A VI (Voltage-Current) probe is a diagnostic sensor that measures the RF voltage and current waveforms at the input to a plasma chamber, enabling determination of plasma impedance, delivered power, and harmonic content. It is the standard tool for monitoring and controlling RF power delivery in plasma processing.

What a VI Probe Measures

• RF Voltage (V): The peak-to-peak or RMS voltage of the RF signal driving the plasma. Measured using a capacitive voltage divider.
• RF Current (I): The current flowing to the electrode/plasma. Measured using a current transformer or Rogowski coil.
• Phase Angle (φ): The phase relationship between voltage and current — determines how much power is absorbed by the plasma vs. reflected.

Derived Parameters

• Impedance: $Z = V/I$ — the complex impedance of the plasma load. Used for impedance matching optimization.
• Delivered Power: $P = V \times I \times \cos(\phi)$ — the actual power absorbed by the plasma (real power). May differ significantly from the RF generator's reported power.
• Reflected Power: Power reflected back to the generator due to impedance mismatch.
• Harmonic Analysis: The VI probe can measure harmonic content of the RF signal — non-sinusoidal waveforms indicate nonlinear plasma behavior.
• Ion Bombardment Energy: Correlated with the voltage waveform, particularly the DC self-bias that develops on the driven electrode.

Applications

• RF Power Calibration: Verify that the actual power delivered to the plasma matches the setpoint — RF generators' built-in sensors may not account for cable and matching network losses.
• Process Monitoring: Track VI probe readings during production to detect process drift — changes in plasma impedance indicate changes in plasma conditions.
• Endpoint Detection: Plasma impedance changes when the material being etched switches from one film to another — the VI probe can detect this transition.
• Chamber Matching: Ensure different chambers receive the same actual RF power and drive the same plasma impedance — critical for tool-to-tool consistency.
• Fault Detection: Detect arcing events, impedance excursions, or power delivery anomalies in real-time.

Where the VI Probe is Installed

• Typically installed between the RF matching network and the electrode feedthrough — measuring the actual power and impedance at the point of entry to the chamber.
• This location captures the true plasma-facing electrical conditions, excluding matching network losses.

Limitations

• Calibration: Must be carefully calibrated for the specific frequency and power range. Calibration drift can cause measurement errors.
• High-Temperature Environments: Proximity to the hot plasma chamber can affect sensor accuracy.

The VI probe is the primary tool for understanding and controlling RF power delivery to plasma processes — it provides the electrical truth that connects generator settings to actual plasma conditions.