Abstract:
A bump structure of a semiconductor package and a method for fabricating the same are provided. The bump structure is used to connect a semiconductor element to a carrier of the semiconductor package. The fabrication method primarily employs an electroplating process to form the bump structure including an under bump metallurgy (UBM) layer, at least one I-shaped conductive pillar, and a solder material. This allows fine-pitch electrical connection pads to be arranged in the semiconductor package, and also provides an enhanced support structure and a sufficient height between the semiconductor element and the carrier.

Description:
FIELD OF THE INVENTION 
     The present invention relates to bump structures of semiconductor packages and fabrication methods thereof, and more particularly, to a bump structure of a flip-chip semiconductor package, and a method for fabricating the bump structure. 
     BACKGROUND OF THE INVENTION 
     Along with the development of light-weight and small-profile electronic products, semiconductor packages serving as core components of the electronic products have accordingly been miniaturized in size. Preferably, the semiconductor packages with a reduced size, fine-pitch interconnections and high-density I/O (input/output) connections, such as a flip-chip package, ball grid array (BGA) package, and chip size package (CSP), etc., have become mainstream package products in the market. 
     For example, in a conventional flip-chip semiconductor package  10  shown in  FIG. 1 , a plurality of bumps  101  are formed on an active surface of a semiconductor element  103  such as a semiconductor chip or wafer, and corresponding electrical connection pads are disposed on a carrier  105  such as a substrate or circuit board. The semiconductor element  103  is mounted on the carrier  105  in a face-down manner that the active surface of the semiconductor element  103  faces downwardly and the bumps  101  are bonded to the electrical connection pads on the carrier  105 , such that the semiconductor element  103  is electrically connected to the carrier  105  via the bumps  101 , and signals from the semiconductor element  103  can be transmitted to the carrier  105 . 
     In order to prevent short circuit caused by melting and bridging of the plurality of bumps  101  during a subsequent high-temperature reflow process, U.S. Pat. No. 6,038,136 has disclosed the use of an underfill material  107  such as resin for filling spaces between the adjacent bumps  101 , so as to isolate the bumps  101  from each other and enhance the bonding strength between the bumps  101  and the carrier  105 . 
     However, since the functionality of the semiconductor element has become more complicated, the bumps are accordingly arranged in higher density. As a result, a pitch between adjacent bumps has been reduced from 250 μm to 200 μm, or even to 150 μm. Particularly for the conventional ball-shaped bumps  101  shown in  FIG. 1 , the spaces between the adjacent bumps  101  would be too small to be completely filled with the underfill material  107  due to the bump shape, and thus causes voids being formed between the adjacent bumps  101 . This adversely affects the reliability of the semiconductor package, and even leads to bridging or short circuit of the bumps  101  after the reflow process. 
     In light of the foregoing drawbacks, U.S. Pat. No. 5,698,465 or 6,555,296 has disclosed the use of pillar bumps to increase spacing between adjacent bumps and maintain a height between the semiconductor element and the carrier, so as to solve the above problem of incomplete filling of the underfill material in the small spaces between adjacent bumps. As shown in  FIG. 2 , for fabricating a bump structure disclosed in U.S. Pat. No. 5,698,465, an under bump metallurgy (UBM) layer  205  is firstly formed on a bond pad  207  of a semiconductor element such as a chip. Then, a specific pillar bump  201  is formed on the UBM layer  205 , and a solder material  203  is applied on the pillar bump  201  by a printing or electroplating technique. Subsequently, the solder material  203  is subjected to a reflow process to electrically connect the pillar bump  201  to a corresponding electrical connection pad on a carrier. 
     The pillar bump  201  can desirably maintain the height between the semiconductor element and the carrier, however it has a limited effect on increasing the spacing between adjacent pillar bumps. Particularly when the number of I/O connections of the semiconductor element is increased and the pitch between adjacent pillar bumps is reduced to below about 150 μm, the similar drawback is produced that the underfill material fails to penetrate and fill small spaces between the adjacent pillar bumps, thereby causing the problem of voids or short circuit, etc. 
     Therefore, the problem to be solved here is to provide a bump structure of a semiconductor package, which can avoid the above drawbacks in the prior art. 
     SUMMARY OF THE INVENTION 
     In light of the drawbacks in the prior art, a primary objective of the present invention is to provide a bump structure of a semiconductor package and a method for fabricating the same, so as to increase spacing between adjacent bumps and reduce formation of voids. 
     Another objective of the invention is to provide a bump structure of a semiconductor package and a method for fabricating the same, so as to maintain a sufficient height between a semiconductor element and a carrier. 
     A further objective of the invention is to provide a bump structure of a semiconductor package and a method for fabricating the same, which can facilitate filling of an underfill material between adjacent bumps. 
     In order to achieve the foregoing and other objectives, the present invention proposes a bump structure of a semiconductor package, for connecting a semiconductor element to a carrier of the semiconductor package, wherein the semiconductor element has at least one electrical connection pad on a surface thereof. The bump structure comprises: an under bump metallurgy (UBM) layer formed on the electrical connection pad; an I-shaped conductive pillar disposed on the UBM layer, wherein a middle portion of the conductive pillar has a width smaller than that of an upper end and a lower end of the conductive pillar respectively; and a solder material applied on the conductive pillar. 
     A method for fabricating the foregoing bump structure of the semiconductor package, comprises the steps of: providing a carrier and a semiconductor element, wherein the semiconductor element has a plurality of electrical connection pads on a surface thereof; performing an under bump metallurgy (UBM) process to form a UBM layer on the electrical connection pads; performing an electroplating process to deposit a conductive pillar on a position of the UBM layer corresponding to each of the electrical connection pads, wherein a middle portion of the conductive pillar has a width smaller than that of an upper end and a lower end of the conductive pillar respectively; and applying a solder material on each of the conductive pillars. 
     The bump structure in the present invention is formed on the semiconductor element, or alternatively can be formed on the carrier. In the latter case, an active surface of the semiconductor element can be attached and electrically connected to the bump structure of the carrier. 
     The semiconductor element can be a semiconductor chip, and the carrier can be a substrate or circuit board. The conductive pillar is I-shaped, comprising a first conductive portion, a second conductive portion, and a third conductive portion, wherein the width of the first conductive portion and the third conductive portion is respectively larger than the width of the second conductive portion. The desirable I-shaped conductive pillar is fabricated by performing a series of exposure and development processes and controlling the size of photoresist openings during each of the exposure and development processes. 
     For example, a first photoresist layer can be firstly formed on the UBM layer and has a plurality of first openings for exposing positions of the UBM layer corresponding to the electrical connection pads. Then, an electroplating process is performed in each of the first openings to form a first conductive portion that is connected to the UBM layer. A second photoresist layer is applied on the first photoresist layer and has a plurality of second openings for exposing the first conductive portions, wherein the second opening is smaller in size than the first opening. Then, the electroplating process is performed in each of the second openings to form a second conductive portion that is connected to the corresponding first conductive portion. Subsequently, an electroless plating or sputtering process is carried out to deposit a thin metallic layer on the second photoresist layer and the second conductive portions. A third photoresist layer is applied on the thin metallic layer and has a plurality of third openings for exposing positions of the thin metallic layer corresponding to the second conductive portions, wherein the third opening is larger in size than the second opening. Afterwards, the electroplating process is performed in each of the third openings to form a third conductive portion that is connected to the thin metallic layer and the second conductive portion. Finally, a solder material is applied on each of the third conductive portions, such that a set of the first, second and third conductive portions form an I-shaped conductive pillar. 
     Since the first conductive portion and the third conductive portion of the I-shaped conductive pillar are wider than the second conductive portion, the I-shaped conductive pillar has an inwardly recessed structure. Thus, the I-shaped conductive pillar not only maintains a sufficient height between the semiconductor element and the carrier, but also provides wider spaces for filling an underfill material between adjacent conductive pillars, such that fine-pitch electrical connection pads can be arranged in the semiconductor package, and the problems in the prior art due to incomplete filling of the underfill material are solved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  (PRIOR ART) is a cross-sectional view of a conventional bump structure; 
         FIG. 2  (PRIOR ART) is a cross-sectional view of another conventional bump structure; 
         FIG. 3  is a cross-sectional view of a bump structure of a semiconductor package in accordance with the present invention; 
         FIGS. 4A to 4L  are schematic diagrams showing a method for fabricating the bump structure of the semiconductor package in accordance with the present invention; 
         FIG. 5A  is a schematic diagram showing the bump structure being formed on a semiconductor element to be connected to a carrier in accordance with the present invention; and 
         FIG. 5B  is a schematic diagram showing the bump structure being formed on the carrier to be connected to the semiconductor element in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 3 to 5B  are schematic diagrams showing a bump structure of a semiconductor package and a method for fabricating the bump structure according to preferred embodiments of the present invention. It should be noted that the drawings are simplified schematic diagrams and only show the basic structure and relevant components according to the present invention. The number, shape and size of the components are not drawn in real scale, and the arrangement of components should be much more complex in practice. 
       FIG. 3  shows the bump structure  1  of a semiconductor package in this embodiment. The bump structure  1  is used for connecting a semiconductor element  2  to a carrier. The semiconductor element  2  has a plurality of electrical connection pads  11  and a passivation layer  13  thereon, with the electrical connection pads  11  being exposed from the passivation layer  13 . The bump structure  1  comprises an under bump metallurgy (UBM) layer  20 , an I-shaped conductive pillar  15 , and a solder material  17 . 
     In this embodiment, the electrical connection pads  11  are formed on a surface of the semiconductor element  2 , and the passivation layer  13  is applied on the surface of the semiconductor element  2  and have openings  131  (shown in  FIG. 4A ) for exposing the electrical connection pads  11 . The UBM layer  20  is formed on each of the exposed electrical connection pads  11 . The passivation layer  13  can be made of a material such as silica, polyimide, silicon nitride, or any other equivalent material that is capable of preventing external air, water and dust from coming into contact with the surface of the semiconductor element  2 . 
     The I-shaped conductive pillar  15  can be made of a metal such as, but not limited to, copper or lead. The I-shaped conductive pillar  15  is formed on an upper surface of the UBM layer  20  located on each of the electrical connection pads  11 , wherein a middle portion of the I-shaped conductive pillar  15  has a width smaller than that of an upper end and a lower end of the conductive pillar  15  respectively, such that the conductive pillar  15  has an inwardly recessed structure and thus increases spacing between the adjacent conductive pillars  15 . The solder material  17  is applied on the I-shaped conductive pillar  15  by a printing or electroplating technique and is then subjected to a reflow process. This completes the bump structure  1  in this embodiment. 
     In this embodiment, a single I-shaped conductive pillar  15  is formed on each of the electrical connection pads  11 . It should be noted that the present invention is not limited to this arrangement. Alternatively, in the present invention, two or more I-shaped conductive pillars  15  can be vertically stacked to be mounted on a corresponding electrical connection pad  11 . 
     The bump structure  1  can be fabricated by procedural steps shown in  FIGS. 4A to 4L . The bump structure  1  is used to connect the semiconductor element to the carrier of the semiconductor package. 
     In this embodiment, the bump structure  1  is firstly formed on the semiconductor element  2  such as a semiconductor chip, and then the semiconductor element  2  is attached to the carrier such as a substrate or circuit board via the bump structure  1 . It should be noted that the present invention is not limited to this arrangement. Alternatively, the bump structure  1  can be firstly formed on the carrier, and then the carrier with the bump structure  1  is connected to the semiconductor element  2 . 
     First referring to  FIG. 4A , the semiconductor element  2  such as a semiconductor chip is provided, which has an active surface  2   a  and a non-active surface  2   b  opposed to the active surface  2   a . A plurality of electrical connection pads  11  made of copper or aluminum and a passivation layer  13  are formed on the active surface  2   a  of the semiconductor element  2 . The passivation layer  13  has a plurality of openings  131  for exposing the electrical connection pads  11 . In this embodiment, the passivation layer  13  can be applied by printing, spin-coating, attaching, or any other equivalent technique. The passivation layer  13  is then subjected to processes of photoresist formation, exposure and development, etc. so as to expose the electrical connection pads  11  from the passivation layer  13 . There are numerous methods well known in the art for fabricating the electrical connection pads and passivation layer on the semiconductor element  2 , which are not the characteristic feature of the present invention and not to be further detailed herein. 
     Next, a UBM layer  20  is formed on the electrical connection pads  11 , which comprises a metallic adhesion layer  21 , a barrier layer  23  and a solder wettable layer  25 . Referring to  FIG. 4B , the metallic adhesion layer  21  made of such as copper or titanium is deposited on the electrical connection pads  11  and the passivation layer  13  by an electroless plating, sputtering or evaporating technique. Then, referring to  FIG. 4C , the barrier layer  23  made of nickel or nickel/vanadium alloy is deposited on the metallic adhesion layer  21  by a sputtering, evaporating or electroplating technique, etc. 
     Referring to  FIG. 4D , a solder wettable layer  25  made of e.g. copper is deposited on the barrier layer  23  by a sputtering technique. The metallic adhesion layer  21 , the barrier layer  23  and the solder wettable layer  25  together form the foregoing UBM layer  20 . 
     Subsequently, referring to  FIG. 4E , a first photoresist layer  31  such as a dry film or liquid photoresist is applied on the topmost solder wettable layer  25  and then subjected to exposure, lithography and etching processes, such that positions of the solder wettable layer  25  corresponding to the electrical connection pads  11  are exposed via first openings  311  formed in the first photoresist layer  31 . 
     Referring to  FIG. 4F , an electroplating process is carried out to form a first conductive portion  51  in each of the first openings  311 . In this embodiment, as shown in  FIG. 4F , a top surface of the first conductive portion  51  is flush with that of the first photoresist layer  31 ; however, the present invention is not limited to such arrangement. 
     Referring to  FIG. 4G , a second photoresist layer  33  is applied on the first photoresist layer  31 , and has a plurality of second openings  331  for partly exposing the first conductive portions  51 , wherein the size of the second opening  331  is smaller than that of the first opening  311 . Then, referring to  FIG. 4H , the electroplating process is performed on the semiconductor element  2  to form a second conductive portion  53  in each of the second openings  331 , wherein the second conductive portion  53  is connected to the corresponding first conductive portion  51  and has a width smaller than that of the first conductive portion  51 . 
     Moreover, referring to  FIG. 4H-1 , an electroless plating or sputtering process is performed to deposit a thin metallic layer  36  on the second photoresist layer  33  and the second conductive portions  53 . This thin metallic layer  36  can be made of such as copper or palladium. 
     Referring to  FIG. 4I , a third photoresist layer  35  is applied on the thin metallic layer  36 , and has a plurality of third openings  351  for partly exposing the thin metallic layer  36 , wherein the size of the third opening  351  is larger than that of the second opening  331 . 
     Referring to  FIG. 4J , the electroplating process is carried out on the semiconductor element  2  to form a third conductive portion  55  in each of the third openings  351 . This third conductive portion  55  is connected to the thin metallic layer  36  and the corresponding second conductive portion  53 . Moreover, the first conductive portion  51 , the second conductive  53 , and the third conductive portion  55  can be made of a metal such as, but not limited to, copper, lead, tin, gold, zinc, or nickel, etc. 
     Subsequently, referring to  FIG. 4K , a solder material  17  is formed on the third conductive portion  55  in each of the third openings  351  by a screen-printing or electroplating technique, etc. The solder material  17  can be an alloy made of a mixture of metals selected from the group consisting of lead, tin, silver and copper. 
     Finally, the semiconductor element  2  is subjected to exposure, development and etching processes to remove all the photoresist layers  31 ,  33 ,  35  and the thin metallic layer  36 , as shown in  FIG. 4L . Thus, a corresponding set of the first conductive portion  51 , the second conductive portion  53  and the third conductive portion  55  completely forms an I-shaped conductive pillar  15  on each of the electrical connection pads  11 , and a reflow process is performed to fix the solder material  17  on the I-shaped conductive pillar  15 . 
     The removal of the first, second and third photoresist layers  31 ,  33 ,  35  and the thin metallic layer  36  employs a conventional technique well known in the art, which is not to be further described herein. Similarly, the processes of applying photoresist, exposure, lithography, etching, screen-printing and electroplating are all well known in the art and thus not to be further detailed herein. 
     In this embodiment, as shown in  FIG. 5A , the bump structure  1  is formed on the semiconductor element  2  such as a semiconductor chip, and then this semiconductor element  2  with the bump structure  1  is connected to the carrier  3 . Alternatively, in another preferred embodiment, as shown in  FIG. 5B , the bump structure  1  can be formed on the carrier  3  such as a substrate or circuit board, and then this carrier  3  with the bump structure  1  is connected to the semiconductor element  2 . 
     The I-shaped conductive pillar  15  comprises the first conductive portion  51 , the second conductive portion  53  and the third conductive portion  55 , and has a larger height than that of the conventional bump structure, such that the I-shaped conductive pillar  15  can provide a sufficient height between the semiconductor element and the carrier. Since the second conductive portion  53  has a width smaller than that of the first conductive portion  51  and the third conductive portion  53  respectively, the I-shaped conductive pillar  15  has an inwardly recessed structure and thus increases spacing between the adjacent conductive pillars  15  for accommodating an underfill material. As a result, when the underfill material is filled in spaces between the adjacent bump structures  1  during an underfill process, it is less likely to form voids in the spaces and the underfill process would not be affected by fine-pitch arrangement of electrical connection pads. 
     The drawings of the present invention only show a part of the electrical connection pads. It should be understood that the number of electrical connection pads and bump structures can be flexibly arranged in the semiconductor package according to the practical requirements. Further, the UBM layer can be formed over a surface of the semiconductor element or carrier having the electrical connection pads, or alternatively, the UBM layer can be individually formed on each of the electrical connection pads. Moreover, the fabrication processes of the bump structure can be applied to one side or both sides of the semiconductor element or carrier, and the present invention is not limited by the above embodiments. 
     Therefore, in the bump structure of a semiconductor package and the method for fabricating the bump structure according to the present invention, the I-shaped conductive pillar is formed by electroplating through the use of multiple photoresist layers and different openings of the photoresist layers, such that the fabricated I-shaped conductive pillar can have an inwardly recessed structure for increasing spacing between the adjacent bump structures to facilitate filling of the underfill material. This solves the prior-art problem of voids being formed between the fine-pitch bumps due to incomplete filling of the underfill material. 
     The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.