Patent Publication Number: US-2007111499-A1

Title: Wafer redistribution structure with metallic pillar and method for fabricating the same

Description:
This application claims the benefit of Taiwan application Ser. No. 94140168, filed Nov. 15, 2005, the subject matter of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates in general to a wafer structure and a method for fabricating the same, and more particularly to a redistribution structure of a wafer structure and a method for fabricating the same.  
      2. Description of the Related Art  
      In response to the requirements with regards to the integration and miniaturization of electronic devices, the packaging technology of integrated circuits (ICs) is headed towards miniaturization and high density. Examples of the packaging technology of high-density integrated circuits including the ball grid array (BGA) technology, the chip-scale package (CSP) technology, the flip chip (F/C) technology, and the multi-chip module (MCM) technology are provided. In terms of a high-density integrated circuit, that is, an integrated circuit having more pins per unit area, the speed of signal transmission will be improved if the length of wiring distribution is shortened. Therefore, the application of the metallic pillar capable of shortening the length of wiring distribution has gradually replaced the application of routing and become a mainstream practice in high-density packaging technology.  
      The solder pads disposed on the chip being so close to each other that the bumps disposed on the solder pads are likely to be electrically connected to each other, causing the occurrences of unexpected short-circuits. Besides, the solder pads being so close to the sawing line of the wafer that the bumps are likely to be sawn when the wafer is being sawn. Therefore, the current bumping technology changes the disposition of the bumps by redistribution. Under certain circumstances, the redistribution layer has to be crisscrossed with the layout path due to the overall layout of the chip, and the three-layered redistribution structure is thus invented.  
      However, in the three-layered redistribution structure, some of the contacts directly form bumps above the original position of the solder pads, making the bumps to be positioned too low. Referring to  FIG. 1 , a cross-sectional view according to a conventional wafer structure having a three-layered redistribution structure is shown. The conventional wafer structure  10  includes a substrate  11 , two solder pads  12  and  14 , a first passivation layer  20 , two first redistribution layers  22  and  24 , a second passivation layer  30 , two second redistribution layers  32  and  34 , a third passivation layer  40 , an under bump metallurgy (UBM) layer  42 , two bumps  52  and  54 . The substrate  11  has several solder pads  12 ,  14  and the first passivation layer  20  disposed thereon. The first passivation layer  20  has several first apertures to expose the solder pads  12  and  14 .  
      The solder pads  12  and  14  respectively adopt different designs of redistribution layers. The solder pad  14  is extended to the right via the first redistribution layer  24  and the second redistribution layer  34 , and forms an under bump metallurgy (UBM) layer  44  and a bump  54  at one end of the second redistribution layer  34 . A component is electrically connected to the solder pad  14  by contacting the bump  54 , thereby redistributing the electrical contacts of the wafer  10 . On the other hand, the bump  52  electrically connected to the solder pad  12  is predetermined to be formed above the original position of the solder pad  12 . Similarly, the solder pad  12  is extended upwardly via the first redistribution layer  22  and the second redistribution layer  32 , and forms the UBM layer  42  and the bump  52  in the second redistribution layer  32 . It is noted that an obvious height drop x 1  exists between the bump  52  and the bump  54 . That is to say, the design of adopting the three-layered redistribution layer and forming the bump  52  on the solder pad  14  would easily cause the bump  52  to be positioned too low, severely affecting the subsequent packaging process.  
      Referring to  FIG. 2 , another cross-sectional view according to the conventional wafer structure having the three-layered redistribution layer is shown. The solder pad  16  is slightly extended to the left via the first redistribution layer  26  and the second redistribution layer  36 , and forms an under bump metallurgy (UBM) layer  46  and a bump  56  in the second redistribution layer  36 . The contact area between the first redistribution layer  26  and the second redistribution layer  36  is too small, so the two layers are likely to come off the structure, severely damaging the electrical characteristics of the components.  
      Generally speaking, despite the conventional three-layered redistribution layer resolves the problem of crisscrossed circuits, however, when the bump is formed above the solder pad or the extension distance is too short, the bumps are likely to come off the structure if the bumps are positioned too low. The above two problems severely restrict the application of the conventional three-layered redistribution layer, and yet the problems of crisscrossed circuits still remain unresolved.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the invention to provide a redistribution structure of a wafer structure and a method of fabricating the same. The design adopts a multi-layered redistribution structure, avoiding the occurrences of short-circuits when the circuits are crisscrossed. The bumps are of the same height, thus avoiding height drop. Furthermore, the redistribution layers of the redistribution structure have good contacts and are not easy to be detached.  
      The invention achieves the above-identified object by providing a wafer structure including a substrate, a redistribution structure, a third passivation layer, an under bump metallurgy (UBM) layer and a bump. The substrate has a solder pad and a first passivation layer. The first passivation layer has a first aperture to expose the solder pad. The redistribution structure is formed on the substrate. The redistribution structure includes at least one redistribution layer, a metallic pillar and a second passivation layer. The redistribution layer is electrically connected to the solder pad, and has at least one top portion and an indented portion. The metallic pillar fills the indented portion and is protruded from the top portion. The second passivation layer is formed on the first passivation layer and surrounds the metallic pillar. The third passivation layer is formed on the redistribution structure and has a third aperture to expose the metallic pillar. The UBM layer is formed in the third aperture and disposed on the metallic pillar with respect to the indented portion. The bump is formed on the UBM layer.  
      The invention further achieves the above-identified object by providing a wafer structure. The wafer structure includes a substrate, a redistribution structure, a passivation layer, an under bump metallurgy (UBM) layer and a bump. The substrate has a solder pad. The redistribution structure is formed on the substrate. The redistribution structure includes at least one copper pillar electrically connected to the solder pad. The passivation layer is formed on the redistribution structure and has an aperture to expose the copper pillar. The UBM layer is formed in the aperture and is disposed on the copper pillar. The bump is formed on the UBM layer.  
      The invention further achieves the above-identified object by providing yet another method of fabricating a wafer structure. The method includes the following steps. (a). Providing the substrate having the solder pad and the first passivation layer, wherein the first passivation layer has a first aperture to expose the solder pad. (b). Forming a redistribution layer on the first passivation layer and the first aperture, wherein the redistribution layer has an indented portion. (c). Forming a metallic pillar to fill the indented portion, wherein the metallic pillar is protruded from the redistribution structure. (d). Forming a second passivation layer on the first passivation layer, wherein the second passivation layer surrounds the metallic pillar. (e). Forming a third passivation layer on the second passivation layer, wherein the third passivation layer has a third aperture to expose the metallic pillar. (f). Forming an under bump metallurgy (UBM) layer in the third aperture and on part of the third passivation layer, wherein the UBM layer is electrically connected to the metallic pillar with respect to the indented portion. (g). Forming a bump on the UBM layer.  
      Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  (Prior Art) is a cross-sectional view according to a conventional wafer structure having a three-layered redistribution structure;  
       FIG. 2  (Prior Art) is another cross-sectional view according to the conventional wafer structure having the three-layered redistribution structure;  
       FIG. 3  is a cross-sectional view of a wafer structure according to a preferred embodiment of the invention;  
      FIGS.  4 A˜ 4 J are flowcharts of a method of fabricating a wafer structure according to the preferred embodiment of the invention; and  
       FIG. 5  is a perspective of the redistribution structure of the wafer structure according to  FIG. 4J . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The invention provides a multi-layered redistribution structure and a method for fabricating the same, avoiding the occurrences of short-circuits even when circuits are crisscrossed. According to the invention, the bumps have the same height and are without height drop. At the contact point of the redistribution layer extended from the original position of the solder pad, a metallic pillar is used to fill the indented portion and elevate the under bump metallurgy (UBM) layer, so that the bump extended from the original position of the solder pad has the same height with the bump positioned elsewhere.  
      Referring to  FIG. 3 , a cross-sectional view of a wafer structure according to a preferred embodiment of the invention is shown. The wafer structure  100  of the present embodiment of the invention includes a substrate  110 , a redistribution structure  234 , a third passivation layer  170 , an under bump metallurgy layer (UBM)  180  and a bump  190 . The substrate  110  has a solder pad  112  and a first passivation layer  118 . The first passivation layer  118  has a first aperture to expose the solder pad  112 . The redistribution structure  234  is formed on the substrate  110 . The redistribution structure  234  includes at least one redistribution layer  120 , a metallic pillar  130  and a second passivation layer  140 . The redistribution layer  120  electrically connected to the solder pad  112  has at least one top portion  120   b  and an indented portion  120   a . The metallic pillar  130  fills the indented portion  120   a  and is protruded from the top portion  120   b . The second passivation layer  140  is formed on the first passivation layer  118  and surrounds the metallic pillar  130 . The third passivation layer  170  formed on the redistribution structure  234  has a third aperture  170   a  to expose the metallic pillar  130 . The UBM layer  180  formed in the third aperture  170   a  is disposed on the metallic pillar  130  with respect to the indented portion  120   a . The bump  190  is formed on the UBM layer  180 .  
      In other words, the redistribution layer  120  is formed in the first aperture and on the first passivation layer  118 . The top portion  120   b  covers the first passivation layer  118 . The indented portion  120   a  is disposed in the first aperture  118   a . Preferably, the metallic pillar  130  is a copper pillar possessing excellent features of conductivity and hardness. The metallic pillar  130 , preferably ranging from 5 μm to 8 μm, fills the indented portion  120   a  of the redistribution layer  120  and is protruded from the top portion  120   b  of the redistribution layer  120 . By doing so, the top surface of the second passivation layer  170  is disposed at the same height with the top surface of the metallic pillar  130 , therefore the UBM layer  180  disposed on the metallic pillar  130  is approximately of the same height with the UBM layer  180  disposed on the second passivation layer  170 . That is to say, the top surface of the redistribution structure formed above or near the original position of the solder pad has the same height with the top surface of the redistribution structure extended outwardly from the solder pad. Therefore, the UBM layer and the bump formed under the same conditions would have the same height.  
      Referring to FIGS.  4 A˜ 4 J, several flowcharts of a method of fabricating a wafer structure according to the preferred embodiment of the invention are shown. Each flowchart includes an upper view and a cross-sectional view viewed along the cross-sectional line of the upper view. The steps of the method for fabricating the wafer structure of the present embodiment of the invention are disclosed below.  
      Firstly, as shown in  FIG. 4A , a substrate  110  having several solder pads  112 ,  114 , and  116  and a first passivation layer  118  is provided, wherein the first passivation layer  118  has a first aperture  118   a  to expose the solder pads  112 ,  114 , and  116 . Furthermore, for the bump of the solder pad  112  to be formed at the original position, the redistribution layer of the solder pad  114  must crisscross with the redistribution layer of the solder pad  116 . Therefore, the redistribution structures of the solder pads  112 ,  114 , and  116  are different from one another.  
      Next, a redistribution layer and a metallic pillar are formed. The preferable way of forming the metallic layer includes the following steps. Referring to  FIG. 4B , a metallic layer  122  is formed on the first passivation layer  118  and the first aperture  118   a . The metallic layer  122  disposed in the first aperture  118   a  forms an indented portion  122   a . The way of forming the metallic layer  122  includes electroless plating, sputtering or chemical deposition. Afterwards, a photo-resist layer  124  is formed on the metallic layer  122 . The photo-resist layer  124  is patterned to form a slot  124   a  to expose the indented portion  122   a . Examples of the photo-resist layer  124  include a photo-resist solution or a dry film. Then, the metallic pillar  130  formed in the slot  124   a  by electroplating fills the indented portion  122   a  and is protruded from the metallic layer  122 . Preferably, the metallic pillar  130  is a copper pillar. Next, the photo-resist layer  124  is removed as shown in  FIG. 4C . Afterwards, another patterned photo-resist layer  126  is formed on the metallic layer  122  as shown in  FIG. 4D . Lastly, the metallic layer  122  is patterned according to the metallic pillar  130  and another patterned photo-resist layer  126  to form several redistribution structures  120 ,  128  and  129  as shown in  FIG. 4E . Referring to  FIG. 4E , the redistribution layer  120  disposed in the first aperture  118   a  forms an indented portion  120   a  and covers the first passivation layer  118  forms a top portion  120   b . To elaborate the above disclosure in greater details, the top portion  120   b  of the redistribution layer  120  of the bump predetermined to be formed at the original position is positioned at the original position, while the top portion  129   a  of the redistribution structure  129  of the bump predetermined to be formed elsewhere extends a distance towards the predetermined position of the bump on the first passivation layer  118 . Particularly, the top portion  128   a  of the redistribution structure  128  of the bump predetermined to be formed elsewhere is also disposed at the original position so as to crisscross with the redistribution structure  129 .  
      On the other hand, the redistribution layer and the metallic pillar can be separately formed. For example, the redistribution layer is first formed on the first passivation layer and the first aperture to form the indented portion of the redistribution layer, and then the metallic pillar is formed to fill the indented portion and is protruded from the redistribution layer. The formation of the redistribution layer includes the following steps: firstly, a metallic layer is formed on the first passivation layer and the first aperture; next, the metallic layer is patterned to form a redistribution layer; the redistribution layer disposed in the first aperture forms the indented portion and covers the first passivation layer to form the top portion. Moreover, the formation of the metallic pillar includes the following steps: a photo-resist layer is formed on the first passivation layer and the redistribution layer; the photo-resist layer is patterned to form a first aperture to expose the indented portion; and a metallic pillar is formed in the first aperture by electroplating, so that the metallic pillar fills the indented portion and is protruded from the redistribution layer.  
      Then, a second passivation layer  140  is formed on the first passivation layer  118  and surrounds the metallic pillar  130  as shown in  FIG. 4F . The formation of the second passivation layer  140  includes rotary coating, chemical deposition, printing, draping, spraying and other methods. Preferably, the top surface of the second passivation layer  140  is disposed at the same height with the top surface of the metallic pillar  130 . Besides, several apertures  140   a  are formed on the second passivation layer  140  to expose the redistribution layers  128  and  129 . Afterwards, the second redistribution layers  132  and  134  are formed according to the above method of formation as shown in  FIG. 4G . Furthermore, the top portion  132   b  of the redistribution layer  132  electrically connected to the solder pad  114  extends a distance on the second passivation layer  140 , while the top portion  134   b  of the redistribution layer  134  electrically connected to the solder pad  116  extends a distance towards another direction on the second passivation layer  140  so as to crisscross with the redistribution layer  132 . Thus, the redistribution structure is completed.  
      Afterwards, a third passivation layer  170  is formed on the second passivation layer  140 . The third passivation layer  170  has a third aperture  170   a  to expose the metallic pillar  130  as shown in  FIG. 4H . The way of forming the third passivation layer  170  includes rotary coating, chemical deposition, printing, draping, spraying and other methods. Meanwhile, the third aperture  170   a  of the third passivation layer also exposes the second redistribution layers  132  and  134  of the redistribution structure. Preferably, all of the first passivation layer, the second passivation layer and the third passivation layer include polyimide (Pl).  
      Next, several UBM layers  180 ,  182 ,  184  are formed in the third aperture  170   a  and on part of the third passivation layer  170  as shown in  FIG. 41 . The UBM layer is normally composed of an adhesion layer, a diffusion barrier layer and a wetting layer. The adhesion layer for providing adhesion between the solder pad and the passivation layer includes aluminum, titanium, chromium, tungsten-titanium alloy and other metals. The diffusion barrier layer for preventing metal diffusion between the bump and the solder pad includes nickel-vanadium alloy, nickel and other metals. The wetting layer for providing draping between the under bump metallurgy layer and the bump includes copper, molybdenum, platinum and other metals. Besides, the UBM layer  180  is electrically connected to the metallic pillar  130  with respect to the indented portion  120   a . Thus, the bottom of the UBM layer  180  formed on the metallic pillar  130  is approximately disposed at the same height with the bottom of the UBM layer  184  formed on the second passivation layer  140 . The cavity volume of the indented portion of the UBM layer  180  formed on the metallic pillar  130  is approximately the same with the cavity volume of the indented portion of the UBM layer  184  formed on the second passivation layer  140 .  
      Lastly, the slot formed by patterning the photo-resist is filled with solder paste or electroplating solder and is disposed on the UBM layers  180 ,  182  and  184 , and then the bumps  190 ,  192 , and  194  are formed via the process of reflowing as shown in  FIG. 4J ; meanwhile, the patterned photo-resist is removed to complete the wafer structure  100  of the present embodiment of the invention. It is noted that the bumps  190 ,  192 , and  194  are substantially of the same height and are without vertical height drop. Since the cavity volume of the indented portion of the UBM layer  180  is approximately the same with the cavity volume of the indented portion of the UBM layer  184 , the amount of the solder paste or the electroplating solder used the indented portion of the UBM layer  180  is approximately the same with amount of the solder paste or the electroplating solder used to fill up the indented portion of the UBM layer  184 . Consequently, the bumps formed according to the process of reflowing substantially have the same size and the same height. Referring to  FIG. 5 , a perspective of the redistribution structure of the wafer structure according to  FIG. 4J  is shown. In the wafer structure  100  of the present embodiment of the invention, the layout path L 1  and the layout path L 2  are crisscrossed but not short-circuited, because the redistribution layer  129  and the second redistribution layer  132  are disposed on different layers.  
      Despite the method of fabricating redistribution structure disclosed above is exemplified by three layers of the passivation layers  118 ,  140 , and  170  and two layers of the redistribution layers, the invention is not limited thereto. The redistribution structure of the invention can be formed by alternately stacking N layers of the redistribution layers and (N−1) layers of the passivation layers, and a metallic pillar is added to the (N−1) th  passivation layer to fill the indented portion of the redistribution structure, so that the top portion of the metallic pillar has the same height with the top portion of the (N−1) th  passivation layer. Therefore, various redistribution structures are disposed at the same horizontal height before the UBM layer is formed. The UBM layers and the bumps are subsequently formed using the same material, having the same specifications and following the same manufacturing process, therefore the bumps have the same height.  
      According to the wafer structure and the method for fabricating the same disclosed in above embodiment of the invention, the bumps formed after redistribution have the same height. In the wafer structure of the invention, a metallic pillar is added to the redistribution structure to fill the indented portion formed in the redistribution structure extended from the original position of the solder pad. Therefore, the UBM layers and the bumps subsequently formed using the same material, having the same specifications and following the same manufacturing process would have the bumps of the same height. Besides, the contact between the redistribution layer and the metallic pillar as well as the contact between the metallic pillar and the UBM layer are excellent, avoiding the problem of detachment.  
      While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.