Patent Grant
6888294
This application is a national phase of PCT/FR00/01101 which was filed on Apr. 26, 2000, and was not published in English.
In a general manner, the present invention concerns a device that uses a field effect electron source (for example, a microdot device) and, more particularly, a field emission device, for example a flat, cathodo-luminescence display screen that is stimulated by field emission, or cold emission, using microdots. It also concerns the manufacture of such a device.
More precisely, the invention involves creating a reducing atmosphere within the interior of the device in order to prevent the oxidation of the microdots (or other electron emitting elements) when the device is working.
Microdot screens are flat cathode ray tubes that operate under vacuum. These screens comprise a cathode (notably made out of cathodic conductors, grids and microdots) and an anode (made out of conductors and luminophors). In order to maintain the vacuum, an element known as a getter is used, as is the case with conventional cathode ray tubes. A getter is an element that, once it has been activated by heating under vacuum, can fix the gases desorbed by the device and maintain the vacuum level required for the device to work properly.
The operational life time of these devices depends, amongst other things, on the life time of the cathodes, which is linked to the drop in the electron current over time. The life time of the cathodes depends, to a large extent, on the amount and type of residual gases that are present in the sealed structure that constitutes the screen.
When a microdot screen of this type is under operation, the electronic bombardment of the anode produces gases through a de-gassing effect, which is very dependent on the type of luminophors used to make up the anode. It has now been clearly demonstrated that the drop in the electron current emitted by the cathode is mainly caused by oxidation of the emissive material that is used in the microdots. This oxidation, caused by the desorbed gases, is particularly noticeable when the microdots are made out of molybdenum.
Depending on the type of luminophors used, this oxidation is more or less severe. In the case of colour screens, which use three different luminophors to obtain emissions in the red, green and blue, the oxidation is generally high and leads to short operational life times, of less than 100 hours.
Therefore, one aims to avoid oxidation of the emissive material. Three approaches may be used to attempt to solve this problem.
According to a first approach, the use of an emissive material that is insensitive to oxidation could be envisaged. This is a long term approach because, at the moment, only molybdenum can be used for making operational cathodes.
According to a second approach, a material that is sensitive to oxidation, such as molybdenum could be used, in the presence of non-oxidisable luminophors. This is possible in the case of monochromic screens by using ZnO as a luminophor. However, in the case of colour screens, this imposes very severe constraints as regards the choice of luminophors and is today very costly.
According to a third approach, a material sensitive to oxidation (such as molybdenum) could be used in the presence of oxidisable luminophors, but by creating, within the interior volume of the screen, a reducing atmosphere that can prevent this oxidation occurring and which can thus maintain the emissive material in its most favourable state. This third approach is particularly interesting, because it allows molybdenum to be used as the emissive material, while at the same time allows a wide choice as regards the type of luminophor.
In conventional applications, the role of a getter is to maintain the vacuum, in other words, to replace a vacuum pump. In the case of field emission flat screens, it has been suggested that a getter is used to carry out two functions: to evacuate the oxidative gases (which is its normal role) and to maintain a partial pressure of hydrogen.
The SAES GETTERS S.P.A. Company, which is specialised in the manufacture of getters, has developed and described materials that are capable of fulfilling this double role. Thus, its patent application WO-A-96/01492 discloses an application process that comprises the following steps:
Document FR-A-755 295 describes an improvement to this process in order to solve the problem of hydrogen loss during the screen assembly phase.
A hydrogen atmosphere effectively makes it possible to stabilise the current in trichromic screens for several thousand hours. However, this hydrogen based process has the following disadvantages.
The getters that are able to maintain a sufficient pressure of hydrogen are specific and have a relatively low evacuating capacity as regards other oxidising gases. The quantity of getter that needs to be introduced is high (around 0.5 g for a 5 inch screen), which can lead to cost and cluttering problems, especially when large screens are involved.
In addition, the amount of hydrogen that the getter has to adsorb beforehand is quite considerable (1333 cm3. Pa to 13330 cm3. Pa, i.e. 10 to 100 cm3. Torr per gram of getter), which, given the volume of the screen, can lead to the manufacturer having to assemble the screen under a hydrogen pressure close to atmospheric pressure.
These conditions are difficult to implement in industrial conditions and, in particular, pose difficult safety problems, which can only be overcome by resorting to costly solutions.
According to the present invention, it is proposed that the disadvantages of the prior art may be overcome by using a NxHy type gas, preferably ammonia NH3, as a reducing gas instead of hydrogen. A partial pressure of NH3 makes it possible to avoid the oxidation of the dots and thus to ensure that the cathodes have a long operational life.
This gas NxHy is not evacuated, or is only evacuated to a very small extent, by the getters and is therefore compatible with getters known to those skilled in the art, which have very good evacuating characteristics. It has, in addition, the advantages of having very low toxicity, is non-explosive and presents no safety problems. It is therefore easy to use industrially.
A first objective of the invention consists in a device comprising at least one field effect electron source within a sealed structure, which encompasses an internal space that contains a reducing gas, whose purpose is to prevent oxidation of the emissive material of the electron source, whereby the reducing gas is a gas with the formula NxHy, or a gaseous mixture based on NxHy, where x=1 and y=3 or x=2 and y=4. Advantageously, the reducing gas is under a pressure of between 10−8 mbar and 10−3 mbar and, preferably, under a pressure of between 10−8 mbar and 10−5 mbar.
Preferably, the gas with formula NxHy is NH3.
In addition, the device may comprise one or several getters that are in communication with the internal space of the device.
The sealed structure may be made out of a first strip that bears a microdot cathode on its internal structural face, a second strip placed opposite the first strip and bearing an anode on its internal structural face, and means for sealing the first strip to the second strip around their edges. Luminophors may, in addition, be spread out on the anode. It may, for example comprises a flat display screen.
A second objective of the invention consists in a manufacturing process for this type of device, which comprises the following stages:
A third objective of the invention consists in another manufacturing process for this type of device, comprising the following stages:
For both of these processes, if the device comprises means for sealing at high temperature, the assembly stage is carried out under vacuum or under a controlled atmosphere, by heating up to the activation temperature of the means used for sealing. Advantageously, once the heating stage is finished, the device is cooled down to ambient temperature and operated for a given time before the other stages are carried out. The processes may, in addition, include the following stages:
The getter may also be activated after the exhaust tube has been sealed off.
A fourth objective of the invention consists in an apparatus for implementing these two processes, comprising:
In order to implement the first process cited above, the intermediate devices may comprise a gas reservoir that is connected up to the said pipe via a second valve and to the source of NxHy via a third valve, with an apparatus for measuring the pressure in the reservoir being provided.
In order to implement the second process cited above, the intermediate devices may simply comprise a valve.
A fifth objective of the invention consists in another manufacturing process for a device of this type, comprising the following stages:
A sixth objective of the invention consists in still another manufacturing process for this type of device, comprising the following stages:
A seventh objective of the invention consists in a device for implementing these two latter processes, comprising:
If necessary, this device can also comprise, in addition, an apparatus for producing a controlled atmosphere that is connected to the interior of the enclosure via a third valve, linked to the apparatus for producing a controlled atmosphere, for example a suitable gas cylinder.
The invention will be more clearly understood and other advantages and distinctive features will become apparent from the following description, given as a non-limiting example, accompanied by the drawings in the appendix, including:
According to the invention, the specific choice of NxHy gas, or a mixture of gases based on NxHy, makes it possible to both prevent the oxidation of the emissive material of the electron source and, at the same time, to avoid deposits on the emissive material. In fact, any decomposition of this gas takes place in the gaseous form, unlike other types of reducing gas (CH4, H2S for example).
Several examples for manufacturing a device according to the invention will now be described.
Amongst the various types of getters that could be used, those commercialised by the S.A.E.S GETTERS S.p.A. Company, for example:
A microdot screen 20, of the type shown in
After sealing and returning to atmospheric pressure, at least one getter 23, for example an ST 171 type getter, is introduced into the interior of the exhaust tube 21.
The device 20 is then placed in the zone 30 of the apparatus shown in
An NH3 gas cylinder 48 is connected to the entry orifice of the reservoir 45 via a pipe 49, fitted with a valve 50. This valve 50 is a needle valve that allows easy adjustment of the flow.
The apparatus set up in this way is also fitted with a gauge 51, that allows the pressure of gas within the reservoir 45 to be measured, and a gauge 52 that allows the pressure at the exit of the screen 20 to be measured.
The following procedure is then followed:
The screen 20 and the reservoir 45 are put under vacuum using the vacuum pump 42, with the valves 44 and 47 open and the valve 50 closed. The screen 20 is then heated up to 360° C. for 16 hours. After cooling down to ambient temperature, the screen 20 is operated for 20 hours. Once this operating stage, which allows the luminophors to be de-gassed, is finished, the getter 23 (or the getters) is activated by radio-frequency heating to a temperature of 800° C. for 4 minutes.
The reservoir 45 is then isolated by closing valve 47. Ammonia is then introduced into the reservoir 45.
The screen 20 is then isolated from the vacuum pump 42 by closing the valve 44. The valve 47 is then opened and ammonia is introduced into the screen 20 at an equilibrium pressure, which depends on the quantity introduced into the reservoir 45 and which is preferably between 10−3 and 10−5 mbar. The screen 20 may then be isolated from the apparatus by sealing off the exhaust tube 21.
According to a variation of example 1, NH3 may be introduced into the screen under dynamic conditions.
In order to do this, once the getter activation phase is finished, a partial pressure of NH3 is adjusted via valve 50, with valves 44 and 47 open.
The partial pressure of NH3 is preferably between 10−8 and 10−5 mbar. After a period of dynamic scanning lasting several minutes to several tens of minutes, the screen is isolated from the apparatus by sealing off the exhaust tube.
According to another implementation method, the screen may be assembled in an integral manner, in other words, the screen is de-gassed then sealed under vacuum or under a controlled atmosphere. The process is such that, after sealing, it remains under vacuum, or under a controlled atmosphere, unlike the previous case (examples 1 and 2) in which, after sealing, the screen is returned to atmospheric pressure and then re-evacuated and heated.
The following procedure may then be followed.
The various parts of the screen (strip bearing the cathode, strip bearing the anode, frit glass, getters, etc.) are placed in position under vacuum then heated to a temperature of around 300° C. to 450° C. for one or several hours. The getters may be placed in position either within the interior of the screen or in an external area such as a sealed off exhaust tube or getter box. During the heating stage, the anode may be laid flat against the cathode or maintained at a certain distance from the cathode. In the latter case, the de-gassing is more efficient.
These operations are carried out in an apparatus schematically shown in
The apparatus shown in
After the heating stage, NH3 is introduced into the enclosure 60 containing the screen at a pressure of between 10−3 and 10−3 mbar.
If the anode and cathode strips are not already in contact, they are brought into contact by means of the frit glass, and the screen is sealed under the NH3 pressure established beforehand at a temperature of between 450° C. and 500° C.
Depending on the type of getter used, the getter must, or must not, be “flashed” or activated after sealing and returning to ambient temperature. It may be advantageous to use a ST 122 type getter, which may be activated during the assembly stage.
According to a variation of the embodiment described in Example 3, one of the parts of the screen (the cathode strip, the anode strip, the getter box) comprises a hole, with a diameter of around 1 millimeter or several millimeters, which allows the interior of the screen and the airtight enclosure 60 to be connected together.
As in Example 3, the various parts of the screen are placed in position under vacuum and then heated.
The sealing stage may be carried out at this time under a controlled atmosphere, which is advantageous when a boro-silicate type glass is used for the screen strips, and the enclosure is re-evacuated after the screen has been assembled.
This type of embodiment may be advantageous even when the sealing is carried out under vacuum, because all of the products de-gassed within the interior of the screen during sealing are removed, which makes it possible to obtain a better vacuum within the interior of the screen.
After the sealing stage and, if necessary, cooling and re-evacuating, NH3 at a pressure of between 10−3 and 10−3 mbar is introduced into the enclosure and, as a consequence, into the screen. The hole connecting the screen and the enclosure is then sealed by any appropriate means.
In this type of embodiment, it may be advantageous to use the same getters as in Example 3.
| Number | Date | Country | Kind |
|---|---|---|---|
| 99 05361 | Apr 1999 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCTFR00/01101 | 4/26/2000 | WO | 00 | 10/24/2001 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO0067285 | 11/9/2000 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3552818 | Benda | Jan 1971 | A |
| 5275840 | Mikami et al. | Jan 1994 | A |
| 5688708 | Kato et al. | Nov 1997 | A |
| 5788551 | Dynka et al. | Aug 1998 | A |
| 5964630 | Slusarczuk et al. | Oct 1999 | A |
| 6100627 | Carretti et al. | Aug 2000 | A |
| 6136670 | Blalock et al. | Oct 2000 | A |
| 6268288 | Hautala et al. | Jul 2001 | B1 |
| 6465952 | Itoh et al. | Oct 2002 | B1 |
| Number | Date | Country |
|---|---|---|
| 0036681 | Sep 1981 | EP |
| 0609815 | Aug 1994 | EP |
| WO 9601492 | Jan 1996 | WO |
| WO 9828769 | Jul 1998 | WO |
