Feature Article - Source and Drain Extensions, Part 2 - By Christopher Henderson

Let’s examine short-channel transistors and halo optimization more closely. In advanced technologies, we want to minimize the diffusion of the dopant atoms. This is especially important with the halo profile. One can improve the halo profile with a flash anneal. This is also important in mixed-signal, digital and RF analog applications. If the halo reaches to the surface, if can alter the effective channel length of the transistor, creating an effect like the reverse short channel effect. As the drain voltage increases, the depletion will spread beyond the surface halo, decreasing the threshold voltage and increasing the saturation drain current. This is similar to a short channel effect, but is instead observed in long channel transistors and degrades the Early voltage.

Another technique used in the lateral asymmetrical channel. This technique, where dopant atoms are implanted at just one fixed angle to form a single halo or pocket, increases the contribution associated with velocity overshoot because of the built-in field at the source end of the transistor. It does improve analog parameters though.

Let’s move on and discuss raised source/drain regions. This technology involves implanting lightly-doped drain regions or extensions to form spacers. The raised portion of the structure is created through selective epitaxial growth on the junction region. One then implants a dopant into this structure and drives it in using rapid thermal anneal. This diagram doesn’t show the salicide layer. This approach drives up the cost of the technology through extra steps, and it is not clear how well this will work for future transistor technologies. However, there are some significant advantages. The implant damage is confined to the epitaxial layer, and it eliminates transient-enhanced diffusion. The junction is formed by driving in the dopants from an elevated location, which helps prevent overdriving. There is also sacrificial silicon for the silicide reaction. It forms a borderless contact, and it reduces the contact aspect ratio.

Stay tuned for the conclusion of this article in November!