Pulsed RF Generator Frequency Control for Fast Impedance Matching
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Solution Overview
Problem
Conventional pulsed RF generators face challenges in rapidly adjusting the driving frequency for impedance matching at high pulse frequencies, leading to unstable plasma due to insufficient time for frequency adjustment, especially when the ON-time period is less than 50 µs.
Innovation Solution
A pulsed variable frequency RF generator with a control loop that predicts and adjusts the driving frequency during the ON-time period by linearly fitting the imaginary part of phase-shifted admittance, setting the frequency to zero, and varying the reactance of the impedance matching network to achieve optimal impedance matching, even at high pulse frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pulse frequency is increased to reduce the on-time period, then the productivity is improved, but the impedance matching cannot be completed due to insufficient time to adjust the driving frequency
Solution Approach 1:
The patent applies preliminary action by predicting the optimal driving frequency for the next pulse based on impedance data from the current pulse. This prediction is performed during the OFF-time period, preparing the frequency setting in advance before the next pulse begins, thus eliminating the need for time-consuming frequency adjustments during the ON-time period.
Solution Approach 2:
The patent implements dynamics by continuously adapting the driving frequency based on real-time impedance measurements. The system dynamically adjusts the frequency prediction for each subsequent pulse based on the actual impedance conditions observed in previous pulses, allowing the frequency to evolve optimally with changing plasma conditions.
2Productivity
If the on-time period is reduced to several milliseconds or less, then the productivity is improved, but the plasma becomes unstable due to insufficient frequency adjustment time
Solution Approach 1:
The system performs frequency prediction and preparation during the OFF-time period, establishing the optimal driving frequency for the upcoming pulse before the plasma is ignited. This preliminary frequency setting ensures that the plasma starts at the optimal frequency immediately, maintaining stability even with very short ON-time periods.
Solution Approach 2:
The patent employs feedback by measuring the impedance during each pulse and using this information to predict and adjust the driving frequency for the next pulse. This closed-loop feedback mechanism ensures that the frequency is continuously optimized based on actual plasma conditions, maintaining plasma stability.
3Productivity
If the pulse frequency is increased to several kHz or higher, then the productivity is improved, but the impedance matching cannot be performed at several milliseconds level
Solution Approach 1:
The patent performs impedance measurement and frequency prediction during the OFF-time period, completing the impedance matching preparation work before the pulse begins. This shifts the time-consuming matching operations to the OFF-time period, enabling the system to operate at high pulse frequencies without sacrificing matching quality.
Data Source
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AI summary
A frequency tuning impedance matching method includes analyzing a start driving frequency, set by a user, and an RF output signal to vary a driving frequency. Specifically, a next frequency may be predicted using susceptance which is an imaginary part of measured admittance in an n-th pulse. Accordingly, impedance matching may be completed at high speed or an optimal frequency may be reached at high speed.