Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J27/00—Ion beam tubes
  • H01J27/02—Ion sources; Ion guns
  • H01J27/20—Ion sources; Ion guns using particle beam bombardment, e.g. ionisers
  • H01J27/22—Metal ion sources
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J27/00—Ion beam tubes
  • H01J27/02—Ion sources; Ion guns
  • H01J27/26—Ion sources; Ion guns using surface ionisation, e.g. field effect ion sources, thermionic ion sources

Definitions

• the present invention relates to the field of liquid metal ion sources, in which ions are produced from a filler metal which covers a refractory metal tip.
• the emissive zone is of the order of a few niti ⁇ for an emission current of about 2 ⁇ A.
• the article by Bell et al. published in "Journal of applied physics", vol. 53, no.7. July 1982, pp. 4602 to 4605 describes a source of aluminum ions consisting of a graphite point fixed on a filament of tungsten heating.
• the graphite tip is treated on the surface by depositing a titanium film.
• graphite is relatively resistant to attack by liquid aluminum, but it is difficult to wet it with such a liquid.
• the use of titanium film solves this problem.
• the source has the form illustrated in FIG. 1, where the reference 2 designates the graphite tip, which has a cylindrical part at its base.
• a heating filament 4 made of tungsten passes through this cylindrical part. The heating of the tip 2 is obtained by the heat released by the Joule effect in the filament.
• This type of source has the advantages on the one hand of simplicity, and on the other of compactness since the graphite tip has a diameter of about 0.8 mm, for a length of 2 mm.
• the present invention seeks to solve these problems.
• a source of liquid metal ions comprising a cylindrical rod of a conductive and refractory material extended by a tip of refractory material, intended to be covered with a liquid filler metal, characterized in that the assembly constituted by the cylindrical rod and the tip passes through a reservoir made of a conductive material, the area where the rod is engaged in the reservoir ensuring electrical contact between the rod and the reservoir, and in that the reservoir is in contact with a conductive filament, the cylindrical rod, the reservoir and the conductive filament being thus connected in series from the electrical point of view.
• Such a constitution for a source of liquid metal ions makes it possible to limit the supply of energy necessary for optimal operation of the source.
• production of a heating only in a very localized zone limited to the part of the cylindrical rod in the vicinity of the point, to the reservoir and to the tungsten filament.
• the most resistive element of the circuit thus formed is the cylindrical part of the rod, which, when crossed by a current of 5 amperes, by Joule effect, reaches a temperature of 700 ° C near its end.
• the power consumed is limited to less than 10 Watts.
• the assembly is of a much less delicate assembly and positioning than in the sources of liquid metal ions known from the prior art. Finally, the mechanical stability is clearly improved. The use of a reservoir also makes it possible to increase the reserve of liquid metal and, therefore, the lifetime of the source.
• the rod and the tip form a single piece
• the rod and the point can be made of graphite.
• the graphite tip is covered with a titanium film.
• the tip is covered with a metallic bonding layer of the same nature as the liquid metal intended to be used with the source.
• the bonding layer can not only cover the tip, but also part of the tank.
• the liquid metal intended to cover the graphite tip is, for example, aluminum
• a surface treatment of the tip makes it possible to improve the wettability of the latter by liquid aluminum.
• the second has the advantage of making it possible to obtain a very homogeneous aluminum film over the entire tip.
• the treatment of the surface of the graphite tip by depositing a titanium film does not allow wetting to be carried out homogeneous of the tip with an aluminum film: one obtains, in fact, a formation of aluminum islands on the surface of the graphite tip and it follows that the function of bringing aluminum to the tip is very disturbed. The emitted current is then unstable and very difficult to keep constant over long periods.
• the second treatment promotes on the one hand the function of supplying the metal to be ionized towards the apex of the tip, and on the other hand makes it possible to increase the quantity of stored filler metal.
• the ion current obtained during the production of aluminum ions is all the more stable over time.
• the cylindrical rod and the tip are adjusted mechanically with tight tolerances inside the zone provided for their passage through the tank. This adjustment has the following advantage.
• FIG. 1 already described, represents a source of liquid metal ions according to the prior art
• FIG. 2 represents a source of liquid metal ions according to the present invention
• FIGS. 3a and 3b represent two examples of a reservoir used in a source 'of liquid metal ions according to the present invention
• - Figure 4 represents an embodiment of a rod for a source according to the invention
• FIG. 5 shows the treatment device for preparing a liquid metal ion source according to the present invention
• FIG. 6 and 7 give examples of results obtained with an ion source according to the present invention.
• FIG. 2 illustrates a particular embodiment of a source of liquid metal ions according to the present invention.
• This source consists of a conductive rod comprising a cylindrical part 10 and a point, the latter itself being constituted by a conical part or end 12 and a cylindrical part 13.
• the cylindrical part 10 consists of a conductive and refractory material.
• graphite is well suited for application to an ion source • of a light metal, such as aluminum. But this does not exclude the use of other materials, such as for example tungsten.
• the rod, in its cylindrical part 10 may have, in the case of graphite, a diameter of the order of a few tenths of a millimeter, for example 0.5 mm, and a length of between 5 and 20 mm, for example 15 mm.
• a simple pencil pencil of the "HB" type has been used, which has been entirely satisfactory.
• the end 12 of the tip is cut into a cone shape, with a half-angle at the top having a value of between 48 and 50 °, for example 49 °, by mechanical polishing in two stages.
• the point is placed in rotation, inclined by about 49.5 °, and it is brought into contact with a plane which will machine the conical part.
• the cone is roughed out on an average roughness surface, around 30 ⁇ m.
• the finishing is carried out on a surface of low roughness, for example of a few microns.
• the tip that is to say the assembly constituted by the cylindrical part 13 and the conical end 12 has a total length of between 5 and 10 mm, the cylindrical part 13 having a length of a few mm
• the assembly forms a single piece.
• the rod is introduced into a reservoir 14, two exemplary embodiments of which are illustrated more precisely in FIGS. 3a and 3b.
• the reservoir designated by the references 14-1 and 14-2 has a shape having substantially a symmetry of revolution about an axis passing through the part 10.
• a cylindrical opening 19 inside the tank allows the passage, but also the maintenance of the rod in a fixed position and contributes to the mechanical stability of the assembly.
• the reservoir 14-2 has a recessed part 18, in the form of a chamfer. The purpose of this recess is to increase the capacity, in liquid metal, of the reservoir.
• An interior recess 17 makes it possible to reduce the volume of material used for the reservoir.
• the reservoir is made of a conductive and refractory material.
• the rod 10 is made of graphite, it will suffice to choose for example graphite as the material for the reservoir.
• a particularly advantageous form of graphite is vitreous carbon. Unlike polycrystalline graphite, which has micropores and microcracks, vitreous carbon has an impermeable structure with closed pores. However, long-term aging of the source has been observed. It manifests itself by the appearance of a graphite powder on the surface of the liquid filler metal, in particular in the case of aluminum.
• the tip is made of glassy carbon, its longevity will be extended and the resistivity will be improved. In addition, the point alone can always be cut electrochemically.
• the rod 10 being used as an electrical conductor, there is, in principle, production of a heating in a very localized zone, limited to the conical part 12, to the cylindrical part 13 and to the reservoir 14.
• the temperature can reach a value higher than that of the temperature at the end of the rod, in the vicinity of the tip 12, because in the vicinity of this tip the presence of liquid metal contributes to dissipating heat.
• the liquid metal may have a tendency to diffuse thermally, along the rod, towards the hot spots, which has the consequence of gradually emptying the reservoir and, therefore, of decreasing the autonomy of the source.
• it may be advantageous to mechanically adjust the cylindrical part 10 of the rod with tight tolerances, without any play, inside the cylindrical opening 19 (see FIGS. 3a and 3b) provided for the passage of the rod inside the reservoir 14. Consequently, the liquid metal cannot escape along the rod. Furthermore, this strengthens the mechanical strength of the assembly.
• a tank was produced with a diameter of approximately 5 mm, for a height of approximately 2 mm. The cylinder 19 is machined to the nominal diameter of the rod 10 and the latter is then forced into it by hand. This is sufficient to provide the required tight fit.
• the reservoir 14 is electrically connected to a heating circuit.
• This circuit can be constituted for example by a tungsten filament 16 wound around the base of the reservoir 14, on the side opposite to the tip 12.
• the ends of this filament are themselves connected to conductive elements 24, 26, such as for example tantalum plates.
• the filament 16 is not in contact with the outer surface of the reservoir 14, but it is introduced into a groove cut in the surface of this reservoir. This makes it possible to limit the contact between the heating filament 16 and possible drops of liquid metal which could diffuse along the external surface of the tank 14. Indeed, certain liquid metals, and in particular aluminum, are extremely corrosive vis -to metals.
• the rod is held at its base by a clamp consisting of two jaws 28, 30 which has the function, on the one hand of ensuring mechanical maintenance of the rod with maximum rigidity without weakening it, and on the other hand of ensure reliable electrical contact with the material of this rod, electrical contacts which allow the circulation of a current of approximately 6 A.
• the heating circuit comprises the rod, with its cylindrical part 10 and its tip, the reservoir 14 and the filament 16. From the electrical point of view, all these elements are connected in series and the assembly operates under a voltage d floating supply of the order of a few volts.
• the most resistive element of the circuit is the cylindrical part 10 of the rod, which, when crossed by a current of 5 A, reaches a temperature of 700 ° C by Joule effect.
• the fact of using the rod as a heating element makes it possible to limit the power consumed to approximately less than 10 Watts.
• the assembly rests on a base 32, crossed by 3 threaded rods 27-1, 27-2, 27-3.
• the central threaded rod 27-2 is extended by the jaws 28, 30; the lateral rods 27-1 and 27-3 are extended by fixing jaws 29-1 and 29-2 of the plates 26 and 24. All of these elements (jaws, threaded rods) are part of the electrical circuit.
• the structure which has just been described gives very good mechanical stability to the assembly, and in particular to the tip 12. This makes it possible to stabilize the emissive zone, at the apex of the tip, and to make the source compatible with the 'use in technical fields where the precision required is extremely high, for example in electrostatic optics.
• the source described is fully compatible with already existing machines or systems on which the sources of liquid metal ions of the prior art are adapted.
• FIG. 4 Another embodiment of the rod is shown in Figure 4.
• the rod passes through a reservoir 15.
• the latter is similar to that described above in connection with Figure 3b, except for the cylindrical opening 33 which has a larger diameter in its upper part than in its lower part, thus defining a shoulder 34.
• the rod also consists of a cylindrical part 35. It is extended by a point which itself consists of a conical part 37, a cylindrical part 39 and a flange 41.
• the rod 35 is of diameter substantially wider than in the first embodiment of the rod.
• the cylindrical part 39 and the end 37 of the tip have themselves substantially the same dimensions as above.
• the cylindrical part and the tip are made of two different materials.
• the rod is made of a conductive and refractory material, for example graphite. As in the first embodiment, it can be carried out for example from a pencil lead. It is introduced over part of the depth of the cylindrical opening 33, so as to be in contact with the tip.
• the tip is made of a refractory material such as boron nitride or alumina. It is introduced so that the rim 41 rests on the shoulder 34 and it is in contact with the end of the cylindrical part 35.
• the tip 37 is cut with a half-angle at the top between 48 and 50 ° (worth for example 49 °) by mechanical polishing using an abrasive such as for example a diamond grinding wheel.
• This second embodiment can also be used in combination with a tank of shape similar to that described in connection with FIG. 3a, provided that the cylindrical opening is adapted in a corresponding manner (opening of the wider cylinder in its upper part than in its lower part). What has been said in the context of the first embodiment, on the mechanical adjustment with tight tolerances applies equally to this second embodiment.
• the operation of the source is the same; there is always electrical contact between the rod 35 and the reservoir 15 and the current flows from the rod to the reservoir and to the heating filament. This current produces a heating of the point by Joule effect.
• a variant of this surface treatment consists in depositing the titanium film by spraying before bringing it under vacuum to a temperature of around 1700 ° C. to melt it.
• this surface treatment a significantly improved homogeneity of the aluminum film compared to the homogeneity obtained in the case where the surface is treated with the method described above. This makes it possible to have a good contribution function towards the apex of the tip, and therefore a more stable ion current over time.
• a third method of treating the surfaces which can be used in the context of the present invention, consists in irradiating with a beam of ions the tip and the reservoir which must receive the liquid filler metal.
• the ion beam is a beam of aluminum ions.
• the irradiation is carried out in a vacuum enclosure.
• FIG. 5 schematically represents the implementation of the irradiation process.
• the source whose end is to be irradiated is shown on the right of the figure, in a vertical position, supported by a support 40, which can pivot around a vertical axis, and which can also be moved in translation along three perpendicular directions in space.
• a support 40 which can pivot around a vertical axis, and which can also be moved in translation along three perpendicular directions in space.
• This source can either be a source identical to that which one seeks to achieve, and to which a similar treatment has already been applied, namely a source such as that described in the prior art, for example in the article by Bell et al., already cited above.
• An extraction electrode 44 makes it possible to accelerate the ions formed by the source 42 in the form of a beam 48 which passes through a window 46 formed in the electrode 44. This window includes an extraction diaphragm 47.
• the emitting tip is brought to a variable high voltage, of about 10-12 kV.
• the assembly formed by the extraction electrode 44 and the source 42 can be oriented in three perpendicular directions in space.
• the ion beam 48 has a conical shape, as shown in FIG. 4, and it is on the central axis of this beam that the maximum of the current is distributed. It is therefore advantageous to position the source 42 so that the part to be irradiated from the source 38 is approximately on the central axis of the ion beam 48.
• the ion dose received by the source 38 during the treatment corresponds approximately to a surface dose of l ⁇ l 8 ions / cm2, that is to say to an irradiation with a current of 2 ⁇ A for 1 hour.
• the acceleration voltage imposed on the aluminum ions formed from the source 42 can vary between a few kilovolts and 20 kilovolts; it can be worth for example around 12 kilovolts.
• the surface treatment is mainly done in two stages: l. a pickling step, during which the source 42 emits a current of a few microamps, between 5 and
• the upper limit value of this current must be chosen in such a way that in the first step, there is essentially emission of simple ions Al + , and practically no emission of aggregates Al + n .
• an acceleration voltage between the emitting tip and the extraction electrode 44 of about ten kilovolts, the aluminum ions Al + will:
• a higher current is emitted at the source, sufficient to form a bundle of metallic aggregates, of Al n + type in the case of aluminum.
• a current with a value greater than 50 ⁇ A is sufficient to form aggregates.
• These aggregates will deposit on the layer of metal ions deposited in the first step on the treated surface and which acts as a bonding layer.
• This layer of metallic aggregates itself forms a bonding layer for the liquid metal which must then be deposited at the end of the source.
• the duration of each of the two stages described above depends on the current used in each stage. With a current of a few microamps, the first stage lasts about 20 minutes; for a current of about 50 ⁇ A, the second step lasts about 40 minutes.
• a step of implantation of the ions and metallic aggregates already deposited on the surface to be treated in the first two steps.
• the source 42 emits a current of a few microamps, so as to emit a beam mainly comprising single ions, and few aggregates.
• These ions are accelerated under a maximum voltage (of the order of 20 kilovolts), so as to "sink" the aggregates deposited during the second stage, in the surface part of the zone which was treated during the preceding stages .
• a protective sheet 50 such as for example a sheet of aluminum foil.
• a protective sheet 50 such as for example a sheet of aluminum foil. This can be important, in the case where the liquid metal with which the source is intended to be used, can present corrosion effects on the metal parts of the source. This is the case in particular of aluminum which, in the liquid state, can easily corrode the parts of the heating system of the tank and the tip external to these, in particular the tungsten filament 16 (see FIG. 2).
• a bonding layer consists of metal aggregates is deposited on these parts during irradiation, the liquid metal will tend, when using the source thus prepared, to also catch on these parts, which will cause rapid corrosion of the metal elements located in the vicinity of these parts. It is for this reason that one limits, in particular in the case of aluminum, the irradiation from the source to the tip, to the front end of the tank, and to the walls of compartment 18 open in the tank ( see figure 3b). After undergoing this treatment, the source is ready for use. It is immersed for example in a bath of liquid aluminum, which wets the irradiated parts and covers them by capillary action in the form of a thin film, perfectly uniform and homogeneous.
• This very good homogeneity promotes, on the one hand, the function of supplying the ionized metal to the apex of the tip, and, on the other hand, making it possible to maximize the quantity of stored filler metal.
• This third treatment, applicable to a source having the structure according to the invention can also be applicable to any type of source of liquid metal ions, in particular to a source having the structure described in the article by Bell et al., And illustrated in FIG. 1. In this case, it suffices to subject the graphite tip 2 to irradiation with, for example, a beam of aluminum ions (Al + then Al n + ). The wettability of graphite is improved compared to the treatment proposed by Bell et al. in the aforementioned article, since the latter leads to the formation of aluminum islands on the surface of the graphite tip.
• the invention has been described in the context of the production of a source of aluminum ions.
• the choice of this element is not limiting and the same structure and the same surface treatment can be used for any source of ions of another nature, for example for a source of boron.
• the surface treatment will then consist in irradiating the source with a beam of boron ions, first of B + ions then of B n + aggregates.
• Boron is, moreover, a corrosive element in the liquid state, like aluminum, and it is therefore preferable to limit the surface treatment to the tip 12 of graphite and to the "front" part of the reservoir 14.
• the structure of the source according to the present invention can also be used for the production of ions from other elements, in particular non-corrosive elements in the liquid state.
• the production of ions from it is carried out using an extraction electrode, mounted in front of the tip, by the same so that the electrode 44 is mounted in front of the source 42 in the assembly of FIG. 5.
• the beam obtained can be more or less rich in metallic aggregates of variable size. In fact this selection depends on the voltage applied to the tip. For this reason, the entire source is brought to a high voltage of around 11 kV, the additional HV power supply being connected to the base of the rod 10 (via the jaws 28, 30 in the representation of Figure 2).
• the modulation of the high voltage leads to a modulation of the current emitted by the tip, this current in turn modulating the distribution of the size of the aggregates emitted.
• the potential difference between the tip and the extraction electrode modulates the kinetic energy of the ions or aggregates emitted.
• Essential applications of the source according to the present invention are: - on the one hand the manufacture of machines with focused ion beams, - on the other hand, the use of such machines in the field of microelectronics and that of preparation of samples for observation by Transmission Microscopy (TEM).
• TEM Transmission Microscopy
• the principle then consists in using the interaction between a beam of very energetic ions, focused in a spot of less than 0.1 micron, and a sample.
• the incident ions will locally spray the surface of the sample, at the spot corresponding to the impact zone.
• FIGS. 6 and 7 Examples of results obtained with an aluminum ion source, designed according to the present invention, are given in FIGS. 6 and 7.
• the source used incorporated a surface treatment with an aluminum bonding layer, as described below. above.
• FIG. 6 is a photograph of a copper grid taken using an electron microscope, in which the electron gun of the microscope was replaced by the aluminum ion source.
• the ion acceleration voltage is 12.5 keV
• the emission current is 16 ⁇ A.
• the wires 60, 62, 64 of the grid have a thickness of approximately 25 ⁇ m.
• the grid must be irradiated with a very stable beam of Al + ions for approximately 1 min 30 seconds. This photograph therefore shows the very good stability, over time, of the current and of the beam of the source according to the invention.
• FIG. 7 is a photograph of an etching carried out on GaAs by a beam of Al + ions of 20 keV of energy (current i ⁇ ll ⁇ A). In this photo, 1 cm represents 100 nm.

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• Combustion & Propulsion (AREA)
• Electron Sources, Ion Sources (AREA)

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• 1994
  • 1994-07-07 FR FR9408395A patent/FR2722333B1/fr not_active Expired - Lifetime
• 1995
  • 1995-07-06 JP JP8504154A patent/JPH10506497A/ja not_active Ceased
  • 1995-07-06 DE DE69509390T patent/DE69509390T2/de not_active Expired - Lifetime
  • 1995-07-06 EP EP95920933A patent/EP0769202B1/de not_active Expired - Lifetime
  • 1995-07-06 WO PCT/FR1995/000903 patent/WO1996002065A1/fr active IP Right Grant
  • 1995-07-06 US US08/765,873 patent/US5936251A/en not_active Expired - Lifetime

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