FIELD OF THE INVENTION 
     The present invention is generally related to a bumping process and bump structure, and more particularly, to a planarized bump structure and a bumping process therefor. 
     BACKGROUND OF THE INVENTION 
     Wire bonding, tape automated bonding (TAB) and flip chip bonding are popular packages for integrated circuits (ICs). Generally, wire bonding is used in low-density package with less than 300 inputs/outputs (I/Os). In high-density packages, up to 600 I/Os may be provided by TAB, and flip chip package provides much higher package density with more than 600 I/Os. In flip chip package, it is required to form bumps on the pads of the integrated circuit for the pressing process in chip-on-glass (COG), chip-on-board (COB), chip-on-film (COF), or other package processes. In order to reduce electrical noises and to increase adhesion and conductivity, gold is typically used for the bump material, which makes the bumping process expensive and difficult. Therefore, improving the bump structure and bumping process becomes an important issue. On the other hand, the density and performance of a package limit the size and performance of a chip. As the size of IC shrinks, the IC package becomes the bottleneck to further shrink the IC, if the density and performance of the package are not enhanced, for example the size and pitch of the bumps are limited or the conductivity of the bumps are not good enough. 
       FIG. 1  shows a conventional gold bump structure  10 , in which on a substrate  12  a pad  14  is partially covered by a passivation layer  16 , an under bump metallization (UBM)  18  is formed on the exposed surface of the pad  14  and the peripheral passivation layer  16 , and a gold film  20  and bump  22  are formed on the UBM  18 . Typically, the material of the pad  14  is aluminum, the passivation layer  16  comprises a layer of silicon dioxide  24  and a layer of silicon nitride  26 , and the UBM  18  is a stacked layer of titanium and tungsten. The gold film  20  is sputtered and has denser crystalline, to increase the adhesion between the gold bump  22  and UBM  18 . The gold bump  22  grows by electro-plating from the gold film  20  and has larger crystalline and higher hardness. Since the passivation layer  16  always has step  28  at the peripheral of the pad  14 , the upper surface of the bump  22  will have step  30  at its edge and therefore, only the central concave region  32  becomes an effective region during the pressing process. The roughness h of the upper surface of the bump  22  is about 2 μm. If a larger effective region  32  is required, the pad  14  has to be larger. However, if only the width of the bump  22  is increased, as shown in  FIG. 2 , the effective region  32  will remain nearly the same because the increased region  34  on the upper surface of the bump  22  is useless due to the uneven upper surface of the bump  22 .  FIG. 3  shows several bumps  22  on the substrate  12 , where the width of the pad  14  is w 1 , the bump gap is g, and the bump pitch is p. The width w 2  of the bump  22  is no greater than the width w 1  of the pad  14 , so the effective region  32  is small compared to the pad  14 . To increase the effective region  32 , it is required to have a larger pad  14 . However, the contact density of the chip is thus lowered and the chip size cannot be minimized. In addition, a larger pad  14  will result in a larger bump pitch p. If the bump gap g remains constant, the only way to obtain an increase in the contact density of the chip is to shrink the pad  14 . But shrinking the pad  14  causes the minimization of the effective region  32 . There&#39;s difficulty to solve this problem using conventional techniques. 
     A conventional bumping process is shown in  FIGS. 4A to 4E . In  FIG. 4A , a passivation layer  16  with a thickness of 1.2 μm is deposited to cover pads  14  on a substrate  12 . In  FIG. 4B , the passivation layer  16  is etched to form openings  36  to expose the pads  14 , and after this step, the passivation layer  16  will have steps  38  at the peripherals of the pads  14 . In particular, the thicker the passivation layer  16  is, the higher the steps  38  are and the deeper the openings  36  are. In  FIG. 4C , Ti/W stack with a deposition thickness of 800 Å is used as UBM  18 , and a gold film  20  with a thickness of 800 Å is deposited thereon. In this step, due to the step  38 , step  40  formed thereon is even wider. The thicker the UBM  18  is, the narrower the concavity  42  is.  FIG. 4D  shows the structure after the UBM  18  and gold film  20  are patterned. In  FIG. 4E , gold bumps  22  are grown up from the gold film  20  and have a thickness of about 17 μm. It is therefore shown by this process that the steps  38  are inevitable. As a result, effective regions  32  always have small areas. The thicker the UBM  18  is, the smaller the effective region  32  is. Moreover, the thicker the passivation layer  16  is, the greater the roughness h is. Even though the semiconductor process is capable to minimize the chip size, the backend package does not catch up with the IC shrinkage and thus limits the minimized size of the chip. 
     Further, a conventional bump structure has drawbacks during the pressing process. Referring to a COG structure  44  shown in  FIG. 5 , while pressing the bump  22  to a wire  48  on a glass substrate  46 , an anisotropic conductive film (ACF)  50  is used therebetween as an interface. The ACF  50  is a polyimide (PI) with conductive particles thereof, and the conductive particles will form a conductive path in the pressing direction between the bump  22  and wire  48  during the pressing process. Since the surface roughness of the bump  22  is about 2 μm, the diameter of the conductive particles  52  within the ACF  50  has to be larger than 3 μm to construct an excellent conduction between the bump  22  and wire  48 . However, if the conductive particles  52  are larger, then there will be fewer of them to be trapped in the effective region  32 , and thus there&#39;s greater contact impedance and poor conduction quality after the pressing process. On the other hand, the conductive particles  56  with larger diameter inside the bump gap  34  will easily cause short or leakage between neighboring bumps  22 , and thus lower the yield of the pressing process. If small conductive particles  52  are used, excellent connection between the bump  22  and wire  48  cannot be reached. Therefore, there&#39;s unbeatable difficulty in conventional technology. To satisfy the requirement of smaller size and higher I/O count of an IC chip, the pad  14  on the chip is required to be shrunk, and the effective region  32  is thus minimized, which causes the drop of the yield of the pressing process and conduction quality of the product. Furthermore, an elemental drawback of flip chip package is the weak mechanical strength at the peripheral region  58  of the bump  22 , and damage happens easily due to lateral force. However, to obtain a smaller roughness h at the pressing surface of the bump  22  will have the step  28  to decrease, and a thinner passivation layer  16  could not overcome the drawbacks in weak mechanical strength. 
     Therefore, it is desired an improved bumping process and bump structure. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a structure and process for a planarized bump to overcome the drawbacks of conventional art. 
     In a bumping process, according to the present invention, it is formed a passivation layer with a planarized surface to cover a pad on a substrate, the passivation layer is etched to form a hole penetrating therethrough to expose a contact surface of the pad, and a bump is formed on the contact surface and the planarized surface. 
     In a bump structure, according to the present invention, a passivation layer covering a portion of a pad on a substrate has a planarized surface, the pad has a contact surface, and a bump contacts the contact surface and the planarized surface. 
     Preferably, the passivation layer comprises several layers with different hardness in stack. 
     Preferably, the contact surface has a shape of stripe. 
     Since the passivation layer has the planarized surface to provide for larger effective region, the pad could be minimized, and the mechanical strength at the peripheral of the pad could be enhanced by increasing the thickness of the passivation layer. During the pressing process, since the bump has a larger effective area, there will be greater selection flexibility for the anisotropic conductive film, and the probabilities of short circuit and current leakage are reduced, thereby improving the yield of the pressing process and the conductive quality for the pad. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of a conventional gold bump structure; 
         FIG. 2  shows an enlarged conventional gold bump structure; 
         FIG. 3  is a schematic diagram of several conventional gold bumps on a substrate; 
         FIGS. 4A to 4E  show a conventional bumping process; 
         FIG. 5  is a schematic diagram of a conventional COG structure; 
         FIGS. 6A and 6B  are two cross-sectional views of a gold bump structure according to the present invention; 
         FIGS. 7A and 7B  are two top views of a gold bump according to the present invention; 
         FIGS. 8A to 8G  show a first bumping process according to the present invention; 
         FIGS. 9A to 9D  show a second bumping process according to the present invention; 
         FIG. 10  is a schematic diagram of a COG structure according to the present invention; and 
         FIG. 11  is a schematic diagram of a gold bump with a thicker passivation layer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 6A and 6B  show a gold bump structure  60  according to the present invention, and  FIGS. 7A and 7B  show the top views, in which  FIG. 6A  is a cross-sectional view along the X direction and  FIG. 6B  is a cross-sectional view along the Y direction. Referring to  FIGS. 6A and 6B , in the gold bump structure  60 , a passivation layer  64  has a planarized surface and covers a portion of each pad  62  on a substrate  12 , a UBM  18  and a gold film  20  are stacked on the pad  62  and passivation layer  64 , and a gold bump  66  is on the gold film  20 . The pad  62  is made of aluminum, aluminum alloy, or other metal or highly conductive alloy, and the passivation layer  64  comprises one or more layers of silicon dioxide, silicon oxide, silicon nitride, silicon oxy nitride, or other superior chemical resistive materials or their combination to protect the circuits within the substrate  12 . The UBM  18  is used mainly to protect the pad  62  from being penetrated by any chemical particles during the following processes to affect the electrical characteristics of the product, and at the same time to improve the adhesion between the gold film  20  and pad  62 . In one embodiment, the pad  62  is made of aluminum, and the UBM  18  comprises titanium (Ti) and tungsten (W) layers in the manner that the titanium layer is at the bottom to have good adhesion with the aluminum pad  62  and the tungsten layer is at the top to have good adhesion with the gold film  20 . As shown in  FIG. 6A , the pad  52  has a width w 1   x  in the X direction that is much smaller than a conventional pad, and the width w 2   x  of the bump  66  in the X direction is also smaller such that the bump pitch p can be minimized. However, in the Y direction, as shown in  FIG. 6B , though the width w 1   y  of the pad  62  is also smaller than a conventional pad, the width w 2   y  of the bump  66  is much larger than the width w 1   y  of the pad  62 . Due to the smaller pad  62 , the concave region  68  at the center of the top surface of the bump  66  is minimized. If the UBM  18  is thicker, the concave region  68  may be completely eliminated. Since the passivation layer  64  has a planarized surface, a planarized region  70  occupies most of the top surface of the bump  66  and can be used as the effective region for pressing. Being different from the conventional bump structure  10 , the effective region of the bump  66  is at the peripheral of the top surface rather the center; in other words, it is mainly at the region above the passivation layer  64 . 
       FIG. 7A  further illustrates the relation between the bump  66  and pad  62 . For comparison, the conventional bump  22  and pad  14  are also shown at the right side of  FIG. 7A . In the conventional bump structure  10 , the pad  14  is larger than the bump  22 , and thus, in order to have enough effective region on the bump  22 , the pad  14  cannot be shrunk. While in the bump structure  60  according to the present invention, the bump  66  is larger than the pad  62 , and therefore the pad  62  can be minimized. In the bump structure  60 , the exposed contact surface  72  on the pad  62  for coupling to the bump  66  has a stripe shape. In the conventional bump structure  10 , the exposed contact surface  74  on the pad  14  for coupling to the bump  22  has almost the same width in both the X and Y directions.  FIG. 7B  shows the high-density bump  66  on the substrate  12 . The bump  66  has a stripe shape extending in the Y direction. In the X direction, since the pad  62  can be minimized, the bump  66  can be more tightly arranged. If more planarized region  70  on the bump  62  is desired, it can be achieved by increasing the width w 1   y  in the Y direction. Since the pad  62  can be minimized, more bumps  66  can be arranged on an IC of the same size to increase the I/O density and pin count. 
       FIGS. 8A to 8G  show a bumping process according to the present invention. In  FIG. 8A , film  76  such as silicon dioxide or silicon oxide is deposited with thickness of 1000 to 1200 Å to cover pads  62  on a substrate  12 , and etching back process such as chemical mechanical polishing (CMP) is used to etch the film  76  to leave a thickness of 600 to 800 Å, which results in a planarized surface  78  as shown in  FIG. 8B . In  FIG. 8C , film  80  such as silicon nitride or silicon oxy nitride is deposited with a thickness of 300 to 500 Å on the films  76 . Since the film  76  has a planarized surface  78 , the film  80  also has a planarized surface  82 . The films  76  and  80  serve as the passivation layer  64  in  FIG. 6A , and preferably, the film  80  is harder than the film  76 . The softer film  76  is used to protect the substrate  12  and the surface of the pad  62 , and the harder film  80  is used against force. As shown in  FIG. 8D , the films  80  and  76  are etched to form an opening  84  that penetrates through the films  80  and  76  from the planarized surface  82  to the top surface of the pad  62 , to expose a contact surface  72  on the pad  62 . In  FIG. 8E , a UBM  18  with a thickness of 800 Å is deposited on the contact surface  72  on the pad  62  and the planarized surface  82  of the film  80  by sputtering titanium and tungsten for example. A gold film  20  is deposited on the UBM  18  thereafter by sputtering. As shown in  FIG. 8F , the gold film  20  and UBM  18  are patterned to define the bumps, and in  FIG. 8G , a gold bump  66  is grown up from the gold film  20  for 15 to 20 μm by electro-plating. Since the passivation layer  76  and  80  has planarized surface, the central concavity  68  on the top surface of the bump  66  is very small or even none, and most of the top surface of the bump  66  is a planarized region  70 . 
       FIGS. 9A to 9D  show another bumping process according to the present invention. In  FIG. 9A , deposited films  76  and  80  cover pads  62  on a substrate  12 , in which the film  80  is preferably harder than the film  76 . The softer film  76  is used to protect the surfaces of the substrate  12  and pad  62 , and the harder film  80  is used against force. For example, the film  76  comprises silicon dioxide or silicon oxide with a thickness of 200 to 800 Å, and the film  80  comprises silicon nitride or silicon oxy nitride with a thickness of 300 to 500 Å. In  FIG. 9B , the films  76  and  80  are etched back by for example CMP, to leave them a total thickness of about 600 to 1000 Å, which results in a planarized surface  86 . In  FIG. 9C , an opening  84  is formed to expose a contact surface  72  on the pad  62 . In  FIG. 9D , sputtering is used for example to deposit titanium and tungsten to a thickness of 800 Å as a UBM  18  on the contact surface  72  and planarized surface  86 , a gold film  20  is deposited by sputtering to a thickness of 800 Å on the UBM  18 , the gold film  20  and UBM  18  are patterned to define the bumps, a gold bump  66  is grown up by electro-plating from the gold film  20  to a thickness of 15 to 20 μm. Since the planarized surface  86  is formed in previous step, the central concavity  68  on the top surface of the bump  66  is very small or even none, and most of the top surface of the bump  66  is a planarized region  70 . 
     In the bumping process according to the present invention, since the passivation layer  64  with a planarized surface is used, on the planarized surface the UBM  18  has an area much larger than that on the contact surface  72  to obtain a maximized effective region  70 . Thus the pad  62  is minimized. 
       FIG. 10  shows a structure  88  where the bump  66  is pressed to a wire  48  on a glass substrate  46 . Being different from the conventional COG structure  44 , the pressing effective region provided by the bump  66  is the planarized region  70 . Since there&#39;s no problem about surface roughness thereof, it will have more flexibility in selecting the diameter of the conductive particles  92  in the ACF  90 , for example 1 to 5 μm. Even though smaller conductive particles  92  are used, excellent conductivity can be still obtained. Since the effective region is the planarized region  70  that has larger area, the effective region  70  is capable to trap more conductive particles  92 . If the conductive particles  92  have smaller diameter, the trapped amount of them is even higher and the conductive quality is much better. On the other hand, if the conductive particles  92  have smaller diameter, it is less possible for the conductive particles  92  within the bump gap  94  to cause short circuit or current leakage during the pressing process. Moreover, since the passivation layer  64  with a planarized surface is used, the mechanical strength at the peripheral  96  of the bump  66  is improved and damage will not easily happen thereto. In the bump structure  60 , since it is used the passivation layer  64  with a planarized surface, the thickness of the passivation layer  64  is not limited.  FIG. 11  shows an embodiment when a thicker passivation layer  64  is used. The passivation layer  64  comprises films  76 ,  80  and  98 , in which the films  76  and  98  are silicon dioxide or silicon oxide, and the film  80  is silicon nitride or silicon oxy nitride. The total thickness of films  76 ,  80  and  98  is up to more than 1.2 μm and thus increases the mechanical strength of the corresponding structure. 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.
A bumping process comprises forming a passivation layer having a planarized surface covering a pad on a substrate, forming a hole penetrating through the passivation layer to expose a contact surface of the pad, and forming a bump on the contact surface and planarized surface. The planarized surface will provide a larger effective area for pressing, thereby minimizing the pad, enhancing the mechanical strength at the peripheral of the pad, providing more selection flexibility for anisotropic conductive film, reducing the possibilities of short circuit and current leakage within the bump gap, and increasing the yield of the pressing process and the conductive quality of the bump.