RF Metrology Sampling Control for Spurious Frequency Correction
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Solution Overview
Problem
Conventional plasma etching systems face signal distortions due to aliasing and intermodulation distortion (IMD) during broadband sampling, where the sample rate of analog/digital converters coincides with the signal of interest, leading to in-band interference and spurious frequencies that existing anti-aliasing filters fail to detect and prevent.
Innovation Solution
A control module comprising multiple analog to digital converters (A/D converters) synchronized with a symmetric phase controller to generate phase-shifted sampling clock signals, increasing the sample rate and using a multiplexer to interleave data, along with a digital processing unit to detect and correct spurious frequency interference by adjusting sample rates and applying mathematical vector corrections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If broadband sampling is used to monitor plasma process chamber, then measurement capability is improved, but aliasing and intermodulation distortion occur causing signal corruption
Solution Approach 1:
The patent applies preliminary action by detecting potential aliasing and intermodulation distortion problems before they corrupt the measurements. The system proactively identifies frequency bands that may be affected by these phenomena and takes corrective action by adjusting sampling rates or frequency settings in advance, preventing signal corruption before it occurs during broadband sampling operations.
2Reliability
If sample rate of analog/digital converter is increased to prevent aliasing, then signal integrity is improved, but device complexity and cost increase
Solution Approach 1:
The patent implements dynamics by making the sampling rate adjustable rather than fixed. The system dynamically changes the sampling rate based on the detected frequency content and potential aliasing conditions. This allows the converter to use lower sampling rates when possible (reducing complexity) while automatically increasing the rate when signal integrity is at risk, achieving both goals adaptively.
3Measurement precision
If anti-aliasing filters are used to prevent frequency folding, then measurement accuracy is improved, but device complexity increases
Solution Approach 1:
The patent applies parameter changes by adjusting the sampling rate parameter rather than adding complex anti-aliasing filter hardware. The system changes the temporal sampling parameter to prevent frequency folding, which is a simpler approach than implementing complex analog or digital filtering circuits. This maintains measurement accuracy through parameter optimization rather than additional filtering components.
4Adaptability or versatility
If multiple frequency sources are used in plasma etching, then process control capability is improved, but intermodulation distortion and in-band interference increase
Solution Approach 1:
The patent implements feedback by continuously monitoring the frequency spectrum and detecting intermodulation distortion products generated by multiple RF sources. The system uses this feedback information to identify when IMD occurs and automatically adjusts the sampling rate or frequency settings to eliminate the interference, maintaining accurate measurements despite the presence of multiple frequency sources used for enhanced process control.
Data Source
AI summary
A system for detecting and correcting for spurious frequencies that may coincide in a bandwidth of interest in an RF metrology system. The system can (1) utilize a deterministic scheme to detect an interference by a spurious frequency and correct the distortion effect or (2) utilize a mixed signal processing architecture to avoid the occurrence of spurious frequency contamination. A detection scheme identifies the event of distortion and triggers either (a) a shift in the analog to digital convert sample rate or (b) a mathematical vector manipulation. The shift of the analog to digital convert sample rate moves an aliased image of the spurious frequency outside of the frequency of interest. The mathematical vector correction removes the distortion and restores the signal of interest.