Giin-Shan Chen¹, Jui-Cheng Chen¹, Jau-Shiung Fang², Yi-Lung Cheng^3
1Feng Chia University, Taichung, Taiwan
²National Formosa University, Yunlin, Taiwan
³National Chi Nan University, Nantou, Taiwan
The implementation of thin films of cobalt (Co) as an alternative to copper (Cu) for interconnect metallization in advanced microchips faces significant challenges due to its inherently high resistivity. This study explores the feasibility of using a cosputtering process to fabricate thin films of Cu-added Co in achieving an electrical resistivity comparable to the bulk value of Co (6.2 µΩ-cm). Three different Co alloy films (typically 60 nm in thickness) with controlled Cu amounts of 0.7, 0.9 and 3.1 at%, denoted CoCu_0.7, CoCu_0.9, CoCu_3.1, were fabricated using unalloyed Co films as a reference. It is found that only the thin CoCu0.9 film (after 600°C annealing) exhibits a minimum resistivity that is unachievable for unalloyed Co, CoCu_0.7 and CoCu_3.1 films. The CoCu_0.9 film has the lowest resistivity of 6.9 µΩ-cm, which is substantially smaller than 10.1 µΩ-cm of the Co film, representing 46% resistivity reduction as compared to the unalloyed CO film; this value is also much smaller than 7.4 and 8.6 µΩ-cm of the CoCu9_0.7 and CoCu_3.1 films, respectively. The mechanism for the resistivity minimum, as observed from the Co film with the optimally added Cu of 0.9 at% will be clarified by in-depth synchrotron X-ray diffraction (XRD) and analytic transmission electron microscopy (XRD) from the viewpoints of grain-texture and phase-structure variations. Moreover, electromigration testing of CoCu_0.9 interconnect lines, using Co lines as a control, reveals that the added Cu greatly extends the electromigration lifetime of Co. The strengthening mechanism for the electromigration mitigation will be elucidated from the results of XRD, TEM and nanoscratch testing, focusing on the effect of the added Cu on the enhancement of film mechanical properties, the retardation of interfacial diffusion/reaction and the refinement of Co grain structure.