Abstract:
A method of forming a plurality of bumps over a wafer mainly comprises providing the wafer having a plurality of bonding pads formed thereon, forming an under bump metallurgy (UBM) layer over the bonding pads wherein the UBM layer includes an adhesive layer, for example a titanium (Ti) layer or an aluminum (Al) layer, and at least one electrically conductive layer formed on the adhesive layer, removing the portions of the electrically conductive layer located outside the bonding pads, forming a plurality of bumps over the residual portions of the electrically conductive layer disposed above the bonding pads, etching the adhesive layer located outside the bumps, and then reflowing the bumps.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention relates to a bumping process. More particularly, the present invention is related to a method for etching the adhesive layer made of a titanium layer or an aluminum layer of the UBM layer (Under Bump Metallurgy layer) through a sulfuric acid solution serving as an etchant. 
   2. Related Art 
   In this information explosion age, integrated circuit 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 circuit package. Moreover, compared with a wire-bonding package or a tape automated bonding (TAB) package, a flip-chip, package uses a shorter electrical path on average and has a better overall electrical performance. In a flip-chip package, the bonding pads on a die 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. 
   Referring to  FIG. 1A , it shows a flow chart illustrating the process flow of a conventional bumping process. The conventional bumping process mainly comprises the following steps. Firstly, a substrate is provided as shown in the step of S 100 . The substrate may be a silicon base or a wafer. Next, an under bump metallurgy layer is formed on the substrate as shown in the step of S 102 . Generally speaking, the under bump metallurgy is made of an adhesive layer, a barrier layer and a wetting layer. Then, the step S 104  of patterning the barrier layer and the wetting layer is performed to define the locations for disposing bumps. Namely, a photo-resist is formed to cover the portions of the wetting layer and the barrier layer for disposing the bumps thereon and then the residual portions of the wetting layer and the barrier layer not covered by the photo-resist layer are removed to expose the adhesive layer. Afterwards, as shown in the step of S 106 , another photo-resist layer is provided to cover the exposed adhesive layer to expose the residual portions of the wetting layer, and the step of plating process is then performed to form solder bumps on the residual portions of the wetting layer. Next, as shown in the step of S 108 , the portions not covered by the solder bumps are removed. Finally, the solder bumps are reflowed (step of S 110 ) to be fixed securely on the residual portions of the wetting layer and shaped into a ball-like shape. 
   Next, referring to  FIG. 1B , it shows a flow chart illustrating the process flow of another conventional bumping process. This conventional bumping process mainly comprises the following steps. Firstly, a substrate is provided as shown in the step of S 200 . The substrate may be also a silicon base or a wafer. Next, an under bump metallurgy layer is formed on the substrate as shown in the step of S 202 . Generally speaking, the under bump metallurgy is made of an adhesive layer, a barrier layer and a wetting layer. Then, the step of S 204  of forming bumps above the bonding pads of the substrate and on the portions of the wetting layer that are not covered by a patterned photo-resist layer is performed. Next, as shown in the step of S 206 , the bumps are served as masks so as to perform the removing process to remove the portions of the wetting layer and the barrier layer not covered by the bumps so as to expose the adhesive layer. Then, as shown in the step of S 208 , the portions of the adhesive layer not covered by the wetting layer and the barrier layer are removed. Finally, as shown in the step of S 210 , the bumps are reflowed to be fixed on the residual portions of the wetting layer and to be shaped into ball-like shape. 
   In the aforementioned conventional bumping processes, the under bump metallurgy layer applicable to the bumping process for the copper wafer mainly comprises a titanium layer, a nickel-vanadium layer and a copper layer. Therein, a hydrogen-fluorine solution (HF) is usually taken as an etchant for patterning the copper layer; a sulfuric acid solution (H 2 SO 4 ) or a dilute phosphoric solution mainly comprising deionized water (DI water), phosphoric acid (CH 3 COOH), acetic acid (H 3 PO4) and hydrogen peroxide (H 2 O 2 ), wherein the composition of said etchant can be refereed to U.S. Pat. No. 5,508,229, is taken as an etchant to define the nickel-vanadium layer. A hydrogen-fluorine solution is also usually taken as the etchant to define or pattern the titanium layer. However, the etchant (said hydrogen-fluorine solution) is very dangerous so that said hydrogen-fluorine solution is not able to be applicable to the bumping process. 
   Besides, the under bump metallurgy layer applicable to the bumping process for the aluminum wafer mainly comprises an aluminum layer, a nickel-vanadium layer and a copper layer. The etchant for patterning the aluminum layer comprises phosphoric acid, acetic acid and deionized water (DI). Therein, 83% of the etchant is phosphoric acid; 11% of the etchant is acetic acid; and 6% of the etchant is deionized water. However, the mentioned-above etchant is able to attack the bumps so as to make the volume of the bumps smaller and smaller. In such a manner, the precision of the volume of the bumps is not able to be well controlled. Accordingly, there are restrictions to take the hydrogen-fluorine solution and the phosphoric acid solution as etchants. 
   Moreover, in the conventional bumping process, when a portion of the photo-resist layer is left, the surface of the bumps will be contaminated. In addition, the etchant taken to remove the UBM layer will also contaminate the surface of the bumps. 
   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 
   In view of the above-mentioned problems, an objective of this invention is to provide a method of forming bumps, and more particularly, a method for etching the adhesive layer made of a titanium layer or an aluminum layer of the UBM layer (Under Bump Metallurgy layer) through a sulfuric acid solution serving as an etchant. 
   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 taking a sulfuric acid solution as an etchant to etch the adhesive layer made of a titanium layer or an aluminum layer of the UBM layer (Under Bump Metallurgy layer). Therein, the concentration of said sulfuric acid solution ranges from about 50% to about 96% and the reaction temperature of said sulfuric acid solution for performing the etching process ranges between about 60° C. and about 90° C. 
   As mentioned above, said etchant in this invention not only avoids attacking said bumps to reduce the volume of the bumps so as to control the volume of the bumps more precisely but also can be applicable to the bumping process for both the aluminum wafer and the copper wafer. Moreover, said etchant is more safe and able to increase the reliability of the bumping process. 
   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 
     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: 
       FIG. 1A  illustrates a flow chart of the process flow of a conventional bumping process. 
       FIG. 1B  illustrates a flow chart of the process flow of another conventional bumping process. 
       FIGS. 2 to 11  are partially enlarged cross-sectional views showing the progression of steps for forming a bump according to the first preferred embodiment of this invention; and 
       FIGS. 12 to 17  are partially enlarged cross-sectional views showing the progression of steps for forming a bump according to the second preferred embodiment of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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. 
     FIGS. 2 to 11  are partially enlarged cross-sectional views showing the progression of steps for forming a bump according to the first preferred embodiment of this invention. 
   As shown in  FIG. 2 , a silicon wafer  300  having a plurality of bonding pads  302  and a passivation layer  304 . Therein, the passivation layer  304  covers the active surface of the silicon wafer  300  and exposes the bonding pads  302 . 
   Next, referring to  FIG. 3 , an under bump metallurgy layer  306  is formed on the active surface of the silicon wafer  300 . Said under bump metallurgy layer  306  mainly comprises an adhesive layer  306   a , for example a titanium layer or a copper layer, a barrier layer  306   b  and a wetting layer  306   c . Said under bump metallurgy layer may comprise a titanium layer, a nickel-vanadium and a copper in sequence. Moreover, the under bump metallurgy  306  layer may also comprise an aluminum layer, a nickel-vanadium layer and a copper layer. In addition, the mentioned-above under bump metallurgy layer  306  can only comprise two layers made of a titanium layer and a copper layer which comprises an adhesive layer and one electrically conductive layer, or said under-bump metallurgy layer comprises four layers made of an aluminum layer, a titanium layer, a nickel-vanadium layer and a copper layer. 
   Next, referring to  FIG. 4 , a photo-resist layer  308  is formed on the portions of the under bump metallurgy layer  306  which are disposed above the bonding pads  302  in order to expose the residual portions of the barrier layer  306   b  and the wetting layer  306   c  not covered by the photo-resist layer. 
   Afterwards, as shown in  FIG. 5 , the photo-resist layer is taken as a mask to remove the residual portions of the barrier layer  306   b  and the wetting layer  306   c  not covered by the photo-resis layer so as to expose the adhesive layer  306   a . For example, the nickel-vanadium layer of the under bump metallurgy layer can be removed by the etchant made of a sulfuric acid solution or a dilute phosphoric solution comprising deionized water (DI water), phosphoric acid, acetic acid and hydrogen peroxide (the composition of said etchant can be refereed to U.S. Pat. No. 5,508,229); and the copper layer of the under bump metallurgy layer can be etched away by the etchant made of a hydrogen-fluorine solution or a aquafortis solution. 
   Referring to  FIG. 6 , after the step of patterning the wetting layer  306   c  and the barrier layer  306   b , the step of removing the photo-resist layer  308  is performed. Next, as shown in  FIG. 7 , another photo-resist layer  310  is formed above the active surface of the silicon wafer  300  and a plurality of openings  312  are formed to expose the predetermined locations for forming bumps. Therein, for example, the openings  312  expose the portions of the wetting layer  306   c  disposed above the bonding pads  302 . 
   Next, referring to  FIG. 8 , a solder material is filled in the openings  312  so as to form a plurality of solder bumps  314 . Therein, the solder material is formed by the method of plating and said solder material may be selected from the material of one of lead-free and solder eutectic. As mentioned above, the adhesive layer  306   a  connects the patterned wetting layer  306   c  and the patterned barrier layer  306   b  so as to perform the plating process to form solder bumps  314 . 
   Moreover, as shown in  FIG. 9  and  FIG. 10 , after the bumps  314  are formed and the photo-resist layer  310  is removed, the bumps  314  will be taken as masks to remove the adhesive layer  306   a  not covered by the bumps  314 . Finally, the passivation layer  304  will be exposed. 
   In this embodiment, the concentration of said sulfuric acid solution ranging from about 50% to about 96% is taken as the etchant to etch the adhesive layer  306   a . Due to the difficulty of the reaction between the sulfuric acid solution and the bumps, the bumps will not be easily attacked by the etchant to cause the volume of the bumps to be reduced. In addition, it is easier to control the volume of the bumps so as to get the more precision of the volume of the bumps. Besides, said sulfuric acid solution is able to be applicable to the bumping process for both the aluminum wafer and the copper wafer. Moreover, said sulfuric acid solution is not harmful to the human and able to increase the reliability of the bumping process. 
   Finally, as shown in  FIG. 11 , a reflowed process is performed to shape the bumps into a ball-like shape. 
   Next, as shown from  FIGS. 12 to 17 , they are partially enlarged cross-sectional views showing the progression of steps for forming a bump according to the second preferred embodiment of this invention. 
   Next, referring to  FIG. 12 , a silicon wafer  400  having a plurality of bonding pads  402  and a passivation layer  404 . Therein, the passivation layer  404  covers the active surface of the silicon wafer  400  and exposes the bonding pads  402 . 
   Afterwards, referring to  FIG. 13 , an under bump metallurgy layer  406  is formed on the active surface of the silicon wafer  400 . Said under bump metallurgy layer  406  mainly comprises an adhesive layer  406   a , for example a titanium layer or a copper layer, a barrier layer  406   b  and a wetting layer  406   c . The under bump metallurgy layer  406  may comprises a titanium layer, a nickel-vanadium layer and a copper layer in sequence. Moreover, the under bump metallurgy layer may also comprise an aluminum layer, a nickel-vanadium layer and a copper layer. In addition, the mentioned-above under bump metallurgy layer  406  can only comprise two layers made of a titanium layer and a copper layer, or said under-bump metallurgy layer comprises four layers made of an aluminum layer, a titanium layer, a nickel-vanadium layer and a copper layer. 
   Next, referring to  FIG. 14 , a photo-resist layer  408  is formed above the active surface of the silicon wafer  400  and a plurality of openings  412  are formed to expose the predetermined locations for forming bumps  410 . Therein, for example, the openings  412  expose the portions of the wetting layer  406   c  disposed above the bonding pads  402 . Then, a solder material is filled in the openings  412  so as to form a plurality of solder bumps  410 . Therein, the solder material is formed by the method of plating and said solder material may be selected from the material of one of lead-free solder and solder eutectic. 
   Moreover, as shown in  FIG. 15  and  FIG. 16 , after the bumps  410  are formed and the photo-resist layer  408  is removed, the solder bumps  410  will be taken as masks to remove the adhesive layer  406   a , the barrier layer  406   b  and the wetting layer  406   c  not covered by the bumps  410  to expose the passivation layer  404 . 
   In this second embodiment, the nickel-vanadium layer of the under bump metallurgy layer  406  can be removed by the etchant made of a sulfuric acid solution or a dilute phosphoric solution comprising deionized water (DI water), phosphoric acid, acetic acid and hydrogen peroxide (the composition of said etchant can be refereed to U.S. Pat. No. 5,508,229); and the copper layer of the under bump metallurgy layer can be etched away by the etchant made of a hydrogen-fluorine solution or a aquafortis solution. 
   As mentioned above, the concentration of said sulfuric acid solution ranging from about 50% to about 96% is taken as the etchant to etch the adhesive layer  306   a . Due to the difficulty of the reaction between the sulfuric acid and the bumps, the bumps will not be easily attacked by the etchant to cause the volume of the bumps to be reduced. In addition, it is easier to control the volume of the bumps so as to get the more precision of the volume of the bumps. Besides, said sulfuric acid solution is able to be applicable to the bumping process for both the aluminum wafer and the copper wafer. Moreover, said sulfuric acid solution is not harmful to the human and able to increase the reliability of the overall bumping process. 
   Finally, as shown in  FIG. 17 , a reflowed process is performed to shape the bumps  410  into a ball-like shape. 
   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.