US3327112A - Electron microscope having a light metal layer on the interior walls to prevent x-ray generation - Google Patents

Description

3,327,112 ELECTRON MICROSCOPE HAVING A LIGHT METAL LAYER ON THE June 20, 1967 HIROSHI AKAHORI INTERIOR WALLS TO PREVENT X-RAY GENERATION Filed Feb. 25, 1964 lNvENToR Hmosm flxfixom Y: 1% m TO United States Patent ELECTRON MICROSCOPE HAVING A LIGHT METAL LAYER ON THE INTERIOR WALLS TO PREVENT X-RAY GENERATION Hiroshi Akahori, Katsuta-shi, Japan, assignor to Hitachi, Ltd., Tokyo, Japan, a corporation of Japan Filed Feb. 25, 1964, Ser. No. 347,276 9 Claims. (Cl. 250-495) This invention relates to an electron discharge device and more particularly to such a device having a layermade of light metal disposed on the inner surface of selective portions of its wall for preventing X-ray generation from secondary electrons.

As X-rays are highly harmful to the human body, electron devices such as electron microscopes and electron machining apparatus must provide means for completely preventing passage of X-rays through the wall of the device.

For the purpose to prevent this X-ray leakage, in the prior art electron discharge device, the walls of both an electron gun and an electron accelerating means are made of heavy metal such as iron or copper primarily to prevent outward leakage, and since outward leakage increases with accelerating voltage the wall of the device has been often increased in thickness to meet such increase in outward leakage.

It has been found, however, from measurement that the outward leakage X-ray dose increases at a faster rate than does the accelerating voltage and the leakage X-ray spectrum comprises apparently a multiplicity of spectra. It has been also found from investigation that, besides primary X-rays which are produced by impingement of primary electrons upon means such as an anode and a specimen arranged in the path of the primary electrons, secondary X-rays are generated at the wall of the device by impingement of recoil and secondary electrons.

Since the dose and intensity of X-rays generated when an electron impinges upon a surface of metal is approximately proportionate to the %th power of the atomic number of the metal in general, the quantity generated increases as the metal becomes heavier. Accordingly, if heavier metal is used for the wall of the device, the quantity of secondary X-ray generated increases still more. As a result, in order to prevent completely the outward leakage of the secondary X-ray in addition to the primary X-ray, the wall of the device must be so increased in thickness that the device becomes disadvantageously largesized and costly.

As there is applied 'a high potential difference between the grid and the wall of an electron gun, the inner surface of its wall is often glazed or chromium plated in order to eliminate pin-holes and irregularities which would cause discharge between the grid and the wall.

Since exceedingly great thickness is required for the wall of the-device in order to prevent completely outward leakage of the primary and secondary X-rays, the inner surface of the wall of the device must be polished after cutting to glaze the surface. Further, since various attachments such as an exhaust pipe must be mounted on the wall of the device, handling during machining is exceedingly troublesome. As a result, the machining operation not only becomes complex but it is so difficult to polish precisely the inner surface of the wall of the device that it can not be avoided that some irregularities and pin-holes remain in the inner surface of the wall of the device. On the other hand, when the inner surface is chromium plated, as the plating layer inherently contains always some pinholes, the plated surface often requires bufiing to remove such pin-holes, But since the wall of the device must be very thick to prevent outward leakage of the primary and secondary X-rays completely and be provided with vari-.

ous attachments such as an exhaust pipe mounted thereon, it is very difficult to finish precisely the inner surface of the wall by bufiing. Moreover, the chromium plating layer is so hard that it is very diflicult to stop the pin-holes. Consequently it can not be avoided that some pin-holes remain and some irregularities are formed in the surface of the wall. As a result, these pin-holes and irregularities cause small discharges to be formed between the grid and the wall of the device. These small discharges effect a disturbance of the accelerating electron beam and result in damage of the wall of the device. Moreover, electrons produced in the small discharge impinges upon the wall of the device resulting in secondary X-ray radiation.

When the wall of the device is damaged by such small discharge, repair or replacement is required. If heavy metal is used as the material of the wall of the device, as done in the prior art, the wall of the device must be very thick to prevent completely outward leakage of secondary X-rays, which results in very high cost and large expenditure in replacement.

Thus the conventional electron device has many difficulties as mentioned above. It is an object of the present invention to overcome such disadvantage inherent in prior art electron devices.

An important feature of the present invention is to prevent outward leakage of X-rays by provision of a layer on the inner surface of the wall of the device which is made of light metal such as aluminium or magnesium to prevent generation of X-rays without making the wall of the device and the layer for preventing X-ray generation unduly thick.

Another feature of the present invention is to reduce generation of discharge by provision of a cylinder for preventing X-ray generation between an electrode and the wall of an electron gun.

Still another feature of the present invention is to minimize economical loss in replacement of the cylinder for preventing X-ray generation by provision of such cylinder which is removably disposed between the electrode and the wall of the electron gun.

The present invention will be now described in detail in conjunction with the accompanying drawings which show illustrative embodiments of the invention and in which:

FIG. 1 shows an elevation in longitudinal cross section of an electron microscope embodying the present invention and FIG. 2 shows an elevation in longitudinal cross section of an electron gun of another embodiment of the present invention.

Referring to FIG. 1 there is illustrated an electron device embodying the invention which comprises a camera section 1 and a microscope section 2. The microscope section 2 includes an electron gun 3 and an electron lens system 4. A cable head 8 is secured to the top of the wall 5 of the electron gun by means of screws 7 and 7 through the intermediary of a packing 6. The cable head 8 is provided with a high voltage insulator 9 which is in turn provided with a cathode 10 and a grid 11. First condenser lens 12 is disposed below the grid 11 and provided with an anode 13 by means of threaded connection. An exhaust pipe 15 is connected with the wall of the electron device at the side thereof. A double condenser lens 17, objective 18, intermediate lens 19 and a projection lens 20 are mounted in described order from the upper side upon the wall 16 of the electron lens system 4. The double condenser lens 17 consists of the first condenser lens 12 and a second condenser lens 21. First specimen chamber 22 is formed betwen the second condenser lens 21 and the objective 18. The first specimen chamber 22 is provided with a specimen holder 24 for supporting a specimen 23. An exhaust pipe 25 is connected to the wall 16 of the device 3 at the side thereof in communicating relation with the first specimen chamber 22. A specimen holder 28 for supporting a specimen 27 can be disposed in a second specimen chamber 26 formed between the intermediate lens 19 and the projection lens 20. An exhaust pipe 29 is connected with the wall 16 of the device at the side thereof in communicating relation with the second specimen chamber 26. Below the projecting lens 20 is arranged an observing chamber in which is disposed a fluorescent screen 31 at the bottom thereof and which is provided with an observing window 32. A layer, 33 for preventing 'X-ray generation which is made of a light metal material such as aluminium or magnesium is mounted on the inner sur-.

face of the walls 5 and 16 of the device in various areas upon which recoil or secondary electrons impinge.

Now another embodiment illustrated in FIG. 2 will be described. Like numerals are used for parts in FIG. 2 which correspond to those shown in FIG. 1 and description is done only for parts which differ from those of FIG. 1. In place of the layer 33 for preventing X-ray generation which is mounted on the inner surface of the wall 5 of the electron gun 3, a cylinder 14 for preventing X-ray generation made of light metal material such as aluminium or magnesium is inserted in fitting relation between the cable head 8 and the first condenser lens 12 and the electron gun wall 5.

When the microscope section 2 of the embodiment of the present invention constructed as mentioned above and shown in FIG. 1 or 2 is highly evacuated through the evacuation pipes 15, 25 and 29, electrons emitting from the cathode travel straight to the anode 13 under the control of the grid 11. The electrons passing through the anode 13 are impinged upon the specimen 23 which is supported in the specimen holder 24 by means of the double condenser lens 17 comprising the first condenser lens 12 and the second condenser lens 21. The electron beam passing through the specimen is enlarged by the objective lens 18, the intermediate lens 19 and the projection lens 20 to impinge upon the fluorescent screen 31. An electron microscope image may be observed on the screen 31 through the observing window 32.

By removing the specimen holder 24 and disposing the specimen holder 28 in the path of electron beam within thesecond specimen chamber 26, an electron diffraction image of high resolving power of the specimen 27 may be observed on the fluorescent screen 31.

Since, in FIG. 1, primary electrons impinge upon the bodies disposed in the path of the electron beam, for example, the specimen holder 24, the anode'13 and the fluorescent screen 31, primary X-ray is radiated from these parts. This primary X-ray is partially absorbed by the layer 33 for preventing X-ray generation and the remainder of the primary X-ray is completely absorbed by the walls 5 and 16 of the device. Further, from the bodies disposed in the path of the electron beam are radiated reflected electrons and secondary electrons to impinge upon the layer 33 for preventing X-ray generation. Since the layer 33 for preventing X-ray generation, however, is made of light metal material such as aluminium or magnesium, secondary X-ray generation by impingement of the reflected and secondary electrons may be maintained at very low level. Consequently even if a very small quantity of secondary X-ray is transmitted through the layer 33 for preventing X-ray generation, it may be completely absorbed by the walls 5 and 16 of the device as energy of this secondary X-ray is very low. As a result, outward leakage of the primary and secondary X-rays can be perfectly prevented. It has been found from experiment that when the walls 5 and 16 are in such thickness that outward leakage of only primary X-ray may be prevented, the layer 33 for preventing X-ray generation of several microns in thickness can prevent perfectly even outward leakage of the secondary X-ray. In other words, by using the layer 33 for preventing X-ray generation of several microns in thickness, outward leakage of the secondary X-ray may be necessarily prevented if the walls 5 and 16 of the device have such thickness that outward leakage of only the primary X-ray can be prevented'Thus the thickness of the walls 5 and 16 may be determined depending upon outward leakage of the primary Xray which has been partially absorbed by the layer 33 for preventing X-ray generation. Consequently the walls 5 and 16 of the device maly be reduced in thickness and manufactured economical y.

The layer 33 for preventing X-ray generation may be formed very easily by applying a foil of light metal onto the inner surface of the walls 5 and 16 or by vacuum spattering or spray of fused metal. The metal utilized in accordance with the invention is selected from the group of metals known as the light metals. As indicated in the Concise Chemical and Technical Dictionary, published by the Chemical Publishing Co., Inc,, 1962, edited by H. Bennett, the light metals are a group of metals of specific gravity up to approximately 3; the alkali metals; the alkaline earth metals, e.g., beryllium, magnesium, and aluminum.

Referring to FIG. 2, primary X-ray is generated by) for example, the anode, disposed in the path of electron beam. This primary X-ray is partially absorbed by the cylinder 14 for preventing X-ray generation and the remainder of the primary X-ray is perfectly absorbed by the wall 5 of the device. Besides the primary X-ray, reflected and secondary electrons are emitted from the bodies disposed in the path of the electron beam to impinge upon the cylinder 14' for preventing X-ray generation. Since the cylinder 14 is made of light metal, such as aluminium or magnesium, generation of secondary X-ray may be suppressed to a minimum. Accordingly even if a very small amount of secondary X-ray is transmitted through the cylinder 14' for preventing X-ray generation, it is absorbed completely by the wall 5 of the electron device. Although the cylinder 14' for preventing X-ray generation may be in thickness of the same order as that of the layer 33 for preventing X-ray generation shown in FIG. 1 is suflicient to perfectly prevent outward leakage of the primary and secondary X-rays, the layer 14' of 1 to 2 mm. thickness is preferable from the view point of mechanical strength.

The cylinder 14 for preventing X-ray generation having thickness to this extent can be easily made by press Work and be process into a definite shape. The inner surface thereof can be finished easily and precisely by buffing, electro-polishing or chemical polishing without any restriction. Moreover, light metal is relatively soft so that the pin-holes may be easily stopped by the above described polishing works. Thus the inner surface of the cylinder 14' for preventing X-ray generation can be finished into substantial speculum without any irregularities and pinholes. Consequently, discharge which may be produced between the grid 11 and the wall 5 of the electron device can be minimized so that disturbance of accelerated electron beam and damage of the cylinder 14' for preventing X-ray generation due to the discharge may be minimized.

Even if the cylinder 14': should fail in any accident, heavy damage as experienced by the prior art devices can be avoided as this cylinder 14' for preventing Xray generation is moderate in price. This cylinder 14' may be replaced easily by removing the cable head 8 from the wall 5 of the electron device by releasing'the screws 7 and 7.

Although heretofore the present invention has been described in connection with an electron microscope having three-lens system, it has been done merely for illustrative purpose and it is to be understood that the invention is applicable to various devices utilizing electron beam, as for example, electron beam machining apparatus and X-ray microanalyser.

As described above in detail, the present invention has many practical advantages that outward leakage of primary and secondary X-rays are perfectly prevented by provision of a layer for preventing X-ray generation without thickening the wall of the electron device and the layer for preventing X-ray generation, occurrence of discharge is minimized by provision of a removable cylinder for preventing X-ray generation and economical loss can be reduced in the case of replacement of the cylinder for preventing X-ray generation.

What is claimed is:

1. An electron microscope comprising an electron gun section and a microscope section for guiding an electron beam emitted by said electron gun section; said electron gun section being provided with electrodes for emitting said electron beam and guiding said electron beam into said microscope section and a wall surrounding said electrodes; said microscope section being provided with first lens means to focus said electron beam, second lens means to expand said focused electron beam, a specimen chamber formed between said first and second lens means, an observation chamber for observing a specimen disposed in said specimen chamber and for guiding said expanded electron beam therethrough and a fluorescent plate disposed in said observation chamber and adapted to receive said expanded electron beam, the wall of said electron gun section and the Wall defining said specimen chamber and said observation chamber being provided on the inner surfaces thereof with a metal layer selected from a group consisting of the light metals to effectively prevent X-ray generation.

2. An electron microscope according to claim 1 wherein said layers for preventing X-ray generation are each composed of aluminum.

3. An electron microscope according to claim 1 wherein the metal layer on the wall surrounding said electrodes is in the form of a cylinder for preventing X-ray generation provided between said wall surrounding the electrodes and said electrodes in said electron gun section.

4. An electron microscope according to claim 3 wherein said cylinder for preventing X-ray generation is made of aluminum.

5. An electron microscope according to claim 3 Wherein said cylinder for preventing X-ray generation is provided in removable fashion.

6. An electron microscope according to claim 5 wherein said removable cylinder for preventing X-ray generation is made of aluminum.

7. An electron microscope according to claim 5 wherein said removable cylinder for preventing X-ray generation is made of a metal from the group essentially consisting of aluminum and magnesium.

8. An electron device comprising an electron gun section and a microscope section for guiding an electron beam emitted by said electron gun section; said electron gun section being provided with electrodes for emitting said electron beam and guiding said electron beam into said microscope section, and a wall surrounding said electrodes; said microscope section being provided with lens means to focus said electron beam, a specimen chamber for guiding said focused electron beam, and a specimen disposed in said specimen chamber to receive said focused electron beam; and the wall of said electron gun section and the wall defining said specimen chamber being provided on the inner surfaces thereof with a metal material layer selected from a group consisting of the light metals to prevent X-ray generation.

9. An electron device according to claim 8 wherein the layer on the wall surrounding said electrodes is in the form of a cylinder for preventing X-ray generation provided between said wall surrounding the electrodes and said electrodes in said electron gun section.

References Cited UNITED STATES PATENTS 2,418,321 4/1947 Smith 250-495 3,018,398 1/1'962 Atlee 31359 3,087,061 4/1963 Dukes et a1 2508'3.3

OTHER REFERENCES An Experimental Electron Microscope for 400 Kilovolts by A. C. van Dorsten et al. from Philips Technical Review, vol. 9, Nov. 7, 1947, pp. 193-201, pp. 199- 201 relied on.

RALPH G. NILSON, Primary Examiner. W. F. LINDQUIST, Assistant Examiner.

Claims (1)

1. AN ELECTRON MICROSCOPE COMPRISING AN ELECTRON GUN SECTION AND A MICROSCOPE SECTION FOR GUIDING AN ELECTRON BEAM EMITTED BY SAID ELECTRON GUN SECTION; SAID ELECTRON GUN SECTION BEING PROVIDED WITH ELECTRODES FOR EMITTING SAID ELECTRON BEAM AND GUIDING SAID ELECTRON BEAM INTO SAID MICROSCOPE SECTION AND A WALL SURROUNDING SAID ELECTRODES; SAID MICROSCOPE SECTION BEING PROVIDED WITH FIRST LENS MEANS TO FOCUS SAID ELECTRON BEAM, SECOND LENS MEANS TO EXPAND SAID FOCUSED ELECTRON BEAM, A SPECIMEN CHAMBER FORMED BETWEEN SAID FIRST AND SECOND LENS MEANS, AN OBERVATION CHAMBER FOR OBSERVING A SPECIMEN DISPOSED IN SAID SPECIMEN CHAMBER AND FOR GUIDING SAID EXPANDED ELECTRON BEAM THERETHROUGH AND A FLUORESCENT PLATE DISPOSED IN SAID OBSERVATION CHAMBER AND ADAPTED TO RECEIVE SAID EXPANDED ELECTRON BEAM, THE WALL OF SAID ELECTRON GUN SECTION AND THE WALL DEFINING SAID SPECIMEN CHAMBER AND SAID OBSERVATION CHAMBER BEING PROVIDED ON THE INNER SURFACE THEREOF WITH A METAL LAYER SELECTED FROM A GROUP CONSISTING OF THE LIGHT METALS TO EFFECTIVELY PREVENT X-RAY GENERATION.

Images