Semiconductor devices, and in particular InP-based laser devices, are usually bonded on a mounting plate (called a submount or heatsink) or directly onto a package. This bonding assembly, which comprises the die-bonding metallic layers, the joint solder and the submount, serves the purpose of heat dissipator, mechanical support and electrical conductor. As such, the quality of both the bonding metallization and the submount are as important as the device die itself to assure the short- and long-term reliable operation of the electronic assembly. Due to its high thermal conductivity, diamond has been used as a very efficient thermal conductor and dissipator under electronic and photonic devices. This application has become even more attractive since the commercialization of the chemically vapor-deposited (CVD)-diamond, due to its lower cost and larger available surface area compared to natural diamond. Therefore it is only natural to use CVD-diamond as the material of choice for mass production of high-power InP-based laser diode submounts. The device is attached to the submount by a metallic bonding medium that contains a few layers, such as an adhesion layer (typically of titanium (Ti) adjacent to the submount), a barrier layer (typically of platinum (Pt)) and capped with hard solder (such as gold-tin (AuSn) eutectic alloy, which has a melting temperature of 278-degrees-C). While offering optimum bonding conditions, the Ti/Pt/AuSn bonding system provides a high quality bond of the laser diode to the CVD-diamond submount. However, the extensive reaction of the AuSn solder with the Pt and Ti layers, even after short bonding periods of 5-10 s, may lead to mechanical deterioration of the bonded assembly, resulting in delamination of the metal and failure of the bond. This failure occurs mainly due to the thermodynamically reactive nature of the Pt-Sn couple, which reacts almost spontaneously even at room temperature. The reaction consumes the Pt layer and an appreciable amount of Sn, leading to the disappearance of the barrier layer and to dilution of Sn from the solder. The variation in the molten solder stoichiometry results in premature solidification of the solder through the bonding cycle after consumption of the entire Pt barrier layer, and dissolution of the Ti adhesion layer, as well. In order to maintain the good wetting performance of the AuSn solder to the Ti/Pt metals, but yet to improve the thermodynamic stability of the metallurgical bonding system, various other metals such as Ni, W, Cr and some of their alloys were evaluated as advanced alternatives to the Pt barrier layer. The quality of the evaluated metallurgical systems was judged upon wetting performance, thermodynamic stability and lack of premature freezing of the molten solder. The Ti/W/W(Ni3Sn4)/Ni3Sn4/Au multilayered structure was finally defined as the optimal and superior metallurgical scheme for the purpose of bonding laser chip to CVD-diamond submount. The time required until the first surface local freezing phenomenon was observed at the AuSn solder (with eutectic composition of 80 wt.% Au) on T/W/W(Ni3Sn4)/Ni3Sn4/Au structure while melted at 320-degrees-C was 200 s, compared to 5-10 s at the Ti/Pt/Au/AuSn system. At the former system the solder was maintained melted at 320-degrees-C for more than 1 h before it was completely frozen, which is much longer than the 30 s measured for the latter system. It was also observed that not only did the AuSn solder on the Ti/W/W(Ni3Sn4)/Ni3Sn4 Multilayer maintain its accurate stoichiometric composition through the entire bonding cycle, but it also provided excellent adhesion and integrity for the entire bonded assembly.
