Ion Implantation

The Ion Implantation Process

Ion implantation shoots dopant atoms into silicon at controlled energies and doses to precisely define doped regions. The process:

• Ion source: A gas (BF₃ for boron, PH₃ for phosphorus, AsH₃ for arsenic) is ionized to create a beam of dopant ions.
• Mass analyzer: A magnet separates ions by mass-to-charge ratio, selecting only the desired species (like a mass spectrometer).
• Accelerator: Electric fields accelerate ions to precise energies (1 keV to 3 MeV), determining how deep they penetrate.
• Scanning: The beam is scanned across the wafer (or the wafer is scanned through the beam) for uniform coverage.

Key control parameters:

• Dose: Number of ions per cm² (typically 10¹¹ to 10¹⁶ cm⁻²). Controls peak concentration.
• Energy: Determines implant depth. Higher energy = deeper penetration.
• Tilt/twist angle: Controls implant direction to avoid channeling along crystal planes.

Channeling and Crystal Damage

Two important effects during implantation:

• Channeling: If ions enter along a crystal axis, they can travel much deeper than intended by "channeling" between rows of atoms. Prevented by tilting the wafer 7° off-axis and/or implanting through a thin screen oxide.
• Crystal damage: Energetic ions displace silicon atoms, creating point defects and even amorphizing the silicon at high doses. This damage must be repaired by annealing.