WO2010010563A2

WO2010010563A2 - Lithium or barium based film getters

Info

Publication number: WO2010010563A2
Application number: PCT/IL2009/000723
Authority: WO
Inventors: Konstantin Chuntonov
Original assignee: Freespace-Materials
Priority date: 2008-07-23
Filing date: 2009-07-23
Publication date: 2010-01-28
Also published as: WO2010010563A3; US20110217491A1; EP2311106A2

Abstract

Two film materials, one of them with the structure of barium eutectic and another one with the structure of lithium solid solution, manufactured by thermal deposition. The mentioned films may give a freedom of choice of the sealing methods starting from the standard bonding processes with heating under vacuum or under inert gas to common gluing at, or around, room temperature in conditions such as: CO2 environment under the pressure of latm.

Description

Lithium or Barium Based Film Getters

Field of the Invention

[001] The present invention generally relates to the field of chemical gas getters. More specifically, the present invention relates to getter films intended for usage in sealed-off long term vacuum chambers.

Background of the Invention

[002] Many types of sealed - off vacuum devices, especially those of the newest generations, are characterized either by a small size (micro electro mechanical systems or MEMS, nano electro mechanical systems or NEMS), or by a big number of parts with highly developed surface and complex chemical composition (field emission displays or FED, Generation III image intensifier tubes or Image Tubes), which are located inside the device. In these devices the value of the ratio of the total surface area of all the interior parts of the device, including the enclosure, to the evacuated volume appears to be very large as well as the specific, (i.e. taken relative to a volume unit) amount of inleaking gases. For sorbing gas streams, which are so intensive, new getters with sorption capacity by orders of magnitude higher than that of currently used ones are required.

[003] There are two ways of increasing the sorption capacity of getter materials radically. In the case of transition metals, where only a near-surface region with the thickness of a few atomic layers takes part in gas sorption at room temperature, the sorption capacity of a getter mass can be increased by increasing the dispersion degree of the material. Thus, at transition from metallic powders of the commercial micron range to nanoparticles the intrinsic sorption capacity of the material grows approximately by three orders of magnitude.

[004] However, metallic nanopowders tend to coalesce, besides, they are extremely reactive. For this reason in practice, e.g. in apparatuses for purification of gas streams from impurities getter mixtures or composites are used. These mixtures or composites consist of two substructures: coarsely dispersed porous bases, having metallic, ceramic, or even polymeric nature, and metallic nanopowders, mainly of Ni and Mn, covering the surface of the mentioned basis and partially filling its pores [Tamhankar S., Weltmer W.R. US Pat. 4713224, Dec.15, 1987; Weber D.K., Vergani G. US Pat. 6521192, Feb.18, 2003; Zeller R., Vroman C. US Pat. 7112237, Sep.26, 2006; Alvarez Jr. D. US Pat.6241955, June 5, 2001]. This solution ensures high degree of gas separation and at the same time has sufficient drawbacks. Volume concentration of nanoparticles, which are the main purification agent, is very low in these materials amounting to only a few percents; and the production technology of getter composites is very complicated, consists of many stages and takes a lot of time, from several tens to hundreds of hours [Watanabe T., Fraenkel D., Torres Jr.R. Pat. WO 03/037484, May 8, 2003]. Besides, the material is immediately destroyed at contact with the air. [005] A recent attempt to apply the described here composites to vacuum problems [Sparks D.R., Najafi N., Newman B.E. US Pat. Application, 2070205720, Sep.6, 2007] looks natural, but no other ideas or experimental data about the structure or properties of the claimed getter material could be found in this paper. Therefore the negative remarks expressed towards gas purification composites are also valid here. Moreover, the point concerning low volume concentration of the nanoconstituent of the getter composite becomes generally critical under the conditions of such devices as MEMS, FED, etc., where there is a deficiency of free space and for this reason getters with maximally high sorption capacity are required.

[006] So, though increasing of the degree of dispersion of getter materials allows sufficiently increase of their sorption capacity, the really achievable values of the capacity appear to be rather far from the theoretical limit. As regards the production technology of getters with metallic nanoparticles, it is a high - cost one; and the price of the end product is very high accordingly.

[007] Another way to increase sorption capacity of getter materials is to replace transition metals with chemically more active metals, alkali and alkali - earth. Gas sorption at room temperature takes place in this case by way of formation of a layer of products on the surface of the metal. This layer grows due to interdiffusion of the reagents till the metallic mass is completely exhausted. That is, these metals react with gases completely, providing maximally high sorption capacity. [008] However, high chemical activity of alkali and alkali - earth metals causes a lot of problems in handling them. Therefore until recently the only possibility of using these metals in the processes of chemical pumpdown of gases was the so called evaporable getters of a flash getter type, which released the vapor of the deposited metal only in an already evacuated and sealed device [Pirani M., Yarwood J. Principles of Vacuum Engineering, Reinhold Publ. Corp., N. Y., 1961, ch.6, pp.251- 291; Turnbull J.C., J. Vac. Sci. Technol., Vol.14, NoI, 1977, 636 - 639]. [009] The disadvantage of this kind of getters is well known - it is the necessity of large free surfaces inside the device. And though for some models of Image Tubes it is possible using design tricks to find the needed space, for example, in a form of a ring gap with the help of auxiliary shields [Thomas N.I. US Pat. Application 20070023617, Feb.l, 2008], there is no such a possibility in devices like MEMS and FED.

[010] Really new vistas for using active metals as getters in small vacuum devices opened up recently with the appearance of multilayer getter films, containing an interior active layer and an exterior protective layer [Kovacs A.L., Peter M.H., Ketola K.S., Linder J.F. US Pat. 6822880, Nov.23, 2004; Sparks D.R. US Pat. 6923625, Aug.2, 2005].

[011] The first of these documents describes a Ti / Pd film. According to the given invention a layer of titanium deposited on the substrate and aimed at sorbing hydrogen is covered from outside with a layer of Pd, which easily lets hydrogen to titanium, but protects the latter from oxidation by the gases from the ambient atmosphere. This is certainly an elegant solution, but it refers to a particular problem - the problem of protecting of a GaAs circuity sealed in a hermetic package from hydrogen. Hydrogen is only one of gaseous species comprising residual gases, which include also O₂, CO, CO₂, H₂O, N₂, etc. and which should be also evacuated. [012]The second document, namely, [Sparks D.R. US Pat. 6923625, Aug.2, 2005], represents by itself the next step in the development of the idea of multilayer getter films. Here the exterior protective layer is preserved only as long as there is a necessity in conservation of the interior active layer. In order to switch the initial film into a working state, it should be heated either in an independent way or by receiving the heat from some concomitant process, e.g. from the operation of sealing the device, which is usually performed at the temperature from 300° to 500⁰C. As the author of US Pat. No. 6923625 believes, the atoms of the inner layer manage to diffuse through the outside layer during the time of sealing and to come to its surface in order to implement their getter functions after that.

[013] US Pat. 6923625 contains a description of two methods of building up film structures, which can be schematically written down as combinations Sub/A/N and Sub/N/A/N, where Sub is a substrate, which is usually glass, silicon, or ceramics, A is layer of a component selected from the ensemble of chemical elements, which the author of the cited patent referred to reactive ones, N is a layer of the component, selected from the rest of the chemical elements (excluding inert gases), which the author called nonreactive.

[014] At this, the second, three-layered, combination is foreseen for the case, when during the treatment the substrate is removed and then the remaining sandwich - like film N/A/N itself provides chemical protection for the layer A from the environment. [015] There is no doubt that following the prescriptions of US Pat. 6923625 it is possible to obtain several high quality getter films of different compositions. However, it is also possible to assert that the degree of generality, which this patent claims for, has no grounds.

[016] Thus, following the classification of the chemical elements into reactive A and nonreactive N, introduced by US Pat. 6923625, and fulfilling the described in this patent operations, let us deposit, e.g. a double layer film on a glass, ceramic of silicon substrate Sub/Li layer 400nm thick / Ag layer 30nm thick hoping to produce a super active getter product of the composition Li - 8.8 at% Ag.

[017] The following heating of the given film in vacuum to any temperature in the interval 300⁰C < T <500°C leads however to chemical destruction of the substrate. (Not mentioning here such negative phenomenon as an intensive evaporation of Li from the film.) This is clear: an alloy Li - 8.8 at% Ag has a melting point of 1450C, that is lower than of pure Li [Pelton A.D., Bull. Alloy Phase Diagrams, Vol.7, No3 (1986) 223]. This alloy is very close in properties to Li and liquid lithium at 300 - 5000C reacts both with glass and ceramics [Borgstedt H.U., Mathews CK. Applied Chemistry of Alkali Metals, Plenum Press, N. Y., 1987] as well as with silicon [Moissan, Cr. Acad, sci., Paris, 134 (1902) 1083]. It should be mentioned that the considered case is not a single one and the number of metals A, which in the temperature range of 300° - 500⁰C are able to react with the prevailing substrate materials like glass, ceramics, or silicon, is approximately equal to 20. [018] Let us take another example. Let's deposit on a suitable substrate a double layer film Sub/Ba layer 400nm thick / In layer 20nm thick and heat it for activation in vacuum to a certain temperature from 300° - 50O⁰C The expected result, according to the ideology of US Pat. 6923625, is obtaining of a getter film with the gross composition Ba - 1 lat% In due to interdiffusion of Ba and In atoms (at equilibrium this would be a very active chemically binary phase film containing a mixture of Ba₁₃In and Ba₃In crystals [Bruzzone G., J. Less-Comm. Met., 11 (1966) 249]). [019] In reality, as experience shows, everything happens in a different way: the process of mixing Ba and In takes a form of a fast proceeding reaction of synthesis of an intermetallic compound Baln4, i.e. proceeds not in equilibrium n Ba(s) + 4 In(I) ^~J→ (n - 1) Ba(s) + Baln4(s) + ΔH , where (s) an (1) indicate an aggregative state of a component, solid and liquid correspondingly and ΔH is a thermal effect of the reaction. Particles of Baln4, being a phase, which is rather stable to gases, are formed with a high volume compression and have a weak physical connection to surface of the barium. This connection becomes still weaker and the particles start peeling off with the beginning of the sorption process, when a layer of compounds (BaO, BaH2, Ba3N2 , etc.) appears between Ba and Baln4 particles. This kind of an A - N system with fusible N comprises a risk group: heating of a binary film A/N in this case can initiate, contrary to the concept given by US Pat. 6923625, not an interdiffusion of the components, but an exothermic reaction of synthesis of an intermetallic phase AN, the particles of which have a weak adhesion to the layer A. Later on these particles easily come off the layer A at mechanical shocks, temperature changes, or during gas sorption. The number of paired combinations A - N, for which the probability of peeling off of the getter film produced following the prescriptions of US Pat. 6923625 is very high, exceeds one hundred according to the data on phase diagrams [Okamoto H. Phase Diagrams for Binary Alloys, ASM International, OH, 2000]. [020] Accordingly:

1. Small long term vacuum devices require new getters, the sorption capacity of which is by two or three orders of magnitude higher than that of sintered HPTF materials or deposited Page films (SAES Getters) type. Getters based on chemically active metals, alkali or alkali - earth, provide this possibility.

2. In principle multilayer getter films allow introducing chemically active metals inside a small vacuum chamber by a method, which is easily compatible with any assembly technology and is applicable to any type of packaging of vacuum devices like MEMS, NEMS, FED and Image Tubes.

3. Among the known developments in the field of multiple getter films there is still no one, which would give a satisfactory solution for the case of chemically active metals. [021] Below a number of getter films with active metals are described. The films are intended for long term maintenance of vacuum in small sealed - off devices.

Summary of the Invention

[022] The present invention is multilayer getter film. According to some embodiments of the present invention, two or more metals having substantially dissimilar gas sorption selectivity and rate characteristics may be adapted to provide mutual complementary gas sorbent abilities. According to some embodiments of the present invention, films may be produced in the form of alternate layers of active sorbents wherein one sorbent may belong to a group of universal gas sorbents such as Ba or Li, and where the other sorbent may belong to a group of getter metals such as Al, Mg, or Pd.

[023] According to some embodiments of the present invention, the layers may be alternately deposited onto a "hot" substrate, after which the last layer may deposited on the cooled film. The total thickness of the deposited film may be determined by the rate of gas leakage and/or by the planned lifetime of the device. The films may be deposited on an inner wall of the vacuum chamber of the device and/or on a suitable metallic strip introduced into the device and afterwards fixed inside at the stage of its assembly.

[024] According to some embodiments of the present invention, a protective cover layer of the multilayered getter film may be stable to Nitrogen and Oxygen that mutually constitute the majority of the volume of air. As Nitrogen is rather inert, stability to Oxygen may be chiefly required. Under conventional conditions, various noble metals (e.g. Ag, Au, pd, Pt) as well as various self passivating ones (e.g. Al, Fe, Mg, Sc, Sm) may be stable to Oxygen. Furthermore, a substantial part of these, or similar, metals may also be able to react with various residual gasses, possibly at room temperature, or to dissolve them in themselves.

[025] -According to some embodiments of the present invention, films of eutectic compositions of Ba and Mg, or, Ba and Al may behave independently with respect to their sorption characteristics. Accordingly, sorption activity may cause grains of a first metal in the composition (e.g. BaMg₂) to react with, and/or dissolve in themselves, mainly Hydrogen, whereas sorption activity may cause grains of a second metal in the composition (e.g. Ba) to react with and/or dissolve in themselves mainly Nitrogen and Oxygen, and/or Nitrogen and Oxygen containing gasses. Furthermore, the eutectic, fine-grained structure may be characterized by a developed net of grain boundaries, and may thus serve as channels, allowing for the migration of gases and the composition's metallic diffusant, maintaining the kinetics of the sorption process at a substantially high level.

[026] According to further embodiments of the present invention, the Ba and Mg, or, Ba and Al films may be deposited on a metallic substrate without an intermediate layer, and/or on an inorganic substrate (e.g. glass, silicone, ceramic) which surface comprises a protection layer (e.g. Cr, Mn). As eutectic Ba — Mg is a fusible alloy, the Ba - Mg films may be brought to a substantially complete homogenization through a relatively short heating to around or above 35O⁰C under Ar at a pressure of around 10^" ² mbar. These films may be sealed-off under vacuum, after cooling to, or to around, room temperature, or, may be immediately sealed-off under residual Argon during their bonding process.

[027] -According to other embodiments of the present invention, solid solutions of Li in some noble metals (e.g. Ag, Au, Cu, Pd) or in their inter-metallic compounds (e.g. AgMg, LiPd₂) may enable for activationless type getters that may sorb gases at, or around, room temperature without the customary activation heating and may also be operate in cases where the getter has been pre-exposed to air. LiPd and LiPd2 with a homogeneity range of around 46 to 52 at% Pd and around 60 to 75 at% Pd, respectively, may be used. Deposited LiPd0.86, LiPdI.5 or substantially similar films, may, at the initial stage of reactions with the residual gases (i.e. at t < tp) get covered with a thin layer of products, mainly lithium oxides, several nanometers thick. Subsequently, the reaction may move into its next stage, where the transfer of Li atoms from the film to the layer of products may become a limiting process for the gas sorption. However, throughout this passivated state Li solid solutions may maintain comparatively high rate of gas sorption, and may therefore be able to withstand the flow of leaking gases from outside for a substantially long time. [028] According to further embodiments of the present invention, this type of getter devices may be sealed at room temperature in CO₂ atmosphere under the pressure of around lbar. As sorption of CO₂ by lithium may take place at a rate approximately 100 times faster than nitrogen sorption, getters may relatively rapidly start working in their usual sorption regime after capturing carbon dioxide, while maintaining vacuum in the chamber by capturing leaking gases. At this stage, hydrogen may be dissolved in a matrix of the LiPd or LiPd₂, while other gases may react with the excess lithium that may diffuse from the film's more internal volume to its surface. Sealing the device at room temperature in CO₂ atmosphere under the pressure of lbar may liberate of the need for vacuum or heating equipment during the sealing procedure and may save the sorption resources of the getter due to the "freezing" of the processes of volume outgassing of the inside parts and walls of the vacuum chamber. Alternatively, the device may be sealed under vacuum conditions at 300° - 500⁰C. [029] -According to other embodiments of the present invention, the opposite sides of a thin metallic strip (e.g. made of stainless steel) may be covered with getter films of different composition, thus allowing for one getter to include a combination of device materials, which may be else wise incompatible. According to some embodiments of the present invention, a Li (e.g. Li - (3.5±1.5) at%Mg) film on one side of the strip and a film of Ti or V on its other side may be used as complementary sorption partners.

[030] According to further embodiments of the present invention, deposition on a first side of the metallic strip may comprise sputtering of a film of a transition metal. Subsequently, the obtained film may be covered with a thin layer of Ag, Au or Pd, (e.g. by a thermal deposition method) without being exposed to the air. The thickness of the protective cover layer may be less then lOnm but should not, in accordance with some embodiments, exceed the maximum of what a solid getter film may dissolve in itself.

[031] According to further embodiments of the present invention, Lithium may be deposited at room temperature on a second side of the metallic strip by thermal deposition with an arbitrary rate, the Li - film may then be covered, possibly at negative temperatures, with a layer of Mg. Both cover layers, the layer of noble metals on Ti or V film and the layer of Mg over the Li film may be deposited in a manner that covers over the boundaries of the lower getter film while covering a small adjoining area of the strip - carrier.

[032] According to further embodiments of the present invention, activation of the getter film may be achieved by raising its temperature to approximately 200⁰C for a period of approximately 15 - 25 minutes. This may cause the Li - Mg film to homogenize as it is close to a liquid state, while the film of the transitional metal (i.e. Ti, V), due to its column structure, may release from its cover layer. Part of this cover layer may dissolve in the volume of the columns and part of it may distribute along the boundaries between the columns. Ti - or V - films are intended mainly for hydrogen sorption while Li - film is intended for most other active gases, accordingly, sorption of a broad range of gasses may be achieved. Brief Description of the Drawings

[033] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

Fig. 1 is a drawing of exemplary Eutectic Getter Films, in accordance with some embodiments of the present invention;

Fig. 2a is a drawing of an exemplary Sorption Mechanism, in accordance with some embodiments of the present invention;

Fig. 2b shows Sorption Kinetics graphs, for exemplary Sorption Mechanisms, in accordance with some embodiments of the present invention;

Fig. 3 is an exemplary graph, of the Growth of Products on the Surface, in accordance with some embodiments of the present invention;

Fig. 4a-4c are drawings of exemplary Strip Getters, in accordance with some embodiments of the present invention; and

Fig. 4d is a drawing of a cross section of an exemplary Getter Strip, in accordance with some embodiments of the present invention.

Detailed Description of the Invention

[034] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the present invention.

[035] Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "sorbing", "getting", "reacting with", or the like, refer to the action and/or processes of a metal based gas getter or gas getter system, or similar chemical device, that may provide gas sorbent abilities.

[036] Embodiments of the present invention may include apparatuses for performing the operations herein. Such apparatus may be specially constructed for the desired purposes, or it may comprise a general-purpose system that may be selectively activated or reconfigured.

[037] The processes and displays presented herein are not inherently related to any particular system or apparatus. Various general-purpose systems may be used in accordance with the teachings herein, or it may prove convenient to construct a more specialized apparatus to perform the desired method. The desired structure for a variety of these systems will appear from the description below.

[038] The concept of multilayer getter films with chemically active material is based on the following premises:

1. Each metal sorbs different gases with a different rate. Different metals are characterized by different sorption selectivity. Hence it is always possible to select a pair of metals in such a way that they should mutually complement each other as gas sorbents providing together practically complete chemical of all active gases.

2. As far as the air consists for 99 vol % of oxygen and nitrogen, and nitrogen is rather inert, it is enough, that a protective layer of the multilayer getter film is stable to oxygen. The number of metals, which are stable to oxygen under conventional conditions, is generally big and among them there are a lot of those, which are able either to react with some representatives of residual gases at room temperature, or to dissolve them in themselves.

[039] According to some embodiments of the present invention, the problem of designing multilayer getter films comes down therefore to a rational selection of sorption partners, which is easier to realize in the case, when one of them belongs to the group of universal gas sorbents, which are Ba and Li, and another one is taken from a group of such getter metals as Al, Mg, or Pd.

[040] In the present invention three methods of combining Ba or Li with Al, Mg, or Pd are used in a way that the principle of additivity is being fulfilled:

(a) films of eutectic composition;

(b) films of solid solutions of lithium in a stable metallic matrix; and

(c) two separate films of different composition.

[041] According to some embodiments of the present invention, Films may be produced in a form of alternating layers A1/A2/A1/A2/ A1/A2 , where Al is

Ba or Li, A2 is Al, Mg or Pd (by using the notations of Al and A2 for manifold layers of the getter film we emphasize that as opposed to US Pat. 6923625 in our case both components are reacting, participating in gas sorption after performing the activation of the film under vacuum conditions) Deposition methods are determined by the nature of the system Al - A2. If the components of the system are characterized by large mixing heats, e.g. Al - Ba and Li - Pd, the films are obtained by simultaneous or alternate deposition of metals Al and A2 with the rate of 0.01 - 1.0 A/s onto a suitable metallic substrate, heated to 150 - 2500C as it is schematically shown below

deposition at T = 200 ± 50⁰C deposition at negative T

[042] According to some embodiments of the present invention, This kind of substrate may be stainless steel, nichrome, molybdenum and other metals. In the case, when the substrate is ceramics, glass or silicon, they should be preliminarily metallized by covering, e.g. with a thin Cr or Mn film.

[043] According to some embodiments of the present invention, The thickness of a single paired layer A1/A2 , if alternate deposition is performed, may be no more than 50nm and the ratio between the thicknesses of the layers Al and A2 inside such a paired layer may correspond to the general ratio between the components Al : A2 in the synthesized product. This technique is taken from the technology of production of alkali photocathodes [Sommer A.H. Photoemissive Materials, John Willey & Sons, N. Y., 1968] and used for getter films to avoid loose particles formation. According to further embodiments of the present invention, when films with the weak interreaction of the atoms of different kinds are deposited, e.g. Mg - Ba films, higher growth rates may be used (of an order of 0.1 - 10.0 A/s) as well as thicker paired layers of A1/A2. [044] According to some embodiments of the present invention, alternate deposition of the layers Al and A2 onto a "hot" substrate may be done till next to last layer of Al inclusively, after which the last layer of A2 may be deposited on the already cooled film according to the above given scheme. The total thickness of the deposited film may be determined by the rate of gas leakage and the planned lifetime of the device. The films may be deposited both on an inner wall of the vacuum chamber of the device and on a suitable metallic strip introduced into the device and afterwards fixed inside the device at the stage of its assembly. Depending on the type of a vacuum device films of different compositions can be employed including films of BaxAll-x , where 0.69 < x < 0.74, BaxMgl-x , where 0.6 < x < 0.7, Lix Pdl-x , where 0.25 < x < 0.40 or 0.48 < x <0.54, and also LixMgl-x , where 0.95 < x < 0.98. The first three of them intensively sorb all active gases at room temperature, while the last one sorbs all gases except hydrogen; the rate of sorbing hydrogen by this film is insignificant. Therefore, Li - Mg films may need a sorption partner, which may be any of the known hydrogen sorbents. According to some embodiments of the present invention, one of the solutions of this problem may be manufacturing of metallic getter strips, one side of which is covered by a LixMgl-x film, where 0.95 < x < 0.98, and the other side - with Ti or V film.

[045] According to some embodiments of the present invention, as the above listed films contain volatile metals, temperature limitations appear for the processes of sealing these devices under vacuum: for Ba - Mg films the maximum heating temperature may be 2500C, for Ba - Al films it may be about ~ 350OC, for Li - Mg films - about -3000C, for Li - Pd films about ~ 4000C. However, the substantially high sorption capacity of the given films may allow avoiding these limitations with the help of low-temperature sealing materials (i.e. materials performing gluing at the temperature from room one to -1500C) if the temperature of their softening (unbrazing) is higher than 2500C. In this case a short heating of the device to 200 - 2500C after the gluing operation may allow completing the activation of the getter film. Moreover, when using Li - Pd films even this heating may not be needed: the sealing can be done at room temperature under normal pressure in CO2 atmosphere. [046] According to some embodiments of the present invention, obtained by the disclosed above method Ba - Mg, Ba - Al, Li - Pd, and Li - Mg films may represent by themselves new effective getters with substantially high utilization factor of the material: both components of the film may participate in reactions with residual gases at room temperature and the reactions themselves at this may proceed to the end. The process of production of this kind of getter films may consist of repeated deposition of thin double layers A1/A2 on a heated substrate, which provides the formation of a product, close to equilibrium, having relatively high mechanical stability and good adhesion to the substrate. Insertion of these getters into small sealed off vacuum devices may allow increasing their lifetime by tens of times.

[047] -According to some embodiments of the present invention, the principle, according to which sorption effects of each constituent of the getter material or each separate part of a getter device summarize intensifying each other, may be embodied in the following exemplary solutions:

[048] (a) Films of Ba - 35 at % Mg or Ba - 28.5 at % Al, as well as films with small deviations from the mentioned composition in both directions refer to eutectic type. A fragment of a binary eutectic structure is shown in Fig. 1: the structure can be considered as a mixture of phases Ba and BaMg2, each of which behaves in sorption respect independently from each other. Like other fine-grained structures eutectic is characterized by a developed net of grain boundaries, which serve as the channels for a rapid migration for gases and the metallic diffusant, maintaining the kinetics of the sorption process on a high level. All this taken together imparts the films with the perfect sorption properties.

[049] (b) LiPd and LiPd2 with the homogeneity range from 46 to 52 at% Pd for the first one and from 60 to 75 at% Pd for the second one [Loebich O., Raub Ch. J. Platinum Metals Rev., 25 (1981) 113] are the representatives of the new getters of the activationless type. Activationlessness here is understood in the narrow sense, that the getter sorbs gases at room temperature without the customary activation heating even if it was already exposed to the air. This is the wonderful feature possessed by solid solution of Li in some noble metals, e.g. in Ag, Au, Cu, Pd or in their intermetallic compounds, e.g. in AgMg, LiPd2 , etc. [050] As - deposited LiPd0.86 or LiPdI.5 films at the initial stage of reactions with the residual gases, i.e. at t < tp , are covered with a thin layer of products, mainly lithium oxides, several nanometers thick (Fig.2a, b). After this the reaction passes into the next stage, where the transfer of Li atoms from the film to the layer of products becomes a limiting process for the gas sorption. As a result the rate of capturing gases by the getter film abruptly slows down and under the normal conditions this is taken for the phenomenon of passivation of the material.

[051] It is important, however, that even in this passivated state Li solid solutions maintain comparatively high rate of gas sorption, and are able therefore to withstand the flow of leaking gases from outside for a long time. It is this sufficiently high sorption rate together with the colossal sorption capacity that allows simplifying the technology assembly and sealing of small vacuum devices performing the mentioned operations under the temperatures as low as room temperature for choice: either under vacuum or under the pressure of latm in CO2 environment.

[052] Sealing of the device not under vacuum conditions at 300 - 5000C, but at room temperature in CO2 atmosphere under the pressure of lbar has certain advantages:^'

- there is no need in vacuum or heating equipment, which simplifies, speeds up and reduces the price of the sealing procedure; and

- the sorption resource of the getter is saved due to the "freezing" of the processes of volume outgassing of the inside parts and walls of the vacuum chamber.

[053] The atmosphere of CO2 is more preferable during sealing than the air as sorption of CO2 by lithium takes place approximately by ~ 100 times faster than nitrogen sorption. After capturing carbon dioxide the getter film starts working in its usual sorption regime maintaining vacuum in the chamber by capturing leaking gases. At this hydrogen is dissolved in LiPd or LiPd2 matrix [Sakamoto Y., Nakamura R., Ura M., J. Alloys Compd., 231(1995) 553] and other gases react with the excess lithium, which diffuses from the film volume to its surface.

[054] (c) Thin metallic strip, preferably of stainless steel, the opposite sides of which are covered with getter films of different composition, allows combining in one getter device materials, which are incompatible by other method. In the present invention a Li - (3.5±1.5) at%Mg film on one side of the strip and a film of Ti or V on the other side are used as complementary sorption partners.

[055] The process of production of this kind of the two — sided getter strip starts with the deposition, by sputtering of a film of a transition metal after which the obtained film without being exposed to the air is covered with a thin layer of Ag, Au or Pd, e.g. by thermal deposition method. The thickness of the protective cover layer should not be less then IOnm but should not at this exceed the maximum of what a solid getter film can dissolve in itself.

[056] Lithium is deposited at room temperature on the other side of the strip by thermal deposition with an arbitrary rate and then the Li - film is covered at negative temperatures with a thin layer of Mg keeping the total atomic ratio in the film equal to approximately Mg : Li = 1: 28. Both cover layers, the layer of noble metals on Ti or V film and the layer of Mg over the Li - film are deposited in such a way that they a little bit run over the boundaries of the lower getter film covering a small adjoining area of the strip - carrier (Fig. 4).

[057] For activation of the getter film its temperature should be raised to —2000C and it should be exposed to this temperature for 15 - 25 minutes. At this the Li - Mg film rapidly homogenizes as it is close to a liquid state and the film of the transitional metal, i.e. of Ti or V, due to its column structure also releases from its cover layer. Part of this cover layer dissolves in the volume of the columns and part of it distributes along the boundaries between the columns. As it was already mentioned above, Ti - or V - films are intended mainly for hydrogen sorption while Li - film is intended for all other active gases.

[058] -According to some embodiments of the present invention, the chemical composition of the getter material, may be one of the technical characteristics of the product. The other important characteristics may be the structure of the material and its dimensional parameters.

[059] According to some embodiments of the present invention, for the continuous getter films their thickness is the dimensional parameter, and the thickness is directly connected with the usage coefficient of the getter material, in other words, with the relative sorption capacity of the getter Cr , which can be defined as a ratio of the amount of the metal atoms really participating in sorption to the total amount of capable of sorption metal atoms.

[060] According to some embodiments of the present invention, the issue of the getter films thickness may be solved with the help of the formal analysis of the sorption kinetics. Thus, in the case of transition metals the sorption rate is described by Elovich equation [Adamson A. W. Physical Chemistry of Surfaces, John Wiley & Sons, New York, 1982]. If the processes of tens and hundreds of thousands of hours are discussed, then on this time scale the curve G(t) for the transition metals will be the line segment (Fig.2b) going from the initial point GO upright down to the point ts , which is located close to the origin of coordinates (curve 1). At t = ts any metallic surface appears to be covered with a monolayer of gases, which for transition metals corresponds to the saturation state (Fig.2b).

[061] It follows herefrom, that it would be efficient to use monoatomic getter films of transition metals. However, during the installation of a getter film into a vacuum device it inevitably passivates and for this reason it has to be activated later on. In order that in the end of the activation practically the complete cleaning of the surface is achieved, the film should be about 300nm thick, taking into consideration the limited solubility of gases in it. The value of Cr in this case is equal to ~ 0.1% and becomes still smaller with the increase of the getter film thickness. [062] According to some embodiments of the present invention, gas sorption by Li - (3.5 ± 5) at % Mg films as well as by Ba - (28.5 ± 2.5) at % Al and Ba - (35 ± 5) at % Mg films follows the parabolic law (curve 2) and even at very big times t the rate of capturing all active gases is high. As far as the reaction with gases lasts to complete exhaustion of the material, the thickness of the getter film is easy to calculate for each concrete application basing on the data about the gas leakage rate Q (Fig.2b) and the planned lifetime of the device.

[063] According to some embodiments of the present invention, the behavior of Li solid solutions (curve 3) can be understood from the point of view of the classical theory of metal oxidation [Hauffe K. Reaktionen in und an festen Stoffen, Springer-Ferlag, Berlin, 1955], but still this is a new case, differing from the previously studied schemes by the following peculiarities: a very low density of gas medium (vacuum conditions), high mobility of the diffusant in the alloy, and a big value of the ratio DLi+ / DLi » 1, where DLi+ is a diffusion coefficient of Li+ cations in the layer of products and DLi is a diffusion coefficient of Li atoms in the alloy. All this taken together leads to an unusual shape of the curve 3, where two different branches may be singled out: the one, which is fast falling in the time interval [0, tp) and another one at t > tp , when the sorption rate more or less stabilizes.

[064] According to some embodiments of the present invention, the getter film maintains the operation of the vacuum device till G > Q (Fig.2b). Therefore the point of intersection of the curve 3 with the line Q determines the lifetime of the device, i.e. the value of tw. Knowing tw it is easy to find the optimal thickness of the getter film for solid solutions of Li.

[065] Fig. 3 shows the curve d = d(t), where d is the thickness of the layer of products, formed on the surface of the getter film by the moment of time t»ts (Fig.2b ). This curve is the result of integration of G = G(t) with respect to t and the further transformation of the obtained dependence C = C (t) into the dependence d = k C (t), where C is the amount of gases captured during the time t and k is the proportionality factor. As is seen from Fig.3 at t = tw the layer of products reaches the thickness of dmax and this thickness dmax with the sufficient for the practical purposes accuracy coincides with the thickness of the getter film h (Fig.2b), at which Cr → 1 and also the lifetime of the device is maximum.

[066] With the appearance of the reliable method of tracing curves 3 [Chuntonov K., Ivanov A., Permikin D. J. Alloys and Compounds, 471 (2009) 211-216] the problem of finding the optimal film thickness h may became easy to solve, since the gas leakage rate Q is usually known.

[067] Thus, the new getter films based on lithium or barium due to the usage of the temporary protective coatings are easily compatible with the existing technologies of assembly and sealing of small vacuum devices. Furthermore, due to the rational selection of the technical parameters of the product, the composition of the getter film and its thickness, it is possible to bring the sorption capacity of these films substantially close to the theoretical limit, excelling in this respect the modern getter films based on transition metals by around 100 times or more. [068] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Detailed Description of the Drawings

[069] Fig. 1 Shows an exemplary surface structure of an eutectic film.

Each of the phase constituents of eutectics of Ba - 28.5 at%Al and Ba - 35 at% Mg takes approximately equal part of the surface of the getter film and each of them preserves its chemical individuality in the sorption process.

[070] Fig. 2a An exemplary mechanism of gas sorption by solid solutions of Li.

For the better understanding of the sorption mechanism a cross section of the getter film is shown on the right and the distribution of lithium concentration cLi in this film for a certain moment of time t ≠ 0 - on the left. Li diffuses from an alloy of "LiPd" or

"LiPd2" through a layer of products to the boundary with gases, where the growth of this layer takes place due to the reaction of Li atoms coming from the volume of the film with gases O2, N2, C O, etc. Hydrogen, on the contrary, diffuses from the gas phase through the layer of products to the boundary with the alloy and further on dissolves in it.

[071] Fig. 2b Shows exemplary graphs of the dependence of sorption rate on time at room temperature for getters of different types.

G is the sorption rate, i.e. the amount of gases sorbed by an area unit during a time unit, GO is the sorption rate at t = 0, Q is the leakage rate through the chamber wall, t is time, h is the thickness of the getter film, d is the thickness of the products layer, 1 is the sorption curve for the films of transition metals, 2 is the sorption curve for the films of Li - (3.5 ± 1.5) at % Mg or barium eutectics, 3 is the sorption curve for Li solid solutions.

[072] The curve 3 has two arms: in the beginning, at t < tp , the process involves a very thin surface layer of the material running practically diffusionlessly. This stage finishes at t D tp , when a layer of products growing on the surface is a few run thick.

Then at t > tp a diffusion stage takes place. It can be described with the help of a term quasi passivation: the appearance of a layer of products on the surface of the getter film slows down but does not stop the sorption process. This makes Li solid solutions so valuable. On the one hand, the layer of products protects the lithium material from destruction at a sudden contact with the air, on the other hand, the sorption rate up to the point t = tw exceeds the leakage rate Q maintaining the vacuum device in the working state. [073] Fig. 3 Shows an exemplary graph of the dependence of the thickness of the growing products layer on time. d - the thickness of the products layer, t - time, dmax - the thickness of the products layer by the end of the operational time, i.e. by the moment t = tw. Assuming for the deposited film the thickness h equal to dmax , we by doing so create the conditions for the most economical usage of the getter material, i.e. conditions for Cr → 1 (if h

« dmax , then Cr → 1, but the lifetime is very short, if h » dmax , then though the lifetime is maximum but Cr «1).

[072] Fig. 4 Shows exemplary getter devices.

1 is a getter film, 2 are free ends of the strip - carrier intended for fixing the getter device inside the chamber. In the case of (a) ends 2 are the terminals for electric contacts, in the cases (b) and (c) they are used for mechanical fixing or for fixing by welding. The cross section s — s shows how getter films 4 and 7, coated by thin protective layers 5 and 6 accordingly, are located on strip 3.

Claims

ClaimsWhat claimed is:

1. a film getter device comprising: two or more layers of a Ba -Al composition; a coating layer of Al, coating said two or more layers; and a substrate, which is protected by a barrier layer of Cr.

2. the film getter device according to claim 1, wherein: the barrier layer is made of Mn.

3. the film getter device according to claim 1, wherein: the substrate is metallic and has no barrier layer.

4. the film getter device according to claim 1 , wherein: the two or more layers are of a Ba-Mg composition; and the coating layer is of Mg.

5. the film getter device according to claim 4, wherein: the substrate is metallic and has no barrier layer.

6. a film getter device comprising: a thin metallic strip; a Li composition with a thin Mg-coating and a Ti composition film with a thin Ag or Pd-coating; and wherein, each of said compositions covers another side of said metal strip.

7. the film getter device according to claim 6, wherein: the Ti composition film is replaced by a V film with a thin Au or Pd- coating.

8. A method of production of film getters, comprising: alternately depositing layers of two different reactive gas sorbent components on a substrate; wherein the last layer deposited also acts as a cover layer.

9. The method according to claim 8, wherein:

The first reactive gas sorbent component consists of Ba; and

The second reactive gas sorbent component consists of one metal selected from a set consisting of Al, Mg or Pd.

10. The method according to claim 9, wherein: the first reactive gas sorbent component consists of Li.

11. The method according to claim 8, further comprising: heating the substrate to a temperature of 150 - 250⁰C for the duration of the deposition of the layers; and cooling the film to a negative temperature for the duration of the deposition of the cover layer.

12. a film getter device comprising: one or more layers of a LiPd composition; a coating layer of Pd, coating said one or more layers; and a substrate, which is protected by a barrier layer of Cr, wherin: said substrate is especially allotted for a deposition area inside a vacuum chamber.

13. the film getter device according to claim 12, wherein: the substrate is protected by a barrier layer of Mn.

14. the film getter device according to claim 12, wherein: the one or more layers are deposited on a metallic substrate without a barrier layer.

15. a method of capturing residual gases in small sealed-off devices using: a film getter device comprising: two or more layers of a Ba -Al composition; a coating layer of Al, coating said two or more layers; and a substrate, which is protected by a barrier layer of Cr.

16. a method of capturing residual gases in small sealed-off devices using: a film getter device comprising: two or more layers of a Ba -Al composition; a coating layer of Al, coating said two or more layers; and a substrate, which is protected by a barrier layer of Mn.

17. a method of capturing residual gases in small sealed-off devices using: a film getter device comprising: two or more layers of a Ba -Al composition; a coating layer of Al, coating said two or more layers; and a metallic substrate.