ion implant species
**Ion Implant Species Selection** is the **choice of which dopant ion (B, BF2, P, As, In, Sb) to implant at each step of the CMOS process** — where the ion's mass, range, diffusivity, and activation behavior determine junction depth, doping profile shape, and electrical characteristics of every transistor region.
**Common Implant Species**
| Species | Mass (amu) | Type | Range in Si | Typical Use |
|---------|-----------|------|-------------|-------------|
| B (Boron) | 11 | p-type | Deep (light ion) | P-well, deep S/D |
| BF2 | 49 | p-type | Shallow (heavy ion) | Ultra-shallow PMOS extension |
| P (Phosphorus) | 31 | n-type | Medium | N-well, NMOS S/D |
| As (Arsenic) | 75 | n-type | Shallow (heavy) | NMOS extension, shallow junction |
| In (Indium) | 115 | p-type | Very shallow | Halo/pocket implant, Vt adjust |
| Sb (Antimony) | 121 | n-type | Very shallow | Retrograde well, buried channel |
**Why BF2 Instead of B?**
- BF2 molecule is 4.5x heavier than B → stops much shallower in Si at same energy.
- At 5 keV: B range ~25 nm, BF2 effective B range ~8 nm.
- For ultra-shallow PMOS extensions (Xj < 15 nm), BF2 is essential.
- F released from BF2 can enhance B activation and reduce transient enhanced diffusion (TED).
**Why As Instead of P for NMOS?**
- As is 2.4x heavier than P → shallower junctions at same energy.
- As has much lower diffusivity in Si — stays where you implant it.
- P preferred for deep n-well (needs to penetrate > 1 μm).
- As preferred for shallow source/drain extensions.
**Ion Implant Parameters**
- **Energy**: Determines implant depth. Higher energy = deeper. Range: 0.2 keV (ultra-shallow) to 3 MeV (deep well).
- **Dose**: Number of ions per cm². Range: 10¹¹ (Vt adjust) to 10¹⁶ (heavy S/D doping).
- **Tilt/Twist**: Angle of implant relative to wafer normal — avoids channeling in crystal lattice.
- **Channeling**: Light ions (B) can travel along crystal channels → deeper than expected. Mitigated by pre-amorphization implant (PAI) with Ge or Si.
**Advanced Implant Techniques**
- **Plasma Doping (PLAD)**: For ultra-shallow, conformal doping of 3D structures (fins, nanosheets).
- **Molecular Beam Implant**: Low-energy implant using molecular ions for shallow junction.
- **Cold Implant**: Wafer cooled during implant to suppress amorphization recovery.
Ion implant species selection is **a foundational process engineering decision** — the choice between B vs. BF2, or P vs. As, at each implant step determines the doping profile that controls threshold voltage, junction leakage, and parasitic resistance for every transistor on the chip.