Silicon carbide based power electronic systems offer significantly reduced size, weight and volume, a 65% volume reduction in a motor drive, for example, over other available systems. Because of the reliability of their device junctions, however, currently available SiC based electronic packages are generally limited to a junction operating temperature of less than about 150° C. These devices are typically assembled using conventional solder materials and techniques to form an electrically conductive path between metal contacts on package substrates. At higher temperatures and during temperature cycling, these soldered connections tend to be unstable, either from operation of the device or the surrounding environment in which the device operates. This causes voids and ultimately debonding in the interconnects, resulting in increased thermal resistance and unreliable operation of the electronics. Thermal resistance can be large due to thick (>1 mil) die attachments and inherently lower thermal conductivity (<40 W/m-K) of the attachment materials. Thermal resistance further increases dramatically during the life of the component due to voiding, which leads to increased device temperatures and accelerates failures. While brazing can form more reliable connections, the higher temperatures need to form the connection (700-1000° C.) can damage the components and devices being assembled. The requirements of high temperature operation, thermal and electrical conductivity, void and creep resistance, and corrosion resistance in devices with joints having low thermal resistance are often in conflict with each other when solder, binary compositions, or brazing materials are used.
TLP bonding can also be used to improve the performance of light-emitting devices. It lowers the device operating temperature due to lower thermal resistance of the bond, resulting in increased light output. It also results in increased mechanical robustness and maintenance of light output through the operating lifetime of the device due to minimal bond degradation during thermal and power cycling and extended operation.
An approach that has been suggested to provide connections between components that are stable at higher temperatures is a bonding process referred to as transient liquid phase (TLP) bonding, which can be accomplished at lower temperatures (less than about 300° C.). Transient liquid phase bonding starts with the use of a high melting point first metal as a contact. A film of a second metal of a much lower melting point is placed on the first metal. The two metals form a system, with the combination of the metals at specific concentrations having an elevated melting point greater than the melting points of the second metal. In U.S. Pat. No. 5,038,996 the metals are then heated to a temperature above the melting point of the mixture, causing an interdiffusion of the metals, forming a bond. The '996 patent discloses the formation of Pb—Sn or Sn—Bi TLP bonded connections between copper leads or surface pads.
U.S. Pat. No. 5,897,341 discloses methods for fabricating semiconductor devices, particularly multi-chip modules, including methods for forming interconnections between an integrated circuit chip and a substrate by solid-state diffusion bonding of dissimilar metals. In particular, a layer of cadmium, gallium, nickel, tin or zinc is formed over aluminum or aluminum coated pads by typical metal deposition techniques, such as evaporation, sputtering, CVD, electroplating or electroless plating. Bonding is then accomplished by solid-state diffusion at 100-150° C. to create a diffusion bond.
The preceding methods, as well as other methods in the prior art, lack the reliability and thermal resistance required for wide band gap (SiC and GaN) based power electronics.
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USPTO Backgrounds