RF Metrology Sampling Clock Control for Spurious Frequency Correction
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Solution Overview
Problem
Conventional plasma control systems fail to effectively prevent or alleviate signal distortions caused by aliasing and intermodulation distortion (IMD) during broadband sampling in plasma etching processes, particularly when the sample rate of analog/digital converters coincides with the signal of interest, leading to in-band interference and spurious frequencies.
Innovation Solution
A control module comprising multiple analog to digital converters (A/D converters) and a symmetric phase controller that generates phase-shifted sampling clock signals, increasing the sample rate through clock synchronization and interleaving data, along with a multiplexer to prevent signal distortion by correcting spurious frequency interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single A/D converter is used with a fixed sample rate, then the device complexity is low, but aliasing and intermodulation distortion cause signal corruption in the frequency bands of interest
Solution Approach 1:
The system divides a single sampling task into multiple parallel A/D converters, each sampling at different phase offsets. This segmentation allows the system to achieve higher effective sample rates and better frequency resolution while maintaining individual converter simplicity.
Solution Approach 2:
Multiple A/D converter outputs are merged through interleaved multiplexing to create a composite digital signal with enhanced sampling characteristics. The combined signal achieves the equivalent performance of a higher-speed single converter while using multiple lower-speed converters.
2Measurement precision
If the sample rate is increased to avoid aliasing, then measurement precision improves, but the risk of in-band interference from intermodulation distortion increases when sample rate coincides with signal frequencies
Solution Approach 1:
The system uses asymmetric phase offset distribution among multiple A/D converters, where each converter is phase-shifted by a different amount (e.g., 0°, 120°, 240°). This asymmetric arrangement creates a comb-like frequency response that strategically places nulls at problematic intermodulation frequencies while maintaining high sample rates.
Solution Approach 2:
The system dynamically adjusts the phase offsets of multiple A/D converters based on the specific signal frequencies being measured. By changing phase relationships, the system can move the null positions in the frequency response to coincide with known intermodulation products, adaptively eliminating in-band interference.
3Measurement precision
If multiple A/D converters with phase-shifted clocks are used, then aliasing is reduced and measurement precision improves, but device complexity and synchronization requirements increase
Solution Approach 1:
The system uses periodic phase-shifted clock signals with fixed relationships (e.g., 120° or 240° phase differences) to drive multiple A/D converters. This periodic structure simplifies synchronization by establishing predictable, repeating sampling patterns that are easier to manage than arbitrary timing schemes.
Solution Approach 2:
A dedicated phase-shifting circuit acts as an intermediary between the master clock and individual A/D converters. This intermediary component automatically generates the required phase offsets, eliminating the need for complex independent timing control for each converter and centralizing synchronization management.
4Measurement precision
If the sample rate is set to avoid aliasing of high frequencies, then high frequency measurement is improved, but intermodulation distortion products fall into the frequency bands of interest causing interference
Solution Approach 1:
The system deliberately allows intermodulation distortion products to be generated at high frequencies, then uses the phase-shifted sampling architecture to create frequency response nulls that specifically attenuate these distortion products. The harmful distortion energy is converted into a predictable pattern that can be systematically eliminated through the sampling design.
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.