Patent Publication Number: US-6669079-B2

Title: Conductive paste and semiconductor component having conductive bumps made from the conductive paste

Description:
This application is a divisional application of U.S. patent application Ser. No. 09/323,464 filed Jun. 1, 1999 now U.S. Pat. No. 6,451,127. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates, in general, to electrical interconnects and, more particularly, to interconnect bumps. 
     Semiconductor manufacturers often form conductive bumps on semiconductor chips to facilitate electrical contact to substrates such as printed circuit boards, Tape Automated Bonding (TAB) substrates, leadframes, etc. Advantages of using conductive bumps include: increased Input and Output (I/O) density, which results in a smaller device “footprint,” increased signal propagation speed due to shorter interconnections, reduced vertical profile, and lower device weight. 
     Typically, the conductive bumps are formed by depositing a layer of masking or stencil material over the substrate, forming openings in the solder mask, disposing a conductive paste in the openings, and reflowing the conductive paste to form the conductive bumps. A disadvantage of presently available conductive pastes is that voids are formed in the conductive bumps after reflowing the conductive paste. These voids weaken the joints between the conductive bumps and the substrates, which result in reliability problems. In addition, residual flux remains after the reflow step that is difficult to remove. This residual flux reduces the adhesive strength between the conductive bumps and the underfill material and increases the device leakage current. 
     Accordingly, it would be advantageous to have a conductive paste for making conductive bumps and a method for making conductive bumps having a reduced number of voids. It would be of further advantage to be able to form conductive bumps that can be easily and cost efficiently cleaned after reflow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a highly enlarged cross-sectional view of a semiconductor wafer at a beginning stage of manufacture in accordance with an embodiment of the present invention; 
     FIG. 2 is a highly enlarged cross-sectional view of the semiconductor wafer of FIG. 1 during manufacture; and 
     FIG. 3 is a highly enlarged cross-sectional view of the semiconductor wafer of FIG. 1 having conductive bumps formed thereon in accordance with an embodiment of the present invention. 
    
    
     For simplicity and clarity of illustration, elements in the drawings are not necessarily drawn to scale, and the same reference numerals in different figures denote the same elements. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Generally, the present invention provides a conductive paste, such as, for example, a solder paste, and a method for forming conductive bumps using the conductive paste. Conductive bumps manufactured using the conductive paste of the present invention have fewer voids than those made with presently available conductive pastes. Further, any residual conductive paste that remains after the reflow step can be conveniently removed using water. 
     In accordance with a first embodiment of the present invention, the conductive paste is formed by combining a tin alloy with a flux composition comprising an aromatic carboxylic acid fluxing agent and a solvent. Suitable tin alloys include: a tin-silver alloy that contains approximately 3.5 weight percent silver (SnAg3.5); a tin-copper alloy that contains approximately 0.7 weight percent copper (SnCu0.7); a tin-silver-bismuth alloy that contains approximately 2 weight percent silver and approximately 2 weight percent bismuth (SnAg2Bi2); a tin-silver-copper alloy that contains approximately 4 weight percent silver and approximately 0.5 weight percent copper (SnAg4Cu0.5); a tin-antimony alloy that contains approximately 5 weight percent antimony (SnSb5); and a tin-lead alloy (SnPb), where the tin content in the tin-lead alloy ranges from approximately 2 weight percent to approximately 70 weight percent. The aromatic-carboxylic acid has the general formula:                    
     where 
     R is hydrogen or an alkyl group comprising from 1 to 16 carbon atoms; 
     and n is an integer ranging from 0 to 3. 
     The polymeric solvent is a non-boiling polymeric solvent having the general formula:                    
     where 
     n is an integer ranging from 6 to 100; 
     R1 is hydrogen or an alkyl group comprising from 1 to 20 carbon atoms; and 
     R2 is hydrogen or an alkyl group comprising from 1 to 20 carbon atoms. 
     Preferably, the fluxing composition comprises approximately 12.5 weight percent 4-hydroxy benzoic acid as the aromatic carboxylic acid fluxing agent and approximately 87.5 weight percent polypropylene glycol ether as the non-boiling polymeric solvent. Further, when using this formula as the flux composition, it is preferable to perform a double reflow step to form the conductive bumps. 
     In accordance with a second embodiment of the present invention, the conductive paste is formed by combining a tin alloy with a flux composition comprising an aromatic carboxylic acid fluxing agent, a flux carrier, and a solvent. Similar to the first embodiment, suitable tin alloys include: a tin-silver alloy that contains approximately 3.5 weight percent silver (SnAg3.5); a tin-copper alloy that contains approximately 0.7 weight percent copper (SnCu0.7); a tin-silver-bismuth alloy that contains approximately 2 weight percent silver and approximately 2 weight percent bismuth (SnAg2Bi2); a tin-silver-copper alloy that contains approximately 4 weight percent silver and approximately 0.5 weight percent copper (SnAg4Cu0.5); a tin-antimony alloy that contains approximately 5 weight percent antimony (SnSb5); and a tin-lead alloy (SnPb), where the tin content in the tin-lead alloy ranges from approximately 2 weight percent to approximately 70 weight percent. The aromatic carboxylic acid has the general formula:                    
     where R is hydrogen or an alkyl group comprising from 1 to 16 carbon atoms; and 
     n is an integer ranging from 0 to 3. 
     The flux carrier has the general formula: 
     
       
         HOCH 2 CH 2 O n H  
       
     
     where n is an integer ranging from 5 to 100. 
     The polymeric solvent is a non-boiling polymeric solvent having the general formula:                    
     where 
     n is an integer ranging from 6 to 100; 
     R1 is hydrogen or an alkyl group comprising from 1 to 20 atoms; and 
     R2 is hydrogen or an alkyl group comprising from 1 to 20 atoms. 
     Preferably, the fluxing composition comprises approximately 12.5 weight percent 4-hydroxy benzoic acid as the aromatic carboxylic acid fluxing agent, approximately 33.3 weight percent polyethylene glycol, i.e., where n=20, as the flux carrier, and approximately 54.2 weight percent polypropylene glycol ether as the non-boiling polymeric solvent. Further, when using this formulation as the flux composition, it is preferable to use a tin-lead alloy as the metal, wherein the weight percent of tin in the tin-lead alloy ranges from approximately 2 weight percent to approximately 70 weight percent. 
     In accordance with a third embodiment of the present invention, the conductive paste is formed by combining a tin alloy with a flux composition as described in the second embodiment, except that the flux of the third embodiment contains a mixture of aromatic and aliphatic acids. The aliphatic acid has the general formula: 
     
       
         HOOCCH 2  n COOH  
       
     
     where n is an integer ranging from 0 to 16. 
     Similar to the second embodiment, the fluxing composition comprises approximately 12.5 weight percent 4-hydroxy benzoic acid as the aromatic carboxylic acid fluxing agent, approximately 33.3 weight percent polyethylene glycol, i.e., n=20, as the flux carrier, and approximately 54.2 weight percent polypropylene glycol butyl ether as the non-boiling polymeric solvent. 
     In accordance with a fourth embodiment of the present invention, the conductive paste is formed by combining a tin alloy with a flux composition comprising an aromatic carboxylic acid fluxing agent, a flux carrier, a secondary flux, and a solvent. Similar to the first and second embodiments, suitable tin alloys include: a tin-silver alloy that contains approximately 3.5 weight percent silver (SnAg3.5); a tin-copper alloy that contains approximately 0.7 weight percent copper (SnCu0.7); a tin-silver-bismuth alloy that contains approximately 2 weight percent silver and approximately 2 weight percent bismuth (SnAg2Bi2); a tin-silver-copper alloy that contains approximately 4 weight percent silver and 0.5 weight percent copper (SnAg4Cu0.5); a tin-antimony alloy that contains approximately 5 weight percent antimony (SnSb5); and a tin-lead alloy (SnPb), where the tin content of the tin-lead alloy is ranges from approximately 2 weight percent to approximately 70 weight percent. The aromatic carboxylic acid has the general formula:                    
     where R is hydrogen or an alkyl group comprising from 1 to 20 carbon atoms; and 
     n is an integer ranging from 0 to 3. 
     The flux carrier has the general formula: 
     
       
         HOCH 2 CH 2 O n H  
       
     
     where n is an integer ranging from 5 to 100. 
     The secondary flux has the general formula: 
     
       
         HOOCCH 2  n COOH  
       
     
     where n is an integer ranging from 0 to 16. 
     The polymeric solvent is a non-boiling polymeric solvent having the general formula:                    
     where 
     n is an integer ranging from 6 to 100; 
     R1 is hydrogen or an alkyl group having from 1 to 18 carbon atoms; and 
     R2 is hydrogen or an alkyl group having from 1 to 18 carbon atoms. 
     Preferably, the fluxing composition comprises 17 weight percent 4-hydroxy benzoic acid as the aromatic carboxylic acid fluxing agent, 12 weight percent polyethylene glycol as the flux carrier, 6 weight percent adipic acid as the secondary flux carrier, and 65 weight percent polypropylene glycol ether as the non-boiling polymeric solvent. 
     Techniques for combining the constituents of the flux composition and the metal alloy are known to those skilled in the art. 
     FIG. 1 is a highly enlarged cross-sectional view of a semiconductor wafer  10  having a surface  11  and a plurality of bond pads  13  formed on surface  11 . In addition, a layer  12  of dielectric material is formed on surface  11  and portions of bond pads  13 . Optionally, an underbump metallization layer  15  is formed on bond pads  13 . Techniques for forming bond pads on the surface of a semiconductor wafer and underbump metallization on a bond pad are well known to those skilled in the art. Further, it is well known that semiconductor wafers are comprised of a plurality of semiconductor chips that contain circuit elements such as transistors, diodes, integrated circuits, passive elements, etc. Bond pads  13  and underbump metallization layers  15  make electrical contact to the appropriate regions of the integrated circuit or semiconductor device. It should be noted that the circuit elements are not shown in FIG.  1 . 
     Still referring to FIG. 1, a layer of solder mask material  14  is formed on surface  11  and bond pads  12 . Solder mask layer  14  is patterned such that underbump metallization layers  15  are exposed. It should be understood the use of a layer of solder mask material is not a limitation of the present invention. For example, a mechanical stencil or the like may be used rather than a layer of solder mask material. 
     Now referring to FIG. 2, a conductive paste  16 , formulated in accordance with the present invention, is applied to the solder mask layer  14  and into the openings in solder mask layer  14 . Preferably, conductive paste  16  is dispensed, spread, or flooded on the surface of solder mask layer  14  in order to roughly cover the surface of semiconductor wafer  10 . A squeegee (not shown) or other suitable instrument is used to sweep conductive paste  16  across solder mask layer  14  and to substantially fill the openings in solder mask layer  14 . Excess conductive paste is removed from the surface of solder mask layer  14 . 
     Now referring to FIG. 3, the solder mask is removed and the conductive paste reflowed to form conductive bumps  18 . Conductive bumps  18  are also referred to as solder bumps. The conductive bumps are then cleaned using water to remove the flux residue. 
     Although conductive bumps  16  are shown and described as being formed on a semiconductor wafer, it should be understood this is not a limitation of the present invention. For example, the conductive bumps can be formed on printed wire boards, flex circuits, metallized ceramic or glass, or the like. 
     By now it should be appreciated that a conductive paste and a method for using the conductive paste have been provided. The conductive paste of the present invention produces conductive bumps having fewer voids than conventional solder pastes. One reason for the reduction in the number of voids is the formation of hydrogen bonds between the hydroxyl and carboxyl groups on the aromatic carboxylic acid fluxing agent and the alkoxy groups on the solvent such that a very high flux concentration can be used in the paste. Another advantage of the present invention is that residual flux can be easily cleaned using water. In addition, to reducing the cost for cleaning the conductive bumps, the use of water is a much more environmentally benign system.