HK1073337B - Porous getter devices with reduced particle loss and method for their manufacture - Google Patents

Description

Porous getter device for reducing particulate loss and preparation method thereof

The present invention relates to a method for preparing a porous getter device with reduced loss of particles and to the device thus prepared.

Getter devices are useful in many process and scientific applications where vacuum maintenance is required, such as flat panel displays (plasma or field emission type), certain types of lamps or particle accelerators for scientific research. Another important field of application of getter devices is gas purification within fluorescent lamps, but mainly in the case of process gases of the microelectronics industry. The invention is also particularly suitable for the preparation of getter devices of the particle type having the shape and size of the substrate to be processed in deposition chambers, for example in process chambers of the microelectronics industry, wherein the devices guarantee a shorter evacuation time and a better purification of the working atmosphere: such getter devices are disclosed in the name of the applicant in the international patent application PCT/IT 00/00136.

The active materials from which these devices are made are mainly zirconium and titanium and their alloys with elements of one or more transition elements and aluminium. These materials have a strong affinity for low molecular weight gaseous species such as oxygen, water, hydrogen, carbon oxides and, in some cases, nitrogen, and are therefore used to remove minute quantities of these gases from spaces where a vacuum needs to be maintained or from gaseous atmospheres or streams that are inert (mostly noble gases) to these materials.

Since gas sorption takes place through the surface of the getter material, it is generally preferred that the surface is as large as possible. In order to obtain this result, while maintaining small device dimensions, it is common to use porous devices made from solidified powders of getter materials, which have a high ratio of the active material exposed surface to the getter device geometric surface.

Various methods of making getter devices have been described in the literature.

Patent GB-B-2077487 describes the preparation of porous getter devices made of a mixture of getter metals, in particular titanium or zirconium powders, and getter alloys; the mixture is pre-pressed and sintered in a vacuum furnace at about 800 to 1100 ℃. The getter alloy with the sintering temperature higher than the metal sintering temperature and the sintering resistance function is added to avoid the reduction of gas adsorption performance caused by the excessive compression of powder.

Patent application DE-A-2204714 discloses a process similar to that of the above-mentioned patent GB-B-2077487, except that in this case graphite powder is used as the anti-sintering agent.

Getter devices having a porosity greater than that obtained by the two techniques described above can be prepared by electrophoretic techniques, as disclosed for example in patent US 5242559. According to this technique, a suspension, usually a hydroalcoholic suspension, of getter material particles is prepared. Two electrodes, one of which is made of metal or graphite, are inserted in the suspension, which also act as carriers for the final getter device. The getter material particles are transferred to the support and attached thereto by applying a potential difference between the two electrodes. The deposit thus obtained is then solidified by sintering heat treatment in a vacuum oven, generally at about 900 to 1000 ℃.

Getter devices with active materials in the form of layers on a planar support can be prepared by screen-printing techniques, as described for example in patent US 5882727. According to this technique, a slurry of particles of getter material is prepared in an aqueous solution containing a low percentage of high-boiling organic compounds acting as binders; this slurry is passed through a suitable mesh and deposited onto the underlying substrate. The deposit is then dried and cured by sintering in a vacuum oven at about 800 to 1000 ℃.

Finally, according to the technique described in patent US5908579, getter devices can be obtained with particularly high porosity. In this case, a mixture of powders of getter material and organic components such as ammonium carbamate is used, the latter evaporating during the thermal treatment of the device curing (treatment at temperatures up to between 900 and 1200 ℃), leaving behind a network of interconnected pores which allow the gases to access the surface of the innermost particles of getter material in the device.

One problem encountered with the getter devices of the known art is the possibility of particle loss, since the surface particles are less bonded than the innermost particles. The presence of free particles is detrimental for the foreseeable applications of most getter devices, since they interfere with the electrical properties (for example in the case of flat displays), they can be between the paths of the radiation or elementary particle beams (in particle accelerators) or they can be deposited on the microelectronic devices produced.

A possible way to eliminate this problem is to increase the sintering temperature, thereby favouring the mutual adhesion of the particles; however, this method not only alleviates this problem but also has the disadvantage of reducing the porosity and the exposed surface of the active material, thus reducing the gas sorption properties of the getter device.

It is an object of the present invention to provide a method for the preparation of porous getter devices with reduced loss of particulate matter, which does not have the drawbacks of the known art, and to provide the resulting devices.

This object is achieved by the process of the present invention, which consists in preparing, on the surface of a porous getter body, a deposit of a material having a thickness of at least 0.5 microns, compatible with the conditions of use envisaged for the getter device, using a technique chosen among evaporation, arc-generated plasma deposition, ion beam deposition and cathodic deposition.

The inventors have found that, contrary to what is believed in the prior art, the deposition of a suitable material of low thickness on the surface of a porous getter body does not compromise its gas sorption properties, while significantly reducing the appearance of particle loss.

The invention is described below with reference to the accompanying drawings, in which:

figure 1 shows a cross-sectional view of a porous getter body before coating by the method of the invention;

fig. 2 shows the same cross section of the porous getter body of fig. 1 after coating by the method of the invention;

figure 3 shows a cross section of several particles of getter material coated by a preferred embodiment of the process of the invention.

Fig. 1 shows a cross-section of a surface portion of a porous getter body 10. The grains 11 of getter material are joined together by "necks" 12, during sintering, the material undergoes micro-scale melting. Due to the rare mechanical resistance of these necks (due to the low temperature of the sintering process) or due to the reduced number thereof, especially in the case of small-sized particles 13, the adhesion of surface particles to the remaining structure can be reduced.

Figure 2 shows the same object of figure 1 coated by the method of the invention. The upper surface of the object 10 is coated with a layer 20 made by one of the techniques described above. These techniques are directional in that the deposit coats only the portion of the object 10 exposed to the source of the deposition material. Certain regions (21) of the surface getter particles (the "shadow regions" in the case of the source of the material to be deposited) therefore remain undeposited. The overall effect is that the deposit 20 acts as a binder for the surface particles, but it does not block the large channels in the getter material particles, allowing the gas access to the innermost particles, the surface of which is not coated by the method of the invention, so that the gas adsorption activity is maintained. The result is a surface coating of the porous getter body 22 by the deposit 20.

The porous getter body on which the deposit 20 is to be formed can be prepared according to any of the techniques described above, namely powder compaction, with or without organic components contained therein, which evaporate during the subsequent thermal treatment of electrophoresis and screen printing, electrophoresis and screen printing.

Getter materials which can be used to prepare the porous bodies are various and generally comprise titanium and zirconium metals, their hydrides or alloys of titanium or zirconium with one or more elements selected from the group of transition elements and aluminum and mixtures of one or more such alloys with titanium and/or zirconium or hydrides thereof. Among the most useful materials for the purposes of the present invention, mention may be made of the alloy Zr — Al disclosed in patent US3203901, in particular the alloy with a weight percentage composition Zr 84% -Al 16% produced and sold by the applicant under the name St 101; the alloy Zr-V-Fe disclosed in patent US4312669, in particular the alloy with a weight percentage composition Zr 70% -V24.6% -Fe 5.4% manufactured and sold by the applicant under the trade name St 707; the alloy Zr-Co-a disclosed in patent US5961750 (where a represents an element selected from yttrium, lanthanum, rare earth elements or mixtures thereof), in particular the alloy manufactured and sold by the applicant under the name St787 with a weight percentage composition zr80.8% -co14.2% -a 5%; the alloy disclosed in patent US4457891 Ti-V-Mn; a mixture comprising 70% by weight of Ti and 30% of alloy St 101; a mixture containing 70% Ti and 30% alloy St 707; a mixture containing 40% Zr and 60% alloy St 707; a mixture comprising 60% Ti and 40% alloy St 707; and mixtures containing 10% by weight of Mo, 80% of Ti and 10% of TiH2, which are produced and sold by the applicant under the name St175, disclosed in patent US 4428856. These getter materials are generally applied in the form of powders having a particle size of less than about 125 microns, preferably from about 20 to 100 microns.

After the getter body has been prepared according to one of the above-mentioned techniques, it can be cured by means of a thermal sintering process in a vacuum or inert atmosphere, generally at a temperature comprised between 800 and 1200 ℃, depending on the materials used.

The getter material thus obtained is subjected to a deposition treatment with a layer thickness of at least 0.5 μm, using a technique chosen from evaporation, plasma deposition by arc generation, ion beam deposition and cathodic deposition.

The evaporation can be carried out by placing the getter sample to be coated and the source of the material to be deposited in the same chamber, the latter being evaporated (under inert gas or vacuum) using known techniques, such as direct heating (for example by passing a current through the material support), indirect heating (for example by induction), electron bombardment or similar methods.

The second technique, more commonly known as arc plasma deposition, is the production of fine droplets of the material to be deposited by melting the solid surface of the material to be deposited with a local electric arc; the droplets thus formed are then accelerated towards the substrate to be coated. This technique enables a dense coating layer to be obtained quickly, for example for coating mechanical tools, in order to improve their hardness properties.

Ion beam-derived deposition techniques, more commonly known as ion beam deposition, are the generation of a plasma of ions of the material to be deposited, which are then accelerated by an electric field moving these ions towards the substrate to be coated.

For the purposes of the present invention, cathodic deposition techniques are preferably used. Cathodic deposition techniques result in thin layers, typically about 10 to 20 microns in thickness, on supports, typically made of different materials. There are many variations of this technique, better known in the art as "sputtering" (which is used in the remainder of this document) or "physical vapor deposition" or the abbreviation "PVD". The sputtering technique is widely known and widely used in industry, and is fundamental in particular to the microelectronics industry, since it enables the preparation of thin layers of active material (for example layers of conductive material) or with passivation functions (for example insulators), and also in many other fields, for example the preparation of aluminium layers for compact disks.

Sputtering and its variants are well known and various and will not be described in detail here. Is composed ofWith the present invention in mind, it is sufficient to remember the basis of this technology. As is well known, in the art vacuum chambers are used in which an electric field can be generated. A target of material to be deposited (usually having the shape of a short cylinder) is placed in the chamber and a support on which a thin layer is to be formed is placed in front of the target. The chamber is first evacuated and then filled with a noble gas, usually argon, at a pressure of 10-2 to 10-5 mbar; applying a voltage of several kilovolts between the support backing and the target (bringing the latter to cathodic potential), electrons and Ar are generated⁺The plasma of (2); these ions move towards the target under acceleration of the electric field, causing impact erosion; the species (usually atoms or "clusters" of atoms) resulting from target erosion are deposited on the support, forming a thin layer. By changing the process parameters, the properties and conditions of the prepared film can be controlled; for example, by increasing the power applied to the electrodes, the thickness produced increases and the morphology of the thin layer produced changes; morphology can be controlled in an even more efficient manner by varying the angle of incidence of deposition relative to the substrate.

The layer thickness deposited on the surface of the porous getter must be at least 0.5 μm, since at lower values the adhesion of the layer is not sufficient to make the grains of getter material hardly adhere to the rest of the device. The upper limit of the deposit thickness is not strictly defined, but it is generally less than 5 μm, since higher thickness values require long process times without particular advantage being obtained. Preferably, the thickness of the deposit is 1 to 2.5 microns.

The material forming the deposit may be any material compatible with the conditions used in the final application of the device. In particular, the material of the deposit must have a low gas evolution and must be able to withstand the temperatures to which the getter device is subjected during the established device manufacturing process steps, without changing, for example, the melting operation of sealing flat displays or lamps; in the case of devices having the shape and dimensions of the substrate to be treated in the deposition chamber disclosed in the above-mentioned international patent application PCT/IT00/00136, the material deposited on the porous getter body must be able to withstand heating at the activation temperature of the getter material; the deposition chamber is subjected to a temperature of at least about 500 c to degas its walls. Typically, the deposited material may be selected from transition metals, rare earth elements, and aluminum. It is also possible to deposit more than one metal simultaneously (e.g. co-evaporation or so-called "co-sputtering" techniques), resulting in a mixture or alloy of said metals.

Preferably, the deposited material is a metal that also has getter properties, such as vanadium, niobium, hafnium, tantalum, or preferably titanium and zirconium, or alloys of these metals. In the case of depositing one of these materials, the gas adsorption performance is improved in addition to reducing particulate loss compared to an uncoated porous body.

Particularly good results are obtained from this point of view if the layer is deposited in a granular or columnar form. An example of a surface of a porous getter body coated with a deposit having this morphology is shown in fig. 3, which shows surface getter particles 11 coated with a plurality of microdeposits 30, in any case the deposit can act as a binding effect for the particles in the region 31, but there are microchannels 32 inside, which improve the accessibility of the gas to the underlying porous getter and also to the surface of the same coated getter particles. The granular or columnar morphology can be obtained by sputtering techniques by controlling the deposition conditions, in particular by operating at high pressure of the rare gases and at low temperature of the substrate (porous getter); preferably, the gas pressure is maintained at about 1X 10^-3-5×10^-2Mbar and the temperature of the substrate was close to room temperature.

Claims (19)

1. A method for producing a porous getter device (22) with reduced loss of particulate matter, said method comprising generating on the surface of a porous getter body (10) a deposit (20; 30, 31) of a material chosen among transition metals, rare earth elements and aluminium, having a thickness of at least 0.5 microns, compatible with the conditions of use of the getter device, using a technique chosen among evaporation, arc-generated plasma deposition, ion beam deposition and cathodic deposition.

2. Process according to claim 1, wherein the porous getter bodies to be coated are prepared by a process selected from the group consisting of powder compaction, electrophoresis and screen printing, in which said powders contain or do not contain organic components that evaporate during the subsequent thermal treatment.

3. A process according to claim 1, wherein the getter material of the porous body is selected from metallic titanium, metallic zirconium, hydrides of titanium, hydrides of zirconium, alloys of titanium or zirconium with one or more elements selected from transition metals and aluminum, and mixtures of one or more of said alloys of titanium and zirconium with one or more of metallic titanium, metallic zirconium, hydrides of titanium and hydrides of zirconium.

4. A method according to claim 3 wherein the getter material is an alloy having a weight percent composition Zr 84% -Al 16%.

5. A method according to claim 3 wherein the getter material is an alloy having a weight percentage composition Zr 70% -V24.6% -fe 5.4%.

6. A method according to claim 3 wherein the getter material is an alloy having a weight percentage composition zr80.8% -co14.2% -a 5%, wherein a represents an element selected among yttrium, lanthanum, rare earth elements or mixtures thereof.

7. A method according to claim 3 wherein the getter material is a mixture comprising 70% by weight Ti and 30% by weight of an alloy having a composition in weight% Zr 84% -Al 16%.

8. A method according to claim 3 wherein the getter material is a mixture comprising 70% by weight Ti and 30% by weight of an alloy having a composition in weight% Zr 70% -V24.6% -fe 5.4%.

9. A method according to claim 3 wherein the getter material is a mixture comprising 40% by weight Zr and 60% by weight of an alloy having a composition in weight% Zr 70% -V24.6% -fe 5.4%.

10. A method according to claim 3 wherein the getter material is a mixture comprising 60% by weight Ti and 40% by weight of an alloy having a composition in weight% Zr 70% -V24.6% -fe 5.4%.

11. A process according to claim 3 wherein the getter material is a composition comprising 10% by weight Mo, 80% by weight Ti and 10% by weight TiH₂A mixture of (a).

12. A process according to claim 1 wherein the getter material is in the form of powders having a particle size of less than 125 μm.

13. A process according to claim 12 wherein the getter material is in the form of a powder with a particle size comprised between 20 and 100 microns.

14. The method of claim 1, wherein said deposit has a thickness of less than 5 microns.

15. The method of claim 14, wherein said deposit has a thickness of 1 to 2.5 microns.

16. The method of claim 15 wherein the material deposited is a metal selected from the group consisting of vanadium, niobium, hafnium, tantalum, titanium or zirconium.

17. A method according to claim 16, wherein the material is deposited by cathodic deposition to give a layer in granular or columnar form.

18. The method according to claim 17, wherein the cathodic deposition is carried out under conditions of 1 x 10^-3-5×10^-2The rare gas pressure in mbar and the temperature of the porous getter body are close to room temperature.

19. Porous getter bodies (22) formed by joining together particles (11) of getter material, wherein at the upper surface of said bodies the getter particles are partially coated with a deposit (20; 30, 31) of a material selected among transition metals, rare earths and aluminum, said deposit having a thickness of at least 0.5 microns.