Abstract:
A method for dispersing conductive particles is provided, which takes advantages of simplified process and lower cost. The present invention is in applying of chemical bonding between metal and thiol with electric charge, thereby makes conductive particles and chip bumps carry charges. The conductive particles are then migrated and fixed to the bonding locations on the bumps of a chip through an electrophoresis procedure. The conductive particles will not come off during subsequent processes. The present invention can be applied to flip-chip packaging or other fine-pitch applications. With the present invention, the distance between bumps is smaller than 20 μm and the density of conductive particles is larger than 15 particles per bump.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to a method for dispersing conductive particles, and more specifically to a electrochemical method that allows conductive particles to be self-positioned and selectively dispersed on the electrodes of a chip or substrate. It is applicable to flip-chip packaging technologies.  
       BACKGROUND OF THE INVENTION  
       [0002]     In semiconductor packaging technologies, flip-chip is the most promising type of packaging method. Related arts and patents have been constantly brought up to propose various improved methods. Among these flip-chip packaging methods, the methods using printing and metal jetting bumping process are the most competitive ones due to their lower costs. However, when these methods are applied to the fine pitch bumping process, short circuit or high junction resistance due to solder joint bridging or missing phenomenon frequently happens. It causes the reliability issues of the packaged IC devives, and reduce the yield of the packaging process.  
         [0003]     Anisotropic conductive film (ACF) is composed of conductive particles and polymer resion. It provide both electrical and mechanical interconnections between chip and substrate, and offer numerous advantages over traditional solders, including flexible and simple process at low temperature, fluxless formulations and environment friendly (lead free process). But due to the size limiation of the conductive particles, when the distances between the bumps on the chip or substrate are too small, the anisotropic conductive property was no longer exist due to the conduction along the X-Y direction and bridge all of the signals within the IC device, For the fine pitch interconnection with electrically conductive adhesive, arrange the conductive particles regularly within the polymer matrix are one of the methods to solve the problem which mentioned above. The regular arrangement of the conductive particles is achieved mostly by using masks to uniformly distribute the conductive particles or disperse the conductive particles only on certain areas of the chip or substrate. An adhesive insulation layer is then coated to fix the conductive particles.  
         [0004]     U.S. Pat. No. 5,221,417 teaches a method using external magnetic field to selectively disperse conductive particles to form an anisotropic conductive film. In this method, a layer of ferromagnetic film is first coated on a temporary substrate. The ferromagnetic film is then patterned using a photo-lithography etching process. An external magnetic field is then applied so that the ferromagnetic film is magnetized with a polarity. Subsequently, conductive particles are dispersed on the ferromagnetic film. As the ferromagnetic film is magnetized, the conductive particles are only collected on the pattern of the ferromagnetic film. The conductive particles are then brought into contact with and thereby adhered to an adhesive layer on a substrate. An anisotropic conductive film with uniformly distributed conductive particles is obtained subsequently after removing the temporary substrate and ferromagnetic film. The foregoing method taught by the U.S. Pat. No. 5,221,417 is more costly as additional ferromagnetic film and photo-lithography etching process are required. In addition, conductive particles would be stacked along the Z direction.  
         [0005]     U.S. Pat. No. 6,042,894 teaches a method using a screen with pores to uniformly disperse conductive particles. In this method, conductive particles are given charges induced either by a strong electric field within a dry environment or by contacting with a roller or brush having a strong electric field. The screen is then applied with an opposite electric field to attract the conductive particles with charges to pass through the pores and fall on a sticking layer of a substrate. An anisotropic conductive film with uniformly distributed conductive particles is thereby obtained. As conductive particles are with charges of same electricity, a stacking of conductive particles along the Z direction is thereby avoided. However, the foregoing method is complicated, difficult to operate, and costly. Moreover, as the diameter of the conductive particles is reduced, the pores of the screen have to be shrunk and the cost for producing the screen is increased as well.  
         [0006]     The two aforementioned methods for uniformly arranging conductive particles both first fix the conductive particles in a bonding film, then bond another chip to establish electrical connections. The present invention provides a method to uniformly and regularly fix the conductive particles on the bumps of a chip. Then a non-conductive bonding film is used to join the chip with another chip. The method provided by the present invention can not only reduce the production cost significantly, but also greatly increase the yield of bonding chips.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a method for dispersing conductive particles, which includes the following steps: preparing a chip having at least a bump conducting a pre-processing procedure, placing the chip in a reactor with a solution inside, conducting a surface processing procedure for a plurality of conductive particles, placing the conductive particles in the reactor, migrating the conductive particles to the bump of the chip and fixing the conductive particles thereto, and bonding to a substrate with nonconductive adhesive.  
         [0008]     The present invention mainly relies on an electrophoresis technology. With an electrophoretic process, the conductive particles according to the present invention are migrated and self-positioned to their targeted positions on a chip. A major criterion for the application of electrophoresis is that the migrated particles or objects must be charged with charges. To achieve this criterion, the present invention utilizes a thiol compound to conduct a surface processing procedure on the conductive particles so that they are charged with charges. The conductive particles themselves are core-shell construction with polymer resin as core, then coated with metal at its surface. The metal layer is usually made by electroless Ni/Au process. The thiol compounds used here were amphiphile molecular, has a thiol functional group (—HS) at one end and a hydrophilic functional group on the other end of its molecular structure. The thiol group may form complex bonding with the metal Gold. These thiol compounds may carry different charges by adopting different hydrophilic functional group. The conductive particles bonded with the thiol compound are therefore induced with either a positive or negative charge.  
         [0009]     The electrophoretic process of the present invention migrates the conductive particles having charges to the bumps of a chip in an alkaline, acid, or neutral aqueous solution by an externally applied electric field. The conductive particles are then fixed to the bumps by a proper fixing procedure so that the conductive particles will not drift away after removing the electric field. There are two ways to fix the conductive particles. One is to use an electrostatic force to fix the conductive particles onto the gold bumps of the chip, with different thiol compounds treatment, the conductive particles and gold bumps will charge with opposite charges. When the conductive particles approaching the gold bump, it will fix by the electrostatic force. The other one is to add an electroplating solution with metallic ions into the reactor and electroplate a metal thin film with a pulse or direct DC current. The conductive particles are thereby fixed to their positions with electroplate metal thin film.  
         [0010]     The present invention is also applicable to conductive particles not carrying any charge. When these conductive particles are added into the solution in the reactor, because their density is larger than that of the water, these conductive particles would sink naturally onto a chip by gravity. An electroplating procedure is then applied to fix the conductive particles onto the bumps.  
         [0011]     According to the present invention, the bumps on a chip can be either a metallic bump or a compliant bump made of a macromolecular polymer wrapped within a metallic layer. In addition, the metallic ions added into the electroplating solution might include Au-ions, Cu-ions, or Ni-ions.  
         [0012]     With the method for fixing the conductive particles according to the present invention, the conductive particles are positioned and fixed to the bumps of a chip precisely. The chip then can be bonded with a substrate using a non-conductive adhesive. The method according to the present invention is not only used in fine-pitch applications but also has advantages such as simplified processes and low costs. The conductive particles fixed by the present invention will not come off during subsequent processing. The present invention is therefore also applicable to the flip-chip packaging.  
         [0013]     The embodiments of the present invention can be classified into the dispersion and chip bonding of negative, positive, or neutral conductive particles. The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a flowchart showing the steps of a method for dispersing conductive particles according to the present invention.  
         [0015]      FIG. 2  is a schematic diagram showing the chemical bonding reactions between conductive particles and thiol compounds.  
         [0016]      FIGS. 3   a  to  3   i  are schematic diagrams showing the steps of dispersing negative charged conductive particles respectively.  
         [0017]      FIGS. 4   a  to  4   i  are schematic diagrams showing the steps of dispersing positive charged conductive particles respectively.  
         [0018]      FIGS. 5   a  to  5   i  are schematic diagrams showing the steps of dispersing neutral conductive particles respectively.  
         [0019]      FIG. 6  is a comparison chart showing the differences between bonding methods using the conventional anisotropic conductive films and the present invention.  
         [0020]      FIG. 7  is a flowchart summarizing the embodiments of the method for dispersing conductive particles according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]      FIG. 1  is a flowchart showing the steps of the method for dispersing conductive particles according to the present invention. At the first step  101 , a chip having at least a pad is placed in a reactor with a solution inside after the chip undergoes a pre-processing procedure. At the second step  103 , a plurality of conductive particles are then placed in the reactor after the conductive particles undergo a surface processing procedure. The steps  101  and  103  can also be conducted in a reverse order. At the third step  105 , the conductive particles undergo a fixing procedure. At last within the fourth step  107 , the chip is bonded with a substrate. According to the present invention, within the pre-processing procedure of the first step  101 , the chip is coated with a metallic layer and then an insulation layer, and the insulation layer on the bump is removed by etching. The surface processing procedure of the second step  103  causes the conductive particles to carry charges or carry no charge. Further details will be given later, using a thiol compound as an example depicted in  FIG. 2 , to describe how conductive particles carrying positive or negative charges are formed respectively. Nevertheless, the fixing procedure of the third step  105  would vary depending on whether the conductive particles carry charges and the electricity of the charges. In the following, the dispersion procedures for conductive particles carrying negative, positive, or no charges will be described in details respectively.  
         [0022]      FIG. 2  is a schematic diagram showing the chemical bonding reactions between conductive particles and thiol compounds. As shown in  FIG. 2 , the thiol compounds  201  carrying a negative charge with sulfite (—SO 3 ) functional group. On the other hand, the thiol compound  202  carrying a positive charge with dimethylamino (—(CH 3)   2 NH) functional group. The conductive particle  203  is made of a macromolecular polymer wrapped within an outer Nickel/Gold film. After the negative charged thiol compound  201  is bonded with the Gold film of the conductive particle  203 , a conductive particle  204  carrying a negative charge is formed. Similarly, after the positive charged thiol compound  202  is bonded with the Gold film of the conductive particle  203 , a conductive particle  205  carrying a positive charge is formed.  
         [0023]      FIGS. 3   a  to  3   i  are schematic diagrams showing the steps of dispersing negative charged conductive particles respectively. As shown in  FIG. 3   a , a chip  301  has a plurality of bumps  302  on a surface of the chip  301  for bonding with a substrate (not shown in  FIG. 3   a ). The chip  301  is then sputtered with a layer of Gold electrode  303 . The Gold electrode  303  and the bumps  302  jointly form the Gold electrode bumps  304 , as shown in  FIG. 3   b . A layer of photoresist  305  is coated on top of the Gold electrode  303  and the Gold electrode bumps  304 , as shown in  FIG. 3   c . A lithography process is then conducted to remove the photoresist  305  on top of the Gold electrode bumps  304 , and residual photoresist  306  is left on the Gold electrode  303 , as shown in  FIG. 3   d . The etched chip  301  is then immersed in a aqueous solution with a thiol compound  202  carrying a positive charge. Through the reaction as depicted in  FIG. 2 , the surfaces of the Gold electrode bumps  304  therefore carry positive charges, as shown in  FIG. 3   e . The chip  301  is then placed in a reactor (not shown) with an electrophoretic solution  310 , along with conductive particles  204  carrying negative charges. As shown in  FIG. 3   f , a positive electric field is applied on the Gold electrode  303  and the Gold electrode bumps  304 . Due to the attraction between the positive and negative charges, the conductive particles  204  carrying negative charges migrate to the surface of the Gold electrode bumps  304  under the electric field. The conductive particles  204  carrying negative charges are therefore tightly coupled with the Gold electrode bumps  304  whose surfaces carry positive charges, and the conductive particles  204  will not drift away even after the electric field is removed, as shown in  FIG. 3   g . After the foregoing electrophoretic positioning and fixing procedure, the electric field is removed and the chip  301  is taken out of the reactor. The residual photoresist  306  and the Gold electrode  303  besides those on the Gold electrode bumps  304  are removed, as shown in  FIG. 3   h . In the last step, a non-conductive adhesive  307  is used to join the chip  301  and the substrate  308  together, as shown in  FIG. 3   i . The bumps  309  on the substrate  308 , conductive particles  204  carrying negative charges, and Gold electrode bumps  304  jointly form the electric connection between the chip  301  and the substrate  308 .  
         [0024]     The positioning and fixing procedure as illustrated in  FIGS. 3   e  to  3   i  is not applicable to conductive particles carrying positive charges because the bonding between Gold and the thiol compound  201  carrying a negative charge would be de-bonded under a negative electric field, causing the thiol compound  201  carrying a negative charge to be detached from the surface of the Gold electrode bumps. Another procedure using electroplating to fix the conductive particles has to be employed.  
         [0025]      FIGS. 4   a  to  4   i  are schematic diagrams showing the steps of dispersing positive charged conductive particles respectively. As shown in  FIG. 4   a , the chip  301  has a plurality of bumps  302  on a surface of the chip  301  for bonding with substrate (not shown in  FIG. 4   a ). The chip  301  is then sputtered with a layer of Gold electrode  303 . The Gold electrode  303  and the bumps  302  jointly form the Gold electrode bumps  304 , as shown in  FIG. 4   b . A layer of photoresist  305  is coated on top of the Gold electrode  303  and the Gold electrode bumps  304 , as shown in  FIG. 4   c . A lithography process is then conducted to remove the photoresist  305  on top of the Gold electrode bumps  304 , and residual photoresist  306  is left on the Gold electrode  303 , as shown in  FIG. 4   d . The etched chip  301  is then placed in a reactor (not shown) with an electrophoretic solution  310 , along with conductive particles  205  carrying positive charges. As shown in  FIG. 4   e , a negative electric field is applied on the Gold electrode  303  and the Gold electrode bumps  304 . The conductive particles  205  carrying positive charges migrate to the surface of the Gold electrode bumps  304  under the electric field. The conductive particles  205  carrying positive charges are loosely coupled with the surfaces of the Gold electrode bumps  304  and, therefore, the electric field has to be maintained, as shown in  FIG. 4   f . In the mean time, a small amount of Au-ion solution is added into the electrophoretic solution  310  to form an electroplating solution  411 . Then, through an electroplating process with direct or pulse current, the conductive particles  205  carrying positive charges and the Gold electrode bumps  304  jointly form a layer of Gold film  412 , as shown in  FIG. 4   g . With the Gold film  412 , the conductive particles  205  carrying positive charges are fixed to the Gold electrode bumps  304  and will not drift away during subsequent cleaning or bonding process. After the foregoing electrophoretic positioning and fixing procedure, the electric field is removed and the chip  301  is taken out of the reactor. The residual photoresist  306  and the Gold electrode  303  besides those on the Gold electrode bumps  304  are removed, as shown in  FIG. 4   h . At the last step, a non-conductive adhesive  307  is used to bond the chip  301  and the substrate  308  together, as shown in  FIG. 4   i . The bumps  309  on the second chip  308 , conductive particles  205  carrying positive charges, Gold film  412 , and Gold electrode bumps  304  jointly form the electric connection between the chip  301  and the substrate  308 .  
         [0026]     In the aforementioned Au-ion solution, the Au-ion can be replaced by other metallic ions such as Cu-ion, Ni-ion, or any combination of the above.  
         [0027]      FIGS. 5   a  to  5   i  are schematic diagrams showing the steps of dispersing neutral conductive particles respectively. As shown in  FIG. 5   a , the chip  301  has a plurality of bumps  302  on a surface of the chip  301  for joining with a substrate (not shown in  FIG. 5   a ). The chip  301  is then sputtered with a layer of Gold electrode  303 . The Gold electrode  303  and the bumps  302  jointly form the Gold electrode bumps  304 , as shown in  FIG. 5   b . A layer of photoresist  305  is coated on top of the Gold electrode  303  and the Gold electrode bumps  304 , as shown in  FIG. 5   c . A lithography process is then conducted to remove the photoresist  305  on top of the Gold electrode bumps  304 , and residual photoresist  306  is left on the Gold electrode  303 , as shown in  FIG. 5   d . The etched chip  301  is then placed in a reactor (not shown) with a aqueous solution  510 , along with conductive particles  203  not carrying any charge, as shown in  FIG. 5   e . The chip  301  is left in the solution  510  for a period of time so that the conductive particles  203  will sink naturally to the surfaces of the Gold electrode bumps  304  and residual photoresist  306  as shown in  FIG. 5   f . A small amount of Au-ion solution is added into the aqueous solution  510  to form an electroplating solution  511 . Then, through an electroplating process with direct or pulse current, the conductive particles  203  not carrying any charge and the Gold electrode bumps  304  jointly form a layer of Gold film  412 , as shown in  FIG. 5   g . With the Gold film  412 , the conductive particles  203  not carrying any charge are fixed to the Gold electrode bumps  304  and will not drift away during subsequent cleaning or joining operations. After the foregoing electrophoretic positioning and fixing procedure, the electric field is removed and the chip  301  is taken out of the reactor. The residual photoresist  306  and the Gold electrode  303  besides those on the Gold electrode bumps  304  are removed, as shown in  FIG. 5   h . In the last step, a non-conductive adhesive  307  is used to join the chip  301  and the substrate  308  together, as shown in  FIG. 5   i . The bumps  309  on the substrate  308 , conductive particles  203  not carrying any charge, Gold film  412 , and Gold electrode bumps  304  jointly form the electric connection between the chip  301  and the substrate  308 .  
         [0028]     In the aforementioned Au-ion solution, the Au-ion can be replaced by other metallic ions such as Cu-ion, Ni-ion, or any combination of the above.  
         [0029]      FIG. 6  is a comparison chart showing the differences between joining methods using the conventional anisotropic conductive films and the present invention. As shown in  FIG. 6 , when using anisotropic conductive films to bond a chip according to prior arts, the distance between the bumps on the chip must be greater than 50 μm. On the other hand, the electrophoretic fixing technique proposed by the present invention can be applied to bond a chip, the distance between the bumps on the chip can be less than 20 μm. The conductive particle density is about 15/bump when using anisotropic conductive films to bond a chip according to prior arts, while the conductive particle density is much greater than 15 per bump when using the electrophoretic fixing technique proposed by the present invention.  
         [0030]      FIG. 7  is a flowchart summarizing the foregoing embodiments of the method for dispersing conductive particles according to the present invention.  
         [0031]     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.