EP0708473A1 - Manufacturing method for micropoint electron source - Google Patents

Abstract

The electron source mfg. procedure involves creating an insulating substrate (32) on which there is at least one cathodic conductor (34). This is covered by an amorphous doped silicon layer (36), an insulating layer (38) and a grid layer (40). A set of holes is formed through the grid layer and the insulating layer. A sacrificial layer (44) is formed on the grid layer by a method of chemical vapour deposition to form an electrolytic deposit. An electron emitting layer (52) is then formed on the assembly and the sacrificial layer is then eliminated by electrolysis, leaving micro-point electron sources (54). The sacrificial layer is chosen from the group of metals including Cr, Fe, Ni, Co, Cd, Cu, Au, Ag and may for example be an alloy of nickel and iron.

Description

TECHNICAL AREA

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

The invention applies to any field where it is likely to use such a source of microtip electrons, in particular the field of flat display devices also called "flat screens".

The invention makes it possible, for example, to manufacture large micropoint flat screens, the surface area of which can be of the order of 1000 cm and can even go up to around 1 m.

STATE OF THE PRIOR ART

(1)

FR-A-2 593 953 correspondant à EP-A-0 234 989 et à US-A-4 857 161

(2)

FR-A-2 623 013 correspondant à EP-A-0 316 214 et à US-A-4 940 916

(3)

FR-A-2 663 462 correspondant à EP-A-0 461 990 et à US-A-5 194 780

(4)

FR-A-2 687 839 correspondant à EP-A-0 558 393 et à la demande de brevet américain du 26 février 1993, numéro de série 08/022,935 (Leroux et al.).

Sources of electrons with microtip emissive cathodes and their manufacturing methods are described in the following documents to which reference will be made:

(1)

FR-A-2 593 953 corresponding to EP-A-0 234 989 and to US-A-4 857 161

(2)

FR-A-2 623 013 corresponding to EP-A-0 316 214 and to US-A-4 940 916

(3)

FR-A-2 663 462 corresponding to EP-A-0 461 990 and to US-A-5 194 780

(4)

FR-A-2 687 839 corresponding to EP-A-0 558 393 and to the American patent application of February 26, 1993, serial number 08 / 022,935 (Leroux et al.).

In particular, document (1) describes a source of microtip electrons with a matrix structure and a method of manufacturing this source.

Documents (2) to (4) relate to improvements to the source described in document (1).

In all the cases considered in these documents, the microtips are produced by a vacuum evaporation method.

This method involves two steps.

These steps are described below with reference to Figure 1 of the accompanying drawings.

A first step is to evaporate, under grazing incidence, a sacrificial layer ("lift off layer" in articles in English), for example nickel.

• un substrat électriquement isolant 2, par exemple en verre,
• des conducteurs cathodiques 4 sur ce substrat,
• une couche électriquement isolante 6 qui recouvre chaque conducteur cathodique, et
• une couche de grille 8 électriquement conductrice qui recouvre cette couche électriquement isolante.

More specifically, there is shown schematically and partially in FIG. 1 a structure comprising:

• an electrically insulating substrate 2, for example made of glass,
• cathode conductors 4 on this substrate,
• an electrically insulating layer 6 which covers each cathode conductor, and
• an electrically conductive grid layer 8 which covers this electrically insulating layer.

After having made this structure, holes 10 are formed through the grid layer 8 and the electrically insulating layer 6, at the level of each cathode conductor 4.

FIG. 1 shows the sacrificial layer which bears the reference 12 and which is formed on the grid layer 8.

The deposition of this layer 12 under grazing incidence makes it possible to selectively deposit the nickel on the grid layer 8 without putting holes at the bottom.

A second step consists in depositing on the entire structure thus obtained a layer 14 of an electron-emitting material such as, for example, molybdenum.

This deposit is made by evaporation of the molybdenum under an almost normal incidence.

Under these conditions, in each hole is formed a microtip 16 made of molybdenum which rests on the cathode conductor corresponding to this hole.

Next, the sacrificial layer 12 is eliminated, which results in the elimination of the molybdenum layer 14.

The major drawback of the technique which has just been mentioned essentially resides in the evaporation of the sacrificial layer under grazing incidence.

Such a step considerably complicates the evaporation equipment and limits the capacity thereof.

In particular, the grazing incidence makes it necessary to place on a crown, in the evaporation device, the structures on which it is desired to form the nickel layer.

This limits the filling rate of this device.

In addition, a tilting system is necessary to switch from grazing to almost normal incidence.

The treatment time is long, in particular because of the evaporation of the nickel which must be done at low speed to avoid splashes.

The material leading to the microtips is evaporated at an angle of incidence of less than 10 ° (almost normal incidence).

Therefore, it is only possible to process, in the evaporation device, substrates whose size (diagonal in the case of rectangular substrates) does not exceed 14 inches (about 35 cm).

It is indeed difficult to use an evaporation distance greater than 1 m in the device.

Beyond this distance of 1 m, it is not easy to obtain the sufficient evaporation rate and the risks of pollution of the evaporated layers are increased.

STATEMENT OF THE INVENTION

The present invention aims to remedy the above drawbacks, by replacing evaporation under grazing incidence by a wet chemical deposit.

• 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,
• on forme des trous à travers la couche de grille et la couche électriquement isolante au niveau de chaque conducteur cathodique,
• on forme sur la couche de grille une couche sacrificielle,
• on dépose sur l'ensemble de la structure ainsi obtenue une couche d'un matériau émetteur d'électrons, d'où la formation, dans chaque trou, d'une micropointe, et
• on élimine la couche sacrificielle, ce qui entraîne l'élimination du matériau émetteur d'électrons placé au-dessus de cette couche sacrificielle,

ce procédé étant caractérisé en ce que la couche sacrificielle est formée par une méthode de dépôt chimique humide.Specifically, the subject of the present invention is a method for manufacturing a microtip electron source, method 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 are formed through the grid layer and the electrically insulating layer at the level of each cathode conductor,
• a sacrificial layer is formed on the grid layer,
• a layer of an electron-emitting material is deposited over the entire structure thus obtained, hence the formation, in each hole, of a microtip, and
• the sacrificial layer is eliminated, which results in the elimination of the electron-emitting material placed above this sacrificial layer,

this process being characterized in that the sacrificial layer is formed by a wet chemical deposition method.

The invention makes it possible in particular to simplify the evaporation device mentioned above and to increase its production capacity, as will be seen more clearly below.

In addition, the present invention makes it possible to deposit microtips on large surfaces.

The following methods can be used as wet chemical deposition methods: electrolytic deposition or chemical deposition in solution.

However, according to a preferred embodiment of the process which is the subject of the present invention, the wet chemical deposition is an electrolytic deposition.

In this case, the grid layer is used as a cathode for this electrolytic deposition.

Preferably, the sacrificial layer is removed by electrolysis.

This sacrificial layer can be made of a material chosen from the group comprising the metals Cr, Fe, Ni, Co, Cd, Cu, Au, Ag and the alloys of these metals.

According to a preferred embodiment of the present invention, this sacrificial layer is made of an alloy of iron and nickel.

The removal of such an iron-nickel layer, after the deposition of the layer of electron-emitting material, is particularly easy.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1, déjà décrite, illustre schématiquement des étapes de fabrication d'une source d'électrons à micropointes selon un procédé connu,
• la figure 2 illustre schématiquement une étape de fabrication d'une telle source selon un procédé conforme à l'invention,
• la figure 3 est une vue schématique et partielle d'un dispositif d'évaporation permettant une évaporation sous incidence rasante d'une couche sacrificielle selon une technique antérieure,
• la figure 4 est une vue schématique et partielle d'un dispositif d'évaporation utilisable pour la mise en oeuvre de la présente invention, et
• les figures 5 et 6 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, schematically illustrates steps for manufacturing a microtip electron source according to a known method,
• FIG. 2 schematically illustrates a step of manufacturing such a source according to a method according to the invention,
• FIG. 3 is a diagrammatic and partial view of an evaporation device allowing evaporation under grazing incidence of a sacrificial layer according to a prior technique,
• FIG. 4 is a schematic and partial view of an evaporation device usable for the implementation of the present invention, and
• Figures 5 and 6 schematically illustrate steps of a particular embodiment of the method of the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

FIG. 2 schematically illustrates a structure which has been discussed in the description of FIG. 1 and which comprises, on the surface, the grid layer 8, this structure not including the layers 12 and 14.

The structure of FIG. 2 has been coated with a sacrificial layer 18 in accordance with the invention, by electrolytic deposition.

As can be seen in FIG. 2, the technique used in the present invention leads to a selective deposition on the grid layer 8, as allowed by evaporation under grazing incidence.

It suffices to polarize the gate layer 8 so that it constitutes a cathode during the electrolysis.

This electrolysis deposition technique, which can be used in the present invention, has the advantage of being rapid and inexpensive since it requires only electrolysis equipment.

FIG. 3 shows a vacuum evaporation device allowing, in accordance with the prior art, the deposition of a sacrificial layer under grazing incidence and the deposition of a layer of electron-emitting material under almost normal incidence.

There is shown very diagrammatically in FIG. 3 a vacuum enclosure 20 and, in this, substrates 22 on which it is wished to first evaporate the sacrificial layer under grazing incidence and then deposit the layer of material electron emitter under almost normal incidence.

A dotted ring 24 is also seen on which the substrates 22 are positioned for deposition under grazing incidence.

Tipping means 26, which are shown diagrammatically by arrows in FIG. 3, are provided for passing from deposition under grazing incidence to almost normal incidence deposition from a source 28 of electron-emitting material.

Figure 4 shows an evaporation device usable in the present invention.

This device is much simpler than that of FIG. 3 since, in a process according to the invention, only the evaporation of an electron emitting material remains, under an almost normal incidence, to form the microtips.

FIG. 4 also shows the enclosure 20 in which the substrates 22 and the source of electron-emitting material 28 are located.

The production capacity of this device is improved, compared to that of the device of FIG. 3, thanks to a shorter processing time and the possibility of putting more substrates in the enclosure 20 than in the case of the FIG. 3.

In fact, in the device of FIG. 4, one is no longer obliged to limit oneself to the arrangement of the substrates on a ring.

In addition, thanks to the electrolysis deposition method which can be used in the invention, the deposition of the sacrificial layer can be easily carried out over large areas.

A method according to the invention is explained below, making it possible to obtain a microtip electron source of the type described in document (3) to which reference will be made.

FIG. 5 shows a structure 29 comprising a glass substrate 30 on which a layer of silica 32 is formed.

Cathodic conductors made of niobium 34 are formed on the silica layer 32.

These cathode conductors 34 have a thickness of 0.2 μm and have a lattice structure with for example square meshes whose pitch is 25 μm.

These niobium cathode conductors 34 constitute the columns of the source of electrons to be formed.

A resistive layer 36 of amorphous silicon doped with phosphorus is deposited on the cathode conductors.

The thickness of this layer 36 is of the order of 1 μm.

An insulating layer 38 of silica is deposited on this resistive layer 36.

The thickness of the silica layer 38 is also of the order of 1 μm.

A metallic layer 40 made of niobium is deposited on the silica layer 38.

This layer 40 constitutes a grid layer.

The thickness of this grid layer 40 is of the order of 0.4 μm.

Holes 42 of 1.4 μm in diameter are etched in the grid layer 40 and in the insulating layer 38.

These holes 42 are placed in the central area of the mesh of the mesh and open onto the resistive layer 36.

In accordance with the present invention, a sacrificial layer 44 made of an alloy of iron and nickel is deposited on the grid layer 40 by electrolysis.

To do this, the structure 29 is placed in an appropriate electrolytic bath 46 and also placed in this electrolytic bath an electrode 48 constituting the anode during the electrolysis.

During this electrolysis, the gate layer 40 serves as a cathode.

An appropriate electrical voltage is applied, thanks to a voltage source 50, between the grid layer 40 and the electrode 48.

• 1) La composition du bain électrolytique est :
  • NiCl₂, 6H₂O : 50g.l⁻¹
  • NiSO₄, 6H₂O : 21,4g.l⁻¹
  • FeSO₄ : 2g.l⁻¹
  • H₃BO₃ : 25g.l⁻¹
  • Saccharinate de Na : 0,8g.l⁻¹
  • Saccharine : 0,8 g.l⁻¹
• 2) Le pH du bain électrolytique est maintenu à 2,5 avec, éventuellement, l'addition de tétraborate de sodium.
• 3) L'électrode 48 servant d'anode (que l'on peut appeler aussi "contre-électrode") est en nickel ou en alliage de fer et de nickel.
• 4) La distance D entre cette électrode 48 et la couche de grille 40 vaut 3 cm.
• 5) Le dépôt de Fe-Ni est effectué à température ambiante, avec une densité de courant voisine de 2 mA/cm.

As a purely indicative and in no way limitative, the deposit conditions are as follows:

• 1) The composition of the electrolytic bath is:
  • NiCl₂, 6H₂O: 50g.l⁻¹
  • NiSO₄, 6H₂O: 21.4g.l⁻¹
  • FeSO₄: 2g.l⁻¹
  • H₃BO₃: 25g.l⁻¹
  • Na saccharinate: 0.8g.l⁻¹
  • Saccharin: 0.8 gl⁻¹
• 2) The pH of the electrolytic bath is maintained at 2.5 with, optionally, the addition of sodium tetraborate.
• 3) The electrode 48 serving as anode (which can also be called "counter-electrode") is made of nickel or an alloy of iron and nickel.
• 4) The distance D between this electrode 48 and the grid layer 40 is 3 cm.
• 5) The deposition of Fe-Ni is carried out at ambient temperature, with a current density close to 2 mA / cm.

There is thus obtained, approximately in 8 minutes, a layer of Fe-Ni 200 nm thick.

Then deposited (FIG. 6), on the sacrificial layer 44, a layer 52 of molybdenum with a thickness equal to approximately 2 μm.

This deposition is carried out by evaporation under almost normal incidence.

Microtips 54 are thus formed in the holes 42.

These microtips 42 rest on the resistive layer 36.

After obtaining the microtips 54, the sacrificial layer 44 is dissolved by electrolysis.

To do this, the structure 53, obtained after the deposition of the molybdenum layer 54, is placed in an appropriate electrolytic bath 56.

By means of an appropriate electrical voltage source 58, an electrical voltage is established between the sacrificial layer 44 and an appropriate electrode 60 placed in the electrolytic bath 52.

The sacrificial layer 44 serves as an anode and the electrode 60 serves as a cathode during the electrolysis.

• 1) L'électrode 60 (que l'on peut appeler "contre-électrode") est en nickel.
• 2) La distance D1 entre cette électrode 60 et la couche sacrificielle 44 est de 3 cm environ.
• 3) L'électrolyte est constitué d'acide chlorhydrique dilué à 10% dans l'eau.
• 4) La dissolution anodique a lieu en maintenant la couche sacrificielle 44 à une tension de + 110 mV par rapport à une électrode de référence au calomel 62 grâce à une source de tension appropriée 64.

As a purely indicative and in no way limitative, the conditions for removing this sacrificial layer 44 and the molybdenum layer 52 are as follows:

• 1) The electrode 60 (which can be called "counter electrode") is made of nickel.
• 2) The distance D1 between this electrode 60 and the sacrificial layer 44 is approximately 3 cm.
• 3) The electrolyte consists of hydrochloric acid diluted to 10% in water.
• 4) The anodic dissolution takes place by maintaining the sacrificial layer 44 at a voltage of + 110 mV relative to a reference electrode to calomel 62 by means of an appropriate voltage source 64.

The voltage applied by the source 58 between the layer 44 and the electrode 60 is approximately 2V.

During the dissolution of layer 44, the electric current flowing between layer 44 and electrode 60 decreases progressively.

Dissolution is complete when this current becomes zero.

The means for measuring this current are not shown in FIG. 6.

The time necessary for the dissolution of the sacrificial layer 44 generally varies between 30 min and 60 min.

To complete the fabrication of the microtip electron source of FIGS. 5 and 6, the grids are then formed, perpendicular to the cathode conductors, by etching of the grid layer.

Claims (5)

Method for manufacturing a microtip electron source, method according to which: - a structure is made up comprising an electrically insulating substrate (2, 32), at least one cathode conductor (4, 34) on this substrate, an electrically insulating layer (6, 38) which covers each cathode conductor, an electrically grid layer conductive (8, 40) which covers this electrically insulating layer, - holes (10, 42) are formed through the grid layer and the electrically insulating layer at the level of each cathode conductor, - a sacrificial layer (18, 44) is formed on the grid layer, a layer (52) of an electron emitting material is deposited over the entire structure thus obtained, hence the formation, in each hole, of a microtip (54), and the sacrificial layer is eliminated, which results in the elimination of the electron emitting material placed above this sacrificial layer, this process being characterized in that the sacrificial layer (18, 44) is formed by a wet chemical deposition method. Method according to claim 1, characterized in that the wet chemical deposition is an electrolytic deposition. Method according to claim 2, characterized in that the sacrificial layer (18, 44) is removed by electrolysis. Method according to any one of claims 1 to 3, characterized in that the sacrificial layer (18, 44) is made of a selected material in the group comprising the metals Cr, Fe, Ni, Co, Cd, Cu, Au, Ag and the alloys of these metals. Method according to claim 4, characterized in that the sacrificial layer (18, 44) is made of an alloy of iron and nickel.

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