Fluxless Bonding Technique of Diamond to Copper Using Silver-Indium Multilayer Structure

In this study we report on successful bonding of chemical vapor deposition (CVD) grown diamond to Cu using a multi-layer Ag-In structure. To manage the large coefficient of thermal expansion (CTE) mismatch between copper and diamond, Ag-rich Ag-In solution is chosen as the final phase in joint. In our previous investigations, we have shown that Ag-In solid solution exhibit superior mechanical properties, such as low yield strength, high tensile strength, and large elongation. Here, we show that by using a fluxless process at vacuum, mechanically robust joints can be formed at 180 °C between copper and diamond. Numerous samples that were bonded with proposed structure show acceptable shear strength and by performing a post bond annealing at 250 °C for 192 hours, we were able to achieve a joint almost fully composed of Ag solid solution with In, with significantly increased shear strength. The deposited multi-layer structure is examined using scanning electron microscopy (SEM) coupled with focused ion beam (FIB) prior to bonding. Following the bonding, samples are sheared and fracture surfaces are examined using energy dispersive X-ray spectroscopy (EDX). Our studies show that Cr/diamond interface, which is the metallization scheme on diamond is a weak interface in the bond design and as the joint becomes stronger by conversion of Ag-In intermetallic compounds into (Ag), more delamination occurs in the Cr/diamond interface. Additionally, it is reported that annealing the Cr/diamond interface can effectively improve its adhesion.