Ion Implantation Acceleration Voltage Control

# Acceleration and Voltage Control: Step 2 — Energy Selection and Beam Formation

The ion source creates dopant ions; Step 2 accelerates them to the energy required for implantation. A high-voltage acceleration stage applies a controlled potential (typically 10 keV to 1 MeV, depending on desired implantation depth) between the ion extraction electrode and the wafer. An ion of charge Q accelerated through potential V gains kinetic energy:

E = Q \cdot V \quad \text{(in eV)}

A phosphorus ion P⁺ accelerated through 100 kV gains 100 keV. A boron ion B⁺ accelerated through 50 kV gains 50 keV. The accelerating voltage directly controls the implantation depth — ions with higher energy penetrate deeper into the crystal before being stopped by collisions with lattice atoms. Step 2 establishes the voltage setpoint and maintains it stable to within ±0.1–0.5%, ensuring that the implantation depth profile is reproducible from wafer to wafer.

## 1. Acceleration Column Design and High-Voltage Stability

v = \sqrt{\frac{2 Q V}{m}} \quad \text{(final velocity after acceleration)}

The acceleration column consists of a series of electrodes at progressively lower potential, creating a uniform electric field that accelerates ions from the extraction voltage (5–20 kV) to the implantation energy (typically another 50–900 kV). The acceleration column must maintain high voltage stability because voltage ripple directly translates to energy spread in the ion beam. A ±1% ripple in the 100 kV acceleration stage produces a ±1 keV energy spread, which causes a ±2–3% uncertainty in implantation depth. Modern implanters use high-voltage power supplies with feedback regulation to keep ripple <0.05%, and acceleration tubes shielded from electromagnetic noise.

## 2. Real Diagram: Acceleration Column and Ion Path Control

## 3. Beam Energy Selection for Different Dopants

Different dopants require different acceleration voltages to achieve the same implantation depth. A heavier ion (arsenic, antimony) at the same voltage penetrates less deeply than a lighter ion (boron, phosphorus) because it loses energy faster through collisions. Conversely, to implant heavy dopants to the same depth as light dopants, higher voltage is required. Step 2 must accommodate this by allowing flexible voltage setting: a production implanter may run 30 keV phosphorus one hour, then switch to 50 keV boron the next hour, with the voltage regulation automatically adjusting to maintain ±0.1% stability at each setpoint.

## 4. Beam Current and Dose Rate

As ions are accelerated and drift toward the wafer, they form a beam of width typically 5–20 cm. The beam current (measured in µA to mA) determines the implantation rate — how many dopant atoms per unit time reach the wafer. For a given wafer area and desired dopant dose (typically 10¹² to 10¹⁶ ions/cm²), the implantation time is:

t_{\text{implant}} = \frac{\text{dose} \times \text{wafer area}}{\text{beam current}}

Higher beam current reduces implantation time. Step 2's voltage control must maintain stability even as beam current varies, because voltage regulation becomes more difficult at very low or very high current.

## 5. Real Diagram: Acceleration Column Design and High-Voltage Stability

## Acceleration and Voltage Control's Place in the Process Lineage

Acceleration and voltage control is step 2 of the ion implantation series — establishing the energy of dopant ions and the implantation depth. Step 3 covers mass separation and beam purity. Steps 4–8 continue through beam scanning, dose control, and wafer handling. Steps 9–25 cover implantation physics and thermal annealing.

Ion Implantation Acceleration Voltage Control

Take ion implantation acceleration voltage control further