EP0712147A1 - Field-effect electron source and manufacturing method; application in display devices with cathodoluminescence - Google Patents

Abstract

In a process for mfg. a field effect electron source having micro-tips (12) of electron emissive metal, the improvement is that the micro-tips (12) are covered with main deposits (13) of diamond(-like) carbon particles, formed by electrophoresis or by electrophoresis or by electrochemical co-deposition of metal and diamond(like) carbon. Also claimed are (i) a field effect electron source made by the above process; and (ii) a cathodoluminescent display device comprising a field effect electron source as above and a cathodoluminescent anode including a layer of cathodoluminescent material.

Description

TECHNICAL AREA

The present invention relates to a method of manufacturing a field effect electron source.

The present invention applies in particular to the field of flat display devices also called "flat screens", as well as to the manufacture of pressure measurement gauges.

STATE OF THE PRIOR ART

Sources of field effect electrons are already known.

These are the sources of microtip electrons.

A microtip electron source comprises at least one cathode conductor on an electrically insulating substrate, an electrically insulating layer which covers this cathode conductor and at least one grid formed on this electrically insulating layer.

Holes are formed through the grid and the insulating layer above the cathode conductor.

The micro-tips are formed in these holes and carried by the cathode conductor.

The apex of each micro-tip is located substantially in the plane of the grid, this grid being used to extract electrons from the micro-tips.

The holes have very small dimensions (they have a diameter of less than 2 µm).

To produce a display device using such a source of microtip electrons, a "triode" type system is produced.

More precisely, there is, opposite the source, a cathodoluminescent anode.

The electrons from the source bombard this cathodoluminescent anode.

Other display devices are also known which have a "diode" type structure.

These other known display devices comprise a cathodoluminescent anode placed facing an electron source comprising layers of diamond or diamond-like carbon intended to emit electrons.

These layers are obtained by laser ablation or by chemical vapor deposition.

Diamond or diamond carbon emits electrons much more easily than the materials conventionally used for the manufacture of microtips.

With diamond or diamond carbon, the minimum electric field from which an electron emission can be obtained can be twenty times weaker than the minimum electric field corresponding to metals such as molybdenum, for example.

Unfortunately, the deposition of diamond or diamond-like carbon layers, with the methods mentioned above, takes place at high temperature (of the order of 700 ° C.).

In addition, it is impossible to obtain micro-tips directly by these methods.

The deposits obtained are continuous layers and not micro-tips.

The resulting display devices are, as seen above, of the "diode" type, which poses a problem as regards their addressing.

Electronic addressing systems must in fact be used to apply voltages of the order of several hundred volts to these devices.

In addition, the high temperature at which the diamond or diamond-like carbon layers are formed prohibits the use of standard glass as a substrate intended to carry these layers.

STATEMENT OF THE INVENTION

The object of the present invention is to remedy the above 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 et 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, et
• on forme, dans chaque trou, une micro-pointe faite d'un matériau métallique émetteur d'électrons,

chacune de ces micro-pointes étant ensuite recouverte d'un dépôt principal de particules de carbone diamant ou de type diamant, ce procédé étant caractérisé en ce que les dépôts principaux sont formés par électrophorèse ou par co-dépôt électrochimique de métal et de carbone diamant ou de type diamant.It relates to a process for manufacturing a field effect electron source, 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 and 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, and
• we form, in each hole, a micro-tip made of a metallic material emitting electrons,

each of these micro-tips then being covered with a main deposit of diamond or diamond-like carbon particles, this process being characterized in that the main deposits are formed by electrophoresis or by electrochemical co-deposition of diamond and diamond-like metal and carbon.

The process which is the subject of the invention can be implemented with large surface substrates and thus makes it possible to obtain electron sources (and therefore display screens) of large surface area (several tens of inches diagonally).

The temperature at which the main deposits are formed is close to ambient temperature, of the order of 20 ° C for electrophoresis and of the order of 40 ° C to 60 ° C for electrochemical deposition.

It is then possible to use, to manufacture a source in accordance with the invention, an ordinary glass substrate (soda-lime), without special precautions.

It will also be noted that these deposits are achievable by simple techniques which require neither a sacrificial layer ("lift off layer") nor vacuum deposition.

In addition, the baths which are necessary for the implementation of these techniques have a long service life (several months).

According to a particular embodiment of the process which is the subject of the invention, the main deposits are covered with a secondary deposit of a metal, for example by electrochemical deposition, in order to consolidate these micro-clusters.

Preferably, the diamond or diamond-like carbon particles have a size of the order of 1 μm or less than 1 μm (but of course less than the size of the microtips).

These particles can be obtained from natural or artificial diamond or by a method chosen from laser synthesis, deposition chemical vapor phase and physical vapor deposition.

The holes formed through the grid layer and the electrically insulating layer may have a circular or rectangular shape.

The size of these holes can be chosen in a range ranging from approximately 1 μm to several tens of micrometers.

The size of the holes that are formed to implement the process which is the subject of the invention can be much larger than that which is necessary for the implementation of a process for manufacturing a conventional source with microtips ( not covered).

This is very advantageous given the difficulties associated with obtaining calibrated holes of small size (less than 2 μm) on large surfaces.

• sur un substrat électriquement isolant, au moins une première électrode jouant le rôle de conducteur cathodique,
• une couche électriquement isolante qui recouvre ce conducteur cathodique,
• au moins une deuxième électrode jouant le rôle de grille, formée sur la couche électriquement isolante, des trous étant formés à travers cette grille et la couche électriquement isolante au-dessus du conducteur cathodique, et
• des micro-pointes qui sont faites d'un matériau métallique émetteur d'électrons et qui sont formées dans ces trous et portées par le conducteur cathodique,

cette source étant caractérisée en ce que chacune de ces micro-pointes est recouverte d'un dépôt principal de carbone diamant ou de type diamant ("diamond like carbon" ou DLC dans les articles en langue anglaise) formé conformément au procédé objet de l'invention.The present invention also relates to a source of field effect electrons, this source comprising:

• on an electrically insulating substrate, at least a first electrode playing the role of cathode conductor,
• an electrically insulating layer which covers this cathode conductor,
• at least one second electrode acting as a grid, formed on the electrically insulating layer, holes being formed through this grid and the electrically insulating layer above the cathode conductor, and
• micro-tips which are made of an electron-emitting metallic material and which are formed in these holes and carried by the cathode conductor,

this source being characterized in that each of these micro-tips is covered with a main deposit diamond or diamond-like carbon ("diamond like carbon" or DLC in articles in English) formed in accordance with the process which is the subject of the invention.

For the same electrical control voltage, the source object of the present invention emits more electrons than a microtip source, due to the use, in the present invention, of deposits of diamond carbon particles or of diamond type which have a higher emissivity than conventional electron emitting materials such as molybdenum.

Thus, in the case of the use of a source in accordance with the invention for manufacturing, for example, a display device, this device has a greater brightness than a conventional device with microtips (not covered), for a same control voltage.

At equal luminosities, this device using a source according to the invention requires a control voltage lower than that which is necessary for such a conventional device with microtips.

In addition, the use of a source according to the invention leads to a “triode” type system which requires control voltages lower than those which are necessary for the “diode” type devices mentioned above, which use layers of diamond carbon or diamond type.

In the present invention, the main deposits can be made of diamond or diamond-like carbon particles or can be made of such particles dispersed in a metal.

In the source object of the invention, each of these main deposits can be covered with a secondary deposit of a metal intended to consolidate these main deposits.

• une source d'électrons à effet de champ, et
• une anode cathodoluminescente comprenant une couche d'un matériau cathodoluminescent,

dispositif caractérisé en ce que la source est celle qui fait l'objet de l'invention.The present invention also relates to a cathodoluminescence display device comprising:

• a source of field effect electrons, and
• a cathodoluminescent anode comprising a layer of a cathodoluminescent material,

device characterized in that the source is that which is the subject of the invention.

We have seen above the advantages of such a device compared to known devices using uncovered micro-tips and to devices comprising layers of diamond carbon or of diamond type.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est une vue en coupe schématique d'une source d'électrons conforme à la présente invention,
• la figure 2 est une vue en coupe schématique d'un dispositif de visualisation utilisant la source de la figure 1,
• la figure 3 illustre schématiquement un procédé de fabrication d'une source d'électrons conforme à l'invention,
• la figure 4 illustre schématiquement la possibilité d'utiliser des trous rectangulaires pour fabriquer une source conforme à l'invention, et
• la figure 5 illustre schématiquement un autre procédé de fabrication d'une source d'électrons conforme à 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 is a schematic sectional view of an electron source according to the present invention,
• FIG. 2 is a schematic sectional view of a display device using the source of FIG. 1,
• FIG. 3 schematically illustrates a process for manufacturing an electron source according to the invention,
• FIG. 4 schematically illustrates the possibility of using rectangular holes to manufacture a source according to the invention, and
• Figure 5 schematically illustrates another method of manufacturing an electron source according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

• sur un substrat électriquement isolant 2, des électrodes 4 jouant le rôle de conducteurs cathodiques (un seul conducteur cathodique est visible sur la figure 1),
• une couche électriquement isolante 6 qui recouvre chaque conducteur cathodique, et
• des électrodes 8 jouant le rôle de grilles et formées sur la couche électriquement isolante 6 (une seule grille est visible sur la figure 1).

The source according to the invention, which is schematically represented in section in FIG. 1, comprises:

• on an electrically insulating substrate 2, electrodes 4 acting as cathode conductors (a single cathode conductor is visible in FIG. 1),
• an electrically insulating layer 6 which covers each cathode conductor, and
• electrodes 8 playing the role of grids and formed on the electrically insulating layer 6 (a single grid is visible in FIG. 1).

Holes 10 are formed through these grids 8 and the insulating layer 6 above the cathode conductors 4.

Micro-tips 12 are formed in the holes 10 and carried by the cathode conductors 4.

Each of these micro-tips 12 is covered with a deposit 13 of diamond or diamond-like carbon particles.

It is specified that the cathode conductors 4 are parallel and that the grids 8 are parallel to each other and perpendicular to the cathode conductors 4.

The holes 10 and therefore the microtips 12 are located in the areas where these grids cross the cathode conductors.

These are the micro-tips of such an area, covered with deposits 13, which emit electrons when an appropriate electrical voltage is applied, by means not shown, between the conductor cathode 4 and grid 8 which correspond to this zone.

A cathodoluminescence display device is schematically represented in section in FIG. 2.

This device comprises the electron source 14 of FIG. 1.

The device of FIG. 2 also comprises a cathodoluminescent anode 16 placed opposite the source 14 and separated from the latter by a space 18 in which a vacuum has been created.

The cathodoluminescent anode 16 comprises an electrically insulating and transparent substrate 20 which is provided with an electrically conductive and transparent layer 22 forming an anode.

This is placed opposite the electron source 14 and coated, opposite this source, with a layer 24 of a cathodoluminescent material or "phosphor" ("phosphor" in publications in English).

Under the impact of the electrons emitted by the micro-tips 12 of the source, covered with deposits 13, this layer 24 emits light which a user of the display device observes through the transparent substrate 20.

• (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.).

It is a device that can be compared to the display devices described in documents (1) to (4) mentioned below, but which has advantages over these devices, as we have seen. upper :

• (1) FR-A-2 593 953 corresponding to EP-A-0 234 989 and 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 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.).

A method according to the invention is explained below, allowing the fabrication of the electron source of FIG. 1, with reference to FIG. 3 which schematically illustrates this method.

• le substrat 2,
• les conducteurs cathodiques 4,
• la couche électriquement isolante 6,
• une couche de grille 25, qui recouvre cette couche électriquement isolante 6,
• les trous 10 formés dans cette couche de grille 25 et la couche électriquement isolante 6,
• les micro-pointes 12 formées dans les trous 10, sur les conducteurs cathodiques.

To make this source, we start by making a structure comprising:

• the substrate 2,
• cathode conductors 4,
• the electrically insulating layer 6,
• a grid layer 25, which covers this electrically insulating layer 6,
• the holes 10 formed in this grid layer 25 and the electrically insulating layer 6,
• the micro-tips 12 formed in the holes 10, on the cathode conductors.

The manufacture of such a structure is known and, on this subject, reference will be made to documents (1) to (4) mentioned above.

It should be noted, however, that the diameter D1 of the holes (substantially circular) formed in the grid 8 and in the electrically insulating layer 6 may advantageously be greater than the diameter of the holes that the microtip electron sources described in the documents (1 ) to (4).

For example, this diameter D1 can take values of the order of 1 μm up to 50 μm.

FIG. 4 schematically illustrates the fact that the holes 10, instead of having a circular shape, can have a rectangular shape.

The width D2 of these holes 10 in FIG. 4, of rectangular shape, can be taken equal to the diameter D1 mentioned above and can therefore also be significantly greater than the diameter of the holes of the microtip sources.

It is then a question of forming, on the microtips 12, the deposits 13 of diamond or diamond-like carbon particles (after which the grids will be formed, perpendicular to the cathode conductors, by etching of the grid layer 25).

To form the deposits 13, a diamond or diamond type carbon powder is used.

This powder can be obtained by chemical vapor deposition from a mixture of hydrogen and light hydrocarbons.

This chemical vapor deposition can be assisted by an electron beam or be assisted by a plasma generated by microwaves.

This powder can also be formed by an ultrasonic spraying process known under the name of "Pyrosol", that is to say, more precisely by pyrolysis of an aerosol of a carbonaceous compound.

This powder can also be synthesized by means of a laser, that is to say, more precisely, by chemical vapor deposition as previously but assisted by laser.

One can also synthesize the powder by physical vapor deposition ("physical vapor deposition"), from carbon targets (graphite for example) and a plasma gas such as argon alone or mixed with hydrogen, hydrocarbons, without dopant or with a dopant such as for example diborane.

This powder can also be obtained by laser ablation.

You can also use a natural diamond powder.

Alternatively, artificial diamonds can be prepared by compacting carbon, at high pressure and high temperature, and then making the powder from these artificial diamonds.

Preferably, these diamond carbon powders and these diamond type carbon powders are chosen so as to have a micron or submicron or nanometric particle size but, of course, less than the size of the microtips.

For example, if these micro-tips have a size of the order of 1 μm, a submicron particle size is used.

It is specified that these diamond or diamond carbon powders can be doped or undoped.

Boron can for example be used as a dopant.

The deposition of the powder (diamond or diamond-like carbon particles) leading to the formation of the deposits 13 can be carried out by electrophoresis (cataphoresis or anaphoresis), optionally supplemented by an electrochemical metallic deposition for consolidation, or by electrochemical co-deposition metal and carbon diamond or diamond type.

In the case of deposition by anaphoresis, the structure provided with micro-tips 12 is placed in an appropriate solution 26 and each micro-tip 12 is brought to a positive potential during this deposition phase.

More specifically, the cathode conductors 4 are brought to this positive potential by means of an appropriate voltage source 28, the positive terminal of which is connected to these cathode conductors 4 while the negative terminal of this source is connected to a platinum counter electrode 32 or stainless steel located in the bath at a distance from the substrate of about 1 to 5 cm.

The fine powder of diamond or diamond-like carbon particles is suspended in solution 26 (before placing the structure in this solution).

• de l'acétone,
• un acide qui peut être de l'acide sulfurique à 8 µl par litre de solution, et
• de la nitrocellulose qui joue le rôle de liant et de dispersant.

Solution 26 includes for example:

• acetone,
• an acid which may be sulfuric acid at 8 μl per liter of solution, and
• nitrocellulose which acts as a binder and dispersant.

The immersion of the structure in this solution and the application of the positive potential to the micro-tips leads to obtaining deposits 13.

The voltage supplied by the source 28 can range up to around 200 V.

In the case of cataphoresis, a negative potential is applied to the microtips.

More specifically, in this case, it is the negative terminal of the source 28 which is connected to the cathode conductors 4 while the positive terminal of the source 28 is connected to a counter-electrode 32 of platinum or stainless steel located in the bath at a distance from the substrate of about 1 to 5 cm.

• de l'alcool isopropylique,
• un liant minéral comme exemple Mg(NO₃)₂, 6H₂O (de concentration 10⁻⁵ mole par litre), et
• un dispersant tel que le glycérol (dont la concentration est de l'ordre de 1% en volume).

The solution 26 then comprises for example:

• isopropyl alcohol,
• a mineral binder, for example Mg (NO₃) ₂, 6H₂O (with a concentration of 10⁻⁵ mole per liter), and
• a dispersant such as glycerol (the concentration of which is around 1% by volume).

A voltage of up to about 200 V is then used.

The same type of deposit is obtained as in the case of anaphoresis.

With the intention of consolidating the deposits 13 obtained by electrophoresis, it is possible, after the formation of these, to produce an electrochemical deposit of a metal for example chosen from Ni, Co, Ag, Au, Rh or Pt or, more generally, among the transition metals, the alloys thereof and the precious metals.

This is schematically illustrated in FIG. 5 where we see the structure which is provided with microtips 12, covered with deposits 13, and which is immersed in a solution 30 allowing such an electrochemical deposit.

A suitable electrical voltage is then applied between the cathode conductors 4 and an electrode 33 placed in this solution, by means of a voltage source 34.

This electrode 33 is for example made of nickel and the solution 30 contains for example 300 g / l of nickel sulphate, 30 g / l of nickel chloride, 30 g / l of boric acid and 0.6 g / l of lauryl sodium sulfate.

For example, an electric current of 4 A / dm is used.

FIG. 5 shows the metal deposit 36 which is formed on each deposit 13 after this electrochemical deposition operation.

The deposits 13 can also be formed by electrochemical co-deposition of metal and of diamond or diamond-like carbon.

To do this, use is made, for example, of a bath containing nickel ions and diamond powder suspended in this bath.

Up to 60% by weight of diamond suspended in the bath can be used.

An appropriate current source is used, for example of the order of 4 A / dm, and the negative terminal of this source is applied to the cathode conductors and the positive terminal of this source to a nickel electrode placed in the bath .

The nickel is deposited on the micro-tips 12, carrying with it the diamond particles, hence the formation of the deposits 13 of nickel and diamond on the micro-tips 12.

Instead of diamond or diamond-type carbon, a powder of particles of silicon carbide or titanium carbide, of micron or submicron size, can be used for the implementation of a process according to the invention, and use the same methods as above (electrophoresis, possibly supplemented by an electrochemical metallic deposition of consolidation, or electrochemical co-deposition of metal and such particles), to form the deposits 13.

Of course, in the present invention, the tops of the microtips 12 covered with deposits 13 (possibly covered themselves with a metal consolidation deposit) are located substantially in the plane of the grids and are not in contact with these grids.

It will be noted that the techniques previously described for forming the deposits 13 are selective: these deposits are formed only on the microtips, no deposit forming on the non-polarized parts of the structure comprising the microtips.

Claims (11)

Method for manufacturing a field effect electron source, method according to which: - a structure is produced comprising an electrically insulating substrate (2), at least one cathode conductor (4) on this substrate, an electrically insulating layer (6) which covers each cathode conductor and an electrically conductive grid layer (25) which covers this electrically insulating layer, - holes (10) are formed through the grid layer and the electrically insulating layer, at the level of each cathode conductor, and - a micro-tip (12) made of an electron-emitting metallic material is formed in each hole, each of these micro-tips (12) then being covered with a main deposit (13) of diamond or diamond-like carbon particles, this process being characterized in that the main deposits (13) are formed by electrophoresis or by co -electrochemical deposit of metal and carbon diamond or diamond type. Method according to claim 1, characterized in that the main deposits (13) are then covered with a secondary deposit (36) of a metal. Method according to claim 2, characterized in that this secondary deposit is formed by electro-chemical deposition. Process according to any one of Claims 1 to 3, characterized in that the diamond or diamond-like carbon particles have a size of the order of 1 µm or less than 1 µm. Process according to any one of Claims 1 to 4, characterized in that the particles are obtained from natural or artificial diamond or by a method chosen from laser synthesis, chemical vapor deposition and physical vapor deposition. Method according to any one of claims 1 to 5, characterized in that the holes (10) have a circular or rectangular shape. Method according to any one of Claims 1 to 6, characterized in that the size of the holes (10) is chosen in a range from approximately 1 µm to several tens of micrometers. Source of field effect electrons, this source comprising: - on an electrically insulating substrate (2), at least a first electrode (4) playing the role of cathode conductor, - an electrically insulating layer (6) which covers this cathode conductor, - at least one second electrode (8) acting as a grid, formed on the electrically insulating layer, holes (10) being formed through this grid and the electrically insulating layer above the cathode conductor, and - micro-tips (12) which are made of an electron-emitting metallic material and which are formed in these holes and carried by the cathode conductor, this source being characterized in that each of these microtips is covered with a main deposit (13) of diamond carbon or of diamond type formed according to the method according to claim 1. Source according to claim 8, characterized in that the main deposits (13) are made of diamond or type carbon particles diamond or are made of such particles dispersed in a metal. Source according to claim 9, characterized in that each of these main deposits (13) is covered with a secondary deposit (36) of a metal. Cathodoluminescence display device comprising: - a field effect electron source (14), and - a cathodoluminescent anode (16) comprising a layer of a cathodoluminescent material (24), device characterized in that the source (14) conforms to any one of claims 8 to 10.

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