US2814729A

US2814729A - X-ray microscope

Info

Publication number: US2814729A
Application number: US613149A
Authority: US
Inventors: Sterling P Newberry; Selby E Summers
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1956-10-01
Filing date: 1956-10-01
Publication date: 1957-11-26
Anticipated expiration: 1974-11-26
Also published as: GB848982A; FR1189217A

Description

United States Patent X-RAY MICROSCOPE Sterling P. Newberry, Schenectady, N. Y., and Selby E. Summers, Milwaukee, Wis., assignors to General Else tric Company, a corporation of New York Application October 1, 1956, Serial No. 613,149

9 Claims. (Cl. 250-495) The instant invention relates to an X-ray microscope, and more particularly to an X-ray microscope of the shadowgraph type.

In an X-ray shadow microscope a beam of electrons provided by an electron gun is projected through an evacuated housing. Positioned within the housing is an electron ray collimating device such as an electron lens which acts on the trajectories of the electrons and produces parallel rays. A second electron lens assembly then focuses the parallel electron rays on an X-ray target which produces X-rays when electrons impinge thereon. A specimen to be examined is positioned in the beam of X-rays produced by the target and an enlarged image thereof is projected onto an X-ray sensitive element to produce an enlarged shadow image of the specimen.

In the hitherto known X-ray microscopes of this type two types of electron lenses for both collimating and focusing the electron rays have been utilized. That is, both electrostatic lenses, in which the electron trajectories are modified by means of electrostatic fields, and electromagnetic lenses in which the trajectories are modified by means of magnetic fields, have been utilized. Because of certain inherent weaknesses in electromagnetic lenses such as extreme sensitivity to ferromagnetic properties of specimens, the requirement of highly stabilized power supplies for the lenses, and certain construction difiiculties in the lens configuration itself, it has been found desirable to use electrostatic lenses in such microscopes.

The first shadow X-ray microscope utilizing an electrostatic objective lens for focusing the electron-beam was described by Von Ardenne in Elektronen-Ubermikroskopie, page 72, Julius Springer, 1940. While instruments of the type disclosed by Von Ardenne and others heretofore known in the art are suitable for some purposes, they have not been entirely satisfactory due to the fact that the X-ray beams produced thereby have not been of suflicient intensity to produce sharp and distinct images of the specimen being examined. The basic limitation in producing high intensity X-rays is the inherent characteristics of electrostatic lenses. Such electron lenses sufier from severe spherical aberration. That is, the outer zone of the lens focus more strongly than the inner zones. As a result, the marginal rays of the electron beams are not focussed at the same point as the paraxial rays. Consequently, it is necessary to eliminate the marginal rays by means of a beam limiting aperture positioned along the electron beam path. This, however, reduces the intensity of the X-rays produced, the image intensity varying approximately inversely as the two thirds power of the aberration constant, inasmuch as a number of electrons are prevented from impinging on the target structure. It has been found, however, that the spherical abberation of an electron lens may be improved to a certain extent by making the focal length of the lens shorter.

In addition to decreasing the spherical aberration of the lens, shortening of the focallength serves the purpose of further increasing the intensity for a given resolution by "ice reducing the electron spot diameter impinging on the X- ray producing target structure without loss of electron current. As a consequence, the intensity of the X-rays produced for a given resolution varies inversely with the square of the focal length of the objective lens. In the previously known instruments of this type, it has been customary to minimize the effects of spherical aberration by providing a beam limiting aperture within the objective lens in order to eliminate all but the paraxial electron rays. In these prior devices, such as that of Von Ardenne referred to previously, the beam limiting aperture is located on the target side of the objective lens assembly and usually on the last electrostatic plate element in the assembly. As a result, the focal length of the objective lens assembly had to be quite large since it included, in addition to the lens elements themselves, a beam limiting aperture.

An improved X-ray shadow microscope of shorter focal length and consequently of higher intensity may be achieved by positioning the beam limiting aperture on the side of the electrostatic field producing lenses closest to the electron beam source. Such a microscope is disclosed in a patent application filed on July 14, 1954, Serial Number 443,384, entitled X-ray Microscope," by Sterling P. Newberry, this patent application being assigned to the General Electric Company of Schenectady, New York, the assignee of the present invention. As shown in that application, the beam limiting aperture is positioned on the electron beam source side of the objective lens assembly. As a consequence, the focal length of the lens assembly is substantially shortened since the aperture assembly is no longer between the lens and the target.

in order to further shorten the focal length of an electrostatic objective lens in such a microscope, it is necessary to control the optical characteristics by controlling the trajectory of the electron beams so as to manipulate the principal optical point of the lens and move it toward the target thus shortening the focal length of the lens element.

Accordingly, it is an object of this invention to provide a new and improved X-ray shadow microscope of increased intensity wherein the focal length of the lens assembly is appreciably shortened.

Another object of this invention is to provide a new and improved X-ray shadow microscope in which the spherical aberration of the lens assembly is reduced.

Yet another object of this invention is to provide an electrostatic lens for an X-ray shadow microscope wherein the optical midpoint is on the target side of the lens mid-plane.

A further object of this invention is to provide an asymmetric electrostatic lens assembly for an X-ray shadow microscope wherein the electron trajectories through the lens are controlled so as to move the principal optical point of the lens closer to the target.

The instant invention, briefly speaking, contemplates an X-ray shadow microscope incorporating an electrostatic lens assembly which because of its construction causes a multiple crossover of the lens optical axis by theelectron rays and consequently causes the optical principal point of the lens to move towards the target and shorten the focal length. The lens assembly is closed at the exit element and asymmetrically spaced along the optical axis thereof and by virtue of this configuration and spacing and its relation to the thickness of one of the lens elements a non-divergent electrostatic field of the proper configuration to cause multiple electron crossover is achieved. I

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however,

both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings which:

Figure 1 is a cross sectional view of an improved X- ray shadow microscope constructed in accordance with the invention;

Figure 2 is an enlarged fragmentary view of the objective lens and target structure of the X-ray shadow microscope of Figure 1;

Figure 3 is a schematic showing of the electrostatic field produced in a lens assembly;

Figure 4 is a diagrammatic showing of the electron trajectory through an asymmetric lens structure of Figure 1.

Referring now to Figure 1 there is illustrated an X-ray shadow microscope embodying the principles of the instant invention. There is provided a suitable housing 1, preferably cylindrically shaped, which is connected to a suitable vacuum pumping system for maintaining the housing under vacuum. Mounted in the lower portion of the housing 1 is an electron beam source 2 consisting of an electron filament 3, an apertured focusing electrode 4, and an apertured accelerating electrode 5 aligned over the filament 3 so as to form the emitted electrons into a beam of electron rays which are accelerated through the apertures in the accelerating electrode 5.

The focusing electrode 4 is fastened to a bottom plate 6 that encloses one end of the housing 1, and is maintained in place by means of set screws 7. The set screws are provided to permit adjustment of the position of the opening in the focusing electrode 4 relative to the accelerating electrode 5 so as to properly position the electron beam emitted therefrom relative to the remaining elements within the housin". The accelerating electrode 5 is insulatingly spaced from the focusing electrode 4. Set screws 8 are provided in order to permit the electron gun assembly to be returned to its previous location after changing a filament. The electron filament 3 may also be adjusted relative to the apertured focusing electrode 4 by means of set screws provided on opposite sides of the filament base.

Disposed along the electron beam path is a collimating device which affects the trajectory of the electron beam and serves to bring the divergent electron rays emitted from the source into a beam of substantially parallel or slightly convergent rays. To this end, a first set of apertured electrostatic field producing elements 9, 10 and 11 are positioned immediately above the electron beam source 2. Each of these elements has its central aperture aligned along the electron beam path and functions to affect the electron trajectory so as to form an electron beam condenser lens assembly. This assembly is maintained in place by means of a cylindrical sleeve member 12 which in addition also functions as a magnetic shield. The sleeve member 12 is supported on a lip portion of accelerating electrode 5, and may be manipulated relative to the electron beam path by means of a screw rod 13 having a knurled knob thereon, the inner end of which engages the electrode element 9. Screw rod 13 acts against a spring-loaded pusher element 14 which provides a balancing-adjusting motion to the assembly that aids in aligning the same. The elements 11 and 9 are fastened by means of bolts to form a unitary structure so that movement of one by adjustment of the knob 13 results in the movement of the other. The central element 10 is supported between the elements 9 and 11 so that it follows any adjustment thereof. Consequently, all three electrostatic field producing elements can be moved in a unitary fashion relative to the electron beam path in order to align it therewith. Operating potential for the lens assembly is supplied to the central element 10 by means of a high voltage connection 15 resiliently attached to the element so that the connection may track movement of the assembly.

Briefly speaking, the operation of the condenser lens assembly may be described by reference to the effect of the electrostatic field upon the electron trajectories. It can be stated that the condenser lens assembly serves to modify the electron trajectories to bend the electron rays into curved paths during passage through the assembly, and through this bending action functions to bring the group of divergent electron rays emitted from the beam source 2 into a beam of substantially parallel or slightly convergent rays. Because of this characteristic action, the structure is termed a condenser assembly. For a more complete description of the manner in which the electrostatic field producing elements 9, 10 and 11 function as a condenser lens assembly, reference is made to the book The Electron Microscope, by D. Gabor, published by The Chemical Publishing (30., Inc., 1948, Brooklyn, New York, and specifically to chapters 2 and 3 thereof. In addition, the book entitled Electron Microscopy, by V. E. Cosslett, Academic Press, Inc., publisher, London and New York (1951), contains an excellent discussion of the principles of operation of condenser lens assembly. In particular, reference is made to chapter 2 of the Cosslett book for an excellent discussion of electrostatic lens theory.

Positioned at the end of the housing 1 opposite to the electron beam source is an electrostatic objective lens assembly Whichis aligned along the eelctron beam path and serves to focus the parallel electron rays transmitted from the condenser lens assembly. This objective lens assembly 16 is constituted of a second set of apertured electrostatic

field producing plates

17, 18 and 19. The objective lens assembly 16 functions to modify the trajectory of the electrons passing therethrough and causes a multiple crossover of the electron rays and thereby producing an extremely short focal length for the lens assembly. The electrostatic

field producing elements

17, 18 and 19 which constitute the lens assembly are asymmetrically spaced along the optical axis with respect to the middle element 18. The precise relationship of the spacing of the elements to the structural configuration thereof will be explained in greater detail with reference to Figure 2. At this point, however, it is sufiicient to say that the asymmetrical configuration of the lens is such as to produce a non-divergent electrostatic field therein which causes a multiple crossover of the electron rays therein and a consequent movement of the lens principal point in a direction away from the electron beam source. As a result the focal length of the electrostatic lens assembly 16 is reduced by an order of magnitude.

Positioned at the focal point of the objective lens assembly is a target window structure 20 which functions to produce X-rays upon the impingement of electron rays thereon. This target structure preferably comprises a fiat plate having good heat dissipating qualitites as Well as being capable of transmitting X-rays without absorption thereof. A thin layer ofan X-ray producting heavy metal, such as tungsten, is coated or plated upon the base member. In a preferred embodiment the base element may be constituted of beryllium and of a thickness of .025 millimeter while the heavy metal may be constituted of tungsten and be of the order of thickness of .001 millimeter. The target structure is preferably secured to the inner surface of the last electrostatic plate member 19 at a point farthest from the electron beam source 2 so that electron rays impinge upon the target structure after passing through the condenser and objective lens structures. Inasmuch as the target structure 20 is flat and substantially flush with the surface of the electrostatic plate element 19, the target window does not in any fashion adversely modify the electrostatic field produced by the lens 16. Furthermore, by positioning the target structure flush with the last element of the objective lens assembly, a non-divergent electrostatic field and consequently a shorter focal length, less spherical aberration and a higher intensity X-ray output, may be achieved.

As has been pointed out previously, electron lens assemblies such as illustrated at 16 have a well-known inability to focus marginal rays of electrons as Well as paraxial rays. That is, the spherical aberration of the lens causes the marginal rays to be focused at a different point than the paraxial rays. Consequently, it is desirable to provide a beam limiting aperture within the microscope to prevent the marginal electron rays from impinging on the target structure. In order to achieve this result, an electron beam limiting aperture 33 is located in a movable member 34 which is positioned between the condenser lens assembly and the objective lens assembly. The member 34 containing the beam limiting aperture is secured to the end of a rotatable control rod 35 journaled in the housing 1. The element 34 may have a plurality of different sized apertures, such as 33, positioned therein so that by rotating the control rod 35 an aperture of any desired diameter can be brought into alignment with the electron beam path. In this manner the marginal rays are eliminated prior to passage of the electron rays through the objective lens asembly 16 thus eliminating to a large extent the spherical aberration of the lens element 16 and its attendant undesirable affects on the X-ray target structure.

This manner of positioning the beam limiting aperture, is disclosed and claimed in Serial 443,384 mentioned previously, accomplishes the elimination of the marginal rays without in any fashion requiring an elongation of the focal length of the objective lens assembly. By this construction the target window structure 20 will produce much higher intensity X-rays since the focal length of the lens assembly has been shortened by the elimination of the beam limiting aperture from the lens structure itself. Inasmuch as the intensity of X-rays is inversely proportional to the square of the focal length of the objective assembly, a substantial improvement in an X-ray microscope may be achieved in this manner.

A specimen holder and X-ray sensitive image reproducing element are positioned on the other side of the target structure 20. The X-rays produced at the target structure 20 pass therefrom through and around a specimen being examined, which specimen is indicated at 21 and is supported in a small cup-shaped specimen holder 22 seated in a manipulator generally indicated at 23. After passing through or around the object 21 being examined, the X-rays impinge on an X-ray sensitive photographic plate, indicated at 24, which is supported in a suitable holder mechanism 25. Since the X-raysdiverge outwardly from the point source on the target structure 20, the X-rays passing around the object 21 will produce an enlarged image of the specimen on the X-ray sensitive photographic plate, thereby producing a magnified image thereof.

In order to make certain that the specimen 21 is properly centered with respect to the X-day beam, and further to assure optimum magnification of the specimen, a manipulator 23 is provided in order to adjust the position of the specimen 21. The manipulator includes a base member 26 having a threaded exterior and which is secured to the housing 1 by means of set screws. A rotatable adjusting ring 27 engages the threaded exterior of the base member and provides a means for rotating the entire structure. The rotatable adjustable ring 27 can be rotated circumferentially by means of a ratchet 28 driven through a suitable adjusting knob mechanism. The rotatable adjusting ring 27 has a grooved upper surface which seats a plurality of pins 29 on which a base plate 30 is supported. The base plate 30 has a central opening therein in which a specimen holder disk 31 is positioned. The specimen holder disk 31 is integral with the cup-shaped specimen holder 22. The opening which accommodates the specimen holder disk 31 is sufiiciently long to allow the disk to be moved in either one of two directions by means of adjusting pins 32. The adjusting pins 32 act against a tension spring, not shown, secured to one corner of the-disk.

In this fashion, it is possible to raise or lower the specimen holder 22 and consequently the specimen 21 with respect to the target structure 20, and to move the specimen holder in any desired direction in the horizontal plane. Consequently, by operation of the manipulating device 23, it is possible to position the specimen holder at the exact point to provide optimum magnification of the specimen.

As has been pointed out previously, in order to shorten the focal length of the objective lens assembly and consequently increase the intensity of the X-rays produced, it is necessary that the principal optical point of the lens assembly be moved towards the exit element of the lens assembly. Prior attempts to achieve a shortening of the focal length by manipulating the position of the principal optical point involved either decreasing the radius of the aperture in the center element or decreasing the voltage applied to said center element and thus controlling the potential at the mid-point of the aperture. However, it was found that, to the contrary, the principal point moved very rapidly in the direction of the electron source rather than towards the exit element of the electrostatic lens. As a consequence, rather than shortening the focal length, the contrary effect appeared. Consequently, it was believed that the minimum focal length possible for focusing at the target with an electrostatic lens was equal to the sum of the working distance between the center element and the exit element plus the optical thickness of the center element, roughly slightly more than half the geometrical thickness of the center high voltage element. Since the working distance is limited by the voltage breakdown in the lens, this minimum focal distance thus depends roughly on one-half of the geometrical thickness of the center element.

It has been found, however, that the focal length may be shortened and the principal optical point moved in the direction of the lens exit element by causing a multiple crossover of the electron rays passing through the lens element. That is, by causing the electron rays to cross the optical lens axis a number of times within the lens field, and by eliminating the diverging field at the exit element, the optical principal point and the optical principal plane containing said point moves towards the front surface of the middle element of the lens and the focal distance of the lens element is appreciably shortened.

Figure 4 illustrates, schematically, the path of a single electron ray through an electrostatic lens of the proper configuration to produce the multiple crossover. An electron ray e moving under the influence of the electrostatic field produced by the lens assembly is caused to cross the optical axis A at the point b, the point c and finally at the point d. The configuration of the lens, which will be explained in greater detail later, is so proportioned that the final crossover point d is positioned at the exit lens element 19 and the target structure 20 positioned therein.

It can be seen from Figure 4 that the electron ray e striking the target 20 at the point :1 appears, to that target, as if it were refracted at the plane P in order to strike the target. Since the target, which contains the focal point of the lens system, sees the electron ray 2 as though it were refracted at the plane P, the lens acts as if its principal point, and the principal plane containing it, were positioned within the plane P which is located near the front surface of the middle element 18. Thus the focal length of the lens assembly as it appears to the target structure 20 is determined by the distance between the plane P, now denominated as the principal plane, and the target structure, a distance illustrated on the drawing as the focal length Closing the exit element of the lens by virtue of the target 20 mounted therein eliminates the divergent electrostatic field which would be present with an open apertured exit element, and the electron rays refracted at the plane P approach the target at a much steeper angle. Thus the rays appear to the target, to have been refracted at a point much closer to the target than would be the case if a divergent field were present and caused the electron rays to approach the target at a lesser angle. Thus it can be seen by causing a multiple crossover of the electron rays and by simultaneously eliminating the divergent field at the exit element by mounting the target therein, the principal point of the lens assembly is moved from the cathode side of the geometric mid-plane M, where it would he with but a single crossover to the plane P near the front surface of the mid element 17.

This multiple crossover of the electron rays passing through the lens assembly can be achieved by controlling the potentials in the center of the apertures of the individual

electrostatic elements

17, 18 and 19. It can be shown, that in a symmetrical lens system, that is, one in which the exit and entrance elements of the lens are equally spaced from the middle high voltage element, the electron rays may be caused to cross the optical axis a multiplicity of times. The optical properties of such a lens, including the focal length, are then defined by a characteristic parameter defined as the ratio of the potentials at the center of the aperture of the middle element to that at the center of either the exit or the entrance element. Thus the crossover of the electron is controlled by the parameter where V is the potential at the center of the middle lens element Whereas V is the potential at the center of the entrance element.

Figure 3 illustrates the potential distribution in a symmetrical lens of this type. Figure 3 shows an apertured entrance element 17, an apertured high voltage middle element 18, and an apertured exit element 19. The lines L, M and N, etc. illustrate equi-potential lines within the lens assembly. The voltages V and V at the center of the apertured members are defined by the following formulae:

where V is the voltage applied to the middle electrostatic element 18, V is the potential applied to the entrance and exit

electrostatic elements

17 and 19, S is the spacing between the middle element 18 and the entrance and exit elements, R is the radius of the aperture of the central element 18 and R is the radius of the aperture of the entrance element 17. For a more complete description of the manner in which multiple crossover of the electron rays may be achieved in a symmetrical electrostatic lens assembly reference is made to the book Electron Optics, by O. Klemperer, Cambridge Monograph on Physics, second edition, published by The Syndics of The Cambridge University Press, 1953, Cambridge, United Kingdom, and specifically chapter 4 thereof.

it is possible in a symmetrical lens assembly to provide a multiple crossover of the third order of the electron rays for a value of X=2.9 However, it has been found in the case of symmetrical lenses, that a very highly regulated and accurate voltage supply for the middle electrostatic element is necessary in order to achieve the proper value for the parameter X to obtain the triple crossover of the electron rays. As a consequence, it has been found extremely diflicult to produce an accurate symmetrical lens assembly in which the multiple crossover of the electron rays may be achieved.

In order to avoid these difiiculties, while yet achieving the shorter focal length possible with multiple crossover, applicants have provided an electrostatic lens assembly in which the divergent field has been eliminated at the exit and which is asymmetrically spaced along the optical axis. Figure 2 shows an enlarged view of the objective lens assembly 16 of Figure 1 embodying the principles of the instant invention and illustrating the relative structural parameters of the asymmetrical lens assembly. There is illustrated an entrance electrostatic element 17 having an aperture therein. An inner apertured high voltage lens element 18 having a circular aperture of radius R extending therethrough and having a thickness dimension t. The exit element of the lens assembly 16 is constituted of a third apertured electrostatic element 19 having the target structure 20 mounted within the aperture in order to eliminate the divergent field. The entrance element 17 and the exit element 19 of the lens assembly are asymmetrically spaced along the optical axis A with respect to the middle or inner electrostatic member 18.

In order to achieve the objects of this invention and provide a lens assembly having a very short focal length in which the electron rays are caused to cross the optical axis a multiple number of times, the following relationship between the spacing of the exit element and the entrance element must be observed. That is, S Z where S is the spacing between the exit element 19 and the middle element 18 whereas Z is the spacing between the entrance element 17 and the middle element. Thus, in order to produce the necessary multiple crossover of the electron rays it is necessary to fulfill the condition that the exit element be positioned closer to the middle element than the entrance element.

In addition, it has been found that certain other relationships between the structural parameters of the lens assembly must be observed in order to achieve the effect. Thus, the following two relationships between the exit element spacing and the thickness and aperture radius of the middle element 18 must be maintained.

t S 5 Z .5 and 1 2 Thus it can be seen that in the asymmetric lens structure of the instant invention the maximum thickness of the middle element can be four times the spacing between said element and the exit element 19. Furthermore, the aperture radius of the center element must be less than the /2 thickness of the middle element. If these relationships between the various structural parameters of the asymmetric lens assembly are observed, it has been found that an electrostatic lens of an extremely short focal length is obtained.

It has further been found that with a construction of this type a triple crossover of the electron ray within the lens assembly may be achieved for a Value of X which equals 1.1 10- If this is compared to the value of X obtained for the third crossover for a symmetric lens it can be seen that there is a difference of 30 orders of magnitude between the values. Since X, as has been pointed out previously, is proportional to the voltages at the center of the apertures and in turn a function of the voltages applied to the electrostatic elements, the degree of regulation necessary with the lens assembly of the instant invention is substantially less than that necessary for a symmetrical assembly. As a consequence, it is possible to achieve an electrostatic lens assembly having a very short focal length without requiring a voltage supply having a very high degree of regulation. As a consequence a much less expensive apparatus having the same order of accuracy may be achieved.

An asymmetric lens assembly embodying the principles of the instant invention was constructed having the following dimensions:-

S=.159 cm. t=.508 cm. R =.127 cm.

Thus the ratios 2 fall within the required limits. The lens assembly thus constructed had a focal length of approximately 1 millimeter which is shorter than that of any hitherto known practical electrostatic lens assembly.

From the foregoing description, it can be appreciated that the instant invention provides an X-ray shadow microscope incorporating an electrostatic objective lens assembly having a shorter focal length than hitherto previously known. As a result, much higher intensity levels of X-rays are produced with the concomitant increase in the sensitivity of the apparatus as a whole.

While a particular embodiment of this invention has been shown it will, of course, be understood that it is not limited thereto since many modifications both in the circuit arrangement and in the instrumentalities employed may be made. It is contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of this invention.

What We claim as new and desire by Letters Patent of the United States is:

1. An X-ray shadow miscroscope including in combination an electron beam source, an X-ray producing target structure positioned to intercept the electron beam, means to focus said beam on said target structure by causing multiple crossover of said beam including asymmetric electrostatic lens means whose principal plane is positioned between the lens mid-plane and the target.

2. An X-ray shadow microscope including in combination an electron beam source, an X-ray producing target window structure positioned to intercept the electron beam, asymmetrical lens means positioned along said electron beam path to produce an odd multiple crossover of said electron beam, comprising electrostatic field producing elements to control the trajectory and focus of said electron beam whereby the principal plane of said lens is shifted in the direction of said target.

3. An X-ray shadow microscope including in combination an electron beam source, an X-ray producing target window structure positioned to intercept the electron beam, a set of asymmetrically spaced electrostatic non-diverging field producing elements positioned along the electron beam path to form an objective lens assembly, said non-divergent electrostatic field causing multiple crossover of said electron beam thereby providing the minimum focal length for said lens.

4. An X-ray shadow microscope including in combination an electron beam source, an X-ray producing target window structure positioned to intercept the electron beam, spaced apertured non-diverging electrostatic field producing elements positioned along the electron beam path forming an objective lens assembly, the spacing of said elements being asymmetrical along the optical axis to control the potential ratio at centers of the element apertures to produce multiple crossover of the electron beam.

5. An X-ray shadow microscope including in combination an electron beam source, an X-ray producing target window structure positioned to intercept the electron beam, spaced apertured electrostatic field producing elements positioned along the electron beam path forming an objective lens assembly, said target being secured to one of said elements whereby the exit of said lens is closed whereby the electrostatic field is non-divergent, the spacing of said elements being asymmetrical along the optical axis to control the potential ratio at the centers of at least two of the element apertures to produce multiple crossover of said electron beam.

6. An X-ray shadow microscope including in combinations an electron beam source, an objective lens assembly positioned along the beam path comprising a first apertured electrostatic field producing element, two apertured electrostatic field producing elements positioned asymmetrically on opposite sides of said first element and constituting entrance and exit lens elements, an X-ray producing target structure positioned to intercept the electron beam secured to and closing said lens exit element so as to maintain the field non-divergent and the potential ratio at the aperture centers of said first and exit elements at a value to produce multiple crossover of said beam.

7. An X-ray shadow microscope including in combination an electron beam source, an objective lens assembly positioned along the beam path comprising a first apertured electrostatic field producing element, a second apertured electrostatic field producing element spaced from said first element and on the beam source side thereof forming an entrance lens element, a third apertured electrostatic field producing element positioned on the opposite side of said first element forming an exit lens element, the spacing between said exit element and said first element being less than that between said entrance element and said first element, an X-ray producing target structure secured to said exit element to close said lens assembly so as to maintain a non-divergent field and whereby multiple crossover of said electron beam is produced.

8. An X-ray shadow microscope including in combination an electron beam source, an object lens assembly positioned along the beam path comprising a first apertured electrostatic field producing thick lens element having thickness t, two electrostatic field producing elements positioned asymmetrically on opposite sides of said first element and forming entrance and exit lens elements, the distance S from said first element to said exit element being less than that to said entrance element, the relationship between said first and exit elements being defined y 9. An X-ray shadow microscope including in combination an electron beam source, an electrostatic lens assembly positioned along the beam path to focus said beam by causing multiple crossover of said electron beam, comprising spaced apertured entrance, middle and exit elements, said middle element comprising a thick lens element of thickness t and having a circular aperture of radius R the spacing of the elements being asymmetric and defined by the relations Z S, where S is the spacing between the exit and middle elements and Z, the spacing between the entrance and middle elements, the relations of the spacing, thickness and aperture radius being defined by S 5 Z .5 and 'R-1 1 2 and an X-ray target structure to intercept said electron Cosslett and Nixon: The X-Ray Shadow Microscope, Journal of Applied Physics, volume 24, Number 5, May 1953, pp. 616 to 623.