Ion Implantation Dose Measurement
# Dose Measurement and Control: Step 5 — Precise Ion Counting and Integration
As the ion beam scans across the wafer, the total number of dopant ions reaching the wafer surface must be precisely measured and controlled. Step 5 implements dose monitoring — real-time measurement of ion current and integration to determine total dose (ions/cm²). The dose is one of the most critical parameters in ion implantation because dopant concentration in the wafer directly determines electrical properties. Too little dose produces insufficient dopant concentration; too much produces dopant precipitation or defects. Step 5 maintains dose uniformity across the wafer to within ±2–3%, ensuring every device receives the correct dopant concentration.
## 1. Faraday Cup and Current Integration
A Faraday cup — a metal electrode in the ion beam path — collects ions and measures the induced current. The current (in amperes) is the number of charges per second; integrating this current over implantation time gives total charge. Dividing by elementary charge (e = 1.6×10⁻¹⁹ C) gives total number of ions. Dividing by wafer area gives dose in ions/cm². Modern implanters use real-time dose feedback — the measured dose is compared continuously to target, and if the dose rate is drifting (due to source instability or beam-steering drift), the scanning parameters are adjusted to maintain constant dose accumulation.
## 2. Real Diagram: Dose Integration and Feedback Control
## 3. Dose Uniformity and Wafer-to-Wafer Repeatability
The dose must be uniform across the wafer (within ±2–3%) and repeatable from wafer to wafer (wafer-to-wafer variation <5%). This requires stable beam current, accurate dose integration, and feedback control. Some implanters use beam normalization — the ion source current is continuously monitored and regulated to stay constant, independent of temperature or gas pressure variations.
## 4. Dose Offset and Correction
Wafer holders and shadowing can cause some areas to receive slightly less ion flux. Step 5 includes dose correction algorithms — the scanning pattern is adjusted to deliver slightly higher dose to regions that naturally receive less, ensuring uniform final dose across the wafer.
## 5. Real Diagram: Faraday Cup and Current Integration
## Dose Measurement's Place in the Process Lineage
Dose measurement and control is step 5 of the ion implantation series — ensuring the correct total number of dopant ions reaches the wafer. Step 6 covers wafer handling and temperature management. Steps 7–8 continue through post-implant processing. Steps 9–25 cover implantation physics and annealing.