US3418465A - Radiation source for reducing specimen contamination in electron microscopes - Google Patents

Description

Dec. 24, 1968 E. HAHN ETAL 3,418,465

RADIATION SOURCE FOR REDUCING SPECIMEN CONTAMINATION IN ELECTRON MICROSCOPES Filed Aug. 17, 1965 Fig. 1

1a 20 I I e #17 19 75 In ventors United States Patent 3,418,465 RADIATION SOURCE FOR REDUCING SPECIMEN CONTAMINATION IN ELECTRON MICROSCOPES Eberhard Hahn and Wolfgang Hoch, Jena, Germany, as-

signors to VEB Carl Zeiss Jena, Jena, Germany Filed Aug. 17, 1965, Ser. No. 482,322 6 Claims. (Cl. 250-495) ABSTRACT OF THE DISCLOSURE An electron microscope is provided with a device for reducing specimen contamination. The reduction is achieved by the exploitation of the contamination effect itself. The specimen is for this purpose located in a chamher, and the chamber walls near the specimen are exposed to electronic, ionic or short-Wave electromagnetic radiation. These walls accordingly produce an adsorbing effect that decreases the hydrocarbon pressure in the vicinity of the specimen.

In electron microscopy, it is very annoying that the specimens undergo contamination when exposed to electronic radiation. This effect is assumed to take place as follows:

Hydrocarbon molecules of the residual gas in the microscope vacuum during the time they linger on the irradiated surface of the specimen are struck and ionized by electrons of the illuminating ray, so that polymerides are produced. These polymerides in the course of further irradiation carbonize and deposit more and more as a sediment on the specimen. The velocity of that contamination of the specimen depends chiefly on the hydrocarbon partial pressure near the specimen, on the temperature of the specimen and on radiation conditions in general. As contamination works a undesired change of the specimen and thus impairs the quality of the image, it is desirable to reduce it or altogether to avoid it. Several attempts in that direction have been made.

For example, the specimen and/or its surroundings are cooled to approximately 80 centigrade, in consequence of which the hydrocarbon partial pressure decreases so much that no deposits can be formed.

The contamination can be reduced also by heating the specimen to several hundred degrees centigrade, which among other things shortens the time the molecules linger on the specimen surface.

In another known device, the specimen is enveloped by a thin layer of gas under a pressure considerably higher than that of the residual gas, so that it is possible even to disintegrate a carbon layer already in existence.

Not only are the said anticontamination measures bound up with constructional difiiculties, but they present the disadvantage of causing intolerable changes, chiefly of organic nature, in the structure of the specimen. In a further known device the specimen accordingly lies in a chamber which, excepting one small aperture for the entry and one for the exit of the illumination ray, is sealed against the remaining residual gas and is connected to a pump for decreasing the hydrocarbon partial pressure.

Finally, attempts have been made to reduce specimen contamination by decreasing the hydrocarbon partial pressure through glow discharge in the near and wider surroundings of the specimen. This means however that the high gas pressure in the immediate neighbourhood of the specimen does not admit microscopy while there is such glowing, and that a considerable decrease of the hydrocarbon partial pressure near the specimen occurs only after the lapse of a comparatively long time. Moreover, for the glow discharge appreciably to reduce contamination, this 3,418,465 Patented Dec. 24, 1968 discharge must take place in closest proximity to the specimen, Which is thus likely to be damaged.

The present invention, which aims at obviating the foregoing disadvantages of the previously known devices for reducing specimen contamination in electron microscopes, sets out from the principle that such reduction or even prevention can be achieved by exploiting the contamination eifect itself. The problem is solved by decreasing the hydrocarbon partial pressure in the vicinity of the specimen in such a manner that, according to the invention, the specimen is located in a chamber in which the walls near the specimen are exposed to electronic, ionic or short-Wave electromagnetic radiation.

The irradiated surfaces produce an absorbing efiect which may be so great that illuminating the specimen, particularly by a fine beam, will not entail any contamination, so that it is not necessary to cool the surroundings of the specimen.

It is convenient to make the apertures in the specimen chamber as small as possible and so to position and dimension them that the imaging primary beam can freely traverse the chamber and that electron or ion beams or short-wave electromagnetic radiation, preferably at right angles to the imaging beam, can 'be included. As against chambers of the foregoing kind having a pump for decreasing the hydrocarbon partial pressure, the chamber of the device according to the invention in the interest of a further reduction of the contamination may have very small ray-entrance apertures, the only condition being the use of suitable electron-optical means concentrating the rays near these apertures.

The locations of the electrodes of an electrostatic lens or the pole shoes of a magnetic lens and/ or the mounting of the specimen may call for another kind of chamber for the specimen. To avoid direct bombardment of the specimen, the additional irradiation is parallel to the surface of the specimen. Moreover, the specimen may be screened from stray radiation, for example when corpuscular radiation with charged particles is used, by raising it to a potential beyond the reach of the charge-carrier beam. As the accelerating voltage of the charge-carrier beam need only be small as compared to the voltage of the imaging electron beam, the imaging relations remain unaffected.

When a magnetic lens is used and there is ionic radiation, the bombardment direction and accelerating voltage in relation to the magnet field in the lens can be so chosen that the ions do not strike the specimen itself but proceed in the range of a girdle round the specimen before they strike the interior wall of the chamber.

Advantageously, the device according to the invention can be combined with the known means for reducing specimen contamination. For example, an increase of the suction velocity of the surfaces struck by the additional beam or, in other words, better absorption of hydrocarbon molecules on the Walls, can be obtained by cooling these walls.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example two embodiments thereof and in Which:

FIG. 1 shows the one embodiment, in combination with an electrostatic lens, and

FIG. 2 shows the other embodiment, in combination with a magnetic lens.

In FIG. 1, a specimen 1 is disposed in a chamber 4. The specimen 1 is illuminated by an electron beam traversing apertures 2 and 3 of the chamber 4 and is imaged by means of an electrostatic lens comprising electrodes 5, 6 and 7.

The specimen 1 is held in position by a displaceable rod 8 mounted in a Tefion seal 9.

The chamber 4 is a hollow cylinder the axis of which lies in the plane of the drawing and is at right angles to the optical axis. The chamber 4 is adjustably held by a copper rod which is at right angles to the plane of the drawing and may at the same time serve as heat conductor for changing the temperature in the chamber. Against the two torus-shaped ends of the chamber 4 are respectively pressed a specimen guide 10 and the housing 11 of an additional electron gun which is represented in the drawing as an incandescent cathode 12 and a Wehnelt cylinder 13, a ring 14 insulating the cylinder 13 from the housing 11 at anode potential.

In the embodiment FIG. 2, a magnetic lens is used into which the specimen is laterally introduced through a slit. Two lens pole- shoes 15 and 16 and a distance ring 17 concentric with the lens axis sufiiciently encase the space containing the specimen 1 to be imaged. Air-tight connection with the remaining microscope space is only through two pole- shoe bores 18 and 19, which may be sufficiently narrow and which throttle the supply of hydrocarbon molecules. One lateral aperture in the ring 17 admits the specimen rod 8 and another lateral aperture in the ring 17 envelops the current of ions which emanates from an additional source 20 and prevents contamination of the specimen to be imaged.

We claim:

1. In an electron microscope comprising at least an electron gun, an electron optical lens system, a specimen arranged in the beam of electrons proceeding from said electron gun to said lens system and a chamber enclosing said specimen and having two apertures for the passage of said electron beam, a device for reducing specimen contamination comprising a source of radiation being so arranged relatively to said chamber that the walls of said chamber which are near said specimen are exposed to the radiation of said source so as to produce an adsorbing efiect and thus to decrease the hydrocarbon pressure in the vicinity of said specimen.

2. A device for reducing specimen contamination as claimed in claim 1, wherein said source of radiation is a source of electrons.

3. A device for reducing specimen contamination as claimed in claim 1, wherein said source of radiation is a source of ions.

4. A device for reducing specimen contamination as claimed in claim 1, wherein said source of radiation is a source of short-wave electromagnetic radiation.

5. In an electron microscope comprising at least an electron gun, an electron optical lens system and a specimen arranged in the beam of electrons proceeding from said electron gun to said lens system and a chamber enclosing said specimen and having two apertures for the passage of said electron beam, a device for reducing specimen contamination comprising a source of radiation being so arranged relatively to said chamber that the walls of said chamber which are near said specimen are exposed to the radiation of said source so as to produce an adsorbing effect and thus to decrease the hydrocarbon pressure in the vicinity of said specimen and means preventing direct and indirect irradiation of said specimen by said radiation directed to said walls of said chamber.

6. In an electron microscope comprising at least an electron gun, an electron optical lens system and a specimen arranged in the beam of electrons proceeding from said electron gun to said lens system and a chamber enclosing said specimen and having two apertures for the passage of said electron beam, a device for reducing specimen contamination comprising a source of radiation being so arranged relatively to said chamber that the walls of said chamber which are near said specimen are exposed to the radiation of said source so as to produce an adsorbing effect and thus to decrease the hydrocarbon pressure in the vicinity of said specimen and means for cooling said irradiated walls.

References Cited UNITED STATES PATENTS 2,467,224 4/1949 Picard 25049.5 2,890,342 6/1959 Columbe 25049.5 3,206,598 9/1965 Wegmann 250-49.5

RALPH G. NILSON, Primary Examiner.

A. L. BIRCH, Assistant Examiner.

US. Cl. X.R.

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