Abstract:
A semiconductor device having reduced self and mutual capacitance of bonded wires is provided by coating the wires with a foamed polymer effectively having a very low dielectric constant. Additional benefits are realized by electrically insulating the wires against short-circuiting, by cushioning the wires with a low modulus sheath, and by protecting chip bond pad metallization 
     
       
         
               
             
                 TABLE 1 
               
                   
               
                 Method of Moments Capacitance Models 
               
                   
               
                   
               
               
               
               
             
                   
                 Wire Dimensions 
                 25 × 25 microns 
               
                   
                 Separation between Wires 
                 63.5 microns 
               
                   
                 Distance to ground 
                  191 microns 
               
               
               
               
               
             
                 Model 
                 Dielectric 
                 Self capacitance 
                 Mutual Capacitance 
               
               
               
               
               
               
             
                   
                 constant of 
                 Wire 1 
                 Wire 2 
                   
               
                   
                 separation 
               
                 Model 
                 Dielectric 
                 Wire 1 
                 Wire 2- 
                 Mutual cap 
               
                   
                 constants 
                 self cap 
                 self cap 
                 pf/cm 
               
                   
                   
                 pf/cm 
                 pf/cm 
               
                 Plastic encased 
                 4.0 
                 1.03 
                 0.54 
                 1.57 
               
                 package 
               
                 Cavity package 
                 4./1.0/4. 
                 0.31 
                 0.12 
                 0.43 
               
                 Foam sheath 
                 4./1./4./1./4. 
                 0.34 
                 0.16 
                 0.50 
               
                 wires/molded 
               
                 Wires - no 
                 1.  
                 0.26 
                 0.13 
                  0.39. 
               
                 package

Description:
This application is a division of Ser. No. 09/989,263, filed Nov. 19, 2001 now U.S. Pat. No. 6,822,340 which claims priority under 35 U.S.C. § 119 based upon Provisional Application Ser. No. 60/252,126, filed Nov. 20, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to semiconductor devices, and more particularly to wire bonded integrated circuits. 
     DESCRIPTION OF PRIOR ART 
     In fabrication of plastic encapsulated semiconductor devices, the electrically conductive pads of an integrated circuit chip are connected to electrically conductive external leads by means of a very thin wire, typically a gold wire. As shown in  FIG. 1   a , during molding to encapsulate chip  10  and leads  12 , using a dielectric plastic molding compound  14 , there is a tendency for wires  11  to distort. This lateral distortion of the wires in the direction of flow of molding compound, represented by arrow  15  is referred to as wire sweep. In  FIG. 1   b , a gold bond wire  11  is formed in an arc shape between the chip pad  13  and the lead finger  12  in order to prevent wire shorting to either the chip edge, or to the supporting chip pad  14 . However, as illustrated in  FIG. 1   c , a wire  21  often does become deformed, not only by mold compound flow, but also by vibration, mechanical damage or other means, and the tendency for distortion is strongly aggravated by decreasing wire thickness and increasing wire length. As control of the arc shape is lost, spacing between wires is no longer in control. 
     As the trend to increase the number of input/output connections on circuits has continuously increased, the spacing between pads on the chip has decreased, double tiers of bonding pads have been included, and the length of wires has increased. Leads cannot be fabricated with the same high density as chip pads, and therefore wire lengths have increased in order to allow connection between the leads and closely spaced pads on the chip. 
     Capacitance loading increases directly with increased wire length, and wire lengths are currently approaching one centimeter. As the wires are brought closer together by design, and/or by wire sweep or other distortion, the separation between wires decreases. All of these factors have a tendency to cause parasitic capacitance coupling between the wires, as well as for short-circuiting of the wires. Increased mutual capacitance between neighboring wires increases electrical noise and affects signal transmission of the circuit. Both self capacitance of long wires and mutual capacitance between wires have become significant obstacles to low cost wire bonded, high speed integrated circuits. 
     Various attempts have been made to electrically insulate bond wires, by coating with a dielectric material either before or after bonding, and thus to prevent shorting. In several instances wires have been coated with polymers which decompose with heat during the bonding operation. Alternately, thin films of silicones, parylene, other polymers, or even plasma enhanced chemically vapor deposited SiO2 have been applied after wire bonding. However, none have been widely accepted because of deleterious side effects. 
     Further, almost no attention has been paid to requirements for, or methods to minimize capacitance coupling, and thus improve both reliability and performance of circuits. As the speed of circuits has increased, the parasitic capacitance of wire bonded circuits has become very serious in light of the fact that wire bonded devices, both now and for a some time in the future, will continue to be the economical and preferred method of interconnecting chips to package leads, and therefore a means to minimize the capacitance issues would be very beneficial to the industry. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a wire bonded integrated circuit device having low mutual capacitance between the wires, and thus minimize parasitic coupling and cross talk attributable to bond wires. 
     It is an object of this invention to provide a method for isolating neighboring wire bonds from each other in order to minimize self and mutual capacitance of the wires. 
     It is an object of the invention to provide a means to insulate bond wires from each other after the wire bonding process has been completed. 
     It is an object of the invention to provide a means to insulate bond wires from each other after the wire bonding process has been completed, without requiring any cleaning of the leads, internal or external to the molded package. 
     It is an object of the invention to provide a very low dielectric medium surrounding bond wires which minimizes capacitance coupling between wires, and which eliminates short-circuiting. 
     It is an object of the invention to provide a dielectric material surrounding bond wires which further effectively has a low modulus of elasticity, and supports enhanced reliability of the device. 
     It is an object of the invention to decrease mutual capacitance between wires by a factor of about 3 from comparably dimensioned plastic molded wire bonded devices. 
     The aforementioned objectives are met by first using an electromagnetic model to analyze the capacitance of neighboring wires separated by epoxy molding compound, of wires separated by air only, and of wires separated by a layer of a very low dielectric constant sheath on the wires prior to embedding in a mold compound. Analysis of the data indicates that only a thin layer, approximately 2.5 microns on all sides, of a very low dielectric constant material surrounding the wires will reduce mutual capacitance by a factor of about 3 from that of epoxy molding compound having a dielectric constant of 4. Air as the dielectric for wires at 40 microns separation would provide a 4.5 times decrease from that of molded epoxy. However, because air separation is not a viable solution for plastic molded devices, or even a reliable solution for cavity packages, a very low dielectric constant dielectric constant sheath is provided as a means of minimizing mutual capacitance, and resulting crosstalk. 
     In order to form a usable, truly low dielectric constant medium surrounding the wire, a foamed polymer having pockets of air or other gas incorporated into the medium is provided. Density of the polymer is decreased, and effectively both the dielectric constant and modulus are decreased by foaming. One method for fabricating such a layer is to react components of a polymer which produce and incorporate gas pockets during curing. Alternate methods for foaming the dielectric medium include adding blowing agents to a polymer prior to curing, thereby capturing air within the medium, and providing the necessary properties. Foamed polymers form low-density embedding materials, reduce the dielectric constant significantly, without creating a rigid coating, such as that found with low dielectric materials, such as polyimides. Further, foamed polymers are processed at temperatures acceptable for wire bonded integrated circuit devices, and they do not require high temperature processing, as do some polymers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows a wire-bonded integrated circuit having wires swept laterally by mold compound flow. (Prior Art) 
         FIG. 1   b  is a section of a wire bonded device. (Prior Art) 
         FIG. 1   c  illustrates a sagging wire bond. (Prior art) 
         FIG. 2   a  is a model of a plastic encapsulated integrated circuit device with neighboring bond wires. 
         FIG. 2   b  is a model of wire bonds surrounded by air, and the assemblage encased in plastic, as in a cavity package. 
         FIG. 3  is a model of a plastic encapsulated integrated circuit device in accordance with the invention having a layer of low dielectric coating surrounding wires. 
         FIG. 4   a  is a section of a wire bonded device illustrating an embodiment of the invention having a low dielectric sheath surrounding the wire. 
         FIG. 4   b  is a cross section of neighboring wire bonds having a dielectric sheath. 
         FIG. 4   c  is a cross section of a wire with a foamed polymer sheath. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to ensure proper signal transmission and timing of integrated circuits, prediction and control of electrical parameters of a packaging system are analyzed using electromagnetic computer modeling and simulation programs. Such electrical modeling programs are both commercially available and have been developed by a number of universities. The geometry and material properties of a series of conductors and insulators are input to the computer program, and the output includes a matrix of capacitance, inductance of the conductors and resulting impedance of the leads in question, and of the surrounding leads. 
     Prediction of capacitance loading in a wire bonded integrated circuit device is analyzed using such a modeling program.  FIG. 2   a  illustrates a pair of wires  21  and  22 , each 25 microns in diameter, and separated by a distance  23 . The separation is filled by a material  24  having a dielectric constant 4.0, which is typical of epoxy molding compounds used for encapsulating wire bonded circuit devices. Bond wire  22  is positioned at a distance represented by an arrow  28  above a ground plane  27 , which is not integral to the package, but assumed to be in the circuit board.  FIG. 2   b  is a model of a device having the same dimensions, but housed in the cavity of a shell  26  having a dielectric constant of 4.0. The cavity and space between wires is filled by air  25  having a dielectric constant of 1.0. This model is representative of a cavity package. 
       FIG. 3  illustrates a model of a device having the same wire dimensions and spacing as in  FIGS. 2   a  and  2   b , but with a low dielectric constant layer  35  surrounding each wire  31  and  32 . The dielectric sheath  35  has a thickness represented by arrows  39  of 2.5 microns on each side. A material  34  having a dielectric constant 4.0 encapsulates the assemblage, and fills the space between the dielectric coated wires. 
     Capacitance results of this model are compared in Table 1 to those of the encapsulated device ( FIG. 2   a ) having no wire coating, the cavity device ( FIG. 2   b ) having air between wires. In addition, for comparison, results of a model of wires having no encapsulant, with air only between the wires and ground is included in Table 1. 
     From these data, the mutual capacitance of wires in the molded device having no wire coating is 1.57 pf/cm or a factor of about 3.65 times greater than that of the cavity type device having air separation between the wires, and about 3.14 times greater than the device of this invention having a thin, very low dielectric constant layer surrounding the wires. Mutual capacitance of wires with only air surrounding and separating from a ground plane is 0.39 pf/cm, as compared to 0.43 pf/cm for a cavity package, and 0.50 pf/cm for a molded device having a sheath of low dielectric constant material. However, the practicality of air as a dielectric between wires is the inability to control spacing, resulting not only in capacitance coupling, but also in wire shorting. Each of the models having a low dielectric constant medium surrounding the wires has significantly lower self and mutual capacitance than the model representing a conventional molded device ( FIG. 2   a ), and provides a significant reduction in probability of parasitic coupling and cross talk in a circuit. However, only the sheathed wires offers a practical, manufacturable solution. 
       FIG. 4   a  is a device of the current invention, having a wire  41  bonded to a chip contact pad  40 , and to a lead  48 , and having a low dielectric constant sheath  45  conforming to the wire. Bond wires are separated by a distance equal to or greater than the wire diameter, typically in the range of 50 to 75 microns. The coating, or sheath  45  which has been applied after the wire bonding process, extends onto both the chip surface and the portion of the lead where the wire is attached. This extension of the dielectric coating provides further reliability enhancement to the device by covering the fragile bond pad metallization. The chip, wire, coating, and inner lead are encased by a molding compound  44 , typical of that used in plastic molded IC packages. 
       FIG. 4   b  is a cross section of a pair of neighboring wires  41  and  42  sheathed in a relatively thin dielectric material  45 , and embedded in molding compound  44 .  FIG. 4   c  provides a more detailed view of the cross section of a low dielectric constant material  45  coating a wire  41 . The polymeric medium  45  filled with pockets  50  of air or gas is a foamed polymer, having both very low density, and effectively a very low dielectric constant resulting from the interspersed pockets of air. 
     In a preferred embodiment, the dielectric coating is a foamed polymer produced in situ by reacting components of a polymer. Such a foamed polymer is a polyurethane having pockets of carbon dioxide incorporated throughout the medium. Polyurethane is produced by a polyol reacting with a diisocyanate, and the foaming is generated by adding water and additional isocyanate. (1) Alternately, many other embedding resins are made into low density foams by adding blowing agents, unless their cure is affected. (2) A blowing agent is impregnated into a thermosetting polymer to create foamed polymers of materials such as polyetherimide, polypropylene, epoxy, or polyimide. 
     The dielectric constant of a polymeric material having a large volume of air pockets dispersed throughout is effectively reduced to approaching that of air, or in the range of 1.0 to 2.3. Results of the wire bond models, given in Table 1 show that a thin film of such a coating reduces mutual capacitance between neighboring wires in a plastic molded package by a factor of about 3, and thus significantly reduces the propensity for parasitic coupling, and cross talk between wires. The dielectric medium thickness is a minimum of 2.5 microns on all surfaces to achieve this level of capacitance change. 
     Placement control of the dielectric material is not critical; a minimum film thickness of 2.5 microns is sufficient to provide the reduction in capacitance, and may be much thicker. Swelling of the dielectric medium during foaming serves to control flow or run out of the polymer, and thus run out onto the leads which extend outside the molded package is of little concern. The dielectric material around the ball bond and surrounding chip bond pad provides a seal against moisture ingress, and thus serves to enhance reliability of the device. The foamed polymer sheath can surround each wire, the sheath covering substantially only the wire and wire connections to the pads on the integrated circuit chip and the conductive leads and not covering other portions of the chip and the conductive leads. 
     The effective elastic modulus of the dielectric material is very low as a result of the embedded air pockets, thus stress on the wires is reduced, as compared to encasing in a more brittle coating, or in conventional molding compound. 
     The method for forming an integrated circuit device having low mutual capacitance between bond wires includes the following; a polymeric material is disposed on each of the wires after the bonding to the chip and lead finger has been completed, the foaming reaction is allowed to proceed, and the polymer is cured prior to over molding with a conventional molding compound, such as epoxy novolac. 
     The preferred embodiment described above is a wire bonded integrated circuit in an over-molded plastic package, wherein the wires are surrounded by a very low density dielectric material in order to minimize capacitance of the wires. It should be noted that in this embodiment, the wires are held in place by the foamed dielectric material, and that sweep or sagging of the wires as a result of mold flow or mechanical damage is decreased. Therefore, not only is the capacitance better controlled, but wire shorting is eliminated. 
     A second embodiment of the current invention is to prevent movement or displacement of bonding wires in cavity packages, and thus eliminate potential shorting between wires or to substrates. Leaded surface mount or Ball Grid Array packages having either plastic or ceramic housing are included in a cavity package embodiment of this invention. Further, the drawings have indicated bond wires to leads which are typically associated with lead frames, but packages having leads on dielectric substrates are also embodiments of this invention. 
     While preferred embodiments and some alternative applications of the invention have been described above, they are not intended to be limited, but instead it should be understood that various modifications may be made from the specific details described herein without departing from the spirit and scope of the invention as set forth in the appended claims.