Patent Publication Number: US-2003222352-A1

Title: Under-bump metallugical structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001] This application claims the priority benefit of Taiwan application serial no. 91111431, filed May 29, 2002.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of Invention  
       [0003] The present invention relates to an under-bump metallurgical structure between the solder pad and the solder bump of a chip or a substrate. More particularly, the present invention relates to an under-bump metallurgical structure between the solder pad and the solder bump of a chip.  
       [0004] 2. Description of Related Art  
       [0005] Flip chip interconnect technology utilizes an area array arrangement to place a plurality of pads on the active surface of a die. Each pad has a bump such as a solder bump and the pads may contact corresponding contact points on a substrate or a printed circuit board (PCB) as the die is flipped over. Because flip chip technology has the capacity to produce high pin count chip packages with a small packaging dimension and short signal transmission path, it has been widely adopted by chip manufacturers. Many types of bumps are currently available including solder bumps, gold bumps, conductive plastic bumps and polymer bumps. However, the most common one is solder bumps.  
       [0006]FIG. 1 is a cross-sectional view of a conventional under-bump metallic layer between the bonding pad of a die and a bump. As shown in FIG. 1, the die  10  has an active surface  12  with a passivation layer  14  and a plurality of bonding pads  16  (only one is shown) thereon and the passivation layer  14  exposes the bonding pads  16 . In fact, the active surface  12  of the die  10  refers to the side where all the active devices are fabricated. Furthermore, there is an under-bump metallic layer  100  over the bonding pads  16  serving as a junction interface between the bonding pad  16  and a bump  18 .  
       [0007] The under-bump metallic layer  100  has a multiple metallic layer structure that mainly includes an adhesion layer  102 , a barrier layer  104  and a wettable layer  106 . The adhesion layer  102  strengthens the bond between the underlying bonding pad  16  and the overhead barrier layer  14 . In general, the adhesion layer  102  is made from chromium, titanium, titanium-tungsten alloy, chromium-copper alloy, aluminum or nickel. The barrier layer  104  prevents cross-diffusion between upper and lower metallic layers. In general, the barrier layer  104  is made from chromium-copper alloy, nickel or nickel-vanadium alloy. The wettable layer  106  is capable of increasing the wetting capacity with the overhead solder bump  1   8 . In general, the wettable layer  106  is made from copper, nickel or gold. Note that if the wettable layer  106  is made from copper, the under-bump metallic layer  100  may further include an oxidation resistant layer (not shown) over the wettable layer  106  for preventing surface oxidation. In general, the oxidation resistant layer is made from gold or other organic surface protective material.  
       [0008] Since lead-tin alloy has good solderability, most solder bumps  18  are made from lead-tin alloy. Note that after the solder bump  18  is properly positioned over the under-bump metallic layer  100  through a plating, a printing or some other method, a reflow operation must be carried out. The reflow operation not only attaches the underside of the solder bump  18  firmly to the wettable layer  106 , but also transforms the solder bump  18  into a lump of material having a roughly spherical profile. Thereafter, the die  10  is flipped over so that the solder bumps  18  on the active surface  12  are able to contact corresponding contact points on a substrate (or a printed circuit board). Another reflow operation is conducted so that the upper surface of the solder bumps  18  are bonded to the contacts on the substrate (or printed circuit board) (not shown).  
       [0009] If the top layer of the under-bump metallic layer  100  is made from copper, nickel, aluminum, silver or gold, after several heat treatment such as reflow, the tin within the solder bump  18  may react chemically with copper, nickel or gold within the under-bump metallic layer  100 . Hence, an inter-metallic compound (IMC) may be formed between the solder bump  18  and the under-bump metallic layer  100 . Lead-copper is the most easily formed inter-metallic compound, tin-nickel is the second most easily formed inter-metallic compound while tin-gold is the third most easily formed inter-metallic compound. Note that the inter-metallic compound is not so conductive layer that may increase the electrical resistance between the solder bump  18  and the under-bump metallic layer  100 . Accordingly, electrical performance of the flip chip package after the die is enclosed within may deteriorate. Moreover, adhesive strength at the junction between the solder bump  18  and the under-bump metallic layer  100  may be weakened.  
       SUMMARY OF THE INVENTION  
       [0010] Accordingly, one object of the present invention is to provide an under-bump metallurgical structure between the bonding pad and the solder bump of a die such that thickness of the layer of inter-metallic compound between the under-bump metallurgical structure and the solder bump is reduced. Hence, mechanical strength and electrical performance of the package that the die is enclosed within is improved.  
       [0011] A second object of this invention is to provide an under-bump metallic layer formed between the bonding pad of a substrate and a solder bump such that thickness of the inter-metallic compound between the under-bump metallic layer (or the bonding pad (copper pad) of a substrate) and the solder bump is reduced. Consequently, electrical performance and mechanical strength of the flip-chip package after packaging the die is improved.  
       [0012] To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an under-bump metallurgical structure between the bonding pad of a die and the solder bump. The solder bump is mainly made from lead-tin alloy. The under-bump metallurgical structure has a metallic layer over the bonding pads and a buffer metallic layer between the metallic layer and the solder pad for reducing the growth of inter-metallic compound between the metallic layer and the solder bump.  
       [0013] According to the second object, this invention also provides an under-bump metallic layer between the bonding pad of a substrate and a solder bump. The solder bump is mainly made from a lead-tin alloy and the bonding pad is mainly made from copper or aluminum. The under-bump metallic layer has a metallic layer over the bonding pads and a buffer metallic layer between the metallic layer and the solder bump for reducing the growth of inter-metallic compound between the metallic layer and the solder bump.  
       [0014] Similarly, according to the second object, this invention also provides an under-bump metallurgical structure between the bonding pad of a substrate and a solder bump. The solder bump can be for example made from lead-tin alloy and the bonding pad can be for example made from copper. The under-bump metallurgical structure has a buffer metallic layer between the bonding pad and the solder bump for reducing the growth of inter-metallic compound between the bonding pad and the solder bump.  
       [0015] However, the bump can also include a lead-free material, such as SnAg, SnAgBi, SnAgBiCu, SnAgBiCuGe, SnAgBiX, SnAgCu, SnBi, SnCu, SnZn, SnCuSbAg, SnSb SnZnBi, and the under-bump metallurgical structure in general can include Sb, Ag, Sn/Ag, Sn/Cu, and so on. However, SnPbAg with lead may also be used for forming the bump.  
       [0016] 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  
     [0017] 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. In the drawings,  
     [0018]FIG. 1 is a cross-sectional view of a conventional under-bump metallic layer between the bonding pad of a die and a bump;  
     [0019]FIGS. 2A to  2 F are schematic cross-sectional views showing different types of under-bump metallurgical structures between the bonding pad of a die and a solder bump according to a first preferred embodiment of this invention;  
     [0020]FIGS. 3A to  3 G are schematic cross-sectional views showing the progression of steps for fabricating the first type of under-bump metallurgical structure as shown in FIG. 2A;  
     [0021]FIGS. 4A to  4 H are schematic cross-sectional views showing different types of under-bump metallurgical structures between the bonding pad of a die and a solder bump according to a second preferred embodiment of this invention;  
     [0022]FIGS. 5A to  5 H are schematic cross-sectional views showing the progression of steps for fabricating the first type of under-bump metallurgical structure as shown in FIG. 4A; and  
     [0023]FIGS. 6A and 6B are schematic cross-sectional views showing respectively the first type of under-bump metallic structure and the second type of under-bump metallic structure according to this invention between the bonding pad of a substrate and a solder pad. 
    
    
     DETAILED DESCRIPTION  
     [0024] 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.  
     [0025]FIG. 2A is a schematic cross-sectional view showing a first type of under-bump metallurgical structure between the bonding pad of a die and a solder bump according to a preferred embodiment of this invention. As shown in FIG. 2A, the die  10  has an active surface  12  with a passivation layer  14  and a plurality of bonding pads  16  (only one is shown) thereon. The passivation layer and the bonding pads  16  are formed over the active surface  12  of the die  10  such that the passivation layer  14  exposes the bonding pads  16 . Note that the active surface  12  of the die  10  refers to the side where all active devices are formed. To provide an interface for joining the bonding pad  16  and the solder bump  18  together, this invention proposes a first type of under-bump metallurgical structure  201  between the bonding pad  16  and the solder bump  18 . The first type of under-bump metallurgical structure  201  includes a metallic layer  210  and a buffer metallic layer (or an inter-metallic compound growth buffer layer)  220 . The metallic layer  210  is formed over the bonding pad  16  and the buffer metallic layer  220  is formed between the metallic layer  210  and the solder bump  18 . In addition, the metallic layer  210  further includes an adhesion layer  212 , a barrier layer  214  and a wettable layer  216 . The adhesion layer  212  is formed over the bonding pad  16 , the barrier layer  214  is formed over the adhesion layer  212  and the wettable layer  216  is formed between the barrier layer  214  and the buffer metallic layer  220 . Since the metallic layer  210  has a material and structural composition identical to the under-bump metallic layer  100  as shown in FIG. 1, detailed description is not repeated here.  
     [0026] In general, the wettable layer  216  is made from a material including copper or gold. If the wettable layer  216  is made from copper, an anti-oxidation layer (not shown) may be coated over the wettable layer  216  to prevent surface oxidation of the copper wettable layer  216 . The anti-oxidation layer is commonly a thin layer of gold. However, if major constituents of the wettable layer  216  are copper, nickel or gold, the tin within the solder bump  18  may easily react chemically with copper, nickel or gold within the under-bump metallic layer  210  after a thermal treatment of the solder bump  18 . Ultimately, a layer of inter-metallic compound is formed between the solder bump  18  and the under-bump metallic layer  210 . In this invention, the buffer metallic layer  220  of the first type of under-bump metallurgical structure  210  is formed between the wettable layer  216  and the solder bump  18  so that growth of the inter-metallic compound is reduced. To prevent the buffer metallic layer  220  from melting during thermal treatment (for example, a reflow operation) and losing its functional capacity, the buffer metallic layer  220  must have a melting point higher than the solder bump  18 . Furthermore, to provide a good bonding strength between the buffer metallic layer  220  and the solder bump  18 , the buffer metallic layer  220  must easily wet the solder bump  18 . Thus, the buffer metallic layer  220  is preferably made from lead, a high melting point lead-tin alloy or some other materials.  
     [0027]FIGS. 2B and 2C are schematic cross-sectional views of the second and the third type of under-bump metallurgical structures between the bonding pad  16  of the die  10  and the solder bump  18 . As shown in FIG. 2B, the second type of under-bump metallurgical structure  202  is very similar to the first type of under-bump metallurgical structure  201 . The second type of under-bump metallurgical structure  202  similarly has the metallic layer  210  in the first type of under-bump metallurgical structure  201 . However, the buffer metallic layer  220  further includes a first buffer metallic layer  222  and a second buffer metallic layer  224 . The first buffer metallic layer  222 , for example, is a lead layer formed over the wettable layer  216 . The second buffer metallic layer  224 , for example, is a tin layer formed between the first buffer metallic layer  222  and the solder bump  18 . As shown in FIG. 2C, the third under-bump metallurgical structure  203  is also similar to the first type of under-bump metallurgical structure  201 . The third under-bump metallurgical layer  203  similarly has the metallic layer  210  of the first under-bump metallurgical structure  201 . However, the buffer metallic layer  220  further includes a first buffer metallic layer  222 , a second buffer metallic layer  224  and a third buffer metallic layer  226 . The first buffer metallic layer  222 , for example, is a lead layer formed over the wettable layer  216 . The second buffer metallic layer  224 , for example, is a tin layer formed over the first buffer metallic layer  222 . The third buffer metallic layer  226 , for example, is a lead layer formed between the second buffer metallic layer  224  and the solder bump  18 .  
     [0028]FIGS. 2D, 2E and  2 F are cross-sectional views of the fourth, fifth and the sixth type of under-bump metallurgical structures between the bonding pad  16  of a die  10  and the solder bump  18 . Since the buffer metallic layer  220  of the first type under-bump metallurgical structure  201  as shown in FIG. 2A is capable of wetting the solder bump  18 , the wettable layer  216  may be omitted to form the fourth type of under-bump metallurgical structure as shown in FIG. 2D. Similarly, the buffer metallic layer  220  of the second under-bump metallurgical structure  202  as shown in FIG. 2B is capable of wetting the solder bump  18 . Hence, the wettable layer  210  may be omitted to form the fifth under-bump metallurgical structure  205  as shown in FIG. 2E. Likewise, the buffer metallic layer  220  of the third under-bump metallurgical structure  203  is capable of wetting the solder bump  18 . Consequently, the wettable layer  216  may be omitted to form the sixth type of under-bump metallurgical structure  206  as shown in FIG. 2F.  
     [0029]FIGS. 3A to  3 G are schematic cross-sectional views showing the progression of steps for fabricating the first type of under-bump metallurgical structure as shown in FIG. 2A. As shown in FIG. 3A, the die  10  has an active surface  12  with a passivation layer  14  and a plurality of bonding pads  16  (only one is shown) thereon. The passivation layer and the bonding pads  16  are formed over the active surface  12  of the die  10  such that the passivation layer  14  exposes the bonding pads  16 . As shown in FIG. 3B, a metallic film layer  302  is globally formed over the active surface  12  of the die  10 , for example, by evaporation, sputtering or plating. The thin metallic layer  302  serves as a seed layer. As shown in FIG. 3C, a photoresist layer  304  is formed over the thin metallic layer  302  exposing a portion of the thin metallic layer  302  above the bonding pads  16 . As shown in FIG. 3D, another metallic layer  306  is formed over the thin metallic layer  302  by plating, evaporation or sputtering, for example. The metallic layer  306  includes an adhesion layer, a barrier layer and a wettable layer. As shown in FIG. 3E, a buffer metallic layer  308  is formed over the metallic layer  306  by plating, for example. As shown in FIG. 3F, the patterned photoresist layer  304  is removed to expose the thin metallic layer  302  underneath but outside the metallic layer  306 . Finally, as shown in FIG. 3G, a short etching operation is conducted to remove the thin metallic layer  302  outside the metallic layer  306 , thereby forming the first type of under-bump metallurgical structure  201  as shown in FIG. 2A.  
     [0030] Note that the aforementioned paragraph only describes one of the processes that can be used to fabricate the first type of under-bump metallurgical structure  210 . Since the steps for producing other types of under-bump metallurgical structures such as  202  to  206  as shown in FIGS. 2B to  2 F are very similar, detail descriptions are omitted. In addition, this invention also permits the formation of a mini bump to replace the buffer metallic layer  220  of the under-bump metallurgical structure  201  in FIG. 2A for a further reduction of the growth of inter-metallic compound between the metallic layer and the solder bump.  
     [0031]FIG. 4A is a schematic cross-sectional view showing a seventh type of under-bump metallurgical structure between the bonding pad of a die and a solder bump according to a preferred embodiment of this invention. As shown in FIG. 4A, the seventh type of under-bump metallurgical structure  401  includes a metallic layer  410  and a mini bump  422 . The metallic layer  410  is formed over a bonding pad  16  and the mini bump  422  is formed between the metallic layer  410  and the solder bump  18 . The metallic layer  410  has a material composition identical to the metallic layer in the first type of under-bump metallurgical structure  201 . Note that material compositions and properties of the mini bump  422  are identical to the buffer metallic layer  220  in FIG. 2A. For example, the mini bump  422  is capable of wetting the solder bump  18  for increasing the bonding strength between the mini bump  422  and the solder bump  18 . Furthermore, the mini bump  422  has a melting point higher than the solder bump  18  to prevent the mini bump  422  from melting away in a high temperature treatment (such as a reflow operation) and incapacitating the capacity to reduce the growth of inter-metallic compound. Due to the aforementioned reasons, the mini bump  422  is preferably made from lead, a lead-tin alloy having a composition of 95% lead with 5% tin or some other materials.  
     [0032]FIG. 4B is a schematic cross-sectional view showing an eighth type of under-bump metallurgical structure between the bonding pad of a die and a solder bump according to a preferred embodiment of this invention. As shown in FIG. 4B, the eighth type of under-bump metallurgical structure  402  has a smaller distribution area compared with the seventh type of under-bump metallurgical structure  401  in FIG. 4A. Hence, the solder bump  18  has a relatively smaller diameter and the pitch between neighboring solder bumps  18  can be reduced.  
     [0033]FIG. 4C is a schematic cross-sectional view showing a ninth type of under-bump metallurgical structure between the bonding pad of a die and a solder bump according to a preferred embodiment of this invention. As shown in FIG. 4C, the buffer metallic structure  420  of the ninth type of under-bump metallurgical structure  403  further includes a mini bump  422  and a buffer metallic layer  424 . The mini bump  422  is formed over the metallic layer  410  and the buffer metallic layer  424  is formed between the mini bump  422  and the solder bump  18 . The buffer metallic layer  424  is a tin layer, for example.  
     [0034]FIG. 4D is a schematic cross-sectional view showing a tenth type of under-bump metallurgical structure between the bonding pad of a die and a solder bump according to a preferred embodiment of this invention. As shown in FIG. 4D, the tenth type of under-bump metallurgical structure  404  has a smaller distribution area compared with the ninth type of under-bump metallurgical structure  403  in FIG. 4C. Hence, the solder bump  18  has a relatively smaller diameter and the pitch between neighboring solder bumps  18  can be reduced.  
     [0035]FIGS. 4E to  4 H are schematic cross-sectional views showing an eleventh, a twelfth, a thirteenth and a fourteenth type of under-bump metallurgical structures between the bonding pad of a die and a solder bump according to a preferred embodiment of this invention. As shown in FIGS. 4E to  4 H, the mini bump  422  of the eleventh to the fourteenth types of under-bump metallurgical structures  405  to  408  is capable of wetting the solder bump  18 . Hence, the wettable layer  416  in the seventh to the tenth under-bump metallurgical structures as shown in FIGS. 4A to  4 D can be omitted to form the eleventh to the fourteenth types of under-bump metallurgical structures. Since the mini bump  422  and the buffer metallic layer  424  has already been explained before, detail description is not repeated here.  
     [0036]FIGS. 5A to  5 H are schematic cross-sectional views showing the progression of steps for fabricating the first type of under-bump metallurgical structure as shown in FIG. 4A. As shown in FIG. 5A, the die  10  has an active surface  12  with a passivation layer  14  and a plurality of bonding pads  16  (only one is shown) thereon. The passivation layer and the bonding pads  16  are formed over the active surface  12  of the die  10  such that the passivation layer  14  exposes the bonding pads  16 . As shown in FIG. 5B, a metallic film layer  502  is globally formed over the active surface  12  of the die  10 , for example, by evaporation, sputtering or plating. The thin metallic layer  502  serves as a seed layer. As shown in FIG. 5C, a photoresist layer  504  is formed over the thin metallic layer  502  exposing a portion of the thin metallic layer  502  above the bonding pads  16 . As shown in FIG. 5D, another metallic layer  506  is formed over the thin metallic layer  502  by plating, evaporation or sputtering, for example. The metallic layer  506  includes an adhesion layer, a barrier layer and a wettable layer. As shown in FIG. 5E, a buffer metallic layer  508  is formed over the metallic layer  506  by plating or printing, for example. As shown in FIG. 5F, the patterned photoresist layer  504  is removed to expose the thin metallic layer  502  underneath but outside the metallic layer  506 . As shown in FIG. 5G, a short etching operation is conducted to remove the thin metallic layer  502  outside the metallic layer  506 . Finally, as shown in FIG. 5H, a reflow operation may be conducted to transform the buffer metallic layer  508  into a mini bump  508   a  that encloses the metallic layer  506 . However, the aforementioned paragraph only describes one of the processes that can be used to fabricate the seventh type of under-bump metallurgical structure  401 . Since the steps for producing other types of under-bump metallurgical structures such as  402  to  408  as shown in FIGS. 4B to  4 H are very similar, detail descriptions are omitted.  
     [0037] The under-bump metallurgical structure of this invention can be applied to a junction interface between the bonding pad and the solder bump of a flip-chip package aside from the junction interface between the bonding pad of a die and the solder bump. FIG. 6A is a schematic cross-sectional view showing the first type of under-bump metallic structure between the bonding pad of a substrate and a solder pad according to this invention. Bonding pads  26  on the substrate  20  are linked by a patterned conductive layer in such a way that each bonding pad  26  is exposed through a solder mask  24  lying over the substrate surface  22 . Because the bonding pads  26  and the conductive layer in the substrate  20  are typically made from copper, the pads  26  can easily react chemically with tin, which is the major ingredient of the solder bump  28 , leading to the growth of inter-metallic compound. Conventionally, a nickel layer  612  or a gold film  614  is formed between the bonding pad  26  and the solder bump  28  to serve as a buffer metallic layer like the structure shown in FIG. 6A. However, both nickel and gold may ultimately react with tin in the solder bump  28  to form inter-metallic compound.  
     [0038] As shown in FIG. 6A, the first type of under-bump metallic layer  601  includes a metallic layer  610  and a buffer metallic layer  620 . The metallic layer  610  is formed over the bonding pads  26  of the substrate  20 . The metallic layer  610  may include a nickel layer  612  and a gold film  614 . The nickel layer  612  is formed over the bonding pads  26 . The gold film  614  is formed between the nickel layer  612  and the buffer metallic layer  620  for reducing the growth of inter-metallic compound. Similarly, the buffer metallic layer  620  formed between the metallic layer  610  and the solder bump  28  reduces the growth of inter-metallic compound between the bonding pad  26  and the solder bump  28 . To prevent melting of the buffer metallic layer  620  during heat treatment (such as a reflow operation) and thus lowering the capacity to reduce the growth of inter-metallic compound, the buffer metallic layer  620  must have a melting point higher than the solder bump  28 . Furthermore, to provide a good bonding strength between the buffer metallic layer  620  and the solder bump  28 , the buffer metallic layer  620  must be a material capable of wetting the solder bump  28  such as lead or some other materials.  
     [0039]FIG. 6B is a schematic cross-sectional view showing the second type of under-bump metallic structure between the bonding pad  26  of a substrate  20  and a solder pad  28  according to this invention. Since the buffer metallic layer  620  already has the capacity to reduce the growth of inter-metallic compound between the bonding pad  26  and the solder bump  28 , the metallic layer  610  in FIG. 6A (comprised of the nickel layer  612  and the gold film  614 ) may be omitted to form the under-bump metallic layer  602  as shown in FIG. 6B. Similarly, the under-bump metallic layer  602  is preferably made of lead or some other materials.  
     [0040] Note that lead has a coefficient of thermal expansion (CTE) much closer to lead-tin alloy than with copper. With less thermal stress between the under-bump buffer metallurgical structure and the solder bump, the solder bumps need to sustain less shear stress and ultimately the chance of having a broken junction is less likely.  
     [0041] The under-bump metallurgical structure according to this invention can be applied to a junction interface between the bonding pad of a die and a solder bump. The principle constituent of the solder bump is lead-tin alloy. The under-bump metallurgical structure includes a metallic layer and a buffer metallic structure. The metallic layer is formed over the bonding pads. The principle constituent of the metallic layer is copper, nickel or gold. The buffer metallic structure is formed between the metallic layer and the solder bump for reducing the growth of inter-metallic compound between the metallic layer and the solder bump. The buffer metallic structure may include a buffer metallic layer, a mini bump or a combination of the two. The buffer metallic structure is capable of wetting the solder bump and has a melting point higher than the solder bump. The buffer metallic structure is preferably made from lead.  
     [0042] The under-bump metallurgical structure according to this invention can also be applied to a junction interface between the bonding pad of a flip-chip package substrate and a solder bump. The principle constituent of the bonding pad is copper and the principle constituent of the solder bump is lead-tin alloy. The under-bump metallurgical structure includes a buffer metallic structure between the bonding pad and the solder bump for reducing the growth of inter-metallic compound between the bonding pad and the solder bump. The buffer metallic structure is capable of wetting the solder bump and has a melting point higher than the solder bump. In addition, the under-bump metallurgical structure further includes a nickel layer and a gold film. The nickel layer is formed over the bonding pad while the gold film is formed between the nickel layer and the buffer metallic layer. The buffer metallic layer is preferably made from lead.  
     [0043] About the material, the foregoing bump can also be made from a lead-free material, such as SnAg, SnAgBi, SnAgBiCu, SnAgBiCuGe, SnAgBiX, SnAgCu, SnBi, SnCu, SnZn, SnCuSbAg, SnSb or SnZnBi, and the under-bump metallurgical structure can include, for example, Sb, Ag, Sn/Ag, Sn/Cu, and so on. However, if the lead is included, it can include, for example, SnPbAg for the bump.  
     [0044] In conclusion, the under-bump metallurgical structure according to this invention is formed between a bonding pad and a solder bump or between the bonding pad of a package substrate and a solder bump. The under-bump metallurgical structure reduces chemical reaction between tin, a principle constituent within the solder bump, with other metallic materials within the under-bump metallic layer or metallic materials within the bonding pad to form inter-metallic compound. By reducing the growth of inter-metallic compound, electrical resistance between the under-bump metallurgical structure and the solder bump is reduced while bonding strength between the under-bump metallurgical structure and the solder bump is increased.  
     [0045] 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.