Abstract:
In a process of fabricating flip chip interconnection, a UBM layer is deposited on an I/O pad of a chip. The UBM layer includes a nickel layer. On the UBM layer is formed a tin-containing solder material. The chip is mounted on a carrier substrate by alignment of the bonding pad with a contact pad of the carrier substrate. A reflow process is performed to respectively turn the tin-containing solder material to a tin-containing solder bump and form a composite intermetallic compound on the nickel layer of the UBM to prevent its spalling.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional application of, and claims the priority benefit of, U.S. application Ser. No. 10/065,297 filed on Oct. 01, 2002. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates generally to flip chip interconnection structure and a manufacture process of the flip chip interconnection structure that prevent spalling effects. More particularly, the invention forms a composite intermetallic compound in the flip chip interconnection structure to prevent spalling effects.  
           [0004]    2. Description of the Related Art  
           [0005]    In response to the concern of environmental pollution by lead (Pb) which seriously affects human health, the industrialized countries have implemented restrictive standards to reduce as much as possible the use of lead-containing substances in industrial products and manufacture processes. In some aspects, those restrictions have met successful results such as, for example, the developments of automobile lead-free fuel and lead-free paint products. In other aspects such as electronic products, more restrictions are expected to accelerate the ban of lead-containing substances in manufactured products. In this purpose, the Ministry of Trade and Industry of Japan has already passed a regulation that prohibits the sale of electric and electronic products in Japan that contain lead substances after the year of 2002. The European Commission proposes the implementation of similar restrictive measures after the year of 2007. Therefore, the particular development of lead-free solder materials, solder materials being widely used in the microelectronic and semiconductor industries, appears to be a technical necessity for the manufacturers that want to conform to the future national and international anti-pollution legislation.  
           [0006]    In the semiconductor packaging process, the use of lead-free solder material has met concrete developments in respect of certain aspects such as for attachments of ball grid array (BGA) packages or surface mount dies. However, many problems remain with respect to some particularly critical techniques that require the use of lead-containing substances. For example, to achieve a controlled collapse chip connection (C 4 ), the skilled artisan knows that the use of lead free solder material for the connecting bumps between the chip and the carrier substrate causes spalling effects of the UBM layer. This consequently deteriorates in substantial manner the strength of the solder joint within the flip chip interconnection structure.  
           [0007]    [0007]FIG. 1 is a schematic view that illustrates a conventional flip chip interconnection structure. As illustrated, a conventional flip chip interconnection structure usually attaches a chip  100  through bumps  108  to a carrier substrate  110 . An underfill material  114  is further filled between the chip  100  and the carrier substrate  110  to buffer the mechanical stresses induced in the bumps  108  due to thermal mismatch between the chip  100  and the carrier substrate  110 .  
           [0008]    An active surface  100   a  of the chip  100  includes bonding pads  102  and a passivation layer  104  that has openings  105  exposing the bonding pads  102 . The carrier substrate  110  includes contact pads  112  that correspond to the bonding pads  102 . To achieve a flip chip interconnection through the bumps  108 , an under bump metallization (UBM) layer  106  is commonly interfaced between each bonding pad  102  and bump  108 .  
           [0009]    A structure of UBM layer  106  known in the prior art comprises a chromium (Cr) layer  106   a,  a copper (Cu) layer  106   b  and a gold (Au) layer  106   c,  while the bumps  108  are commonly made of a solder material. After the UBM layers  106  and the solder material of the bumps  108  have been formed, the chip  100  is mounted on the carrier substrate  110  and a reflow process is performed to turn the solder material to solder bumps  108 .  
           [0010]    Besides Cr/Cu/Au triple-layers structure, a double-layers structure of the UBM layer comprising chromium/nickel is also known in the prior art. When the UBM layer comprises a triple-layers structure of Cr/Cu/Au, tin (Sn) in the solder material will typically react with copper of the UBM layer to form copper-tin (Cu—Sn) intermetallic compound. In the case of a double-layers structure Cr/Ni of the UBM layer, tin (Sn) of the solder material will typically react with nickel (Ni) of the solder material to form a tin-nickel (Sn—Ni) intermetallic compound.  
           [0011]    Regardless whether it is composed of Cu—Sn compound or Sn—Ni compound, the known intermetallic compound has high interfacial energy with chromium of the UBM layer. As a result, during reflowing, the Cu—Sn or Sn—Ni intermetallic compound spalls and becomes spherical, and then peels from the surface of the chromium layer of the UBM layer to flow into the solder material. This spalling and peeling effect seriously deteriorates the strength of the solder joint in the flip chip package.  
         SUMMARY OF INVENTION  
         [0012]    An aspect of the invention is therefore to provide a flip chip interconnection structure and a process for fabricating the same flip chip interconnection structure, in which a composite intermetallic compound is formed to prevent spalling effects.  
           [0013]    To accomplish the above and other objectives, a flip chip interconnection structure of the invention comprises at least a tin-containing solder bump that is selectively formed between a bonding pad of an electronic chip and a contact pad of a carrier substrate. The tin-containing solder bump is formed on an under bump metallization (UBM) layer itself formed on the bonding pad of the chip. A composite intermetallic is interfaced between the tin-containing solder bump and the UBM layer to prevent the occurrence of spalling effects of the UBM layer. An underfill material is further filled between the chip and the carrier substrate to encapsulate the tin-containing solder bump.  
           [0014]    In accordance with the above and other objectives, the invention further provides a manufacture process of a flip chip interconnection structure suitable for flip chip bonding of an electronic chip to a carrier substrate. A UBM layer is formed on at least a bonding pad of the chip, and a tin-containing solder material is formed on the UBM layer. The chip is mounted on the carrier substrate by alignment of the bonding pad with a contact pad of the carrier substrate. A copper reservoir is further provided in proximity of the tin-containing solder material. A reflow process then is performed to respectively turn the tin-containing solder material to a tin-containing solder bump and form a composite intermetallic compound between the tin-containing solder bump and the UBM layer to prevent spalling effects.  
           [0015]    According to a preferred embodiment of the invention, the UBM layer includes a nickel (Ni) layer that reacts with tin (Sn) of the solder material and copper (Cu) of the copper reservoir during reflowing to form the composite intermetallic compound. A portion of the composite intermetallic compound adjacent to the UBM layer thereby includes Cu 6 Sn 5  while a portion of the composite intermetallic compound adjacent to the tin-containing solder bump includes (Cu, Ni) 6 Sn 5 . Via the formation of this composite intermetallic compound, the nickel layer of the UBM layer is isolated from the solder material, which therefore prevents spalling of the UBM layer.  
           [0016]    According to one aspect of the invention, the copper reservoir is constituted via introducing copper particles in the tin-containing solder material formed on the UBM layer.  
           [0017]    According to another aspect of the invention, the copper reservoir is constituted via having the contact pad of the carrier substrate made of copper.  
           [0018]    It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0019]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
         [0020]    [0020]FIG. 1 is a schematic view that illustrates a conventional flip chip interconnection structure;  
         [0021]    [0021]FIG. 2 is a flow chart that illustrates a manufacture process of a flip chip interconnection structure according to an embodiment of the invention;  
         [0022]    [0022]FIG. 3 is a schematic view of a stage in the manufacture process according to an embodiment of the invention, in which a tin-containing solder material includes copper particles as copper reservoir;  
         [0023]    [0023]FIG. 4 is a schematic view of another stage in the manufacture process of the invention subsequent to the stage shown in FIG. 3, in which a bump and a composite intermetallic compound are formed by reflow process;  
         [0024]    [0024]FIG. 5 is a flow chart that illustrates a manufacture process of a flip chip interconnection structure according to another embodiment of the invention;  
         [0025]    [0025]FIG. 6 is a schematic view of a stage in a manufacture process according to an embodiment of the invention, in which a copper contact pad of the carrier substrate is used as copper reservoir; and  
         [0026]    [0026]FIG. 7 is a schematic view of another stage in the manufacture process of the invention subsequent to the stage shown in FIG. 6, in which a bump and a composite intermetallic compound are formed by reflow process. 
     
    
     DETAILED DESCRIPTION  
       [0027]    The following detailed description of the embodiments and examples of the present invention with reference to the accompanying drawings is only illustrative and not limiting. Furthermore, wherever possible in the description, the same reference symbols will refer to similar elements and parts unless otherwise illustrated in the drawings.  
         [0028]    Reference now is made to FIG. 2 through FIG. 4 to describe the manufacture process of a flip chip interconnection structure according to an embodiment of the invention.  
         [0029]    [0029]FIG. 2 is a flow chart of the manufacture process of the flip chip interconnection structure. As illustrated, the manufacture process comprises:  
         [0030]    (a) providing a chip;  
         [0031]    (b) forming an under bump metallization (UBM);  
         [0032]    (c) forming a solder material including copper particles;  
         [0033]    (d) providing a carrier substrate;  
         [0034]    (e) mounting the chip on the carrier substrate;  
         [0035]    (f) performing a reflow process in a manner to form a composite intermetallic compound for preventing spalling effects; and[0033] (g) forming an underfill material.  
         [0036]    Referring to FIG. 3, an active surface of a chip  200  includes at least a bonding pad  202  and, for example, a passivation layer  204  that covers the active surface  200   a  and is provided with at least an opening  205  that exposes the bonding pad  202 . The bonding pad  202  may be made of, for example, aluminum, copper, or other adequate metals.  
         [0037]    A UBM layer  206  is formed on the bonding pad  202 . The UBM layer  206  is, for example, a double-layers structure preferably including a chromium (Cr) layer  206   a  (adhesion layer) and a nickel (Ni) layer  206   b.  The chromium layer  206   a  is, for example, 500 Å-thick, and the UBM layer  206  is, for example, 2000 ÅA thick. On the UBM layer  206  is formed a tin-containing solder material  208   a  that includes copper particles  220 . The solder material  208   a  is formed via, for example, screen printing. The solder material  208   a  is preferably lead-free solder such as, for example, SnAg 3.5 , and the copper particles  220  may consist of, for example, high-purity (99.99%) copper.  
         [0038]    Furthermore, a provided carrier substrate  210  includes at least a contact pad  212 . The contact pad  212  can be made of adequate metals such as copper. The surface of the contact pad  212  may be further plated with a nickel film  216 . The chip  200  then is mounted on the carrier substrate  210  by alignment of the bonding pad  202  with the contact pad  212 . A reflow process is subsequently performed.  
         [0039]    As illustrated in FIG. 3 and FIG. 4, to substantially limit spalling of the UBM layer  206 , a copper reservoir is provided in proximity of the solder material  208   a  to trigger a reaction that forms a protective composite intermetallic compound. In the example illustrated in FIG. 3, the copper reservoir is constituted via the introduction of copper atoms under the form of particles  220  in the solder material  208   a . During the subsequent reflow process, the added copper atoms diffuse within the entire solder material, and a nickel-containing (Ni—Sn) intermetallic compound is formed on the nickel layer  206   b.  As the copper atoms diffuse in the entire solder material, the solder material further reacts with the copper atoms to progressively deposit a copper-tin (Cu—Sn) intermetallic compound on the nickel-containing intermetallic compound, forming a composite intermetallic compound  218 .  
         [0040]    The above deposition of Cu—Sn intermetallic compound on the nickel-containing intermetallic compound isolates the nickel layer  206   b  from the solder material and effectively prevents a reaction there between. Furthermore, a solidification reaction occurs between the nickel layer  206   b  and the Cu—Sn intermetallic compound and the nickel-containing intermetallic compound, which further slows down the reaction involving the nickel layer  206   b.  The consumption of the nickel layer  206   b  is thereby prevented, which therefore substantially limits spalling of the UBM layer  206 .  
         [0041]    Referring to FIG. 4, after reflowing, the tin-containing solder material  208   a  is turned to a tin-containing solder bump  208   b  while the composite intermetallic compound  218  between the solder bump  208   b  and the nickel layer  206   b  includes two different types of material. A portion of the composite intermetallic compound adjacent to the nickel layer  206   b  includes Cu 6 Sn 5  while a portion of the composite intermetallic compound adjacent to the solder bump  208   b  includes (Cu, Ni) 6  Sn 5 , formed by solidification.  
         [0042]    After reflowing to form the solder bump  208   b , an underfill material  214  is formed between the chip  200  and the substrate  210 . The underfill material  214  can be, for example, polyimide. The underfill material  214  buffers the mechanical stresses induced in the solder bump  208   b  due to thermal mismatch between the chip  200  and the carrier substrate  210 , which thereby improves the reliability of the interconnection structure.  
         [0043]    Reference now is made to FIG. 5 through FIG. 7 to describe the manufacture process of a flip chip interconnection structure that reduces the spalling effects according to another embodiment of the invention.  
         [0044]    [0044]FIG. 5 is a flow chart illustrating the manufacture process of this variant embodiment of the invention. As illustrated, the manufacture process of this variant embodiment comprises:  
         [0045]    (a) providing a chip including at least a bonding pad thereon;  
         [0046]    (b) forming a UBM layer on the bonding pad;  
         [0047]    (c) forming a solder material on the UBM layer;  
         [0048]    (d) providing a carrier substrate having at least a copper bonding pad thereon;  
         [0049]    (e) forming a passivation layer on the carrier substrate to expose the copper bonding pad;  
         [0050]    (f) mounting the chip on the carrier substrate;  
         [0051]    (g) performing a reflow process in a manner to form a composite intermetallic compound for preventing spalling; and  
         [0052]    (h) forming an underfill material.  
         [0053]    This embodiment differs from the previous embodiment in that the copper reservoir needed to form the composite intermetallic compound is embodied through a contact pad of the carrier substrate that is made of copper.  
         [0054]    As illustrated in FIG. 6, an active surface of a chip  300  includes at least a bonding pad  302  and a passivation layer  304  that covers the active surface  300   a  and is provided with at least an opening  305  that exposes the bonding pad  302 .  
         [0055]    A UBM layer  306  is formed on the bonding pad  302 . The UBM layer  306  may have, for example, a double-layers structure including a chromium (Cr) layer  306   a  and a nickel (Ni) layer  306   b.  On the UBM layer  206  is formed a tin-containing solder material  308   a  by, for example, screen printing. The tin-containing solder material  308   a  is preferably lead-free solder such as, for example, SnAg 3.5 .  
         [0056]    Furthermore, a provided carrier substrate  310  includes at least a contact pad  312  preferably made of copper. Copper of the contact pad  312  preferably has a high purity of about 99.99%. A nickel film  316  is formed on the surface of the copper contact pad  312  and includes an opening  317  that partially exposes the copper contact pad  312 . The chip  300  is mounted on the carrier substrate  310  by alignment of the bonding pad  302  with the copper contact pad  312 . A reflow process then is performed.  
         [0057]    During the reflow process, the copper reservoir of the copper contact pad  312  provides the copper atoms needed to form a composite intermetallic compound  318  in a manner similar to that described in the previous embodiment.  
         [0058]    As illustrated in FIG. 7, after reflowing, the solder material  308   a  is turned to a tin-containing bump  308   b.  An underfill material  314  then is formed between the chip  300  and the carrier substrate  310 .  
         [0059]    As described above, the invention therefore provides a structure of flip chip interconnection that favorably prevents spalling of the UBM layer by interfacing a composite intermetallic compound between the UBM layer and the solder bump. The composite intermetallic compound is formed through a reaction between nickel from a nickel layer of the UBM layer, tin from the solder material, and copper from a copper reservoir. The copper reservoir may be embodied under different forms.  
         [0060]    According to one embodiment, the copper reservoir is formed via adding copper particles in the solder material of the tin-containing bump.  
         [0061]    According to another embodiment, the copper reservoir is embodied through a contact pad of the carrier substrate that is made of copper.  
         [0062]    It should be apparent to those skilled in the art that other structures that are obtained from various modifications and variations of different parts of the above-described structures of the invention would be possible without departing from the scope and spirit of the invention as illustrated herein. Therefore, the above description of embodiments and examples only illustrates specific ways of making and performing the invention that, consequently, should cover variations and modifications thereof, provided they fall within the inventive concepts as defined in the following claims.