US3346736A - Electron probe apparatus having an objective lens with an aperture for restricting fluid flow - Google Patents

Description

Oct. 10, 1967 H. NEUHAUS 3,346,736

ELECTRON PROBE APPARATUS HAVING AN OBJECTIVE LENS WITH AN APERTUHE FORRESTRICTING FLUID FLOW Filed Sept. 22, 1964 ,0 (yr/w will My My ll FIG. I

EXHAUST DIFFI PUMP [as FORE PUMP 94 m 20- J EXHAUST AUXILIARY PUMP EXHAUST/ 92 I ATTORNEY United States Patent ration of Delaware Filed Sept. 22, 1964, Ser. No. 398,280 1 Claim. (Cl. 250-495) This invention relates to a novel instrument for rapid chemical analysis of selected microscopic regions of a specimen by X-ray emission techniques, and, more particularly, to a novel arrangement for focusing an electron beam upon the specimen and for supporting the specimen.

Instruments of the type with which the invention is concerned are known as microprobes, and generally include an electron gun, means for bringing the electron beam emitted by the gun to a line focus, means for supporting a specimen to be analyzed with its surface at the focus of the electron beam, a microscope for viewing the surface of the specimen at the point where the electron beam impinges upon it, and one or more X-ray spectrometers for measuring the X-rays of various difierent wavelengths emitted by the specimen in response to the electron irradiation. Instruments of this type are currently gaining wide acceptance in industry and are being found to be advantageous for use in many difierent fields. Heretofore, however, they have been relatively expensive, and have not been well suited for analyzing materials that cannot withstand exposure to a hard vacuum.

Accordingly, one important object of the present invention is to provide a novel X-ray instrument for microanalysis, which is of relatively simple and inexpensive construction.

Another object is to provide an instrument of this type including means for maintaining the electron beam system within a hard vacuum, while simultaneously allowing the specimen to be maintained in an atmosphere at appreciable pressure, thereby to permit analysis of materials that cannot satisfactorily withstand exposure to a hard vacuum.

The foregoing and other objects and advantages of the invention will become apparent in the following detailed description of a presently preferred embodiment thereof, taken in conjunction with the drawing, wherein:

FIGURE 1 is a cross-sectional view of an X-ray analytical instrument according to the invention, particularly showing the electron beam focusing arrangement and manner of supporting the specimen.

FIGURE 2 is an exploded, perspective view of a specimen holder according to the invention, together with a specimen prepared to fit within the holder.

FIGURE 5 is a partly schematice plan view, partly in section,and on a reduced scale of the instrument shown 1 in FIGURE 1.

Briefly, the invention contemplates an instrument in which the electron beam focusing system is enclosed and sealed except for an exhaust port and an aperture through which the electron beam is emitted. The aperture for the electron beam opens into an outer housing, which may be maintained at a pressure greater than the hard vacuum required for protection of the electron gun and for proper focusing of the electron beam.

In operation, leakage of air or other gas into the inner enclosure is restricted by a relatively small aperture at the objective electron lens to a value which is easily handled by the exhaust diifusion pump Without permitting a significant increase in pressure in the inner enclosure.

The instrument does not require an internal stage for mounting and positioning the specimen under examination. Instead, there is an aperture in the wall of the outer 3,346,736 Patented Get. 10, 1967 housing in alignment with the electron beam, and the specimen is sealed against the outer surface of the outer housing for examination, either by itself or within a cup, the rim of which seals against the housing wall. The arrangement is simple and inexpensive, yet fully practical and highly convenient.

A novel optical arrangement is also provided for illuminating and for viewing the specimen during analysis without interfering with the electron beam or the X-ray detectors.

Referring now to the drawing, the instrument shown therein includes an electron gun (not shown) mounted within an upper housing portion 10 and positioned to.

direct an electron beam downwardly along the central vertical axis of the instrument. The beam passes first through a condenser lens 12, then an objective lens 14, and finally impinges upon the specimen 16.

Although from one viewpoint, an electron lens consists of the magnetic or electric field which acts upon electrons passing through it, it has become common usage to refer to the physical parts used for producing the field as the lens, and the term is used in this latter sense in this application.

The two lenses 12 and 14 constitute the electron beam focussing system. They are fixed to a cylinder 18, which fits partly into the main housing 20 of the instrument and supports the lenses therein. The condenser lens 12 is secured in the cylinder 18 upon a ring 30, which rests upon an inwardly projecting shoulder 31 in the inner wall of the cylinder. An outer flange 22 of the lens 12 rests upon the ring 30, and is firmly secured thereto by a retainer ring, which overlies the lens and is screw attached to the ring 30. A tube 32 constituting a diaphragm aperture for the electron beam is fitted within the bore 34 of the condenser lens for limiting the beam.

The objective lens 14 is secured against the lower end of the tube 18 in sealing engagement with an inner ring 36. The ring 36 is welded or brazed to the inner wall of the cylinder 18, and the seal between the ring 36 and the top of the objective lens 14 is accomplished by an O-ring 38. The objective lens 14 is retained in pressure engagement against the O-ring 38 by a retainer ring 40 which is screw attached to the ring 36. A viewing window 41 is provided in the wall of the cylinder 18 near its upper end for viewing a retractable fluorescent screen 43. The screen 43 is used during preliminary adjustment of the instrument, and is retracted out of the electron beam for normal operation.

The structure so far described constitutes the inner en clsoure assembly of the instrument, and is removably fitted through an aperture 42 in the top wall of the housing 24 A ring 44 is brazed or otherwise secured around the outside of the cylinder 18 in sealing engagement with it, and an O-ring 46 is positioned to seal between the bottom surface of the ring 44 and the top surface of the housing 20.

The cylinder 18 also fits sealingly through an aperture (not separately designated) in a shelf 48 within the housing 20, thereby being held in proper vertical alignment within the housing. One or more ports 50 in the cylinder 18 provide communication between the interior of the cylinder and the region 52 within the housing between the shelf 48 and the top wall of the housing. In operation the region 52 is at the same pressure as the interior of the cylinder 18. Electrical leads 54 for the coil of the objective lens 14 are trained through one of the ports 50 around the edge of the condenser leans 12, thereby permitting the leads for both of the coils to be connected to a common jack 55 in the wall of the cylinder 18.

In order to permit maintenance of a substantial atmosphere in the outer housing 20 while still maintaining a hard vacuum within the cylinder 18, a pair of diaphragm aperture plates 56 are fitted within the bore of the objective lens 14 just above the magnetic gap '70. The bore of the gap, in the embodiment illustrated is 0.2" and the diameter of the apertures in the plates 56 is 0.3 mm. The aperture plates 56 restrict the flow of atmosphere so that a diffusion pump of conventional size, such as one rated at 300 liters per second at 10- mm. Hg pressure can still maintain a hard vacuum of less than mm. Hg when the pressure within the outer housing is as high as about 100 microns Hg.

A second shelf 53 is brazed, or otherwise sealed across the housing 20 just beneath the lower tip of the objective lens 14 to provide an air lock 5%, which may be vented to the ambient atmosphere without raising the pressure within the major part of the housing 20. A central aperture 62 is provided in the lower shelf 58, and a gate valve 64 is arranged for selectively opening and closing the aperture 62. An aperture 66 is formed in the bottom wall of the housing 20 to permit the electron beam to pass through the wall and impinge upon the specimen 16 when the gate valve 64 is open.

For economy in manufacture, the condenser lens 12 and the objective lens 14 are preferably of substantially identical construction. The peculiar shape of the lenses is, however, dictated by considerations afiecting principally the objective lens 14. For optimum focussing of the electron beam, the bombarded surface if the specimen 16 should be normal to the beam, and in order to minimize the effects of self absorption, and of anomalies and irregularities in the surface of the specimen 16, the take-off angle should be high, that is, the X-rays selected for detection should be those emitted by the specimen at as large an angle from the specimen surface as possible. For this purpose, the coil housing of the objective lens 14 is extended downwardly and tapered toward the specimen. This permits the detection of those X-rays that pass upwardly from the surface of the specimen relatively close to the axis of the electron beam. The tapered wall 72 is arranged to lie generally along a cone having its apex at the focal point of the electron beam, thereby permitting the achievement of the maximum takeoff angle consistent with the diameter of the cylindrical portion 74 of the coil housing and the focal length of the lens. In the embodiment of the invention illustrated herein, the focal length of the objective lens 14 was about 1 /2", and the conical angle of the tapered portion 72 of the lens was about 60, permitting a take-off angle close to 60 from the specimens surface.

The sample holding arrangement of the present invention does away with the need to provide an expensive stage arrangement within the evacuated housing 24) for supporting the specimen. As illustrated, the specimen 16 may be cut to size and placed Within a shielded cup '78, which is merely placed against the bottom of the housing 20 with an O-ring 8h interposed to seal it. The air lock 69 is evacuated. The gate valve 64 is opened, and the atmospheric pressure against the cut '78 is sufficient to hold the cup and specimen in place against the bottom of the housing 20. The specimen then may be moved by hand when it is desired to analyze different portions of its surface, or any desired cross slide (not shown) may be used to control its motion. Alternatively, the specimen 16, itself, may be sealed against the bottom of the housing 20. This arrangement is especiaily useful when it is desired to work with large specimens, or to avoid cutting a sample from a large workpiece.

Another important advantage of the specimen holding arrangement of the invention it its ready adaptability to illumination of the specimen by light transmitted through it, as is often desired, particularly in geological work. For this purpose, a window 77 is provided in the bottom of the cup '78, and a bottom illuminator 79 is aimed at the specimen through the window. The illuminator 79 is not in the housing 20, but in the ambient atmosphere, so it may be of any desired power and heat dissipation without the need of considering the special problems that would arise if it were within the evacuated housing.

The exhaust port 82 of the cylinder 18 is connected to the input of a diffusion pump 84 so that the electron gun and the electron focusing assembly is maintained at a desired hard vacuum pressure. The interior of the main housing 20, however, outside of the inner assembly included in the cylinder 18 may be maintained at any desired pressure from a hard vacuum up to about 100 microns Hg by adjustment of a modulatable valve 86, which is connected between this exhaust port of the housing 20 and the inputs of the diffusion pump 84 and an auxiliary forepump 89. It is believed that for many specimen materials, the preferred pressures will be from a hard vacuum up to about one micron Hg.

In order to avoid unacceptable attenuation of X-rays emitted by light elements, the pressure within the outer housing should be kept below about 100 microns Hg. Also, when high voltages are present within the outer housing 20, such as are used in certain types of X-ray detectors, it becomes necessary to maintain the pressure within the housing 2% below about 10 microns Hg.

Even with pressures as high as 100 microns Hg in the outer housing 20, the aperture plates 56 restrict the flow of atmosphere into the cylinder 18 sufficiently to allow the diffusion pump 84 to maintain a satisfactory hard vacuum within the cylinder 18.

The optical system for illuminating and viewing the upper surface of the specimen includes prism mounted with one of its faces against the bottom of the objective lens 14, and centrally apertured to permit the electron beam to pass through it without interference. The other two faces 9%) and 90" of the prism are reflective. The illumination enters from the left, as viewed in the drawing, and is reflected to the specimen by the left hand face 90 of the prism, while the viewing optical axis is bent by the right hand face 90". A microscope 91 is sealed through the wall of the housing 20 aimed at a slight downward angle so that its projected optical axis is reflected by the prism 90 to intersect the electron beam at its focal point. The illuminator 92, similarly to the bottom illuminator 79 is situated outside of the housing 20, thus avoiding cooling problems and enabling the provision of high intensity illumination with convenience.

The optical system is not only arranged for the convenient use of high intensity illumination in either the reflective or transmission modes of illuminating the specimen, but also permits optical viewing simultaneously with the electron bombardment and the taking of X-ray measurements. This is an important feature, which is lacking in many of the microprobe instruments heretofore known. Moreover, the optical system of the present invention is extremely simple and inexpensive, yet flexible and highly satisfactory in use.

One or more spectrometers are arranged within the outer housing 20 for detecting X-rays emitted by the specimen 16 responsively to electron bombardment. The spectrometer, or spectrometers 94, typically two as shown in FIGURE 3 may be of any desired type, and form no part of the present invention.

What is claimed is:

Apparatus for bombarding an object with an electron beam to cause the object to emit X-rays, said apparatus comprising:

(1) an electron gun,

(2) a magnetic condenser lens,

(3) a magnetic objective lens,

(4) a tubular housing for mounting said gun and said lenses in coaxial alignment with each other, said objective lens being mounted at the opposite end 0f said housing from said gun and being sealed to said housing so that the sole opening at said opposite end is through the magnetic gap of said objective lens,

(5) an outer housing for receiving said tubular hous- 5 6 ing with a major portion of said tubular housing (9) an X-ray spectrometer mounted in said outer houswithin said outer housing and with said objective lens ing for detecting X-rays emitted by an object ex- Within said outer housing and with said gun outside posed to the electron beam, and of said outer housing, (10) an optical system for visually observing an object (6) aperture means for restricting fluid flow through 5 exposed to the electron beam.

the magnetic gap of said objective lens, whereby in References Cited operation a hard vacuum may be maintained within said tubular housing while a substantial pressure is UNITED STATES PATENTS present in said outer housing, 2,241,432 5/ 1941 Von Ardenne et al. 25049.5 (7) means sealing said tubular housing to said outer 10 2,899,556 8/1959 Schopper et al 250-49.5 housing, 2,944,172 7/ 1960 Opitz et al 25049.5 X (8) means for exposing an object to an electron beam 2,968,723 1/1961 Steigerwald 250-495 passing through the magnetic gap of said objective OTHER REFERENCES lens out of said tubular housing, said exposing means n including an aperture through the exterior Wall of 15 The Encyclopedlo of Y and Gamma Rays, edlted Said Outer housing aligned with the magnetic gap by G. L. Clark, Reinhold Publishing Corp., New York, of said Objective lens to allow the electron beam to July 1963: PP- 842 and pass out of said outer housing and to impinge upon AL H G NILSON, Primary Examiner an ob'ect exteriorl th r of nd s al (1 t th exterirwan, y e e a e e agams e 20 W. F. LINDQUIST, Assistant Examiner.

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