NMOS Ring oscillators with cobalt-silicided P-diffused shallow junctions formed during the "Poly-plug" contact doping cycle

Composite silicided source-drains are being developed to provide low-resistance shallow junctions for high performance fine-line circuits. The junctions are usually formed either by implantation and drive prior to silicide formation or else by implantation immediately after, followed by a heat cycle. This paper describes a novel approach for the fabrication of CoSi2/n⁺-p junctions (2 . 5 Ω/□ sheet resistance), wherein the junctions are doped by diffusion through the contact windows using the conventional "poly-plug" doping cycle [1], [2]. LPCVD poly-Si is deposited on windows to previously silicided gate and source-drain regions, and exposed to PBr3at an elevated temperature. Since the diffusivity of dopants in silicides is higher than in bulk Si, this step transports the P through the poly-Si via the windows laterally into the silicide, to form uniformly doped junction surrounds. This poly-Si doping scheme for junction fabrication eliminates an ion-implant step, provides an independent means of tailoring channel length, and can potentially result in low-resistance contacts even if the window etch step has punched through the silicide, Electrical characteristics of 1.25-µm gate-length ring oscillators are similar to those of circuits processed with the conventional As implant and drive. Transistor \´s and subthreshold behavior remain unaffected by the silicide doping process. Junction depth and leakage are sensitive functions of the poly-plug thermal cycle, with a 950°C 30-min drive resulting in 0.3-µm junctions. For a 1-µm design rule circuit layout, 30 to 45 min at 950°C is judged adequate.