Abstract:
A method is provided for forming a microelectronic package at a wafer level. Such method can include providing a semiconductor wafer having a surface with a pattern of electrical contacts thereon. An interposer component can be provided which has a compliant dielectric layer bonded to a conductive layer. A pattern of holes can be formed through the compliant dielectric layer and the conductive layer which corresponds to the pattern of electrical contacts. The compliant dielectric layer can be contacted with the semiconductor wafer surface so that the pattern of holes is in an aligned position with the pattern of contacts and the compliant dielectric layer and the semiconductor wafer surface then bonded in the aligned position to unite the semiconductor wafer and the interposer component to form a wafer level semiconductor package. The wafer level semiconductor package can be diced to form individual semiconductor chip packages.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/005,607 filed Dec. 6, 2007, the disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter of the present application relates to microelectronic assemblies and methods for making microelectronic packages and assemblies at a wafer level. 
         [0003]    Microelectronic devices such as semiconductor chips typically require many input and output connections to other electronic components. The input and output contacts of a semiconductor chip or other comparable device are generally disposed in grid-like patterns that substantially cover a surface of the device (commonly referred to as an “area array”) or in elongated rows which may extend parallel to and adjacent each edge of the device&#39;s front surface, or in the center of the front surface. Typically, devices such as chips must be physically mounted on a substrate such as a printed circuit board, and the contacts of the device must be electrically connected to electrically conductive features of the circuit board. 
         [0004]    Semiconductor chips are commonly provided in packages that facilitate handling of the chip during manufacture and during mounting of the chip on an external substrate such as a circuit board or other circuit panel. For example, many semiconductor chips are provided in packages suitable for surface mounting. Numerous packages of this general type have been proposed for various applications. Most commonly, such packages include a dielectric element, commonly referred to as a “chip carrier” with terminals formed as plated or etched metallic structures on the dielectric. These terminals typically are connected to the contacts of the chip itself by features such as thin traces extending along the chip carrier itself or by fine leads or wires extending between the contacts of the chip and the terminals or traces. 
         [0005]    The dielectric substrate utilized in such a package can be made from a material such as a polyimide or other polymeric sheet. Although the thickness of the dielectric substrate will vary with the application, the dielectric substrate most typically is about 10 μm-100 μm thick. The terminals and conductive traces may be formed on the sheet from any electrically conductive material, but most typically are formed from copper, copper alloys, gold or combinations of these materials. The thickness of the traces will also vary with the application, but typically is about 5 μm-25 μm. 
         [0006]    In a surface mounting operation, the package is placed onto a circuit board so that each terminal on the package is aligned with a corresponding contact pad on the circuit board. Solder or other bonding material is provided between the terminals and the contact pads. The package can be permanently bonded in place by heating the assembly so as to melt or “reflow” the solder or otherwise activate the bonding material. 
         [0007]    Many packages include solder masses in the form of solder balls, typically about 0.1 mm and about 0.8 mm (5 and 30 mils) in diameter, attached to the terminals of the package. A package having an array of solder balls projecting from its bottom surface is commonly referred to as a ball grid array or “BGA” package. Other packages, referred to as land grid array or “LGA” packages are secured to the substrate by thin layers or lands formed from solder. Packages of this type can be quite compact. Certain packages, commonly referred to as “chip scale packages,” occupy an area of the circuit board equal to, or only slightly larger than, the area of the device incorporated in the package. This is advantageous in that it reduces the overall size of the assembly and permits the use of short interconnections between various devices on the substrate, which in turn limits signal propagation time between devices and thus facilitates operation of the assembly at high speeds. 
         [0008]    Microelectronic devices such as semiconductor chips normally are made by processing a large body of material, commonly referred to as a “wafer”, to form features such as electronic circuits in numerous regions of the wafer and then severing of “dicing” the wafer to yield individual devices. Microelectronic packages also include wafer level packages. In manufacture of a wafer-level package, the wafer is subjected to a number of additional process steps to form the package structure and the wafer is subsequently diced to free the individual chips. Wafer level package processing provides an advantage in that the cost of the packaging processes is divided among the various chips on the wafer. Testing of individual chips poses another formidable problem with respect to individual handling and interfacing of the test equipment with the relatively delicate unpackaged chips. Wafer level package processing facilitates wafer level testing. 
         [0009]    Assemblies including packages can suffer from stresses imposed by differential thermal expansion and contraction of the device and the substrate. During operation, as well as during manufacture, a semiconductor chip tends to expand and contract by an amount different from the amount of expansion and contraction of a circuit board. Where the terminals of the package are fixed relative to the chip or other device, such as by using solder, these effects tend to cause the terminals to move relative to the contact pads on the circuit board. This can impose stresses in the solder that connects the terminals to the contact pads on the circuit board. As disclosed in certain embodiments of U.S. Pat. Nos. 5,679,977; 5,148,266; 5,148,265; 5,455,390; and 5,518,964, the disclosures of which are incorporated by reference herein, semiconductor chip packages can have terminals that are movable with respect to the chip or other device incorporated in the package. Such movement can compensate to an appreciable degree for differential expansion and contraction. 
         [0010]    Certain embodiments disclosed in the aforementioned patents contemplate fabrication of the package structures in a wafer-level process. However, even further improvements would be desirable. 
       SUMMARY 
       [0011]    In accordance with an aspect of the invention, a method is provided for forming a microelectronic package at a wafer level. Such method can include providing a semiconductor wafer having a surface with a pattern of electrical contacts thereon. An interposer component can be provided which has a compliant dielectric layer bonded to a conductive layer. A pattern of holes can be formed through the compliant dielectric layer and the conductive layer which corresponds to the pattern of electrical contacts. The compliant dielectric layer can be contacted with the semiconductor wafer surface so that the pattern of holes is in an aligned position with the pattern of contacts and the compliant dielectric layer and the semiconductor wafer surface can be bonded in the aligned position to unite the semiconductor wafer and the interposer component to form a wafer level semiconductor package. The wafer level semiconductor package can be diced to create individual semiconductor chip packages. 
         [0012]    In accordance with such aspect of the invention, the compliant dielectric layer can be a b-stage polymer prior to bonding with the semiconductor wafer surface. 
         [0013]    In addition, the compliant dielectric layer can be further polymerized during bonding with the semiconductor wafer surface. 
         [0014]    In accordance with one or more further aspects of the invention, such method may include one or more further additional steps. For example, a second interposer component can be provided which has a second compliant dielectric layer bonded to a second conductive layer. A second pattern of holes can be formed which extend through the second compliant dielectric layer and the second conductive layer corresponding to the pattern of holes through the compliant dielectric layer and the conductive layer. The second compliant dielectric layer can be contacted with the conductive surface so that the second pattern of holes is in an aligned position with the pattern of contacts. The second compliant dielectric layer can be bonded with the conductive surface in the aligned position to unite the second interposer component and the interposer component. 
         [0015]    In accordance with one or more further aspects of the invention, the holes in the second interposer component can be larger than the holes in the interposer component, whereby at least a portion of the conductive surface is exposed. 
         [0016]    In accordance with an aspect of the invention, a method can be provided for forming microelectronic packages. In accordance with such aspect, a subassembly can be provided which includes a wafer having a pattern of contacts thereon. A dielectric layer may cover the wafer, and a conductive layer can cover the dielectric layer, with holes extending through the conductive layer and dielectric layer in alignment with the contacts. The subassembly can be treated so as to remove portions of the conductive layer and form individual conductive features. At least some of the conductive features can be wire-bonded to the contacts of the dielectric layers. The wire bonds can extend through the holes. The subassembly can then be severed to form a plurality of individual units. 
         [0017]    In accordance with an aspect of the invention, the subassembly can be provided in a manner which includes forming the holes in the dielectric layer and conductive layer and then uniting the dielectric layer and conductive layer with the wafer. 
         [0018]    The holes may be formed in a manner that includes punching a starting material including the conductive layer and the dielectric layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a sectional view of a portion of a wafer component during a manufacturing process in accordance with an embodiment; 
           [0020]      FIG. 2  is a sectional view of a portion of an interposer component during a manufacturing process in accordance with an embodiment; 
           [0021]      FIG. 3  is a sectional view illustrating an interposer such as shown in  FIG. 2  at a later stage of a process in accordance with an embodiment; 
           [0022]      FIG. 4  is a sectional view illustrating an interposer component as shown in  FIG. 3  as assembled to a wafer component (as illustrated, e.g., in  FIG. 1 ) at a later stage of an assembly process; 
           [0023]      FIGS. 5-11  are sectional views illustrating stages in a method of manufacturing in accordance with one embodiment; 
           [0024]      FIG. 12  is a sectional view illustrating an assembly according to another embodiment; and 
           [0025]      FIGS. 13-19  are sectional views illustrating stages in a method of manufacturing an assembly such as illustrated in  FIG. 12  in accordance with one embodiment. 
       
    
    
       [0026]    In the various figures of this specification, the various layers of materials are not drawn to scale but rather are depicted in various thicknesses for clarity or convenience of depiction. Also, the lateral boundaries of the various wafer, interposer, conductive layers and other packaging features show only a representative portion of the wafer scale package and will be understood by a person of skill in the art to be substantially replicated across the extent of a typical wafer. 
       DETAILED DESCRIPTION 
       [0027]    Referring to  FIG. 1 , a wafer component  8  includes electrical contacts  4  on an upper surface  5  of a semiconductor wafer  2  containing integrated circuits attached together at dicing lanes (now shown). A contact  4  is typically fashioned of aluminum and is plated with a layer  6  of electro-less nickel in order to make the contact compatible with other metals to be deposited in subsequent steps. Although only one contact is visible in RIG.  1 , the wafer contains a pattern of numerous chips and each chip has a pattern of numerous contacts. Once the contacts  4  are plated, the wafer component  8  is prepared for subsequent laminating with protective and signal carrying layers to form a plurality of chip packages at the wafer level for subsequent separation (dicing). Throughout this description, terms such as “upper,” “lower,” “top” and “bottom” do not refer to any gravitational positioning but instead refer to directions in the frame of reference of the components. 
         [0028]    As seen in  FIG. 2 , the stock for an interposer component  10  has a dielectric element  12  positioned adjacent a conductive top layer  14  with an optional bottom layer  16  disposed remote from the top layer  14 . The dielectric element  12  is typically a polymer such as a polyimide sheet. The conductive top layer  14  can be a metallic sheet, which can include or consist essentially of copper. In a particular example, the thickness of the metallic sheet can range between 12 and 18 micrometers. The polymer dielectric element  14  may be only partially cross-linked, that is, B-stage, material which is tacky. Bottom layer  16  may optionally be a release film. One suitable stock is commercially available under the definition of TC-F-100 from Hitachi Chemical Co., Ltd. 
         [0029]    The fabrication of an interposer component from a B-stage polymer dielectric layer  12  having a conductive sheet  14  attached thereto provides several advantages. Firstly, no adhesive may be needed to assemble the dielectric layer  12  to the sheet  14  or may be required when the dielectric layer is laminated to another element or substrate in subsequent operations, because the B-stage material is tacky and only partially cured. Secondly, the B-stage dielectric material is conformable to the substrate to allow proper adhesion and dimensional control of the overall thickness of the assembly until the B-stage material is fully polymerized. 
         [0030]    In one stage of the process, as shown in  FIG. 3 , a pattern of through holes  18  is created through the entire thickness of the interposer component  10  including the conductive layer  14 . The pattern forms clearance holes  18  that will correspond with the pattern of contacts  4  on the wafer. In this case, the through holes  18  are formed prior to assembling the interposer component  10  with the wafer component  8 . The holes  18  may be formed by many methods such as physically removing material by mechanical, chemical or physical means. For example, the pattern of holes  18  may be punched with a matching die set, i.e., a punch and die having an opening such that the punch travels through the material into the opening of the die. The punching operation can be performed, in one example, with an additional polymer film (not shown) placed between the bottom sheet  16  and the die of such die set during the punching operation, so that the punch passes through the additional die layer before entering the die. 
         [0031]    Once the interposer component  10  is punched, the bottom sheet  16  is peeled and discarded. As shown in  FIG. 4 , the B-stage dielectric layer  12  is placed on top of the wafer component  8  with the pattern of holes  18  aligned with the pattern of contacts  4 . The holes  18  may be sized sufficiently larger than the contacts  4  to readily provide sufficient pattern matching tolerances. A further advantage is that the positions of the contacts  4  are visible through the holes  18  to allow a machine vision system to directly optimize the placement of the pattern of holes  18  relative to the pattern of contacts  4  rather than rely on secondary datums and predicted positions of the contacts. 
         [0032]    The components  8  and  10  are then laminated and the B-stage dielectric material is cured by known polymer curing methods to form an initial wafer level assembly  20 . Thus wafer level assemblies  20 , each having a resilient dielectric layer  12  and a conductive layer  14 , are formed after assembly of an interposer component  10  with the wafer component  8 . The assembly  20  provides access to the wafer contacts  4  without the need for patterning the holes  18  after assembly of an interposer component  10  with the wafer component  8 . Moreover, the process of patterning the holes can be performed rapidly and economically by punching. 
         [0033]    During the next phase of processing the conductive layer  14  is patterned or separated into distinct electrically conductive runs and terminals. As shown in  FIG. 5  a resist mask  22  is created by processes such as applying electro-pneumatically deposited photoresist (EPDR), selectively exposing the photoresist to light, and developing the resist to leave the resist over those portions of the conductive layer  14  that will ultimately be removed in order to create the conductive features of the interposer. 
         [0034]    A layer of electroless or electroplated nickel  24  is then plated on the previously nickel plated surface  6  of the contacts  4  and on the exposed portions of the conductive layer  14  that are not covered by photo resist  22 . A layer of gold  26  is then plated on the contacts and on the exposed potions of the conductive layer, as seen in the stage of the assembly  20  depicted in  FIG. 6 . The photoresist structures  22  are then stripped and an etchant such as nitric acid is used to remove the underlying conductive layer  14  in the portions of the package formerly covered by the photoresist. This etching forms channels  28  ( FIG. 7 ) through the electrically conductive layers  14 ,  24  and  26  to create electrically distinct, separate conductive elements  27  in patterns constituting traces and terminals as shown in  FIG. 7 . 
         [0035]    Referring to  FIG. 8 , solder mask  30  is then applied to the upper face of the wafer level assembly  20  to cover the conductive elements except in those areas of the conductive elements that form bond pads  32  and terminals  36 . For example, the solder mask covers portions  34  of the conductive elements which serve as traces connecting the bond pads  32  and contacts  36 . Next, as shown in  FIG. 9 , wire bonds  38  are used to electrically connect the plated contacts  4  of the wafer  2  to the bond pads  32 . The wire bonds  38  extend from the contacts  2  through the holes  18  in the interposer component  10  to the bond pads  32 . Encapsulant  40  is then used to fill, seal and protect the holes  18 , wire bonds  38  and the bond pads  32  resulting in the wafer level assembly  20  as shown in  FIG. 10 . Fabrication of the wafer level assembly  20  optionally may include attaching an array of solder balls  42  to the terminals  36  as depicted in  FIG. 11 . 
         [0036]    The process discussed above is substantially insensitive to manufacturing variations and thus provides a robust “tolerance budget.” The holes  18  ( FIG. 9 ) can be formed in a precise pattern in the stock using a relatively inexpensive punching process. Moreover, because the interposer element includes a continuous metal layer, it has predictable and substantially isotropic thermal expansion characteristics, and does not tend to stretch or shrink in horizontal dimensions parallel to the plane of the stock. This facilitates registering the holes  18  with numerous sets of contacts on a large wafer. Additionally; the holes  18  can be larger than the contacts on the wafer, so that any deviation from ideal placement of the punched interposer element on the wafer does not cause the interposer element to cover the contacts. Because the masking and etching processes used to form the individual conductive features from the conductive layer are performed after the interposer element is united with the wafer, tolerances on placement of the interposer element do not affect the location of the bond pads  32  ( FIGS. 8 and 9 ) relative to the contacts. For example, the photolithographic process used to pattern the photoresist can be performed in registration with the wafer  2 , so that the resulting conductive features, including the bond pads, are in a precise positional relationship with the bond pads. Additionally, the wire bonding process can compensate for any minor misalignment between the bond pads and the contacts. Some wire-bonding systems incorporate machine-vision systems which can detect the location of each contact, or each group of closely-spaced contacts on the wafer, and the location of the corresponding bond pads, and which can modify the motion of the wire-bonding tool accordingly. 
         [0037]    The completed wafer level assembly  20  may then be tested and diced into individual packaged chips. In the dicing step, the wafer is cut into individual chips and the interposer component attached to the wafer is cut with the wafer into individual package elements assembled together with each chip. The circuitry and performance of each chip may be tested at the wafer level prior to dicing. This testing can be conducted by establishing electrical contacts for the test equipment through the terminals  36 , before or after the addition of the solder balls  42 . 
         [0038]    Turning to an embodiment depicted in  FIG. 12  and subsequent figures, the process previously described is enhanced by steps to incorporate an additional interposer component  10 ′ to the interposer component  10  to provide the wafer scale assembly  50  having a ground plane. 
         [0039]    Second interposer component  10 ′ has a dielectric element  12 ′ positioned adjacent an exposed conductive top layer  14 ′. The second interposer component  10 ′ typically has the same materials and characteristics as previously described for the interposer component  10 , although variations in dielectric and conductive materials, as well as layer thickness can be used. By methods disclosed in the description of  FIG. 3 , a first pattern of through holes  18  is created through the first interposer component  10  and a second pattern of through holes  18 ′ can be formed to extend through the second interposer component  10 ′. The second pattern forms clearance holes that will correspond with the pattern of contacts  4  on the wafer and the first pattern of holes  18  created through the interposer component  10 . The creation of through holes  18  and  18 ′ occurs prior to assembling the interposer components  10  and  10 ′ with the wafer component  8  to form the assembly  50 . The holes  18 ′ can have larger horizontal dimensions than the holes  18 . 
         [0040]    As discussed above, if the stock used to form the interposer components  10 ,  10 ′ includes a release layer, the release layer is peeled and discarded. As shown in  FIG. 12 , the interposer component  10  is assembled with the wafer component  8  by placing the B-stage dielectric layer  12  on top of the wafer component  8  with the pattern of holes  18  aligned with the pattern of contacts  4 . The interposer component  10 ′ is placed on the interposer component  10  with the B-stage dielectric layer  12 ′ on top of the conductive layer with the pattern of holes  18 ′ aligned with the pattern of holes  18 . If desired, the interposer components  10  and  10 ′ may also be assembled together prior to being united with the wafer component  8 . Because the holes  18 ′ are sufficiently larger than the holes  18 , a land region  52  of the conductive layer  14  remains exposed to provide access for electrical connections to the conductive layer  14 . As discussed below, layer  14  will ultimately serve as a ground plane. Here again, the method of forming holes through interposer components prior to assembly provides tolerance advantages. Creating stepped holes  18  and  18 ′ with lands  52  would be difficult after the interposer components were united with the wafer component. 
         [0041]    The components  8 ,  10  and  10 ′ are then laminated by curing the B-stage dielectric by known polymer curing methods to form an initial wafer level assembly  50 , as seen in  FIG. 12 . Thus, a wafer level assembly  50  having resilient dielectric layers  12  and  12 ′, conductive layer  14  and conductive ground plane layer  14 ′ is formed after assembly of the interposer components  10  and  10 ′ with the wafer component  8 . 
         [0042]    During the next phase of processing the conductive layer  14 ′ is patterned or separated into distinct electrically conductive traces and terminals using steps similar to those discussed above. As shown in  FIG. 13  a resist mask  22  is created by processes such as applying an electro-pneumatically deposited photoresist (EPDR), selectively exposing the photoresist to light, and developing the resist to leave the resist over those portions of the conductive layer  14 ′ that are ultimately to be removed in order to create the conductive runs and terminals. A layer of electroless or electroplated nickel  24  and a layer of gold  26  are then plated on the previously nickel plated surface  6  of the contacts  4 , the lands  52  and the exposed portions of the conductive layer  14 ′ that are not covered with resist mask  22 , to result in the stage of the assembly  50  depicted in  FIG. 14 . The photoresist structures  22  are then stripped as shown in  FIG. 15  and an etchant such as nitric acid is used to remove the underlying conductive layer  14 ′ in the portions of the assembly formerly covered by the photoresist. This etching create channels  28  through all the electrically conductive layers  14 ′,  24  and  26  to create electrically distinct and conductive elements  27  in patterns suitable for traces and terminals as shown in  FIG. 16 . 
         [0043]    Solder mask  39  can then be applied to the upper face of the assembly  50  in order to establish portions of the conductive elements  27  as the bond pads  32 , traces  34  and terminals  36  ( FIG. 16 ). Next, as shown in  FIG. 17  wire bonds  38  are used to electrically connect the plated contacts  4  of the wafer  2  to the bond pads  32  or to connect the lands  52 , as required by the chip circuitry and functions. Encapsulant  40  ( FIG. 18 ) is then used to fill, seal and protect the holes  18  and  18 ′, wire bonds  38  and the bond pads  32  resulting in the wafer level assembly  50  shown in  FIG. 18 . Fabrication of the wafer level assembly  50  optionally may include attaching an array of solder balls terminals  36  with the terminals  36  exposed at a top surface of the assembly  50 , as depicted in  FIG. 19 . The assembly  50  may then be diced into individual chip packages each containing one or more semiconductor chips diced from the original wafer  2 . 
         [0044]    In the process illustrated in  FIGS. 12-19 , the conductive layer  14  of the lower interposer element  10 , disposed closest to the wafer surface, is not etched, but instead, is left as a continuous ground plane in the finished structure. In a variant of this process, conductive layer  14  may be formed into individual conductive features such as traces, bond pads and terminals. The upper interposer element  10 ′ may be provided with additional holes (not shown) in alignment with terminals formed from the conductive layer of the lower interposer element  10 . In further variants, more than two interposer elements can be assembled together with the wafer in each wafer level assembly. In a particular embodiment, more than two interposer elements can be stacked vertically overlying the chip in each completed chip package. 
         [0045]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. For example, the stock for an interposer component  10  can be formed in many ways, such as by utilizing a fully cross-linked dielectric element  12  and adhesives to adhere the conductive top layer  14  and to bond the dielectric element  12  to the wafer. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention.