Abstract:
A fan out package includes a molding compound, a conductive plug in the molding compound, a dielectric covering the molding compound and a portion of the conductive plug, and an interconnect disposed over the dielectric and contacted with the conductive plug, wherein a width of the interconnect contacting the conductive plug is substantially smaller than a width of the conductive plug in the molding compound, and a width of the interconnect disposed over the dielectric is substantially greater than the width of the conductive plug in the molding compound.

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
       [0001]    This application is a continuation application of U.S. application Ser. No. 13/935,167 filed on Jul. 3, 2013, entitled “Semiconductor Device and Manufacturing Method Thereof”, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     FIELD 
       [0002]    The disclosure relates to a semiconductor device, and more particularly to a three dimensional integrated fan out package. 
       BACKGROUND 
       [0003]    Semiconductor device is widely adopted in various applications. The geometry is trending down rapidly as user&#39;s demands increases on the performance and functionality. For example, a 3G mobile phone presented in the market is expected to be capable of telecommunicating, capturing images and processing high stream data. In order to fulfill the requirements, the 3G mobile phone needs to be equipped with different devices such as a processor, a memory and an image sensor in a limited space. 
         [0004]    Combining several semiconductor devices in one package is an approach to enhance the performance by integrating devices with various functions into a single component. Roadmap in the field shows a three dimensional package with a multi-level structure for a superior and miniature-sized semiconductor component. 
         [0005]    A three dimensional integrated semiconductor package contains several different sub-structures. The sub-structures are arranged in a stack manner and are either in contact with each other or linked by interconnects. However, on the other hand, different properties of the sub-structures also create challenges to a designer. Compared to a two dimensional semiconductor package, failure modes increase for a comparatively more complex three dimensional integrated semiconductor package. As such, improvements in the structure and method for a three dimensional semiconductor package continue to be sought. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
           [0007]      FIG. 1  is a schematic of a three dimensional semiconductor structure. 
           [0008]      FIG. 2  is a 3D semiconductor structure which has a composite stress buffer between a molding compound  200  and a conductive plug. 
           [0009]      FIG. 3  is a semiconductor structure including a liner as a stress buffer. 
           [0010]      FIG. 4  is a semiconductor structure including a liner as a stress buffer and the liner is under a bottom surface of a molding compound. 
           [0011]      FIG. 5  is a semiconductor structure including a recess on a top corner of a conductive plug. 
           [0012]      FIG. 6  is a semiconductor structure including a recess on a top corner of a conductive plug. 
           [0013]      FIG. 7A-7M  are operations of a method of manufacturing a three dimensional semiconductor structure. 
           [0014]      FIG. 8A-8D  are operations of a method of manufacturing a three dimensional semiconductor structure. 
           [0015]      FIG. 9  is an integrated 3D IC package  600  according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    In the present disclosure, a three dimensional (3D) semiconductor structure is designed to prevent a crack generated from a location in the 3D semiconductor structure. The 3D semiconductor structure provides a package for a semiconductor chip. The semiconductor chip is enclosed inside the 3D semiconductor structure and electrically connected to an external circuit through interconnects in the structure. In some embodiments, the 3D semiconductor structure is a fan out package. In some embodiments, the 3D semiconductor structure is an integrated fan out package-on-package (POP) device. 
         [0017]    The 3D semiconductor structure is composed of two or more different substructures. In some embodiments, the substructures are dielectric, molding material, electrical interconnects, filled vias or plugs, and contact pads. In some embodiments, the dielectric is disposed between two conductive layers and formed with a polymer material such as epoxy, polyimide, polybenzoxazole (PBO), and the like. In some embodiments, the dielectric is disposed on the semiconductor chip that is placed in the 3D semiconductor structure. The dielectric can also include spin-on glass (SOG), silicon oxide, silicon oxynitride, or the like, by any suitable method such as spin coating or vapor deposition. 
         [0018]    The molding material is a compound and formed with composite materials including epoxy resins, phenolic hardeners, silicas, catalysts, pigments, and mold release agents. Material for forming a molding compound has a high thermal conductivity, a low moisture absorption rate, a high flexural strength at board-mounting temperatures, or a combination of these. 
         [0019]    The electrical interconnects are conductive lines or films routed inside the 3D semiconductor structure. In some embodiments, the electrical interconnects are redistribution layers (RDL). The RDLs are used for a fan-in or a fan-out process. In some embodiments, the electrical interconnects are formed with a conductive material such as gold, silver, copper, nickel, tungsten, aluminum, and/or alloys thereof. 
         [0020]    In some embodiments, a filled via or conductive plug in the present disclosure is a conductive post. The filled via or plug is conductive and is disposed in a substructure such as a carrier, a substrate, or a molding compound. The conductive filled via or plug is arranged to extend through the substructure and provides an electrical communication between a top surface and a bottom surface of the substructure. 
         [0021]    In some embodiments, a contact pad is disposed on a top surface of the 3D semiconductor structure. A top surface of the contact pad receives a solder ball or solder paste and acts as a terminal to connect the 3D semiconductor structure to an external circuit. A bottom surface of the contact pads is connected to an interconnect, such as an RDL. In some embodiments, the contact pad is an under bump metallization (UBM). A solder ball or solder paste is placed on the top surface of the UBM so that the 3D structure can be electrically connected to an external device. In some embodiments, the UBM is formed with a conductive material such as gold, silver, copper, nickel, tungsten, aluminum, and/or alloys thereof. 
         [0022]    In some embodiments, a 3D semiconductor structure has a conductive pillar disposed on a semiconductor chip. The semiconductor chip is placed in the 3D semiconductor structure. The conductive pillar is electrically connected with a bond pad of the semiconductor at one end. The conductive pillar is electrically connected with an interconnect such as an RDL at the other end. In some embodiments, a conductive pillar is a conductive bump. The conductive pillar is formed with a conductive material such as gold, silver, copper, nickel, tungsten, aluminum, tin and/or alloys thereof. Formation of the conductive pillar can be by a process such as evaporation, electroplating, vapor deposition, sputtering or screen printing. 
         [0023]    In some embodiments, a 3D semiconductor structure is manufactured using a wafer scale package (WSP) operation. In some embodiments, the 3D semiconductor structure is manufactured using a chip level package operation. In some embodiments, the 3D semiconductor structure is manufactured using a flip chip operation. 
         [0024]    The 3D semiconductor structure has a layer disposed between two different substructures. The layer is a stress buffer, also called a liner. The stress buffer or liner is designed to avoid a crack caused by an internal stress. In some embodiments, the internal stress originates from a difference in thermal expansion between the two different substructures. The thermal expansion difference is because a difference in a coefficient of thermal expansion (CTE) of the substructure materials between the two different substructures. 
         [0025]    The term “CTE” in the present disclosure is a property of an object when it is heated or cooled. An object&#39;s length changes by an amount proportional to the original length and the change in temperature. The unit of CTE is ppm/K, which stands for 10-6 m/m K. The CTE unit is shortened to “ppm” hereinafter. In the present disclosure, the CTE of pure copper is 16.6 ppm, and the CTE of pure silicon is 3 ppm. CTE of each material may be in a range according to the concentration of the impurity in the material. For a molding compound material, the CTE is dependent on the composition of the molding compound and may be a wide range up to over hundred ppm. 
         [0026]    In some embodiments, a 3D semiconductor structure has a stress buffer or liner between at least two substructures. The stress buffer or liner has a CTE that is between a CTE of one substructure and a CTE of the other substructure. The stress buffer or liner may be formed with an electrically conductive or electrically insulating material. In the present disclosure, the stress buffer or liner is, but is not limited to, immersion tin, electroless nickel electroless palladium immersion gold (ENEPIG), electroless nickel electroless palladium (ENEP), polybenzoxazole (PBO), polyimide and the like. In some embodiments, the stress buffer or liner is a composite film including at least two different films. 
         [0027]      FIG. 1  is a 3D semiconductor structure  12 . The 3D semiconductor structure  12  has a semiconductor chip  100 . The semiconductor chip  100  is located at the bottom of the structure  12 . In certain embodiments, the semiconductor chip  100  is on a die attached film (DAF). The semiconductor structure  12  is a package of the semiconductor chip  100 . The semiconductor chip  100  has a front side and a backside. A bond pad  154  of the semiconductor chip  100  is disposed on the front side. In some embodiments, the backside of the semiconductor is bonded with a heat dissipation layer, an adhesion layer, or a buffer layer. The semiconductor chip  100  has a passivation  152  on the front side around the bond pad  154 . The passivation  152  is formed with dielectric materials, such as spin-on glass (SOG), silicon oxide, silicon oxynitride, silicon nitride or the like. The passivation  152  provides an electrical isolation and a moisture protection for the semiconductor chip  100 . The passivation is formed with a vapor deposition or a spin coating process. 
         [0028]    Molding compound  200  is formed to surround sidewalls of the semiconductor chip  100 . The molding compound  200  has a top surface  202  and a bottom surface  204 . In some embodiments, the bottom surface  204  and the backside of the semiconductor chip  100  form a surface of the semiconductor structure  12 . For embodiments having a DAF under the semiconductor chip, the bottom surface  204  and the DAF form a surface of the semiconductor structure  12 . The molding compound  200  can be a single layer film or a composite film stack. For certain embodiments, the 3D semiconductor structure  12  has a bas buffer under the semiconductor chip  100  and the molding compound  200 . 
         [0029]    The 3D semiconductor structure  12  has a conductive filled via or plug  300  in the molding compound  200 . In some embodiments, the conductive filled via or plug  300  extends from the top surface  202  of the molding compound  200  to the bottom surface  204  of the molding compound  200 . The conductive filled via or plug  300  is connected with an interconnection  472  at one end of the conductive filled via or plug  300 . For the other end of the conductive filled via or plug  300 , the conductive filled via or plug  300  is available as a terminal for the 3D semiconductor structure  12  to be electrically connected with an external circuit located at the backside of the 3D semiconductor structure. In certain embodiments, one end of the filled-via  30  is connected to the cover  40  and the other end is connected with an external circuit located at a buffer layer. The buffer layer is disposed on the bottom surface of the semiconductor structure  10 . 
         [0030]    A conductive pillar  410  is disposed on the top surface of the bond pad  154 . The conductive pillar  410  is electrically connected to the bond pad  154  of the semiconductor chip  100  at one end. The conductive pillar  410  is electrically connected with an interconnect  471  at the other end. A first dielectric  501  is disposed on the semiconductor chip  100 . The conductive pillar  410  is surrounded by the first dielectric  501 . In some embodiments, the first dielectric  501  is a buffer between the passivation  152  and other dielectric layers over the first dielectric  501 . 
         [0031]    Interconnects such as  471  and  472  are included in the semiconductor structure  12 . Interconnects labeled with a same number in a same drawing are formed during a same operation. The interconnects  471  and  472  are electrical connections to and/or between the semiconductor chip  100  and an external circuit. In  FIG. 1 , an interconnect  471  on the first dielectric  501  is electrically connected with the conductive pillar  410  at one end. The interconnect  471  is electrically connected with an interconnect  472  at the other end. For certain embodiments as in  FIG. 1 , the interconnects  471  and  472  has a seed layer  475 . 
         [0032]    A UBM  480  is placed on a top surface of the semiconductor structure  12 . The UBM  480  has a bottom portion connected with an RDL  472 . The UBM  480  has a top surface  482 , which receives a solder ball or a solder paste. 
         [0033]    In  FIG. 1 , the 3D semiconductor structure  12  has a second dielectric  502 , and a third dielectric  503 . The second dielectric  502  is on the dielectric  501 , the conductive filled via or plug  300  and the molding compound  200 . The second dielectric  502  provides isolation between the RDL  471  and the RDL  472 . The second dielectric  502  has a through structure  512  filled with interconnect  472  and third dielectric  503 . The RDL  472  is electrically connected with the RDL  471  in the through hole  512 . The third dielectric  503  is formed on the second dielectric  502  and the RDL  472 . The third dielectric  503  protects the RDL  472  from being exposed to ambient conditions. The third dielectric  503  has a through structure  513  filled with UBM  480 . The UBM  480  is formed in the through structure  513  and electrically connected with an RDL  472 . 
         [0034]    The 3D semiconductor structure  12  has a liner  50  interposed between the molding compound  200  and the conductive plug  300 . The liner  50  acts as a stress buffer between the molding compound  200  and the conductive filled via or plug  300 . The liner  50  has a CTE that is between a CTE of the molding compound  200  and the conductive plug  300 . When a thermal process (heating or cooling) is applied on the 3D semiconductor structure  12 , dimension change for the molding compound  200  is greater than for the conductive plug  300 . For example, in some embodiments, a 3D semiconductor structure has a molding compound formed with epoxy with a CTE of 55 ppm and a conductive plug formed with copper with a CTE of 16 ppm. The large CTE mismatch (over 3 times) between the molding compound and the conductive filled via or plug generates an internal stress in the semiconductor structure, especially at an interface between the molding compound and the conductive plug. With a stress buffer layer, such as tin (CTE is around 23.4 ppm), disposed between the molding compound and the conductive plug, the gradient of CTE across the interface is reduced. Because the CTE of the stress buffer layer is between the molding compound and the conductive plug, the dimension change for the stress buffer layer is between that of the molding compound and the conductive plug. The internal stress is reduced because the CTE mismatch between adjacent films is decreased. In some embodiments, the CTE of the stress buffer layer is between about 9 ppm and about 90 ppm. In some embodiments, the CTE of the stress buffer layer is between about 25 ppm and about 70 ppm. 
         [0035]    In some embodiments, a liner as a stress buffer between a molding compound and a conductive filled via or plug is a composite film. The composite stress buffer has two or more stress buffer layers. In some embodiments, the stress buffer layer disposed closest to the conductive filled via or plug has a smallest CTE among all the stress buffer layers. In  FIG. 2 , a 3D semiconductor structure  12  has a composite stress buffer between a molding compound  200  and a conductive plug  300 . The composite stress buffer has a first stress buffer layer  51  and a second stress buffer layer  52 . The second stress buffer layer  52  is between the molding compound  200  and the first stress buffer layer  51 . The first stress buffer layer  51  is between the second stress buffer layer  52  and the conductive plug  300 . The second stress buffer layer  52  has a CTE that is between the CTE of the molding compound  200  and the CTE of the first stress buffer layer  51 . The first stress buffer layer  51  has a CTE that is between the CTE of the second stress buffer layer  52  and the conductive filled via or plug  300 . 
         [0036]    In some embodiments, the second stress buffer layer  52  is tellurium, which has a CTE around 37 ppm. The first stress buffer layer  51  is strontium, which has a CTE around 22.5 ppm. In some embodiments, the first stress buffer is nickel and the second stress buffer is immersion gold, palladium, of combinations thereof. The molding compound  200  is epoxy having a CTE around 75 ppm, and the conductive plug  300  is copper having a CTE around 16.6 ppm. The gradient of CTE change between the molding compound  200  and the conductive plug  300  is further reduced by the design of composite stress buffer. In some embodiments, the composite stress buffer has more than two different stress buffer layers between the molding compound and the conductive plug in order to change the gradient of CTE from the molding compound to the conductive filled via or plug. 
         [0037]    In some embodiments, a 3D semiconductor structure has a composite stress buffer and the composite stress buffer has one stress buffer layer formed with electrically conductive material and another stress buffer layer formed with electrically insulating material. For example, in some embodiments, a composite stress buffer has a stress buffer layer formed with polypropylene and another stress buffer made with silver. In certain some embodiments, a 3D semiconductor structure has a composite stress buffer and the composite stress buffer has all stress buffer layers formed with an electrically insulating material. 
         [0038]    In some embodiments, a liner or stress buffer disposed between a molding compound and a conductive plug is not a continuous layer. The liner or stress buffer may have several separated sections. In  FIG. 3 , a 3D semiconductor structure  12  has a molding compound  200 , a conductive plug  300  and a liner  50 . The liner  50  is between the molding compound  200  and the conductive plug  300 . The liner  50  has two separated sections for each side. In some embodiments, the liner  50  has at least one section extending to the top surface  202  of the molding compound  200 . In some embodiments, the liner has at least three separated sections for each side. 
         [0039]    In various embodiments, a liner or stress buffer disposed between a molding compound and a conductive plug. The liner or stress buffer does not cover a portion of the interface between the molding compound and the conductive plug. Thus, a portion of the conductive plug contacts the molding compound. 
         [0040]    In some embodiments, a liner as a stress buffer is further disposed on a bottom surface of the molding compound. In  FIG. 4 , a liner  50  is between the molding compound  200  and the conductive plug  300 . The liner  50  is further disposed on a bottom surface  204  of the molding compound  200  and the bottom of the die  100 . In certain embodiments, the liner  50  is disposed on a backside of a DAF. In still other embodiments, the liner  50  is disposed on a base buffer. The base buffer is disposed on the backside of the molding compound and the backside of the DAF. 
         [0041]    In some embodiments, thickness of a liner or stress buffer is between 0.2 μm and about 5 μm. In some embodiments, thickness of a liner or stress buffer is between 1 μm and 4 μm. In some embodiments, thickness of a liner or stress buffer is between 1.5 μm and 3.5 μm. 
         [0042]    For some embodiments as in  FIG. 5 , a 3D semiconductor structure  12  has a stress buffer  50  between a molding compound  200  and a conductive filled via or plug  300 . The top surface of the conductive via or plug  300  is recessed below the top surface of the molding compound  200 . The top surface of the conductive via or plug  300  is below the top surface of the molding compound  200 . In certain embodiments, the conductive filled via or plug  300  has a recessed top surface  310  around a top corner of the conductive filled via or plug  300 . The recessed top surface  310  is filled with a second dielectric  502 . In some embodiments, the recessed top surface  310  is in a ring shape from a top view perspective. In certain embodiments, the top surface of the liner or stress buffer  50  is coplanar with the top surface of the molding compound  200 . In certain embodiments, the top surface of the liner of stress buffer  50  is coplanar with the top surface of the recessed top surface of the conductive plug  300 . In certain embodiments, the top surface of the liner or stress buffer  50  is between the top surface of the molding compound  200  and the recessed top surface of the conductive plug  300 . In certain embodiments as in  FIG. 5 , the top surface of the conductive pillar  410  is recessed. The top surface of the conductive pillar  410  is lower than the top surface of a first dielectric  501 . 
         [0043]    For some embodiments as in  FIG. 6 , a 3D semiconductor structure  12  has a liner or stress buffer  50  between a molding compound  200  and a conductive filled via or plug  300 . A semiconductor die  100  is placed at the bottom of the 3D semiconductor structure  12 . The liner or stress buffer  50  is further disposed on the bottom surface of the molding compound  200  and the bottom of the semiconductor die  100 . The top surface of the conductive via or plug  300  is recessed below the top surface of the molding compound  200 . The top surface of the conductive via or plug  300  is below the top surface of the molding compound  200 . The conductive plug  300  has a recessed top surface  310  around a top corner of the conductive plug  300 . The recessed top surface  310  is filled with a second dielectric  502 . In some embodiments, the recessed top surface  310  is in a ring shape from a top view perspective. In certain embodiments, the top surface of the liner or stress buffer  50  is coplanar with the top surface  202  of the molding compound  200 . In certain embodiments, the top surface of the liner of stress buffer  50  is coplanar with the top surface of the recessed top surface of the conductive plug  300 . In certain embodiments, the top surface of the liner or stress buffer  50  is between the top surface of the molding compound  200  and the recessed top surface of the conductive plug  300 . 
         [0044]    A method of manufacturing a 3D semiconductor structure and the semiconductor structure has a liner designed as a stress buffer between two different substructures. The method includes a number of operations and the description and illustration are not deemed as a limitation as the order of the operations. 
         [0045]    A term “patterning” or “patterned” is used in the present disclosure to describe an operation of forming a predetermined pattern on a surface. The patterning operation includes various steps and processes and varies in accordance with the features of embodiments. In some embodiments, a patterning operation is adopted to pattern an existing film or layer. The patterning operation includes forming a mask on the existing film or layer and removing the unmasked film or layer with an etch or other removal process. The mask is a photo resist, or a hardmask. In some embodiments, a patterning operation is adopted to form a patterned layer directly on a surface. The patterning operation includes forming a photosensitive film on the surface, conducting a photolithography process and a developing process. The remaining photosensitive film is retained and integrated into the 3D semiconductor structure. 
         [0046]    A term “plating” or “plated” is used in the present disclosure to describe an operation of forming a film or a layer on a surface. The plating operation includes various steps and processes and varies in accordance with the features of embodiments. The film or layer been plated on the surface is a single film or a composite stack. In some embodiments, a plating operation is adopted to form a metallic film. In some embodiments, a plating operation includes forming a seed layer and electroplating a metallic film on the seed layer. In some embodiments, a plating operation is a vapor deposition process. In some embodiments, a plating operation is a sputtering process. 
         [0047]    A term “filling” or “filled” is used in the present disclosure to describe an operation of forming material in a hole. The filling operation includes various steps and processes and varies in accordance with the features of embodiments. In some embodiments, a filling operation includes forming a conductive material in a hole. In some embodiments, a filling operation includes forming a liner on the sidewalls of the hole and forming a conductive film on the liner. In some embodiments, a filling operation includes a electroplating process. In some embodiments, a filling operation includes a vapor deposition process. In some embodiments, a filling operation includes a sputtering process. 
         [0048]    In  FIG. 7A , a carrier  700  is provided to form a 3D semiconductor structure thereon. The carrier  700  is a substrate designed to from the 3D semiconductor structure thereon. Shape of the carrier can be different according to the design. In some embodiments, the shape of the carrier is round. In some embodiments, the carrier is silicon or silicon oxide. In some embodiments, the carrier is removed after the 3D semiconductor structure is formed. For some embodiments as in  FIG. 7A , a glue layer  702  is disposed on the carrier  700 . The glue layer  702  acts as a detachable bonding layer to bind a film or structure to the carrier  700 . The carrier  700  can be removed from the structure attached thereon by degrading the glue layer  702 . A base buffer  704  is disposed on the glue layer  702 . The base buffer layer  704  is formed with material such as polyimide, PBO, SR, LTHC (light to heat conversion film), wafer backside coating tape, and ABF, or the like. In some embodiments, the base buffer layer  704  includes at least two layers with different materials. 
         [0049]    In  FIG. 7B , a seed layer  705  is formed over the top surface of the carrier  700 . The seed layer  705  is a single layer or a composite stack and formed with material such as copper, tantalum, tin, titanium/copper, tin/copper, tantalum/copper, or the like. The seed layer  705  provides growing sites to promote an electroplating operation. In some embodiments, a sputtering or vapor deposition process is employed to form a seed layer over the carrier  700 . 
         [0050]      FIG. 7C  is an operation of forming a patterned layer  708  on the seed layer  705 . In some embodiments, the patterned layer  708  is formed with a photoresist such as polyimide by a spin coating process. The patterned layer  708  has two or more through structures  718 . 
         [0051]    In  FIG. 7D , the through structures  718  is filled with an electrically conductive material  710 . An electroplating or a sputter process can be adopted to form conductive material in the through structures. 
         [0052]      FIG. 7E  is an operation of stripping the patterned layer from the top surface of the seed layer  705 . The stripping operation is a selective cleaning process to remove only the patterned layer without damaging the electrically conductive material. Post-like electrically conductive material  710  is preserved after the stripping operation.  FIG. 7F  is an operation of removing the seed layer that is located between the conductive posts and only the portion under the conductive posts still remains. In some embodiments, the seed layer is a metal; to remove the seed layer between conductive plugs can prevent electrically communication between the conductive plugs. The remained seed layer and conductive post form several conductive plugs  300  on the base buffer  704 . 
         [0053]      FIG. 7G  is an operation of forming a liner or stress buffer  50  on the conductive plugs  300  and the base buffer  704 . The liner or stress buffer  50  is a stress buffer layer. In some embodiments, the liner or stress buffer  50  is formed with an electroless plating process. A metal layer is formed on the conductive plugs  300  and the base buffer  704  before electroless plating the liner or stress buffer  50 . In some embodiments, the metal layer is tin. In some embodiments, the liner or stress buffer  50  is formed with a vapor deposition process. In some embodiments, the liner or stress buffer  50  is a polymer. Spin coating is employed to form a liner on the conductive plugs for some embodiments. Materials such as spin-on glass or polyimide are adopted to form the liner. 
         [0054]    In some embodiments, a liner is designed as a stress buffer including at least two different films. As in  FIG. 7H , the liner is a composite film including a first stress buffer layer  51  and a second stress buffer  52 . The first stress buffer layer  51  is disposed on the conductive plug  300  and the top surface of the base buffer  704 . The second stress buffer layer  52  is disposed on the first stress buffer layer  51 . In some embodiments, a different process forms each stress buffer layer. For example, the first stress buffer layer is formed by electroless plating a tin layer on the conductive plugs and the base buffer. The second stress buffer is formed by spin coating a PBO layer on the first stress buffer layer. In some embodiments, the first stress buffer layer and the second stress buffer layer are formed in a same process. For example, the first stress buffer layer and the second stress buffer are formed by a vapor deposition. The first stress buffer layer is formed by disposing titanium on the conductive plugs and the base buffer. The second stress buffer layer is formed by disposing a titanium nitride on the first stress buffer layer. In some embodiments, an in-situ recipe is designed to form two different buffer layers in same manufacturing equipment. 
         [0055]    For embodiments with an electrically conductive liner or stress buffer, an additional removal operation is necessary to prevent short circuit between the conductive plugs. For example, if the liner is a tin film, an operation as in  FIG. 7I  is introduced to remove a portion of the liner or stress buffer that is disposed between the conductive plugs  300 . The operation includes a step to mask the conductive plugs  300  before removing a portion of the liner  50 . For embodiments with an electrically insulating liner, such as a PBO liner, the removal operation as in  FIG. 7I  is optional. 
         [0056]    For certain embodiments, a liner or stress buffer is formed on the conductive plugs  300  without a seed layer disposed on the carrier  700 . The liner of stress buffer is formed on the conductive plugs  300  by electroless plating. The liner or stress buffer is selectively formed on the conductive plugs  300 . Thus, the removal operation as in  FIG. 7I  is skipped. 
         [0057]    In  FIG. 7J , a semiconductor chip  100  is placed on the carrier  700  and located between the conductive plugs  300 . For certain embodiments, a die attached film (DAF) is disposed between the semiconductor chip  100  and the base buffer  704 . The semiconductor chip  100  is covered by a first dielectric  501 . A conductive pillar  410  is disposed on the semiconductor chip  100  in order to electrically communicate with an interconnect. In some embodiments, the first dielectric  501  is formed on the semiconductor chip  100  after the semiconductor chip is placed on the carrier  700 . In some embodiments, the first dielectric  501  is pre-formed on the semiconductor chip  100  before placing the chip  100  is placed on the carrier  700 . 
         [0058]      FIG. 7K  is an operation of disposing molding compound on a carrier. Molding compound  200  is formed by coating, injection, or compress and placed on the carrier  700 . The molding compound  200  also covers the conductive plugs  300 . For some embodiments, if the conductive plugs are with small pitch, a liquid molding compound (LMC) is selected to fill in small gaps. A curing process can be implemented after forming the molding compound in order to harden the molding compound. 
         [0059]    The method of manufacturing a 3D semiconductor structure includes a grinding process as in  FIG. 7L  to expose the conductive pillar  410 . The grinding process is a blanket removal process so that the molding compound  200 , the conductive plugs  300  are ground to a same level as the conductive pillar  410 . 
         [0060]    In some embodiments, the conductive plug  300  is made with a softer material than the molding compound  200 . Debris of removed conductive filled via or plug material is trapped in the molding compound  200 . A clean operation is adopted to selectively remove a certain amount of conductive plug to ensure that no electrically conductive residues are embedded in the molding compound  200 . As in  FIG. 7M , the conductive filled via or plug  300  has a recessed top surface, which is below the top surface of the molding compound  200 . 
         [0061]      FIG. 8A  is a 3D semiconductor structure according to present disclosure. The 3D semiconductor structure has several conductive filled vias or plugs  300  and each conductive plug has a recessed top surface. A second dielectric  502  is formed on a molding compound  200  and a first dielectric  501 . The second dielectric  502  also fills in the recesses of the conductive plugs  300 . In some embodiments, the second dielectric  502  is formed with a material same as the first dielectric  501 . In some embodiments, the material for the second dielectric  502  is different from the first dielectric  501 . 
         [0062]    In  FIG. 8B , the second dielectric  502  is patterned to have several through structures  512 . The top surface of the conductive filled via or plug  300  is exposed at the bottom opening of the through structures  512 . In  FIG. 8C , a conductive film  725  is formed on the second dielectric  502 , the conductive filled vias or plugs  300  and in the through structures  512 . The conductive plug  300  includes a recess  310  at a top corner of the conductive plug  300 . The recess  310  is filled with the second dielectric  502 . The recess  310  has a ring shape. 
         [0063]    The conductive film is patterned to be an RDL  571  as in  FIG. 8D . In some embodiments, a 3D semiconductor structure includes RDL distributed in different layers. 
         [0064]      FIG. 9  is an integrated 3D IC package  600 . The integrated 3D IC package  600  includes the 3D semiconductor structure  12  as in  FIG. 1  and a memory chip  11 . The 3D semiconductor structure  12  has a liner or stress buffer layer  50 . The memory chip  11  is electrically connect with the 3D semiconductor structure  12 . 
         [0065]    A fan-out package includes a molding compound, a conductive plug and a stress buffer. The conductive plug is in the molding compound. The stress buffer is between the conductive plug and the molding compound. The stress buffer has a coefficient of thermal expansion (CTE). The CTE of the stress buffer is between a CTE of the molding compound and a CTE of the conductive plug. A method of manufacturing a three dimensional includes plating a post on a substrate, and disposing a stress buffer on the sidewall of the post. The method further includes surrounding the stress buffer with a molding compound. 
         [0066]    A semiconductor structure includes a molding compound, a filled via and a liner. The filled via is in the molding compound. The liner is between the molding compound and the filled via. The liner is tin, tungsten, zirconium, gold, palladium, polyimide, ENEPIG, ENEP, or PBO. 
         [0067]    A method of manufacturing a three dimensional semiconductor package. The manufacturing method includes plating a post on a substrate, and disposing a stress buffer on the sidewall of the post. The method further includes surrounding the stress buffer with a molding compound. 
         [0068]    The methods and features of this invention have been sufficiently described in the above examples and descriptions. It should be understood that any modifications or changes without departing from the spirit of the invention are intended to be covered in the protection scope of the invention.