EP0689222B1 - Method of manufacturing micropoint electron source - Google Patents

Description

The present invention relates in a way general to cathodic emissive systems using electronic emission by field effect such as by example those of the matrix flat screens used for display of images; it relates more precise, to a process making it possible to improve the characteristics of microtip cathodes and their uniformity over large areas.

Such a microtip emissive system and its manufacturing process are described in detail by example in document FR-A-2 593 953 of 24/01/1986. We will begin by recalling the known technique for manufacturing such microtips in a structure of this kind, as it emerges from aforementioned document with reference to Figures 1, 2 and 3 attached.

Figure 1 shows an already developed structure, comprising on a substrate 6 surmounted by an insulator 7, a system of cathode conductors 8 and grids 10a superimposed in cross form with insulation intermediate 12 and a layer for example of nickel 23 deposited on the surface to serve as a mask during microtip production operations. This layer 23 of nickel, the grids 10a and the insulator 12 are drilled with holes 16, at the bottom of which is to come and deposit the future microtips made of an electrically conductive metal with the cathode electrode 8.

For the realization of microtips, we takes in the following way, referring to the figure 2. We start first by performing for example the deposition of a molybdenum layer 18a on the whole of the structure. This layer 18a has a thickness about 1.8 µm. It is filed under incidence normal with respect to the surface of the structure; this deposition technique makes it possible to obtain cones 18 made of molybdenum housed in holes 16 having a height 1.2 to 1.5 µm. We then carry out the dissolution of the nickel layer 23 by a process electrochemical so as to release, as shown in FIG. 3, the grids, for example made of niobium 10a perforated and to show the microtips 18 electron emitters.

With a few technological variants, the known method thus described with reference to the figures 1, 2 and 3 is always the one that we apply until today to realize the microtips of the systems to emissive cathode.

Unfortunately, microtips as well obtained have certain defects. these faults first come from the fact that the previous method makes it difficult to obtain microtips, the shape is reproducible from one point to another and / or from one cathode to another, especially on large surfaces during mass production. They also come from the fact that, on the other hand, microtips obtained are far from always having the perfect conical shape which we have represented under the reference 18 in FIGS. 2 and 3. Most often in indeed, they have shape inequalities and a majority has a much too large radius of curvature, this which gives them a domed profile as we can see in Figure 4. This domed profile notably decreases their emissivity in a considerable way, that is to say the current density emitted for a grid voltage determined microtip.

On the other hand, the realization of the cathode requires at least one photolithography step intervening after the realization of the points in particular for the definition of the conductive strips forming the grids. This step creates pollution risks important on the tips (organic residues, traces of cleaning,...).

However, the emissivity of a point varies so exponential with the shape of the tip and its state of area.

Under these conditions, only a small proportion of microtips ensures current system electronics; therefore the effect of average plays badly and the broadcast is not uniform on the whole cathode.

According to patent application EP -434330, it is known to make a spike attack after their manufacturing in order to refine their radius of curvature. But this process does not work well for large cathodes surfaces.

Statement of the invention

The object of the present invention is precisely a process for producing electron sources using microtips which allows both to standardize the state of surface and to refine the geometry of the microtips.

This process thus makes it possible by greatly reducing the dispersions of characteristics from point to point the other and from one source to another to compensate for previous disadvantages and make it easier the production of microtip cathodes with uniform and reproducible characteristics, as well than a high emission level.

• une première étape de nettoyage,

puis à :

• une étape d'affinage par gravure superficielle

More specifically, the present invention relates to a method for producing a microtip electron source comprising a system of cathode conductors, grids superimposed with an intermediate insulator, and microtips, the grids being geometrically comprised between a lower plane and an upper plane, characterized in that the microtips are subjected to:

• a first cleaning step,

then to:

• a refining step by surface etching

In other words, after the manufacturing of microtips, as explained for example in document FR-A-2,593,953, the invention proposes carry out, as a first step, a cleaning which makes it possible to standardize the surface condition and, secondly, a refining step which consists of an additional engraving to give microtips a profile as close as possible to the ideal desired, i.e. with a radius of curvature as small as possible (less than a few tens of nanometers).

In practice, this optimization consists of search, for microtips, of a profile as close as possible to a tapered cone tapered, in other words in search of an effect of tips increased to guarantee a large amplitude of the electric field.

Advantageously, the step is followed refining of a second cleaning step, consisting of wet chemical cleaning.

Preferably, the first cleaning step comprises a first wet chemical cleaning sub-step and a second cleaning sub-step with a plasma, for example with O ₂ plasma.

According to the invention, the refining step by surface etching can be achieved by one any of the known methods which are in particular controlled chemical or electrochemical attack, the attack by reactive ion etching and the attack by ion bombardment.

According to an implementation characteristic of the process which is the subject of the invention, the surface attack microtips is performed over a thickness of a few tens to a few thousand Angstroms.

One of the advantages of the process which is the subject of the present invention, is that it applies to treatment very large emissive surfaces, such as meets precisely in flat screens display. The process thus makes it possible to correct very simply the approximate shape of the microtips obtained to date and, by removing the dispersal of peak emission characteristics to the other, to allow a level of emission very high and significantly increased electronics compared to those of the prior art, and therefore allow the reduction of the supply voltage required between the grids and cathode conductors for extract the electrons.

In short, the principle of the invention consists in choose a method for producing microtips which gives them an approximate form (more easy to carry out on large surfaces and less expensive) then cleaning the microtips and finally improve and homogenize their radius of curvature to using, in particular, reactive ion etching or other methods of chemical etching or electrochemical.

• une première partie servant de base, de forme sensiblement tronconique, et étant constituée d'un premier matériau conducteur, choisi de telle façon qu'il ne soit pas ou très peu attaqué par l'étape d'affinage,
• une deuxième partie constituant la pointe proprement dite et étant déposée sur la première partie, cette deuxième partie étant constituée d'un second matériau conducteur choisi de telle façon à ce qu'il soit attaqué par l'étape d'affinage.

The invention is particularly advantageous to implement when the microtips are produced respectively in at least two parts:

• a first part serving as a base, of substantially frustoconical shape, and consisting of a first conductive material, chosen so that it is not or very little attacked by the refining step,
• a second part constituting the point proper and being deposited on the first part, this second part consisting of a second conductive material chosen so that it is attacked by the refining step.

Also included in the term "material conductive ", semiconductor materials.

Preferably, the first part (base) is height such that its top is about the same level than the lower plane of the grid.

The advantage of implementing the invention in this particular case is as follows.

When the microtips consist of a unique material, sensitive to the refining stage, the ripening time must be checked: if it is too important, the top of the tip can quickly find below the lower plane of the grid, this which is very unfavorable to electronic transmission. If it is too small, the radius of curvature is not optimum and the effect sought by the refining is not achieved.

On the contrary, when the microtips are consist of two parts, as described above, the ripening time must be sufficient to obtain the optimum radius of curvature of the tip, but if it is longer, the top of the tip always stays above of the lower plane of the grid since it rests on unattacked or slightly attacked material.

According to an exemplary embodiment, the first part is niobium (Nb), the second part is molybdenum, or chromium, or silicon, or iron, or nickel.

The invention applies to sources in which microdots are not deposited directly on the cathode conductors but by example on a resistive layer inserted between the microtips and cathode conductors.

Brief description of the figures

• les figures 1 à 3 illustrent différentes étapes de formation de micropointes, selon un procédé connu de l'art antérieur,
• la figure 4 représente schématiquement la forme des micropointes obtenues par un procédé connu,
• la figure 5 représente schématiquement le profil en cône idéal souhaité,
• les figures 6a et 6b illustrent l'émissivité des micropointes d'une part avant et après traitement d'affinage et d'autre part avant et après la seconde étape de nettoyage,
• les figures 7a, 7b et 7c montrent schématiquement les formes obtenues pour une micropointe en un seul métal, dans le cas d'un affinage respectivement trop poussé, insuffisant et optimal,
• les figures 8a à 8c illustrent le procédé d'affinage pour une micropointe en deux parties.

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:

• FIGS. 1 to 3 illustrate different stages of microtip formation, according to a method known from the prior art,
• FIG. 4 schematically represents the shape of the microtips obtained by a known method,
• FIG. 5 schematically represents the desired ideal cone profile,
• FIGS. 6a and 6b illustrate the emissivity of the microtips on the one hand before and after refining treatment and on the other hand before and after the second cleaning step,
• FIGS. 7a, 7b and 7c schematically show the shapes obtained for a microtip made of a single metal, in the case of refining respectively too much, insufficient and optimal,
• Figures 8a to 8c illustrate the refining process for a two-part microtip.

Detailed example of embodiments

• dépôt par pulvérisation cathodique sur le substrat 6, d'une couche d'oxyde de silicium 7 (voir figure 1), d'environ 100 nm,
• dépôt par pulvérisation cathodique, sur la couche 7, d'une première couche conductrice en oxyde d'indium dans laquelle seront réalisées les conducteurs cathodiques 8 (épaisseur environ 160 nm),
• gravure de la première couche conductrice pour former des premières bandes conductrices parallèles ou conducteurs cathodiques 8,
• dépôt chimique en phase vapeur (à partir des gaz de silane, phosphine, oxygène) d'une seconde couche isolante d'oxyde de silicium d'épaisseur environ 1 µm (12),
• dépôt, par évaporation sous vide, sur la couche d'oxyde de silicium, d'une troisième couche, conductrice, dans laquelle seront formées les grilles 10a (niobium, épaisseur environ 0,4 µm),
• ouverture de trous 16 (diamètre environ 1,3 µm) dans la troisième couche conductrice, par gravure ionique réactive (GIR) en utilisant un plasma de SF₆, et dans la seconde couche 12 par gravure ionique réactive dans un plasma de CHF₃ ou par attaque chimique dans une solution d'acide fluorhydrique et de fluorure d'ammonium.
• dépôt d'une couche de nickel 23 (figure 2) par évaporation sous vide, sous incidence rasante par rapport à la surface de la structure. L'angle α formé entre l'axe d'évaporation et la surface de la couche 10a est voisin de 15°. La couche de nickel présente une épaisseur d'environ 150 nm,
• formation des micropointes par un procédé décrit dans l'introduction de la présente demande, en liaison avec les figures 2 et 3,
• gravure de la troisième couche pour former des deuxièmes bandes conductrices parallèles aux grilles.

The steps of the method according to the invention supplement the known methods of forming electron-emitting microtip cathodes. Such a method is described for example in document FR-A-2,593,953 (corresponding American patent: US-A-4,857,161). In summary, it includes the following steps:

• sputtering on the substrate 6, a layer of silicon oxide 7 (see FIG. 1), of about 100 nm,
• sputtering deposition, on layer 7, of a first conductive layer of indium oxide in which the cathode conductors 8 will be produced (thickness about 160 nm),
• etching of the first conductive layer to form first parallel conductive strips or cathode conductors 8,
• chemical vapor deposition (from silane gases, phosphine, oxygen) of a second insulating layer of silicon oxide with a thickness of approximately 1 μm (12),
• deposition, by vacuum evaporation, on the silicon oxide layer, of a third conductive layer, in which the grids 10a (niobium, thickness approximately 0.4 μm) will be formed,
• opening of holes 16 (diameter approximately 1.3 μm) in the third conductive layer, by reactive ion etching (GIR) using a plasma of SF ₆ , and in the second layer 12 by reactive ion etching in a plasma of CHF ₃ or by chemical attack in a solution of hydrofluoric acid and ammonium fluoride.
• deposition of a layer of nickel 23 (FIG. 2) by evaporation under vacuum, under grazing incidence relative to the surface of the structure. The angle α formed between the evaporation axis and the surface of the layer 10a is close to 15 °. The nickel layer has a thickness of approximately 150 nm,
• formation of the microtips by a process described in the introduction to the present application, in conjunction with FIGS. 2 and 3,
• etching the third layer to form second conductive strips parallel to the grids.

• un nettoyage chimique humide dans un bain de lessive (TFD4 à 10% dans de l'eau), à 60°C, assisté par ultrasons, le tout pendant une durée de 5 minutes environ,
• un nettoyage par gravure ionique réactive dans un plasma d'oxygène, par exemple à l'aide d'un équipement vendu dans le commerce sous l'appellation NEXTRAL 550.

According to the invention, these steps are followed first by a cleaning step which has the function of standardizing the surface condition before any other step. This cleaning step can include two sub-steps:

• wet chemical cleaning in a detergent bath (TFD4 at 10% in water), at 60 ° C, assisted by ultrasound, all for a period of approximately 5 minutes,
• cleaning by reactive ion etching in an oxygen plasma, for example using equipment sold commercially under the name NEXTRAL 550.

This last operation which lasts about ten minutes is done for example with a power of 250 Watts, a plasma pressure of 100 millitorrs and a flow rate of 100 cm ³ / min.

The cleaning step is followed by a step of refining or etching the tips, for example for molybdenum tips by reactive ion etching in an SF ₆ plasma (same equipment as that mentioned above). This step eliminates a layer of molybdenum oxide which may have formed during cleaning under O ₂ plasma. It also allows microtip etching to modify their shape and in particular to reduce their radius of curvature. The conditions of action of the sulfur hexafluoride plasma are for example the following: the operation lasts approximately 20 seconds with a power of 400 W, a flow rate of 40 cm ³ / min under a plasma pressure of 30 millitorrs. At the end of this treatment, a high proportion of microtips have the same profile which approximates the ideal cone profile of FIG. 5 and a very uniform surface condition.

Figure 6a is a curve showing emissivity of microtips before treatment ripening (dotted curve) and after treatment ripening (solid line curve). On this graph, current density in microamps per millimeter square is plotted on the ordinate and the grid-microtip voltage in volts is plotted on the abscissa. The increase in emissivity following treatment immediately appears to be considerable. We therefore effectively obtains microtips, for which the radius of curvature of the end is less than a few tens of nanometers.

Figure 6b shows emissivity (same units as in FIG. 6a) of the microtips after refining, but before (dotted curve) and after the second cleaning step (curve in solid line). We see that this second cleaning step still allows improve the emissivity of an important factor.

Other tip refining processes can be used as an alternative to the one described above, for example by chemical attack (or electrochemical) controlled or by ion bombardment.

Advantageously, it is also possible to carry out the second stage of wet chemical cleaning in the laundry bath mentioned above, for a period about 30 minutes.

The duration, during which the stage is carried out must be checked in the event that the microdots are made of a single sensitive metal refining, for example molybdenum.

The grid 10a is geometrically understood between or delimited by two planes, a lower plane (I) and an upper plane (S) (see figure 7a, on which, as in FIGS. 7b, 7c, 8a-c, the references 6, 8, 10a, 12 have the same meaning as in Figures 1 to 5).

If the ripening time is too long, the top of point 18 can quickly be found, as illustrated in figure 7a, below the plan lower I of grid 10a, which is very unfavorable to the broadcast.

If the ripening time is too short, the radius of curvature is not optimum (see Figure 7b) and the desired effect is not achieved.

In fact, with the single metal structure, the ripening time should be long enough to obtain the optimum radius of curvature, but despite everything not too long for the tip to remain above the lower plane I of the grid (FIG. 7c).

On the contrary, when the point is made up at least two superimposed metals, the ripening time is as we will see, much less critical.

• une première partie ou base 20 qui a une forme tronconique, de hauteur H. Elle est constituée d'un premier matériau choisi de telle façon qu'il ne soit pas ou très peu attaqué par l'étape d'affinage décrite ci-dessus. Ce matériau peut être par exemple du niobium,
• une deuxième partie 22 qui constitue la pointe proprement dite. Elle est déposée directement sur la première partie. Elle est constituée d'un deuxième matériau sensible à l'étape d'affinage, par exemple du molybdène, ou du chrome (Cr), ou du silicium (Si), ou du fer (Fe), ou du nickel (Ni).

The structure of the tip before refining is illustrated in FIG. 8a, and comprises:

• a first part or base 20 which has a frustoconical shape, height H. It consists of a first material chosen so that it is not or very little attacked by the refining step described above. This material can for example be niobium,
• a second part 22 which constitutes the point proper. It is placed directly on the first part. It consists of a second material sensitive to the refining step, for example molybdenum, or chromium (Cr), or silicon (Si), or iron (Fe), or nickel (Ni).

A method for obtaining microtips having this structure is derived from the process already described in introduction to making microtips made of a single material. We start with deposit a layer 18a for example by niobium on the nickel layer 23, by evaporation under vacuum at normal incidence, as in the figure 2. There is a direct relationship between the height of material deposited in hole 16 and duration vacuum evaporation. We can therefore interrupt this evaporation when the desired height H of the trunk of cone forming base 20 is reached, and continue then evaporation with the second material such as molybdenum so as to obtain the second part 22. The whole then has the overall shape substantially conical of Figure 8a.

In fact, the height H of the base 20 must be sufficient for the vertex A of the cone obtained to be located above the lower plane of the grid 10a. Of preferably, A will be located, after the deposit operations which have just been described, above the plan upper of the grid 10a; for this purpose, the height H will be substantially equal to the thickness of the insulation 12, that is to say in this embodiment, at the distance separating the cathode conductor 8 from the plane lower of the grid 10a.

If a resistive layer is interposed between microdots and cathode conductors there will obviously have to take into account the thickness of this resistive layer.

• une première partie, sensiblement tronconique, de hauteur H, H est de préférence sensiblement égale à la distance séparant le conducteur cathodique 8 du plan inférieur I de la grille 10a, c'est-à-dire sensiblement égale à l'épaisseur e de l'isolant 12 ; par exemple H sera comprise entre 0,8e et 1,1e, (là encore, il faut tenir compte de la présence éventuelle d'une couche résistive entre les micropointes et les conducteurs cathodiques),
• une deuxième partie conique, dont la base est de diamètre d inférieur au diamètre D de la section supérieure du tronc de cône 20.

We can then proceed to the cleaning and refining operations which have been previously described. Due to the initial choice of materials from which parts 20 and 22 are made, the only part attacked by refining is part 22. The structure obtained by the process (Figures 8b or 8c) has the following form:

• a first part, substantially frustoconical, of height H, H is preferably substantially equal to the distance separating the cathode conductor 8 from the lower plane I of the grid 10a, that is to say substantially equal to the thickness e of l 'insulator 12; for example H will be between 0.8e and 1.1e, (here again, account must be taken of the possible presence of a resistive layer between the microtips and the cathode conductors),
• a second conical part, the base of which is of diameter d less than the diameter D of the upper section of the truncated cone 20.

The time during which the ripening is carried out must be sufficient to obtain the radius of curvature sought (Figure 8b), but if this duration is longer long, the vertex A 'of the point always remains above of the lower plane of the grid 10a, since the part 22, attacked by refining, rests on the part 20, not attacked by ripening. The tip does could therefore disappear only after an attack time significantly longer.

• l'isolant est en silice d'une épaisseur proche de 1 µm,
• la grille est en niobium (Nb) d'une épaisseur d'environ 0,4 µm ; les trous dans la grille ont un diamètre de l'ordre de 1,4 µm,
• le métal constituant la base 20 de la pointe est en Nb d'une épaisseur comprise entre 0,8 et 1,1 µm,
• la partie 22 est en molybdène d'une épaisseur suffisante pour constituer la pointe, par exemple 1 µm avant affinage, l'affinage de cette partie pouvant se faire de la même façon que décrit précédemment dans l'exemple de réalisation où les micropointes sont entièrement en molybdène.

According to a nonlimiting exemplary embodiment:

• the insulation is made of silica with a thickness close to 1 μm,
• the grid is made of niobium (Nb) with a thickness of approximately 0.4 μm; the holes in the grid have a diameter of the order of 1.4 μm,
• the metal constituting the base 20 of the tip is made of Nb with a thickness of between 0.8 and 1.1 μm,
• the part 22 is made of molybdenum of sufficient thickness to constitute the tip, for example 1 μm before refining, the refining of this part can be done in the same way as described previously in the embodiment where the microtips are entirely in molybdenum.

Finally, a microtip cathode obtained by the method described in the present invention can be associated with a structure comprising at least one anode and a cathodoluminescent material to achieve a display device as described in US Patents 4,857,161 (FR-2,593,953), US 4,940,916, US.5 225 820 (FR-2 633 763) or US.5 194 780 (FR-A-2 663 462).

Claims (12)

1. Process for the production of an electron source comprising a system of cathode conductors (8), grids (10a) superimposed with an intermediate insulator (23) and microtips (18), the grids being geometrically located between a lower plane (I) and an upper plane (S), characterized in that the microtips undergo a first cleaning stage and then to a finishing stage by surface etching.
2. Process according to claim 1, the refining stage being followed by a second cleaning stage consisting of a wet chemical cleaning.
3. Process according to either of the claims 1 and 2, the first cleaning stage comprising a first, wet chemical cleaning substage and/or a second plasma cleaning substage.
4. Process according to either of the claims 2 and 3, the chemical cleaning being carried out in a basic lye bath.
5. Process according to claim 1, the surface etching finishing stage being performed by any of the methods from among controlled chemical or electrochemical etching, reactive ionic etching and ionic bombardment.
6. Process according to claim 5, the finishing stage being a reactive ionic etching stage based on a SF₆ plasma.
7. Process according to any one of the claims 1 to 6, the surface etching of the microtips being performed over a thickness of a few dozen to a few thousand Angstroms.
8. Process according to any one of the claims 1 to 7, characterized in that, prior to the cleaning and finishing stages, the microtips are respectively produced in at least two parts:

   a first part (20) serving as a base and made from a first conductor material chosen in such a way that it is not or is only very slightly etched by the finishing stage,

   a second part (22) constituting the actual tip and deposited on the first part, said second part being made from a second conductor material chosen in such a way that it is etched by the finishing stage.
9. Process according to claim 8, the base being of height (H) such that its apex is substantially at the same level as the lower plane (I) of the grids (10a).
10. Process according to claim 8, the first part (20) being of niobium (Nb).
11. Process according to claim 8, the second part (22) being of molybdenum (Mo), chromium (Cr), silicon (Si), iron (Fe) or nickel (Ni).
12. Process for the production of a display means by cathodoluminescence, involving the production of an electron source by a process according to any one of the claims 1 to 11.

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