super steep retrograde (ssr) well

**Super Steep Retrograde (SSR) Well** is an **extreme version of the retrograde well profile** — with an abrupt transition from near-intrinsic surface doping to heavily doped subsurface, providing the ultimate balance of high mobility (undoped surface) and strong SCE control (deep doping step). **What Is SSR?** - **Profile**: Nearly undoped channel (surface ~$10^{15}$ cm$^{-3}$) with an abrupt step to high doping (~$10^{18}$ cm$^{-3}$) at a depth of ~50-100 nm. - **Challenge**: Achieving a steep profile requires minimizing dopant diffusion during subsequent thermal steps. - **Formation**: High-energy implant + minimal thermal budget. Carbon co-implant can suppress boron diffusion. **Why It Matters** - **Best of Both Worlds**: Maximum carrier mobility (undoped channel) + maximum SCE control (deep doping wall). - **SRAM**: Excellent for SRAM cells where both speed and low $V_t$ variability are needed. - **Precursor to Undoped Channels**: SSR represents the conceptual bridge to fully undoped channels used in FinFET/FD-SOI. **SSR Well** is **the cliff edge doping profile** — an abrupt subsurface wall of dopants that catches punch-through while leaving the channel surface pristinely undoped.