Abstract:
A system for bonding a die to a high power dielectric carrier such as a ceramic dielectric core with double-sided conductive layers is described. In the system, the upper conductive layer has a first area whose surface has a first wettability. A second area that at least partially surrounds the first area has a surface with a second wettability that is greater than the first wettability. During bonding, an adhesive material bonding a chip to the substrate spreads among the first area by a downward force placed on the chip. Due to the difference in wettability, the adhesive material then spreads among the second area by a wetting force generated by the greater second wettability of the second area surface causing the chip to be drawn down until reaching a predetermined position. The predetermined position can be determined by substrate protrusions or substrate cavities.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of electronics packaging and, more particularly, to a system for attaching a high power die to a substrate with a bond line of consistent and accurate thickness along the entire die. 
     BACKGROUND 
     Various advances in high power and high switching frequency electronic devices have been increasingly used in power applications in transportation systems, appliances, energy systems, and motor control. Such applications require power on the order of megawatts with operating temperatures on the order of 200° C. Exemplary high power devices are insulated gate bipolar transistors (IGBTs) that are semiconductor devices with four alternating layers and have a metal-oxide semiconductor gate structure. Due to the operating conditions of these devices, high dielectric breakdown voltage and high thermal conductivity are required in the device packaging. Typical substrates are ceramic-based direct bonded copper with flat copper. A bond line to such substrates is on the order of 0.4 mil to 3 mils. 
     Due to the high-power operation of these devices, it is important that the bond line be reliably uniform across the entire area of the bonded die. However, such reliably uniform bond lines have proven difficult to achieve with thin and/or uneven bondline thicknesses resulting in cracking as a result of high power switching that leads to thermal cycling, resulting in inelastic creep strain and crack growth. This results in partial or complete debonding of the die from the substrate. 
     Thus there is a need in the art for improved bonding systems that will maintain the required high dielectric breakdown voltage and high thermal conductivity necessary for high power and high frequency device applications. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a system for bonding a die to a high power dielectric carrier such as a ceramic dielectric core with double-sided conductive layers. In the system, the upper conductive layer has a first area whose surface has a first wettability. A second area that at least partially surrounds the first area has a surface with a second wettability that is greater than the first wettability. During bonding, an adhesive material bonding a chip to the substrate spreads among the first area by a downward force placed on the chip. Due to the difference in wettability, the adhesive material then spreads among the second area by a wetting force generated by the greater second wettability of the second area surface causing the chip to be drawn down until reaching a predetermined position. 
     In one embodiment, the predetermined position is determined by protrusions in the substrate that act as stops for the placement of the die. In other configurations, an etched cavity defines the flow stop for the adhesive material. In an exemplary embodiment, the adhesive material includes a metal solder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts problems with a conventional die bonding system. 
         FIG. 2  depicts a die bonding system according to the present invention. 
         FIG. 3  depicts alternative die bonding systems of the present invention 
       In  FIG. 4 ,  FIGS. 4A and 4B  depict increased bonding reliability for the bonding systems of the present invention. 
         FIG. 5  depicts an electrical package aspect of the present invention. 
       In  FIG. 6 ,  FIGS. 6A to 6E  depict formation of cavities and protrusions. 
       In  FIG. 7 ,  FIGS. 7A to 7G  illustrate protrusion/column patterns. 
       In  FIG. 8 ,  FIGS. 8A to 8D  show alignment feature patterns. 
     
    
    
     DETAILED DESCRIPTION 
     Turning to the drawings in detail,  FIG. 1  depicts problems with conventional die bonding systems determined during the course of creating the present invention. In the conventional system of  FIG. 1A , a die  10  is to be bonded to substrate  20  (such as a direct bonded copper substrate) through a bonding/adhesive medium such as solder  30  or another type of bonding/adhesive material. In  FIG. 1B , pressure is applied in an attempt to create an even bond across the entire die area. In  FIG. 1C , the pressure is released. However, before the bond is permanently solidified, adhesive backflow occurs as shown in  FIG. 1D . The present inventors determined the backflow occurred as the result of a low wettability of the bonding material to the substrate, such that surface tension of the bonding material causes an uneven bond line thickness to be formed. 
     To prevent the adhesive backflow depicted in  FIG. 1D , the system of  FIG. 2  was created. In the system of  FIG. 2 , the die placement region includes regions having different wettabilities. “Wettability” relates to degree of ability of a liquid to contact a solid surface and is often designated in terms of the contact angle between the liquid and the solid with lower contact angles indicating that the liquid more readily wets the solid surface. When interfacial tension (adhesive/substrate) is larger than the substrate surface tension, the adhesive tends to ball up with a high contact angle. In contrast, when interfacial tension (adhesive/substrate) is smaller than the substrate surface tension, the adhesive tends to spread on the substrate surface due to the low contact angle. 
     Note that the degree of wettability is determined not only by the materials involved but also by the surface morphology of the surface to be wetted. For example, for materials of the same composition, a rough surface promotes wetting. Surfaces can be deliberately micromachined (chemically, mechanically, or through chemical-mechanical techniques) to induce different wettability characteristics. Surfaces with high wettability encourage liquid spreading which is important for forming a continuous layer. In contrast, surfaces with low wettability encourage de-wetting, a situation in which a liquid layer, once spread on a surface, forms discontinuities as the liquid “balls up” due to the interfacial tension being substantially higher than the substrate surface tension. 
     As seen in  FIG. 2A , substrate  20  includes a first region  22  having a first wettability and a second region  24  having a higher wettability than first region  22 . In the embodiment of  FIG. 2A , the first wettability region  22  includes surface protrusions  26  that act as a self-aligning feature for the die  10  as it is bonded to the substrate  20 . Although  FIG. 2A  shows protrusions  26  within region  22 , it is understood that protrusions can be formed in other regions of the substrate/dielectric carrier conductive layer depending upon the overall desired bonding configuration. 
     In  FIG. 2B , adhesive material  30  spreads due to applied pressure from the die  10 . As the adhesive material  30  spreads towards the second region  24  having the higher wettability, die  10  is drawn downward due to the wetting force, shown in  FIG. 2C . In  FIG. 2D , the die has been pulled down into contact with protrusions  26  to form a uniform and predefined bondline by wetting force and protrusions. 
     The first region  22  and the second region  24  each have a different wettability that can be from two different materials with different levels of wettability or can be from two of the same materials having different surface morphologies that create different wettability (or a combination of different materials and different morphologies), depending upon the amount of wettability contrast desired. The selected amount of wettability difference between region  22  and region  24  is determined by factors such as the size of the die, the desired thickness of the final bondline, the desired thermal conductivity and the operation parameters of the finished device. Typically, the difference in wettability is selected to be in a range on the order of 100% to 400% greater (in terms of the ratio of the higher contact angle to the lower contact angle for the same adhesive material on surfaces with different wettabilities), more particularly 300% to 400% Surface morphology variations can range from a rough surface in region  24  and a relatively smoother surface in region  22  to a micromachined surface having a surface structure featuring micropillars or microcolumns on the order of a micron (in addition to the larger surface protrusions  26 ). 
     When different materials are selected for region  22  and  24 , exemplary material combination includes ( 22 / 24 : Cu/Cu, Cu/Au, Cu/Ag, Ni/Au, Ni/Ag). Adhesive material  30  is selected based on the materials used for the bonding region. Typical adhesive material, region  22  and  24  combinations include adhesive: (SnAg, SAC, SnPb), 22 : Cu,  24 :(Cu, Au,Ag)). However, it is understood that any combination of materials can be used as long as the combination meets the conditions specified above. 
     Various other configurations can be formed according to the present invention, particularly additional structures that will assist in forming a reliable and uniform bondline and creating self-aligning features for the die being bonded to the substrate. As seen in  FIG. 3 , various cavity structures and combinations of regions with various wettabilities can be used to both confine the adhesive material and create an even and reliable bond line. In  FIG. 3A  a cavity  40  is defined in a lower wettability material  22  including the protrusions  26 . On the sidewalls  42  of cavity  40  a higher wettability material  24  is formed. As in the embodiment of  FIG. 2 , the die  10  is drawn downward by the wetting force, contacting protrusions  26 . 
     In  FIG. 3B , the higher wettability region  24  is formed in a central cavity with protrusions  26  also formed from the same higher wettability material  24 . To assist in confining the adhesive material  30  in the cavity, lower wettability sidewalls  22  are used to confine adhesive material  30 . As the material flows towards lower wettability sidewalls  22 , the contact angle increases and the material remains within cavity  40 . 
     In  FIG. 3C  a variant of the invention is shown in which the entire cavity, including protrusions  26 , is formed from the same material with a high wettability surface  24 . In this embodiment, the configuration of the cavity and the protrusions permits formation of an even bond line between die  10  and substrate  20 . For the purposes of this embodiment, a high wettability material  24  is defined as one in which the adhesive material  30  forms a contact angle with material  24  of less than approximately 30 degrees. 
       FIG. 4  depicts various simulated mechanical performance improvements due to the bonding system of the present invention. In  FIG. 4A , a 38% reduction in inelastic creep strain for a protrusion/column height of 8 mils is depicted. In  FIG. 4B , for an even bond line thickness of 250 microns the fatigue life is increased by a factor of three. 
     The die attachment system of the present invention has further applications in forming an internal electrical path between the die  10  and input-output points through a conductive portion without the need for additional wiring. Turning to  FIG. 5 , input-output points  50  are defined by insulating solder mask  60 . The solder mask is adjacent to die  10  which is set on protrusions  26  surrounded by high-wettability material region  24 . One or more electrical bridges is constructed across the conductive portions on the substrate  20  via the input-output points  50 . This is due to the fact that the bonded die  10  electrically contacts protrusions  26  to create an electrical path between the die and the input-output points  50  through the conductive portion formed on the substrate surface. Input-output points are connected to further elements typically through solder balls. Eliminating the need for additional wire bonding substantially reduces cost of the overall package as well as increasing performance, yield and reliability. 
       FIG. 6  depicts formation of cavities and protrusions for the bonding system of the present invention including optional alignment features/stops. In  FIG. 6A , cavities  40  are formed by half-etching into a metallic surface of substrate  20 . Simultaneously, protrusions/microcolumns  26  are defined in the cavity. In  FIG. 6B , high wettability material layer  24  is deposited. In  FIG. 6C , alignment features  70  are bonded in such a manner that they partially overhang the edges of cavities  40 . Bonding material  30  is positioned within cavities  40  in  FIG. 6D . In  FIG. 6E , dies  10  are placed and compressed guided by the alignment features  70  and the bond lines are solidified. 
       FIG. 7  illustrates a wide variety of protrusion/column  26  patterns that can be formed (in embodiments with or without optional alignment stops  70 .  FIG. 7B  depicts a corner pattern  FIG. 7C  depicts a matrix pattern,  FIG. 7D  depicts a matrix-asterisk pattern,  FIG. 7E  depicts a face-centered pattern,  FIG. 7F  depicts an asterisk pattern, and  FIG. 7G  depicts a cross pattern. Note that these are only some examples of the many patterns that can be formed; a particular pattern can be selected based on die size, adhesive material and wettability material considerations along with other manufacturing considerations. 
       FIG. 8  depicts various arrangements for alignment features/stops  70 .  FIG. 8B  is an edge-center arrangement of alignment features  70 ;  FIG. 8C  is a diagonal arrangement of alignment features  70 , and  8 D is a peripheral arrangement of alignment features  70 . 
     According to the present invention, micro-features compatible with DBC processes are formed to enhance reliability and maintain a high dielectric breakdown voltage and high thermal conductivity. A wetting enhancing surface treatment on copper is optionally used to achieve the new structures depicted in the FIGS. While the foregoing invention has been described with respect to various embodiments, such embodiments are not limiting. Numerous variations and modifications would be understood by those of ordinary skill in the art. Such variations and modifications are considered to be included within the scope of the following claims.