Work function controllability of metal gates made by interdiffusing metal stacks with low and high work functions

Work function (ϕm) of metal transistor gates was controlled by using Al-Ni and Ni-Ta alloys made through an interdiffusion process, in which stacks of different metals were annealed. In addition to conventional capacitance-voltage measurements for ϕm evaluation, ϕm uniformity was also measured microscopically. The average ϕm of the Al-Ni alloy was successfully changed compared to that of pure Al by interdiffusion at 500 °C. The ϕm nonuniformity of Al-Ni, however, was problematic. Though Ni-Ta alloys required interdiffusion at a higher temperature of 700 °C, the Ni-Ta alloys had better ϕm uniformity. Thus, the Ni-Ta alloy is a more suitable candidate for use in metal gates of high-integrity transistors.