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.