Ion Implantation Beam Scanning
# Beam Scanning and Spatial Distribution: Step 4 — Uniform Implantation Across Wafer
The ion beam emerging from the accelerator and mass analyzer is typically narrow — only a few centimeters wide. Step 4 distributes this narrow beam across the entire wafer (10–30 cm diameter) through beam scanning — electromagnetically deflecting the beam back and forth, allowing it to cover the entire wafer surface. Two scanning modes are common: raster scanning (scanning in parallel lines, like a television) and circular scanning (rotating the beam in concentric circles). The scanning system ensures that every point on the wafer receives the same implant dose, within tight tolerances (typically ±2–5%).
## 1. Raster and Circular Scanning Techniques
In raster scanning, the beam is swept horizontally across the wafer (using a fast-deflecting coil), then stepped vertically (using a slow-stepping coil), then swept horizontally again in the opposite direction, covering the wafer in horizontal strips. The dwell time at each line and the line spacing are adjusted so that the dose accumulated at each point equals the target. In circular scanning, the beam is rotated in a spiral or circular pattern, accumulating dose as it sweeps. Circular scanning is often faster and more uniform for large wafers because the beam reaches all points symmetrically.
## 2. Real Diagram: Raster vs. Circular Scanning and Dose Uniformity
## 3. Edge Effects and Dose Correction
The wafer edges receive fewer ions than the center because the scanning pattern may not extend fully to the edge, or the beam cannot be focused sharply at extreme angles. Advanced scanning systems use edge-correction algorithms — the scanning pattern is adjusted near the wafer edges to increase dwell time and compensate for geometric factors, ensuring uniform dose at the edges.
## 4. Beam Current Stability During Scanning
As the beam is scanned, the current measured at the wafer may fluctuate due to geometric factors (beam spot size changes with deflection angle) or source instability. Step 4 includes feedback control — the measured beam current is continuously monitored, and if it deviates from target, the scanning parameters are automatically adjusted to maintain constant dose rate.
## 5. Real Diagram: Raster and Circular Scanning Techniques
## Beam Scanning's Place in the Process Lineage
Beam scanning is step 4 of the ion implantation series — distributing the narrow ion beam uniformly across the wafer. Step 5 covers dose measurement and control. Steps 6–8 continue through wafer handling and thermal processing. Steps 9–25 cover implantation physics and annealing.