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Real-Time Voltage and Ion Energies

Introduction

In semiconductor manufacturing, process control can be improved with feedback and control based on real time measurements of the system. The control system makes output voltage and current measurements internal to the power system. These can be used to provide real-time wafer voltage and ion energy distribution (IED) information. This allows for automated feedback and control, improved process performance, reduced processing time.

Wafer Voltage

Understanding how the wafer voltage changes can lead to insight for improving a process. Unfortunately, it is impossible to measure the wafer potential with traditional RF generators due to the complexity of the matching networks. Additionally, it is not feasible to include wafer voltage measurements on a production process chamber.

EHT pulse generators calculate the wafer voltage in real time based on internal voltage and current measurements, known circuit parameters, and simple assumptions about the bulk plasma. While the actual IED involves computing the momentum function across the wafer sheath, which requires computationally intensive PIC or Monte-Carlo algorithms, this simplified analysis yields estimate of the IED that can be computed in real time.

In the figure below, the measured output voltage from Spartan™ is shown in red on two different timescales. Using this data, the voltage on the electrode (wafer) was calculated and is shown in blue. Since this data was collected on an experimental test chamber, the electrode voltage could also be measured with a oscilloscope probe (green). There is excellent agreement between the measured and calculated electrode (wafer) voltages.

Wafer voltage

Ion Energy Distribution

Today, RF power is the primary data for processing plasmas. Using measurements internal to the power system, the EHT Semi power systems can estimate the ion energy distribution in real time. This information can be used as a control parameter, which would allow process engineers to use IEDs to control their process rather than the simplistic measure of power delivered to the plasma.

The figures below are an example from Perseus™ operating in a low-voltage regime. The left-hand column shows the measured Perseus' output waveforms at increasing set voltages. The middle column shows the calculated wafer voltage and plasma potential. The right-hand column shows the estimated IED for the different set voltages. This data shows the center point of the IED can be shifted with confidence.

IED for ALE

Conclusion

EHT Semi pulse generators provide significantly more information that can be used to better control semiconductor manufacturing processes. EHT power systems:

  • Measure output voltage and current
  • Calculate plasma potential and wafer voltage
  • Estimate the ion energy distribution

This advanced process information allows process engineers to use the desired IED as a control parameter rather than just power delivered to the plasma. This information can be used to improve the etch rate, selectivity, or the minimum critical dimensions.