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Reactive getters for MEMS applications
Vacuum 123 (2016) 42e48
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Reactive getters for MEMS applications K. Chuntonov a, *, J. Setina b a b
NanoShell Consulting, Nitzanim 9, 2351305, Migdal Haemek, Israel Institute of Metals and Technology, Lepi pot 11, SI-1000, Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2015 Received in revised form 7 October 2015 Accepted 8 October 2015 Available online xxx
The vacuum problem in MEMS devices has not been solved yet. The attempts of many years to support the reliability of these devices using getter films based on transition metals did not lead to success. Getters and the standard bonding process appeared to be incompatible due to strong volume outgassing of all materials within the cavity during their heating. The way out in this situation is seen in lowering the temperature of the sealing process and replacing the traditional getters with the getters of the more reactive nature, the sorption capacity of which at room temperature is higher by orders of magnitude. This becomes possible now due to the development of low temperature sealing materials and new packaging machines, which operate without any heating of the bonding zone. The purpose of this paper is to show that the combination of the new packaging technologies and reactive getters will allow increasing the lifetime of vacuum package MEMS devices to 20 years by using simple engineering solutions. © 2015 Elsevier Ltd. All rights reserved.
Keywords: MEMS Vacuum cavity Reactive getters Getter housings Long lifetime
1. Introduction Vacuum package MEMS devices are capable of functioning during a long time under the condition that their encapsulated microchamber is reliably insulated from the environment and contains an effective getter with high sorption capacity. The experience shows, however, that the integration of a getter material into a vacuum hermetic package while preserving the sorption potential of the getter is not an easy task. Problems here appear from both sides, from the side of the sealing process and from the side of the getter. The sealing processes, which are now widely used in MEMS industry, provide high hermiticity of the package but this is achieved due to heating to 300 С and higher. Examples of such techniques can be anodic bonding, fusion bonding, eutectic bonding, glass frit bonding, etc. [1,2]. Their disadvantages are well known and all of them are connected with the use of heating; these are high production costs, thermal stress, and incompatibility of the bonding process with the procedure of activating the getter material [3]. This incompatibility should be understood in the sense that the manufacturing equipment, which combines encapsulation of the
* Corresponding author. E-mail address: [email protected] (K. Chuntonov). http://dx.doi.org/10.1016/j.vacuum.2015.10.012 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
vacuum cavity and activation of the getter into one thermal procedure, is not capable of supporting long term performance of MEMS devices. Intensive volume outgassing of all components of the cavity at the temperatures of the bonding process poisons the getter, which is activated at heating and is rapidly saturated with the released gases. This situation became one of the reasons behind the search of low temperature sealing materials as well as for attempts of making new packaging machines, where the bonding process and the activation of the getter would be separated both in space and time. The significant event in the recent years was the appearance of several new solutions suggesting both low temperature materials [4e8] and room temperature hermetic/vacuum packaging for MEMS [9e13]. The new sealing materials require heating only to 150 ± 30 С, which is lower than the temperatures of intensive volume outgassing. However, in the technologies, where the getter receives the thermal energy for its activation from the sealing process, the employment of the getters on the basis of transition metals becomes impossible as for their activation heating to the temperature not lower than 300 С is necessary. The second group of solutions is based on the different principles and is more ambitious. In Refs. [12,13] jointed surfaces are preliminarily cleaned by atom or ion beams in vacuum, after which they are brought into direct contact for bonding due to dangling bond. In technologies by Stellarray Inc [9e11]. a method of compression bonded hermetic packaging is used, which consists in
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the cold weld or the swage sealing process, where the sealing material is a metal gasket. In these machines separate chambers are provided for activation of the getter, which eliminates any limitations concerning activation temperatures. At the same time the requirement of the high sorption capacity remains in force, so the attempts to prolong the lifetime of MEMS devices with the help of the traditional getters do not make sense because their sorption capacity at normal temperature is extremely low [3]. So, if the goal for increasing the lifetime of vacuum package MEMS devices to ~20 years is to be fulfilled, as was planned in the beginning of the 2000-ies, this can be achieved only with the involvement of reactive getters, the material basis for which are alkaline and/or alkali-earth metals. These metals do not require activation and their sorption capacity at room temperature is unachievably high for other metals [3,14e17]. The advantages of reactive metal films were recognized already at the dawn of the getter technologies [18e22]. Reactive getters always, whenever the proper efforts for that were made, managed to take the dominant position in the vacuum devices of their time. Lamps, CRT, Dewar vessels, solar collectors [23e27], etc. can be mentioned as examples of this kind of applications, in which Ba films produced in-situ with the aid of RF heating were used and are still being used for vacuum maintenance. However, the radical miniaturization of vacuum chambers in the newest opto- and microelectronic devices lead to strict temperature limitation, which made previous methods of producing the reactive getter film impossible. The present paper provides the results of the measurements of the sorption properties of reactive alloys which are intended particularly for sealed-off vacuum chambers. Making the change from films made of a pure metal to the films on the basis of alloys appeared to be a compulsory measure. In miniature devices with the deficiency in space it is possible to introduce the reactive material without loses into a vacuum cavity and to structure it there in an optimal way only if we deal with alloys and not with pure metals. The substitution of this kind allows integrating the reactive metal into the cavity due to the operations, which are possible only by using phase mixtures of different composition. In our opinion, taking into account the above said about reactive getters and their compatibility with the new packaging technologies, all the conditions are now present to start the practical development of MEMS devices with the life span of about 20 years. 2. Experimental methods and first results The alloys were prepared in cylindrical steel crucibles of SS 304 from the metals purchased from Alfa Aesar of the purity: Ba-99.9%, Ga-99.999%, In-99.999% and Li-99.9%. The mixture of the components was slowly heated in high vacuum to ~300 С, exposed for 1 h and then the temperature was raised again till complete melting of all the products of mixing in the atmosphere of Ar under the pressure P ¼ 10 PMe, where P Me is equilibrium vapor pressure of the most volatile component of the alloy. Usually for homogenization of the melt the ingot was re-melted two-three times, keeping track of the reactions taking place in it and of phase transitions with the help of DTA recording. Three ingots were produced, each of the volume of 30 cm3; two of them answered intermetallic compounds of Ba8Ga7 and LiGa and the third one eutectics Ba-20 at% In. The measurements of the sorption properties were made on powder samples with an adapted gas-sorption method [28], which is a modification of the standard dynamic sorption method [29], and which was developed for testing getters intended for the usage in MEMS devices. In the given research one of the constituents of the measurement system, namely, the preparation chamber, was substituted with a milling machine or a disintegrator (Fig. 1), which
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Fig. 1. Milling chamber.
allowed producing reactive powder inside the system by milling a monolithic ingot in controlled atmosphere, i.e. in vacuum, or in the medium of inert, or studied gas [30e32]. This kind of organization of the process sufficiently widens instrumental possibilities of the measuring equipment and also presents interest in the application respect. In the “mechanochemical” regime, when the measurements are performed simultaneously with milling of the ingot, the sorption activity of the powder is extremely high due to dynamic excitations. This super reactivity of as-made powders can be used both in research, e.g., in the study of the nature of mechanochemical reactions in systems gas/metal, and in industry, e.g., in the production of ultra pure gases. In the “delayed” regime, when the time interval Dt is set between the production of the powder and the sorption measurements, there is a possibility to define experimentally relaxation time of certain sorption factors and distinguish between their contributions by performing the measurements at different Dt. Fig. 2 shows experimental sorption curves of the powders of Ba20 at% In obtained at room temperature for three gases, N2, CO2 and H2. In the case of sorption of nitrogen the getter mass was 0.12 g, the gas flow was 8 106 mbar L/s and the gas pressure above the getter powder was from 4 104 mbar to 2.5 102 mbar. The measurements were started 24 h after the preparation of the powder, i.e. Dt ¼ 24 h. For the sorption tests with CO2 and H2 the same powder with the mass of 0.25 g was used. At CO2 sorption the gas flow was equal
Fig. 2. Gas sorption by powders of eutectic alloy Ba0.8In0.2 at room temperature.
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to 3.5 105 mbar L/s, the gas pressure above the getter was from 1.5 105 mbar to 4.5 104 mbar and Dt ¼ 24 h. Then the powder was pumped at 2 108 mbar at room temperature during 72 h, heated to 90 ± 50С for 20 h, exposed at room temperature for 48 h and then tested with H2. The conditions for measurements of hydrogen sorption were: the gas flow was 6.7 105 mbar L/s, the gas pressure above the getter was from 1.2 104 mbar to 1.35 104 mbar and Dt 170 h. One more group of sorption data is shown in Fig. 3, which contains the sorption test results which were also obtained at room temperature [33]. Here curve 1 describes nitrogen sorption by such well known high porous getter material as St.122, curve 2 describes nitrogen sorption by powders of Ba8Ga7, curve 3 describes nitrogen sorption by powders of LiGa, and curve 4 describes oxygen sorption by powders of Ba8Ga7. Although in this series of measurements the main task was to get information about the gettering properties of reactive alloys there was also an attempt to find the influence of the parameter Dt on sorption behavior of the getter material. This possibility appears at the comparison of curves 2 and 3: the powders of Ba8Ga7 of the mass of 0.035 g were exposed under high vacuum for about two weeks (Dt x 340 h) before the beginning of the measurements, while the LiGa powders of the mass of 0.020 g were produced directly before the measurements (Dt / 0).
3. Discussion of the results Our analysis is based on the natural classification of getter materials built up with the differentiation of these materials according to the sorption mechanism [3]. In the essence it is an indirect recital of the known empirical laws formulated earlier for the systems gas/ metal [34e37]. Using this approach each getter class can be related to its kinetic law Q ¼ Q(t), where Q is the quantity of gas sorbed by a unit surface area of metal by the moment of time t (Fig. 4). In fact, interaction of gases with transition metals like Ti, V, Zr, Fe, Ni, etc. (curve 1) at room temperature completely stops after the saturation of the surface with active gases at t ¼ tp. The ultimate value of the capacity Q*1 ¼ k1hpx1, where k1 is a conversion factor and x1 is the concentration of gases in the passivated layer with the thickness hp, corresponds to this state. On the opposite, reactive metals follow the parabolic law Q3 ¼ k3t1/2 (curve 3) or the linear law Q4 ¼ k4t (curve 4) continuously sorbing gases at any t by growing on their surface of a layer of compounds MeX according to the reaction Me þ X ¼ MeX, where Me is metal and X is gas (k3 and k4 are the rate constants).
Fig. 3. Results of sorption tests for porous getter St.122, powders of Ba8Ga7 and LiGa:1 e sorption of nitrogen by St.122, 2 e sorption of nitrogen by powders of Ba8Ga7, 3 e sorption of nitrogen by powders of LiGa, 4 e sorption of oxygen by powders of Ba8Ga7.
Fig. 4. Classification of getters according to kinetic law Q(t): 1 e passivating metals, 2 e metals - absorbents, 3 and 4 e reactive metals with parabolic and linear growth of product accordingly.
Curve 2 describes one more sorption mechanism, when gases are dissolved in the volume of getter material asymptotically nearing with time the value of Q*2 ¼ k1h (Kp)1/2, where h is the film thickness, K is an equilibrium constant, and p is the partial pressure of gas molecules X2 above the getter. This is the case of sorption of hydrogen by transition metals at room temperature or sorption of other gases at heating of the metal. The reason for the failure of the getter film during the volume outgassing lies exactly here, when the gases released from all the heated materials of the cavity are immediately sorbed by the getter according to curve 2. Taking into consideration that Q*3 ¼ Q*4 ¼ k1hx3, where x3 is concentration of gas in MeX, and h is the film thickness and also having in mind that x1 x x3, hp ≪ h and that the ultimate solubility of gas in the metal at Troom is much lower than concentration of gases in the compound MeX, we come finally to the correlation Q*4 ¼ Q*3 > Q*2 >> Q*1. The given correlation, which is the direct consequence of the abovementioned laws about the interaction of gases with metals [34e37], shows that at room temperature the sorption capacity of reactive metals (Q*3 and Q*4) is by orders of magnitude higher than the sorption capacity of transition metals (Q*1) and sufficiently higher than that of the latter even after their heating (Q*2). This means that there is no alternative to reactive getters in those applications, where the definitive requirement is high sorption capacity of the getter material.
3.1. Sorption capacity of reactive powders Let us now compare the sorption properties of reactive powders with the properties of the current getter product. Powders of transition metals or porous sintered powder materials, the particle size of which is close to that of the reactive powders, can serve as the structural analog of reactive powders. The experimental data for both of them is shown in Figs. 5 and 6. Fig. 5 shows the curves for sorption of nitrogen, one of the most difficult for removal gases. It is seen that at room temperature the sorption capacity of the high porous getter St.122, which was preliminarily activated at 450 C in vacuum 107 mbar for 1 h, is approximately by 20 times lower than of LiGa powder and by 500 times lower than of Ba-20 at% In powder (see also Figs. 2 and 3). In these measurements the size of reactive powders was in the range from ~50 to 250 microns; the getter mass in each case was 0.12 g, while the film double e sided getter St.122 [38] had powder mass of 0.24 g. Sorption properties of powders St.707 [39] are compared with the properties of barium alloys in Fig. 6. Oxygen and carbon oxides,
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Fig. 5. Sorption of nitrogen at room temperature. Getter St.122 activated at 450 C in vacuum 107mbar for 1 h; powder of LiGa tested immediately after milling; powder of Ba0.8In0.2 tested 24 h after milling.
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For reactive particles with the size of 50e250 microns the thickness of the reacted layer according to the calculations varied in the range of 0.5e5.0 microns. From here we conclude that at room temperature the sorption potential of reactive getters in the media of residual gases can be used to the end under the condition that the typical size of the sorption material will not exceed 5 microns. In other words, in the case of powders of needle or pseudospherical shape the particle diameter should not be larger than 10 microns; and in the case of a continuous film on a substrate its maximal thickness should not be larger than 5 microns if we want the metallic residual to become negligibly small by the moment when sorption rate goes down to 0.001 (L/s)/cm2 (Figs. 2 and 3). The above said is of fundamental importance for the future small sealed-off vacuum chambers. Reactive metals and alloys opposite to other gas sorbents capture gases at room temperature not only with their surface but also by way of chemical reactions involving the entire volume of the getter material into the sorption process. This provides reactive getters with a great technical and economical advantage. However, the sorption capacity of the getter material is not the only factor determining the lifetime of the vacuum device; an important role in this respect belongs to sorption kinetics. Sorption rate during the planned life time should not go below the rate of gas leakage from outside [3,41]. Besides, at high sorption activity of the getter material the problem of peeling off of the products of reactions with gases inevitably appears. These issues need separate consideration. 3.2. Estimation of the life time of vacuum cavity The pressure of the residual gases p inside the vacuum cavity (Fig. 7) changes with time according to the correlation of two values, the gas sorption rate J ¼ dQ/dt and the gas leakage rate, which is proportional to the difference of their pressure P0 e p outside and inside the cavity. The value p can be calculated from the equation of the material balance
Fig. 6. Sorption of oxygen and carbon oxides at room temperature. Powder St.707 activated at 500 C for 10 min, test gas CO [39]; powder Ba8Ga7 tested 340 h after milling, test gas O2; powder Ba0.8In0.2 tested 24 h after milling, test gas CO2.
the sorption behavior of which is so similar that they are exchangeable in this kind of tests, were used as test gases. The mass of the getter powder St.707 was 0.2 g with the particle size 40e128 microns [39]; the mass of reactive powders was 0.25 g and the particle size was in the range of 50e250 microns, the same as in the case with nitrogen. As is seen from Fig. 6, the specific sorption capacity of the powders Ba8Ga7 and Ba-20 at% In towards oxygen containing gases is by two orders of magnitude higher than that of the getters on the basis of transition metals. Earlier the same advantage of reactive alloys over the traditional getter material was demonstrated with the participation of dendritic granules Ca3In and Sr2Sn [40]. That is, the obtained result is not singular and was expected, but it is not the final result. While transition metals after tens of years of intensive development of getter technologies have reached their potential, for reactive metals everything is only starting. The quantitative analysis of the above given data shows that in the case of nitrogen (Fig. 5) only 2e4% of the powder mass reacted by the end of the measurements; and in the case of oxygen containing gases (Fig. 6) 6e10% accordingly. That is, sorption capacity of a reactive material can be increased not less than by an order of magnitude and thus reach 1000-fold superiority over the getters produced from transition metals.
V dp K ¼ zrpc þ Z l ðPo pÞ; RT dt Dd
(1)
where V is the inside volume of the cavity, Z is the area of the inner surface of the cavity, t is time, Dd is the thickness of the cavity wall, z is the surface area of the getter film, Kl is the coefficient of transparency of the wall, c is the concentration of the reactive component in the product layer MeX on the boundary with gas X, r is the coefficient of a chemical reaction equal to ayx (RT/2pM)1/2, a is a sticking coefficient, nx is partial molar volume of a gaseous constituent in the layer MeX, R is a gas constant, T is temperature, M is gas molar mass. The analytical solution of the equation (1) in the dimensionless form has a form [42] of
rðtÞ ¼
1
G G t expðHUtÞ þ 1þ ; HU HU U
(2)
Fig. 7. Cavity of MEMS device: 1 e bottom, 2 e device, 3 e seal, 4 e getter film, 5 e cap, 6 e inside space, P0 e outside pressure, p e inside pressure.
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where t ¼ tD/L2, r(t) ¼ p(t)/P0, G ¼ L2SKlRT/VDd, H ¼ rLs/D, U ¼ LzRT/VP0, D is a coefficient of diffusion of Me in MeX, L is the thickness of the getter film, S ¼ S/L, S is the thickness of the layer of products MeX, s is the concentration of gas in the medium above the layer MeX, 4 is the concentration of gas in the layer MeX. The results of the calculation of the function r(t) made for the case of small chambers with the assumption that p (0) ¼ P0 and HU > G are given in Fig. 8. The physical meaning of the conditions mentioned is simple: the first one means that the cavity with the getter are sealed at atmospheric pressure without preliminary pumping down and the second one means that the pumping effect of the getter (parameters H and U) prevails over the leakage (parameter G). According to the calculations the gas pressure r(t) in a hermetically closed chamber due to the getter rapidly goes down practically to zero and then stabilizes at this level (Fig. 8). However, with time the gas pressure will very slowly raise due to the second summand in equation (2). A large number of experimental curves p(t) of the same form as the curves r(t) in Fig. 8 are given in Refs. [43,44]. They were obtained in the process of measuring the pressure of different atmospheric gases after their entry to the vacuum vessel which was filled with particles of barium alloys. Equation (2) allows calculating the lifetime for each particular vacuum cavity having been given the maximal value of the pressure p, at which the device is still able to function. These calculations made for the case when V ¼ 0.1 cm3, 1015 m2/s Kl 1012 m2/s, L ¼ 10 micron and pmax ¼ 102 mbar predict lifetime of 20 years if the volume of the getter material is equal to approximately 0.5% of the inner volume of the cavity [42]. At this according to the initial conditions the getter at the first stage of the process captures all the active gases in the cavity, thus fulfilling the work of the outside pump, and then maintains vacuum in the cavity by removing the leaking gases. The scheme of the calculations used above was to show the readiness to the worst variant, when due to refusal from the standard bonding processes the getter had to spend part of its sorption potential for the creation of the initial vacuum. However, the emergence of the new packaging machines relieves the getter from the functions of the outside pump and this only strengthens the conclusion about the ability of reactive getters to provide a long lifetime of a vacuum device under the discussed conditions. Let us also pay attention to the fact that the calculated thickness of the getter film L ¼ 10 micron appeared to be close to the recommended value of 5 micron, which we found experimentally (see chapter 3.1).
Why the preference is given to barium and not to other reactive metals can be understood from Fig. 9, where the calculation results for thermodynamic equilibriums in systems Ba e H e O e C e N obtained with the aid of the program FACT-SAGE [46] are given. No other reactive metal provides such low pressure of residual gases at room or close to room temperatures as barium does. Besides, barium as a gas sorbent is universal, it reacts with all active and low activity gases while lithium, for example, does not sorb hydrogen at room temperature, calcium and magnesium do not sorb nitrogen, etc. So, two getter solutions are suggested:- binary films with activation at ~150 С intended for use both with low temperature sealing materials and with room temperature packaging machines; - porous bulk bodies with activation at ~250 С intended for use in room temperature packaging technologies. Film getters. Getter films of the compositions Ba e (35 ± 5) at % Mg and Ba e (28.5 ± 2.5) at % Al with a cover layer of Al or Mg correspondingly are manufactured by multiple alternate deposition of thin layers of Ba/Al/Ba/Al … Ba/Al or Ba/Mg/Ba/Mg … Ba/Mg onto a hot substrate at 200 ± 50 С, where the last layer of Al or Mg is deposited at a temperature lower than 0 С to create a protective layer [47]. The ability of Al and Mg to self passivation was already used in the past for protection of barium from the air [20]. The idea of synthesizing chemically homogeneous getter films in the process of deposition of several components onto the heated substrate is taken from the technology of production of photocathodes of Cs3Sb type [48]. According to numerous experimental data on sintering of disperse materials, for consolidation and homogenization of powders or multilayer films it is enough to heat them to Tammann temperature Т T ¼ 0.45Т f, where Т f is the melting temperature of the metallic material in Kelvin [36]. In the case of BaeAl and BaeMg getter films the value of Т T is close to ~150 C, which allows obtaining of equilibrium getter film at the above mentioned temperature and further, after inserting the film into the working place in the cavity, to get rid of the cover layer during the sealing process at the same 150 C. During the activation the lower, metal, sublayers of the cover layer diffuse into eutectics and the upper oxide film of the cover layer, Al2O3 or MgO, which was formed at the contact of the getter with the air at the stage of packaging, loses its protective functions. This happens due to the misfit of crystal lattices: directly under the oxide film Al2O3 eutectic Al5Ba4 þ Ba appears instead of Al and under the oxide film MgO the eutectic BaMg2 þ Ba appears instead of Mg.
3.3. Two getter types for MEMS The replacement of barium with its alloys allows developing once again a mass getter product similar to what Ba e EG had been for tens of years [23,45] in the time of television CRTs, and to what binary alloys BaeMg and BaeAl or ternary alloys BaeMgeNa can become in application to MEMS.
Fig. 8. Calculated dependence of r on t.
Fig. 9. Partial pressure Pj of some vapors and gases over solid Ba.
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Eutectic films BaeAl and BaeMg are manufactured with the help of simple methods of thermal deposition of components in vacuum. High volatility of these components, especially of Ba and Mg, simplifies the task of production of a getter film with the thickness of about 5 micron. Micro ingots of the intermetallic compound BaGa2, which easily withstand a short contact with air [49,50], showed good results as barium vapor sources. Porous bulk bodies. Reactive 3D getters with end-to-end micro channels [50] can become competitors of reactive getter films when there is deficiency of free surface in a vacuum cavity. It is a special variation of porous materials with the structure of a dendritic carcass obtained by quenching of metallic droplets during their movement in the cooling medium [40,51] or during their casting into metallic moulds [50] succeeded by vacuum evaporation of the volatile component, which leads to the formation of porosity. Fig. 10 shows a mould with ingot 1 placed inside getter housing 2, which is part of vacuum cavity 3. The wall of the getter housing is made of porous sintered material, which allows gases but entraps small solid particles. The activation of an ingot having a composition (BaMg2)1-xNax, where 0.10 x 0.15, takes place in a specialized vacuum chamber foreseen in new packaging machines for thermal treatment of getters (see e.g. Ref. [13]). The mould with the ingot is heated to ~250 С for evaporation of Na. Na evaporates from the ingot leaving behind voids (interdendritic spaces), which provide gases with the access to bared intermetallic carcass BaMg2. A thin oxide layer on the surface of the ingot formed at the stage of introduction of the mould into the chamber does not hinder evaporation of sodium but makes its contribution in sorption of moisture. After activation of the getter, lid 4 together with getter housing 2 is transported through the sluice system to the next vacuum chamber, where at room temperature lid 4 is hermetically connected with the lower part of cavity 6 using room temperature sealing process. The assembled in this way device will withstand leakage of the outside gases for ~20 years. At that the volume of a reactive ingot should be not less than 0.005 V, where V is the volume of the cavity and the rate of quenching the droplets in the production of the ingots should be dT/dt 40 K/s. The first of these two conditions is responsible for the sorption capacity of the getter material and the second one is responsible for sorption kinetics. As is known, dendrite arm spacing ds and the rate of cooling the droplets at quenching have a correlation dT/dt a2/ d2s , where a is a proportionality coefficient [52,53]. Identifying the typical sorption size of a getter body having a structure of a dendritic carcass with the value ds/2 and taking the latter equal to 5 micron in accordance with chapter 3.1 we come to the above mentioned condition for the quenching rate. For metallic droplets of the radius of several hundreds of microns the cooling rate of
Fig. 10. Vacuum cavity with bulk porous getter: 1 e mould, 2 e getter housing, 3 e cavity, 4 e lid, 5 e dendritic carcass BaMg2, 6 e bottom, 7 e sensor.
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about 40 K/s is typical for quenching in gas medium but it can be many-fold increased at quenching into metallic mould [54]. 3.4. Porous receptacle Barium alloys in the form of continuous films or permeable for gases at room temperature bulk bodies are able to spend their entire sorption potential for chemical capturing of residual gases with sufficient for the practical needs rate provided that the typical sorption size of such getter is close to 5 microns. Under the typical size we understand here, as can be seen from the above said, the way of the reactive front, which it passes by the moment of completing of the sorption process. In the case of the films it is such geometrical value as the thickness of the film L; in the case of a porous body with the structure of a dendritic carcass it is a radius of a dendrite arm ds/2. So, in the light of the above said it would be difficult to disagree that at the present moment favorable conditions have been formed for the attempts of creation of the first samples of vacuum package MEMS devices having life spans of tens of years. The prerequisites of the success in such an undertaking are new getters on the basis of reactive metals and new generation of packaging equipment allowing hermetization of a vacuum cavity without the heating, which destroys or sufficiently weakens the sorption potential of getter material. The only unsolved issue of a technical character restraining immediate realization of a project of this kind is connected with the problem of peeling off of any getter materials. If a reaction between the getter material and gases runs to completion its end product will be powder, the particles of which possess all the degrees of freedom. Since unrestricted spread of solid products of the reaction MeX inside the cavity is unacceptable an additional design element is required, which would separate the getter from the rest part of the cavity but would allow residual gases to the getter. This element is well known under the name of a receptacle, partition or getter housing; it is already used in the production of MEMS devices [55e57]. That is, the problem of peeling off from getter films and from porous getter bodies is not new; moreover, there are solutions when the sorption material is from the beginning introduces in a form of powder, which is separated from the sensors by a porous partition [57]. In general the formation of small solid particles and their coming off are the natural consequence of the sorption work of any getter material independent of its shape or nature. On the other hand, the amount of the powder produced by the getter is proportional to the intensity of the sorption process: the larger the share of the used getter material the more free powder particles are formed. For this reason a getter housing, with a porous wall permeable for gases but retaining solid particles, should become an essential member of any vacuum MEMS device claiming for extended lifetime. Conversely, the absence of such getter housing clearly indicates the absence of any noticeable contribution from the getter in the increase of reliability of a MEMS device. As far as in MEMS packaging technologies porous getter housings are a known and mastered way of keeping solid particles in a designated for them restricted space [55e59], there are no reasons to expect any complications connected in particular with barium alloys. The choice of materials for a porous wall is rater wide: these can be sintered metal powders, ceramics, glass; and the temperature of heating of the cavity at activation of the getter does not exceed 150 C for binary getter films and 250 C for micro ingots of ternary alloy BaeMgeNa. In the first case the heating temperature is by 200 C lower than the melting point of the film and in the second case a metallic or ceramic partitions are chemically stable to Na vapor till 700 C and 500 C accordingly [60].
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There are good reasons to expect that in the nearest time if the results, which we have presented, stimulate interest from the side of MEMS manufacturers, practical activities can be started on manufacturing and testing the first model of a getter housing with a reactive micro ingot [50]. 4. Conclusion 1. Getters on the basis of reactive metals excel getters on the basis of transition metals in sorption capacity at room temperature by two orders of magnitude and have an additional potential for its further increase not less than by 10 times. 2. Getters on the basis of reactive metals can be produced in a form of films or porous bulk bodies with the activation temperature of ~150 C or ~250 C correspondingly. 3 The analysis of the situation in the field of MEMS technologies shows that the usage of reactive getters combined with low or room temperature sealing techniques and getter housings inside vacuum cavity will allow the creation of long living MEMS devices capable of working for tens of years. Acknowledgments The authors would like to thank Prof. G. Voronin, Moscow State University, for his assistance in the thermodynamic issues and Mr. Gary Douglass, Agile Chemistry, Inc., Elmhurst, USA for his help. References [1] V. Dragoi, E. Cakmak, E. Pabo, Metal wafer bonding for MEMS devices, Rom. J. Inf. Sci. Tech. 13 (1) (2010) 65e72. [2] B. Bhushan (Ed.), Handbook of Nanotechnology, third ed., Springer-Verlag, Berlin, 2010. [3] K. Chuntonov, S. Yatsenko, Recent Patents on Materials Science, Getter Films for Small Vacuum Chambers, 6, Bentham Science Publishers, 2013, pp. 29e39. Number 1. [4] R. Agarwal, Low Temperature Hermetically Sealed 3-D MEMS Device for Wireless Optical Communication, Dissertation, University of South Florida, 2007. [5] L. Yan, C. Lee, D. Yu, W.K. Choi, A. Yu, S.U. Yoon, J.H. Lau, A Hermetic Chip to Chip Bonding at Low Temperature with Cu/In/Sn/Cu Joint, IEEE ECTC, Florida USA, 2008, pp. 1844e1848. [6] H. Ishida, T. Ogashiwa, T. Yazaki, T. Ikoma, T. Nishimori, H. Kusamori, J. Mizuno, Low-temperature Wafer Bonding for MEMS Hermetic Packaging Using Sub-micron Au Particles, 3, Trasaction Jap. Inst. Electronics Packaging, 2010, pp. 62e67. No.1. [7] M. Antelius, Wafer-scale Vacuum and Liquid Packaging Concepts for an Optical Thin-film Gas Sensor, Doctoral Thesis, KTH Royal Institute of Technology, Stockholm, Sweden, 2013. [8] R. Straessle, Y. Petremand, D. Briand, M. Dadgras, N.F. de Rooij, J. Micromech. Microeng 23 (Number 7) (2013) 075007, 23. [9] Curtis N.P. Hermetic MEMS Package and Method of Manufacture. US Patent 7538106, 2008. [10] Curtis N.P. Cold Weld Hermetic Package and Method of Manufacture. US Patent 7576427, 2009. [11] Eaton M.F, Curtis N.P, and Miner A. Cold Weld Hermetic Package and Method of Manufacture. US Patent 8394679, 2013. [12] T. Itoh, H. Okada, H. Takagi, R. Maeda, T. Suga, Room temperature vacuum sealing using surface activated bonding method, in: The 12th Int. Conf. Solid State Sensors, Actuators and Microsystems, Boston, 2003, pp. 1828e1831. [13] Room Temperature Wafer Bonding Machine BOND MEISTER https://www. mhi-global.com/products/detail/wafer_bonding_machine.html [accessed 11.09.15]. [14] J.J.B. Fransen, H.J.R. Perdijk, The Adsorption of Gases by Barium Getter Films applied as a Tool, Vacuum 10 (1960) 199e203. [15] J.C. Turnbull, Barium, strontium, and calcium as getter in electron tubes, J.Vac. Sci. Technol. 14 (N1) (1977) 636e639. [16] B. Ferrario, Chemical pumping in vacuum technology, Vacuum 47 (1996) 363e370. [17] K. Chuntonov, J. Setina, New lithium gas sorbents: I. The evaporable variant, J. Alloys Compd 455 (2008) 489e496. [18] Holst G, Oosterhuis E. Improved Process for Removing Gas Residues and for Purifying Inert Gases in Electric Vacuum Tubes, Incandescent Lamps and the like. GB Patent 151611, 1921. [19] W. Espe, M. Knoll, M.P. Wilder, Getter Materials, Electronics (1950) 1e11. [20] Gabbrielli E. Vacuum Tube Getter Body Material. US Patent 2778485, 1957.
[21] M. Pirani, J. Yarwood, Principles of Vacuum Engineering, Reinhold Publishing Corp., New York, 1961. [22] H.J.R. Perdijk, A Compilation of Gases Reactions as Observed in Electron Tubes, Suppl. Nuovo Cimento 5 (1967) 73e92. [23] P. della Porta, Gettering an Integral Part of Vacuum Technology, Technical Paper TP 202, in: AVS 39th, National Symposium, Nov. 9-13, 1992. [24] M. Wutz, H. Adam, W. Walcher, K. Jousten, Handbuch Vakuumtechnik. Theorie und Praxis, Vieweg & Sohn, Braunschweig, 2000. [25] A. Giedraitis, S. Tamulevicius, R. Gudaitis, M. Andrulevicius, Kinetics of Growth and Sorption Properties of Evaporable Barium Getter Films, Mater. Sci. (Medziagotyra) 16 (N1) (2010) 12e23. [26] Horie H, Fukuda Y, Kato H, Nakashima N, and Makino Y. Getter Material and Evaporable Getter Device Using the Same, and Electron Tube. US Patent 7927167, 2011. [27] D. Mishra, N.K. Saikhedkar, A Study and Theoretical Analysis of Evacuated Tube Collectors as Solar Energy 37Conversion Device for Water Heating, ISSN(Print): 2349 e 1094, ISSN(Online): 23-1108, 1 (3) (2014) 30e39. [28] B. Erjavec, J. Setina, Investigations of a method for determining pumping speed and sorption capacity of nonevaporable getters based on in situ calibrated throughput, J. Vac. Sci. Technol. A 29 (2011), 051602e051611-10, 051602, http://dx.doi.org/10.1116/1.3626535. [29] ASTM Standard, Standard Practice for Determining Gettering Rate, Sorption Capacity, and Gas Content of Nonevaporable Getters in the Molecular Flow Region, 2002. F 798-97. [30] Chuntonov K. Sorption Pump with Mechanical Activation of Getter Material and Process for Capturing of Active Gases. US Patent Appl. 20130078113, 2013. [31] K. Chuntonov, M.K. Lee, Mechanochemical Sorption Apparatuses. Advanced Materials Research 875e877, Trans Tech Publications, Switzerland, 2014, pp. 1106e1110. [32] Chuntonov K. Sorption Apparatuses for the Production of Pure Gases. US Patent 9095805, 2015. [33] Chuntonov K, Setina J. Patent pending, 2015. [34] O. Kubaschewski, B.E. Hopkins, Oxydation of Metals and Alloys, Butterworths, London, 1962. [35] K. Hauffe, Reaktionen in und an festen Stoffen. 2. Auflagen, Springer, Berlin, 1966. [36] K. Meyer, Physikalisch-chemische Kristallographie, Verlag fur Grundstoffindustrie, Leipzig, 1968. [37] E. Fromm, E. Gebhardt, Gase und Kohlenstoff in Metallen, Springer-Verlag, Berlin, 1976. [38] Brochure SAES Getters: Solutions for Flat Panel Displays, 2004. [39] Toia L, and Boffito C. Non-evaporable Getter Alloys. US Patent 6521014, 2003. [40] Chuntonov K. Gas sorbents on the Basis of Intermetallic Compounds and a Method for Producing the Same. US Pat. Appl. 20060225817, 2006. [41] K. Chuntonov, A.O. Ivanov, D. Permikin, Getter films with a reactive component, Vacuum 85 (2011) 755e760. [42] K. Chuntonov, A.O. Ivanov, D. Permikin, New lithium gas sorbents: IV. Application to MEMS devices, J. Alloys Compd 471 (2009) 211e216. [43] Boffito C, and Schiabel A. Process for the Sorption of Residual Gas by Means of a Non-evaporated Barium Getter Alloy.US Patent 5312606, 1994. [44] Schiabel A, and Boffito C. Process for the Sorption of Residual Gas by Means by a Non-evaporated Barium Getter Alloy. US Patent 5312607, 1994. [45] J.M. Lafferty, Foundations of Vacuum Science and Technology, John Wiley & Sons, New York, 1998. [46] FactSage 5.4.1; www.factsage.com. [47] Chuntonov K. Lithium or Barium Based Film Getters. US Patent Appl. 20110217491, 2011. [48] A.H. Sommer, Photoemissive Materials, John Wiley & Sons, New York, 1968. [49] K.A. Chuntonov, A.O. Ivanov, Evaporators of Reactive Metals and Droplet Casting. Recent Patents on Materials Science, Bentham Sciece Publ. 6 (2013) 214e228. [50] Chuntonov K. Apparatus and Method for Droplet Casting of Reactive Alloys and Applications. US Patent Appl. 20140290897, 2014. [51] Chuntonov K. Safe Gas Sorbents with High Sorption Capacity on the Basis of Lithium Alloys. US Patent. 8529673. [52] S.Q. Armster, J.-P. Delplanque, W.-H.L. Lai, E.J. Lavernia, Monosize Droplet Deposition as a Means to Investigate Droplet Behavior during Spray Deposition, Metallurgical Mater. Trans. B 31B (2000) 1333e1344. [53] J.F. Seconde, M. Suery, Effect of solidification conditions on deformation behaviour of semi-solid Sn e Pb alloys, J. Mater. Sci. 19 (1984) 3995e4006. [54] E.J. Lavernia, J.D. Ayers, T.S. Srivatsan, Rapid solidification processing with specific application to aluminium alloys, Int. Mater. Rev. 37 (No.1) (1992) 1e44. [55] T. Aikiyo, T. Kimura, Y. Ikegami, Optical Device, Electronic Device Enclosure, and Getter Assembly, EP 1022781, 2000. [56] Seppala B, DCamp J, and Glenn M. Getter on die in an upper sense plate designed system. US Patent 7800190, 2010. [57] Wakelin S. System and Method for Gettering Gas-phase Contaminants within a Sealed Enclosure. US patent 7160368, 2007. [58] Boroson ML, Serbicki JP, Bessey PG, Irvin GC, Rowley LA, and Kaminsky CJ. Desiccants and Desiccant Packages for Highly Moisture-sensitive Electronic Devices. US Patent 6740145, 2004. [59] Chen C-H, Craid DM, and Schwinabart TD. Placement of Absorbing Material in a Semiconductor Device. US Patent 7045885, 2006. [60] H.U. Borgstedt, C.K. Mathees, Applied Chemistry of the Alkali Metals, Plenum Press, New York, 1987.
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Reactive getters for MEMS applications K. Chuntonov a, *, J. Setina b a b
NanoShell Consulting, Nitzanim 9, 2351305, Migdal Haemek, Israel Institute of Metals and Technology, Lepi pot 11, SI-1000, Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 August 2015 Received in revised form 7 October 2015 Accepted 8 October 2015 Available online xxx
The vacuum problem in MEMS devices has not been solved yet. The attempts of many years to support the reliability of these devices using getter films based on transition metals did not lead to success. Getters and the standard bonding process appeared to be incompatible due to strong volume outgassing of all materials within the cavity during their heating. The way out in this situation is seen in lowering the temperature of the sealing process and replacing the traditional getters with the getters of the more reactive nature, the sorption capacity of which at room temperature is higher by orders of magnitude. This becomes possible now due to the development of low temperature sealing materials and new packaging machines, which operate without any heating of the bonding zone. The purpose of this paper is to show that the combination of the new packaging technologies and reactive getters will allow increasing the lifetime of vacuum package MEMS devices to 20 years by using simple engineering solutions. © 2015 Elsevier Ltd. All rights reserved.
Keywords: MEMS Vacuum cavity Reactive getters Getter housings Long lifetime
1. Introduction Vacuum package MEMS devices are capable of functioning during a long time under the condition that their encapsulated microchamber is reliably insulated from the environment and contains an effective getter with high sorption capacity. The experience shows, however, that the integration of a getter material into a vacuum hermetic package while preserving the sorption potential of the getter is not an easy task. Problems here appear from both sides, from the side of the sealing process and from the side of the getter. The sealing processes, which are now widely used in MEMS industry, provide high hermiticity of the package but this is achieved due to heating to 300 С and higher. Examples of such techniques can be anodic bonding, fusion bonding, eutectic bonding, glass frit bonding, etc. [1,2]. Their disadvantages are well known and all of them are connected with the use of heating; these are high production costs, thermal stress, and incompatibility of the bonding process with the procedure of activating the getter material [3]. This incompatibility should be understood in the sense that the manufacturing equipment, which combines encapsulation of the
* Corresponding author. E-mail address: [email protected] (K. Chuntonov). http://dx.doi.org/10.1016/j.vacuum.2015.10.012 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
vacuum cavity and activation of the getter into one thermal procedure, is not capable of supporting long term performance of MEMS devices. Intensive volume outgassing of all components of the cavity at the temperatures of the bonding process poisons the getter, which is activated at heating and is rapidly saturated with the released gases. This situation became one of the reasons behind the search of low temperature sealing materials as well as for attempts of making new packaging machines, where the bonding process and the activation of the getter would be separated both in space and time. The significant event in the recent years was the appearance of several new solutions suggesting both low temperature materials [4e8] and room temperature hermetic/vacuum packaging for MEMS [9e13]. The new sealing materials require heating only to 150 ± 30 С, which is lower than the temperatures of intensive volume outgassing. However, in the technologies, where the getter receives the thermal energy for its activation from the sealing process, the employment of the getters on the basis of transition metals becomes impossible as for their activation heating to the temperature not lower than 300 С is necessary. The second group of solutions is based on the different principles and is more ambitious. In Refs. [12,13] jointed surfaces are preliminarily cleaned by atom or ion beams in vacuum, after which they are brought into direct contact for bonding due to dangling bond. In technologies by Stellarray Inc [9e11]. a method of compression bonded hermetic packaging is used, which consists in
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the cold weld or the swage sealing process, where the sealing material is a metal gasket. In these machines separate chambers are provided for activation of the getter, which eliminates any limitations concerning activation temperatures. At the same time the requirement of the high sorption capacity remains in force, so the attempts to prolong the lifetime of MEMS devices with the help of the traditional getters do not make sense because their sorption capacity at normal temperature is extremely low [3]. So, if the goal for increasing the lifetime of vacuum package MEMS devices to ~20 years is to be fulfilled, as was planned in the beginning of the 2000-ies, this can be achieved only with the involvement of reactive getters, the material basis for which are alkaline and/or alkali-earth metals. These metals do not require activation and their sorption capacity at room temperature is unachievably high for other metals [3,14e17]. The advantages of reactive metal films were recognized already at the dawn of the getter technologies [18e22]. Reactive getters always, whenever the proper efforts for that were made, managed to take the dominant position in the vacuum devices of their time. Lamps, CRT, Dewar vessels, solar collectors [23e27], etc. can be mentioned as examples of this kind of applications, in which Ba films produced in-situ with the aid of RF heating were used and are still being used for vacuum maintenance. However, the radical miniaturization of vacuum chambers in the newest opto- and microelectronic devices lead to strict temperature limitation, which made previous methods of producing the reactive getter film impossible. The present paper provides the results of the measurements of the sorption properties of reactive alloys which are intended particularly for sealed-off vacuum chambers. Making the change from films made of a pure metal to the films on the basis of alloys appeared to be a compulsory measure. In miniature devices with the deficiency in space it is possible to introduce the reactive material without loses into a vacuum cavity and to structure it there in an optimal way only if we deal with alloys and not with pure metals. The substitution of this kind allows integrating the reactive metal into the cavity due to the operations, which are possible only by using phase mixtures of different composition. In our opinion, taking into account the above said about reactive getters and their compatibility with the new packaging technologies, all the conditions are now present to start the practical development of MEMS devices with the life span of about 20 years. 2. Experimental methods and first results The alloys were prepared in cylindrical steel crucibles of SS 304 from the metals purchased from Alfa Aesar of the purity: Ba-99.9%, Ga-99.999%, In-99.999% and Li-99.9%. The mixture of the components was slowly heated in high vacuum to ~300 С, exposed for 1 h and then the temperature was raised again till complete melting of all the products of mixing in the atmosphere of Ar under the pressure P ¼ 10 PMe, where P Me is equilibrium vapor pressure of the most volatile component of the alloy. Usually for homogenization of the melt the ingot was re-melted two-three times, keeping track of the reactions taking place in it and of phase transitions with the help of DTA recording. Three ingots were produced, each of the volume of 30 cm3; two of them answered intermetallic compounds of Ba8Ga7 and LiGa and the third one eutectics Ba-20 at% In. The measurements of the sorption properties were made on powder samples with an adapted gas-sorption method [28], which is a modification of the standard dynamic sorption method [29], and which was developed for testing getters intended for the usage in MEMS devices. In the given research one of the constituents of the measurement system, namely, the preparation chamber, was substituted with a milling machine or a disintegrator (Fig. 1), which
43
Fig. 1. Milling chamber.
allowed producing reactive powder inside the system by milling a monolithic ingot in controlled atmosphere, i.e. in vacuum, or in the medium of inert, or studied gas [30e32]. This kind of organization of the process sufficiently widens instrumental possibilities of the measuring equipment and also presents interest in the application respect. In the “mechanochemical” regime, when the measurements are performed simultaneously with milling of the ingot, the sorption activity of the powder is extremely high due to dynamic excitations. This super reactivity of as-made powders can be used both in research, e.g., in the study of the nature of mechanochemical reactions in systems gas/metal, and in industry, e.g., in the production of ultra pure gases. In the “delayed” regime, when the time interval Dt is set between the production of the powder and the sorption measurements, there is a possibility to define experimentally relaxation time of certain sorption factors and distinguish between their contributions by performing the measurements at different Dt. Fig. 2 shows experimental sorption curves of the powders of Ba20 at% In obtained at room temperature for three gases, N2, CO2 and H2. In the case of sorption of nitrogen the getter mass was 0.12 g, the gas flow was 8 106 mbar L/s and the gas pressure above the getter powder was from 4 104 mbar to 2.5 102 mbar. The measurements were started 24 h after the preparation of the powder, i.e. Dt ¼ 24 h. For the sorption tests with CO2 and H2 the same powder with the mass of 0.25 g was used. At CO2 sorption the gas flow was equal
Fig. 2. Gas sorption by powders of eutectic alloy Ba0.8In0.2 at room temperature.
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to 3.5 105 mbar L/s, the gas pressure above the getter was from 1.5 105 mbar to 4.5 104 mbar and Dt ¼ 24 h. Then the powder was pumped at 2 108 mbar at room temperature during 72 h, heated to 90 ± 50С for 20 h, exposed at room temperature for 48 h and then tested with H2. The conditions for measurements of hydrogen sorption were: the gas flow was 6.7 105 mbar L/s, the gas pressure above the getter was from 1.2 104 mbar to 1.35 104 mbar and Dt 170 h. One more group of sorption data is shown in Fig. 3, which contains the sorption test results which were also obtained at room temperature [33]. Here curve 1 describes nitrogen sorption by such well known high porous getter material as St.122, curve 2 describes nitrogen sorption by powders of Ba8Ga7, curve 3 describes nitrogen sorption by powders of LiGa, and curve 4 describes oxygen sorption by powders of Ba8Ga7. Although in this series of measurements the main task was to get information about the gettering properties of reactive alloys there was also an attempt to find the influence of the parameter Dt on sorption behavior of the getter material. This possibility appears at the comparison of curves 2 and 3: the powders of Ba8Ga7 of the mass of 0.035 g were exposed under high vacuum for about two weeks (Dt x 340 h) before the beginning of the measurements, while the LiGa powders of the mass of 0.020 g were produced directly before the measurements (Dt / 0).
3. Discussion of the results Our analysis is based on the natural classification of getter materials built up with the differentiation of these materials according to the sorption mechanism [3]. In the essence it is an indirect recital of the known empirical laws formulated earlier for the systems gas/ metal [34e37]. Using this approach each getter class can be related to its kinetic law Q ¼ Q(t), where Q is the quantity of gas sorbed by a unit surface area of metal by the moment of time t (Fig. 4). In fact, interaction of gases with transition metals like Ti, V, Zr, Fe, Ni, etc. (curve 1) at room temperature completely stops after the saturation of the surface with active gases at t ¼ tp. The ultimate value of the capacity Q*1 ¼ k1hpx1, where k1 is a conversion factor and x1 is the concentration of gases in the passivated layer with the thickness hp, corresponds to this state. On the opposite, reactive metals follow the parabolic law Q3 ¼ k3t1/2 (curve 3) or the linear law Q4 ¼ k4t (curve 4) continuously sorbing gases at any t by growing on their surface of a layer of compounds MeX according to the reaction Me þ X ¼ MeX, where Me is metal and X is gas (k3 and k4 are the rate constants).
Fig. 3. Results of sorption tests for porous getter St.122, powders of Ba8Ga7 and LiGa:1 e sorption of nitrogen by St.122, 2 e sorption of nitrogen by powders of Ba8Ga7, 3 e sorption of nitrogen by powders of LiGa, 4 e sorption of oxygen by powders of Ba8Ga7.
Fig. 4. Classification of getters according to kinetic law Q(t): 1 e passivating metals, 2 e metals - absorbents, 3 and 4 e reactive metals with parabolic and linear growth of product accordingly.
Curve 2 describes one more sorption mechanism, when gases are dissolved in the volume of getter material asymptotically nearing with time the value of Q*2 ¼ k1h (Kp)1/2, where h is the film thickness, K is an equilibrium constant, and p is the partial pressure of gas molecules X2 above the getter. This is the case of sorption of hydrogen by transition metals at room temperature or sorption of other gases at heating of the metal. The reason for the failure of the getter film during the volume outgassing lies exactly here, when the gases released from all the heated materials of the cavity are immediately sorbed by the getter according to curve 2. Taking into consideration that Q*3 ¼ Q*4 ¼ k1hx3, where x3 is concentration of gas in MeX, and h is the film thickness and also having in mind that x1 x x3, hp ≪ h and that the ultimate solubility of gas in the metal at Troom is much lower than concentration of gases in the compound MeX, we come finally to the correlation Q*4 ¼ Q*3 > Q*2 >> Q*1. The given correlation, which is the direct consequence of the abovementioned laws about the interaction of gases with metals [34e37], shows that at room temperature the sorption capacity of reactive metals (Q*3 and Q*4) is by orders of magnitude higher than the sorption capacity of transition metals (Q*1) and sufficiently higher than that of the latter even after their heating (Q*2). This means that there is no alternative to reactive getters in those applications, where the definitive requirement is high sorption capacity of the getter material.
3.1. Sorption capacity of reactive powders Let us now compare the sorption properties of reactive powders with the properties of the current getter product. Powders of transition metals or porous sintered powder materials, the particle size of which is close to that of the reactive powders, can serve as the structural analog of reactive powders. The experimental data for both of them is shown in Figs. 5 and 6. Fig. 5 shows the curves for sorption of nitrogen, one of the most difficult for removal gases. It is seen that at room temperature the sorption capacity of the high porous getter St.122, which was preliminarily activated at 450 C in vacuum 107 mbar for 1 h, is approximately by 20 times lower than of LiGa powder and by 500 times lower than of Ba-20 at% In powder (see also Figs. 2 and 3). In these measurements the size of reactive powders was in the range from ~50 to 250 microns; the getter mass in each case was 0.12 g, while the film double e sided getter St.122 [38] had powder mass of 0.24 g. Sorption properties of powders St.707 [39] are compared with the properties of barium alloys in Fig. 6. Oxygen and carbon oxides,
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Fig. 5. Sorption of nitrogen at room temperature. Getter St.122 activated at 450 C in vacuum 107mbar for 1 h; powder of LiGa tested immediately after milling; powder of Ba0.8In0.2 tested 24 h after milling.
45
For reactive particles with the size of 50e250 microns the thickness of the reacted layer according to the calculations varied in the range of 0.5e5.0 microns. From here we conclude that at room temperature the sorption potential of reactive getters in the media of residual gases can be used to the end under the condition that the typical size of the sorption material will not exceed 5 microns. In other words, in the case of powders of needle or pseudospherical shape the particle diameter should not be larger than 10 microns; and in the case of a continuous film on a substrate its maximal thickness should not be larger than 5 microns if we want the metallic residual to become negligibly small by the moment when sorption rate goes down to 0.001 (L/s)/cm2 (Figs. 2 and 3). The above said is of fundamental importance for the future small sealed-off vacuum chambers. Reactive metals and alloys opposite to other gas sorbents capture gases at room temperature not only with their surface but also by way of chemical reactions involving the entire volume of the getter material into the sorption process. This provides reactive getters with a great technical and economical advantage. However, the sorption capacity of the getter material is not the only factor determining the lifetime of the vacuum device; an important role in this respect belongs to sorption kinetics. Sorption rate during the planned life time should not go below the rate of gas leakage from outside [3,41]. Besides, at high sorption activity of the getter material the problem of peeling off of the products of reactions with gases inevitably appears. These issues need separate consideration. 3.2. Estimation of the life time of vacuum cavity The pressure of the residual gases p inside the vacuum cavity (Fig. 7) changes with time according to the correlation of two values, the gas sorption rate J ¼ dQ/dt and the gas leakage rate, which is proportional to the difference of their pressure P0 e p outside and inside the cavity. The value p can be calculated from the equation of the material balance
Fig. 6. Sorption of oxygen and carbon oxides at room temperature. Powder St.707 activated at 500 C for 10 min, test gas CO [39]; powder Ba8Ga7 tested 340 h after milling, test gas O2; powder Ba0.8In0.2 tested 24 h after milling, test gas CO2.
the sorption behavior of which is so similar that they are exchangeable in this kind of tests, were used as test gases. The mass of the getter powder St.707 was 0.2 g with the particle size 40e128 microns [39]; the mass of reactive powders was 0.25 g and the particle size was in the range of 50e250 microns, the same as in the case with nitrogen. As is seen from Fig. 6, the specific sorption capacity of the powders Ba8Ga7 and Ba-20 at% In towards oxygen containing gases is by two orders of magnitude higher than that of the getters on the basis of transition metals. Earlier the same advantage of reactive alloys over the traditional getter material was demonstrated with the participation of dendritic granules Ca3In and Sr2Sn [40]. That is, the obtained result is not singular and was expected, but it is not the final result. While transition metals after tens of years of intensive development of getter technologies have reached their potential, for reactive metals everything is only starting. The quantitative analysis of the above given data shows that in the case of nitrogen (Fig. 5) only 2e4% of the powder mass reacted by the end of the measurements; and in the case of oxygen containing gases (Fig. 6) 6e10% accordingly. That is, sorption capacity of a reactive material can be increased not less than by an order of magnitude and thus reach 1000-fold superiority over the getters produced from transition metals.
V dp K ¼ zrpc þ Z l ðPo pÞ; RT dt Dd
(1)
where V is the inside volume of the cavity, Z is the area of the inner surface of the cavity, t is time, Dd is the thickness of the cavity wall, z is the surface area of the getter film, Kl is the coefficient of transparency of the wall, c is the concentration of the reactive component in the product layer MeX on the boundary with gas X, r is the coefficient of a chemical reaction equal to ayx (RT/2pM)1/2, a is a sticking coefficient, nx is partial molar volume of a gaseous constituent in the layer MeX, R is a gas constant, T is temperature, M is gas molar mass. The analytical solution of the equation (1) in the dimensionless form has a form [42] of
rðtÞ ¼
1
G G t expðHUtÞ þ 1þ ; HU HU U
(2)
Fig. 7. Cavity of MEMS device: 1 e bottom, 2 e device, 3 e seal, 4 e getter film, 5 e cap, 6 e inside space, P0 e outside pressure, p e inside pressure.
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where t ¼ tD/L2, r(t) ¼ p(t)/P0, G ¼ L2SKlRT/VDd, H ¼ rLs/D, U ¼ LzRT/VP0, D is a coefficient of diffusion of Me in MeX, L is the thickness of the getter film, S ¼ S/L, S is the thickness of the layer of products MeX, s is the concentration of gas in the medium above the layer MeX, 4 is the concentration of gas in the layer MeX. The results of the calculation of the function r(t) made for the case of small chambers with the assumption that p (0) ¼ P0 and HU > G are given in Fig. 8. The physical meaning of the conditions mentioned is simple: the first one means that the cavity with the getter are sealed at atmospheric pressure without preliminary pumping down and the second one means that the pumping effect of the getter (parameters H and U) prevails over the leakage (parameter G). According to the calculations the gas pressure r(t) in a hermetically closed chamber due to the getter rapidly goes down practically to zero and then stabilizes at this level (Fig. 8). However, with time the gas pressure will very slowly raise due to the second summand in equation (2). A large number of experimental curves p(t) of the same form as the curves r(t) in Fig. 8 are given in Refs. [43,44]. They were obtained in the process of measuring the pressure of different atmospheric gases after their entry to the vacuum vessel which was filled with particles of barium alloys. Equation (2) allows calculating the lifetime for each particular vacuum cavity having been given the maximal value of the pressure p, at which the device is still able to function. These calculations made for the case when V ¼ 0.1 cm3, 1015 m2/s Kl 1012 m2/s, L ¼ 10 micron and pmax ¼ 102 mbar predict lifetime of 20 years if the volume of the getter material is equal to approximately 0.5% of the inner volume of the cavity [42]. At this according to the initial conditions the getter at the first stage of the process captures all the active gases in the cavity, thus fulfilling the work of the outside pump, and then maintains vacuum in the cavity by removing the leaking gases. The scheme of the calculations used above was to show the readiness to the worst variant, when due to refusal from the standard bonding processes the getter had to spend part of its sorption potential for the creation of the initial vacuum. However, the emergence of the new packaging machines relieves the getter from the functions of the outside pump and this only strengthens the conclusion about the ability of reactive getters to provide a long lifetime of a vacuum device under the discussed conditions. Let us also pay attention to the fact that the calculated thickness of the getter film L ¼ 10 micron appeared to be close to the recommended value of 5 micron, which we found experimentally (see chapter 3.1).
Why the preference is given to barium and not to other reactive metals can be understood from Fig. 9, where the calculation results for thermodynamic equilibriums in systems Ba e H e O e C e N obtained with the aid of the program FACT-SAGE [46] are given. No other reactive metal provides such low pressure of residual gases at room or close to room temperatures as barium does. Besides, barium as a gas sorbent is universal, it reacts with all active and low activity gases while lithium, for example, does not sorb hydrogen at room temperature, calcium and magnesium do not sorb nitrogen, etc. So, two getter solutions are suggested:- binary films with activation at ~150 С intended for use both with low temperature sealing materials and with room temperature packaging machines; - porous bulk bodies with activation at ~250 С intended for use in room temperature packaging technologies. Film getters. Getter films of the compositions Ba e (35 ± 5) at % Mg and Ba e (28.5 ± 2.5) at % Al with a cover layer of Al or Mg correspondingly are manufactured by multiple alternate deposition of thin layers of Ba/Al/Ba/Al … Ba/Al or Ba/Mg/Ba/Mg … Ba/Mg onto a hot substrate at 200 ± 50 С, where the last layer of Al or Mg is deposited at a temperature lower than 0 С to create a protective layer [47]. The ability of Al and Mg to self passivation was already used in the past for protection of barium from the air [20]. The idea of synthesizing chemically homogeneous getter films in the process of deposition of several components onto the heated substrate is taken from the technology of production of photocathodes of Cs3Sb type [48]. According to numerous experimental data on sintering of disperse materials, for consolidation and homogenization of powders or multilayer films it is enough to heat them to Tammann temperature Т T ¼ 0.45Т f, where Т f is the melting temperature of the metallic material in Kelvin [36]. In the case of BaeAl and BaeMg getter films the value of Т T is close to ~150 C, which allows obtaining of equilibrium getter film at the above mentioned temperature and further, after inserting the film into the working place in the cavity, to get rid of the cover layer during the sealing process at the same 150 C. During the activation the lower, metal, sublayers of the cover layer diffuse into eutectics and the upper oxide film of the cover layer, Al2O3 or MgO, which was formed at the contact of the getter with the air at the stage of packaging, loses its protective functions. This happens due to the misfit of crystal lattices: directly under the oxide film Al2O3 eutectic Al5Ba4 þ Ba appears instead of Al and under the oxide film MgO the eutectic BaMg2 þ Ba appears instead of Mg.
3.3. Two getter types for MEMS The replacement of barium with its alloys allows developing once again a mass getter product similar to what Ba e EG had been for tens of years [23,45] in the time of television CRTs, and to what binary alloys BaeMg and BaeAl or ternary alloys BaeMgeNa can become in application to MEMS.
Fig. 8. Calculated dependence of r on t.
Fig. 9. Partial pressure Pj of some vapors and gases over solid Ba.
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Eutectic films BaeAl and BaeMg are manufactured with the help of simple methods of thermal deposition of components in vacuum. High volatility of these components, especially of Ba and Mg, simplifies the task of production of a getter film with the thickness of about 5 micron. Micro ingots of the intermetallic compound BaGa2, which easily withstand a short contact with air [49,50], showed good results as barium vapor sources. Porous bulk bodies. Reactive 3D getters with end-to-end micro channels [50] can become competitors of reactive getter films when there is deficiency of free surface in a vacuum cavity. It is a special variation of porous materials with the structure of a dendritic carcass obtained by quenching of metallic droplets during their movement in the cooling medium [40,51] or during their casting into metallic moulds [50] succeeded by vacuum evaporation of the volatile component, which leads to the formation of porosity. Fig. 10 shows a mould with ingot 1 placed inside getter housing 2, which is part of vacuum cavity 3. The wall of the getter housing is made of porous sintered material, which allows gases but entraps small solid particles. The activation of an ingot having a composition (BaMg2)1-xNax, where 0.10 x 0.15, takes place in a specialized vacuum chamber foreseen in new packaging machines for thermal treatment of getters (see e.g. Ref. [13]). The mould with the ingot is heated to ~250 С for evaporation of Na. Na evaporates from the ingot leaving behind voids (interdendritic spaces), which provide gases with the access to bared intermetallic carcass BaMg2. A thin oxide layer on the surface of the ingot formed at the stage of introduction of the mould into the chamber does not hinder evaporation of sodium but makes its contribution in sorption of moisture. After activation of the getter, lid 4 together with getter housing 2 is transported through the sluice system to the next vacuum chamber, where at room temperature lid 4 is hermetically connected with the lower part of cavity 6 using room temperature sealing process. The assembled in this way device will withstand leakage of the outside gases for ~20 years. At that the volume of a reactive ingot should be not less than 0.005 V, where V is the volume of the cavity and the rate of quenching the droplets in the production of the ingots should be dT/dt 40 K/s. The first of these two conditions is responsible for the sorption capacity of the getter material and the second one is responsible for sorption kinetics. As is known, dendrite arm spacing ds and the rate of cooling the droplets at quenching have a correlation dT/dt a2/ d2s , where a is a proportionality coefficient [52,53]. Identifying the typical sorption size of a getter body having a structure of a dendritic carcass with the value ds/2 and taking the latter equal to 5 micron in accordance with chapter 3.1 we come to the above mentioned condition for the quenching rate. For metallic droplets of the radius of several hundreds of microns the cooling rate of
Fig. 10. Vacuum cavity with bulk porous getter: 1 e mould, 2 e getter housing, 3 e cavity, 4 e lid, 5 e dendritic carcass BaMg2, 6 e bottom, 7 e sensor.
47
about 40 K/s is typical for quenching in gas medium but it can be many-fold increased at quenching into metallic mould [54]. 3.4. Porous receptacle Barium alloys in the form of continuous films or permeable for gases at room temperature bulk bodies are able to spend their entire sorption potential for chemical capturing of residual gases with sufficient for the practical needs rate provided that the typical sorption size of such getter is close to 5 microns. Under the typical size we understand here, as can be seen from the above said, the way of the reactive front, which it passes by the moment of completing of the sorption process. In the case of the films it is such geometrical value as the thickness of the film L; in the case of a porous body with the structure of a dendritic carcass it is a radius of a dendrite arm ds/2. So, in the light of the above said it would be difficult to disagree that at the present moment favorable conditions have been formed for the attempts of creation of the first samples of vacuum package MEMS devices having life spans of tens of years. The prerequisites of the success in such an undertaking are new getters on the basis of reactive metals and new generation of packaging equipment allowing hermetization of a vacuum cavity without the heating, which destroys or sufficiently weakens the sorption potential of getter material. The only unsolved issue of a technical character restraining immediate realization of a project of this kind is connected with the problem of peeling off of any getter materials. If a reaction between the getter material and gases runs to completion its end product will be powder, the particles of which possess all the degrees of freedom. Since unrestricted spread of solid products of the reaction MeX inside the cavity is unacceptable an additional design element is required, which would separate the getter from the rest part of the cavity but would allow residual gases to the getter. This element is well known under the name of a receptacle, partition or getter housing; it is already used in the production of MEMS devices [55e57]. That is, the problem of peeling off from getter films and from porous getter bodies is not new; moreover, there are solutions when the sorption material is from the beginning introduces in a form of powder, which is separated from the sensors by a porous partition [57]. In general the formation of small solid particles and their coming off are the natural consequence of the sorption work of any getter material independent of its shape or nature. On the other hand, the amount of the powder produced by the getter is proportional to the intensity of the sorption process: the larger the share of the used getter material the more free powder particles are formed. For this reason a getter housing, with a porous wall permeable for gases but retaining solid particles, should become an essential member of any vacuum MEMS device claiming for extended lifetime. Conversely, the absence of such getter housing clearly indicates the absence of any noticeable contribution from the getter in the increase of reliability of a MEMS device. As far as in MEMS packaging technologies porous getter housings are a known and mastered way of keeping solid particles in a designated for them restricted space [55e59], there are no reasons to expect any complications connected in particular with barium alloys. The choice of materials for a porous wall is rater wide: these can be sintered metal powders, ceramics, glass; and the temperature of heating of the cavity at activation of the getter does not exceed 150 C for binary getter films and 250 C for micro ingots of ternary alloy BaeMgeNa. In the first case the heating temperature is by 200 C lower than the melting point of the film and in the second case a metallic or ceramic partitions are chemically stable to Na vapor till 700 C and 500 C accordingly [60].
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There are good reasons to expect that in the nearest time if the results, which we have presented, stimulate interest from the side of MEMS manufacturers, practical activities can be started on manufacturing and testing the first model of a getter housing with a reactive micro ingot [50]. 4. Conclusion 1. Getters on the basis of reactive metals excel getters on the basis of transition metals in sorption capacity at room temperature by two orders of magnitude and have an additional potential for its further increase not less than by 10 times. 2. Getters on the basis of reactive metals can be produced in a form of films or porous bulk bodies with the activation temperature of ~150 C or ~250 C correspondingly. 3 The analysis of the situation in the field of MEMS technologies shows that the usage of reactive getters combined with low or room temperature sealing techniques and getter housings inside vacuum cavity will allow the creation of long living MEMS devices capable of working for tens of years. Acknowledgments The authors would like to thank Prof. G. Voronin, Moscow State University, for his assistance in the thermodynamic issues and Mr. Gary Douglass, Agile Chemistry, Inc., Elmhurst, USA for his help. References [1] V. Dragoi, E. Cakmak, E. Pabo, Metal wafer bonding for MEMS devices, Rom. J. Inf. Sci. Tech. 13 (1) (2010) 65e72. [2] B. Bhushan (Ed.), Handbook of Nanotechnology, third ed., Springer-Verlag, Berlin, 2010. [3] K. Chuntonov, S. Yatsenko, Recent Patents on Materials Science, Getter Films for Small Vacuum Chambers, 6, Bentham Science Publishers, 2013, pp. 29e39. Number 1. [4] R. Agarwal, Low Temperature Hermetically Sealed 3-D MEMS Device for Wireless Optical Communication, Dissertation, University of South Florida, 2007. [5] L. Yan, C. Lee, D. Yu, W.K. Choi, A. Yu, S.U. Yoon, J.H. Lau, A Hermetic Chip to Chip Bonding at Low Temperature with Cu/In/Sn/Cu Joint, IEEE ECTC, Florida USA, 2008, pp. 1844e1848. [6] H. Ishida, T. Ogashiwa, T. Yazaki, T. Ikoma, T. Nishimori, H. Kusamori, J. Mizuno, Low-temperature Wafer Bonding for MEMS Hermetic Packaging Using Sub-micron Au Particles, 3, Trasaction Jap. Inst. Electronics Packaging, 2010, pp. 62e67. No.1. [7] M. Antelius, Wafer-scale Vacuum and Liquid Packaging Concepts for an Optical Thin-film Gas Sensor, Doctoral Thesis, KTH Royal Institute of Technology, Stockholm, Sweden, 2013. [8] R. Straessle, Y. Petremand, D. Briand, M. Dadgras, N.F. de Rooij, J. Micromech. Microeng 23 (Number 7) (2013) 075007, 23. [9] Curtis N.P. Hermetic MEMS Package and Method of Manufacture. US Patent 7538106, 2008. [10] Curtis N.P. Cold Weld Hermetic Package and Method of Manufacture. US Patent 7576427, 2009. [11] Eaton M.F, Curtis N.P, and Miner A. Cold Weld Hermetic Package and Method of Manufacture. US Patent 8394679, 2013. [12] T. Itoh, H. Okada, H. Takagi, R. Maeda, T. Suga, Room temperature vacuum sealing using surface activated bonding method, in: The 12th Int. Conf. Solid State Sensors, Actuators and Microsystems, Boston, 2003, pp. 1828e1831. [13] Room Temperature Wafer Bonding Machine BOND MEISTER https://www. mhi-global.com/products/detail/wafer_bonding_machine.html [accessed 11.09.15]. [14] J.J.B. Fransen, H.J.R. Perdijk, The Adsorption of Gases by Barium Getter Films applied as a Tool, Vacuum 10 (1960) 199e203. [15] J.C. Turnbull, Barium, strontium, and calcium as getter in electron tubes, J.Vac. Sci. Technol. 14 (N1) (1977) 636e639. [16] B. Ferrario, Chemical pumping in vacuum technology, Vacuum 47 (1996) 363e370. [17] K. Chuntonov, J. Setina, New lithium gas sorbents: I. The evaporable variant, J. Alloys Compd 455 (2008) 489e496. [18] Holst G, Oosterhuis E. Improved Process for Removing Gas Residues and for Purifying Inert Gases in Electric Vacuum Tubes, Incandescent Lamps and the like. GB Patent 151611, 1921. [19] W. Espe, M. Knoll, M.P. Wilder, Getter Materials, Electronics (1950) 1e11. [20] Gabbrielli E. Vacuum Tube Getter Body Material. US Patent 2778485, 1957.
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