the alumina. Thus, stray electrical fields can be formed which can affect the functioning of the beam. To isolate such fields from the beam, an extension member 64 on the sleeve 45 is positioned between the plug and the beam. Since this extension is made of the conductive material (molybdenum 1n the example shown) any electric fields will be 7 shorted to ground and shielded from intersecting the beam path.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

I claim:

1. A system for sensing the operating conditions of an electron beam column for directing a beam onto a target, said system comprising:

a plurality of beam-sensing elements positioned along said beam axis and adapted to generate a signal indicative of the passage of the beam with one of the said sensing elements being an electrically-isolated aperture-forming plate including means for generating a signal indicative of a portion of the beam striking said plate, and

a control for indicating the operating condition of said column acting in response to said signals generated by said sensing elements,

an electric heating means for heating said aperture plate, said heating means being electrically isolated from said aperture plate.

2. A control system as defined in claim 1 including an electrical circuit connecting with said plate for conducting away any beam current resulting as the beam strikes the plate and including means for measuring the conduction of said current through said circuit.

3. A control system as defined in claim 1 including a beam-sensing element positioned at the target end of said beam to supply a signal to said control.

4. A control system as defined in claim 1 including electric current insulating means positioned between said aperture-forming plate and said heating means and at a lower voltage potential than said plate thereby to conduct stray electric currents originating at said heating means away from said plate.

5. A control as defined in claim 4 wherein said electric current insulating means is shielded to prevent any charge thereon from generating electric fields which intersect said beam axis.

6. A heated aperture assembly for use in an electron beam column comprising:

a body member formed to interfit with said column, an aperture forming plate, a heat conductive member holding said plate, insulating means mounting said heat conductive member on saidbody member; and an electric heating element mounted on said heat conductive member in heat-conducting relationship with said aperture-forming plate whereby said plate and heating element are electrically and heat insulated from said body member and column, an electrical isolating means positioned between said heating element and said aperture plate to limit the direct conduction of electric current therebetween, said isolating means includes an electrically conductive member at a lower voltage potential than said aperture-plate thereby to intercept any stray electric currents flowing from said electric heating element before reaching said plate. 7. An aperture assembly as defined in claim 6 wherein said aperture-forming plate is removably mounted on said heat conductive member.

References Cited UNITED STATES PATENTS 3,03 8,993 6/1962 Masuda 250-495 3,170,116 2/1965 Farrington 324-70 3,295,008 12/1966 Gallaro et a1. 315-3 3,293,429 12/1966 Leboutet et a1 250-41.9 3,239,664 3/1966 Farrell 25049.5 3,345,529 10/1967 Loefiier et al 3l384 JOHN W. HUCKERT, Primary Examiner B. ESTRIN, Assistant Examiner US. Cl. X.R.

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