METHOD FOR CREATING AND KEEPING A CONTROLLED ATMOSPHERE IN A FIELD EMITTER DEVICE BY USING A GETTER MATERIAL

Abstract: There is described a method for creating and keeping a controlled atmosphere in a FED, essentially free of oxidizing gases and including hydrogen at a pressure comprised between 10<-7> and 10<-3> mbar, which comprises the step of arranging inside the FED, before it is frit sealed, a getter material previously charged with hydrogen gas. Subsequently, the two parts forming the FED are frit sealed along their perimeter and the FED itself is evacuated during this operation or later, through a suitably arranged tail, which is hermetically closed after being evacuated through a "tip-off". The getter material is charged by exposing it to hydrogen gas at a pressure comprised between 10<-4> and 2 bar.

Description: "METHOD FOR CREATING AND KEEPING A CONTROLLED ATMOSPHERE IN A FIELD EMITTER DEVICE BY USING A GETTER MATERIAL" The present invention relates to a method for creating and keeping a controlled atmosphere in a field emitter device by using a getter material. The field emitter devices are studied for many uses, among which there is the production of flat displays, called FED (Field Emitter Display). These displays, which are in the course of development, are destined in general for the showing of images, and in particular to provide flat television screens. A FED is generally obtained by sealing along their perimeter two plan parts made of glass; the sealing is carried out by melting a glass paste having a low melting point, with an operation called "frit sealing". The final structure consists of two parallel surfaces at a distance of few hundreds μm. The space inside the FED is kept under vacuum. On the inner surface of the rear part there is a plurality of pointed microcathodes (microtips) made of a metallic material, for example molybdenum, which emit electrons, and a plurality of grid electrodes, placed at a very short distance from said cathodes, so as to generate a very high electric field; this electric field extracts electrons from the point of the microtips, thus generating an electronic current which is accelerated toward the phosphors, placed on the inner surface of the front part (the real display). The luminescence intensity of the so excited phosphors, and therefore the display brightness, are directly proportional to the current emitted by the microtips. Until now it was considered necessary, for the good working of the FED, to keep the pressure under 10 "5 mbar inside the vacuum space between the microtips and the phosphors; for this purpose many patent applications proposed the use of getter materials, such as BaAI 4 , mentioned in EP-A-443865, metals such as Ta, Ti, Nb or Zr mentioned in EP-A- 572170, and combinations of powdered Ti, Zr, Th and their hydrides with Zr-based alloys, to be employed in the shape of porous layers, as described in the Italian patent application MI94-A-000359. Recent studies, however, have shown that not all the gases have a detrimental effect on the working of the FEDs. In particular, hydrogen may be present in the device at pressures higher than 10 '5 mbar. Spindt et al., in "IEEE Transactions on Electron Devices", vol. 38, No.10 (1991), p. 2355-2363, and Mousa, in "Vacuum", Vol. 45, No. 2-3 (1994) p. 235-239, have shown, by measuring the current emitted by the microtips at a constant voltage according to the gaseous environment, that hydrogen does not damage the electronic emission even for long times if present in the FED at a pressure up to 1.5 x 10 "2 mbar. Furthermore, introducing hydrogen into an "aged" FED, i.e. a FED whose electronic emissivity has decreased in time, takes the emissivity itself back to the initial values. The aforesaid article of Spindt et al. also shows that the oxidizing gases, in particular air, have the expected negative effect on the current emission of the microtips. In the aforesaid article of Mousa it is also pointed out that, with pressures higher than 2 x 10 "1 mbar, hydrogen has a negative effect on the electronic emissivity, probably due to the erosion of the microtips due to the bombardment of hydrogen ions which occurs at these relatively high pressures. In conclusion, from these studies it seems clear that an optimal gaseous environment inside the FED should be free of oxidizing gases and contain a little partial pressure of reducing gases, in particular hydrogen. Even if, as seen above, the effects of hydrogen are generally known, there is at present no industrially useful method for determining controlled quantities of hydrogen inside the FED. The studies carried out until now have followed laboratory procedures, in which hydrogen is introduced into the FED through a suitable pipe (tail) formed in the structure of the FED itself. The procedure derivable from the laboratory tests, not applicable in practice to an industrial production line, should have the following steps: closing the FED by frit sealing a glass paste having a low melting point at the edges of the two plan parts (front and rear) made of glass which form the device itself; evacuating the FED through the glass tail generally placed at the rear part of the FED itself; introducing hydrogen in a measured quantity through the tail; closing the tail with a hot compression ("tip-off'). Such a process has at least the following disadvantages: it is hard to reproduce the determination of the low partial pressures through a hydrogen line; - the local heating which occurs during the "tip-off' process could cause important hydrogen leaks. Therefore, it is an object of the present invention to provide a method for creating and keeping inside the FEDs a gaseous environment optimal for their working, in particular an environment essentially free of oxidizing gases and including hydrogen at a pressure comprised between 10 "7 and 10 "3 mbar approximately, and in any case higher than the pressure of the oxidizing gases. A further object of the present invention is to provide a method for introducing hydrogen into a FED, so that it occurs, during the closing step of the FED itself by frit sealing, an overpressure of hydrogen which keeps a reducing environment on the microtips and helps the expulsion of the oxidizing gases which are potentially detrimental. These and other objects are obtained according to the present invention through a method for creating and keeping inside the FEDs an environment essentially free of oxidizing gases and including hydrogen at a pressure comprised between 10 "7 and 10 "3 mbar, comprising the following steps: - charging a getter material with gaseous hydrogen by exposing it to this gas at a pressure comprised between 10 "4 and 2 bar; - arranging the getter material saturated with hydrogen into the FED before it is frit sealed; - frit sealing along their perimeter the two parts which form the FED at a temperature comprised between 400 and 500°C with a glass paste having a low melting point; evacuating the FED, either during the frit sealing step or later through a suitably arranged tail, which is hermetically closed after the evacuation through a "tip-off'. The term "charging", as used in the text and in the claims, means the introduction of hydrogen into a getter material, which is performed by exposing the getter material, at a fixed temperature, to hydrogen at a fixed pressure; the quantity of hydrogen thus introduced into the getter material is not necessarily the saturation quantity at the operating temperature. The invention will be now described with reference to the attached drawings and diagrams of the figures, wherein: Fig. 1 shows a closed FED; Fig. 2 shows the inner surface of the rear glass part of a FED, i.e. the surface on which the microtips are arranged; Fig. 3 shows the cross-section along the l-l line of a FED of Fig. 1, obtained according to the "chamber" process as explained later; Fig. 4 shows the cross-section of a FED obtained through an alternate way, according to the "tail" process explained later; - Fig. 5 shows in a schematic way a system for the treatment of the gas employed for charging the getter materials with hydrogen; Fig. 6 shows in a schematic way a system for measuring the quantities of gas sorbed or released by the getter materials; in this system it is possible to simulate the frit sealing process employed for sealing the FEDs; Fig. 7 shows two C0 2 sorption curves for two samples of getter material differently treated. In detail, Fig. 1 shows a finished FED (10), consisting of a plan front part (11) made of glass and a plan rear part (12) made of glass, sealed along the perimeter with a glass paste (13) having a low melting point; Fig. 1 also points out by hatching the area (14) on which the phosphors are arranged on the inner surface of part 11. Fig. 2 shows in a schematic way the inner surface (20) of the rear part (12) of a FED, and points out the area (21 ), opposite and corresponding, at the interior of the FED, to the area 14 on which the microtips are arranged. These are produced with planar building techniques typical of the technology of the solid state devices, and may reach a density amounting to tens of thousands of microtips per square millimeter. The evacuation of the FED may be carried out either during the frit sealing step of the glass paste 13, by operating in a vacuum chamber (chamber process), or by arranging inside the FED a glass tail through which the sealed FED is evacuated and which is afterwards hermetically closed through a "tip-off. Fig. 3 shows the cross-section (not in scale) along the l-l line of a FED of Fig. 1 , which shows the typical configuration obtained in the chamber process. In this process the two glass parts, front (11 ) and rear (12), forming the FED are introduced into a chamber kept under vacuum during the whole process, juxtaposed, and heated up to the melting temperature of paste 13 which performs the sealing. In this process, the most suitable configuration for the getter material is in the shape of a strip (30) arranged along one or more sides of the area in which the microtips are housed; for the details about the deposition methods of the getter material, which must have a large surface area and therefore must preferably be present in a porous form, reference is made to the patent application MI94- A-000359 in the name of the applicant. Fig. 3 also points out microtips (31), built on a silicon base (32); grid electrodes (33), separated from the base (32) by a layer (34) of a dielectric material; phosphors (35); and the inner space (36) of the FED to be kept in a controlled atmosphere. The sizes of the parts are not in scale, because the two glass parts 11 and 12 may be some millimeters thick, space 36 is few hundreds of microns thick, while the cathodic structure (microtips and grid electrodes) is generally few microns high. The electric loops for feeding the device are not shown in the drawing. As an alternative, the FED may be produced with the "tail" process, in which the two glass parts are frit sealed in a non-evacuated environment. The evacuation of the FED is carried out in a second step, through a glass pipe (tail) suitably arranged on either part of the FED, generally the rear one. Fig. 4, analogous to Fig. 3, shows a cross-section of a FED produced with the tail process; in this case the getter material (40) is arranged, generally in a supported form, on the part of the tail (41) closer to the FED, which remains after the "tip-off' operation. The chamber process may result preferable because it is cleaner and can be automated more easily. In both processes, however, during the frit sealing the glass paste which has a low melting point releases a non negligible quantity of gases and oxidizing vapors, in particular water, which could considerably decrease the electronic emissivity of the microtips. During this step the getter material releases part of the hydrogen it was previously charged with, and this hydrogen allows to keep a reducing environment on the microtips; furthermore, the overpressure of hydrogen which is generated in this step has also a mechanical expulsion effect on the oxidizing gases, thus helping to keep a reducing environment. The getter material is present in the FED in a supported form, for example rolled on a metallic tape or as powder pressed inside an open container. The getter materials which may be employed as- a "tank" of hydrogen may be very different, tiut they must preferably have a relatively high equilibrium pressure of hydrogen at a temperature close to the room temperature (the working temperature of the FEDs), in order to obtain a pressure of hydrogen comprised between 10 "7 and 10 '3 mbar inside the FED, after being closed with a frit sealing. In a preferred embodiment of the invention, the support may be heated during the life of the FED, in order to increase the emission of hydrogen if a decrease in time of the device efficiency is noticed. The heating element may be a resistor placed on the face of the support opposite to the face on which the getter material is fixed, or it is possible to exploit the resistance itself of the material forming the support. This preferred embodiment allows to have a better control on the pressure of hydrogen inside the FED during the life of the device. Getter materials employable for the objects of the invention generally are: binary alloys comprising a first element chosen between Zr or Ti and a second element chosen among V, Mn, Fe, Co, Ni and Cr; ternary alloys comprising a first element chosen between Zr or Ti and a second and a third element chosen among V, Mn, Fe, Co, Ni and Cr. Among the above mentioned class of compounds, the following alloys are particularly useful: - ZrM 2 alloys, where M is a transition metal chosen among Cr, Mn, Fe, Co or Ni and their mixtures, described in US patent 5,180,568 in the name of the applicant; - the intermetallic compound ZnMniFei, manufactured and sold by the applicant with the name St 909; - the Zr-V-Fe alloys described in US patent 4,312,669 in the name of the applicant, whose percent composition by weight, when brought into a ternary composition diagram, is comprised within a triangle whose vertices are the following points: a) Zr 75% -V20% - Fe 5%; b) Zr 45% - V 20% - Fe 35%; c) Zr 45% - V 50% - Fe 5%, and in particular the alloy having the percent composition by weight Zr 70% - V 24.6% - Fe 5.4%, manufactured and sold by the applicant with the name St 707; the intermetallic compound ZnViFβi, manufactured and sold by the applicant with the name St 737; - the Ti-rich Ti-Ni alloys, in particular the Ti-Ni alloys comprising 50 to 80% by weight of Ti; the Ti-V-Mn alloys described in US patent 4,457,891. The charging of hydrogen into the above mentioned alloys is carried out by operating at the room temperature in hydrogen at a pressure comprised between 10 "4 and 2 bar, and requires a time varying between 1 and 60 minutes approximately. The values of the hydrogen pressure to be employed depend on the particular getter material which is considered; the significant ranges for the above mentioned materials are the following: between 0.5 and 2 bar; - Zr 70% - V 24.6% - Fe 5.4% alloy: between 10 "4 and 0.1 bar; - ZriViFei: between 0.01 and 0.1 bar; Ti-Ni alloys: between 0.01 and 0.1 bar; - Ti-V-Mn alloys: between 10 "4 and 0.1 bar. Inside these ranges, the particular value of the hydrogen pressure during the alloy charging step depends on the frit sealing operation of the FED: in fact, as said, during this operation the getter material is indirectly heated and releases part of the hydrogen contained therein. The released quantity of hydrogen depends on the thermal cycle the FED is subject to, and in particular on the time it remains at the highest temperature. The knowledge of the details of the frit sealing process and of the equilibrium pressure of hydrogen above the various alloys in function of the temperature allows to exactly measure the quantity of hydrogen to be initially introduced into the getter material so that, after the frit sealing, the remaining part could generate an equilibrium pressure comprised in the range of the pressures desired in the FED. An example of determination of the hydrogen charging - 9 - conditions for an alloy is reported in the examples. The following examples have a purely explanatory purpose of the features of the invention and in any case should not be considered as limiting the scope of the invention itself. EXAMPLE 1 In this example there is described a hydrogen charging test of a getter alloy. The employed system is schematically shown in Fig. 5 and consists of a main hydrogen tank (50) connected, through a line (51) and a valve (52), to a first chamber (53) provided with a pressure gauge (54). Chamber (53) is connected, through a line (55) and a valve (56) to a second chamber (57) in which a housing (58) for the sample is present. The temperature of housing (58) is controlled through a heating element (59) and measured with a thermocouple (60). Chamber (57) is connected through line (61 ) and valve (62) to the vacuum pump system (63). The test is performed on a sample of St 707 alloy having the aforesaid composition. 130 mg of said alloy are introduced into a ring holder and pressed. The sample is then introduced into the described system for the charging of hydrogen. After the sample has been evacuated and activated at 200°C, it is cooled down to 50°C approximately. At this temperature the hydrogen is introduced into chamber (57) at a pressure of 0.67 mbar. The sample sorbs 4.3 mg approximately of hydrogen per gram of alloy. The charged getter material is sample 1. EXAMPLE 2 This example reports a test in which there are simulated the frit sealing process of the FEDs and the hydrogen release of a getter material charged with this gas. The test is performed in a vacuum system consisting of a chamber (70) to which a pressure gauge (71) and, through a line (72) and a valve (73), a vacuum pump system (74) are connected; chamber (70) is also connected, through line (75) and valve (76), to a C0 2 tank (77) which is employed in a subsequent test; the system is schematically shown in Fig. 6. Sample 1 is introduced into chamber 70. Chamber 70 is evacuated and degassed for one night. A frit sealing simulation is then performed. The treatment is carried out by heating the sample at 450°C for 20 minutes; during this operation, valve 73 is throttled, thus reducing the flow of gases evacuated by the pump system 74; the conditions of the gas emission outside the FED perimeter during the sealing operation are thus simulated. At the end of this treatment valve 73 is closed. The remaining pressure in chamber 70 is 1.3 x 10 "3 bar. By letting the sample cool down to the room temperature, the pressure progressively decreases down to 4 x 10 "6 mbar. EXAMPLE 3 After the test reported in example 2, a gas sorption test of the getter material is performed according to the procedures of the ASTM F 798-82 Standard test. Chamber 70 is connected to a C0 2 tank (77), while keeping valve (73) closed and opening valve (76), so as to keep in the chamber a constant pressure of C0 2 at 4 x 10 '5 mbar. The proceeding of the C0 2 sorption speed (G) (cc per second) is recorded as a function of the sorbed quantity (Q) (cm 3 x mbar at normal conditions). The results of the test are reported in Fig. 7 ("a" curve). EXAMPLE 4 (COMPARATIVE) The test of example 2 is repeated, except for substituting the sample of getter material charged with hydrogen with a sample having the same composition, weight and size, but not charged with hydrogen. At the end of the test the pressure measured in chamber 70 is 8 x 10 "7 mbar approximately. On this sample there has been then performed a sorption test as in example 3, whose results are reported in Fig. 7 ("b" curve). Curves "a" and "b" look substantially similar. The result of this test confirms that the final pressure measured during test 2 is due to the presence of hydrogen, and that the getter material is capable of standing the frit sealing at the reported conditions. As can be taken from the examination of the above mentioned examples, the method of the present invention allows to keep inside the FED an optimal environment for the operation of the device. In particular, the presence of a getter material charged with hydrogen allows to obtain a pressure of hydrogen in the desired range; furthermore, the charging of the getter material with hydrogen does not interfere with the action of sorbing gases other than hydrogen, thus helping to keep an environment substantially free of oxidizing gases during the life of the FED (example 3).

Claims: CLAIMS 1. Method for creating and keeping in a FED a controlled atmosphere essentially free of oxidizing gases and including hydrogen at a pressure comprised between 10 "7 and 10 "3 mbar, comprising the following steps: - charging a getter material with gaseous hydrogen by exposing it to this gas at a pressure comprised between 10 "4 and 2 bar; arranging the getter material saturated with hydrogen into the FED before it is frit sealed; - frit sealing along their perimeter the two parts which form the FED at a temperature comprised between 400 and 500°C with a glass paste having a low melting point; evacuating the FED, either during the frit sealing step or later through a suitably arranged tail, which is hermetically closed after being evacuated through a "tip-off. 2. Method for introducing hydrogen into a FED according to claim 1, wherein the getter material is chosen among: binary alloys comprising a first element chosen between Zr or Ti and a second element chosen among V, Mn, Fe, Co, Ni and Cr; ternary alloys comprising a first element chosen between Zr or Ti and a second and a third element chosen among V, Mn, Fe, Co, Ni and Cr; and the hydrogen charging of the alloy is carried out at the room temperature at a pressure comprised between 10 "4 and 2 bar for a time comprised between 1 and 60 minutes. 3. Method according to claim 2, wherein the getter material is the intermetallic compound, charged with hydrogen at a pressure comprised between 0.5 and 2 bar. 4. Method according to claim 2, wherein the getter material is a Zr-V- Fe alloy, whose percent composition is Zr 70% - V 24.6% - Fe 5.4% charged with hydrogen at a pressure comprised between 10 "4 and 0.1 bar. 5. Method according to claim 2, wherein the getter material is the ZηViFei intermetallic compound, charged with hydrogen at a pressure comprised between 0.01 and 0.1 bar. 6. Method according to claim 2, wherein the getter material is a Ti-Ni alloy charged with hydrogen at a pressure comprised between 0.01 and 0.1 bar. 7. Method according to claim 6, wherein the Ti-Ni alloy comprises 50 to 80% by weight of Ti; 8. Method according to claim 2, wherein the getter material is a Ti-V-Mn alloy charged with hydrogen at a pressure comprised between 10 "4 and 0.1 bar. 9. Method according to claim 1, wherein during the frit sealing operation there is generated an overpressure of hydrogen which keeps a reducing environment on the microtips and helps the expulsion of the oxidizing gases which are potentially detrimental. 10. Method according to claim 1, wherein the getter material charged with hydrogen is introduced into the FED supported on a strip or in an open container which can be heated by means of an electric current flow, so as to adjust the temperature of the getter material and, as a consequence, the hydrogen emission thereof.

Inventors: CARRETTI CORRADO, FERRARIO BRUNO, CARRETTI, CORRADO, FERRARIO, BRUNO
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1995
Main classification: H01J29
IPC codes: C22C14, C22C16, H01J7, H01J9, H01J17, H01J29, H01J31