Abstract:
A micro-electromechanical systems (MEMS) package that includes a substrate onto which is disposed or otherwise formed an active MEMS device, a first barrier wall for preventing sealant from contaminating the MEMS device, a second barrier wall for preventing sealant from contaminating unintended areas of the substrate, and a cap for hermetically sealing the MEMS package with a particular gas or mixtures thereof which enhance the MEMS performance.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of integrated circuit packaging and in particular to an integrated package for chip level MEMS devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    Packaging of electrical circuits is a key element in the technological development of any device containing electrical components. With microelectromechanical systems (MEMS), the packaging is critically important as oftentimes it must provide for the isolation of a functional element, such as a circuit or actuator, from its environment. 
         [0003]    More particularly, because MEMS devices tend to have moving parts, they typically cannot be packaged in the same manner used for purely electronic components. Instead, a hermetically sealed enclosure or “cavity” is oftentimes formed around the MEMS device itself. 
         [0004]    One challenge in creating MEMS packages therefore, is to create this hermetically sealed cavity and provide one or more external electrical connections thereto while—at the same time—not damaging the microelectromechanical structures contained therein. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with the principles of the invention, an integrated package for a MEMS or other device is achieved through the use of a pair of perimeter barrier walls surrounding a MEMS device disposed upon or part of a substrate, and a mating cap. Advantageously, the present invention provides mechanical robustness, a hermetic seal, ease of fabrication and low probability of damage/contamination to the packaged MEMS. 
         [0006]    In accordance with yet another aspect of the present invention, the MEMS device is hermetically sealed in an environment containing an electronegative gas or gases, either alone or in combination with other electronegative gases or other inert gases. 
     
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]    In the drawing: 
           [0008]      FIG. 1(A)  is a perspective view of an assembled MEMS package according to the present invention; 
           [0009]      FIG. 1(B)  is a side view of the MEMS package of  FIG. 1(A) ; 
           [0010]      FIG. 2(A)  is a partially-exploded-perspective view of the MEMS package according to the present invention; 
           [0011]      FIG. 2(B)  is a side view of the MEMS package of  FIG. 2(A) ; 
           [0012]      FIG. 3(A)  is a fully-exploded-view of the MEMS package according to the present invention; and 
           [0013]      FIG. 3(B)  is a side view of the MEMS package of  FIG. 3(A) . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. 
         [0015]    Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
         [0016]    Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
         [0017]    Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the invention. 
         [0018]      FIG. 1(A)  is a perspective view of a MEMS package  100  according to the present invention.  FIG. 1(B)  is a side view of that same package  100 . With simultaneous reference to  FIGS. 1(A) and 1(B) , there is shown a MEMS device  115  disposed upon an upper surface of, or alternatively formed as part of a substrate  110 . Inner barrier wall  135  and outer barrier wall  130  are disposed upon the upper surface of the substrate  110 . 
         [0019]    While not specifically shown in FIGS. ( 1 A) or  1 (B), in a preferred embodiment the inner barrier wall  135  completely surrounds the perimeter of the MEMS device  115 . Similarly, the outer barrier wall  130 , completely surrounds the perimeter of the inner barrier wall  135 . 
         [0020]    As can be appreciated by those skilled in the art, the resulting structure has the MEMS device  115  innermost, an inner barrier wall  135  surrounding the perimeter of the MEMS device  115 , and an outer barrier wall  130  surrounding the perimeter of the inner barrier wall  135 . The relative position of the two barrier walls define a “gap” or “moat”  132  between the two walls. 
         [0021]    According to the present invention, the depth and width of the gap or moat is variable—depending upon the particular application. Furthermore, while the gap is shown having a uniform width, it could nevertheless have a variable width as one traverses its perimeter and such variations are well within the contemplations of the present invention. 
         [0022]    Disposed within the moat  132  is bonding block  140  to which the package cover or “cap”  160  is bonded through the effect of bonding material  150 . Accordingly, and according to the present invention, when the cap  160  is affixed a perimeter seal is created as the cap  160  is bonded by the bonding material  150  to the bonding block  140 . 
         [0023]    When positioned in this manner, a space or “cavity”  175  is created in an area proximate to the MEMS chip  115 . As we will discuss later and according to the present invention—this cavity  175  is preferably filled with one or more strongly-electronegative gasses or a mixture thereof. Advantageously, the perimeter seal formed by the cap  160  and the bonding block  140  through the effect of the bonding material  150 , seals the electronegative gas(ses) within the cavity  175 , permanently. 
         [0024]    Of further advantage, and according to the present invention, the two barrier walls  130 ,  135  serve to contain the bonding material within the moat  132  as the cap  160  is pressed into place. As can be appreciated by those skilled in the art, placing the cap  160  onto the bonding block  140  acts to “squeeze” or compress some of the bonding material  150 . Absent one or both of the barrier walls  130 ,  135  the bonding material so squeezed would tend to “run” or otherwise foul the surface of the substrate  110 , or worse, the MEMS chip  115  itself. Significantly, and as can now be readily appreciated by those skilled in the art, when a eutectic or similar bonding material is employed the barrier walls  130 ,  135  act to contain any bonding material  150  which is so squeezed. 
         [0025]    Turning now to  FIG. 2(A)  and  FIG. 2(B)  it can be seen how the cap  160  fits together with the structures disposed upon the substrate  110 . More particularly, it may be observed that the cap  160  engages the moat region  132  until the bonding material  150  and the bonding block  140  are fully engaged and therefore sealed. When the barrier walls  130 ,  135  are appropriately sized (as in  FIG. 4 ), they serve as additional mechanical “stops” to the engagement of the cap  160  within the moat.  FIGS. 3(A) and 3(B)  offer “exploded” views of the components employed. 
         [0026]    Those skilled in the art will quickly appreciate that the particular shapes and relative sizes of the components are matters of design choice, and wide variations are possible. In particular, it has been shown in  FIGS. 1-3  that the cap engages the moat region upon placement. Such arrangements are advantageously not required according to the present invention. 
         [0027]    More particularly, with reference now to  FIG. 4 , it is shown that the cap  160  does not have such a shape that it engages the moat region. Instead, its bottom, sealing surface  161  is substantially flat so that it uniformly contacts both barrier walls  130 ,  135  simultaneously. As a result, when the cap  160  is placed upon the barrier walls  130 ,  135 , it is mechanically stopped from further downward movement while still permitting the bonding material  150  to provide an effective seal along the bottom surface of the cap  160  and the length of the bonding pad  140 . Still further, the inner barrier wall  130  prevents significant amounts of bonding material  150  from contaminating the cavity  175  in which the MEMS chip  115  becomes encased. Finally, the outer barrier wall  135  prevents significant amounts of bonding material  150  from being displaced onto external surfaces of the substrate  110 . While this  FIG. 4  shows a preferred embodiment fo the present invention, those skilled in the art will quickly realize that modifications to this preferred embodiment are within the scope of the invention. More particularly, alternatives to the configuration shown in  FIG. 4  are shown in  FIGS. 5(A) ,  5 (B) and  6 (A),  6 (B). 
         [0028]    As noted earlier, particular gas(ses) are hermetically sealed within the MEMS cavity along with the MEMS device(s). More particularly, a non-flammable gas such as nitrogen or carbon dioxide may be employed, or in a preferred embodiment, an electronegative gas may be permanently sealed within such MEMS cavity. 
         [0029]    In particular, and according to the present invention, a strongly electronegative gas such as sulfur hexafluoride (SF 6 ) in a range of concentrations and pressure(s) is a particularly useful gas for the MEMS cavity. Pressures as low as 0.1 ATM up to and including many ATM are well within the operating range of the present invention. In addition, concentrations as low as 1 PPM may show marked improvement over devices which do not include such an electronegative gas. Finally, while sulfur hexafluoride is particularly disclosed herein, it is to be understood that other electronegative gases or other halogen containing gases may be used in combination with other gases such as FREONS, Carbon Tetrachloride (CCl 4 ), HALONS (chloro-fluorohydrocarbons), or dicarbon hexafluoride. 
         [0030]    Advantageously, the MEMS package described according to the present invention permits the MEMS to withstand relatively high electrical voltages with very small gaps. As such, MEMS switches constructed and packaged according to the present invention operate over a very broad range of electrical voltages—as high as 500 volts with a gap of only a few microns. 
         [0031]    At this point, while the present invention has been shown and described using some specific examples, those skilled in the art will recognize that the teachings are not so limited. Accordingly, the invention should be only limited by the scope of the claims attached hereto.