Suppression of oxidation-enhanced boron diffusion in silicon by carbon implantation and characterization of MOSFETs with carbon-implanted channels

Abstract

In NMOS transistors with boron-doped channels, Oxdation-Enhanced Diffusion (OED) is a key contributor to boron profile broadening. Starting with the arguments presented in several recent reports on the role of carbon in silicon as a sink for self-interstitials, we have explored the feasibility of using carbon in the Metal Oxide Silicon Field Effect Transistor (MOSFET) active region to retard boron diffusion during gate oxidation. A highly effective suppression of OED of boron was observed providing more than an order of magnitude reduction in boron diffusivity. MOSFETs with carbon- and boron-implanted channels have been fabricated to evaluate the impact of carbon on the electrical properties of Si. Boron diffusion, activation, and critical electrical parameters including subthreshold swing, threshold voltage, off-state leakage current, and channel mobility have been evaluated as a function of the carbon dose. While our results show that carbon can effectively suppress boron diffusion daring gate oxidation, carbon can also lead to poor boron activation and degradation in MOSFET performance when carbon dose levels above a threshold of /spl sim/10/sup 14/ cm/sup -2/ are utilized. Our results, however, indicate considerable improvement in boron activation with increases in the thermal budget. We show that if carbon implantation damage is annealed out prior to boron implantation, not only is boron activation improved, but carbon continues to serve as a sink for self-interstitials, thereby effectively suppressing OED.