US2939954A - X-ray shadow microscope - Google Patents

Description

June 7, 1960 s. P. ONG 2,939,954

X-RAY SHADOW MICROSCOPE Filed Jan. 3, 1958 2 Sheets-Sheet 1 FIG.4

\NVENTOR SING POEN ONG w June 7, 1960 s. P. oNG X-RAY SHADOW MICROSCOPE 2 Sheets-Sheet 2 Filed Jan. 3, 1958 INVEN TOR.

SING POEN 0N6 X-RAY SHADOW MICROSCOPE Sing Poen Ong, Delft, Netherlands, assignor to North American Philips ompany, Inc., New York, N.Y., a corporation of Delaware Filed Jan. 3, 1958, Ser. No. 706,924

hired States Fatent O F Claims priority, application Netherlands Feb. 16, 1957 5 Claims. (Cl. 250-495) This invention relates to X-ray shadow microscopes-provided with special electron-optical devices.

An X-ray shadow microscope is an X-ray tube in which the focal spot, the source of the X-rays, is formed on a thin metal window and by means of which an image of a small article is produced on a display screen which is spaced away from the focal spot by a distance which is a multiple of the distance by which the article is spaced away from the focal spot. This focal spot must be very small, for otherwise the X-ray image is not sharp. The

higher is the required resolving power, the more exacting this requirement becomes. Such a small focal spot is obtained by focussing the beam of electrons travelling from the cathode to the window (the target) by means of a highly reducing electron-optical system.

By varying the energization of the electron-optical systour or the shapes or mutual spacings of the system components, the image of a difierent sectional plane of the electron beam is projected onto the target each time. The problem is to reduce the sectional area of the beam on the target to a minimum by means of this variation. Thus, the correct adjustment is achieved.

T 0 this end, one would like to observe the focal spot during the adjustment of the electron-optical system. However, since this cannot be done in a simple manner for a variety of reasons, use is frequently made of a metal gauze arranged adjacent the focal spot. The shadow which this gauze, when iradiated by X-rays, casts onto a fluorescent screen, is observed, the electron-optical system being adjusted so that the lines of this shadow image have maximum definition.

Owing to the comparatively low voltage by which in an X-ray shadow microscope the electronsare accelerated, the brightness of the fluorescent image is so small that the room in which the screen is arranged must be completely darkened and the observer must accustom his eyes to the darkness for a prolonged period of time before he can observe the image. With this low brightness, the instant at which, during the variation of the image, the lines are defined most sharply, cannot be determined with certainty. Therefore, the conventional method of focussing adjustment must be regarded as insufficient. The present invention obviates the said disadvantage.

An electron microscope is known in which an image of an article is produced on a pick-up screen by electrons which are reflected at the article. The electrons irradiating the article pass through the lens serving to produce an image of the article. Thus, this lens has two functions, that is to say, it increases the electron density at the article of which an image is to be formed and it produces an image of this article on a pick-up screen.

According to the present invention, the phenomenon upon which the operation of the said electron microscope is based, is used to achieve a difierent purpose, namely to render the focal spot in an X-ray shadow microscope perceptible in order to enable this spot to be sharply adjusted.

In the apparatus in accordance with the invention, elec- Fatented June 7, 1960 trons, which, by the action of the rays impinging upon the target, are emitted from the focal spot, are passed through the optical system or through part of the stages which this system comprises and are collected on a fluorescent observation screen. This observation screen must be disposed at a point where, with correct adjustment, the lens produces an electron-optical image of the focal spot. 7

The electrons in the returning beam have different velocities. However, the curve, which is a graphical representation of the amountrof electrons per unit of time as a function of their velocities, has a peak at the value of the velocity equal to that at which the electrons travelling to the target reach this target. The present invention utilizes the occurrence of this peak.

In an X-ray shadow microscope in which the axes of ,the system stages coincide with the beam axis, the electrons which return from the target with no or substantially no energy loss, finally come back to the electron ejector. Thus, the observation screen must have an aperture of passage for the electrons .going to the focal spot. This aperture forms a blind spot centrally of the image to be observed. If the size of this aperture is not excessive, at misadjustment of the objective, the returning electrons can be detected in a zone surrounding the beam travelling to the target by means of this screen. However, with the required adjustment of the objective at the point of ob servation, the image just falls within the beam travelling to the target and consequently is no longer perceptible.

In order to avoid the said blind spot, at least to shift it from the centre of the image so that the observation is improved, the electron beam which is emitted from the target and passes through the optical system, or through part of the stages which this system comprises, can be separated from the beam striking the target. This separation can be efiected by magnetic fields which act upon the rays.

To achieve this beam separation, provision can be made of a magnet system which produces a magnetic field the lines of force of which are at right angles to the axis of the ray beam. When the electron-optical imageproducing system includes a magnetic lens, the beams may alternatively be separated from one another by arranging this lens so that its optical axis is at a slight angle to the axis of the electron beam which travels from the cathode to the target.

Thus, an electrically conductive fluorescent screen may be interposed between the electron ejector and the magnetic lens, which screen has an aperture of passage for the electrons going from the cathode to the target and must be at a positive potential with respect to the cathode.

In order that the invention may readily be carried out, two embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows diagrammatically the electron-optical system of an apparatus in accordance with the invention which is provided with a separate magnet system for the separation of the electron beams;

Fig. 1a shows a microscope including the electronoptical system shown in Fig. l; V

Fig. 2 shows diagrammatically a system in which use is made of a lens disposed obliquely relatively to the concentrating the electrons. t together 'con'stitute'the'electr'ofn ejector; "Ihecldfrcid' we 50f X-ray shad-ow microscope. in operation, a

3, or the last electrode of the ejcctorsystem, can be at the same potential as the target '2, ahe'heretna'tter this iron 'paths'are deflected so that the terminatein a very 'srnallfocal spot '6 which usually; is circular. From this focalspot, soft X ray'sare emitted in every direction, i.e.'

from both sides of the target 2. X-rays emitted'from the side of the target remo'tel'from thepoin't of impactof the "electron beam are useful for X-ray microsc py, 'While thoseemitted 'fromlhe side of impact are useless. The

size oi the focalspot not only depends upon the physiv cal dnnensions of the cathode, the relative spacings and the potential di's't-ributiombut'also upon the lens adjustmerit. even less.

The diameter of the focal spot may he l ,u. or a The X-ray'semanating -fromfthc focal spot.6 penetrate V the's ecimen 26 which,fby means of a holder 27-, is supported .by the wall 2 8 of the partition which encloses the specimen chamber. The end Wall-2 9 of the chamber includes a' fluorescent screen Sit on which an enlarged image of'the structure of the specimen is produced. 1

From the focal spot 15, secondary electrons ejected from the target by the impact of primary electrons leave "the target with substantilly thesame energy, and hence netic transverse; field. "One of these magnet poles,'whic h assumed to be" arranged behind the beam, is designated In this event, the electron lens cannot be adjusted with the aid of an X-ray image, since there is no visible fluorescent X-ray image. Even in this case, the adjustment can be performed by the use of the invention which renders the adjustment independent of the X-ray image. The number of secondary electrons returning from the target at a substantially"uhdiminished velocity is even relatively increased at lower voltages (lower efficiency of the production of X-rays) V The imagewhich is produced on the target at correct adjustment of the lens, is the image o'f'the smallestbeam cross-section, and can be observed through "the window 32 which is covered by a viewing glass. A negatively charged. diaphragm 4 is arranged in front of the cathode and the annular or cylindrical anode 3 which is spaced away from the cathodeby a slightly larger distance.

Neglecting any deviations owing to lens defects, there is formed on the plate 11 a visible image which with respect to itsshape and size is an exact copy or" the beam cross-section, an i-m'ag'e of which is produced on the target Z, Thefdimensions of-the image on the plate 11 .will be smalL however, with the aid of optical magnification it's degree of definition can always befoh'served.v The image also shows whether the-arrangement er the electrodes of the ejector system relative to one another :or to a lei-1's mustbe corrected, since not only the definition but also the "shape of the image can be examined. The spot which is visible on the screen ll is evenly illuminated and, unlike the image *produced' by an electron-microscope, 'doesnot'show a certain pattern. V

Adjustment of the image definition can be very accur'at'e. When the focuss'ing of the ray beam from the electron ejector on-th'etarg'et 2 is not correct, but is such that the lack of definition cannot be observed visu- "11 becomes visibly blurred and 'son disappears when 'defocussi'ng 'is continued. This is inter alia due to the 7 in Fig. 1. It is the south pole. V, The'noith pole (not p shown) is disposed in front of the beam.

This'magneticltransverse:field produces a'deflectio'rl of the paths of the electrons travelling mm the 'ejector to the target. are deflected by the magnetic transversejfieldiin the same sense of rotation-, the returning beam 9 is detached in this field from the beam 8 travelling to the target and beyond thetransverse fieidgoes its separate way. The

i change in focussing produced by the magnetic transverse field is very slight, so. that the 'returning'electrons are concentrated onto a small area surrounding a point 10 at which a luminescent layerl'll is arranged on a pickupplatell of conductive'material on the'tube wall is disposed. .Ihis plate'is given a positive" potential with Since'the pathsof the returning electrons. V

respect to the cathode, for example, by connecting it to the anode '3.

The plate 11 is coated with 'a fiuorescentmaterial so that a visible image of the. focal spot is produced. However, this image is many times the size of the focal-spot itself, since it is magnified by the lens. The brightness 7 of this image materially exceeds the brightness of the X- ray image by which the 'deiinitlon'of the focal spot has been judged hitherto. Frequently, there will be no need in a 'dark room. In some cases, the intensity of the X- rays Wlll be insufiicient to produce visible fluorescence of a fiuorcscent screen even in a completely darlg room.

'for the observation of the focus. image to be performed fact-that the defect in focu'ssing is repeated on the way' is produced by rays striking the pick-upplate through a considerable area so that'any definition is out of the question. g t

Hence, a brightly illuminatedand sharp spat becomes visible on the plate 11 only with'accurateadjust-me'nt of the el'ec'tron-optical. system; Even at a slight rnisa'djust ment, the bright spot becomes a shapel'es s' glimmer of appreciably attenuated fluorescent light .ofr'adially decreasing brightness. w V

V Fig. 2, in which parts corresponding "to those at the arrangement shown in Figl" are'desi'g'nftedfby like reference "numeralsfiliustrate's an alternative 'inethod of separa'tihg the beam of returning'el'ectron's from the beam er electrons travelliiig ftolhe target. Ill this'em'bodiment, "the "magnetic less 5 is disposed so that its axis is at a slight angle of, fo'r'exam'ple', 0.5 t6 the s of the electron beam. This lens can beass'urh'ed as, euig composed areas the axis of which lies in the plane of't'he dmiwtag and er a'inagneticfpole pair producing a field the cs ral vector or" which is at i'ightahgles to the plane of the drawing. For the "sake er clantgthe's'e two c'oinpoililfs areshdwn se arately in lll qllivhlfi't dlgl 'afil 0f .3. The} hie lille lcffon li'is { 3016s 12 and 13 and a winsiiigia) and span er poles 1s and 16. It is assumed that the diieetieii Of the energizin current in the vvindin 14 is chosen so that 12 and hence 15 is a north pole and 13 and hence ll a south pole. T his implies-a deflection of the beam o felectrons generatd by the source and of the beam of returning electrons to the right. Consequently, the latter beam leaves the lens in a difierent direction. Although in actual fact the beam axes 17, 18 and 19 may not lie in one plane, as is assumed here, this is not important for an understanding of the operation of the apparatus in accordance with the invention; it will be appreciated that in any case the beams diverge from one another.

Adjacent the electron ejector there is arranged a pickup plate 2%) having an aperture 21 for the passage of the electron beam going to the target. This plate is coated with a layer of fluorescent material and is electrically connected to the anode 3. This implies that the plate must either be made of metal, at least of a conductive material, or must be coated with a conductive layer. The plate may alternatively be the image-forming diaphragm of the electron-optical system.

Fig. 4 shows what will be visible on the pick-up plate. The plate or a large part thereof is covered by a weak glow (in the region 22) produced by dispersion and chromatic aberration. With correct adjustment of the focal distance of the lens, the electrons which return from the focal spot at undiminished or substantially undiminished velocity concentrate in a brightly illuminated spot 23 beside the aperture 21. With respect to its shape and size, this spot corresponds to the cross-section of the beam of which an image is produced on the target 2. The correct adjustment of the focal distance of the lens is that at which the spot 23 is as small as possible. A change in the focal distance causes the spot to spread and to become blurred.

As has been mentioned hereinbefore, the smallest beam cross-section, an image of which is required to be produced on the target 2, usually lies between the anode 3 and the diaphragm 4 of the ejector system. In this event, a pick-up plate disposed as shown in Fig. 2 is spaced away from the lens by a smaller distance than the object of which an image is to be produced. However, when the difference in distance is not excessive, it does not appreciably affect the image definition, since, unlike the region between the lens and the target 2, in the region between the lens and the pick-up plate 20, the beams of the rays converging into an image point are very slender, so that a sufliciently sharp image is produced even in a plane which does not pass exactly through the beam peaks.

There may be a larger difierence between the distance by which the image formed by the returning beam is spaced away from the target and the distance by which the electron ejector is spaced from the target. This is the case when the optical system comprises at least two lenses and the returning beam does not pass through all the lenses of the system. In Fig. 1, for example, a second lens may be interposed between the anode 3 and the magnet system 7, which lens produces an intermediate image. If this intermediate image is real, the magnet system 7 must be located between the intermediate image and the lens 5. Consequently, the pick-up plate will be nearer the target 2. If the intermediate image associated with the ray beam travelling to the target is virtual, the beam of returning electrons produces a real image at a larger distance than in the absence of the second lens.

While I have described my invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. An X-ray shadow microscope comprising, an electron beam source, a target, an electric lens for focussing the electron beam along a given axis and forming a focal spot on said target, and magnetic means interposed between the source and the target for producing a magnetic field at right-angles to the axis of the electron beam whereby secondary electrons emitted by said focal spot are focussed on a fluorescent screen disposed outside the path of the electron beam, said screen being located so that the electrons emitted by said focal spot produce an optical image of the focal spot thereon when the electron beam is focussed for producing the focal spot.

2. An X-ray shadow microscope comprising, an electron beam source, a target, an electron lens for focussing the electron beam along a given axis and forming a focal spot on said target, and magnetic means interposed between the source and the target for producing a magnetic field at right-angles to the axis of the electron beam whereby secondary electrons in said beam emitted by said focal spot are separated and focussed on a fluorescent screen disposed outside the path of the electron beam, said screen being located so that the electrons' emitted by said focal spot produce an optical image of the focal spot thereon when the electron beam is focussed for producing the focal spot.

3. An X-ray shadow microscope comprising, an electron beam source, a target, an electron lens for focussing the electron beam along a given axis and forming a focal spot on said target, and means to produce a magnetic field at right angles to the axis of the electron beam and at right angles to the axis of a beam of secondary electrons emitted by said focal spot interposed between the source and the target for focussing secondary electrons emitted from the target on a fluorescent screen disposed outside the path of the electron beam generated by said source, said screen being located so that the secondary electrons emitted by said focal spot produce an optical image of the focal spot thereon when the electron beam generated by the source is focussed for producing the focal spot.

4. An X-ray shadow microscope comprising, an electron beam source, a target, an electron lens for focussiug the electron beam along a given axis and forming a focal spot on said target, and means to produce a magnetic field at right angles to the axis of the electron beam and at right angles to the axis of a beam. of electrons emitted by said focal spot interposed between the source and the target for focussing secondary electrons emitted from the target on a fluorescent screen disposed outside the path of the electron beam emitted by said source, said lens having an optical axis forming an angle with the axis of the electron beam generated by said source, said screen being located so that the secondary electrons emitted by said focal spot produce an optical image of the focal spot thereon when the electron beam emitted by the source is focussed for producing the focal spot.

5. An X-ray shadow microscope comprising, an electron beam source, a target, an electron lens for focussing the electron beam along a given axis and forming a focal spot on said target, an electrically conductive fluorescent screen member disposed between said source and said electron lens, said screen member having an aperture therein for the passage of the electron beam generated by said source, and means to produce a magnetic field at right angles to the axis of the electron beam generated by said source and at right angles to the axis of a beam of secondary electrons emitted by said focal spot for focussing the secondary electron beam emitted from the target onto said fluorescent screen whereby an optical image of the focal spot is produced thereon when the electron beam generated by the source is focussed for producing the focal spot.

References Cited in the file of this patent UNITED STATES PATENTS 2,238,577 Boersch Apr. 15, 1941 2,348,031 Rajchman May 2, 1944 2,356,633 Von Ardenne Aug. 22, 1944 2,440,640 Marton Apr. 27, 1948 2,777,958 LePoole Jan. 15, 1957 2,799,779 Weissenberg July 16, 1957 2,814,729 Newberry et a1 Nov. 26, 1957

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