EP0697710A1 - Manufacturing method for a micropoint-electron source - Google Patents

Abstract

A micro-tip electron source prodn. process involves: (a) producing a structure comprising an insulating substrate (10) bearing one or more cathode conductors (12) covered with an insulating layer (14) and then a conductive grid layer (16), holes (18) being formed through these layers (14,16) to each cathode conductor (12); and (b) forming a micro-tip of electron-emissive metal on the cathode conductor (12) in each hole (18), and an insulating protective layer (50) on the grid layer (16), depositing (pref. electrolytically) electron-emissive metal to overfill the holes, removing the protective layer and electrolytically etching the metal deposit (60) to form the micro-tips.

Description

The present invention relates to a method for manufacturing a microtip electron source ("microtips").

It applies in particular to the manufacture of flat display devices.

When a potential difference is applied between two electrodes, one of which is pointed, the electric field thus generated can easily reach, at the end of this pointed electrode, a value of the order of 10⁷ V / cm, sufficient value so that electrons are extracted from this electrode.

Such a principle is used to produce cold sources of electrons, capable of replacing the electron-emitting heating filaments, since such cold sources have a faster response, lower electrical consumption and are capable of more great miniaturization than these heating filaments.

One of the most important applications of these cold sources of electrons, also called "microtip sources", is the manufacture of flat television tubes.

The principle of these flat tubes, or flat screens, is recalled, with reference to FIGS. 1 and 2.

Figure 1 is a schematic and partial sectional view of such a flat screen and Figure 2 is a schematic and partial perspective view of this flat screen.

The flat screen of FIGS. 1 and 2 comprises a source of microtip electrons 2 and a glass substrate 4 which is separated from the source 2 by a thin space in which a vacuum has been created.

The substrate 4 carries, opposite the source 2, a transparent, electrically conductive layer 6, for example of indium tin oxide, this layer 6 itself carrying cathodoluminescent elements 8, also called "phosphors".

The microtip source 2 comprises, on an electrically insulating substrate 10, for example made of glass, a set of parallel cathode conductors 12 which constitute the columns of the screen.

These cathode conductors are covered by a layer 14 of an electrically insulating material such as silica.

A set of other parallel electrical conductors 15 is placed above the insulating layer 14 and these other conductors 15, or grids, are perpendicular to the cathode conductors 12 to form the lines of the screen.

At the intersections between the cathode conductors and the grids, holes 18, 19 are formed through the insulating layer 14 and these grids 15 and microtips 20 made of an electron-emitting material are formed in these holes and rest on cathode conductors 12.

The phosphors 8 are formed on the transparent conductive layer 6, facing these intersections, as seen in FIG. 2.

The electrons are extracted by applying appropriate electrical voltages between the grids and the microtips, then these electrons are accelerated thanks to appropriate applied voltages. between the grids and the conductive layer 6 constituting the anode of the screen.

Each phosphor 8 excited by electrons 22 emits light 24.

An appropriate voltage sweep on the rows and columns of the screen makes it possible to form an image.

Only the microtips located at the intersection of a line and a column supplied with voltage emit electrons to form an image element or pixel.

Each pixel is in fact "excited" by several hundred microtips whose dimensions are of the order of 1 μm, generally 1.5 μm, and which are spaced from one another by a distance of the order of a few micrometers, typically 5 µm.

These small dimensions are essential for, on the one hand, not having to use excessively high voltages between the grids and the microtips (voltages of the order of 50 V) and, on the other hand, for having an emission of sufficiently high current per unit area (approximately 1 mA / mm).

A flat screen thus typically uses around 10,000 microtips per square millimeter on areas of several square decimetres.

The flat screens currently manufactured have surfaces of the order of 5 dm and it is envisaged to manufacture flat screens whose surfaces would go up to approximately 1 m.

However, it is not easy to obtain microtip sources having such large areas with the known methods of manufacturing microtips.

The process most used to make these microtips is the so-called Spindt process (from the name of its inventor).

We will consult on this subject for example the following document:

(1) C.A. Spindt, J. Appl. Phys., Vol.39, p.3504, 1968.

FIG. 3, which schematically illustrates this process, shows a structure comprising the insulating substrate 10 on which the cathode conductors 12 are formed, and the insulating layer 14 which is formed on these cathode conductors and which carries a grid layer 16 electrically conductive.

The grids themselves are obtained from this grid layer 16, after having formed the microtips as will be seen.

After having etched the holes 18 and 19 respectively in the insulating layer 14 and in the grid layer 16, a nickel layer 16a is deposited on the grid layer 16 by evaporation under vacuum and under grazing incidence.

The microtips 20 are obtained by evaporation of an electron-emitting material 26.

A layer 28 of this material then forms on the surface of the grid layer 16a.

As a result, the holes 19 formed in these layers 16 and 16a decrease progressively as the thickness of the layer 28 increases.

Since evaporation is very directive, the diameter of the material deposits 26 in the holes 18 of the insulating layer 14 varies like the diameter of the holes in the layer 16a and the grid layer 16, which leads to the point shape of the deposits in the holes 18, that is to say to the microtips 20.

The layer 28 is then eliminated by selective dissolution of the nickel layer 16a, which reveals these microtips.

The main advantage of this known method is that it does not require precise alignment of microlithography masks since it is the holes in the grid layer which themselves define the microtips.

It would indeed be almost impractical to first etch the microtips and then the holes in the grid layer by conventional microlithography methods, with alignment accuracy greater than a micrometer over large areas.

Another known method for manufacturing microtips is described in the following document:

(2) Oxidation-Sharpened Gated Field Emitter Array Process, N.E. McGruer et al., IEEE Transactions on Electron Devices, (38) 1991 October, n ° 10.

This other method is schematically illustrated in FIG. 4.

This FIG. 4 shows a silicon substrate 30.

We begin by oxidizing this substrate superficially then discs 32 are formed from the silica layer which results from this oxidation.

Reactive ion etching of the silicon substrate 30 then allows the formation of silicon pedestals 34, the disks 32 serving as masks.

A layer of silica 36 is then formed on the substrate 30 by evaporation of silica 38.

A layer 40 of silica is then formed on each disc 32.

The pedestals 34 are then thermally oxidized, which leads to the formation of microtips 42 from these pedestals.

A grid layer 44 is then formed by evaporation of an electrically conductive material on the silica layer 36.

During this evaporation, a layer 46 of this material also forms on the layer 40 of silica associated with each disc 32.

Then removing the silica which covers the microtips 42 as well as the discs 32 and the corresponding layers 40 and 46.

The disadvantage of the known methods which have just been described is that they require very directive evaporations.

Using for example the example of FIG. 3, the angle of incidence θ of an evaporation beam F varies as a function of the position of the holes 19 of the grid layer 16, which leads to the phenomenon illustrated on FIG. 5, that is to say to microtips whose axes Y are less perpendicular to the surface of the substrate 10 the greater the angle of incidence θ.

This results in a variation in the shape of the microtips, a variation which induces a dispersion of the emission characteristics of the electrons, and, ultimately, a short circuit between microtips and the grid layer.

To solve this problem, one can consider increasing the distance L between the evaporation source 48 (containing the material 26) and the surface of the structure on which this material 26 is evaporated, in order to keep the angle θ within limits. acceptable.

However, this leads to too large an increase in the size of the microtip manufacturing equipment as well as to a too large decrease in the deposition rate.

The object of the present invention is to remedy these drawbacks.

• on fabrique une structure comprenant un substrat électriquement isolant, au moins un conducteur cathodique sur ce substrat, une couche électriquement isolante qui recouvre chaque conducteur cathodique, une couche de grille électriquement conductrice qui recouvre cette couche électriquement isolante, des trous étant formés à travers cette couche de grille et la couche électriquement isolante, au niveau de chaque conducteur cathodique, et
• on forme, dans chaque trou, une micropointe qui est faite en un matériau métallique émetteur d'électrons et qui repose sur le conducteur cathodique correspondant à ce trou,

ce procédé étant caractérisé en ce que la formation des micropointes comprend les étapes suivantes :

• on forme une couche de protection électriquement isolante sur la couche de grille,
• on forme un dépôt chimique, de préférence électrolytique, du matériau métallique émetteur d'électrons au fond des trous jusqu'à ce que ce matériau métallique déborde de ceux-ci,
• on élimine la couche de protection, et
• on réalise une attaque électrolytique du matériau métallique déposé, de manière à obtenir les micropointes à partir de ce matériau métallique.

It relates to a process for manufacturing a microtip electron source, a process according to which:

• a structure is made up comprising an electrically insulating substrate, at least one cathode conductor on this substrate, an electrically insulating layer which covers each cathode conductor, an electrically conductive grid layer which covers this electrically insulating layer, holes being formed through this layer grid and the electrically insulating layer, at the level of each cathode conductor, and
• a microtip is formed in each hole which is made of an electron-emitting metallic material and which rests on the cathode conductor corresponding to this hole,

this process being characterized in that the formation of the microtips comprises the following stages:

• an electrically insulating protective layer is formed on the grid layer,
• a metallic deposit, preferably electrolytic, of the metallic material emitting electrons is formed at the bottom of the holes until this metallic material overflows from them,
• the protective layer is removed, and
• an electrolytic etching of the metallic material deposited is carried out, so as to obtain the microtips from this metallic material.

According to a particular embodiment of the process which is the subject of the invention, preferred for its simplicity of implementation, the grid layer is used as a cathode for the electrolytic attack on the metallic material.

During this dissolution phase, it is advantageous to renew, by any known means, the electrolyte located around the metallic material so as to avoid an over-concentration in metallic ions which could slow down the dissolution and cause a significant redeposition of this material on the grid around microdots in formation.

A small redeposition or a controlled redeposition which spans the entire grid is tolerated; it results in a significant reduction in the diameter of the holes, which is rather favorable to the emission of electrons by the microtips.

The protective layer can be formed by depositing, under grazing incidence, a layer of an electrically insulating material on the grid layer.

However, this protective layer is preferably formed by anodic oxidation of the gate layer.

The grid layer can be made of a material selected from the group comprising niobium, tantalum and aluminum.

The metallic material can be chosen from the group comprising iron, nickel, chromium, Fe-Ni, gold, silver and copper.

The protective layer can be removed by chemical attack.

This protective layer can also be removed by reactive ion etching.

• la figure 1, déjà décrite, est une vue en coupe schématique et partielle d'un écran plat,
• la figure 2, déjà décrite, est une vue schématique et partielle en perspective de cet écran plat,
• la figure 3, déjà décrite, illustre schématiquement un procédé connu de fabrication des micropointes d'une source d'électrons à micropointes,
• la figure 4, déjà décrite, illustre schématiquement un autre procédé connu de fabrication des micropointes d'une source d'électrons à micropointes,
• la figure 5, déjà décrite, illustre schématiquement des inconvénients de ces procédés connus, et
• les figures 6A à 6E illustrent schématiquement des étapes d'un mode de mise en oeuvre particulier du procédé objet de l'invention.

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 and partial sectional view of a flat screen,
• FIG. 2, already described, is a schematic and partial perspective view of this flat screen,
• FIG. 3, already described, schematically illustrates a known method of manufacturing the microtips of an electron source with microtips,
• FIG. 4, already described, schematically illustrates another known method for manufacturing the microtips of an electron source with microtips,
• FIG. 5, already described, schematically illustrates drawbacks of these known methods, and
• FIGS. 6A to 6E schematically illustrate steps of a particular mode of implementation of the method which is the subject of the invention.

According to this particular mode of implementation, one begins by forming (FIG. 6A) a structure 49 of the kind of that represented in FIG. 3 and which comprises the electrically insulating substrate 10 on which the cathode conductors 12 are formed, the layer electrically insulating 14 formed on these cathode conductors and the gate layer 16 formed on this electrically insulating layer 14 (it being understood that, in other particular embodiments, the structure could comprise only one cathode conductor).

We also see the substantially circular holes 18 and 19 respectively formed through the insulating layer 14 and through the grid layer 16.

The methods for obtaining such a structure are known in the state of the art.

For example, the substrate 10 is made of glass, the cathode conductors consist of a bilayer of chromium and copper, the layer 14 is made of silica and the grid layer 16 is made of niobium, tantalum or aluminum.

A protective layer is then formed on the grid layer 16 (FIG. 6B).

To do this, it is possible to evaporate silica, under grazing incidence, on the grid layer 16, to cover the latter with silica.

However, preferably, anodic oxidation of the gate layer 16 is carried out, which leads to the formation of a layer 50 of niobium oxide or tantalum oxide or aluminum oxide in the example. considered, which covers the remaining part of the grid layer 16, as seen in FIG. 6B.

This anodic oxidation leads to a more reliable covering of the grid layer than evaporation under grazing incidence mentioned above and is simpler to use.

An electrolytic deposition of a metallic material is then carried out at the bottom of the holes 18 until this metallic material overflows from these holes as can be seen in FIG. 6C, part of this material then being above the layer. 50.

To do this, the structure 49, comprising the protective layer 50, is placed in an appropriate electrolytic bath 54 (containing ions of the metallic material to be deposited) and also placed in this electrolytic bath a block 56 of this metallic material.

NiCl₂, 6H₂O

50 g.l⁻¹

NiSO₄, 6H₂O

21 g.l⁻¹

FeSO₄

2 g.l⁻¹

H₃BO₃

25 g.l⁻¹

Saccharinate de Na

0,8 g.l⁻¹

When this metallic material is fernickel, the electrolytic bath can be used, the composition of which is as follows:

NiCl₂, 6H₂O

50 gl⁻¹

NiSO₄, 6H₂O

21 gl⁻¹

FeSO₄

2 gl⁻¹

H₃BO₃

25 gl⁻¹

Na saccharinate

0.8 gl⁻¹

An appropriate electrical voltage is then applied, thanks to a voltage source 58, between the cathode conductors 12 and this block 56.

In the case where the metallic material is iron-nickel, the following conditions can be used for the electrolytic deposition:
current density: 0.5 to 2 mA / cm
voltage: 1 to 2V
ambient temperature.

For electrolysis, the cathode conductors 12 serve as the cathode and the block 56 serves as the anode.

The electrically conductive elements 60, which result from the deposition of the metallic material at the bottom of the holes 18, are in contact with the cathode conductors but are electrically insulated from the grid layer 16 thanks to the protective layer 50 which covers the latter.

This protective layer 50 is then removed by chemical attack or by reactive ion etching (FIG. 6D).

An electrolytic attack is then carried out on the electrically conductive elements 60 so as to form the microtips 62 therefrom (FIG. 6E).

To do this, the structure is placed, where the protective layer 50 has been removed, in an appropriate electrolytic bath 64 (containing for example 10% HCl at 37% and 90% H₂O for the dissolution of nickel iron) and, at by means of a suitable electrical voltage source 66, an electrical voltage is established (for example 1 to 2 V for the dissolution of iron-nickel) between the cathode conductors 12 which, in this case, serve as an anode, and the layer of grid 16 which serves as a cathode.

Preferably, the electrolyte is renewed by stirring and / or by circulation, so as to avoid a concentration of ions around the material of the elements 60.

During the electrolysis, the material of the elements 60 is eliminated in a substantially symmetrical manner around the axis Z of the holes 18 and the metal ions produced by the chemical attack of the material of the elements 60 are partly eliminated thanks to the renewal of the electrolyte and partly redeposited on the grid layer.

Depending on the material of the elements 60 and the rate of renewal of the electrolyte, the redeposited fraction of the ions is more or less large and can be controlled.

• d'éléments pointus qui affleurent sensiblement à la surface de la couche de grille 16 et constituent les micropointes 62, et
• de parties 68 qui se détachent de ces micropointes et restent dans le bain électrolytique comme on le voit sur la figure 6E.

The wear of the conductive elements 60 by electrolysis leads to obtaining:

• pointed elements which are substantially flush with the surface of the grid layer 16 and constitute the microtips 62, and
• parts 68 which detach from these microtips and remain in the electrolytic bath as seen in FIG. 6E.

Preferably, this step of forming the microtips is carried out with the glass substrate above and the electrolytic bath below, so as to allow the parts 68 to fall into the electrolytic bath.

The formation of the microtip electron source is then terminated by producing, in a known manner, from the grid layer 16, parallel grids (not shown) making an angle with the cathode conductors (but if there is no had only a cathode conductor, we would keep the grid layer as it is).

The advantage of the process which is the subject of the present invention is that it allows the manufacture of self-aligned microtips on the holes of the grid layer 16, by means of a non-directive technique, in isotropic liquid medium (electrolytic bath 64).

This process which is the subject of the invention is therefore independent of the surface of the structure where it is desired to form the microtips.

Claims (9)

Method for manufacturing a microtip electron source, method according to which: - a structure (49) is produced comprising an electrically insulating substrate (10), at least one cathode conductor (12) on this substrate, an electrically insulating layer (14) which covers each cathode conductor, an electrically conductive grid layer (16 ) which covers this electrically insulating layer, holes (18, 19) being formed through this grid layer and the electrically insulating layer, at the level of each cathode conductor, and - a microtip (62) is formed in each hole which is made of an electron emitting metallic material and which rests on the cathode conductor corresponding to this hole, this process being characterized in that the formation of the microtips comprises the following stages: - an electrically insulating protective layer (50) is formed on the grid layer (16), a chemical deposit of the metallic material emitting electrons is formed at the bottom of the holes until this metallic material overflows from them, - the protective layer (50) is eliminated, and - An electrolytic attack is carried out on the metallic material deposited, so as to obtain the microtips (62) from this metallic material. Method according to claim 1, characterized in that the chemical deposition of the metallic material emitting electrons is an electrolytic deposition. Method according to any one of claims let 2, characterized in that one uses the grid layer (16) as a cathode for the electrolytic attack of the metallic material. Method according to any one of Claims 1 to 3, characterized in that the protective layer (50) is formed by depositing, under grazing incidence, a layer of an electrically insulating material on the grid layer (16). Method according to any one of Claims 1 to 3, characterized in that the protective layer is formed by anodic oxidation of the gate layer (16). Method according to any one of Claims 1 to 5, characterized in that the grid layer (16) is made of a material chosen from the group comprising niobium, tantalum and aluminum. Process according to any one of Claims 1 to 6, characterized in that the metallic material is chosen from the group comprising iron, nickel, chromium, Fe-Ni, gold, silver and copper. Method according to any one of Claims 1 to 7, characterized in that the protective layer (50) is removed by chemical attack. Method according to any one of Claims 1 to 7, characterized in that the protective layer (50) is removed by reactive ion etching.

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