The demand for getters with high sorption efficiency has generated a need for resources to assist in qualification of getter materials for their practical use. This paper discusses innovative steps which should provide a dramatic improvement in the selection and application of getter technologies used in various processes. The first step was to build a natural classification of chemisorbents, from which we obtain a corresponding order of suitability related to known getter products. The classification system suggested by the authors is based on criteria which are directly connected with the sorption behavior of the material. This has lead to the challenge of developing of a computing algorithm for characterization of sorption properties of getter materials and for solving the inverse problem—the problem of designing a chemisorbent based on the requirements of a fully realized application. The employment of the new methodology is demonstrated in the example of the calculations supporting the selection of getter films for MEMS.
Metallic chemisorbents, commonly known as getters, are used in vacuum chambers for capture of residual gases or in different types of sorption columns for the removal of gas impurities. A great wealth of knowledge about the selection and production of getter technologies has been accumulated during the long period of their existence [1] - [6] .
The state of knowledge about getter materials has been analyzed [7] - [9] and shows that it is possible to support the innovative development of getter applications. This is done by reconsidering concepts and facts which were obtained previously in the fundamental studies of gas/metal systems. Here we intend to create a clear and rational classification of getter materials, which will be useful for both the manufacturers and users of getters. Naturally, such a classification should be based on the essential features of getter materials, which are directly connected with their sorption properties.
Several attempts had been made toward systemization of getter selection in the past. According to one of them, getters are divided into three groups―evaporable getters (EGs), non-evaporable getters (NEGs) and sputtering NEGs (SNEGs) [10] - [13] . While the given classification comes into common use, it is somewhat misleading and it appears to have become a source of terminological confusion. The supporters of this classification define a getter as a material which chemically sorbs active gases and then conveys these properties to objects of a different nature, which is incorrect. For example, in the review [10] , we find that both the container with powder mixture of BaAl4 + Ni and a Ba-film, which subsequently appears as a product of heating of the mentioned container, are getters. To be sure, this kind of description is unacceptable. This is because a container with a powder mixture is neither a material nor a getter; but is actually a getter device, while the powder mixture is a precursor material.
In other well known reviews, e.g. [11] [12] , Ba and Ti are called representatives of the getter class EGs, which is not possible as Ba and Ti are elemental substances and not materials. Even rods of Ba and Ti created to set dimensions, for which the notion of a material can be applicable, are not yet getters. This is because they are not able to provide a rate of gas sorption which we expect from getters. It is appropriate to only use the notion getter after we have transformed the substances to powders or films of Ba and Ti, that is, to sorption materials with greater specific surface area.
Another method of classifying getter materials, which is used in [14] , looks more reasonable. According to this document any getter film can be assigned to one of two getter classes, either to bulk getters, the sorption properties of which depend on the thickness of the film, or to surface getters, the sorption properties of which do not depend on the thickness of the film.
This second classification is better able to influence the choice of the technological solutions in getter manufacturing as opposed to the classifications EGs-NEGs-SNEGs. For instance, it directly follows from [14] that for the increase of sorption capacity of bulk getters, it is sufficient to increase the thickness of getter layers while in the case of surface getters, there is only one way possible for achieving the same target―increasing their surface area. However, this classification was not developed further.
The third variant of getter classification [7] was proposed as an intermediate solution: from two factors determining the sorption process, the chemical and the structural, only the chemical one was taken into consideration. This is the usual way in a scientific analysis of complex problems, to which sorption phenomena certainly belongs.
The starting point of the mentioned third variant lies within those concepts which were formed as a result of the fundamental studies of the interaction of gases with metals [15] - [18] . With the help of these concepts all chemisorbents can be distributed according to their properties into three different classes: adsorbents, absorbents and reactants [7] - [9] . This classification reduces the sorption process to such well studied phenomena as adsorption, dissolution and chemical reactions, each of which can be quantitatively described. A new situation is being created, which sharply changes the role and importance of the getter classification: instead of fulfilling the usual reference information functions the classification system gets a possibility to become part of a special computer program, which is capable of designing getter materials for different applications.
In order to fully realize the possibility which we are proposing it is necessary to expand the third variant of the classification adding new classes. This variant will now include size and structural features of the chemisorbent. A trial version of this type of classification is shown below, and suggests some practical consequences as a result.
2. The Natural Classification
The sorption process depends not only on the sorption mechanism, but also on the geometrical shape and size of the sorbing material unit. With other factors being equal, the sorption rate is proportional to the surface area which is available for gases; and it is usually sought to make this surface area value as large as possible. There are three methods of shaping a solid material into a form which will maximize its specific surface area: to produce it in a form of a powder, a film, or a 3D body with end-to-end channels. It is these kinds of materials which have become entrenched in existing applications as standard getter products; therefore, it would be reasonable to introduce these powders, films and bulk porous bodies into our classification system as a well-established method for classifying and identifying chemisorbents according to the size-structural feature. This classification system, which combines both ways of delimitation of getter materials, i.e. according to the chemical and the structural features, is schematically shown in Figure 1.
In Figure 1, getter materials are distributed into three groups, the group of adsorbents, the group of absorbents, and the group of reactants. These three groups further fall into three groups each, into powders, films, and bulk porous bodies. As a result we come to nine getter classes covering the entire multitude of metallic chemisorbents. In the bottom row of the given scheme a representative of a getter product is provided for each class as an example.
2.1. The Sorption Mechanism
The sorption behavior of a getter is quantitatively described by the kinetic law Q = Q(t), where Q is amount of gas captured by a unit of the surface area of the chemisorbent by the moment of time t. Most often two values are of practical interest, the sorption rate J = dQ/dt, which is also called gettering rate, and the ultimate sorption capacity Q*, which is equal to the amount of gas sorbed by the moment, when the value of J goes down to 0 or to the level when the process loses its significance. The task is to find the method of calculation of J and Q* with the help of Figure 1. Table 1, which contains the equations necessary for defining the sorption properties of chemisorbents, is the first consequence of Figure 1.
In compliance with Table 1, in the case of adsorbents (I), the Yelovich equation [28] [29] can be used, according to which the sorption process continues as long as vacant sites remain on the surface.
If the sorption takes place by way of dissolving gases in the volume of solids (II) the selection of the concrete dependence Q = Q(t) is carried out taking into account the limiting stage of the process. However, it is necessary to mention sufficient limitations, which are inherent to getters of this class: only hydrogen is able to dissolve in metals at room temperature in substantial quantities, while for absorption of other gases metals have to be heated (which will result in the release of hydrogen gas).
In the case of reactants (III), which sorb gases according to the reaction Me + X = MeX, where Me is a metal and MeX is a nonvolatile chemical compound, a kinetic law can change depending on the composition of the material as in the case of absorbents. Common for all the reactants is that the interphase boundary MeX/Me with
Classification system
Empirical laws of sorption gases by metals
Adsorbents (I)
Absorbents (II)
Reactants (III)
Kinetics
Kinetics
Kinetics
, where k1 and k2 are constants [28] [29] .
, if the process is limited by the dissociation of molecules; , if the process is limited by the transition; , if the process is limited by the diffusion of atoms of X in [X]s. Here K is an equilibrium constant, p is vapor pressure of gas X2 above the getter, k3, k4 and k′ are coefficients, [X]s is solid solution of gas X in metal [18] .
Q = k5(t1/2), or Q = k6t depending on the nature of the metal. Here k5 and k6 are rate constants [16] - [18]
time continuously moves in the direction of Me till the entire getter material is consumed.
In order to make use of the equations in Table 1 it is necessary to know the coefficients ki (i = 1, 2, 3…), which are included in the equation and behind which the geometrical factor is standing. Hence the question arises, which structural parameter influences the value of one or another coefficient ki most of all? To answer this question one should have a general understanding about the structure of a getter.
2.2. The Material Structure
Geometrical data on getters of three structural classes, powders, films and bulk porous bodies are given in Table 2. This table is the second consequence of Figure 1.
In the first approximation, powder materials (I) can be assumed to consist of monolithic particles (balls, whiskers, and laminas). When the minimal size d of these kind of particles (Table 2) and the value of the porosity ε of the getter mass, which fills the getter housing or the sorption column, are known, it is possible to calculate the specific surface area of the getter material, the mass and all the sorption properties provided that there is information about the sorption mechanism of the material. Here we define ε as a ration of the factual volume of the getter mass to its apparent volume, i.e. ε = (m/η)/(SxH), where m is the mass of the getter, which is found by weighting, η is the theoretical density of getter material, S is the area of the getter basis and H is its height (see Table 2).
Films (II) can also be considered as consisting of the same structural units as powders. Monolithic films produced by methods of PVD or CVD [30] - [32] are similar to expanded lamina taking the entire surface of the substrate. Films with columnar structure produced by sputter deposition [31] [33] [34] can be presented as whiskers, oriented along the normal to the substrate, which are tightly pressed up to each other from the from the side surface. Porous films produced by diffusion deposition [35] or by sintering [36] have approximately the same structure as close-packed spheres, which are held together by diffusion necks in the points of contacts between the particles.
Finally, bulk porous bodies (III) can be reduced to powders. Thus, sintered powder materials [21] do not differ much from the usual powder mass if their average particle sizes and the values of ε coincide. The same can be said about the porous materials with the structure of a dendritic carcass [7] [27] the peculiarity of which is only that their structural analog is particles of whisker type.
The structure of any of the modern getters can be described with the help of powder particles of the simplest shape, which makes solving the calculation problem easier by reducing it to the dependence on one value, which is the parameter d (the minimal linear size of the powder particle). At the same time it should be mentioned that the effect of the parameter d on the sorption behavior of the getter material depends on the sorption mechanism.
In this way, it is easy to show that adsorbents, the sorption capacity of which is determined by the specific surface area, have the value of Q* ~ aε /d, where a ≈ 4.5, if a powder mixture is used as a getter. That is, here d serves as a measure of the ultimate sorption capacity of a gas sorbent. Another matter is absorbents and reactants, where the process is determined by mass transfer in the volume of the material. Here parameter d (or d/2) acquires the meaning of the typical sorption size [9] , which is either equal to the distance, which the gas atoms have to overcome while dissolving inside the crystal, or equal to the path which the interface boundary MeX/Me passes during sorption of gases by reactive metals.
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