US 3763346 A
This invention relates to a method of shaping, resharpening, or cleaning tips, particularly the ends of filaments used as electrodes emitting electric particles by field effect. This process consists in the steps of placing said tip under a high vacuum and of subjecting it to a bombardment by electrons so that the temperature of the tip is lower than the fusion temperature.
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Description (OCR text may contain errors)
l0--2-73 OR 1ln1ted States Patent Drechsler et al.
Oct. 2, 1973 METHODS OF SHAPING RESHARPENING OR CLEANING TIPS Inventors: Michael Drechsler, Marseille;
Jean-Paul Prulhiere, Courbevoie, both of France LAgence Nationale De Valorisation De La Recherche (Anvar), Courbevoie, France Filed: July 29, 1971 Appl. No.: 167,245
lForeign Application Priority Data July 31, 1970 France 7028388 11.5. CI. 219/121 EM lint. Cl 823k 15/00 lField of Search 219/121 EB, 121 EM;
250/495 R, 49.5 JE; 29/630 R  References Cited UNITED STATES PATENTS 3,385,949 5/1968 Hcil 219/121 EB Primary Examiner-.l. V. Truhe Assistant Examiner-Gale R. Peterson Att0rneyCurtis, Morris & Safford  ABSTRACT This invention relates to a method of shaping, resharpening, or cleaning tips, particularly the ends of filaments used as electrodes emitting electric particles by field effect. This process consists in the steps of placing said tip under a high vacuum and of subjecting it to a bombardment by electrons so that the temperature of the tip is lower than the fusion temperature.
4 Claims, 4 Drawing Figures METHODS OF SHAPING RESHARPENING R CLEANING TIPS The present invention relates to methods of shaping, resharpening, or cleaning tips with a very small radius of curvature, particularly tips used as electrodes emitting electric particles by field effect.
it also relates to devices for carrying out these methods.
According to the invention, in order to shape, resharpen or clean a tip, it is bombarded under a high vacuum by a beam of electrons. The filament, at the end of which said tip is located, is placed in a sealed chamber, in which a high vacuum prevails. It is taken to an anodic potential. A beam of electrons, coming from an auxiliary cathode, is focussed on the end of said filament. it causes said latter to heat up and the removal of matter so that a sharp profile is finally obtained which is in the form of a tip.
it is known that tips with a very small radius of curvature, of the order of a micron or even smaller, are used in the vacuum as sources of electric particles. For example, in the electron guns of electronic microscopes of certain cathode tubes, the emitter cathode is constituted by such tips taken to high voltage, which emit a beam of electrons under the influence of the electric field.
A first application of the invention will therefore be the shaping of these tips either to manufacture them or to resharpen them after a certain period of use.
Another application will be the cleaning of these tips in order to remove impurities which are deposited on their surface, these operations being carried out in situ.
llt frequently happens that such an emitter tip is deteriorated during its use. The changing of the tip is then a complicated operation which necessitates much time, thus limiting the applications of the electron guns equipped with these tips despite their other advantages.
The invention eliminates this disadvantage by enabling the deteriorated tips to be resharpened and cleaned in situ and in vacuo.
Tips are also used as source of ions in mass spectrometers with field ions and the invention may be applied to the manufacture, resharpening or cleaning of such a tip in the chamber itself where it is used in vacuo.
In electronic or ionic microscopes, by field effect, metal tips are used for example as samples to be observed, which are taken in a high vacuum to a high potential and the image of the electron or ion beam is observed on a fluorescent screen.
Obviously, such a tip must have regular geometric shapes and must generally be free of any impurity.
One application of the invention resides in the making, reshaping and cleaning of these tips in situ.
The list of the possible applications of the tips made according to the invention is non-limiting. The invention may be applied in all cases where use must be made of sharp tips, with a small radius of curvature, for example, tips for diodes.
To this day, the micro-tips used as emitter electrodes by field effect or as objects of observation in a microscope with field emission, are made by electrolytic attack of the end of a filament at atmospheric pressure. Very irregular profiles are thus obtained. These tips are then positioned in the evacuated chamber where they are to be used and the profile is made regular by taking the filament to high temperature in vacuo.
The disadvantage of this method lies in the fact that all the filament must be heated in order to heat the tip. The temperature to which the tip may be taken is then limited as it is lower than that of the filament and the temperature at which the filament breaks must be avoided.
it was previously proposed to make, by electronic bombardment, cathodes for thermo-electronic emission having a spheric end. For manufacturing such spheric cathodes, the end of a very thin wire is bombarded by a beam of electrons. This bombardment supplies, by a process of fusion, the formation of a spheric swelling to the end of the wire.
The inventors have observed that it is possible to obtain tips, by sharpening the end of a wire by electronic bombardment.
This sharpening is produced first essentially by localized evaporation of the material, this evaporation being attended and chiefly followed by surface and eventually volume diffusion, this process producing under the influence of an intense electric field, a shape still sharper. This process, excludes naturally that the end of the wire may be set to the fusion temperature.
The method according to the invention consists to heat, by electronic bombardment, the end of a tip in order to take it locally to a temperature so high as to produce removing of material by evaporation and displacing of material toward the end of the tip by surface and eventually volume diffusion, the temperature of the end being lower than the fusion temperature.
This method permits to obtain very sharp tips with a radius of curvature smaller than 1 micron. The intensity of the electric field about the end of the tip is important. The inventors have observed that with an electric field whose intensity is higher than 2 l0 volts/centimeter it is possible to make the tips, the radius of curvature of which is smaller than 1 micron.
Another advantage of the method according to the invention resides in the fact that the shaping may be instantaneously stopped by causing the electronic bombardment to cease.
Another important advantage of the invention resides in the fact that the shaping operations may be eas ily controlled and automatically stopped as soon as the sought after radius of curvature is reached. To this end, it is sufficient to periodically interrupt the electronic bombardment and to feed the tip being shaped with a high negative voltage in order to render it emitting by field effect.
The beam emitted may be observed on a fluorescent screen in order to check the cleanness of thetip. The intensity of the current produced by the beam of electrons emitted by .the tip may be measured. As the law which connects this intensity with the voltage and the radius of curvature of the tip is prefectly known, it is easy to stop the bombardment automatically as soon as the desired intensity of emission is achieved and auto-' matically to start the resharpening when the intensity of the emission falls below a reference threshold.
The tips shaped or resharpened according to the invention are of perfect revolution, have a very regular profile and their radius of curvature may be very small, smaller than 0.1 micron.
The different characteristics and advantages of the invention will be more readily understood upon reading the following description of several embodiments given by way of example, without limiting character, refer ence being made to the accompanying drawings, in which:
FIG. 1 shows a device for shaping tips according to the invention;
FIG. 2 shows the photograph taken by electronic microcope of a tip made with the device of FIG. 1;
FIG. 3 schematically shows another device in accor dance with the invention, used in an electronic or ionic microscope with field effect; and
FIG. 4 schematically shows the combination of a device according to the invention with'the electron gun of an electronic microscope with cathode tip.
Referring now to the drawings, FIG. 1 shows a filament 1, for example a tungsten filament with a diameter of I microns, the end 2 of which has to be machined into a tip. This filament is placed in a chamber 3 evacuated by means of a vacuum pump 4, enabling a vacuum to be obtained which is comprised for example between 10 and 10 torr. In the chamber 3 there is disposed a source of electrons, for example, a cathode 5 with thermic emission and electrostatic lenses constituted by a plate 6 perforated with a hole and by a ring 7. The filament 2 is taken to a positive potential with respect to the cathode 5. The anodes 6 and 7 are taken to a negative potential with respect to the cathode 5 and focus the electron beam 8 issuing from 5 on the end 2 of the filament 1.
Under the effect of this electronic bombardment, the end 2 heats up and becomes sharp. The temperature and speed of shaping may be adjusted by regulating the intensity of the electron beam, so that the temperature of the end of the tip be lower than the fusion temperature.
FIG. 2 shows the photograph of a tip made with the device of FIG. 1. The radius of curvature of the tip is smaller than onetenth micron and the total angle of aperture of the cone is of the order of 12. The tip is of perfect'revolution and the profile very regular.
The device of FIG. I enables tips to be shaped, in filaments whose diameter is about 100 microns, whose radius may vary between 0.1 micron and 100 microns.
FIG. 3 shows the combination of a device according to the invention with an electronic or ionic microscope with field effect. In such a microscope, the sample observed is constituted by the end of a filament 9 cut to a point. This tip, placed in an evacuated chamber 10 is rendered emissive by taking it to a high potential with respect to the screen 12. The electron or ion beam 11 which is emitted is received on a fluorescent screen 12 where it gives a visible image of the tip. This technique is used for example for metallographic studies. For example, it may permit the individual atoms to be visualized or the study of the monoatomic absorption layers.
At the present, the tips used are electrolytically shaped at atmospheric pressure. Observation is rendered difficult by the impurities which are deposited on the surface of the sample.
The device according to the invention consists in disposing about the tip 9 an electron source constituted by a hot filament l3 heated by a current source 24. Two cylindric electrostatic screens 14 and 15 are disposed about the tip 9 in order to deflect the electron beam 16 and to focus it near the end of the tip.
The device comprises switch means 26 for varying the potential supplied to the tip 9, the auxiliary cathode I3 and the two screens 14 and 15 by the DC. source and by the potentiometers 27 and 28.
The tension of the source 25 is variable between 3 and 10 Kilovolts. During the period of observation, the switches are in the position shown in continuous line. The tip 9 is set to a negative potential whereas the screens 14 and 15 and the auxiliary cathode 13 are set to a positive potential. The tip 9 emitts normally the electron beam 11.
When it is necessary to clean the end of the tip 9 or to reshape it, switch means 26 are set in the position shown by a dotted line.
The tip 9 is set to a positive or anodic potential in comparison to the potential of the screen 15, the cathode 13 and the screen 14 which are set to potentials more and more negative. The auxiliary cathode l3 emitts an electron beam 16 which is directed and focused near the end of filament 9.
The potentiometers 27 and 38 permit to reduce during the bombardment, the intensity of the electron beam in order to avoid the fusion of the tip.
During the period of observation, the tip 9 is taken to a negative potential. When it is desired to clean it or reshape it, it is sufficient to take it to a positive potential with respect to that of the cathode 13. Cleaning in vacuo of the impurities is carried out very rapidly in 10 to 10 seconds.
In the physical or chemical studies with emitting tips, operation must sometimes be carried out at a constant temperature in order that a process can develop. Usually, the tip is heated by conduction by means of an electric current passing in a loop. The disadvantage of this process comes from the temperature gradient provoked by the radiation of the tip and the crystal studied, placed at the end of the tip, is at a relatively low temperature. As the impurities tend to move towards the relatively colder regions of the surface, there is contamination of the crystal studied. On the contrary, if the tip is heated by electronic bombardment according to the process of the invention, the temperature gradient is partially reversed and the above-mentioned drawback is avoided.
FIG. 4 represents the combination of a device according to the invention with the electron gun of an electronic microscope equipped with a cathode tip 18. Reference numeral 17 designates the evacuated chamber 19, the Wehnelt cylinder and 20 the acceleration anode of the microscope. The cathode tip 18 is mounted on a heating loop 21 enabling the gun to be used at high temperatures.
According to the invention, there is a permanent auxiliary electron source constituted for example by a thermic emission filament 22.
When it is desired to regenerate or clean the cathode tip 18, it is sufficient to take it to an anodic potential, to start the auxiliary cathode 22 and adjust the potentials of the Wehnelt and the electrode 20, so that the electron beam 23 is deflected towards the end of the tip of the cathode 18. This example shows the simplicity that there is in carrying out the invention which, in this latter case, necessitates only the addition to the electron gun of an auxiliary cathode 22 and of devices enabling the feed voltages of the cathode tip and the electrostatic lenses to be modified. This device enables a constant electron source of high quality and a relatively long life duration to be had.
On the other hand, this device enables cathodes with field emission to be used in a less high vacuum, for example in a vacuum of between 10 and 10" torr.
In the examples described, the electrons are deflected and focussed by electrostatic screens or lenses. This same function could be replaced by magnetic lenses.
in certain cases, the sharpening operation of the tip may he completed by a simultaneous chemical reaction. A gas, for example oxygen, is introduced in the evacuated chamber at a predetermined partial pressure. At the high temperature reached, the substance of the tip and the gas, for example oxygen, react on each other and the products of the reaction are evapo rated.
lln certain cases of application, it is necessary to obtain tips having a well-determined radius of curvature. The invention enables the value of the radius of curvature to be very simply checked in situ and the electronic bombardment to be automatically stopped as soon as the desired radius is reached.
To this end, electronic circuits are disposed of for feeding the tip and the auxiliary cathode which enable the bombardment of the tip to be periodically interrupted and enable it to be rendered emissive. These circuits may cover all known forms. They are controlled for example by a cyclic programmer or by a trigger circuit which receives pulses. The tip is rendered emissive by field effect by applying thereto a high negative voltage of known value. The current emitted is measured and is compared with a reference value corresponding to the desired radius of curvature. As soon as this value is reached, the shaping process is automatically stopped.
A device according to the invention used for regenerating the deteriorated tips in the vacuum, may comprise circuits for automatic starting. If a tip is used as source of electric particles by field effect, as in the examples corresponding to H68. 3 and 4, a supplementary electric circuit periodically interrupts the working voltages and starts the checking system with emissions by field effect.
The intensity of the electronic beam transmitted is compared with a second reference intensity which corresponds to a given radius of curvature, beyond which the tip must be regenerated. As soon as this reference value is reached, the shaping process is automatically stopped as previously.
ln certain cases, for example, if a tip is to be successively regenerated, it is useful to be able to control, from the outside of the evacuated chamber, a relative displacement of the tip and the electronic lenses, in order to be able to place the end of the tip near the focus of the electronic beam. Known devices are used, for example devices constituted by deformable metallic membranes.
An additional advantage of the invention resides in that not only can the impurities located on the surface be cleaned but also the impurities absorbed which diffuse towards the surface. The cleaning that is effected in a few seconds in vacuo is much better than that obtained by other methods.
The devices according to the invention are used mainly for sharpening metal tips. They may also serve to cut tips in other substances such as semi-conductors or oxides.
What is claimed is:
l. The method of shaping, resharpening, or cleaning a pointed tip located at the end of a filament comprising, the steps of placing said end inside a sealed chamber in which a high vacuum is created and bombarding said end by means of an electron beam so that the end of said filament is at a temperature lower than the fusion temperature, said temperature being so high as to remove said material by evaporation, and to displace it towards the end by diffusion.
2. The method as defined in claim ll wherein said electronic bombardment is periodically interrupted, said tip is taken to a high negative potential in order to render it emissive by field effect, the electrons emitted by said tip are picked up on a fluorescent screen placed in front of it and the enlarged image of said tip formed by the electrons is observed on said screen in order to check the condition of said tip.
3. The method as defined in claim 1 wherein the radius of curvature of the tip must be smaller than limiting value, and wherein the electronic bombardment is periodically interrupted, said tip is rendered emissive by field effect by taking it to a high negative potential; the intensity of the current emitted by said tip is compared with the intensity of a reference current which corresponds to that which is emitted, under the same conditions, by a tip having the radius of curvature chosen as limit and the bombardment is stopped when the intensity emitted reaches that of the reference current.
4. The method according to claim 1 wherein a cathode tip is resharpened or cleaned so that its emissive characteristics remain higher than a given limit, and wherein said cathode is rendered periodically emissive by field effect by taking it to a high negative potential; the intensity of the current emitted is compared with a first reference current whose intensity corresponds to said lower limit and, if it is lower, an electronic bombardment of said cathode tip is automatically started; this bombardment is periodically interrupted; after each interruption, the tip is rendered emissive by field effect; the intensity of the current emitted is compared with a second reference current having an intentity higher than that of the first reference current and, as soon as this second intensity is reached, the electronic bombardment is automatically interrupted.