Abstract:
A metallic composition including a solvent and a plurality of metal nanoparticles dispersed therein is formulated such that curing of the metallic composition on a substrate provides a metallic conductor with a resistivity of about 5×10 −4  Ω·cm or less. Electrical components of an assembly can be interconnected by a metallic conductor formed by curing the metallic composition on a substrate. A metallic composition including metal nanoparticles can be deposited on a substrate and solidified. The metallic composition can be contacted with a metal wire before or after solidification of the metallic composition and secured to the solidified metallic composition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. Nos. 61/077,711 to Roundhill et al. and 61/081,539 to Roundhill et al., the entire disclosures of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This document relates to metallic pastes and inks that can be used to form metallic conductors with improved conductivity. 
       BACKGROUND 
       [0003]    Metallic pastes, like metallic inks, can be formulated with metal nanoparticles, liquid vehicles, dispersants, and other additives. Additives can be included to alter physical properties such as viscosity, wetting, and contact angle on chosen substrates. The higher viscosity of pastes (e.g., about 10,000 cP to about 60,000 cP) compared to inks (e.g., less than about 5000 cP) facilitates persistent dispersion of metal nanoparticles. Use of a metallic ink or paste can be based on a number of factors including printing method and substrate. Inks with a low viscosity (e.g., less than about 20 cP, or between about 10 cP and about 20 cP) may be ink-jet printed or aerosol printed. Pastes are too viscous to be ink-jet printed and may be applied by screen printing or other methods that are suitable for higher viscosities. 
         [0004]    Some metallic pastes or inks, however, may require heating at elevated temperatures in an inert atmosphere-conditions which can be unsuitable for certain applications, such as flexible electronics (e.g., with polymeric substrates). Additionally, some metallic pastes or inks include one or more liquid components with a high boiling point. When a metallic paste or ink with a high boiling point component is cured by a slow thermal sintering process in air, the high boiling point components can form non-volatile products that remain in the cured conductor. Thermal decomposition to non-volatile products can also during thermal sintering in an inert atmosphere, yielding contaminated conductors and relatively high resistivity. In some cases, organic residues from the liquid vehicle degrade the adhesion between the conductor and the substrate, reducing the quality of the metallic conductor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a flowchart that illustrates preparation of a metallic composition. 
           [0006]      FIG. 2  is a flowchart that illustrates a process for forming metallic conductors by a process including photosintering. 
           [0007]      FIG. 3  is a plot of resistivity vs. process temperature for copper conductors formed by thermal sintering and photosintering. 
           [0008]      FIG. 4  illustrates an assembly having chips that are interconnected by printed conductors made from a metallic composition. 
           [0009]      FIG. 5  illustrates an assembly having chips that are interconnected through vias by printed conductors made from a metallic composition. 
           [0010]      FIG. 6  illustrates a wire secured to a photosintered metallic bump. 
           [0011]      FIG. 7  is a flowchart that illustrates steps in a formation of a photosintered metallic bump. 
           [0012]      FIG. 8  illustrates a metallic wire secured between photosintered metallic bumps. 
           [0013]      FIGS. 9A-9F  show photographs of various stages of securing a copper wire to a photosintered copper bump. 
           [0014]      FIG. 10  illustrates a compliant photosintered metallic bump. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Metallic compositions (e.g., inks and pastes) formulated for low temperature processing are suitable for use in the manufacturing of printed electronics, as conductive adhesives, or in other applications including the manufacture and assembly of various electrical components and circuits, such as electrodes and interconnects. The metallic compositions include metal nanoparticles (e.g., copper, nickel, silver, gold, aluminum, cobalt, molybdenum, zinc, and the like) in optically transparent vehicles suitable for photosintering. The nanoparticles in these composition may be selected according to size and passivation coating, and the composition may be formulated to allow precision printing. The nanoparticles in the printed composition may be cured (e.g., photosintered, thermally sintered, or both) into bulk metallic films or lines at temperatures compatible with plastic substrates. 
         [0016]    Metallic compositions described herein are formulated to yield cured conductors with reduced amounts of organic residue from the liquid vehicle. The metallic compositions may be sintered (e.g., in less than about 5 msec, less than about 2 msec, or less than about 1 msec) to produce metallic conductors using a photosintering process. In this photosintering process, a high-intensity light pulse (e.g., about 50,000, 100,000, or 150,000 lux or higher) is absorbed by the metal nanoparticles in the composition and then converted into heat. As a result, the metallic composition may be subjected to a short, high thermal pulse that rapidly evaporates organic components before these components undergo thermal oxidation or decomposition. This photosintering of metallic compositions (e.g., pastes and inks) yields conductors with high conductivity that may be formed at lower temperatures, and lower resistivities than some thermal sintering processes alone. 
         [0017]    In some embodiments, the metallic compositions described herein may be formulated such that photosintering and/or thermal sintering of the composition on a polymer substrate (e.g., in air or in a forming gas) provides conductors that have a lowered resistivity. For example, copper conductors made from copper pastes described herein may have a resistivity between about 1×10 −3  Ω·cm and about 1×10 −6  Ω·cm. That is, the copper conductors may have a resistivity of less than about 1×10 −3  Ω·cm, less than about 1×10 −4  Ω·cm, less than about 1×10 −5  Ω·cm, or greater than about 1×10 −6  Ω·cm. 
         [0018]    An embodiment of preparation of metallic compositions used to form conductors with high conductivity (low resistivity) is illustrated in  FIG. 1 . Preparation  100  may be performed in an inert atmosphere. In step  102 , a desired amount (e.g., volume) of the liquid vehicle is prepared. The liquid vehicle may include, for example, aromatics, aliphatic ketones and alcohols, esters, and any combination thereof. If the vehicle includes more than one component, the components may be mixed together. In step  104 , one or more dispersants may be added to the vehicle, and the mixture agitated (e.g. stirred, shaken, mixed, sonicated, or the like) to achieve homogeneity. Dispersants may include, for example, short chain polymeric amines, alcohols, sulfonates, phosphates, and any combination thereof. In step  106 , an amount of metal nanoparticles (e.g. in the form of a nanopowder) may be added to the vehicle to achieve the desired copper loading. In some embodiments, the nanopowder may be about 30 wt % to about 70 wt %, or about 50 wt % to about 80 wt %, of the composition. This heterogeneous mixture is then agitated to form a paste. In step  108 , the paste may be sonicated to form a homogeneous dispersion. The sonication may be achieved with low energy and short duration (e.g., for less than about 10 minutes, about 2 to 8 minutes, or about 5 minutes). The sonication bath liquid may be cooled (e.g., to about 0° C.). If the sonication time is too long, or the sonication energy is too high, cavitation may cause localized heating to unacceptably high temperatures, leading to aggregation or agglomeration of the metal nanoparticles. Inhibition of aggregation or agglomeration of the metal nanoparticles may be facilitated by cooling the metallic paste (e.g., to about 0° C.) during the sonication procedure. Aggregation or agglomeration of the metal nanoparticles can lead to larger particle sizes and lowered performance in the sintering step. Aggregation can be problematic for copper nanoparticles, because copper-copper chemical bond formation may occur. These copper-copper bonds may not be broken by subsequent mechanical action. In step  110 , additives selected to adjust properties such as viscosity, wetting, or contact angle may be added and mixed with the paste. The additives may be covalent compounds, ionic compounds, or any combination thereof. 
         [0019]    A metallic paste prepared as described in  FIG. 1  may be formed into a conductor (e.g., a conductive film) by a photosintering process. For example, the metallic paste may be coated onto a substrate (e.g., a KAPTON® polyimide film available from DuPont) by a drawdown process. The photosintering process may include a pre-drying step prior to the light flash that photosinters the metal nanoparticles into, for example, a metallic film. For example, the coating may be dried at about 100° C. in air for about 60 min. This step promotes removal of volatile components from the printed paste. If the volatile components are allowed to remain in the paste, rapid vaporization during the photosintering step may cause metal nanoparticles to be blown off the substrate. After drying, the resistivity of the dried paste may be, for example, as high as 100 Ω·cm. In some embodiments, drying time may be reduced by using a higher temperature, carrying out the drying step under vacuum, or heating under infrared or microwave radiation. For copper pastes that do not contain volatile liquid components, the drying step may be eliminated to reduce the processing time. 
         [0020]    The dried paste may be cured in a forming gas or in air. For example, the dried paste may be thermally sintered for about 60 min at about 350° C. in a mixture of up to about 10 vol % hydrogen in nitrogen (e.g., about 3-5 vol % hydrogen in nitrogen). For a copper paste with nanoparticles in a range of about 20 nm to about 200 nm, a resistivity of the thermally sintered film may be about 3×10 −4  Ω·cm. The forming gas may reduce copper oxides in the dried paste to copper. For example, the hydrogen component in the forming gas reacts with the copper oxides to form copper and water as shown below: 
         [0000]      CuO+H 2 →Cu+H 2 O 
         [0000]      and 
         [0000]      Cu 2 O+H 2 →2Cu+H 2 O 
         [0000]    The water vapor can be carried off in the forming gas. 
         [0021]    The thermally sintered metallic composition may be photosintered in a forming gas or in air to reduce its resistivity. Photosintering includes subjecting the metallic composition to a flash of light. The intensity (as measured by the voltage) and duration (as measured by the pulse width) of the light flash may be selected to reduce blow off of the metal particles from the substrate, to reduce resistivity of the resulting conductor, and to increase adhesion of the resulting conductor to the substrate. In an example, after photosintering of a thermally sintered copper conductor in air, a thickness of the conductor may be about 1 μm, and a resistivity of the conductor may be about 2×10 −5  Ω·cm. 
         [0022]    A metallic paste may be dried in air or in an inert atmosphere. A metallic paste, or a dried metallic paste, may be thermally cured to form a metallic conductor. The thermally sintered conductor may be photosintered to reduce the resistivity of the conductor. In some cases, a dried metallic paste can be photosintered without undergoing thermal sintering.  FIG. 2  illustrates a process  200  for forming a low resistivity conductor from a metallic composition. In step  202 , a metallic paste is applied to a substrate. In step  204 , the substrate may be heated (e.g., in an oven for about 60 min at about 100° C.) to dry the paste. In step  206 , the dried paste may be thermally sintered. 
         [0023]    In an example, thermal sintering may include the following steps. A substrate with a dried metallic paste is loaded into a quartz tube at room temperature. The quartz tube is evacuated (e.g., to about 100 mTorr). The quartz tube may be heated (e.g., to about 350° C.) and purged with a forming gas (e.g., about 4 vol % hydrogen mixed with nitrogen) until the temperature is stabilized. The coated substrate may be heated for about 60 min at 350° C. After the forming gas and heater are turned off, and the tube may be purged with an inert gas (e.g., nitrogen) to cool the substrate (e.g., to below 100° C.). The substrate with the thermally sintered conductor may be removed from the quartz tube. 
         [0024]    In step  208 , the dried or thermally sintered metallic paste is photosintered. A high voltage flash xenon lamp may be used for photosintering. Photosintering may be achieved at temperatures of less than about 100° C. (e.g., ambient temperature, or about 20° C.), to yield a conductor with reduced electrical resistivity and increased adhesion to the substrate. U.S. Patent Application Publication No. 2008/0286488, which is incorporated by reference herein, describes a photosintering process. 
         [0025]    A comparison between photosintering and thermal sintering is shown in Table 1. 
         [0000]                                                                          TABLE 1                   Comparison between photosintering and thermal sintering.                Photosintering                    (after                   thermal sintering   (after thermal           Thermal sintering   in forming gas)   sintering in air)                        Substrate   Ceramic   Polyimide   Polyimide   Polyimide   Polyimide       Coating   Blade   Drawdown   Drawdown   Drawdown   Drawdown       method       Material   ANI-2   ANI-1   ANI-1   ANI-1   ANI-1       Cu particle   ~200   ~50   ~50   ~50   ~50       size (nm)       Drying   70° C.   100° C.   100° C.   100° C.   100° C.       temperature       Drying time   60 min   60 min   60 min   60 min   60 min       Curing   300° C.   100° C.   350° C.   Ambient (~20° C.)   Ambient (~20° C.)       temperature   350° C.   200° C.           500° C.   300° C.       Curing gas   Forming gas   Air   Forming gas   Air   Air           (4 vol % H 2  in N 2 )       (4 vol % H 2  in N 2 )       Curing time   60 min   60   60 min   1.2 msec   1.2 msec       Resistivity   4 × 10 2  (300° C.)   4.50 × 10 3     3 × 10 −4     2 × 10 −5     2.00 × 10 −4         (Ω·cm)   2 × 10 2  (350° C.)           2 × 10 −4  (500° C.)       Final film   ~10   ~1   ~1   ~1   ~1       thickness (μm)                    
The formulations for copper pastes ANI-1 and ANI-2 are shown in Table 2.
 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Copper paste formulations. 
               
             
          
           
               
                   
                 Vehicle 
                 Solvent 
                 Cu material 
                 3-roll 
                 Viscosity 
                   
                   
               
               
                 Paste 
                 (wt %) 
                 (wt %) 
                 (wt %) 
                 mill 
                 (cP) 
                 Dispersant 
                 Additive 
               
               
                   
               
               
                 ANI-1 
                 None 
                 MMB 1   
                 Copper 
                 Yes 
                 11,000 
                 1.5% 
                 0.5% 
               
               
                   
                 (0 wt %) 
                 (29 wt %) 
                 nanopowder 2   
                   
                   
                 DISPERBYK ®- 
                 DISPERBYK ®- 
               
               
                   
                   
                   
                 (69 wt %) 
                   
                   
                 111 
                 181 
               
               
                 ANI-2 
                 DJB-715 3   
                 Terpineol 
                 Copper 
                 No 
                 16,000 
                 none 
                 none 
               
               
                   
                 (12.5 wt %) 
                 (37.5 wt %) 
                 nanopowder 4   
               
               
                   
                   
                   
                 (50 wt %) 
               
               
                   
               
               
                   1 3-methoxyl-3-methyl-1-butanol 
               
               
                   2 Mean diameter 50 nm 
               
               
                   3 Daejoo organic vehicle from Daejoo Fine Chemical Co., LTD. (Korea) 
               
               
                   4 Mean diameter 200 nm 
               
             
          
         
       
     
         [0026]      FIG. 3  shows resistivity (Ω·cm) vs. sintering temperature for the copper conductors in Table 1. Point  300  indicates a resistivity of about 3×10 −4  Ω·cm for a conductor formed by thermally sintering copper paste ANI-1 in a forming gas environment. Point  302  indicates a resistivity of 2×10 −5  Ω·cm for the conductive film formed by thermally sintering copper paste ANI-1 in a forming gas environment (point  300 ) followed by photosintering in air at about 20° C. (1.2 msec, 1200 V). The photosintering step thus reduces the resistivity by about an order of magnitude. 
         [0027]    Points  304 ,  306 , and  308  indicate a resistivity of about 4.5×10 3  Ω·cm for conductive films formed by thermally sintering copper paste ANI-1 in air at about 100° C., about 200° C., and about 300° C. Point  310  indicates a resistivity of about 2×10 −4  Ω·cm for a conductive film formed by thermally sintering copper paste ANI-1 in air (points  304 ,  306 , and  308 ) followed by photosintering in air at about 20° C. (1.2 msec, 1200 V). The photosintering step thus reduces the resistivity by over seven orders of magnitude. 
         [0028]    Points  312 ,  314 , and  316  indicate resistivities of about 4×10 2  Ω·cm, about 2×10 2  Ω·cm, and about 2×10 4  Ω·cm for conductive films formed by thermally sintering ANI-2 paste (nanoparticle size 200 nm, no additives or dispersants) in a forming gas (4 vol % H 2  in N 2 ) at 300° C., 350° C. 500° C., respectively. Thus,  FIG. 3  indicates that, below a certain temperature (e.g., about 400° C. in some cases), photosintering can be used to achieve lower resistivities than can be achieved with thermal sintering alone. 
         [0029]    In some embodiments, a dried metallic paste may be photosintered without an intermediate thermal sintering step. This may be advantageous for substrates damaged by higher thermal sintering temperatures. Because of the low temperatures involved in the photosintering process (e.g., below 100° C.), photosintering can be used to form conductors (with or without the use of a forming gas environment) with a resistivity in the order of 10 −5  Ω·cm or less from metallic pastes and inks on substrates including polymers such as polyethylene, polyester, Flame Retardant  4 , and the like, without damaging the substrate. 
         [0030]    Metallic compositions (e.g., pastes or inks) may be used to make interconnects on printed circuit boards.  FIG. 4  illustrates an assembly  400  having printed metal interconnects  402  formed with a photosintered metallic paste. The assembly  400  may include a substrate  404  (e.g., a silicon, ceramic or flexible organic substrate such as polyimide, polyester, liquid crystalline polymer, or the like) with two or more chips  406 . Each chip  406  may have one or more metallic leads  408  that may be connected with other chip leads  408 . Because the photosintering step can be carried out at low temperature, the metallic interconnects  402  may be directly coupled to the chip leads  408  to provide interconnections among the chips  406  assembled onto the substrate  404 . 
         [0031]      FIG. 5  illustrates an assembly  500  in which interconnects  402  formed from a photosintered metallic composition (e.g., paste or ink) are used to fill vias  502  between chip metallic leads  408  and metallic interconnects  402 . The substrate  404  (e.g., a silicon, ceramic or flexible organic substrate such as polyimide, polyester, liquid crystalline polymer, or the like) may include two or more chips  406  disposed thereon, each having one or more metallic leads  408 . A conformal coating  506  may be printed onto the substrate  404  to cover the chips  406 . Vias  502  to metallic chip leads  408  may be drilled and filled with the conductive copper paste. Copper interconnects  402  may then be printed onto the top of the vias  502 , thereby connecting the chip leads  408 . This process may be repeated in order to fabricate multi-layer circuits. 
         [0032]    High viscosity copper pastes (e.g., about 10,000 cP to about 60,000 cP) may be prepared with high copper loadings (e.g., about 50 wt % to about 80 wt %) to allow for printing of thick lines. Thick lines with a low resistivity can carry high current density that may be required for many electronic devices. The copper pastes may be printed into the desired form of the interconnect features  402  in  FIGS. 4 and 5  or used to fill the vias  502  in  FIG. 5 . The viscosity and wetting characteristics of the copper pastes may be modified for different interconnect dimensions or different via diameters and depths. 
         [0033]    The metallic pastes described herein may be used to make multi-layer interconnections to reduce interconnect length and electrical resistance. As a result, a high density interconnect with lighter weight, smaller real estate, less noise and less loss on electrical signals can be formed to yield improved chip to chip connects. The metallic pastes described herein may also be used to eliminate wire bonding processes so as to increase performance and reliability of circuit design for chip to board interconnects, as well as removing the risk of tin whisker growth risk (and subsequent short circuiting) that can be caused by the use of lead-free solder. By using a metallic paste that can be directly deposited (e.g., screen printed) and photosintered, manufacturing of multi-layer boards is simpler and less costly. 
         [0034]    In some cases, a conductive bump formed from a metallic paste or ink may be used to create an interface between an integrated circuit and other electronic circuitry.  FIG. 6  illustrates an electronics device  600  in which substrate  404  supports integrated circuit  406 . The integrated circuit  406  is electrically connected to a metallic bump  602  formed on the substrate  404 . Metallic bump  602  is connected to integrated circuit  406  with wire  604 . In some embodiments, metallic bump  602  is formed from a copper paste or ink, and wire  604  is a copper wire. 
         [0035]      FIG. 7  illustrates a process  700  for forming the assembly in  FIG. 6 . In step  702 , a droplet of metallic ink or paste is deposited on a substrate with a method selected according to a property of the ink or paste (e.g., viscosity). The substrate may be a polymer such as, for example, a polyimide. In step  704 , a portion of the wire (e.g., an end of the wire) is inserted into the droplet. In step  706 , the droplet  602  is cured (e.g., thermally sintered, photosintered, or the like). Another portion of the wire (e.g., the other end) may be secured, for example, to an integrated circuit. The photosintering promotes the elimination of metal oxide (by, for example, the reduction of to copper oxide to copper), together with the fusion of the metal nanoparticles into a metal bulk film. After the sintering step, the wire is secured in the solid metal bump, resulting in an electrical connection between the wire and the substrate to which the metal bump is adhered. 
         [0036]      FIG. 8  illustrates a device  800  including conductive lines or pads  802  formed from a metallic ink or paste deposited onto a substrate  404  and photosintered. Conductive lines or pads may be, for example, about 0.1 μm thick. Substrate  404  may include a polymeric material, such as KAPTON®. Tape bonding wires  804  are positioned on top of the photosintered copper pads  802 . The bonding wires  804  may be, for example, about 25 μm in diameter. Droplets of metallic ink or paste (e.g., about 40-60 μm in diameter)  806  are deposited on the conductive pad/bonding wire junction  808  and baked (e.g., for 30 min at 100° C.) to dry the droplets. The droplets are then photosintered, and the resistance is measured through the wire junctions  808 . 
         [0037]      FIGS. 9A-9F  are photographs of various stages in the process described with respect to  FIG. 8 .  FIGS. 9A and 9C  show copper wires  804  inserted in copper droplets  806  before photosintering. FIGS.  9 B and  9 D-F show the copper bumps  900  after photosintering. 
         [0038]    In some cases, as illustrated in  FIG. 10 , the metallic bumps may be compliant. Compliant bumps  1000  may be formed by depositing a compliant polymer material  1002  onto substrate  404 . Then, metallic ink or paste  1004  is deposited over the polymer material  1002 . A wire may be inserted into the metallic ink or paste  1004 , and the droplet cured (e.g., photosintered) to secure the wire to the cured metallic conductor. This process may be performed at a temperature less than 100° C. 
         [0039]    Manufacturing costs can be reduced when metallic bumps are formed of copper, for example, rather than gold, for several reasons. First, copper droplets can be positioned precisely with selected printing techniques (e.g., copper inks can be inkjet printed). Second, since the copper bumps can be cured (e.g., photosintered) at less than 100° C., a wider array of substrate materials may be used without being damaged by a high-temperature process. Third, since the copper bumps can be cured at low temperatures, the sintering can occur in an air environment (rather than an inert environment), with minimal oxidation of the metal occurring during bonding between the chip pads and the conductive bumps. 
         [0040]    It is to be understood that while the invention has been described in conjunction with the detailed description thereof; the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.