Abstract:
An integrated getter structure and a method for its formation and installation in a circuit module enclosure ( 24 ). The integrated structure includes a hydrogen getter structure ( 10 ) and selected quantities of a material ( 20 ) that is formulated to provide both a particle getter function and an RF absorber function. In one embodiment, the material ( 20 ) is placed in discrete quantities over the hydrogen getter structure ( 10 ). In another embodiment, the hydrogen getter structure ( 10 ) is formed over a sheet of the material ( 20 ) and is provided with apertures ( 30 ) to expose the material ( 20 ).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to radio-frequency (RF) circuit modules and, more particularly, to structures for reducing the effects of unwanted materials or radiation within circuit modules or packages. In a chemical or metallurgical process, a “getter” is a material added in small amounts during the process to absorb impurities. In this specification, the term “getter” is used to encompass any material or structure for reducing the effects of unwanted materials or radiation, whether by absorption, scattering, dispersion, or some other process.  
         [0002]     In the electronics packaging industry, hydrogen is known to affect electrical performance adversely, but hydrogen is an unavoidable component in the electronics package module, such as in plated nickel layer. With the passage of time, especially in a space environment or in the low pressure environment, hydrogen can diffuse out from the materials in which it was originally contained, and cause permanent damage or adversely affect the electrical performance of the electronics module. Hydrogen getters have been developed to alleviate this problem, as described in U.S. Pat. No. 6,423,575 to Dean Tran et al., and U.S. Pat. No. 6,369,442 to Yoshio Saito. For completeness, the disclosures or these patents are incorporated by reference into this specification. A hydrogen getter structure typically includes a layer of a material known to absorb hydrogen by chemical reaction, such as titanium, and an overlying layer of a material such as palladium, which facilitates entry of hydrogen into the titanium layer but prevents oxidation of the titanium. There may also be a layer of a contact metal, such as silver or gold, underlying the titanium. One of more hydrogen getters of this type are formed of appropriate size and shape and then incorporated into the RF module, such as by affixing them to a module enclosure surface.  
         [0003]     Other potential unwanted contaminants in RF circuit modules take the form of particles. Loose particles can arise from a number of sources during the fabrication and assembly processes and may remain in the sealed package. The need to deal with particle contamination in sealed electronics, superconducting or photonics modules has given rise to the development of particle getter structures that must also be incorporated into the module package before sealing. These may take the form of silicone materials that act in part as adhesives, to catch any loose particles that would otherwise be free to move about the sealed package. Particle getter structures in the past have been separately formed and installed in the enclosure or elsewhere in the package.  
         [0004]     The need for absorption of radio frequency (RF) radiation in an electronics module is also well known. Especially at very high frequencies, unwanted resonances can occur within module cavities, effectively precluding or adversely affecting operation at certain frequencies. RF absorbing structures are known in the art, and have in the past also been separately formed and installed in the module package. For example, a commercially available material, ECCOSORB® RF, manufactured by Emerson &amp; Cuming Microwave Produces, Randolph, Mass., is an RF absorber that can be machined to appropriate shapes for inclusion in an RF package housing.  
         [0005]     Prior to the present invention, hydrogen getter structures, particle getter structures and RF absorbing structures were individually formed and installed in an electronics module package for RF application. The costs of manufacturing and assembly of the three types of getter/absorber are, therefore, a significant proportion of the total product cost of each circuit module. Moreover, each type of getter adds to the total weight of the module. For some applications, such as in spacecraft, module weight is critically important because it impacts the cost of launching the spacecraft.  
         [0006]     Accordingly, it will be appreciated that there is still a significant need for a structure and related method that would perform the functions of a hydrogen getter, a particle getter and an RF absorber, but without imposing a requirement to form and install each type of getter/absorber separately. This would result in an RF circuit module that was less costly, more compact and lighter in weight. The present invention satisfies this need.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention resides in a getter structure that combines at least two of the three principal getter functions in an electronic module. The structure of the invention may be defined as an integrated multi-purpose getter structure suitable for installation in an electronics module, especially an RF package. Briefly, and in general terms, the getter structure comprises a quantity of material providing a particle getter function and an RF absorber function. This quantity of material is formable into portions of appropriate size and shape for installation in the RF circuit module.  
         [0008]     More specifically, the integrated multi-purpose getter structure further comprises a hydrogen getter structure integrated with the portions of the quantity of material. The multi-purpose getter structure thereby performs the combined functions of particle getter, hydrogen getter and RF absorber.  
         [0009]     As disclosed by way of example in one embodiment of the invention, the hydrogen getter structure is formed over an enclosure surface of the RF circuit module; and the portions of the quantity of material are formed over the hydrogen getter structure. In an alternative embodiment of the invention, the quantity of material is formed as a generally continuous layer over an enclosure surface of the RF circuit module; and the hydrogen getter structure is formed over the continuous layer and includes a plurality of apertures through which the quantity of material is exposed to provide the particle getter and RF absorber functions.  
         [0010]     More specifically, the material combining the functions of particle getter and RF absorber comprises a silicone suspension to provide the particle getter function, mixed with a powdered metal-containing material to provide the RF absorber function. The powdered metal-containing material may be any suitable selection or combination of transition elements, such as the iron, cobalt and rhodium in combination, scandium, tantalum, vanadium, or a metal-organo compound in which a metal atom is linked into a carbon chain or carbon branch structure or polymer that has polar helix structure. Preferably, the powdered metal is iron. In a specific embodiment of the invention, powdered iron approximately in the size range 6-10 microns is mixed with a silicone suspension such that approximately 60-70 percent, by weight, of the resulting material is iron.  
         [0011]     The hydrogen getter structure in the integrated getter typically comprises a hydrogen-absorbing metal layer and a facilitating over-layer allowing one-way pass-through of hydrogen to the absorbing underneath layer and protecting the absorbing metal from oxidation. Specifically, the hydrogen-absorbing layer may be titanium and the protective over-layer may be palladium or rhodium. The hydrogen getter structure may further comprise a metal contact layer on which the titanium metal layer is formed.  
         [0012]     The present invention may also be defined in terms of a method of forming an integrated multi-purpose getter structure in a radio-frequency (RF) circuit module. In brief, the method comprises the steps of taking a silicone dispersion material; mixing a metal-containing powdered material in the silicone dispersion material; placing the mixed silicone dispersion material in an RF circuit module enclosure; and curing the mixed silicone dispersion material to provide a combined particle getter function and RF absorber function. To provide the additional hydrogen getter function, the method further comprises the step of integrating the mixed silicone dispersion material with a hydrogen getter structure in the RF circuit module enclosure, to provide combined hydrogen getter, particle getter and RF absorber functions in the same structure.  
         [0013]     It will be appreciated from the foregoing summary, that the invention provides a significant advance in the field of manufacture and packaging of RF circuit modules. In particular, the invention combines the functions of at least two of a hydrogen getter, a particle getter and an RF absorber. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is cross-sectional view of a hydrogen getter of the prior art.  
         [0015]      FIGS. 2A and 2B  are cross-sectional and plan views, respectively, of an integrated getter structure in accordance with the present invention.  
         [0016]      FIG. 3A and 3B  are views similar to  FIGS. 2A and 2B , respectively, but depicting an alternate embodiment of the invention.  
         [0017]      FIGS. 4A and 4B  are views similar to  FIGS. 2A and 2B , respectively, but depicting another alternate embodiment of the invention.  
         [0018]      FIG. 5  is a cross-sectional view similar to  FIG. 4A , but depicting an alternate assembly scheme.  
         [0019]      FIG. 6  is a cross-sectional view similar to  FIG. 2A , but depicting the alternate assembly scheme used in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     As shown in the drawings for purposes of illustration, the present invention pertains to a novel approach to performing the functions of a hydrogen getter, a particle getter and an RF absorber, in a radio frequency (RF) circuit module. As noted above, an RF circuit package sealed in an enclosure is subject to contamination by hydrogen and by loose particles trapped within the enclosure, and is also subject to the effects of RF signals propagating within the package and producing cavity resonance effects and unwanted RF losses. In the past, each of these three types of contamination has been dealt with by installing special structures in the RF package, namely a hydrogen getter structure, a particle getter structure and an RF absorber structure.  
         [0021]     In accordance with the present invention, at least two of these three structures are combined into one integrated structure, thereby simplifying manufacture and assembly, reducing cost and reducing module weight.  FIG. 1  shows, by way of example, a hydrogen getter structure  10  of the prior art. The structure includes a layer  12  of a metal, such as titanium, that absorbs hydrogen by chemical reaction, and an overlying layer  14  of a protective material, such as palladium, which allows entry of hydrogen but protects the titanium layer  12  from oxidation. The titanium layer  12  may also have an underlying layer  16  of a contact metal, such as silver or gold, which performs no part in the hydrogen getter function, but may be used to facilitate attachment of the structure  10  inside an RF module enclosure.  
         [0022]     An important aspect of the invention is that the functions of particle getter and RF absorber are combined in a single material. More specifically, the material, shown at  20  in  FIGS. 2A, 2B ,  3 A,  3 B,  4  and  5 , is formed by starting with a base silicone material that functions as a particle getter, and mixing a powdered metal-containing material into the silicone, using, for example, a roller mixer to ensure a uniform dispersion within the material. Good results were obtained using a silicone dispersion as the base material, specifically a silicone dispersion designated MED 1356, manufactured by NuSil Technology, Carpinteria, Calif. The powdered metal-containing material may be any suitable transition metal that functions as an RF absorber, such as iron, cobalt and rhodium in combination, scandium, tantalum, vanadium, or any of a number of metal-organo compounds in which a metal atom is linked into a carbon chain structure or polar helix polymer. Good results were obtained using an iron powder with a 6-10 micron particle size and 99.5% purity. The iron particles were mixed with the silicone material such that the iron was 60-70% by weight of the resulting mixture, which may then be placed in a suitable mold or otherwise positioned in the module enclosure or its lid. Then the mixture is allowed to cure at ambient temperature.  
         [0023]     As shown in  FIGS. 2A and 2B , in one embodiment of the invention the iron-impregnated silicone material  20  is formed as a plurality of generally round “buttons” on the palladium upper surface  14  of the hydrogen getter  10 . For purposes of illustration, the hydrogen getter  10  is shown as being installed on an enclosure lid  24 .  
         [0024]     In the embodiment of  FIGS. 3A and 3B , the hydrogen getter, indicated by  10 ′, is formed in a sinuous wave configuration. That is to say, the layers of titanium  12 ′, palladium  14 ′ and contact metal  16 ′ are not planar but have a sinuous shape as viewed in cross section. As viewed from above, as in  FIG. 3B , the palladium layer is corrugated into parallel ridges and troughs. The iron-impregnated silicone material  20  is formed as longitudinal stripes  28  along the crests of the corrugations in the underlying hydrogen getter structure  10 ′.  
         [0025]     The embodiment of  FIGS. 4A and 4B  is one in which composite particle getter and RF absorber material  20  is first formed on the lid  24 , and then a hydrogen getter structure  10 ″ is formed over the material  20 , but with apertures through the structure  10 ″ to expose regions  30  of the material  20 .  
         [0026]     In the embodiments of  FIGS. 2A-2B ,  3 A- 3 B and  4 A- 4 B, the hydrogen getter structure  10 ,  10 ′ and  10 ″, respectively, is secured to the lid  24  by a circumferential bead of solder  32 . In an alternate assemble scheme, shown in  FIGS. 5 and 6 , a dipped solder wrap  34  extends around the peripheral edges of the hydrogen getter structure  10 ″ or  10 .  
         [0027]     In experiments testing various thicknesses of the iron-impregnated silicone material  20 , RF absorbing performance was found to be as good as or better than that of a conventional layer of ECCOSORB® material, which, of course, has no particle absorbing properties. More specifically, the material  20  was formed to various thicknesses in an RF module package lid. The thicknesses used in the test were 185 mil (4.7 mm), 205 mil (5.3 mm), 225 mil (5.7 mm). Measurements were also taken using a layer of ECCOSORB® material of thickness 225 mil (5.7 mm), and for comparison the same RF measurements were taken without any RF absorber present. Without an RF absorber of any kind, the test package showed large resonances at two frequencies of interest. Use of the iron-impregnated silicone material  20  reduced these resonant effects just as well as the ECCOSORB® material. In a further test, the iron impregnated material  20  was coated with an additional coating of the same silicone material (without iron). The coating did not significantly affect the RF absorbing performance of the structure. Therefore, an option is to coat the iron-impregnated material  20  with additional silicone, which will not affect RF performance but may increase particle getting performance.  
         [0028]     In summary, the invention provides a material that functions both as an RF absorber and a particle getter. This material may be combined in various ways with a conventional hydrogen getter structure, to provide an integrated hydrogen getter, particle getter and RF absorber. The resulting RF circuit module may be enclosed in a more compact package because the integrated getter/absorber uses the enclosure volume more efficiently. Also, manufacturing costs are reduced significantly and the module weight is also reduced by use of the invention.  
         [0029]     It will be appreciated from the foregoing that the present invention represents a significant advance in the field of RF circuit modules or packages. In particular, the invention provides an RF module at lower cost and lower weight and volume, by combining the functions of hydrogen getter, particle getter and RF absorber into a single structure. It will also be appreciated that, although specific embodiments of the invention have been illustrated and described by way of example, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.