EP0769202B1 - Liquid metal ion source - Google Patents

Description

Technical area

The present invention relates to the field sources of liquid metal ions, in which ions are produced from a filler metal which covers a refractory metal tip.

By applying an intense electric field, under vacuum, between this tip and an extraction electrode, there is emission of ions according to a field evaporation mechanism. This emission is located at the apex of the point. The size of the emissive zone is of the order of a few nm ² for an emission current of approximately 2 μA.

These sources are used in machines focused ion beams which occupy a part of more and more important in the techniques of microelectronic manufacturing.

Currently, these machines use almost exclusively liquid metal ion sources gallium. The use of lighter ions is interesting because it makes it possible to reduce the size of ion probes. This increases the resolution existing machines, by simply changing the source. In addition, for layout applications, a light element is interesting because it penetrates more deep in the material that a heavy element, to equal energy.

State of the art

The article by Bell et al. published in "Journal of applied physics ", vol. 53, n ° 7, July 1982, pp. 4602 at 4605 describes a source of aluminum ions consisting a graphite point fixed on a filament of tungsten heating. The tip of graphite is surface treated by depositing a titanium film. In graphite is relatively resistant to attacks from liquid aluminum but it's difficult to wet it with such a liquid. The use of titanium film solves this problem. The source has the form illustrated in Figure 1, where reference 2 designates the graphite tip, which has a cylindrical part at its base. A filament heater 4, made of tungsten, passes through this part cylindrical. The heating of tip 2 is obtained by the heat released by the Joule effect in the filament.

This type of source has the advantages of on the one hand simplicity and on the other hand compactness since the graphite tip has a diameter about 0.8 mm, for a length of 2 mm.

However, there are a number of disadvantages. First, the assembly and positioning of the graphite tip on a filament whose diameter is less than 0.2 mm are very delicate, even random. The set lacks mechanical stability, which causes drifts thermal incompatible with applications in the microelectronics. On the other hand, the heating of the emissive zone is poorly controlled. Gold, the higher the thermal power used by the source important, plus the thermal drifts due to radiation to the environment are high. This, in consequence, can modify the mechanical centering of the emissive point relative to a target or relative to an extraction electrode. This centering influences the direction of ion emission and is inaccessible during operation. Its variation therefore leads to loss of precision; as part of a use in the field of microelectronics, resolution of the etched structures obtained with a such a source is considerably affected. A another problem is that of the limited lifespan of source. Indeed, the reserve of liquid metal input is low, and cannot be used in whole.

WO 86/06210 describes another type of ion source, comprising a tip and an element heating in the form of a ribbon. A hole in the ribbon heater allows molten liquid to flow into tip direction.

Statement of the invention

The present invention seeks to resolve the problems mentioned above.

Its object is a source of metal ions liquid comprising a cylindrical rod made of a material conductor and refractory extended by a point in refractory material, intended to be covered with a liquid filler metal, characterized in that the assembly constituted by the cylindrical rod and the point passes through a tank made of a material conductive and refractory, the area where the rod is engaged in the reservoir ensuring electrical contact between the rod and the tank, and in that the tank is in contact with a conductive filament, the rod cylindrical, reservoir and conductive filament thus being connected in series from the electrical point of view.

Also, it relates to a process for preparing a sample using such a source. Such a constitution for an ion source made of liquid metal helps limit energy intake necessary for optimal functioning of the source. There is indeed, production of a heating only in a very localized area, limited to the part of the cylindrical rod near the tip, at tank and tungsten filament. The element more resistive of the circuit thus formed is the part cylindrical of the rod, which when crossed by a current of 5 amperes, by Joule effect, reached a temperature of 700 ° C near its end. In addition, the power consumed is limited to less than 10 Watts.

The set is an assembly and a much less delicate positioning than in sources of liquid metal ions known in the art prior. Finally, the mechanical stability is clearly improved. The use of a tank allows by elsewhere to increase the supply of liquid metal and, therefore, the lifetime of the source.

According to a particular embodiment of the invention, the rod and the point form a single and same room

For example, the stem and tip can be graphite.

According to an even more embodiment particular, the graphite tip is covered with a titanium film.

Alternatively, the tip is covered a metallic bonding layer of the same type as liquid metal intended for use with the source.

The bonding layer can not only cover the tip, but also part of the tank.

In the case where the liquid metal intended for covering the graphite tip is, for example, aluminum, a tip surface treatment improves its wettability by liquid aluminum. Two surface treatments set out above, the second has the advantage of allow a very homogeneous aluminum film to be obtained across the entire tip. Indeed, the processing of graphite tip surface by film deposition titanium does not allow wetting homogeneous tip with aluminum film: we actually obtains aluminum island formation at the surface of the graphite tip and it results that the function of bringing aluminum to the point is very disturbed. The emitted current is then unstable and very difficult to keep constant over long periods.

On the other hand, the second treatment promotes a share the contribution function of the metal to be ionized towards the apex of the point, and on the other hand allows to increase the quantity of filler metal stored. The stream ion obtained during the production of aluminum ions is even more stable over time.

According to another particular embodiment of the invention, the cylindrical rod and the tip are mechanically adjusted with tight tolerances to inside the area planned for their passage in the tank.

This adjustment has the following advantage. While the source is operating, it is possible than hot spots, other than those located at near the tip, appear on the stem cylindrical, on the other side of the tank with respect to The point. So if there is some play between the cylindrical rod and the reservoir, the liquid metal there introduces and goes up towards these hot spots, starting which it can evaporate, which shortens the duration of source life. The adjustment, without any play of the rod inside the tank, helps to remedy this disadvantage.

Other complementary aspects of the invention appear in the dependent claims.

Presentation of the figures

• la figure 1, déjà décrite, représente une source d'ions à métal liquide selon l'art antérieur,
• la figure 2 représente une source d'ions à métal liquide selon la présente invention,
• les figures 3a et 3b représentent deux exemples de réservoir utilisé dans une source d'ions à métal liquide selon la présente invention,
• la figure 4 représente un mode de réalisation d'une tige pour une source selon l'invention,
• la figure 5 représente le dispositif de traitement pour préparer une source d'ions à métal liquide selon la présente invention,
• les figures 6 et 7 donnent des exemples de résultats obtenus avec une source d'ions selon la présente invention.

In any case, the characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to the exemplary embodiments, given by way of explanation and without limitation, with reference to the appended drawings in which:

• 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,
• FIG. 4 represents an embodiment of a rod for a source according to the invention,
• FIG. 5 represents the treatment device for preparing a source of liquid metal ions according to the present invention,
• Figures 6 and 7 give examples of results obtained with an ion source according to the present invention.

Detailed description of embodiments of the invention

Figure 2 illustrates a particular mode of production of a liquid metal ion source according to the present invention.

This source consists of a rod conductive comprising a cylindrical part 10 and a point, the latter itself being made up of a part or conical end 12 and part cylindrical 13. The cylindrical part 10 is constituted of a conductive and refractory material. In general, the graphite is well suited for an application to a source of light metal ions, such as aluminum. But this does not exclude the employment of others materials, such as for example tungsten. The stem, in its cylindrical part 10, can have, in the case graphite, a diameter of the order of a few tenths of a millimeter, for example 0.5 mm, and a length between 5 and 20 mm, for example 15 mm. In fact, in various exemplary embodiments, a single pencil type "HB" was used, which gave full satisfaction.

The end 12 of the tip is cut in cone shape, half angle at the top having a value between 48 and 50 °, for example 49 °, by mechanical polishing in two stages. The tip is placed in rotation, inclined by about 49.5 °, and it is brought into contact with a plane that will machine the part conical. In a first step, the cone is outlined on an average roughness surface, around 30 µm. In a second step, the finishing is carried out on a surface of low roughness, for example a few microns.

Thus, a reproducible result is obtained radius of curvature at the apex of the tip in the order of ten micrometers.

The tip, that is to say the set made up 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 (for example 3 mm).

In order to avoid, during operation of the source, a rupture of the liquid metal film on the edge at the base of the cone, where the game begins cylindrical 13, it can be interesting to chamfer the connection zone 11 between the cylindrical part 13 and the conical end 12.

If the tip is made of the same material that the cylindrical part 10 of the rod, the assembly forms a single piece.

The rod is introduced into a reservoir 14, two examples of which are illustrated more specifically in Figures 3a and 3b. On each of these figures, the reservoir designated by the references 14-1 and 14-2, has a shape having substantially a symmetry of revolution around an axis passing through the part 10. In the two examples, an opening cylindrical 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 all. In the example of Figure 3b, the tank 14-2 has a recessed part 18, in the form of chamfer. The purpose of this recess is to allow increase the liquid metal capacity of the tank. An interior recess 17 makes it possible to reduce the volume of material used for the tank.

In all cases, the tank is made up of a conductive and refractory material. If the rod 10 is made of graphite, just choose for example the graphite as material for the tank.

A particularly graphite form advantageous is vitreous carbon. Unlike the polycrystalline graphite, which has micropores and microcracks, glassy carbon has a waterproof structure with closed pores.

Now, an aging of the source, long term, has been observed. It is manifested by the appearance graphite powder on the surface of the liquid metal filler, in particular in the case of aluminum.

The grains of this powder are, it seems, torn away by the constraints and the chemical action of molten aluminum, which creeps into the micropores and the microcracks of polycrystalline graphite, however specifically developed for applications refractory.

• l'absence de particules de carbone à la surface du film de métal liquide (aluminium, notamment), ce qui laisse penser que la longévité de la source sera augmentée,
• une résistivité de l'ensemble pointe-réservoir nettement plus importante, ce qui permet de diminuer la valeur du courant de chauffage par un facteur 2. Ce courant est donc réduit à environ 2 Ampères.

Tests conducted with a tip and a glassy carbon tank have shown:

• the absence of carbon particles on the surface of the liquid metal film (aluminum, in particular), which suggests that the longevity of the source will be increased,
• a significantly higher resistivity of the tip-tank assembly, which makes it possible to reduce the value of the heating current by a factor of 2. This current is therefore reduced to approximately 2 amperes.

In addition, the possibility of electrochemically trim the tip of the tip glassy carbon to adapt the threshold value of ion emission to a specific device.

It is possible to work only with the tip or the glassy carbon tank.

If only the tank is made of glassy carbon, we still notes that there is no more corrosion of the tank. The resistivity is less good than in the previous case but better than if the tank is in "normal" graphite.

Similarly, if only the tip is made of carbon glassy, its longevity will be extended and the resistivity will be improved. Furthermore the tip alone can always be cut electrochemically.

Rod 10 being used as a conductor electric, there is, in principle, production of a heating in a very localized area, limited to the conical part 12, to the cylindrical part 13 and to the tank 14. However, we cannot sometimes avoid the formation of "hot spots" on the cylindrical part 10, and near the base of the reservoir 14, by example at a point such as point A represented on Figure 2. At such a hot spot, the temperature can reach a value greater than that of the temperature at the end of the rod, near the point 12, because in the vicinity of this point the presence of liquid metal helps to dissipate heat. However, liquid metal can tend to spread thermally, along the stem, direction of hot spots, which results in gradually empty the tank and, therefore, decrease the autonomy of the source. For these reasons, it may be interesting to mechanically adjust the part cylindrical 10 of the rod with tight tolerances, without any play, inside the opening cylindrical 19 (see Figures 3a and 3b) provided for the passage of the rod inside the reservoir 14. By therefore, the liquid metal cannot escape the along the stem. In addition, this strengthens the holding mechanics of the assembly. As an indication, a tank was made with a diameter of about 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 It is then forced into it by hand. This sufficient to provide the required tight fit.

As illustrated in FIG. 2, the reservoir 14 is electrically connected to a heating circuit. This circuit can be constituted for example by a filament tungsten 16 wrapped around the base of the tank 14, on the side opposite to the point 12. The ends of this filament are themselves connected to elements conductors 24, 26, such as for example plates tantalum. According to a variant, not shown on the Figures, the filament 16 is not in contact with the outer surface of the tank 14 but it's introduced into a groove cut into the surface of this tank. This limits the contact between the heating filament 16 and possible drops of liquid metal that could diffuse along the outer surface of the reservoir 14. Indeed, some liquid metals, especially aluminum, are extremely corrosive to metals.

The rod is held at its base by a clamp consisting of two jaws 28, 30 which has for on the one hand, to provide mechanical maintenance of the rod with maximum rigidity without weakening it, and secondly to ensure reliable electrical contact with the material of this rod, electrical contacts which allow the circulation of a current of approximately 6 A.

Thus constituted, the heating circuit comprises the rod, with its cylindrical part 10 and its tip, the reservoir 14 and the filament 16. From the point of electrical view, all these elements are connected in series and the whole works under a tension floating supply of the order of a few volts. The most resistive element of the circuit is the part cylindrical 10 of the rod, which, when it is crossed by a current of 5 A, reaches a temperature of 700 ° C by Joule effect. Furthermore, the fact using the rod as a heating element, allows 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 threaded rod central 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 these elements (jaws, threaded rods) make part of the electrical circuit.

The structure which has just been described confers very good overall mechanical stability, and especially at point 12. This stabilizes the emissive area, at the apex of the tip, and make the source compatible with use in fields techniques where the precision required is extremely high, for example in electrostatic optics. In besides, from a congestion point of view, the source described is fully compatible with machines or systems already existing on which metal ion sources prior art liquids are suitable.

Another embodiment of the rod is shown in Figure 4. In this figure, the rod passes through a reservoir 15. The latter is similar to that described above in connection with FIG. 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 stem is still made up of a part cylindrical 35. It is extended by a point which itself consists of a conical part 37, of a cylindrical part 39 and a rim 41. The rod 35 is significantly larger in diameter than in the first embodiment of the rod. The cylindrical part 39 and the end 37 of the point have, them, substantially the same dimensions as before.

In this embodiment, the part cylindrical and the tip consist of two different materials.

The rod is made of a conductive material and refractory, for example graphite. As in the first embodiment, it can be achieved by example from a pencil lead. She 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 material refractory 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 vertex between 48 and 50 ° (worth for example 49 °) by mechanical polishing using an abrasive such as example a diamond wheel.

This second embodiment can also be used in combination with a form tank similar to that described in connection with FIG. 3a, provided that the cylindrical opening is adapted to corresponding way (cylinder opening more wider in its upper part than in its part lower). What was said in the first embodiment, on mechanical adjustment with tight tolerances apply as well to this second embodiment.

The operation of the source is the same; he there is always electrical contact between the rod 35 and the tank 15 and the current flows from the rod to the tank and heating filament. This current produces a heating of the point by Joule effect.

Whatever the shape of the stem, and so to improve the wettability of the tip 12 and of the tank by the liquid metal filler it is interesting to carry out a surface treatment of this tip and tank. Beforehand, surfaces to be treated may first have been cleaned in a boiling trichlorethylene bath, then degassed by vacuum heating to a temperature about 1000 ° C.

A surface treatment has been described in the article by Bell et al. already cited above. This treatment consists in depositing an aqueous solution of titanium powder on the tip. After drying, the point is brought under vacuum to a temperature of approximately 1700 ° C to melt the titanium. This treatment is compatible with the structure of the source according to the present invention, as described above.

A variant of this surface treatment consists of depositing the titanium film by spraying before bringing it under vacuum to a temperature about 1700 ° C to melt it. In the case of the use of the source with liquid aluminum, it was found, with this surface treatment, a significantly improved homogeneity of the aluminum film by compared to the homogeneity obtained in the case where the surface is treated with the method described above. This allows to have a good contribution function towards the apex of the point, and therefore an ion current more stable over time.

A third method of processing surfaces, usable in the context of this invention consists in irradiating with an ion beam the tip and the tank to receive the metal filler liquid. In case the source is intended to be used with liquid aluminum, the ion beam is an aluminum ion beam. The irradiation is carried out in a vacuum enclosure. The Figure 5 shows schematically the implementation of the irradiation process. The source whose end must be irradiated is shown on the right of the figure, in vertical position, supported by a support 40, which can pivot about a vertical axis, and which can also be moved in translation along three perpendicular directions of space. In order to achieve the desired irradiation of the end of the source 38, you need another source of ions 42. This source can either be a source identical to the one we are trying to achieve, and to which we have already applied a similar treatment, either a source such as that described in the prior art, by example in the article by Bell et al., already cited above. An extraction electrode 44 allows to accelerate the ions formed by the source 42 in the form of a beam 48 which passes through a window 46 made in electrode 44. This window has a extraction diaphragm 47. In order to adjust the current source 42, the emitting point is brought to a variable high voltage, about 10-12 kV. The assembly constituted by the extraction electrode 44 and the source 42 can be oriented according to three perpendicular directions of space. In general, the ion beam 48 has a conical shape, such as shown in Figure 4, and it's on the central axis of this beam that is distributed the maximum of the current. It is therefore advantageous to position the source 42 of such so that the part to be irradiated from source 38 located approximately on the central axis of the beam of ions 48.

The ion dose received by the source 38 during the treatment corresponds approximately to a surface dose of 10 ¹⁸ ions / cm ² , that is to say to an irradiation with a current of 2 μA 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.

1. une étape de décapage, pendant laquelle la source 42 émet un courant de quelques microampères, entre 5 et 10 µA, que l'on règle en ajustant la haute tension à laquelle est portée la pointe émettrice de la source 42. En fait, la valeur limite supérieure de ce courant doit être choisie de telle façon que dans la première étape, on ait essentiellement émission d'ions simples Al⁺, et pratiquement pas d'émission d'agrégats Al⁺ _n. Avec une tension d'accélération entre la pointe émettrice et l'électrode d'extraction 44, d'environ une dizaine de kilovolts, les ions aluminium Al⁺ vont :

• décaper les surfaces de la source 38 qui sont exposées sur leur trajet (la pointe 52 et l'extrémité du réservoir 54),
• former une couche sur la surface décapée, qui servira de couche d'accrochage pour les agrégats qui seront déposés au cours de la deuxième étape,

2. dans une seconde étape, on fait émettre à la source un courant plus élevé, suffisant pour former un faisceau d'agrégats métalliques, de type Al_n ⁺ dans le cas de l'aluminium. En général, un courant d'une valeur supérieure à 50 µA est suffisant pour former des agrégats. Ces agrégats vont se déposer sur la couche d'ions métalliques déposée dans la première étape sur la surface traitée et qui joue le rôle de couche d'accrochage. Cette couche d'agrégats métalliques forme elle-même une couche d'accrochage pour le métal liquide qui doit être ensuite déposé à l'extrémité de la source.

The surface treatment is mainly done in two stages:

1. a stripping step, during which the source 42 emits a current of a few microamps, between 5 and 10 μA, which is adjusted by adjusting the high voltage to which the emitter tip of the source 42 is brought. In fact, the upper limit value of this current must be chosen in such a way that in the first step, there is essentially emission of single ions Al ⁺ , and practically no emission of aggregates Al ⁺ _n . With an acceleration voltage between the emitting tip and the extraction electrode 44, of about ten kilovolts, the aluminum ions Al ⁺ will:

• pickling the surfaces of the source 38 which are exposed on their path (the tip 52 and the end of the reservoir 54),
• form a layer on the pickled surface, which will serve as a bonding layer for the aggregates which will be deposited during the second step,

2. in a second step, 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. In general, a current with a value greater than 50 µA 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 step. With a current of a few microamps, the first stage has a duration of about 20 minutes; for a current of approximately 50µA, the second stage lasts approximately 40 minutes.

It is possible to add to these two steps a implantation step for ions and aggregates metallic already deposited on the surface to be treated in the first two steps. During this stage source, the source 42 emits a current of a few microamps, so as to emit a beam mainly comprising single ions, and few of aggregates. These ions are accelerated under a voltage maximum (around 20 kilovolts), so that "sink" the aggregates deposited during the second stage, in the superficial part of the area which has been processed in the previous steps.

In order to limit the surface treatment to the tip 52, and at the front part of the reservoir 54 (part referenced by 20 and 31 in Figures 2 and 4, and which is limited, in these same figures, by a line in dotted), it is possible to interpose between the beam 48 and the parts of the source 38 which are not does not want to radiate, a protective sheet 50, such than for example a sheet of aluminum foil. This can be important, in case the liquid metal with which the source is intended to be used, may exhibit corrosion effects on parts source metal. This is particularly the case for aluminum which, in the liquid state, can easily corrode the parts of the heating system of the tank and tip outside of these, in particular the tungsten filament 16 (see FIG. 2). If, therefore, a bonding layer consists of metallic aggregates is deposited on these parts during of the irradiation, the liquid metal will tend, when of the use of the source thus prepared, to also hold onto these parts, which will cause rapid corrosion of elements metal located in the vicinity of these parts. It is for this reason which one limits, in particular in the aluminum, irradiation from source to tip, at the front end of the tank, and at the walls compartment 18 open in the tank (see Figure 3b). After undergoing this treatment, the source is ready to use. We immerse it for example in a bath of liquid aluminum, which wets the irradiated parts and cover them by capillary action under the shape of a thin film, perfectly uniform and homogeneous. This very good homogeneity promotes share the function of supplying the ionized metal to the apex of the tip, and on the other hand allows to make maximum the quantity of filler metal stored.

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.

Source structure according to this invention can also be used for the production of ions from other elements, in particular non-corrosive elements in the liquid state.

After preparation and once the source is wetted by liquid metal, the production of ions at from this is carried out using a extraction electrode, mounted in front of the tip, in the same way as the electrode 44 is mounted in front from 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 makes this selection depends on the voltage applied to The point. 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 rod 10 (via jaws 28, 30 in the representation of Figure 2). The modulation of high voltage leads to modulation of the current emitted by the tip, this current modulating at turn the size distribution of the aggregates issued. The potential difference between the tip and the extraction electrode modulates, the kinetic energy of the emitted ions or aggregates.

• d'une part la fabrication de machines à faisceaux d'ions focalisés,
• d'autre part, l'utilisation de telles machines dans le domaine de la microélectronique et celui de la préparation d'échantillons pour l'observation par Microscopie par transmission (TEM).

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 the preparation of samples for observation by transmission microscopy (TEM).

The principle then consists in using interaction between an ion beam very energetic, focused in a spot of less than 0.1 micron, and a sample.

Incident ions will locally spray the surface of the sample, at the spot corresponding to the impact area.

We control the erosion process according to X, Y axes parallel to the surface of the sample in scanning this surface with the beam, and we control the depth of this engraving by a displacement of the machine along a z axis perpendicular to the surface. We can, with a device incorporating a source according the present invention, to develop structures having sizes on the order of 70 to 80 nanometers.

Examples of results obtained with a aluminum ion source, designed according to the present invention, are given in Figures 6 and 7. The source used incorporated surface treatment with an aluminum bonding layer, such as described above.

Figure 6 is a photograph of a copper grid taken using an electron microscope, in which the electron gun of the microscope has been replaced by the source of aluminum ions. The ion acceleration voltage is 12.5 keV, and the emission current is 16 µA. The wires 60, 62, 64 of the grid have a thickness of approximately 25 μm. To make such a shot, 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≈11 µA). In this photo, 1 cm represents 100 nm.

Claims (18)

1. A liquid metal ion source including a cylindrical rod (10, 35) made of a conductive and refractory material, extended by a point (12, 13; 37, 39) made of refractory material, intended to be covered by a liquid supply metal, characterized in that the assembly made up by the cylindrical rod and the point passes through a reservoir (14, 15) made of a conductive and refractory material, the area (19, 33) where the rod is inserted into the reservoir ensuring an electrical contact between the rod and the reservoir, and in that the reservoir is in contact with a conductive filament (16), the cylindrical rod, the reservoir and the conductive filament being thereby connected in series from the electrical point of view.
2. A liquid metal ion source according to claim 1, the rod (10) and the point (12, 13) forming one and the same piece.
3. A liquid metal ion source according to claim 2, the rod (10) and the point (12, 13) being made of graphite.
4. A liquid metal ion source according to claim 3, the point (12, 13) being covered with a film of titanium.
5. A liquid metal ion source according to claim 1, the rod (35) being made of a different material to that from which the point (37, 39) is made.
6. A liquid metal ion source according to claim 5, the rod (35) being made of graphite.
7. A liquid metal ion source according to one of claims 5 or 6, the point (37, 39) being made of alumina or boron nitride.
8. A liquid metal ion source according to claim 1, the point (12, 13; 37, 39) being covered by a metal priming coat, of the same kind as the liquid metal it is intended to use with the source.
9. A liquid metal ion source according to claim 8, the priming coat covering the point, and a part (20, 31) of the reservoir (14, 15).
10. A liquid metal ion source according to one of claims 8 or 9, the priming coat being susceptible to being obtained by ionic bombardment.
11. A liquid metal ion source according to claim 10, the ionic bombardment comprising two stages:

    an etching stage, during which a simple beam of ions is directed onto the parts of the ion source to be irradiated, virtually no aggregates being included,

    a second stage in the course of which a beam of ions comprising essentially metal aggregates is directed onto the parts of the source to be irradiated.
12. A liquid metal ion source according to claim 11, an implantation stage being additionally provided, the parts of the source to be irradiated being bombarded by a beam comprising mainly simple ions accelerated under a high voltage, and few aggregates.
13. A liquid metal ion source according to one of claims 1 to 6 or 8 to 12, the point (12, 13, 37, 39) being made of vitreous carbon.
14. A liquid metal ion source according to one of the preceding claims, the reservoir (14, 15) being made of vitreous carbon.
15. A liquid metal ion source according to one of the preceding claims, the rod and the point (10, 13; 35, 39) being adjusted mechanically with tight clearances to the inside of the area (19; 33) provided for their passage in the reservoir (14; 15).
16. A liquid metal ion source according to one of the preceding claims, the reservoir (14) having a cutaway hollow (18).
17. A liquid metal ion source according to one of the preceding claims, the heating filament (16) being introduced into a throat at the periphery of the reservoir (14; 15).
18. A method of preparing a sample using a liquid metal ion source according to one of the preceding claims, a beam of metal ions being produced using this source and directed onto the sample.

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