Abstract:
Proposed is a package structure having a micro-electromechanical (MEMS) element, including a chip having a plurality of electrical connecting pads and a MEMS element formed thereon; a lid disposed on the chip for covering the MEMS element; a stud bump disposed on each of the electrical connecting pads; an encapsulant formed on the chip with part of the stud bumps being exposed from the encapsulant; and a metal conductive layer formed on the encapsulant and connected to the stud bumps. The invention is characterized by completing the packaging process on the wafer directly to enable thinner and cheaper package structures to be fabricated within less time. This invention further provides a method for fabricating the package structure as described above.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a package structure and fabrication methods thereof, and more particularly to a package structure having a micro-electromechanical (MEMS) element built therein and fabrication methods thereof. 
       BACKGROUND OF THE INVENTION 
       [0002]    MEMS (Micro Electro Mechanical System) packages can contain many electrical and mechanical components in one device. Through various micro process techniques, a MEMS package is typically formed by disposing a micro-electromechanical element on a chip/die and then packaging the MEMS chip/die with a protection mask or an underfill to protect it from mechanical damage and hostile environments. The package serves to integrate all of the components required for a system application in a manner that minimizes size, cost, mass and complexity. The three main functions of the MEMS package include: mechanical support, protection from the environment, and electrical connection to other system components.  FIGS. 1A through 1D  are cutaway views depicting several conventional package structures having micro-electromechanical, MEMS elements formed thereon. 
         [0003]    The illustration of  FIG. 1A  is disclosed by U.S. Pat. No. 6,809,412, characterized in that a chip  14  having a micro-electromechanical element  141  formed thereon is disposed on a substrate  10 , wherein the chip  14  is electrically connected to the substrate  10  by bonding wires  11 , and finally a glass lid  12  is disposed on the substrate  10  to hermetically encapsulate the chip  14 , the MEMS element  141  and bonding wires  11 . 
         [0004]    The illustration of  FIG. 1B  is disclosed by U.S. Pat. No. 6,303,986, characterized in that a glass lid  12  is disposed on a chip  14  having a MEMS element  141  formed thereon for hermetically encapsulating the MEMS element  141  therein for protection; the chip  14  is disposed on a leadframe  10 ′ to be carried; the leadframe  10 ′ is electrically connected to the chip  14  by bonding wires  11 ; and the bonding wires  11 , the lid  12 , and the chip  14  are hermetically encapsulated with a packaging material  15 . 
         [0005]    One common drawback of the above package structures share is that both require the use of a carrier (such as the substrate  10  in  FIG. 1A  and the leadframe  10 ′ in  FIG. 1B ), thereby adversely and undesirably increasing the dimensions of the overall structure and thus failing to comply with the demand for device miniaturization. Consequently, the so-called carrier-free package structures have been developed to counter the profile issue. 
         [0006]    As depicted in  FIG. 1C , a carrier-free package structure is disclosed by U.S. Pat. No. 7,368,808 characterized in that a glass lid  12  formed with conductive apertures  120  is disposed on a chip  14  having electrical connecting pads  140  and a MEMS element  141  formed thereon for encapsulating the MEMS element  141  therein, wherein the conductive apertures  120  comprise contact pads  122  formed on both sides thereof, the contact pads  122  formed on the inner side being electrically connected to the electrical connecting pads  140  and those formed on the outer side having solder balls  16  formed thereon for allowing the chip  14  to electrically connect with other electronic components via solder balls  16 . 
         [0007]    In the MEMS package structure depicted in  FIG. 1D  and disclosed by U.S. Pat. No. 6,846,725, a glass lid  12  formed with conductive apertures  120  is disposed on a chip  14  having electrical connecting pads  140  and a MEMS element  141  formed thereon for hermetically encapsulating the MEMS element  141  therein, wherein the electrical connecting pads  140  further comprise solder bumps  142  and the conductive apertures  120  have contact pads  122  formed on both sides thereof, the contact pads  122  formed on the inner side of the conductive apertures  120  being electrically connected to solder bumps  142  such that the chip  14  can be electrically connected with other electronic components via the contact pads  122  formed on the outer side thereof. 
         [0008]    While carrier-free packages are advantageous and favorable for device miniaturization, it nevertheless requires that conductive apertures  120  be fabricated in the lid  12  which then gives rise to issues of high-cost glass drilling and misalignment of the contact pads  122  formed on both sides of conductive apertures  120  or an insecure bonding therebetween, thereby adversely causing inferior electrical connection which in turn degrades the electrical performance of the chip  14  connecting with external electronic components consequently. Therefore, it is desirable to provide a novel package structure that improves on the drawbacks of the prior art as mentioned above. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the foregoing drawbacks associated with the conventional technology, the present invention proposes a package structure having a MEMS element built therein, comprising a chip having a plurality of electrical connecting pads and a MEMS element formed thereon; a lid disposed on the chip for covering the MEMS element; a stud bump disposed on each of the electrical connecting pads; an encapsulant formed on the chip for hermetically encapsulating the stud bumps and the electrical connecting pads, wherein the encapsulant comprises a protruding portion encapsulating the lid, and part of the stud bumps is exposed from the encapsulant; and a metal conductive layer formed on the encapsulant and connected to the stud bumps, wherein the metal conductive layer comprises contact pads. 
         [0010]    In one aspect, the encapsulant comprises a protruding portion flush with the top of the lid and part of the stud bumps is exposed from the encapsulant. 
         [0011]    In the foregoing package structure having the MEMS element, the chip may be made of a silicon material; the MEMS element includes gyroscopes, accelerometers or radiation frequency micro-electromechanical (RF MEMS) elements; the lid may be made of glass or silicon materials; the stud bumps may be made of metallic materials consisting of Gold (Au), Copper (Cu), or a soldering material such as Tin; and the encapsulant may be made of thermal solid resins. 
         [0012]    Additionally, the foregoing package structure having the MEMS element may further comprises a solder ball disposed on each of the contact pads. 
         [0013]    In the foregoing package structure having the MEMS element, the protruding portion has a side wall tilting outwards, and preferably, the included angle of the protruding portion with respect to the side wall is from 45 to 90 degrees. Further, the stud bumps may be disposed at the periphery of the lid. 
         [0014]    In the foregoing package structure having the MEMS element, the metal conductive layer is provided with a solder mask layer formed with solder mask openings for allowing each of the contact pads to be exposed therefrom, wherein each of the contact pads has a solder ball formed thereon. In one aspect, an under bump metallurgy (UBM) layer may further be formed between the solder balls and contact pads, and the under bump metallurgy (UBM) layer may be of a multi-layered structure composed of Gold, Nickel, Nickel/Gold, or Gold/Nickel/Gold. 
         [0015]    In one aspect of the package structure having the MEMS element, the protruding portion may be formed with an encapsulant opening for allowing the MEMS element to be exposed from the lid, the metal conductive layer may be formed to extend to the lid in the encapsulant opening, and the contact pads are formed on the lid. 
         [0016]    In one aspect of the package structure having a protruding portion flush with the top of the lid, the metal conductive layer may be formed to extend to the lid and the contact pads are formed on the lid. 
         [0017]    The present invention further discloses a method for fabricating the package structure having the MEMS element as described above, comprising: providing a wafer whereon a plurality of electrical connecting pads and a plurality of MEMS elements are disposed; disposing a plurality of lids on the wafer each covering on a respective MEMS element; forming a stud bump on each of the electrical connecting pads; forming an encapsulant on the wafer for hermetically encapsulating the lids, the stud bumps and electrical connecting pads therein; cutting the encapsulant disposed at the periphery of the lids to form side walls and trenches between adjacent lids and reveal partial surfaces of the stud bumps therefrom; forming a metal conductive layer connected to the stud bumps on the encapsulant and comprising a plurality of contact pads; and performing a singulation process to form complete and individual package structures by cutting along the contour of the trenches. 
         [0018]    In the foregoing method of fabricating the MEMS package structure, the encapsulant at the periphery of the lids is cut off by means of a bevel cut, and the trenches have a larger width on the tops than the bottoms thereof. 
         [0019]    In one aspect, the method of fabricating the package structure having the MEMS element further comprises, prior to the singulation process, disposing a solder ball on each of the contact pads. 
         [0020]    In the fabrication method of the package structure having the MEMS element, the wafer may be made of a silicon material; the MEMS element includes gyroscopes, accelerometers or radiation frequency, RF MEMS elements; the lids may be made of glass or silicon materials; the stud bumps may be made of a metallic material consisting of Gold (Au), Copper (Cu) or a soldering material such as Tin, wherein the stud bumps may be formed by means of wire-bonding or ball-planting; and the encapsulant may be made of thermal solid resins. 
         [0021]    In the fabrication method of the package structure having the MEMS element, the included angle of the side wall with respect to the top of the encapsulant is preferably 45 to 90 degrees, and the bevel cut may be performed by means of angled cutting knifes or laser grooving. 
         [0022]    In one aspect, the method of fabricating the package structure having the MEMS element further comprises forming a solder mask layer on the metal conductive layer with solder mask openings, for allowing each of the contact pads to expose from each solder mask opening, and further forming a solder ball on each of the contact pads. In addition, prior to formation of solder balls, an UBM (under bump metallurgy) layer may be formed on the contact pad between the solder balls and contact pads, and the UBM (under bump metallurgy) layer may be of a multi-layered structure comprised of Gold, Nickel, Nickel/Gold, or Gold/Nickel/Gold. 
         [0023]    In one aspect of the fabrication method of the package structure having the MEMS element, the encapsulant formed above the MEMS element may further comprise an encapsulant opening for exposing the lid therefrom to reveal the MEMS element therefrom. Further, the metal conductive layer may be formed to extend to the lid in the encapsulant opening and the contact pads are formed on the lid. 
         [0024]    In one aspect of the fabrication method of the package structure having the MEMS element, the encapsulant may be flush with the top of the lid, and the metal conductive layer may be formed to extend to the lid and the contact pads are formed on the lid. 
         [0025]    Summarizing from the above, the package structure having the MEMS element of the present invention is characterized in that the packaging process is completed directly on the wafer without the use of any additional carriers which allows the overall package thickness/size to be minimized. Moreover, the present invention avoids the need to drill holes on the glass lid that helps to simplify the manufacturing process and thus reduces the costs, and also the invention employs the technique of wafer-level packaging that reduces the manufacturing steps to a minimum as well as the time required as a result. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The present invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
           [0027]      FIGS. 1A  through  FIG. 1D  are cutaway views illustrating different embodiments of conventional package structures having the MEMS elements; 
           [0028]      FIGS. 2A through 2H  are cutaway views illustrating a first embodiment of the package structure having the MEMS element according to the present invention, wherein  FIG. 2F  is a top view and FIG.  2 F′ is a cutaway view of  FIG. 2F ; 
           [0029]      FIG. 3  is a cutaway view illustrating a second embodiment of the package structure having the MEMS element according to the present invention; 
           [0030]      FIG. 4  is a cutaway view illustrating a third embodiment of the package structure having the MEMS element according to the present invention; 
           [0031]      FIG. 5  is a cutaway view illustrating a fourth embodiment of the package structure having the MEMS element according to the present invention; 
           [0032]      FIG. 6  is a cutaway view illustrating a fifth embodiment of the package structure having the MEMS element according to the present invention; 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    The present invention is described in the following with specific embodiments, such that one skilled in the pertinent art can easily understand other advantages and effects of the present invention from the disclosure of the invention. The present invention may also be implemented and applied according to other embodiments, and the details may be modified based on different views and applications without departing from the spirit of the invention. 
         [0034]    The following embodiments describe a package structure and a fabrication method thereof. The drawings are simplified to show the special features of the invention in an easily understandable format, and only the components directly related to the invention are shown, details for the remaining components being omitted for brevity. 
       First Preferred Embodiment 
       [0035]      FIG. 2A  through  FIG. 2H  are cutaway views illustrating a first embodiment of the package structure having the micro-electromechanical (MEMS) element and a fabrication method thereof according to the present invention, wherein  FIG. 2F  is a top view and FIG.  2 F′ is a cutaway view of  FIG. 2F ; 
         [0036]    As shown in  FIG. 2A , a wafer  20  is provided whereon a plurality of electrical connecting pads  201  and a plurality of the MEMS elements  202  are formed, wherein the wafer  20  may be made of a silicon material, and the MEMS elements include gyroscopes, accelerometers or radiation frequency (RF MEMS) elements. 
         [0037]    As illustrated in  FIG. 2B , a plurality of lids  21  are disposed on the wafer  20  each covering on a MEMS element  202 , wherein the lids  21  may be formed by glass or silicon materials. 
         [0038]    As shown in  FIG. 2C , a stud bump  22  is formed on each of the electrical connecting pads  201 , wherein the stud bump  22  is made of a metallic material, and preferably, a soldering material such as Gold, Copper or Tin, and the stud bumps  22  may be formed by means of wire-bonding or ball-planting. 
         [0039]    As shown in  FIG. 2D , an encapsulant  23  is disposed on the wafer  20  for encapsulating the lids  21 , the stud bumps  22  and electrical connecting pads  201 , wherein the encapsulant  23  may be made of thermal solid resins of dielectric compound materials, such as Epoxy or Epoxy Molding Compound, EMC. In one aspect, the encapsulant  23  may be formed by covering a side surface of the lid  21  and is flush with the top of the lids  21 . 
         [0040]    As shown in  FIG. 2E , the encapsulant  23  is cut by means of a bevel cut along the periphery of the lids  21  to form a trench  230  having a wider top than the bottom thereof between two adjacent lids  21  and side walls  231  as tilting sides thereof, and partial surfaces of the stud bumps  22  are exposed therefrom, wherein the included angle of the side walls  231  with respect to the top surface of the encapsulant  23  is preferably 45 to 90 degrees, and the bevel cut is performed by means of angled cutting knifes or laser grooving. 
         [0041]    As illustrated in  FIG. 2F  and FIG.  2 F′, a metal conductive layer  24  electrically connected to the stud bumps  22  is formed on the encapsulant  23  with one end thereof extending towards the MEMS element  202  and further forming contact pads  241  at the terminal thereof. 
         [0042]    As illustrated in  FIG. 2G , a solder ball  25  is disposed on each of the contact pads  241  prior to performing the singulation process. In one aspect, prior to forming the solder balls  25  on the contact pads  241 , a solder mask layer (not shown) formed with solder mask openings (not shown) may be disposed on the metal conductive layer  24  for allowing each of the contact pads  241  to be revealed from each solder mask opening. Further, an under bump metallurgy, UBM layer may be formed before solder balls are formed, wherein the UBM layer may be of a multi-layered structure consisting of Gold, Nickel, Nickel/Gold, or Gold/Nickel/Gold. In that the manufacture of the UBM layer is a prior technique well-known in the art, further depiction thereof are omitted herein for the sake of brevity. 
         [0043]    As shown in  FIG. 2H , a singulation process is performed along the contour of each trench  230  to thereby form multiple complete package structures  2  each having a MEMS element  202  built therein. 
         [0044]    By the foregoing fabrication method, the present invention also proposes a package structure having the MEMS element as described above, comprising a chip  20 ′ having a plurality of electrical connecting pads  201  and a MEMS element  202  formed thereon; a lid  21  disposed on the chip  20 ′ for covering the MEMS element  202 ; a stud bump  22  disposed on each of the electrical connecting pads  201 ; an encapsulant  23  disposed on the chip  20 ′ and hermetically encapsulating the lid  21 , the stud bumps  22  and electrical connecting pads  201 , wherein parts of the stud bumps  22  are exposed from the encapsulant  23 , the encapsulant  23  comprising a protruding portion  233  hermetically encapsulating the lid  21 ; and a metal conductive layer  24  disposed on the encapsulant  23  and connected to the stud bumps  22 , wherein the metal conductive layer  24  extends from the end of stud bumps  22  toward the MEMS element  202 , and a contact pad  241  is formed above the MEMS element  202  or the lid  21 , wherein each of the contact pads  241  has a solder ball  25  disposed thereon. 
         [0045]    In one aspect of the foregoing package structure having the MEMS element, the chip  20 ′ may be made of a silicon material; the MEMS element  202  includes gyroscopes, accelerometers or radiation frequency (RF MEMS) elements; the lid  21  may be made of glass or silicon materials; the stud bumps  22  are preferably made of a metallic material consisting of Gold (Au), Copper (Cu), or a soldering material such as Tin; and the encapsulant  23  may be made of a dielectric material of thermal solid resins, such as Epoxy or Epoxy Molding Compound, EMC. 
         [0046]    In one aspect of the foregoing package structure having the MEMS element, the included angle of side walls  231  of the protruding portion  233  with respect to the top thereof is preferably 45 to 90 degrees, and further the stud bumps may be disposed at the periphery of the lid  21 . 
         [0047]    In one aspect of the foregoing package structure having the MEMS element, a solder mask layer formed with solder mask openings (not illustrated) is formed on the metal conductive layer  24  for allowing each of the contact pads  241  to be exposed therefrom, wherein each of the contact pads  241  has a solder ball  25  formed thereon. In one aspect, an under bump metallurgy, UBM layer (not illustrated) may be formed between solder balls  25  and contact pads  241 , and the under bump metallurgy, UBM layer may be of a multi-layered structure comprised of Gold, Nickel, Nickel/Gold, or Gold/Nickel/Gold. 
       Second Preferred Embodiment 
       [0048]      FIG. 3  is a cutaway view illustrating a second embodiment of the package structure having the MEMS element according to the present invention. In this embodiment, the package structure  3  is similar to the one depicted in  FIG. 2H  and only differs in that the protruding portion  233  of the encapsulant  23  formed on the MEMS element  202  is provided with an encapsulant opening  232  for allowing the MEMS element  202  to be exposed therefrom. The fabrication method of the package structure  3  is substantially identical to that of the first embodiment and the description thereof is omitted herein for brevity. 
       Third Preferred Embodiment 
       [0049]      FIG. 4  is a cutaway view illustrating a third embodiment of the package structure having the MEMS element according to the present invention. In this embodiment, the package structure  4  is similar to the one depicted in  FIG. 3  and only differs in that the metal conductive layer  24  is formed to extend to the lid  21  in the encapsulant opening  232 , and the contact pads  241  are formed on the lid  21 . The fabrication method of the package structure  4  is substantially identical to that of the first embodiment and the description thereof is omitted herein for brevity. 
       Fourth Preferred Embodiment 
       [0050]      FIG. 5  is a cutaway view illustrating a fourth embodiment of the package structure having the MEMS element according to the present invention. In this embodiment, the package structure  5  is similar to the one depicted in  FIG. 2H  and only differs in that the top of the encapsulant  23  is flush with the top of the lid  21 , meaning the protruding portion  233  is flush with the top of the lid  21 . The fabrication method of the package structure  5  is substantially identical to that of the first embodiment, in which the encapsulant  23  as illustrated in  FIG. 2D  may be formed to have a larger height than that of the lid  21 , or, the encapsulant  23  may be formed to cover a side surface of the lid  21  and be flush with the top of the lid  21  in the step of forming the encapsulant  23 . 
       Fifth Preferred Embodiment 
       [0051]      FIG. 6  is a cutaway view illustrating a fifth embodiment of the package structure having the MEMS element according to the present invention. In this embodiment, the package structure  6  is similar to the one depicted in  FIG. 5  and only differs in that the metal conductive layer  24  is formed to extend to the lid  21  and the contact pads  241  are formed on the lid  21 . The fabrication method of the package structure  6  is substantially identical to that of the first embodiment and the description thereof is omitted herein for brevity. 
         [0052]    In conclusion, the package structure having the MEMS element of the present invention yields advantages over prior techniques in that the packaging process can be completed on the wafer directly without the use of any additional carrier member to have a desirable package profile. Moreover, the present invention avoids the need to drill holes on the glass lid that helps to simplify the manufacturing process and thus reduces the costs, and also the invention employs the technique of wafer-level packaging that reduces the manufacturing steps to a minimum as well as the time required.