US2467224A - Neutralization of electrostatic charges in electron-optical instruments - Google Patents

Description

G. PICARD Filed Feb. 21, 1948 IN ELECTRON OPTICAL INSTRUMENTS NEUTRALIZATION OF ELECTROSTATIC CHARGES April 12, 1949.

lNVENTOR ROBERT E. FIEAHD ATTORNEY Patented Apr. 12, 1949 UNITED STATES PATENT OFFICE NEUTBALIZATION F ELECTROSTATIC CHARGES IN ELECTRON-OPTICAL IN- STRUMENTS of Delaware Application February 21, 1948, Serial No. 10,084

Claims. 1

This invention relates to improvements in electron optical instruments of the type wherein the specimen under examination is bathed with electrons from an auxiliary source for the purpose of preventing the specimen from going positive as a result of the release therefrom of secondary electrons upon bombardment of the specimen with high velocity probe electrons.

When materials are bombarded with a high velocity electron beam, as occurs when a specimen is examined in an electron microscope, electron diffraction camera or similar instrument, electrons are knocked out of the material and, consequently, it acquires an electrically positive charge. This charge is purely a surface phenomenon, as the penetrating power of the electron beam commonly used (50-100 kilovolts) is extremely small, generally less than l0 mm.

The magnitude and durationof such a positive charge depends on both the causative electron beam and the nature of the bombarded material. Beam current and accelerating voltage influence the total magnitude of the charge. The surface and volume conductivities of the specimen and the potential relative to ground of the specimen mount and support determine the duration and equilibrium value of the charge.

The charge, in turn, influences the path of the beam through subsequent portions of the apparatus by imposing an electrostatic field which the beam must traverse. This effect is of particular importance in electron optical instruments which must image certain charge distributions. The optical constants of the instrument are not calculated to compensate for this unknown electric field, and, furthermore, the field is seldom suffici-ently stable to permit optical elements to counteract it.

This charging effect is noticeable in electronmicroscopy as a loss of resolution, shifting of the specimen, or distortion of the specimen image. In some cases apparently sharp images may be obtained, but the sizes of objects are so distorted that accurate measurements cannot be made. In electron diffraction, a charge on the specimen may shift the pattern, cause unsharpness in the lines or spots and, in many cases, completely destroy the pattern or prevent its formation.

It is obvious that specimens which are good electrical conductors will cause the least image distortion; in fact, unless the specimen is a good insulator, electrostatic effects are negligible in electron microscopy. This condition exists largely because the specimen is very small and is bathed by the electron beam, and a, uniform distribution of charge is speedily obtained. Insulating materials, however, can be studied with ease in the electron microscope if a Very thin film of metal is first deposited on the surface of the material.

The most pronounced charging effects are observed with reflection specimens in electron diffraction. In the study of these specimens, the electron beam strikes one surface only and, if

the specimen is a poor conductor, the charge ac-- cumulated on the bombarded surface cannot leak off. Not only the specimen but its method of mounting is important 'if charging effects are to be avoided. The mounting method should permit specimens to be well grounded, but even this precaution will not overcome electrostatic effects on insulating specimens.

A well known and frequently used method of neutralizing the charge on insulating specimens involves spraying the specimen with a stream of low velocity electrons from an auxiliary electron gun. The auxiliary guns heretofore used for this purpose have been of conventional construction. Usually the apertured cap (i. e. the anode) of the gun is maintained at ground potential and the filament at a negative potential to simplify mounting the gun in a port in the wall of the electron-microscope or dilTract-ion camera. Electrons from the filament are accelerated through a potential of a few hundred volts, pass through the aperture in the anode and are directed toward the specimen. The charge on the specimen surface is neutralized by these neighboring low velocity electrons, and the space charge due to the electron stream produces no measurable deflection of the high velocity primary electron beam.

It has been found that low velocity electron space current at the specimen for effective charge neutralization varies widely with the specimen, but is generally of the order of 1 to 10 ma. An auxiliary electron gun of the conventional type heretofore employed can readily supply this current. However, a very large fraction of the total auxiliary gun beam current does not pass through the anode aperture, and hence does not contribute to charge neutralization at the specimen. This necessitates running the filament at a high temperature to provide adequate emission so that suflicient number of electrons will be available at the specimen for neutralization. This condition gives rise to a second serious disadvantage in the use of such a gun.

The high temperature at which the filament is operated causes considerable evaporation of tungsten from the filament, and a beam of tungsten atoms is directed at the specimen and deposits thereon. This coating of tungsten builds up rapidly on the specimen, masks detail in electron microscopy and, in reflection studies in electron diifraction, soon builds up to a thickness that cannot be penetrated by the primary electron beam and may mask the specimen completely. This effect often occurs before a satisfactory diffraction pattern can be photographed, and often renders the specimen completely useless, as cleaning 01? the tungsten deposit would also remove the surface condition to be studied.

Accordingly, the principal object of the present invention is to obviate the foregoing and other less apparent disadvantages incident to the use of present day electron-guns in the neutralization of electric charges upon microscope and diffraction specimen-materials and, more specifically, to provide an improved method of and apparatus for neutralizing such charges by the use of an electron cloud which is substantially free of metal vapor.

The invention is described in detail in connection with the accompanying drawing, wherein: Figure 1 is a fragmentary view partly in section of so much of an electron-difiraction camera as is necessary for an understanding of the principle of the invention, and including a charge-neutralizing gun, and Figure 2 is a sectional view of the charge-neutralizin electron-gun of Figure 1.

In Fig. 1 of the drawing l designates, generally, the casing of an electron-optical instrument which may be employed as an electron microscope but which, in the instant case, is set-up for use as an'electron-diffraction camera. The casing I has a central electron-optical axis x-rr along which electrons travel at high velocities (say, 50-100 kilovolts) from a source exemplified by a conventional electron-gun 2, through a collimating aperture 3 and a series of two magnetic lenses 4 and 5 to a specimen 6 which is cemented or otherwise affixed to an adjustable diffraction-specimen holder 7. The holder 7 may conveniently be of the construction described in U. S. Patent 2,418,- 903 to Frank E. Runge.

As is-well understood, the high velocity electrons from the gun 2 are diffracted from the specimen in' a pattern determined by the material of which the specimen is composed and the diffracted electrons proceed to a fluorescent screen or other target, not shown. As also is well understood the specimen 6, ordinarily acquires an electrically positive charge as a result of the impact thereon of the high velocity electrons from the gun 2 and, in the absence of same means for neutralizing the said charge or field about the specimen the electron-image or pattern of the specimen will be distorted.

It has heretofore been proposed (see Journal of Applied Physics, vol. 16, Feb. 1945, pp. 94-96) to prevent such image defects by neutralizing the charge or field about the specimen by bombarding the specimen with electrons from an auxiliary source of electrons, indicated generally at 8, mounted in port 9 a side wall of the casing l in register with the specimen' As above set forth, an electron-gun of conventional construction may be used for neutralizing the positive charge acquired by a specimen under bombardment by high velocity electrons, but the use of such a gun gives rise to other serious image defects. The present invention is predicated upon the discovery that these other image defects arise from the deposition of metal vapor (from the cathode of the gun) upon the specimen, and the invention may be said to reside in the hereinafter described method of and apparatus for minimizing this undesired deposit of metal vapor.

The generation of metal vapor is minimized, in accordance with the present invention, by operating the filament or other thermionic electronsource at the lowest temperature required to produce electrons in a quantity sufficient to neutralize a positive charge of a given intensity and by utilizing substantially all of the said electrons for neutralization purposes. This is best achieved by the use of an electron-gun of the novel construction shown in Fig. 2.

Before proceeding with a description of the electron-gun of Fig. 2 it may be well to point out that a tungsten filament in a gun of conventional design is ordinarily operated within the temperature range of from, say 1500 to, say, 3000 Kelvin and that the rate of evaporation of the tunesten increases over the said range by a factor of 10 while the rate of emission of electrons increases merely by a factor of 10 Thus, while a lower operating temperature provides a lower rate of emission there is a disproportionate decrease in.

the quantity of metal evaporated from the filament at a low operating temperature. day guns, however, are unable to take advantage of this phenomenon because a very large percentage of the total quantity of electrons emitted by the filament fails to pass through the anode aperture and it is therefore necessary to operate the filament at a high temperature (and hence with disproportionately high metal-vapor emission) to provide an adequate supply of electrons adjacent to the specimen or target at which the gun is aimed. As will hereinafter more fully appear, the electron-gun of Fig. 2 is especially constructed to permit the egress of substantially all of the electrons which are emitted by its filament and therefore can be operated at a relatively low temperature (with a disproportionately low emission of metal vapor) to provide enough electrons for adequate neutralization or the electrostatic charge which the specimen acquires while under bombardment by the high-velocity electrons from the main gun of the instrument.

The electron gun shown in Fig. 2 is designed to be received within a port 9 (Fig. l) in the metal 7 wall of the casing I (Fig. 1) in register with the specimen holder 1 and, to this end, comprises a metal socket ill having an attachment flange H which is provided on its inner surface with a groove l2 for a vacuum-tight gasket 13. When the wall of the specimen chamber of the casing l is thick enough the inner surface of the bore of the port may comprise a portion of the barrel of the gun, in which case the leading end or muzzle of the gun, may comprise a cylindrical sleeve I 3 which may be fitted into the saidbore. Alternatively, as shown in Fig. 1 this sleeve or barrel It may extend all the way through the port-and be attached to the socket it. Supported concentrically within the barrel of the gun, as on one or more rod-like conductive leads I5 is a cylindrical focusing electrode It and, within this open ended cylinder, is a bowed filament I '1 whose opposite ends are affixed by small metal collars l8 and It to a pair of similar conductive supports 28 and 2|, respectively. Each of these conductive supports I5, 26 and 2| extends through a vacuumtight glass--to-metal seal 22, 23, 24, respectively,

to the outer side of the socket where the filament supports 20 and 2i are connected to current supply leads 25 and 26 respectively. It will be ob- Present:

served that in the instant case one of the filament leads is connected by a short lead 27 to the focusing cylinder It which is thus maintained at the same potential as the filament ll. The casing l of the instrument and hence the barrel or anode M of the gun are at ground potential and the filament ll and the focusin electrode it are ordinarily maintained at the same negative potential (say 200 to 1000 volts negative) with respect to ground. However, a separate external lead (not shown) may be provided for the electrode it in the event that it becomes necessary or expedient to confine the cloud of electrons which are released by the filament to a limited area about the specimen.

It will be observed upon inspection of both Figs. 1 and 2 that the anode or barrel M of the neutralizing gun 8, unlike that of the conventional probe-gun 2, is not constricted at its muzzle but is simply covered with a screen or grid 28 of open work construction. Thus, the only barrier to the passage of the electrons through the muzzle of the neutralizing gun comprises the thin (stainless steel) wires of which the screen is composed. These wires may be of the order .001 of an inch thick and need comprise no more than, say, 25 percent of the total area of the screen; hence the path of the electrons between the filament and the specimen is substantially unobstructed and the vast majority of the electrons released from the filament are available for neutralizing purposes. In one successful embodiment of the invention wherein the neutralizing gun comprised a .006" tungsten filament mounted in a focusing cylinder of diameter, with the said cylinder enclosed in an anode having a bore of one inch satisfactory neutralization of the electrostatic charge about a specimen, mounted about two and one-half inches from the end of the gun, was achieved when its filament was operated at a temperature about 500 K. lower than the filament temperature required when a gun of conventional design was employed for the same purpose and, as a result of the disproportionate decrease in the quantity of metal evaporated from the filament at the said lower operating temperature, neutralization was achieved without injury to the specimen and without producing any noticeable electron-optical image defect.

What is claimed is:

1. The combination in an electron-optical instrument of the type wherein a specimen is subjected to bombardment by electrons of a velocity sufliciently high to cause the release of secondary electrons from the said specimen and the consequent development of an electrically positive charge adjacent to the surface thereof, of a thermionic cathode for generating a substantially metal-free cloud of electrons within said instrument, and a foraminous electrode mounted intermediate said source and said specimen through which said vapor-free cloud of electrons is adapted to be drawn to said specimen for the purpose of neutralizing said positive charge.

2. The invention as set forth in claim 1 and wherein said foraminous electrode comprises the only barrier to the passage of electrons in the space between said cathode and said specimen.

3. Method of operating an electron-optical instrument of the type wherein a specimen is bombarded by electrons of a velocity sufilciently high to cause the release of secondary electrons and the consequent development of an electrosataic charge adjacent to the surface of said specimen, said method comprising: bathing the region about said specimen with a substantially metal-free cloud of low-velocity electrons while subjecting said specimen to bombardment bysaid high-velocity electrons.

4. Method of operating an electron-optical instrument of the type wherein a specimen is bombarded by electrons of a velocity sufliciently high to cause the release of secondary electrons and the consequent development of a positive electrical charge adjacent to the'surface of said specimen, said method comprising: subjecting a thermionic electron-emissive metal to an emissive temperature sufficiently low to produce a cloud of electrons substantially free of metal vapor, and then bathing the region about said specimen with said metal-free electron cloud while bombarding said specimen with said high velocity electrons.

5. Method of operating an electron-optical instrument of the type wherein a specimen is bombarded by electrons of a velocity sufficiently high to cause the release of secondary electrons and the consequent development of a positive electrical charge adjacent to the surface of said specimen, said method comprising: producing within said instrument in a region remote from said specimen a cloud of electrons substantially free of metal vapor, establishing a substantially unobstructed path for said cloud of electrons between the region of origin of said cloud and said specimen, directing said cloud of electrons along said substantially unobstructed path to said specimen and then bombarding said specimen with said high velocity electrons.

ROBERT G. PICARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,003,301 Michelssen et al. June 4, 1935 2,088,419 Knoll et a1. July 2'7, 1937 2,129,005 Hudec Sept. 6, 1938 2,233,876 Skellett Mar. 4, 1941 2,271,990 Ramberg et a1. Feb. 3, 1942 2,289,071 Ramo July 7, 1942

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