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.