Angled (Tilt) Ion Implantation is the ion implant technique where the wafer is tilted relative to the ion beam by 7–45°, allowing dopants to be introduced beneath overhanging structures (such as gate electrodes) to form halo pocket implants, adjust lateral channel profiles, or control LDD extension geometries — a critical process for controlling short-channel effects, threshold voltage roll-off, and drain-induced barrier lowering (DIBL) in sub-100nm transistors.
Why Tilt Implant Is Used
- Vertical (0°) implant: Dopants land directly below beam direction — cannot reach under gate overhang.
- Tilt implant: Ion beam enters at an angle → some ions pass beneath the gate edge → reach channel region under the spacer.
- Applications requiring tilt:
- Halo/pocket implants: P-type dopant angled under gate edges for NMOS → raises VT locally near channel ends → suppresses short-channel effects.
- LDD extension: Light tilt ensures S/D extension junction is self-aligned to gate edge with controlled lateral straggle.
- Well engineering: Retrograde well profile formed by high-energy angled implant.
Halo Pocket Implant
- Purpose: Counter-dope channel edges near S/D with opposite polarity → raise VT near channel pinch-off points → reduce DIBL.
- NMOS halo: B or BF₂ at 7–30° tilt → p-type halo pockets just under gate edge → VT raised near source and drain.
- PMOS halo: As or P at tilt → n-type halos → same effect.
- Rotation: 4-rotation implant (0°, 90°, 180°, 270°) → symmetric halos on all four sides of gate (especially important for 2D gates).
- Impact: Can reduce DIBL from 100 mV/V to <30 mV/V for a given gate length.
Implant Tilt Angle Effects
| Tilt Angle | Shadow Under Gate | Lateral Straggle | Use |
|-----------|-----------------|-----------------|-----|
| 0° | None | Vertical only | Source/drain, deep implants |
| 7° | Small | Small | LDD extension |
| 15–25° | Moderate | Moderate | Halo pocket |
| 30–45° | Large | Large | Well punch-through stop |
Process Considerations for Tilt Implant
- Shadowing: At high tilt angles, gate electrode shadows the implant on one side → asymmetric halos → needs multi-rotation.
- Pattern dependency: Nearby structures can shadow implant in dense arrays → layout-dependent VT variation.
- Dose correction: At tilt angle θ, effective dose on vertical surface = D × cos(θ) → must adjust dose for equivalent horizontal dose.
- Wafer rotation: Mechanical rotation of wafer between implant passes → 2-rotation (symmetric S/D direction), 4-rotation (all directions), or continuous rotation.
Tilt Implant in FinFET
- FinFETs have 3D fins → tilt implant geometry changes dramatically:
- Fin sidewalls need different tilt angles than fin tops.
- Shadowing from adjacent fins in dense arrays → halo under-dose on shielded sides.
- Effective tilt implant into fin sidewalls at 45–90° from wafer normal required.
- Alternative: Anti-punch-through (APT) vertical implant into fin bottom before fin formation — eliminates tilt shadow issue.
LDD Extension Tilt
- NMOS LDD: P → AsH₃ or PH₃, 7° tilt, low energy (1–10 keV) → shallow junction just under spacer.
- Tilt ensures extension aligns self-consistently to gate edge → controlled overlap capacitance.
- At 28nm: Extension junction depth Xj ~ 8–12 nm → tilt angle chosen to minimize Xj while maintaining lateral overlap.
Boron Channeling and Tilt
- B along <100> crystal direction → channels deep if 0° implant and wafer aligned.
- Tilt to 7° off major crystal axis → breaks channeling → more reproducible junction depth.
- BF₂ alternative: Heavier molecular ion → inherently less channeling, even at low tilt.
Angled tilt implantation is the three-dimensional dopant engineering tool that allows transistor designers to sculpt charge distributions in regions inaccessible to vertical beams — by directing ions beneath gate overhangs to create halo pockets, control channel profiles, and suppress short-channel effects, tilt implant has been essential to maintaining electrostatic transistor control as gate lengths scaled from 250nm to 20nm over three decades of CMOS development.