Abstract:
A hermetically sealed wafer scale package for micro-electrical-mechanical systems devices. The package consists of a substrate wafer which contains a microstructure and a cap wafer which contains other circuitry and electrical connectors to connect to external applications. The wafers are bonded together, and the microstructure sealed, with a sealant, which in the preferred embodiment is frit glass. The wafers are electrically connected by a wire bond, which is protected by an overmold. Electrical connectors are applied to the cap wafer, which are electrically linked to the outputs and inputs of the microstructure. The final package is small, easy to manufacture and test, and more cost efficient than current hermetically sealed microstructure packages.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor devices, and more particularly, to a protective cover arrangement for a semiconductor substrate containing micro-mechanical devices, wherein the cover arrangement offers more desirable protection and manufacturing characteristics. 
     BACKGROUND OF THE INVENTION 
     A variety of semiconductor devices may be formed on a substrate according to several known techniques. The semiconductor devices may perform, for examples, electrical, mechanical, optical, or other functions, or combinations of such functions. 
     Often, a semiconductor wafer serves as a substrate for such devices. The devices may be fabricated from the semiconductor material of the substrate wafer itself using a variety of known processes, such as growth of various material layers on a surface of the substrate, ion implantation, diffusion, oxidation, photolithography, etching and many other processes. During fabrication, typically, at least some portion of each device is formed “within” the substrate wafer, below the surface of the substrate, and may additionally include particular topographic or structural features on the substrate surface. 
     Alternatively, semiconductor devices may be fabricated from a first semiconductor wafer, and subsequently mounted on a second semiconductor wafer or other type of material that serves as a substrate. The semiconductor devices fabricated from the first wafer may be mounted on the substrate either as individual devices or groups of devices. In this case, the devices “reside on” the substrate, as opposed to being “fabricated from” the substrate, as discussed above. 
     For purposes of the present invention, either of the foregoing examples of substrates, namely, substrates on which semiconductor devices reside, and/or from which semiconductor devices are fabricated, is referred to as a “device” substrate. A device substrate may include one or a large number of devices. 
     Many known semiconductor devices are extremely fragile and/or sensitive to environmental hazards. Some examples of such hazards include contamination by dust or other particles, moisture, and inadvertent scratching or other damage to portions of the surface of the devices. 
     Functional defects may result from one or more environmental hazards, as discussed above, and are a major cause of low device yield and other malperformance characteristics. Device damage due to any number of such hazards may occur, for example, during the process of “dicing” (separating the device substrate into individual devices), as well as during packaging of devices. Often, the number of functioning devices remaining after dicing and packaging is markedly reduced due to defects resulting from environmental hazards. 
     Various techniques are known in the art for protecting semiconductor devices on a substrate from such hazards. One of these techniques includes bonding a protective semiconductor cap wafer to a device substrate before dicing the substrate into individual devices. This technique has been employed particularly with wafer substrates of semiconductor devices that include micro-machined parts or microscopic mechanisms fabricated on the surface of the substrate, such as micro-electrical-mechanical systems, or MEMS. These devices are also referred to as microstructures. 
     According to one known technique for protecting semiconductor micro-mechanical devices, an entire device substrate wafer is capped with another wafer using a pattern of glass-like posts called “frit glass” as a bonding agent. In this technique, the micro-mechanical devices are hermetically sealed inside an open cavity formed by the frit glass pattern, the device substrate and the cap wafer. Hermetic sealing refers to a particular standard for sealing that is known in the art. Accordingly, any micro-mechanisms are free to move within a cavity while simultaneously being protected from various environmental hazards, such as particulate contamination. 
     According to the technique discussed above, the cap wafer is typically another semiconductor wafer of the same type as that used for the device substrate (for example, silicon or gallium arsenide). As a result, the cap wafer has essentially identical thermal characteristics to that of the device substrate. Such a choice of cap wafer results in avoiding most mechanical stresses that may result from a thermal mismatch between the cap wafer and the device substrate. For example, extreme mechanical stress can occur during a high temperature heat treatment necessary to ensure adequate bonding of the frit glass to the cap wafer and the device substrate. Any mechanical stress can severely damage or even destroy the devices on the substrate, and usually would degrade the accuracy of many delicate micro-machined devices, such as microscopic mechanical sensors or other MEMS. 
     After the micro-mechanism has been protected by wafer capping, the individual devices must be prepared for connection to external circuits. This process consists of separating the individual devices through dividing the substrate into individual devices, called “singulating” or “dicing.” After singulation, the devices are placed into individual containers that provide mechanical protection and electrical connections to the microstructures. This process is called “packaging.” This process of fabricating micro-mechanism packages is time consuming, expensive and results in large devices, as a consequence of processing the individual devices as individual devices. 
     FIG. 1 shows a cross-sectional view of sheet of devices before singulation. FIG. 2 shows a cross-sectional view of one of these singulated but unpackaged devices  5 . A microstructure  12  rests on a wafer substrate  10  and is enclosed by glass frits  22  and a cap wafer  20 . The electrical connector  14  connects to the microstructure  12  through the wafer substrate  10 . The singulated device  6  is then connected to a lead frame  8  by the electrical connectors  4 . This is shown in FIG.  3 . This device is then packaged and results in the singulated MEMS package  6  as shown in FIG.  4 . The package  2  surrounds the device  5  while the electrical leads  4  provide an electrical connection from the device  5  to the outside of the package  2 . The package  2  serves to the protect the device  5  from external hazards. 
     In order to create the individual packaged device  6 , each device  5  needs to be placed in the package  2  individually. The final singulated device package  6  is considerably larger than the singulated device itself  5 . 
     Accordingly, what is needed, therefore, is improved packaging which still provides the necessary protection, space and connectibility to the microstructure contained inside. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a hermetically sealed wafer scale package for MEMS devices. According to the method and apparatus of one embodiment of the invention, a MEMS substrate wafer with microstructures is hermetically sealed to a cap wafer with fabricated circuitry to form an assembly. The cap wafer is preferably patterned before being attached to the device substrate to form the assembly. A wafer saw is then used to singulate the cap wafer and wire bond is attached to electrically connect the cap wafer to the MEMS wafer. Overmold is then applied to the structure to protect the wire bond. Solder balls are then attached to the cap wafer and the wafer saw is used to singulate the assembly into packaged MEMS devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. 
     FIG. 1 is a cross-sectional view of a MEMS package assembly according to the prior art; 
     FIG. 2 is a cross-sectional view of a single MEMS package according to the prior art; 
     FIG. 3 is a plan view of a capped MEMS die connected to a lead frame prior to being molded into a package, according to the prior art; 
     FIG. 4 is a cross-sectional view of a MEMS packaged device, fully contained with lead wires, according to the prior art; 
     FIG. 5 is a cross-sectional view of a hermetically sealed MEMS device in a wafer scale package according to one embodiment of the invention; 
     FIG. 6 is a diagram of a plan view of a microstructure substrate; 
     FIG. 7 shows a cross-sectional view of a MEMS wafer substrate, in a step of the invention; 
     FIG. 8 shows a cross-sectional view of a further step of the invention; 
     FIG. 9 shows a cross-sectional view close-up of the cap wafer according to a further step of the invention; 
     FIG. 10 shows a cross-sectional view of an alternative embodiment of a further step of the invention; 
     FIG. 11 shows a cross-sectional view close up of the cap wafer according to one step of the invention; 
     FIG. 12 shows a cross-sectional view of a further step of the invention; 
     FIG. 13 shows a cross-sectional view of a further step of the invention; 
     FIG. 14 shows a cross-sectional view of a further step of the invention; 
     FIG. 15 shows a cross-sectional view of a further step of the invention; 
     FIG. 16 shows a cross-sectional view of a completed invention, a singulated hermetic wafer scale package with a MEMS device. 
    
    
     DETAILED DESCRIPTION 
     In view of the foregoing, it would be advantageous to protect semiconductors such as MEMS devices, on a “wafer” level; namely, simultaneously protecting one or more devices residing on or fabricated from a device substrate, as opposed to protecting individual devices after the device wafer has been diced. Furthermore, it would be advantageous to manufacture these devices in such a way that they will be easily connected to their external applications using a minimum of space and a minimum of manufacturing processes. 
     In the preferred embodiment of the invention, circuitry is fabricated directly on the top surface of the cap wafer. This can be done either before or after bonding, although before is preferable. If the wafer is conductive, the circuitry needs to be insulated by oxide or a deposited dielectric. Finally, the circuitry needs to be electrically connectable to other elements. This metal connection layer is covered by a protective dielectric layer. This layer has gaps which are filled by solder balls. These balls serve as the electrical connectors to the final packaged device. 
     In an alternative embodiment, a flex circuit is attached to the cap wafer rather than the circuitry being fabricated directly on the surface of the cap wafer. In this embodiment the flex circuit is attached to the cap wafer using an adhesive material. This flex circuit consists of a first level dielectric, a metalization layer, and a second level dielectric. The first level dielectric serves to secure the flex circuit to the adhesive. The metalization layer provides the electrical connection along the flex circuit. The second level dielectric serves to protect the metalization from outside interference and to contain the solder ball which will provide the package&#39;s electrical connection to external applications. 
     A cap wafer attached to the device substrate prior to dicing and packaging provides more robust devices by preventing particles from collecting over sensitive surfaces of the devices and scratching the surfaces. 
     By fabricating circuitry directly onto the cap wafer, or alternatively, by attaching a flex circuit to the cap wafer, and then by connecting this cap layer to the MEMS layer, the structure allows external devices to connect directly to the cap layer without the need of a further package structure or step, thereby creating a much smaller, more manageable, and faster produced single packaged device than is available in the prior art. 
     FIG. 5 shows a cross-section of a hermetically sealed wafer scale MEMS package  60 . A microstructure  12 , that is being protected, rests atop a wafer substrate  10  and is attached thereto. The wafer substrate is connected to a cap layer  20  by frit glass  22 . The cap wafer  20 , frit glass  22 , and wafer substrate  10 , serve to protect and hermetically seal the microstructure  12  so that it may operate without hazards caused by foreign objects. An electrical signal allows the microstructure  12  to communicate with the external world. The electric signal from the microstructure  12  is carried through the wafer substrate to the electrical connector  14 . The wire bond  40  connects the microstructure electrical connection  14  to additional circuitry on the cap wafer  30 . An overmold  50  protects the wire bond  40  and electrical connector  14  from external damage. The overmold  50  is typically an epoxy, filled with silica to reduce thermal expansion. It is poured in between the frit glass posts  22  and then hardens to cover and hold in place the electrical connectors  14 . Electrical connectors connect the cap circuitry  30  to external devices. Solder balls  52  provide a means of electrically connecting the flex circuit to external circuitry. By connecting to the cap circuitry  30 , the electrical connectors are also connected to the microstructure  12 . The device of the invention may be manufactured by the inventive process described below. 
     The process of creating a hermetically sealed wafer scale MEMS package begins with a wafer substrate containing the microstructures. The application of frit glass is the next step of manufacturing for a hermetically scaled wafer package for MEMS device. 
     FIG. 6 shows a plan view of a wafer substrate  10  containing microstructures  12 . 
     The left-most microstructure is surrounded by a ring of frit glass  22 . FIG. 7 shows a cross-sectional view of the same wafer substrate. The microstructures  12  are fabricated onto the wafer substrate  10 . Electrical connections  14 , which provide connections to the microstructure, are also fabricated into the substrate. 
     At the next step of the process, the frit glass  22  is placed on the wafer substrate so that it surrounds the microstructure. The cap wafer  20  then is pressed to the frit glass  22  to hermetically seal the microstructures, as shown in FIG.  8 . The entire assembly then is cooled so that the frit  22  becomes solid glass and forms a seal surrounding the microstructure  12 . 
     According to the preferred embodiment of the invention, prior to being sealed to the frit glass  22 , the cap wafer  20  is prefabricated with individual circuits  30  along its surface. These circuits  30  are lined up directly above the microstructure  12  when the cap wafer  20  is sealed to the substrate wafer  10 . 
     FIG. 9 shows a close-up of the cap wafer  20  according to the preferred embodiment of the invention. A dielectric  33  is attached above the cap layer  20  in such a way that it protects the electrical connector  31  while creating an opening  25  which allows external devices to electrically connect to the circuitry on the cap wafer. The external devices connect at the dielectric gap  35  to the electrical connector  31 , which, in turn, connects to the circuitry  30  fabricated on the cap wafer  20 . 
     FIG. 10 shows a cross-sectional view of the substrate wafer  10  and the cap wafer  20  according to an alternative embodiment of the invention. According to this embodiment, a flex circuit  38  containing circuitry  32  is attached to the cap wafer  20 . FIG. 11 shows a close up of the cross-sectional view of the flex circuit  38  attached to the cap wafer  20 . The flex circuit consists of a first level dielectric  34 , circuitry  32 , and a second level dielectric  36 . An adhesive layer  24  connects the flex circuit  38  to the cap wafer  20 . The first level dielectric serves to protect the circuitry  32  from the adhesive bond layer  24 . The second level dielectric  36  serves to protect the circuitry  32  from external hazards and is arranged in a way so that is creates a gap  35  in the second level dielectric  36  which allows access to the circuitry  32 . External devices connect to the circuitry  32  through this gap  35  in the second level dielectric  36 . This alternative embodiment may be used in lieu of direct fabrication of circuitry onto the cap wafer. 
     After the cap wafer  20  is sealed to the substrate wafer  10 , the next step in the process of the invention is to separate the cap wafer so that the unnecessary portions of the cap wafer are removed and access can be gained to the underlying portions of the substrate wafer. FIG. 12 shows this step of the invention. In the preferred embodiment of the invention, the cap wafer  20  is separated by a wafer saw so that only the necessary portions of cap wafer remain. The next step of the invention process is shown in FIG.  13 . During this step, wire bond  40  is attached to the cap wafer circuit  30  and is connected to the electronic connection  14  of the microstructure  12 . The wire bond is typically applied by the use of a capillary which bonds the wire to the circuitry on the cap wafer, spools the wire to the electrical connectors  14  on the microstructure wafer and bonds the wire there. The capillary then severs the wire and moves to the next microstructure. In the preferred embodiment, the wire bond is made of gold but can be made of other similar materials such as aluminum. At the base of the connection of the wire bond  40  to the MEMS connector  14 , a ball of wire bond  42  is formed by sealing the wire bond  40  to the connector  14 . 
     During the next step of the process, an overmold  50 , preferably an epoxy in liquid form filled with silica to reduce thermal expansion, is poured between the microstructure glass seals  22 . The overmold then cools and then serves to protect the wire bond  40  which connects the cap wafer  20  to the substrate wafer  10 . The completion of this step is shown in FIG.  14 . 
     The next step is to attached solder balls  52  to the cap wafer circuit  30 . This is shown in FIG.  15 . External applications connect through the solder balls  52  to the cap wafer and through it connect to the microstructure  12 . 
     The final step is to singulate the hermetically sealed wafer scale packaged MEMS. In the preferred embodiment, a wafer saw is used to cut through the overmold  50  and create the singulated packages  60 . The result of this step is shown in FIG.  16 . The final packages are wafer scale, or “chip-sized.” This means that the final packages are the same size as the singulated devices. There is no external casing required which changes the size of the final package. 
     In between any of the steps above, the manufacturing process could be halted and testing could be performed on the individual microstructure packages or on the entire wafer. This ability to test at the wafer level contributes to the low cost of these microstructure devices. 
     Having thus described at least one illustrated embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be in the spirit of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting.