Dual Work Function Metal Gate Technology for Future CMOS Devices

Polycrystalline silicon (poly-Si) has been used as the gate material for MOSFETs for several decades. This is because it is highly compatible with CMOS processing, and its work function can be selectively modified by ion implantation of the appropriate dopants. The gate-depletion effect, which increases the equivalent SiO2 thickness (EOT) of the gate dielectric by several Angstroms and thereby degrades transistor performance, becomes a significant issue for poly-Si gate technology as the EOT is scaled to 1 nm and below in sub-50 nm devices, however. Because of this, alternative gate-electrode materials which can either reduce or eliminate the gate depletion effect have been investigated by many researchers. While refractory metals and their metallic derivatives are attractive candidates, there are numerous process integration challenges that must be overcome before a viable metal-gate CMOS technology can be developed. This paper highlights some of these challenges and reviews three different process integration schemes.