diffusion process semiconductor

**Diffusion** — the thermal process by which dopant atoms migrate into a semiconductor lattice driven by concentration gradients, historically the primary doping method before ion implantation. **Physics** - Atoms move from high concentration to low concentration (Fick's Law) - Diffusion coefficient: $D = D_0 \exp(-E_a / kT)$ — exponentially dependent on temperature - Typical temperatures: 900–1100°C - Diffusion depth: $\sqrt{Dt}$ (proportional to square root of time × diffusivity) **Two-Step Process** 1. **Pre-deposition**: Expose wafer surface to dopant source at constant surface concentration. Creates a shallow, heavily doped layer 2. **Drive-in**: Heat wafer without dopant source. Dopants redistribute deeper into the silicon with Gaussian profile **Dopant Sources** - Gas phase: PH₃ (phosphorus), B₂H₆ (boron), AsH₃ (arsenic) - Solid sources: Spin-on dopants, doped oxide layers **Modern Role** - Ion implantation replaced diffusion for primary doping (better depth/dose control) - Diffusion still occurs during every high-temperature step (anneal, oxidation) - Thermal budget management: Minimize total heat exposure to prevent unwanted dopant spreading - At advanced nodes: Even a few nanometers of unintended diffusion can ruin a transistor **Diffusion** is a fundamental transport mechanism that chip designers must carefully control throughout the entire fabrication process.