Abstract:
A bumping process mainly comprises the steps of providing a wafer having a plurality of bonding pads, forming a patterned adhesive layer over the bonding pads, forming a barrier layer and a wetting layer on the patterned adhesive layer and the surface of the wafer, removing the barrier layer and the wetting layer not covering the patterned adhesive layer, forming a plurality of bumps on the patterned wetting layer and reflowing the bumps.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    This invention relates to a bumping process. More particularly, the present invention is related to a method of increasing the height of bumps without utilizing increasing the thickness of the photo-mask.  
           [0003]    2. Related Art  
           [0004]    In this information explosion age, integrated circuits products are used almost everywhere in our daily life. As fabricating technique continue to improve, electronic products having powerful functions, personalized performance and a higher degree of complexity are produced. Nowadays, most electronic products are relatively light and have a compact body. Hence, in semiconductor production, various types of high-density semiconductor packages have been developed. Flip chip is one of the most commonly used techniques for forming an integrated circuits package. Moreover, compared with a wire-bonding package or a tape automated bonding (TAB) package, a flip-chip package has a shorter electrical path on average and a better overall electrical performance. In a flip-chip package, the bonding pads on a chip and the contacts on a substrate are connected together through a plurality of bumps formed by the method of bumping process. Accordingly, the technology of bumping process becomes more and more important in the advanced packaging fields.  
           [0005]    As mentioned above, the bumping process mainly comprises forming an under bump metallurgy layer on the bonding pads of the wafer and forming bumps on the under bump metallurgy over the bonding pads. After the wafer with bumps formed thereon is singulated into a plurality of individual bumped chips, the bumped chips are attached to the substrates through bumps respectively. However, when the gap between the bumped chip and the substrate is not small, the bumps connecting the substrate and the chip will bear larger shear stress and more easily damaged for that the coefficient of thermal expansion (CTE) of the chip is different from that of the substrate. In such a manner, forming higher bumps to have a larger gap between the chip and the substrate will have the bumps to be able to bear larger shear stress and enhance the mechanical strength of the bumps.  
           [0006]    [0006]FIG. 1 to FIG. 4 are partially enlarged cross-sectional views showing the progression of steps in a conventional method of forming a bump on a surface of a chip.  
           [0007]    As shown in FIG. 1, a wafer  100  is provided. The wafer  100  has a passivation layer  102  and a plurality of bonding pads  104  (only one of the bonding pads is shown) exposed out of the passivation layer  102 . Next, an under bump metallurgy layer  106  is formed on the wafer  100  to cover the passivation layer  102  and the bonding pads  104 . Generally speaking, the under-ball metallurgy layer  106  mainly includes an adhesion layer  106   a , a barrier layer  106   b  and a wetting layer  106   c . Then, the, referring to FIG. 2, solder bumps  110  are formed by providing a patterned photo-resist layer  108  on the under bump metallurgy layer  106  to form a plurality of openings  108   a  to expose the portions over the bonding pads  104  and filling solder material in the openings  108   a  to dispose on the under bump metallurgy layer  106  not covered by the photo-resist layer  108 . Moreover, the volume of the solder bump  110  is increased through utilizing increasing the thickness of the photo-resist layer  108 . Therein, the solder material is filled into the opening  108   a  of the patterned photo-resist layer  108  through electro-plating to form higher solder bumps  110  by said thicker photo-resist layer. Generally speaking, the opening  108   a  of the patterned photo-resist layer  108  is ranged between about 100 μm and about 120 μm in size and the thickness of the photo-resist layer  108  is about 100 μm or about 120 μm.  
           [0008]    Next, referring to FIG. 3 and FIG. 4, after photo-resist layer  108  is moved, the solder bumps  110  are taken as a mask to etch the under bump metallurgy layer  106  not covered by the solder bumps  110  to form patterned under bump metallurgy layer  106 ′ until the passivation layer  102  is exposed. Finally, the solder bumps  110  are reflowed to form ball-like solder bumps  112  and enhance the attachment of the ball-like solder bumps  112  to the patterned under bump metallurgy layer  106 ′.  
           [0009]    As we know, the height of the solder bumps after reflowing is pertinent to the volume of the solder material filled into the opening defined by the thickness of the photo-resist layer and the area of the under bump metallurgy for disposing the solder material thereon. Accordingly, when the opening of the patterned photo-resist layer is smaller, there are usually needed more thicker patterned photo-resist layers to be stacked with each other in order to meet the volume of the openings for filling the solder material on condition that the area of the under bump metallurgy layer for disposing the solder material thereon keeps unchanged. However, in photolithography process, it is difficult to form smaller and deeper openings, such as the diameter or width of the opening smaller than 100 μm and the depth of the opening ranged between 100 μm and 140 μm, in the photo-resist layer. Thus, the reliability of forming higher bumps will be lowered.  
           [0010]    On the contrary, when the opening of the patterned photo-resist layer becomes larger, the area with solder material disposed thereon to be regarded as the mask for etching the under bump metallurgy will become larger. Accordingly, the area of the patterned under bump metallurgy layer over the bonding pad will become larger and the height of the solder bumps after reflowing will become smaller.  
           [0011]    Therefore, providing another method for forming bumps to solve the mentioned-above disadvantages is the most important task in this invention.  
         SUMMARY OF THE INVENTION  
         [0012]    In view of the above-mentioned problems, this invention is to provide a method of forming bumps to increase the height of each bump so as to enhance the mechanical reliability of bumps without increasing the thickness of the photo-resist layer.  
           [0013]    To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of forming bumps. Firstly, a wafer having a passivation layer exposing a plurality of bonding pads formed thereon is provided. Next, an adhesive layer and a barrier layer are formed on the wafer in sequence. Then, portions of the barrier layer are removed to leave the patterned barrier layer disposed over the bonding pads. Moreover, a wetting layer is formed on the patterned barrier layer and the adhesive layer. Next, a plurality of bumps are formed on the wetting layer disposed over the bonding pads through forming patterned photo-resist layer with a plurality of openings, filling conductive materials, such as solder materials, into the openings and removing the patterned photo-resist layer. Afterwards, the bumps are taken as masks to remove the portions of the wetting layer not covered by the bumps to form a patterned wetting layer. Then, a reflowing process is performed to shape the bumps into spheres or balls. Finally, the bumps, the patterned barrier layer and the patterned wetting layer are regarded as masks to remove the portions of the adhesive layer to form a patterned adhesive layer.  
           [0014]    As mentioned above, the height of each bump formed by the solder material is pertinent to the volume of the opening formed in the patterned photo-resist layer for filling conductive material and the area of the under bump metallurgy layer with conductive material disposed thereon. Accordingly, in this invention, there are needed larger area of the under bump metallurgy with conductive material disposed thereon to reduce the thickness of the patterned photo-resist layer when filling solder material in the openings to meet the predetermined volume. In addition, the height of each bump will become larger due to the reduction of the area of the under bump metallurgy layer with conductive material disposed thereon.  
           [0015]    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 THE DRAWINGS  
       [0016]    The invention will become more fully understood from the detailed description given herein below illustrations only, and thus are not limitative of the present invention, and wherein:  
         [0017]    FIGS.  1  to  4  are partially enlarged cross-sectional views showing the progression of steps for forming a bump according to the conventional invention;  
         [0018]    FIGS.  5  to  12  are partially enlarged cross-sectional views showing the progression of steps for forming a bump according to the preferred embodiment of this invention; and  
         [0019]    FIGS.  13  to  17  are partially enlarged cross-sectional views showing the progression of steps for forming a bump according to another preferred embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The method of forming bumps according to the preferred embodiment of this invention will be described herein below with reference to the accompanying drawings, wherein the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
         [0021]    FIGS.  5  to  12  are partially enlarged cross-sectional views showing the progression of steps for forming a bump according to the preferred embodiment of this invention.  
         [0022]    Firstly, referring to FIG. 5, a wafer  200  having a plurality of bonding pads  202  (only one of the bonding pads is shown) and a passivation layer  204  is provided. Therein, the passivation layer  204  is formed on the wafer  200  and exposes the bonding pads  202 .  
         [0023]    Next, referring to FIG. 6, a dielectric layer  205 , such as polyimide (PI) and benzocyclobutene (BCB), is formed on the passivation layer  204  and exposes the bonding pads  202 . Afterwards, a first electrically conductive layer  206  is formed on the dielectric layer  205  and covers the bonding pads  202 .  
         [0024]    Then, referring to FIG. 6 again, a patterned photo-resist layer  207  is formed on the first electrically conductive layer  206  disposed over the bonding pads  202  so as to take the patterned photo-resist layer  207  as masks to remove portions of the first electrically conductive layer  206 . Thus, a patterned first electrically conductive layer  206 ′ is formed over the bonding pads  202  through leaving the potions of the first electrically conductive layer  206  as shown in FIG. 7.  
         [0025]    Moreover, referring to FIG. 8, a second electrically conductive layer  208  is formed above the patterned first electrically conductive layer  206 ′ and the dielectric layer  205  and another patterned photo-resist layer  209  is formed above the second electrically conductive layer  208 , wherein the patterned photo-resist layer  209  has a plurality of openings  209   a  therein and located correspondingly to the bonding pads  202  to define the area of the second electrically conductive layer  208  for forming bumps thereon. Therein, the width or diameter Wp of the opening  209   a  is larger than the width or diameter Wu of the patterned first electrically conductive layer  206 ′ located over the bonding pad  202  and the thickness Hp of the patterned photo-resist layer  209  so as to fill more conductive materials therein. Specifically, the second electrically conductive layer  208  comprises two layers, the barrier layer and the wetting layer. Therein, the barrier layer is formed on the patterned first electrically conductive layer  206 ′ and the dielectric layer  205 , and the wetting layer is formed on the barrier layer.  
         [0026]    Next, referring to FIG. 9, there is filled the conductive material into the opening  209   a  to form a bump  210 . Therein, the conductive material is filled into the opening  209   a  through plating method. To be noted, the conductive material can be a solder material. Specifically, the solder material may comprise eutectic solder and lead-free solder. As mentioned, the plating process is performed through patterned first electrically conductive layer  206 ′ electrically connecting the second electrically conductive layer  208 .  
         [0027]    Moreover, referring to FIG. 10 and FIG. 11, after the bump  210  is formed, the patterned photo-resist layer  209  is removed and then the bumps  210  are taken as masks to remove the portions of the second electrically conductive layer  208  not covered by the bumps  210  to expose the dielectric layer  205  to form the patterned second electrically conductive layer  208 ′. Therein, the patterned first electrically conductive layer  206 ′ is at least covered by the patterned second electrically conductive layer  208 ′. Namely, the projection area of the patterned first electrically conductive layer  206 ′ over the bonding pad  202  is substantially the same as the projection area of the patterned second electrically conductive layer  208 ′ over the bonding pad  202  and smaller than the projection area of the bump  210  over the bonding pad  202 .  
         [0028]    Finally, referring to FIG. 12, a reflowing process is performed to shape the bump  210  into the sphere or ball  212 .  
         [0029]    As mentioned above, when the first electrically conductive layer  206  is an adhesive layer directly attaching to the bonding pad  202 , wherein the adhesive layer comprise titanium layer or aluminum layer, and the second electrically conductive layer  202  comprises a barrier layer and a wetting layer, the adhesive layer is firstly formed on the wafer and then the portions of the adhesive layer not coving the bonding pad is removed to form a patterned adhesive layer. Next, the barrier layer and the wetting layer are disposed on the patterned adhesive layer and the dielectric layer in sequence. Next, a bump is formed on the wetting layer and then the portions of the barrier layer and the wetting layer not covered by the bump are removed to form a patterned barrier layer and a patterned wetting layer. Then, a reflowing process is performed to shape the bump into a sphere or a ball.  
         [0030]    Finally, the sphere or the ball, the patterned barrier layer and the patterned wetting layer is taken as mask to remove the portions of the adhesive layer to form a patterned adhesive layer. Therein, before the bump is reflowed, the projection area of the patterned barrier layer and the projection area of the patterned wetting layer is substantially the same as the projection area of the patterned adheisve layer and smaller than the projection area of the bump.  
         [0031]    Besides, as mentioned above, a first electrically conductive layer can also be deemed as the under bump metallurgy layer, and the second electrically conductive layer is deemed as a bump defined layer for adapting the photo-resist layer to define the volume of the opening for filling the conductive material therein. In other words, firstly, a wafer having an under bump metallurgy layer is provided. Next, a bump defined layer is disposed on the under bump metallurgy layer and above the wafer, and a photo-resist layer is then provided to form a plurality of openings having the size of the opening be larger than the projection area of the under bump metallurgy layer over the bonding pad. Afterwards, the photo-resist layer is removed to have the bump and the under bump metallurgy layer define the bump defined layer. Thus, the area of the patterned bump defined layer over the bonding pad will be substantially the same as that of the under bump metallurgy layer over the bonding pad. Namely, the area of the patterned bump defined layer is substantially the same as that of the under bump metallurgy layer and smaller than the projection area of the bump.  
         [0032]    Moreover, referring to FIG. 13 to FIG. 17, when the first electrically conductive layer  306  is an adhesive layer directly attaching to the bonding pad  302 , wherein the adhesive layer comprise a titanium layer or an aluminum layer, and the second electrically conductive layer comprises a barrier layer  307  and a wetting layer  308 , the adhesive layer  306  and the barrier layer  307  are firstly formed on the wafer  300  and then the portions of the barrier layer  307  not coving the bonding pad  302  is removed to form patterned barrier layer  307 ′. Next, the wetting layer  308  is disposed on the patterned barrier layer  307 ′ and the adhesive layer  306  layer in sequence as shown in FIG. 14. Next, a conductive bump  310 , such as a solder bump, is formed on the wetting layer  308  and then the portions of the wetting layer  308  not covered by the bump  310  are removed to form patterned wetting layer  308 ′ for exposing adhesive layer  306  until the area of the patterned wetting layer  308 ′ over the bonding pad is substantially the same as the area of the patterned barrier layer  307 ′ over the bonding pad. In such a manner, a patterned second electrically conductive layer comprising the patterned barrier layer  307 ′ and patterned wetting layer  308 ′ is formed. Namely, the area of the patterned barrier layer  307 ′ is substantially the same as the area of the patterned wetting layer  308 ′ over the bonding pad and smaller than the area for disposing the bump  310  thereon as shown in FIG. 16.  
         [0033]    In addition, a reflowing process is performed to have the bump shape into a sphere or a ball  312 . Finally, the sphere or the ball  312 , the patterned barrier layer  307 ′ and the patterned wetting layer  308 ′ are taken as mask to remove the portions of the adhesive layer  306  to form a patterned adhesive layer  306 ′.  
         [0034]    In the embodiments as shown above, the height of the bump formed by the conductive or solder material is pertinent to the volume of the opening formed in the patterned photo-resist layer for filling the conductive or solder material and the area of the patterned under bump metallurgy layer over the bonding pad. Accordingly, in this invention, there are provided larger area of the patterned under bump metallurgy over the bonding for disposing bump thereon to reduce the thickness of the photo-resist layer when filling conductive or solder material in the opening to meet the predetermined volume. In addition, the height of the bumps will also beome larger due to the reduction of the area of the patterned under bump metallurgy layer over the bonding pad for disposing thereon.  
         [0035]    Although the invention has been described in considerable detail with reference to certain preferred embodiments, it will be appreciated and understood that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.