LOW PARASITIC RESISTANCE CONTACTS FOR SCALED ULSI DEVICES

Analysis of the components of parasitic series resistance in ULSI devi ces shows that interfacial contact resistivities less than 10(-7) Ohm cm(2) will be required for sub 100-nm ULSI devices in order to stay on the historical performance trend. With dimensional scaling, the serie s resistance-width product decreases because channel lengths are scale d, while it increases in contacts because the contact length is decrea sed. Unless the contact resistivity is also reduced, the contact resis tance ultimately becomes higher than the channel resistance, and no pe rformance advantage will be obtained by making the device smaller. The challenge in meeting the contacting requirements in the 1997 National Technology Roadmap for Semiconductors is especially difficult in ligh t of the desire to simultaneously contact both n(+) and p(+) junctions with a single material and given the trend towards lower processing t emperatures, in which the equilibrium dopant electrical activity is lo wer. Several techniques, such as dielectric capping during junction an nealing, are effective in reducing contact resistivity by maximizing i nterfacial dopant concentrations and minimizing contact barrier height s. Higher saturated drive currents, due to lowered parasitic series re sistance, are observed in deep submicron devices made using silicides as diffusion sources (SADS); this technique eliminates the interfacial dopant segregation that is associated with conventional silicidation. The use of elevated source drains (ESD) also allows the use of thicke r silicides while minimizing the consumption-induced increase in conta ct resistivity that normally accompanies silicidation; as a result, ES D devices give higher drive currents. The recrystallization of amorpho us layers has been observed to result in non-equilibrium dopant activa tion which can be many times the equilibrium value. Finally, the use o f heterojunction contacts using Si-Ge in the context of elevated sourc e/drain devices presents another way to achieve lower contact resistan ce. (C) 1998 Elsevier Science S.A. All rights reserved.