Au/(Ti—W) and Au/Cr metallization of chemically vapor-deposited diamond substrates for multichip module applications

Abstract

Since diamond obtained by chemical vapor deposition (CVD) has an extremely high thermal conductivity, it holds great promise in solving thermal management problems in high performance multichip modules (MCMs). Consequently, there is a need to develop a reliable metallization system for CVD diamond. Refractory metals such as Ti, Mo, Ta and W are known to form adhering carbide layers at high temperatures. Also, transition metals such as Cr, Ni and Ni—Cr are widely used in other MCM technologies involving Si, AlN, SiC and alumina substrates. In the work reported here, adherent Au/Cr and Au/(Ti—W) metallization systems were produced at low temperatures using d.c. magnetron sputtering and electron beam evaporation techniques. Adhesion at low temperature is essential since CVD diamond could lose its thermal and electrical properties at high temperatures. Furthermore, interaction between metal layers may cause an increase in conductor trace resistivity and delamination. Adhesion was measured using a Sebastian V-A thin film stud pull tester. The deposition parameters were optimized to give maximum adhesion using a statistical design software package, Echip. In the case of the sputtered metallization, pre-sputter cleaning of diamond surface improved adhesion significantly. Values above 9 klbf in−2 were obtained in the case of Au/(Ti—W) and 11.8 klbf in−2 in the case of Au/Cr. Post-deposition annealing was performed in nitrogen ambient to investigate the effect of post-metallization processing on adhesion and also to test for any possible interaction between the metals at high temperatures. Annealing temperatures were limited to 450 °C since MCM substrates are seldom exposed to temperatures higher than these. Energy-dispersive spectroscopy (EDS) analysis indicated outdiffusion of W through Au at 400 °C. No interdiffusion was observed in the case of Au/Cr as per optical microscopy and EDS analysis. Auger electron spectroscopy results indicate interaction between the metals in both Au/(Ti—W) and Au/Cr metallizations at 450 °C.