Abstract:
A bump structure on a contact pad and a fabricating process thereof. The bump comprises an under-ball-metallurgy layer, a bonding mass and a welding lump. The under-ball-metallurgy layer is formed over the contact pad and the bonding mass is formed over the under-ball-metallurgy layer by conducting a pressure bonding process. The bonding mass having a thickness between 4 to 10 μm is made from a material such as copper. The welding lump is formed over the bonding mass such that a sidewall of the bonding mass is also enclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims the priority benefit of Taiwan application serial no. 91120545, filed on Sep. 10, 2002.  
         BACKGROUND OF INVENTION  
         [0002]    1. Field of Invention  
           [0003]    The present invention relates to a bump and fabricating process thereof. More particularly, the present invention relates to a bump having an improved barrier layer mechanical strength and fabricating process thereof.  
           [0004]    2. Description of Related Art  
           [0005]    In this information-hungry society, electronic products are used almost everywhere to meet our demands for communication, business transactions, education, recreation and much more. The principle drivers behind all these electrical devices are specially designed integrated circuits. As electronic technologies continue to advance, increasingly complex, functionally powerful and highly personalized electronic products are produced. Rapid progress in design also has brought about the current trend of product miniaturization. Many types of high-density semiconductor packages are developed using flip-chip technique. Since a flip-chip package utilizes the bump on each contact pad of a chip to make direct electrical contact with a substrate, average circuit length is shorter than other types of packages connected through the wire bonding or the tape automated bonding (TAB) method. The shortened circuit length improves overall performance of a flip-chip package over other conventional packages. Furthermore, the backside of the chip in a flip-chip package may be exposed by design to increase heat dissipation. Because of these advantages, flip-chip techniques for fabricating packages are adopted by most semiconductor package producers.  
           [0006]    [0006]FIG. 1 is a magnified cross-sectional view of a portion of a conventional flip-chip package structure. As shown in FIG. 1, the flip-chip structure  100  includes a silicon chip  100  and at least a bump structure  170  (only one is shown in FIG. 1). The bump structure  170  includes an under-bump-metallurgy (UBM) layer  142  and a bump  160 . The chip  110  has an active surface  112 . The active surface  112  of the chip  110  has a passivation layer  114  and at least one contact pad  116  thereon. The passivation layer  114  has at least one opening  118  that exposes the contact pad  116 . The under-ball-metallurgy (UBM) layer  142  is formed on the contact pad  116  of the chip  110 . The UBM layer  142  includes an adhesion layer  120 , a barrier layer  130  and a wettable layer  140 . The adhesion layer  120  sits directly on the contact pad  116 , the barrier layer  130  is over the adhesion layer  120  and the wettable layer  140  is over the barrier layer  130 . The adhesion layer  120  is made from a material such as titanium or aluminum, the barrier layer  130  is made from a material such as nickel-vanadium alloy and the wettable layer  140  is made from a material such as copper. The bump  160  sits on the wettable layer  140 . The bump  160  is made from a material such as lead-tin alloy.  
           [0007]    In general, the aforementioned flip-chip package structure  100  has a thin wettable layer  140  of between 0.3 to 0.8 μm. Moreover, the copper in the wettable layer  140  may react quickly with the tin inside the bump  160 . At the end of the copper-tin reaction, the tin within the bump  160  may further react with the nickel inside the barrier layer  130 . Since the inter-metallic layer formed by the relatively slow reaction (more than 30 seconds) between tin and nickel is lumpy and discontinuous, ultimate contact with the adhesion layer  120  will be poor. Hence, the bump  160  may easily peel off from the upper surface of the chip  110 .  
         SUMMARY OF INVENTION  
         [0008]    Accordingly, one object of the present invention is to provide a bump structure and a fabricating process thereof capable of increasing mechanical strength of a barrier layer within the bump structure and hence preventing the bump structure from peeling off the chip via the barrier layer.  
           [0009]    Before starting out to describe this invention, the spatial preposition “over” or “above” needs to be clarified. When the preposition “over” or “above” is used, the relationship between the two objects concerned may or may not have direct contact with each other. For example, an object A is “over” or “above” an object B may mean either object A is above object B and directly touching object B or object A is in the space above object B but without touching object B.  
           [0010]    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 bump structure over a contact pad. The bump structure includes an under-bump-metallurgy (UBM) layer, a bonding mass and a bump. The under-ball-metallurgy layer sits over the contact pad. The bonding mass is formed over the under-ball-metallurgy layer by pressure bonding. The bonding mass is made from a material such as copper and has a thickness ranging between 4 to 10 μm. The bump is above the bonding mass and covers a sidewall of the bonding mass.  
           [0011]    According to one preferred embodiment of this invention, the under-ball-metallurgy layer may further include an adhesion layer and a barrier layer. The adhesion layer sits over the contact pad and is made from a material such as titanium, titanium-tungsten alloy, aluminum or chromium.  
           [0012]    The barrier layer sits over the adhesion layer and is made from a material such as nickel-vanadium alloy or nickel. The bonding mass sits over the barrier layer. In addition, the bump may be made from a material such as lead-tin alloy. The bump may be made from a lead-free material such as a single metallic substance or an alloy of metallic substances selected from tin, gold, silver, copper, bismuth, antimony, indium or zinc.  
           [0013]    This invention also provides a process for fabricating a bump structure. First, an under-ball-metallurgy layer is formed over a wafer. Thereafter, a pressure bonding process is conducted to form a bonding mass over the under-ball-metallurgy layer. A solder material is applied over the bonding mass. A reflow process is conducted to solidify the solder material and hence form a bump over the bonding mass.  
           [0014]    In brief, since the bonding mass is a relatively thick layer ranging between 4 to 10 μm, reaction time between copper and tin is extended. Hence, the amount of platy and discontinuous inter-metallic material resulting from the reaction between nickel and tin is restricted. Ultimately, bonding strength between the bump and the chip is increased.  
           [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 DRAWINGS  
       [0016]    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.  
         [0017]    [0017]FIG. 1 is a magnified cross-sectional view of a portion of a conventional flip-chip package structure.  
         [0018]    FIGS.  2  to  10  are magnified cross-sectional views showing the progression of steps for fabricating a bump structure according to one preferred embodiment of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
         [0020]    FIGS.  2  to  10  are magnified cross-sectional views showing the progression of steps for fabricating a bump structure according to one preferred embodiment of this invention. First, as shown in FIG. 2, a silicon wafer  210  is provided. The wafer  210  has an active surface  212 . The active surface  212  has a passivation layer  214  and a plurality of contact pads (only one is shown in FIG. 2) thereon. The passivation layer  214  has a plurality of openings  218  each exposing a contact pad  216 . The passivation layer  214  may be an inorganic compound layer such as a silicon oxide layer or a phosphosilicate glass (PSG) layer. Alternatively, the passivation layer  214  may be a composite layer comprising a stack of the aforementioned inorganic compound layers. Furthermore, the passivation layer  214  may also be an organic compound layer such as a polyimide layer.  
         [0021]    A sputtering process is next carried out to form an adhesion layer  220  over the active surface  212  of the wafer  210 . The adhesion layer  220  covers the contact pad  216  and the passivation layer  214  to form a structure as shown in FIG. 3. The adhesion layer  220  is made from a material such as titanium, titanium-tungsten alloy, aluminum or chromium. Another sputtering or an electroplating process is conducted to form a barrier layer  230  over the adhesion layer  220 , thereby forming a structure as shown in FIG. 4. The barrier layer  230  is made from a material such as nickel-vanadium alloy or nickel. The adhesion layer  220  and the barrier layer  230  together constitute an under-ball-metallurgy layer  240 .  
         [0022]    Thereafter, a plurality of bonding masses (only one is shown in FIG. 5) is formed over the barrier layer  230 . As shown in FIG. 5, a conventional stud-bump-forming machine is utilized to attach a bonding mass onto the barrier layer  230 . The stud-bump-forming machine has a bonding head  260  with a capillary  262  therein. The capillary  262  accommodates a bonding wire  264 . The bonding wire  264  is free to slide inside the capillary  262 . A point discharge method is used to generate heat at one end  266  of the conductive wire  264  so that the heated end melts. Due to intermolecular adhesion between the metallic atoms within the wire, the heated end of the wire  264  transforms into a spherical ball  268 . Throughout the point discharge process, nitrogen and hydrogen are passed to prevent any oxidation on the surface of the spherical ball  268  due to high temperature.  
         [0023]    The spherical ball  268  is pulled down to press against the upper surface of the barrier layer  230  before the spherical ball  268  solidifies as shown in FIG. 6. Ultrasound may also be applied to facilitate the joining of the spherical ball  268  with the barrier layer  230 . At this moment, the spherical ball  268  and the barrier layer  230  melt into each other so that the spherical ball  264  is eventually fastened firmly onto the upper surface of the barrier layer  230 . The bonding head  260  is immediately raised so that the wire  264  detaches from the spherical ball  268  to form the structure as shown in FIG. 7. Hence, a bonding mass  270  is formed over the barrier layer  230 . The bonding mass  270  is made from a material such as copper. Preferably, each bonding mass  270  has an overall thickness  272  between about 4 to  10 μm.    
         [0024]    Using the bonding mass  270  as an etching mask, the under-ball-metallurgy layer  240  is removed by etching to expose the active surface  212  of the wafer  210 . However, a residual under-ball-metallurgy layer  240  remains underneath the bonding mass  270 . In other words, a portion of the adhesion layer  220  and the barrier layer  230  remains underneath the bonding mass  270  to form a structure as shown in FIG. 8.  
         [0025]    A screen-printing method is used to form solder blocks  280  (only one is shown) over the bonding mass  270  as shown in FIG. 9. The solder blocks contain material made by mixing together metallic particles and flux agents. Thereafter, a reflow process is carried out so that the metallic particles inside each pasty solder block  280  melt and coalesce together into a bump  290  (only one is shown) over the bonding mass  270 . The flux agents serve to remove any oxide material from the surface of the bump  290  and the bonding mass  270  so that the bump  290  and the bonding mass  270  are tightly bonded together to form a structure as shown in FIG. 10. The bump  290  can be made from a material such as lead-tin alloy or a lead-free material such as tin, gold, tin-copper alloy, tin-technetium alloy, tin-bismuth alloy, tin-indium alloy, tin-zinc alloy, tin-silver alloy, tin-bismuth-silver alloy, tin-bismuth-technetium alloy, tin-bismuth-zinc alloy, tin-bismuth-indium alloy or tin-silver-copper alloy. Since a portion of the non-volatile flux agents may remain after the bump  290  is formed, a special solvent is applied to the wafer to remove any residual flux agents from the surface of the bump  290  and the active surface  212  of the wafer  210 . The under-ball-metallurgy layer  240 , the bonding mass  270  and the bump  290  together constitute a complete bump structure  292 . In addition, the bump  290  encloses the sidewalls  274  of bonding mass  270  as well.  
         [0026]    In the aforementioned bump structure  292 , the bonding mass  270  is a relatively thick layer having a thickness ranging between 4 to 10 μm. With such thickness, reaction time between copper and tin is extended, thereby reducing the formation of a platy and discontinuous inter-metallic layer through prolonged reaction between nickel and tin. Ultimately, bonding strength between the bump  290  and the wafer  210  is improved.  
         [0027]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.