ion implantation semiconductor
**Ion Implantation** is the **CMOS doping technique that accelerates ionized dopant atoms (boron, phosphorus, arsenic, or other species) to precise energies (0.2 keV to 3 MeV) and drives them into the silicon substrate at controlled doses (10¹¹ to 10¹⁶ atoms/cm²) — enabling exact placement of dopant profiles that control transistor threshold voltage, well doping, channel doping, halo/pocket implants, and latch-up prevention, with implantation being one of the few processes that provides true three-dimensional control of dopant concentration versus depth in the silicon**.
**Ion Implantation Physics**
- An ion source ionizes the dopant gas (BF₃ for boron, AsH₃ for arsenic, PH₃ for phosphorus).
- Ions are extracted, mass-separated by a magnetic analyzer (selects only the desired isotope), and accelerated to the target energy.
- The ion beam scans across the wafer (electrostatic or mechanical scanning) for uniform dose delivery.
- Ions penetrate into Si and lose energy through nuclear collisions (displacing Si atoms — creating crystal damage) and electronic stopping (exciting electrons without displacing atoms).
**Key Parameters**
- **Energy**: Determines implant depth (projected range, Rp). Low energy (0.2-5 keV): ultra-shallow junctions for S/D extensions. High energy (200 keV-3 MeV): deep well implants.
- **Dose**: Total number of ions per unit area (atoms/cm²). Controls peak concentration. Dose uniformity: ±0.5% across 300 mm wafer.
- **Tilt/Twist**: Angle of incidence relative to wafer surface/crystal planes. Typical: 7° tilt to avoid channeling (ions traveling along crystal planes penetrate much deeper than predicted by amorphous stopping models).
- **Beam Current**: Determines throughput. High-current implanters: 5-25 mA for high-dose implants (S/D). Medium-current: 0.1-5 mA for precision implants (Vth adjust).
**Implant Applications in CMOS**
| Implant | Species | Energy | Dose | Purpose |
|---------|---------|--------|------|---------|
| Well implant | P (n-well), B (p-well) | 200-500 keV | 10¹³ cm⁻² | Define n/p-type tubs |
| Channel/Vth adjust | B, BF₂, As | 5-30 keV | 10¹²-10¹³ cm⁻² | Set Vth precisely |
| S/D extension | BF₂, As, P | 0.5-5 keV | 10¹⁴-10¹⁵ cm⁻² | Ultra-shallow S/D |
| S/D deep | As, P, B | 10-50 keV | 10¹⁵-10¹⁶ cm⁻² | Low-resistance S/D |
| Halo/pocket | B, In (NMOS), As, Sb (PMOS) | 20-80 keV | 10¹³ cm⁻² | Control short-channel effects |
| PAI (pre-amorphization) | Ge, Si | 10-40 keV | 10¹⁴-10¹⁵ cm⁻² | Amorphize Si to prevent channeling |
**Ultra-Low Energy Challenges**
At advanced nodes, S/D extension implants require energy <1 keV for junction depth <10 nm:
- Space charge: At low energy, mutual repulsion of ions in the beam causes "beam blow-up" — loss of beam quality and uniformity.
- Molecular implants: Implant BF₂⁺ (49 amu) instead of B⁺ (11 amu). At the same total energy, B atoms enter with only 11/49 of the energy — effective B energy is 4.5× lower.
- **Plasma Doping (PLAD)**: Instead of a focused beam, immerse the wafer in a BF₃ or AsH₃ plasma and apply a negative bias to the wafer. Ions are accelerated directly into the surface from all angles. Achieves ultra-shallow profiles at very high dose rates. Used for FinFET conformal doping.
Ion Implantation is **the precision dopant delivery system of semiconductor manufacturing** — the process that determines the electrical character of every region of silicon in a chip, from the deep wells that separate circuit blocks to the ultra-shallow junctions that define transistor switching speed and leakage current.