EP0550335B1 - System to control the form of a charged particle beam - Google Patents

Description

The present invention relates to a system making it possible to control, or more exactly to control, a beam of charged particles, for example electrons.

This system makes it possible to control the appearance, or more exactly the shape, of the beam of charged particles and, in certain particular cases, also to control the orientation of this beam of charged particles.

The present invention applies in particular to the focusing of an electron beam coming from a planar source, in particular from a microtip source, or from a heating filament.

In addition, the invention applies equally to electron guns for cathode ray tubes as to beams scanned by laser excitation or to sources for vacuum gauges.

Systems are already known which allow the shape of a beam of charged particles to be controlled.

Thus in most electron guns, a focusing optic (also called "refocusing") is used to obtain a beam of reduced diameter or in which the electrons have parallel paths.

Many applications, for example the cathode ray tube of a television screen, indeed require the use of a fine electronic brush to obtain an image of sufficient resolution in the case of television.

In other cases, for example in reverse photoemission (see documents (1) and (2) which, like the other documents cited below, are mentioned at the end of this description), the trajectories of the electrons must be parallel so that all the electrons have the same velocity vector, which is an important parameter of the measurement.

Finally, in some cases, it may be useful to obtain a spot as small as possible at a specific point in order to have a high current density locally.

This is for example the case of a compact semiconductor laser, of the electronically pumped type.

There are several techniques for focusing an electron beam.

A first known technique consists in accelerating this beam, which decreases the relative angular momentum of each trajectory of the electrons.

This can be obtained simply by means of an electrode known as the Wehnelt which faces the source of electrons (this is what one does in a conventional cathode-ray tube of television) or by a whole more or less complex of electrostatic lenses (see document (3)) obtained for example by juxtaposing several cylindrical electrodes centered on the electron beam.

This first technique is very often used for high energy electrons (like those used in conventional cathode ray tubes) because the focusing and acceleration functions are in this case combined.

A second known technique consists in pushing, towards the propagation axis desired for the electrons, those which diverge, that is to say which deviate from this axis.

It is thus known to use, to focus electron beams coming from naturally divergent sources, an electrode which is called Pierce electrode and which is schematically represented in FIG. 1.

We see in this figure 1 an electronic source 2 which emits electrons in the direction of a collecting electrode 4 (anode) which is planar in the case of figure 1.

The axis of propagation desired for the electrons carries the reference X and this axis is perpendicular to the anode 4.

In fact, the source 2 emits a divergent electron beam 6 whose initial opening (opening that the beam would have in the absence of refocusing) bears the reference a in FIG. 1.

The Pierce electrode which makes it possible to focus this divergent beam emitted by the source 2 carries the reference 8.

This electrode 8, which is brought to an adjustable and negative potential with respect to the potential level at which the electrons are emitted, has the shape of a truncated cone whose axis is the X axis and whose half-angle at vertex is 67.5 ° (so that the trace of this truncated cone in a plane containing the X axis makes, with a plane perpendicular to this X axis, an angle b which is 22.5 °).

In addition, the electrode 8 generates with the electrodes 2 and 4 an electric field which exerts on each divergent electron a force f which repels, in the direction of the axis X, this divergent electron.

It is specified that the system shown diagrammatically in FIG. 1 can also comprise an accelerator anode, not shown, which plays the conventional role of a Wenhelt.

Regarding Pierce's optics, we can refer to documents (4), (5) and (6).

• elle ne nécessite pas l'utilisation de hautes tensions et, de ce fait, est préférable à la première technique connue lorsqu'on souhaite par exemple disposer d'électrons faiblement énergétiques,
• pour focaliser les électrons divergents, elle ne fait appel qu'à une cathode (électrode 8) et une anode et éventuellement un Wenhelt, et
• elle permet d'obtenir de très bons résultats pour la focalisation.

This second known technique has various advantages:

• it does not require the use of high voltages and, therefore, is preferable to the first known technique when it is desired, for example, to have low energy electrons,
• to focus the diverging electrons, it only uses a cathode (electrode 8) and an anode and possibly a Wenhelt, and
• it provides very good results for focusing.

However, this second known technique has drawbacks, particularly in the case where flat emissive cathodes are used, in particular cold cathodes (see documents (7), (8) and (9)).

A first drawback is the difficulty of mounting: the electron source and the focusing system must be rigorously positioned and maintained relative to each other, which can give rise to complex and fragile assemblies.

A second drawback lies in the size of this focusing system which can be very large and which adversely compensates for the small size of the planar sources.

A system of a different typ is known from document (10). This system consists of an electrically resistant part in cylindrical shape and two conductive strips, making electrical contact with the resistant part and arranged at the ends of said cylinder. This system can be performed directly on the cathode ray tube.

The object of the present invention is to remedy these drawbacks by proposing a system making it possible to control the shape of a beam of charged particles, this system being capable of having a small footprint and of being manufactured by the techniques of micro- electronic.

Thus, in particular in the case of a microtip source which can be produced by these microelectronics techniques, it is possible to produce, at the same time as this source, the system which is the subject of the invention and to integrate the latter into the substrate on which this source is deposited.

In this way it is no longer necessary to mechanically assemble the source and the system as was done in the second known technique, mentioned above.

Specifically, the subject of the present invention is a system making it possible to control the shape of a beam of charged particles, which comes from a source of these particles, this source being associated with a collecting electrode provided for collecting these particles, this system being characterized in that it comprises at least one resistive zone and at least two control electrodes, this resistive zone and these control electrodes being arranged substantially at the same level as the source, these control electrodes being further placed on either side of the resistive zone and provided for polarizing the latter, the electrical resistance profile of the resistive zone being chosen so as to have the potential distribution making it possible to obtain the desired shape of the beam coming from the source when the control electrodes are suitably polarized.

The shape of the beam of charged particles is modified as a function of the application chosen for this beam and, in particular, as a function of the structure of the collecting electrode (which depends on this application).

It is specified that, in the present invention, this collecting electrode can be planar or, on the contrary, non-planar.

The number and shape of the control electrodes as well as the electrical potentials their surface can be determined experimentally as a function of the shape to be given to the particle beam, possibly using simulation software.

Note that it is only possible to reconstruct the field imposed by a non-planar refocusing electrode, for example of the Pierce electrode type, only if the potential distribution in a plane located at the source is continuously variable. A set of separate electrodes makes it possible to obtain only approximately such a distribution. On the contrary, the present invention, which uses at least one resistive zone, makes it possible to obtain this continuously variable distribution or even a continuously variable distribution by parts.

According to a particular embodiment of the system which is the subject of the invention, this system comprises a plurality of resistive zones respectively placed between two control electrodes, these resistive zones then being separated by at least one control electrode.

In the case where two resistive zones are separated from each other by two separate control electrodes, a discontinuous potential profile can be produced at the location of these electrodes. In the case where two resistive zones are separated by a single control electrode, it is possible to produce a profile of continuous potential but not necessarily monotonic.

According to another particular embodiment, the different resistive zones have different resistance profiles.

According to another particular embodiment, each resistive zone comprises a plurality of elementary resistive zones, the resistances of which respective electrical are different from each other and which are between two control electrodes.

In the latter case, the respective thicknesses of these elementary resistive zones and / or the respective resistivities of these elementary resistive zones and / or the respective resistive surfaces of these elementary resistive zones of equal length can be different from one another and chosen so to obtain the desired electrical resistances.

In a particular embodiment of the invention, the source comprises a grid for extracting the particles and this grid constitutes one of the control electrodes and is located in the center of the system, each other control electrode surrounding the source.

In another particular embodiment, each control electrode surrounds the source.

At least one control electrode which surrounds the source may be discontinuous and form a plurality of elementary control electrodes. In this case, in addition to controlling the shape of the beam, the system also makes it possible to control the orientation of this beam by using suitable electrical potentials, as will be seen below.

In another particular embodiment, the source comprises a grid for extracting the particles and has an elongated shape in one direction, the grid constitutes one of the control electrodes and is located in the center of the system and the system comprises at least one other control electrode, elongated in this direction, located on each side of the source and at least one resistive zone which extends between the grid and the other control electrode on each side of the source.

In another particular embodiment, the source has an elongated shape in a direction and the system comprises at least two control electrodes, of elongated shape in this direction, located on each side of the source, and at least one resistive layer, placed between the two electrodes on each side of the source.

In these last two particular embodiments, the system also makes it possible to control the orientation of the beam in addition to the shape of this beam.

Furthermore, in the case of a source of elongated shape the number of electrodes and resistive layers on each side of the source is not necessarily the same.

In an advantageous embodiment of the system which is the subject of the invention, this system is integrated into the source of charged particles when said source is a planar source.

Finally, the control electrodes can be flat and located substantially in the plane of this source.

• la figure 1, déjà décrite, est une vue schématique d'un système connu, permettant de focaliser un faisceau d'électrons divergents,
• la figure 2 est une vue schématique d'un système de focalisation utile pour la compréhension de l'invention et comportant des électrodes de commande en forme d'anneaux concentriques,
• la figure 3 est une vue schématique d'un autre système de focalisation utile pour la compréhension de l'invention et comportant des électrodes de commande de forme allongée,
• la figure 4 est une vue schématique d'un mode de réalisation particulier du système objet de l'invention, comportant une zone résistive comprise entre deux électrodes de commande,
• la figure 5 illustre schématiquement, à l'aide de graphiques, diverses façons de réaliser cette zone résistive,
• la figure 6 est une vue schématique d'un autre mode de réalisation particulier du système objet de l'invention, comportant d'autres électrodes de commande en plus des deux électrodes entre lesquelles se trouve la zone résistive,
• la figure 7 illustre de façon schématique et partielle divers modes de réalisation particuliers du système objet de l'invention,
• la figure 8 illustre schématiquement divers modes de réalisation des couches résistives par dépôt avec électrodes enterrées ou non et pour des couches d'épaisseur homogène ou non,
• les figures 9 et 10 illustrent schématiquement et partiellement d'autres systèmes conformes à l'invention,
• la figure 11 représente la simulation de la focalisation d'un faisceau d'électrons provenant d'une source ponctuelle, au moyen d'un système de focalisation,
• la figure 12 représente la simulation de la focalisation en un point, d'un faisceau d'électrons provenant d'une source ponctuelle, à l'aide d'un autre système de focalisation, et
• la figure 13 represente la simulation de la déflexion et de la focalisation en un point simultanées d'un faisceau d'électrons issu d'une source ponctuelle, au moyen d'un autre système de focalisation comportant des électrodes de commande de forme allongée.

The present invention will be better understood on reading the description of exemplary embodiments given below by way of purely indicative and in no way limiting, with reference to the appended drawings in which:

• FIG. 1, already described, is a schematic view of a known system, making it possible to focus a beam of divergent electrons,
• FIG. 2 is a schematic view of a focusing system useful for understanding the invention and comprising control electrodes in form of concentric rings,
• FIG. 3 is a schematic view of another focusing system useful for understanding the invention and comprising elongated control electrodes,
• FIG. 4 is a schematic view of a particular embodiment of the system which is the subject of the invention, comprising a resistive zone comprised between two control electrodes,
• FIG. 5 schematically illustrates, using graphics, various ways of making this resistive zone,
• FIG. 6 is a schematic view of another particular embodiment of the system which is the subject of the invention, comprising other control electrodes in addition to the two electrodes between which the resistive zone is located,
• FIG. 7 schematically and partially illustrates various particular embodiments of the system which is the subject of the invention,
• FIG. 8 schematically illustrates various embodiments of the resistive layers by deposition with or without buried electrodes and for layers of homogeneous thickness or not,
• FIGS. 9 and 10 schematically and partially illustrate other systems in accordance with the invention,
• FIG. 11 represents the simulation of the focusing of an electron beam coming from a point source, by means of a focusing system,
• FIG. 12 represents the simulation of the focusing at a point, of an electron beam coming from a point source, using another focusing system, and
• FIG. 13 represents the simulation of the deflection and of the focusing at a simultaneous point of an electron beam coming from a point source, by means of another focusing system comprising control electrodes of elongated shape.

FIG. 2 schematically shows a system which is useful for understanding the invention and which makes it possible to focus an electron beam emitted by an electronic source 10.

This source 10 is for example a flat source such as a source with a microtip 12 but, in other embodiments, there could of course be several microtips.

The source can therefore be punctual, that is to say with an emissive surface that is small compared to that of the refocusing means, or on the contrary extended.

The source 10 corresponds to the source 2 of FIG. 1 and we can still see in FIG. 2 the X axis along which we want the beam emitted by the source to propagate, as well as the initial opening a of this beam. of electrons.

These electrons are collected by an anode 14 which is still planar in the example shown in FIG. 2.

We still try to focus the electron beam by pushing the diverging electrons towards the X axis but the force f which pushes these electrons towards the X axis is obtained thanks to control electrodes 16 which are brought to potentials. appropriate electrical.

The control electrodes 16 therefore replace the Pierce electrode 8 of FIG. 1.

This electrode 8 is brought to an electric potential V making it possible to generate an electric field which pushes the diverging electrons towards the X axis.

The control electrodes 16 which, in the example shown, are in the plane of the source 10 and which are spaced apart from each other, simulate approximately a continuous distribution of electrical potential in the plane of the source 10 and make it possible to recreate approximately the shape of the equipotential V which is imposed by the Pierce electrode 8 of FIG. 1.

We then obtain approximately the same form of electric field and therefore the same effect on the electron beam which is emitted by the source and which is initially divergent.

FIG. 2 also shows polarization means 18 which make it possible to bring the various control electrodes 16 to electrical potentials which are different from each other and which are negative with respect to the potential level at which the electrons are emitted.

FIG. 2 also shows the electrical connections 20 which respectively connect the control electrodes 16 to the polarization means 18.

In the example shown in this FIG. 2, the electron source 10 comprises an electrically insulating substrate 22, for example made of glass, on which is formed a cathodic contact layer 24 for example made of chromium.

On the latter is formed a resistive layer 26 for example of silicon.

On this layer 26 is formed an electrically insulating layer 28, for example made of silica, which has a hole in which the microtip 12 is located, for example made of molybdenum, this microtip being formed on the resistive layer 26.

Regarding such a source structure, reference may be made to document (7).

The source 10 also includes an extraction grid 30.

Polarization means 19 are provided for bringing this grid 30 to a potential allowing the electrons to be extracted from the micro-tip 12.

In the example shown in FIG. 2, the axis of the microtip 12 is the X axis and the extraction grid 30 has the shape of a disc whose axis is also this X axis and which is pierced in its center to let the electrons extracted from micro-tip 12 pass.

In the example shown in FIG. 2, the control electrodes 16 are planar electrodes, in the form of rings which are concentric and which have the X axis as their common axis.

They are thus centered on the electron source.

The control electrodes 16 are formed, at the same time as the extraction grid 30, on the insulating layer 28.

Thus the system which is represented in FIG. 2 is integrated into the source 10.

In Figure 3, there is shown schematically another system which is useful for understanding the invention and which is intended to focus an electron beam emitted by a flat and elongated source 32.

It is a source with microtips comprising a row of microtips 34 which are aligned (but, in other embodiments, one could have several rows of such microtips).

The source shown in FIG. 3 comprises an insulating substrate 36, for example made of glass, on which is formed a cathodic contact layer 38, for example chromium.

On the latter is formed a resistive layer 40 for example of silicon.

On this resistive layer is formed an insulating layer 42 for example of silica.

This layer 42 has a row of holes in which the microtips 34 are located, which are for example made of molybdenum and which are formed on the resistive layer 40.

The source shown in FIG. 3 also includes an extraction grid 44 pierced opposite the microtips 34 and making it possible to extract the electrons from these microtips 34.

This grid 44 has the form of a bar which extends in the direction of alignment of the microtips 34, as seen in FIG. 3.

An anode 46, provided for collecting the electrons emitted by the micro-tips, is arranged opposite the source 32.

The system which is shown in FIG. 3 also includes control electrodes 48 which extend in the direction of alignment of the microtips 34 and which are arranged on either side of the extraction grid 44.

These electrodes 48 are formed at the same time as the extraction grid 44 on the insulating layer 42.

The system of FIG. 3 is thus integrated into the electron source 32.

In the example shown in FIG. 3, the structure formed by this system and the source is symmetrical with respect to the row of microtips 34 (there are as many electrodes 48 on one side of the grid as there are on the other side of this grid).

FIG. 3 also shows polarization means 49 provided for bringing the grid 44 to an appropriate potential, allowing the extraction of electrons, and also polarization means 50 provided for bringing the control electrodes 48 to independent potentials, different from each other, making it possible to create an electric field which causes the focusing of the electron beam emitted by the source 32.

The systems which have been described with reference to FIGS. 2 and 3 make it possible to simulate approximately a continuous variation of electrical potential using several control electrodes.

However, these systems do not allow the field obtained by a non-planar refocusing electrode to be reconstructed. The invention also proposes systems using at least one resistive zone between control electrodes in order to obtain a determined potential distribution, adapted to the desired shape of the particle beam coming from the source.

FIG. 4 shows a particular embodiment of the system which is the subject of the invention which is simpler and uses only two control electrodes 52 and 54 as well as a resistive area 56 which is between these electrodes 52 and 54 and polarized by them.

The system shown in FIG. 4 is designed to focus an electron beam emitted by an electron source 58 which, in the example shown, is still a microtip source 60.

As before, this source 58 comprises an insulating substrate 62 surmounted by a cathode layer 64, itself surmounted by a resistive layer 66 on which the microtips are formed. 60.

An insulating layer 68 covers the layer 66 and has holes in which the microtips 60 are located.

As in the case of Figure 2, the source of Figure 4 is circular.

It comprises, on the insulating layer 68, a disk-shaped grid 70 which is pierced opposite the microtips 60.

The system according to the invention, which is shown in FIG. 4, is still integrated into the source 58.

The control electrodes 52 and 54 form concentric rings whose axis is that of the disc formed by the extraction grid 70.

These electrodes 52 and 54 are formed at the same time as this grid 70, on the insulating layer 68 after which the resistive zone 56, for example made of silicon, is deposited on the layer 68, between the electrodes 52 and 54 while being in contact with these electrodes 52 and 54.

The potential distribution depends on the resistance profile of zone 56.

An anode, not shown, is provided opposite the source 58 to collect the electrons emitted by the microtips 60.

Polarization means, not shown, are also provided for polarizing the grid 70 so as to extract the electrons and for also polarizing the electrodes 52 and 54 so as to focus the divergent beam emitted by the source.

There are several solutions for obtaining the electrical resistance profile of zone 56, which allows the focusing of the electron beam.

These various solutions are illustrated by the parts A to F of figure 5.

Part A of FIG. 5 is a sectional view of the electrodes 52 and 54 and of the resistive zone 56 by a plane which contains the axis of the source.

The distance d from a point to the axis of this source is identified on an axis parallel to the cross section that can be seen in this part A of FIG. 5.

In the example shown on the latter, it has been assumed that the resistive area 56 is formed of three adjacent sections of elementary resistive areas T1, T2 and T3 of respective lengths L1, L2 and L3 (which are counted along the axis shown on Part B of Figure 5).

The respective electrical resistances of these sections T1, T2 and T3 are respectively denoted R1, R2 and R3.

An example of an electrical resistance profile is shown in the graph in part B of FIG. 5 where the resistance R1 of the section T1 (closest to the electron source) is less than the resistance of the sections T2, itself lower than the resistance of section T3.

There are actually three solutions to obtain such an electrical resistance profile, due to the formula: $$\text{R = rx L x S⁻¹.}$$

This formula is applicable to each section by giving the parameters R, r, L and S the appropriate index (1 or 2 or 3).

These parameters R, r, L and S respectively represent the electrical resistance, the resistivity of the material used, the length and the section of this section (section S being in fact a cylindrical surface in the example shown).

The graph in part C of FIG. 5 shows that the desired resistance profile is obtained by giving the sections T1, T2 and T3 thicknesses e1, e2 and e3, respectively: $$\text{e1>e2> e3.}$$

In the example shown, e1 is the double of e2 and e2 is the double of e3 (S1 is the double of S2 which is the double of S3).

The same electrical resistance profile is obtained, as seen in part D of FIG. 5, by giving the sections T1, T2 and T3 resistivities r1, r2 and r3 respectively such that: $$\text{r1 <r2 <r3.}$$

With the example we took, r1 is half of r2 and r2 is half of r3.

Such resistivity variations are obtained by appropriately varying the doping of the material constituting the resistive zone 56 or by using, for each section, different materials.

Part E of FIG. 5, which shows part of the resistive zone in plan view, illustrates the fact that the same electrical resistance profile can be obtained by etching in places the resistive zone of initially homogeneous thickness, of so that the resistance of the section thus defined increases in inverse ratio to the remaining resistive surface.

Using the example above, with equal lengths of sections, the section T2 has a resistive surface twice less than the section T1 and twice as large as the section T3, so that it is twice as strong as the section T1 and twice as weak as the section T3.

In fact, it will be noted that part E of FIG. 5 applies in all rigor to a resistive zone comprised between two rectilinear control electrodes, which constitutes a particular embodiment applicable to an elongated source of the kind of that of the figure 3 for example.

It is however possible to transpose the example of part E of FIG. 5 to the case of FIG. 4 by replacing the restilinear sections by sections of discs.

The graph in part F of FIG. 5 shows the variation of the potential V as a function of the distance d from the electron source.

The potential of the electrode 52 is noted V1 and the potential of the electrode 54 is noted V2.

With the resistance profile represented in part B of FIG. 5 and V1> V2, it will be noted that the potential decreases monotonically when one goes from the section T1 to the section T3.

The system according to the invention, which is schematically shown in Figure 6, differs from the system which is shown in Figure 3 in that a resistive area 72 is formed between the two control electrodes 48 which are located one side of the row of micro-tips and which are closest to these micro-tips, a resistive zone 72 is also formed between the two control electrodes which are closest to the row of micro-tips but placed from the on the other side of the latter.

There are thus not only two control electrodes between which is the resistive zone but also other control electrodes (which are brought to electrical potentials suitable for focusing the electron beam).

In another particular embodiment, not shown, derived from the system of FIG. 2, the resistive zone is formed between the two concentric electrodes which are closest to the electron source 10.

The system which is the subject of the invention can be used for other applications than the focusing of an electron beam.

In the case where the electron source is of reduced dimensions compared to the system according to the invention with which this source is provided, it is possible to obtain, instead of a parallel electron beam, a concentrated spot or refocused in a specific place.

It is possible to pinch the beam at the desired location by appropriately adjusting the potentials of the control electrodes.

In the case where one works with a system according to the invention, of elongated shape (case of FIG. 6), it is possible to impose different electrical potential profiles on two or more resistive zones situated on either side. else from the electron source.

The electron beam then undergoes a deflection in addition to a focusing.

We thus have a system allowing the deflection of an electron beam.

The advantage of the control electrodes that can be seen in FIGS. 4 and 6 lies in the fact that they can be formed directly on an emissive cathode.

There is thus a focusing optic which is integrated into the electron source, which can be perfectly positioned by etching and which does not add bulk to that, already small, of the source.

A very compact assembly is therefore obtained which, in certain cases, also allows the deflection of the electron beam.

In the following, various particular embodiments of the invention are considered.

We have seen that the invention makes it possible to control a beam of charged particles originating from a source of these particles (a source of ions for example) by recreating the equipotentials which would be obtained by using one or more non-planar refocusing electrodes. like Pierce's cathode. These non-planar cathodes are so far the only ones which make it possible to rigorously refocus beams of any shape.

The invention makes it possible, in particular in the case of plane sources, to recreate the field necessary for controlling the emitted beam, using a system which can be formed by microelectronics techniques.

The planar sources whose beams are to be controlled can be of any shape, depending on the possibilities of the technology for manufacturing these sources.

The forms of the systems allowing the control of the beams coming from these sources can also be any.

In what follows, we show by way of example embodiments of the invention for point or elongated sources, with systems for controlling respectively concentric or elongated beams on either side of the sources.

To obtain a potential distribution continuously variable in the plane of such sources, we have seen that the invention uses at least one resistive zone polarized at its ends by two electrodes.

These can be visible (see part A of FIG. 8 where the electrodes bear the references 74 and 76, where the resistive zone bears the reference 78 and where the substrate on which this zone and the electrodes are formed bears the reference 79 ) or these electrodes can be buried (see part B of figure 8).

To obtain the desired potential distribution, it is possible, as we have seen previously, to use one or more possibly different resistive zones formed respectively by one or more elementary resistive zones of different resistivities obtained by various techniques and / or one can use one or more resistive zones (separated by one or more control electrodes) of different or identical resistivities.

1) The technique which consists of forming one or more resistive zones separated by polarized electrodes at well chosen potentials is schematically illustrated by parts A to F of FIG. 7; parts A, C and E relate to circular electrodes surrounding a source 80 while parts B, D and F relate to rectilinear electrodes placed on either side of a rectilinear source 82. The reference 81 represents the substrate on which the source and the electrodes are formed. In the figures presented, the resistive zones are arranged symmetrically on either side of the source, but it is also possible to use means for controlling the asymmetrical beam.

One can thus obtain linear potential profiles by parts.

In a first embodiment (parts A and B of FIG. 7), a single resistive zone 84 is defined polarized by two electrodes 86 and 88 (it will be noted that, for part B, this structure 84-86-88 is repeated on either side of the source 82).

In a second embodiment (parts C and D of FIG. 7), two resistive zones 90a and 90b are shown separated by a polarization electrode 92, two other polarization electrodes 94 and 96 are deposited on either side resistive zones to finally make it possible to obtain two independent zones where the potential gradients can be imposed independently of one another.

It will be noted that, for part D, the structure 90a, 90b-92-94-96 is repeated on either side of the source 82.

As a variant, two zones 90a and 90b of different resistances could be formed, separated by the electrode 92 and surrounded by the electrodes 94 and 96.

There could also be several resistive zones framed respectively by control electrodes distinct from the resistive zones with the control electrodes associated with these zones and by control electrodes not associated with resistive zones.

In a third embodiment (parts E and F of FIG. 7), the source 80 or 82 has an extraction grid 98 or 100.

This grid plays the role of a polarization electrode which can, as shown in these figures, be associated with a resistive zone.

We can also design a combination of the second and third embodiments.

2) The second technique which consists in depositing between two electrodes a plurality of elementary resistive zones of different resistances in order to obtain profiles of linear potentials by parts and monotones is schematically illustrated by the parts G, H and I, J of the figure 7 where two elementary resistive zones 84a and 84b appear with different resistances. These parts G, H, I, J are respectively the counterparts of parts A, B, E, F of FIG. 7, the zone 84 here being replaced by the zones 84a and 84b.

• a) déposer plusieurs matériaux de résistivités différentes (voir la partie D de la figure 5) ou un seul matériau dopé différemment dans chaque zone élémentaire,
• b) ou déposer une couche d'épaisseur homogène d'un seul matériau et en le gravant localement (voir la partie E de la figure 5),
• c) ou déposer plusieurs couches d'épaisseurs différentes d'un même matériau (voir la partie C de la figure 5),
• d) ou en combinant 2 ou 3 de ces façons.

Several ways of implementing this second technique are possible and have in fact already been considered in the description of FIG. 5: to obtain zones of different resistances, it is possible to

• a) depositing several materials of different resistivities (see part D of FIG. 5) or a single material doped differently in each elementary zone,
• b) or deposit a layer of homogeneous thickness of a single material and by locally etching it (see part E of FIG. 5),
• c) or deposit several layers of different thicknesses of the same material (see part C of FIG. 5),
• d) or by combining 2 or 3 in these ways.

For the procedure indicated in paragraph c), it is possible to deposit independently of each other layers of different thickness (see part C of FIG. 8 where we see two layers 78a and 78b of different thicknesses ) or to form extra thicknesses by overlapping layers (see part D of the figure 8 where layer 78c is covered by layer 78d).

In certain particular embodiments, the invention makes it possible both to focus and to deflect a beam coming from a source.

In the case of an ion source of elongated shape (of the kind of FIG. 6), with resistive zones situated on either side of this source, the beam is deflected on one side by imposing a slightly more attractive to ions (ie higher in the case of negatively charged ions, lower in the case of positively charged ions) to the resistive zone situated on this same side.

In the case of a source surrounded by one or more resistive zones (see parts A and C of FIG. 7), the deflection can be combined with refocusing by breaking up the resistive zone by dividing a control electrode into several elementary polarization electrodes imposing profiles of different potentials on the different sides of this area.

In the case of FIG. 9, the central polarization electrode 86 and the four peripheral elementary polarization electrodes 88a, 88b, 88c and 88d make it possible to define four elementary resistive zones.

The elementary resistive zone defined by the central polarization electrode 86 and the refocusing electrode 88b is here a quarter of the resistive zone 84.

We see that by imposing thanks to the electrode 88b a more attractive potential for the ions in this zone, the beam is deflected on the side of this electrode 88b.

A particularly embodiment advantageous of the invention is shown in Figure 10.

In this embodiment, an attempt is made to control the electron beam from a field effect microtip source according to document (7) for example.

On a substrate 102 are deposited the microtips 104 at the bottom of holes 106 formed through the extraction grid 108 in an insulating layer 110.

The tips 104 are supplied by the electrodes 112 through the resistive layer 114.

Around the area delimited by the edges 116 of the grid 108 and grouping together several microtips, it is possible to deposit a beam control system in accordance with the invention, consisting of the resistive layer 118 above which the polarization electrode is deposited. 120.

The extraction grid 108 plays in this case the role of central polarization electrode of the resistive zone.

The contact of the extraction grid (not shown) can in this case be easily brought from the periphery of the source thanks to the stacking of the layers and the difference in level between the electrodes 108 and 120.

To make a system according to the invention, simulation software can be used.

Each of Figures 11 to 13 illustrates a simulation of focusing of an electron beam.

For the sake of simplification, the details of the constituent elements of the means for controlling the electron beam have not been shown (control electrodes and resistive zones).

In each of these figures, we have considered a microtip source provided with an extraction grid G.

FIG. 11 corresponds to the case of a point source or very limited in relation to the dimensions of the refocusing optic), that is to say which uses one or a few microtips as in the case of FIG. 2 .

The means MF for controlling the beam (control electrodes and resistive zones) in the case of this figure are brought to potentials such that the trajectories of the electrons are parallel to each other.

Figures 12 and 13 further illustrate the case of a microtip source which is point or very small and which is provided with a focusing system comprising control electrodes.

In the case of FIG. 12, potentials are applied to the control electrodes such that the beam is focused at a point situated on the collecting anode AC facing the source. In the case of FIG. 13, the possibility of deflecting this beam towards another point of the anode is shown without losing the focusing property.

We can thus focus the beam emitted by the electron source.

In the case of FIG. 13, the source is of the kind of that of FIG. 3, the control electrodes thus having an elongated shape.

CITED DOCUMENTS

• (1) G. Chauvet, R. Baptist, Journ. of Electr. Spec. and Rel. Phen. 24, 255 (1981).
• (2) Th. Fauster, F.J. Himpsel, J.J. Donelon and A. Marx, Rev. Sci. Instr. 54, 68 (1983).
• (3) P. Grivet. Electronic optics. Vol.I: Electrostatic lenses. (Bordas, Paris, 1955).
• (4) J. R. Pierce. Theory and Design of Electron Beams. (D. Van Norstand, New-York, 1954).
• (5) H.Z. Sar-El. Revised Theory of Pierce-type Electron Guns. Nucl. Instr and Meth. 203 (1982) 21-33.
• (6) H.C. Lange. Entwicklung und Aufbau eines Spektrometers für inverse Photoemission mit variabler Photonenenergie. Freien Universität Berlin (1988).
• (7) French patent application n ° 8715432 of November 6, 1987 - see also US-A-4940916.
• (8) H. Gundel, H. Riege, J. Handerek and K. Zioutas. Low pressure hollow cathode switch triggered by a pulsed electron beam emitted from ferroelectrics. Appl. Phys. Letters, 54 (21), 2971-3 (1989).
• (9) G.G.P. Van Gorkom and A.M.E. Hoeberechts. Silicon cold Cathodes. Philip Technical Review. Vol 43, n ° 3, Jan 1987.
• (10) US-A-4,881,006.

Claims (14)

1. System making it possible to control the shape of a charged particle beam, which has come from a particle source (32, 70, 80, 82), said source being associated with a collecting electrode for collecting these particles, said system comprising in a plane level with the source at least one resistive zone (56;72;78;78a,78b;78c,78d;84;84a,84b;90a, 90b;118) and at least two control electrodes (48;52,54;74,76; 88;86, 88a to 88b; 92,94,96;88,98;88,100;108,120), said resistive zone and said control electrodes being arranged substantially at the same level as the source, said control electrodes also being placed on either side of the resistive zone and serve to polarize the latter, the electric resistance profile of the resistive zone being chosen so as to have the potential distribution making it possible to obtain the desired shape of the beam from the source, when the control electrodes are appropriately polarized.
2. System according to claim 1, characterized in that it comprises a plurality of resistive zones respectively placed between two control electrodes, said resistive zones then being separated by at least one control electrode.
3. System according to claim 2, characterized in that the different resistive zones have different resistance profiles.
4. System according to claim 1, characterized in that each resistive zone comprises a plurality of elementary resistive zones (84a,84b), whose respective electrical resistances are different from one another and which are between two control electrodes (86,88; 88,98).
5. System according to claim 4, characterized in that the respective thicknesses of these elementary resistive zones differ from one another and are chosen in such a way as to obtain the desired electric resistances.
6. System according to either of the claims 4 and 5, characterized in that the respective resistivities of these elementary resistive zones differ from one another and are chosen so as to obtain the desired electric resistances.
7. System according to any one of the claims 4 to 6, characterized in that the respective resistive surfaces of these elementary resistive zones with the sane length differ from one another and are chosen so as to obtain the desired electric resistances.
8. System according to any one of the claims 1 to 7, characterized in that the source comprises a particle extracting gate (98, 108), said gate constituting one of the control electrodes and is positioned in the centre of the system, each other control electrode (88, 120) surrounding the source.
9. System according to any one of the claims 1 to 7, characterized in that each control electrode surrounds the source.
10. System according to either of the claims 8 and 9, characterized in that at least one control electrode surrounding the source is discontinuous and forms a plurality of elementary control electrodes (88a to 88d).
11. System according to any one of the claims 1 to 7, characterized in that the source comprises a particle extraction gate (100) and has an elongated shape in one direction, said gate constituting one of the control electrodes and is positioned in the centre of the system, the system having at least one other control electrode (88), which is elongated in said direction and is positioned on each side of the source and at least one resistive zone (84) extending between the gate and the other control electrode of each side of the source.
12. System according to any one of the claims 1 to 7, characterized in that the source (32, 82) has an elongated shape in one direction, the system comprising at least two control electrodes (48; 86, 88) elongated in said direction and located one each side of the source, and at least one resistive layer (72, 84) placed between the two electrodes of each side of the source.
13. System according to any one of the claims 1 to 12, characterized in that it is integrated into the charged particle source, which is a planar source.
14. System according to any one of the claims 1 to 13, characterized in that the control electrodes are planar and located substantially in the plane of the source.

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