Abstract:
A device may be provided in a sealed package by aligning a seal ring provided on a first surface of a first semiconductor wafer in opposing relationship with a seal ring that is provided on a second surface of a second semiconductor wafer and surrounds a portion of the second wafer that contains the device. Forcible movement of the first and second wafer surfaces toward one another compresses the first and second seal rings against one another. A physical barrier against the movement, other than the first and second seal rings, is provided between the first and second wafer surfaces.

Description:
This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     FIELD 
     The present work relates generally to wafer-level packaging and, more particularly, to seal ring bonding. 
     BACKGROUND 
     Various types of conventional electrical and electromechanical components require packaging in a clean hermetically sealed volume to function properly. For example, Radio frequency microelectromechanical (RF MEMS) technology provides moving sub-millimeter-sized components with RF functionality. Examples of RF MEMS components include resonators, oscillators, switches, switched capacitors, varactors, etc. As mentioned above, the functionality of components from RF MEMS and other technologies depends on the ability to provide the components in hermitically sealed environments. 
     In some conventional approaches, often referred to as wafer-level packaging, the components are provided within respective cavities formed in a surface of a silicon wafer. A set of seal ring metallization stacks (also referred to herein simply as seal rings) is provided on the wafer surface (e.g., using a metal lift-off process) in surrounding relationship to the respective cavities. Another wafer is provided with a similar set of seal ring metallization stacks on its surface. The pair of wafers is positioned in opposing relationship with their respective sets of seal rings aligned such that each seal ring on one wafer is in opposed relationship to a corresponding seal ring on the other wafer. This is illustrated in  FIG. 1 , which shows an opposed pair of seal rings  13  and  14  on an aligned pair of wafers  10  and  11 , with the seal ring  14  surrounding a cavity  12  in wafer  11  where the component (also referred to herein as the “device”) is provided. 
     The opposed pairs of seal rings are then moved forcibly into contact with one another, as shown in  FIG. 2 , and bonded together using a suitable bonding technique, such as eutectic bonding, thereby packaging the components between the two wafers  10  and  11 , with each component hermetically sealed within its cavity  12  by the associated pair of aligned and bonded seal rings  13  and  14 . In some instances, the resulting bonded wafer assembly is then sawed to singulate the packaged components for individual deployment. In other instances, the entire bonded wafer assembly is deployed in a larger assembly. Various conventional connection arrangements (not explicitly shown in  FIG. 2 ) are available to permit electrical access to the packaged components externally of their sealed cavities. The configuration of the connection arrangement depends on the mode, of deployment of the packaged component. 
     Conventional seal ring bonding processes forcibly compress the opposed seal ring pairs against one another. This may cause the seal ring metallizations to deform beyond what is necessary for bonding, with various attendant problems. 
     It is therefore desirable to provide for controlling compression during seal ring bonding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  diagrammatically illustrate, in cross-sectional view, steps in conventional wafer-level packaging. 
         FIGS. 3 and 4  diagrammatically illustrate, in cross-sectional view, the use of stop surfaces that permit compression control during seal ring bonding according to example embodiments of the present work. 
         FIGS. 5 and 6  diagrammatically illustrate, in cross-sectional view, seal rings placed in trenches to produce stop surfaces that permit compression control during seal ring bonding according to example embodiments of the present work. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments of the present work provide on each wafer an arrangement of stop surfaces. The stop surface arrangement includes inner and outer stop surfaces for each seal ring on the wafer. These stop surfaces extend generally parallel to the bonding surface of the associated seal ring. The inner stop surface is located inside the associated seal ring metallization stack, between the seal ring and the component cavity. The outer stop surface is located outside the associated seal ring, with the seal ring interposed between the outer stop surface and the cavity. When a seal ring pair is aligned in opposing relationship for bonding as described above, an associated pair of inner stop surfaces is aligned in opposing relationship to one another, as is an associated pair of outer stop surfaces. 
     The stop surfaces are positioned such that, as the wafers are forcibly pressed together for seal ring bonding, the opposed bonding surfaces of the seal ring pair contact one another first, before the opposed pairs of inner and outer stop surfaces engage one another, so the seal rings begin to compress against each other. As the wafers continue to move toward one another and seal ring compression continues, the opposed pairs of inner and outer stop surfaces eventually contact one another and thus create a physical barrier that prevents further movement of the wafers toward one another, thereby preventing further compression of the seal rings. As a result, the seal ring metallizations are compressed sufficiently to achieve the desired bonding, but the eventual positioning of the inner and outer stop surface pairs in mutual engagement prevents further unnecessary compression of the seal ring metallizations, which further compression would otherwise occur without the stop surface engagement. This further compression occurs in the prior art approach of  FIGS. 1 and 2 . The present work thus controls the bonding process to limit the compression suitably. 
       FIG. 3  diagrammatically illustrates the aforementioned stop arrangement according to example embodiments of the present work. As shown, the wafer  10  has deposited thereon stop structures (or “stops”)  31  and  32 , which provide respective stop surfaces  33  and  34 . One of the stop surfaces  33  and  34  is the aforementioned inner stop surface, and the other of the stop surfaces  33  and  34  is the aforementioned outer stop surface. Similarly, the wafer  11  has deposited thereon stop structures  35  and  36 , which provide respective stop surfaces  37  and  38 . In some embodiments, all of the stops  31 ,  32 ,  35  and  36  are configured as continuous rings on the wafer surface having generally the same shape as the seal rings  13  and  14 . Various embodiments provide the stops in various configurations. In some embodiments, all of the stops  31 ,  32 ,  35  and  36  (and thus their associated stop surfaces) are separated from the associated seal ring by a generally common lateral distance. In the example of  FIG. 3 , all of the stops  31 ,  32 ,  35  and  36  (and thus their associated stop surfaces) are shown having generally the same width, which is approximately four times the width of the seal rings. Various embodiments have stops of various widths. 
     In various embodiments, the stop structures  31 ,  32 ,  35  and  36  are, for example, silicon, or suitable oxide or nitride materials. The engaging pairs of stop surfaces  33 / 37  and  34 / 38  contact one another under the forcible compression and temperature/vacuum conditions of a conventional (e.g., eutectic) bonding environment. This interaction between engaging pairs of relatively smooth stop surfaces creates bonding between the associated pairs of stop structures  31 / 35  and  32 / 36 . This stop structure bonding advantageously provides additional seals both inside and outside the seal created by bonding the associated pair of metal seal rings  13 / 14 . 
     Although it is useful for a given pair of cooperatively engageable stop surfaces to have a common width, the pair of inner stop surfaces may have a common width that differs from a common width of the pair of outer stop surfaces. A specific example of such a configuration is shown in  FIG. 4 , which is generally similar to  FIG. 3 , except that stops  31  and  35  are replaced by narrower stops  41  and  45  having approximately the same width as seal rings  13  and  14 . Depending on the embodiment, the stop surfaces  43  and  47 , respectively provided by the stops  41  and  45 , may function as either the inner stop surfaces or the outer stop surfaces, with the stops  32  and  36  functioning as the other pair of stop surfaces.  FIG. 4  also shows that the narrower stops  41  and  45  are spaced laterally further from the respectively associated seal rings  13  and  14  than are the wider stops  32  and  36 . In various embodiments, the narrower stops  41  and  45  are located at various distances from their associated seal rings. 
     In various embodiments of the type shown in  FIG. 3 , the common lateral distance between the seal rings and the stops  31 ,  32 ,  35  and  36  has various values, ranging, for example, from 20-50 um. In various embodiments of the type shown in  FIG. 4 , the common lateral distance between the seal rings and the stops  32  and  36  has various values, ranging, for example, from 20-50 um, while the common lateral distance between the seal rings and the stops  41  and  45  has various values, ranging, for example, from 50-500 um. 
     Although all stops of  FIGS. 3 and 4  are shown at generally the same height, and both seal rings are shown at generally the same height, the relative heights of these features may be varied in various embodiments. Consider, for example, the arrangement of  FIG. 3 . When the respective bonding surfaces  30  and  39  of the seal rings  13  and  14  first contact one another, the stop surfaces  34  and  38  remain separated by a first separation distance equal to the amount by which stop surface  34  is offset from bonding surface  30 , plus the amount by which stop surface  38  is offset from bonding surface  39 . Similarly, the stop surfaces  33  and  37  remain separated by a second separation distance equal to the amount by which stop surface  33  is offset from bonding surface  30 , plus the amount by which stop surface  37  is offset from bonding surface  39 . If these first and second separation distances are equal, then the engagement of stop surfaces  34  and  38  will occur approximately simultaneously with the engagement of stop surfaces  33  and  37 , regardless of whether all (or any) of the stops  31 ,  32 ,  35  and  36  have the same height, and regardless of whether the seal rings  13  and  14  have the same height. This separation distance condition also applies in  FIG. 4 . The combination of seal ring heights and stop structure heights in  FIGS. 3 and 4  may be determined based on the particular value that is desired for the above-defined separation distance between stop surfaces. Smaller separation distance values limit seal ring compression relatively more, and larger separation distance values limit seal ring compression relatively less. In various embodiments, the separation distance has various values, ranging, for example, from 0.5-2 um. 
       FIG. 5  diagrammatically illustrates another arrangement for controlling seal ring compression according to further example embodiments of the present work. In the example of  FIG. 5  (showing packaging of single component only), stop surfaces  33 A,  34 A,  37 A and  38 A respectively functionally corresponding to the stop surfaces  33 ,  34 ,  37  and  38  of  FIG. 3 . The stop surfaces  33 A,  34 A,  37 A and  38 A are provided by depositing the seal rings  13  and  14  at generally central locations on the bottom surfaces of respective trenches  51  and  52  formed in respectively corresponding wafers  50  and  54 . The trenches  51  and  52  have respective depths that are less than the heights of the respective seal rings  13  and  14 . Thus, the stop surfaces  33 A and  34 A are integrally part of the wafer  50 , and the stop surfaces  37 A and  38 A are integrally part of the wafer  54 . The stop surfaces  33 A,  34 A,  37 A and  38 A are each offset by a common offset distance from the corresponding seal ring bonding surface. Thus, when the seal ring bonding surfaces first engage, the cooperating stop surfaces on wafers  50  and  54  remain separated by a separation distance equal to twice the common offset distance. 
       FIG. 6  diagrammatically illustrates a packaged component after seal ring bonding is completed. In generally the same manner described above relative to  FIGS. 3 and 4 , forcible engagement of the stop surface pairs  33 A/ 37 A and  34 A/ 38 A under conventional bonding conditions produces additional seals both inside and outside the metal seal that is formed by the bonding of the associated seal rings  13  and  14 . 
       FIGS. 5 and 6  show trenches  51  and  52  having generally the same depth, and seal rings  13  and  14  having generally the same height. However, the relative dimensions may vary in various embodiments, as long as the separation distance between the stop surfaces is the same on both sides of the seal ring pair when the seal ring bonding surfaces meet. The combination of trench depths and seal ring heights in  FIGS. 5 and 6  may be determined based on the particular desired value of the separation distance. In various embodiments, the separation distance has various values, for example, in the same separation distance range mentioned above with respect to  FIGS. 3 and 4 . In various embodiments, the trench depths have various values, ranging, for example, from 0.2-5 microns. In various embodiments, the trench widths have various values, ranging, for example, from 5-500 microns. In some embodiments, the trenches  51  and  52  are formed using a suitable conventional etching process (e.g., Bosch etching), and the seal rings are deposited onto the trench bottoms using suitable conventional techniques (e.g., a metal lift-off process). 
     Although example embodiments of the present work are described above in detail, this does not limit the scope of the present work, which can be practiced in a variety of embodiments.