Integrated circuits and discrete components are formed on wafers that are sawed into individual chips often referred to as dies. The dies are mounted in sealed packages having standard pin arrays that connect the die to a larger circuit. One or more of the dies may be bonded on a package substrate, referred to as a chip carrier, using a layer of conductive material known as a “die attach layer”.
The die attach process can materially affect both the performance and reliability of the packaged semiconductor component. The die, the substrate and the die attach layer are normally formed from materials that have different thermal coefficients of expansion (TCE). Mismatches in the TCE of the die, substrate and attach layer may cause mechanical stresses on the bond, especially in those applications involving high-power semiconductor components where higher current flows produce higher levels of heat, or in applications where performance is required over a wide range of temperatures. These mechanical stresses may cause cracks and/or delaminations between the die, attach layer and substrate, resulting in device detachment or die cracking.
In order to reduce the problems caused by mismatch of the TCEs, customized alloys have been formulated for use as the die attach layer in order to better match the TCE between the die and the die attach, as well as between the die attach and the chip carrier package. This metallurgical solution is not entirely satisfactory for at least two reasons. First, the TCE of the die and the package substrate change with temperature, often nonlinearly, in a manner that is specific to each element (the die or the package substrate). Further, the customized TCE of the specially formulated die attach layer also changes with temperature in a manner different from either the die or package substrate. Accordingly, it may not be practical to match the TCEs of the elements using a single alloy. The selection of a suitable alloy for use as the die attach is made more challenging by the fact that it is necessary to limit the selection of an alloy to those that have working temperatures in a range that will not result in damage to the die. Die attach materials with high working temperatures may require processes that diminish die reliability, or damage the die, particularly the sensitive upper oxide layer on the die.
Second, the die and the package substrate require surface metallization layers that adhere strongly to the semiconductor die, and the package substrate, which is typically ceramic. High operating temperatures may cause the die attach layer to interdiffuse with these surface metallization layers. This natural diffusion process effectively mixes the interfacial materials, thereby changing the composition of the customized die attach alloy, as well as its thermal and mechanical properties.
Accordingly, there is a need for a die attach that reduces or eliminates the need for specialized metal alloys and overcomes the problems associated with interfacial diffusion of the materials. The disclosed embodiments are intended to satisfy this need.