WO2004055853A1

WO2004055853A1 - Field emission device, method of manufacturing such a device, and display device comprising a field emission device

Info

Publication number: WO2004055853A1
Application number: PCT/IB2003/005171
Authority: WO
Inventors: Josephus P. A. Deeben; Jan De Graaf; Jos Toonen
Original assignee: Koninklijke Philips Electronics N.V.; Lg Philips Displays Nl
Priority date: 2002-12-13
Filing date: 2003-11-12
Publication date: 2004-07-01
Also published as: AU2003276559A1; AU2003276559A8

Abstract

A field emission device (100) comprises a cathode electrode (120) and a gate electrode (140). Between the electrodes, a dielectric layer (130) is provided which is generally patterned. According to the invention, the dielectric layer (130) comprises a siloxane matrix and/or a silicon oxide matrix, which are formed from a precursor layer of a sol-gel material of an organosilane compound and/or an organosilicate compound respectively. Such a dielectric layer has advantageous electric, mechanical and chemical properties and is particular useful for use in a Field Emission Display (FED) due to its ability to withstand high temperatures.

Description

Field emission device, method of manufacturing such a device, and display device comprising a field emission device

The invention relates to a method of manufacturing a field emission device. The invention further relates to a field emission device and to a display device comprising such a field emission device.

The field emission device may be used as an electron source for a flat-panel type display, the so-called Field Emission Display (FED). The FED is a vacuum electronic device, sharing many common features with the well-known Cathode Ray Tube (CRT), such as low manufacturing costs, good contrast and viewing angle and no required back-lighting. Field emission is a quantum-mechanical phenomenon in which electrons tunnel through a potential barrier at an outer surface of a suitable emitter material, as a result of an applied electric field. The presence of the electric field makes the width of the potential barrier at said outer surface finite, so that this potential barrier is permeable for electrons. Thus, electrons may be emitted from the field emitter material.

A field emission device commonly employs a gate structure (also called triode structure). The gate structure includes field emitter material and two electrodes, namely a cathode electrode and a gate electrode. Between these electrodes, in operation, an electric field is formed which allows emission of electrons from the field emitter material, which is usually located adjacent to the cathode electrode .

In a field emission display, the field emission device employs two sets of electrodes, more particularly a set of cathode electrodes and a set of gate electrodes. The sets of electrodes generally define a passive matrix structure of rows and columns. Thereby, the electric field, and thus the electron emission current, may be modulated independently for each pixel on the display screen of the field emission display.

For obtaining a sufficiently high strength of the electric field over the field emitter material, the cathode and gate electrodes should generally be close to each other. To achieve this, a dielectric layer is provided between the sets of electrodes. Such a dielectric layer is then usually patterned.

For example, in a normal configuration of the gate structure, a cathode electrode is provided on a substrate, and a dielectric layer and a gate electrode are arranged over the cathode electrode. Gate holes are provided extending through the dielectric layer and the gate electrode. The field emitter material is provided adjacent the cathode electrode, at the bottom of the gate holes. Thus, the dielectric layer (and the gate electrode) have to be provided with these gate holes, through which emitted electrons pass. Conventionally the deposition of the dielectric layer is carried out by means of an expensive and time-consuming chemical vapor deposition (CND) technique .

It is an object of the invention to provide a method of manufacturing a field emission device that is faster and cheaper than prior art methods. This object has been achieved by means of the method according to the invention as specified in the independent Claim 1. Further advantageous embodiment of the methods are specified in the dependent Claims 2-8.

According to the invention, the dielectric layer is manufactured from a liquid layer by means of a sol-gel process. The liquid layer is easily provided onto the substrate, for example by means of spray coating, screen printing, spinning or dip-coating.

The invention is, amongst others, based on the recognition that an insulating material based on a silicon oxide matrix or more preferred a siloxane matrix is a particularly good dielectric material for use in a field emission display.

In a FED, this dielectric material is able to withstand the process of sealing of the display panel, which process is carried out at a temperature of approximately 450 degrees Celsius, without cracking or deforming. The dielectric material has a relatively high elasticity, so that the thermal expansion coefficient of the substrate material is not required to match that of the insulating material. This allows for a larger freedom in choosing the substrate material, for example low-cost soda lime glass may be applied. At the same time, this dielectric material has the advantage that it may be patterned with relative ease. Patterning of the dielectric layer is required in a FED.

Moreover, the dielectric constant of the material is relatively low. This is advantageous, because in this case the capacitance of the field emission device is also low. In a field emission display, this is advantageous as a reduced capacitance allows for low power driving. The field emission display then consumes less energy.

An insulating material based on a silicon oxide matrix is conveniently obtainable by converting a sol-gel material comprising an organosilicate compound like TEOS (tetra ethyl orthosilicate : Si(CH₃-CH₂-O)₄ ). During the conversion, a condensation reaction then takes place between water present in the sol-gel material and the ethoxy groups. Thereby, an alcohol is split out and the sol-gel condenses to form silicon oxide bonds (Si-O-Si). Each Si-atom has four reactive sites allowing for good crosslinking of the material. The dielectric layer formed comprises a silicon oxide matrix. Si-OH groups remain present in the matrix, which groups are hydrophilic. A dielectric constant in between 4 and 5 can be obtained for the dried material, which is a suitable value for a field emission device. With this material, the match in thermal expansion coefficient with the substrate plate is comparatively critical, since the crosslinked silicon oxide matrix has a relatively low modulus of elasticity.

A more preferred insulating material based on a crosslinked siloxane matrix is conveniently obtainable by converting a sol- gel material comprising an organosilane compound. The conversion mechanism of the organosilane compound is similar to the conversion mechanism of the organosilicate compound.

In this case, crosslinking is complete after treatment at a temperature below 500°C and yields a hydrophilic matrix also including Si-R groups (R= organic group). Preferably, the organosilane compound comprises ethoxy or methoxy groups. If the number of reactive sites on each Si-atom (i.e. methoxy and/or ethoxy groups) is sufficiently high, the material crosslinks and a siloxane matrix is formed.

In a preferred embodiment, the organosilane compound comprises MTMS (methyl trimethoxy silane ; CH₃-Si(CH₃-O)₃ ). Although each Si-atom has only three reactive sites, crosslinking is still good. A siloxane based matrix is formed wherein methyl groups are present, the basic structure of this matrix being CH₃-Si-Oι.₅. Each silicon atom is now bound to a single methyl group, and this Si-CH₃ binding is mechanically more flexible than the crosslinked Si-O bonds. Therefore, this siloxane matrix has an increased elasticity modulus compared to an silicon oxide matrix. This implies that the match in thermal expansion coefficient between the matrix and the substrate is less critical. The processing temperature and elasticity of this material enable the use of low cost soda-lime substrate glass.

Moreover, the dielectric constant is reduced further to a value of about 3.5 . It is assumed that this is caused by the fact that less water remains in the dielectric layer after the condensation process. A further preferred embodiment involves converting a sol-gel material comprising a combination of an organosilane compound and an organosilicate compound as described above. Thereby, an optimal balance may be struck between the mechanical, electrical and chemical properties of both pure silicon oxide and siloxane matrices as described above. Preferably, the sol-gel material further comprises an inorganic filler material in suspension. During the conversion, this filler material is embedded in the siloxane matrix being formed. Using the filler material, a thicker dielectric layer can be obtained. Preferably, the inorganic filler material comprises colloidial silica (e.g. Ludox TM50 ex Dupont). Using the silica particles, a dielectric layer having a thickness of 20 micrometers may be obtained for a siloxane matrix from MTMS. A dielectric layer of approximately 2 micrometers can be formed from TEOS with this filler material.

Moreover, the silica particles have a dielectric constant that matches the dielectric constant of the silicon oxide and siloxane matrix comparatively well, so that the dielectric constant of the dielectric layer is hardly affected by using this particular filler material.

In a field emission device, the dielectric layer should be patterned. This is for example done using an etching process. When applying the method according to the invention, the patterning may be carried out after converting the sol-gel layer to the dielectric layer. For example, a Reactive Ion Etching (RIE) technique is known to efficiently etch structures in silicon oxide layers, and can also be used for etching siloxane matrices of a different composition.

However, more preferably, the sol-gel layer is patterned before the converting step to the dielectric layer takes place. This allows the layer to be patterned by means of a simple and cheap liquid embossing technique, wherein an elastomeric stamp being provided with the desired patterned is contacted with the sol-gel layer (see the copending European patent application PHNL021230).

DESCRIPTION OF THE FIGURE In a Field Emission Display as shown in the Figure, a vacuum envelope comprises a field emission device 100 according to the invention. The field emission device opposes a display screen 180 provided with phosphor tracks 185. The display screen 180 comprises picture elements 182. The field emission device 100 is used as an electron source, for generating the electrons that impinge on the phosphor tracks 185, thereby illuminating picture elements 182.

Each picture element (pixel) 182 of the display screen 180 is addressable individually, therefore the cathode electrodes and gate electrodes define a passive matrix structure. For each row 184 of pixels 182, a row cathode electrode 120a,b,c is provided, and for each column 186 of pixels 182, a column gate electrode 140a,b,c is provided. The cathode electrodes 120a,b,c are separated from the column gate electrodes 140a,b,c, by a patterned dielectric layer 130. This dielectric layer comprises a siloxane matrix and has a dielectric constant of approximately 3.5. The layer is formed from a sol-gel type material comprising an organosilane compound and preferably an inorganic filler material such as colloidal silica. Said sol-gel type material is converted to form the dielectric layer.

The patterning is then done by means of an etching technique, for example Reactive Ion Etching. Alternatively, the liquid layer is patterned by means of liquid embossing using an elastomeric stamp, and subsequently converted into a patterned dielectric layer. In the Figure, the pattern of the dielectric layer 130 is a pattern of gate holes

135. At the bottom of each gate hole 135, emitter particles (not shown) are provided which emit electrons when a suitable electric field is applied. The gate holes 135 extend through the dielectric layer 130 and the gate electrodes 140a,b,c.

The power consumption of the Field Emission Display should be as low as possible, so it is desirable to have a low voltage difference between the cathode electrode and the gate electrode. Also, the dielectric constant of the dielectric layer should be low, so that the capacitance of the field emission device is also relatively small. The thickness of the dielectric layer has to strike a balance between having a relatively high electric field at said low voltage difference on the one hand, and having a relatively low capacitance on the other hand.

In a preferred embodiment, the dielectric constant of the dielectric layer 130 is 3,5 or 4. The thickness of the dielectric layer 130 is about 20 micrometers. In this case, a voltage difference between cathode electrode and gate electrode of about 100 Volts allows for a sufficiently high electric field over the emitter particles at the bottom of the gate holes 135, so that these particles are able to emit electrons.

As described earlier, the field emission device is, in a Field Emission Display, generally operated pixelwise, whereby each pixel of the field emission device corresponds to a pixel 182 of the display screen 180. A pixel 182 is addressed by switching on the row voltage Nrowl,2,3 of the row cathode electrode 120a,b,c corresponding to that pixel and simultaneously switching on the column voltage Ncoll,2,3 of the column gate electrode 140a,b,c, corresponding to that pixel.

At the intersection of the selected cathode and gate electrodes, an emitter pixel is defined, wherein the emitter particles are activated and emit electrons. The electrons pass through the gate holes in the emitter pixel, and are accelerated towards the display screen 180. For this purpose, the display screen 180 is supplied, in operation, with an anode voltage of for example 10 kVolts. The accelerated electrons land on a pixel 182 of the display screen 180, whereby the part of a phosphor track 185 within said pixel 182 is energized and illuminates. By way of example, when row voltage Nrowl and column voltage Ncol3 are switched on, electrons are released from a plurality of gate holes indicated in the drawing by reference numeral 136, and land on the display screen 180 at a selected pixel indicated by 188. Because of this, the phosphor track 185 within the selected pixel 188 illuminates, rendering the selected picture element 188 visible to a viewer. The manufacturing process of the dielectric layer 130 will be described in more detail in the following example.

Example of preparing the dielectric layer

The following sol-gel was made:

200 gram colloidal silica (Ludox TM50 ex Dupont) was mixed with

200 gram methyl trimethoxy silane (CH₃Si(O-CHs)₃) (MTMS)

18 gram acetic acid and

5 gram tetraethyl orthosilicate (TEOS).

A layer was administered on a substrate using the so-called doctor blade method.

A silicone rubber stamp, provided with a pattern of for example cylindrical features for forming the gate holes in the dielectric layer, is engaged with the sol-gel layer. The layer is then heated to 70 degrees Celsius for 3 minutes, after which the stamp is removed. The layer now maintains the embossed pattern of for example the gate holes.

After heating to 400 degrees Celsius a clear well-adhering dielectric layer with a thickness of 20 micrometer is formed. This layer has a dielectric constant of approximately

3,5. By means of the heating step, during the formation of the dielectric layer the methoxy groups of the MTMS and the ethoxy groups of the TEOS become reactive, forming oxygen bridges between adjacent silicon atoms. In the example, the organosilane compound of the sol-gel is predominantly MTMS. The methyl groups in the MTMS do not participate in these reactions but are incorporated in the dielectric layer. Their presence in the dielectric layer is shown in infra-red spectroscopic measurements. In the measured infra-red spectrum a band around 3000 cm^"1 is present corresponding to the C-H vibrational band.

The presence of Si-CH₃ bonds results in a better mechanical flexibility of the dielectric layer. As a result, during the heating step, differences in thermal expansion between the dielectric layer and the substrate are easily accommodated and as a result the layer shows relatively few cracks.

In an inert gas atmosphere like for instance nitrogen, the material could alternatively be heated to 500 to 550 degrees Celsius, which is advantageous for having a complete out gassing of reactants, solvents and products of the sol-gel conversion. Using the doctor blade method a maximum thickness of approximately 20 micrometer is achievable. This relatively large thickness is obtainable due to the presence of the Ludox colloidal silica filler material, which is embedded into the siloxane matrix during the firing step.

Alternatively, the sol-gel layer is provided by means of a dip-coating technique. The dip-coating technique has shown to result in layers which have a better controlled thickness. In the dip coating technique the substrate is dipped in a solution and slowly withdrawn from the solution. A layer of solution remains on the substrate surface.

As a further alternative, the sol-gel layer may be provided by means of spincoating. As a further alternative, the sol-gel layer may be provided by means of spray coating.

Instead of patterning the sol-gel layer by engaging an elastomeric stamp with the layer, it is alternatively possible to firstly convert the sol-gel layer to a solid dielectric layer, and then pattern said solid dielectric layer, for example by means of a Reactive Ion Etching technique. Thereby, the use of fluorocarbons provides good dry etching of the converted sol-gel material in the presence of an energetic ion bombardment. This technique has a good selectivity and a high grade of anisotropy.

For example, a patterned gate electrode of the field emission device is provided over the converted sol-gel layer, by means of conventional photolithography. This patterned gate electrode is subsequently used as a mask for reactive ion etching of the dielectric layer. Thereby, a pattern of gate holes is formed in the dielectric layer. Finally, emitter particles are deposited inside the gate holes, for example by means of a printing process. The drawing is schematic and is not drawn to scale. While the invention has been described in connection with a preferred embodiment and example, it should be understood that the invention should not be construed as being limited to the preferred embodiment and/or example. Rather, it includes all variations which could be made thereon by a skilled person, within the scope of the appended claims.

Summarizing, a field emission device (100) comprises a cathode electrode (120a,b,c) and a gate electrode (140a,b,c). Between the electrodes, a dielectric layer (130) is provided which is generally patterned. According to the invention, the dielectric layer (130) comprises a siloxane matrix and/or a silicon oxide matrix, which are formed from a precursor layer of a sol-gel material of an organosilane compound and/or an organosilicate compound respectively. Such a dielectric layer has advantageous electric, mechanical and chemical properties and is particular useful for use in a Field Emission Display (FED) due to its ability to withstand high temperatures.

Claims

CLAIMS:

1. A method of manufacturing a field emission device comprising a dielectric layer between a cathode electrode and a gate electrode, the method comprising the steps of preparing a sol-gel material including an organosilane compound and/or an organosilicate compound; providing a layer of said sol-gel material on a substrate and converting said layer of said sol-gel material into said dielectric layer.

2. The method of Claim 1, wherein the organosilane compound comprises methoxy or ethoxy groups.

3. The method of Claim 2, wherein the organosilane compound comprises methyl tri methoxy silane (MTMS).

4. The method of Claim 2, wherein the organosilicate compound comprises tetra ethyl orthosilicate (TEOS).

5. The method of Claim 1, wherein the sol-gel material further comprises an inorganic filler material in suspension.

6. The method of Claim 5, wherein the inorganic filler material comprises colloidial silica (Ludox).

7. The method of Claim 1, wherein the method further comprises the step of patterning the converted dielectric layer.

8. The method of Claim 1 , wherein the method further comprises the step of patterning the layer of sol-gel material before the converting step.

9. A field emission device, comprising a cathode electrode and a gate electrode; a dielectric layer between said cathode electrode and said gate electrode, said dielectric layer comprising a crosslinked siloxane matrix.

10. The field emission device of Claim 8, wherein methyl groups are present in the dielectric layer.

11. The field emission device of Claim 8, wherein particles of an inorganic filler material are embedded in the crosslinked siloxane matrix.