Amorphous metallic thin films as copper diffusion barrier for advanced interconnect applications

Cu metallization has been widely applied in back-end-of-line (BEOL) of integrated circuit fabrication, as well as in advanced packaging such as 3-D interconnects and through silicon via (TSV). As the device feature size further shrinks copper diffusion barrier layers with high conductivity, good thermal stability and low Cu diffusion coefficient are to be developed. Amorphous metal alloy films of Ta-transition metal (TM = Ni, Cr, Ti) are proposed and examined as a potential copper diffusion barrier. All Ta-TM films deposited on Si showed lower resistivity compared to the conventional Ta nitride films. Ta-Ni films containing up to 86 wt% Ta were found to have as-deposited amorphous phase. Ta-Cr films also contained glassy phase in all studied composition range. However, crystalline phase was observed in the as-deposited Ta-Ti films. The glassy Ta-Ni thin films showed high stability up to 800°C. Beyond this temperature, crystallization of Ta, Ni3Si2, Ta2O5 and Ta5Si3 were detected. As-deposited glassy Ta-Cr also maintained the amorphous phase up to 800°C, with Ta2O5 and Ta crystalline peak observed. For Ta-Ti films, a solid phase amorphourization was observed when films were annealed at 600°C. The amorphous phase was stable up to 800°C, with TaxTi1−xO2 crystalline phase has appeared. Therefore it is concluded that Ta-Cr and Ta-Ni has higher glass forming ability and higher thermal stability compared to Ta-Ti films. Copper diffusion barrier performance of Ta-TM films were studied on Cu/Ta-TM/Si stack at different temperatures, ranging from 600 to 800°C. For Cu/Ta-Ti/Si, Solid phase amorphourization of Ta-Ti at 600°C was observed. No Cu3Si peaks were observed for all samples until 700°C. XRD study showed that at 700°C, fast reaction between Cu and Si was observed in Ta-Cr and Ta-Ti barriers, while very low Cu3Si peak could be observed in Ta-Ni barrier. TEM observation showed that Ta-Ti films lost continuity while Ta-Cr and Ta-Ni still maintained integrity at 700°C. It is therefore concluded that the studied Ta-TM binary metallic thin films can be applied as a good amorphous copper diffusion barrier, with low electrical resistivity, high thermal stability and good copper diffusion retardation performance. In order to reduce the diffusion rate of Cu in barrier, Ta-Ni barrier is preferred, followed by Ta-Cr and Ta-Ti.