Abstract:
A paste is described for capping electrodes with an oxide free metal layer incorporating a solvent, an unzippable polymer and particles. The electrode could be an interconnect such as a C 4  bump. A method for forming a coating and for testing integrated circuit chips is also described. The invention overcomes the problem of interconnecting Pb containing electrodes that are covered with an insulating oxide on integrated circuit chips by coating the Pb containing electrode with Au to provide an oxide free surface for testing and interconnection.

Description:
This is a division of application Ser. No. 08/965,227, filed Nov. 6, 1997 now U.S. Pat. No. 6,013,713. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a paste and more particularly to a paste including an unzippable polymer, solvent and conductive particles for coating electrodes such as C 4  bumps with oxide free conductive particles for applications such as low temperature interconnections between an integrated circuit chip and a substrate made of polymer/filler composites, such as an Fr 4  printed circuit board. 
     BACKGROUND OF THE INVENTION 
     With more and more transistors being placed on a chip to increase the number of functions, the number of input/output (I/O) pads per integrated circuit chip has increased significantly. The increase in the number of I/o pads per chip are making traditional bonding methods, such as wire bonding (WB) and tape automated bonding (TAB) difficult. Flip chip attach (FCA), which is usually an area array in contrast to a peripheral array for WB and TAB, is becoming increasingly pervasive due to the number of pads. In FCA, the chip is bumped with a lead-rich Pb/Sn alloy ball using metal deposition through a resist-mask, for example. The bonding of this chip is achieved by self alignment and placing the chip on the substrate which has been covered with high viscosity flux to reduce oxides. The chip is held in place by the flux. The whole assembly (chip and substrate) is subsequently heated in the range from 350 to 400° C. to a temperature which melts the solder forming an interconnect between balls or bumps on a chip and respective pads on a substrate. 
     Conventionally the substrates were multi-layer ceramic (MLC) structures that could withstand temperatures up to 400° C. Dictated by both the number of pads and lower cost, there is a growing need to attach similar C 4  bumped integrated circuit chips to organic substrates made of polymer/filler composites, such as FR 4 . Such organic substrates degrade at solder reflow temperatures above 300° C. Thus a low temperature joining material is needed to attach the C 4  bumps of a chip to respective substrate pads. 
     One method to attach electrodes such as C 4  bumped chips to an organic substrate is by capping the C 4  bumps first with a low temperature melting Pb/Sn-eutectic solder such as described in U.S. Ser. No. 08/710992 filed Sep. 25, 1996 by Berger et al. entitled “Method for Making Interconnect for Low Temperature Chip Attachment” (YO996073) and assigned to the assignee herein. The Pb/Sn solder cap over the C 4  bump may be accomplished by vapor depositing the metal components through a resist mask, followed by solder reflow step. The masking process requires expensive alignment and lithographic steps, and the vapor deposition process is costly due to high vacuum processing. The bonding is accomplished by reflowing the Pb/Sn-eutectic solder at temperatures below 250° C. using acidic flux. Subsequently, the flux is removed using organic solvents that may be chloro-fluoro-carbon (CFC) based. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a paste is described comprising a solvent for an unzippable polymer, an unzippable polymer dissolved in the solvent to form a solution, and particles suspended in the solution. The particles may be suitable for coating the surface of a selected material; and the polymer may comprise 10 wt. % or greater of the solution. 
     The invention further provides a coating comprising a substrate and a layer of the paste mentioned above that leaves a layer of particles which may be heated to form an alloy with the substrate material. 
     The invention further provides a method for testing the integrated circuit chip with C 4  bumps that are coated using the disclosed method. The particles in the paste being conductive and adherent to the C 4  surface will provide a conductive path between the C 4  bumps and conductive pads on the test probe. 
     The invention further provides a method for coating C 4  electrodes or bumps on an integrated circuit chip comprising the steps of applying a paste mentioned above to the surface of the C 4  electrodes, the particles in the paste being conductive and adherent to the surface of the substrate, and heating the paste to remove the solvent and the unzippable polymer wherein the particles may alloy with the Pb in the C 4 . 
     The invention provides a low cost C 4  capping method. 
     The invention provides a method that does not require any lithography, alignment or vacuum processing steps. 
     The invention provides a bonding process that uses conductive adhesive on the substrate that does not require any flux, hence no cleaning step. The conductive adhesive may typically be a silver or gold filled epoxy. 
     The invention provides a bonding process using a conductive organic composite that allows for greater thermal mismatch between the chip and the substrate than a solid metal solder joint would. 
     The invention provides a bonding process that allows flip chip to pads on organic substrates where the mismatch in the thermal coefficient of expansion (TCE) is significantly larger than the TCE between an integrated circuit chip and a ceramic substrate. 
     The invention provides a testing process that would allow the C 4  bumps to be tested by conventional methods of pressure contact before bonding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features, objects, and advantages of the present invention will become apparent upon consideration of the following detailed description of the invention when read in conjunction with the drawing in which: 
     FIG. 1A shows one embodiment of the invention. 
     FIG. 1B shows an intermediate step in using the embodiment of FIG.  1 A. 
     FIG. 1C shows the final result of using the embodiment of FIG.  1 A. 
     FIGS. 2-4 show a sequence of steps for placing a coating of unzippable paste on C 4  bumps of an integrated circuit. 
     FIG. 5 is a SEM image of a C 4  bump after a heating cycle having a metal coating as in FIG.  1 C. 
     FIG. 6 is a graph of data from Energy Dispersive X-ray Analysis of a gold coated site on the C 4  bump after the coating process, and 
     FIG. 7 is a graph of data from Energy dispersive X-ray Analysis of an uncoated site of lead on the C 4  bump in FIG.  6 . 
     FIG. 8 shows the final structure of a C 4  bump coated with oxide free particles to be bonded to a paste bumped substrate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1A, a substrate  10  and a coating of unzippable paste  13  is shown. Unzippable paste  13  comprises an unzippable polymer  15  and/or their blends dissolved in solvent  18 , particles  17  such as oxide-free metal. An oxide-free metal or surface is one that does not form an oxide layer to permit a conductivity greater than 1 ohm-cm or the oxide layer is conducting with a conductivity greater than 1 ohm-cm. For example, C 4  bumps of an alloy of 97 wt. % Pb and 3 wt. % Sn typically have an oxide coating or layer which provides a contact resistance which is greater than 1 ohm-cm without reflow of the solder in the C 4  bump. Unzippable polymer  15  is a polymer that completely volatilizes above a certain temperature defined as the unzipping temperature, T unzip . Some examples of unzippable polymers  15  include poly(methyl methacrylate) (PMMA), poly(∝-methyl styrene) (PAMS), poly(propylene carbonate), poly(ethylene carbonate), and poly(chloral). All these polymers have a T unzip  below 400° C. 
     Solvent  18  is one that may be organic and dissolves the above mentioned polymers. For example, N-methyl pyrrolidinone (NMP) is a good solvent for PMMA and PAMS. 
     Particles  17  may be composed of one or more metals. At least one of the metal components should form an alloy with Pb or Sn wherein the alloy melts in the temperature range from 150 to 400° C. For example, particle  17  may be Au, Sn, or Au/Sn alloy. The particle should be oxide free or with an oxide that is conductive. The particles could also be made of composite material where the inner core may be ceramic or metal and the outer layer is an oxide free metal or a conductive oxide. Particles in the paste may be in the range from 1 to 50 volumetric %. 
     Solvent  18  in paste  13  is removed by drying at T dry  under a selected environment, leaving polymer  15  and particles  17  on the surface of substrate  10 . The temperature is then raised above the unzipping temperature T unzip  of unzippable polymer  15  which results in the degradation and evaporation of the polymer components. 
     FIG. 1C shows the final structure of substrate  10  coated with particles  17  after unzipping. The T dry  is more than 50° C. below the T unzip . The selected environment may be air, N 2 , vacuum, forming gas, Ar, He or a combination thereof. Coating  12  may be patterned or uniform over substrate  10 . Selective coating  12  may be achieved by a standard dispensing process such as screening, stenciling, etc. Particles  17  shown in FIG. 1C may be in a monolayer or multilayer. Particles  17  may form an interconnected percolating structure or discrete islands. 
     FIGS. 2-4 show steps for coating C 4  bumps of an integrated circuit with unzippable paste  13 . Unzippable paste  13  is screened on a smooth, clean surface of a solid substrate  22  through an interposer  24 . Substrate  22  may be glass, silicon or ceramic. Interposer  24  may comprise polyimide, polyester or other organic material or metal such as molybdenum, nickel, stainless steel, etc. Interposer  24  may have a plurality of openings or vias  26  which are slightly larger than the diameter of the C 4  bumps  27  to be coated. The thickness of interposer  24  is less than the height of the C 4  bump  27 . Typically, the diameter of C 4  bump  27  is 75 μm. The diameter of opening  26  may be in the range from 85 to 100 μm and the thickness of interposer  24  may be 50 μm. FIG. 2 shows openings  26  filled with unzippable paste  13 . Interposer  24  can be made by chemical etch, laser ablation or other conventional means to match the pattern of C 4  bumps on a chip or wafer. 
     Some other method of transferring the unzippable paste  13  to C 4  bumps  27  may be by pin transfer, dipping, brushing, etc. 
     Substrate  22  is then placed over or under a chip or wafer  34  of undiced or embedded chips having a plurality of C 4  bumps  27  thereon. Openings  26  filled with unzippable paste  13  are aligned with corresponding bumps  27  on wafer  34 . The alignment is done by self aligning the chip with the vias  26  in interposer  24 . Substrate  22  is brought closer to chip or wafer  34  so that C 4  bumps  27  are pressed into openings  26  and in contact with unzippable paste  13  as shown in FIG.  3 . Substrate  22  is then separated from or moved away from chip or wafer  34  so that C 4  bumps  27  are separated from contact with unzippable paste  13  in openings  26 . A thin layer  29  of unzippable paste  13  adheres to C 4  bumps  27  as the chip or wafer  34  is removed from proximity of substrate  22  as shown in FIG.  4 . In this example, the geometry of each C 4  is well defined dimensionally and by composition,the contact area to unzippable paste  13  is nominally identical for each bump. As a result, the thickness and size of the paste film forming thin layer  29  on the C 4  bumps  27  are nominally of the same thickness. The thickness of thin layer  29  may be in the range from 0.1 to 50 μm with a thickness variation in the range from 0.05 to 25 μm. The process can be repeated a number of times by heating to T dry  to increase the amount of particles adhering to the C 4  bumps. 
     Thin layer  29  is first dried by heating to T dry , then heated above T unzip  and the melting temperature of the solder metals in the C 4  bumps  27 . The particles embed into the molten C 4  or react with solder metal, to form a strong bond between the particle and the C 4 . 
     Depending upon the particular unzippable polymer, metal, alloy, or other material selected. T unzip  may be above or below the melting temperature T melt  of the metal, alloy or other material. Where T unzip  is above T melt  heating may be above T unzip  in one step. Where T unzip  is below T melt , heating may first be above T unzip  and below T melt  and then raised above T melt . 
     Best practice is to completely remove the unzippable polymer prior to raising the temperature to T melt  or above. 
     Heating is done typically in an oven or zone furnace with a nitrogen or preferably forming gas environment. During this thermal cycle, the unzippable paste  13  unzips and vaporizes and the metal filler particles  17  fuse with respective C 4  bumps forming an oxide-free conductive coating  32  on C 4  bumps  27 . 
     FIG. 5 shows an image of conductive coating  32  on a C 4  bump  27  taken by Scanning Electron Microscopy (SEM). The image in FIG. 5 was taken after the heating cycle was completed at 360° C. for 30 mins. in a forming gas environment. Conductive coating  32  as shown in FIG. 5 is a uniform layer or coating of particles  17  which are Au. 
     FIG. 6 is a graph of Energy Dispersive X-ray Analysis of a site on conductive coating  32  on the C 4  bump  27  shown in FIG.  5 . In FIG. 6 the ordinate represents X-ray intensity and the abscissa represents energy in keV. Curve  42  shows the X-ray intensity as a function of energy from 0 to 20 keV. Peak  44  shows that the top surface of conductive coating  32  is Au with very small traces of Pb. Curve  44  illustrates that conductive coating  32  has a high coverage of gold over C 4  bump  27 . 
     FIG. 7 is a graph of Energy Dispersive X-ray Analysis of a site on C 4  bump  27  without conductive coating  32 . FIG. 7 is formed from data obtained from the C 4  bump  27  shown in FIG.  5 . In FIG. 7, the ordinate represents X-ray intensity and the abscissa represents energy in kev. Curve  46  shows the X-ray intensity as a function of energy from 0 to 20 keV. Peak  48  shows that the surface of C 4  bump  27  is Pb. 
     A process for bonding chips  50  to a substrate  52  is now described. The chips  50  to be bonded would be processed to form, as described above, a layer  32  of particles  17  on its C 4  bumps  27 . A conductive adhesive paste  54  comprising a polymer binder and noble metal particles would be placed on electrical pads  55  on substrate  52 . The polymer binder may be a thermoplastic or thermoset polymer. The paste may also contain a solvent system that dissolves the polymer binder. Chip  50 , after C 4  bumps  27  are processed to contain a Au layer  32 , is bonded to paste  54  or paste bump  56  on substrate  52  by a method shown in FIG.  8 . An example of a conductive paste suitable for use herein is described in U.S. Pat. No. 5,086,558 by Grube et al. which issued Feb. 11, 1992 and in U.S. U.S. Pat. No. 5,866,044 which issued Feb. 2, 1999 by R. Booth et al. filed Oct. 21, 1996, both of which are incorporated herein by reference. Typically if paste  54  is epoxy based, chip  50  is bonded by heating the assembly in a temperature range from 150 to 250° C. with 0 to 100 psi pressure. If paste  54  is thermoplastic, the temperature range is the same but the pressure can range from 10 to 100 psi. The attached chip  50  is then underfilled by an encapsulant to form a typical finished assembly. 
     A process of testing the chip is now described. The C 4  bumps of the chip to be tested would be coated with layer  32  by a process described above. The C 4  bumps are electrically connected to the pads of the test probe by either physical contact using force or bonding using the thermoplastic paste method described above. For both methods, the contact pads of the test probe are oxide free. 
     C 4  bumped chips are needed to be attached to Fr 4  or other organic substates. Such assemblies will be used in computers, office equipment, automobiles and trucks, control systems, cellular phones, etc. 
     While there has been described and illustrated a process for forming a metal coating on a structure such as capping C 4  bumps with Au to provide an oxide free surface on C 4  bumps on an integrated circuit chip, it will be apparent to those skilled in the art that modifications and variations are possible without deviating from the broad scope of the invention which shall be limited solely by the scope of the claims appended hereto.