SEM Image Noise Correlation Control for Accurate CD Measurement
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Solution Overview
Problem
Scanning electron microscope (SEM) images often contain noise, leading to errors in critical dimension (CD) measurements for semiconductor patterns, which are critical for precise semiconductor manufacturing.
Innovation Solution
A method to improve SEM images by determining the noise correlation length and adjusting the aperture signal until it is substantially zero, followed by white noise cancellation and optimal measurement condition selection using unsupervised learning to enhance image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If measurement speed of SEM image is increased, then productivity is improved, but noise in SEM image increases leading to measurement precision degradation
Solution Approach 1:
The patent applies preliminary action by measuring the noise correlation length before performing CD measurements. The system characterizes the noise properties of the SEM image in advance, determines the correlation length, and uses this information to guide subsequent measurement processes. This preliminary characterization enables the system to compensate for noise effects and maintain measurement precision even when using faster imaging modes that introduce more noise.
2Measurement precision
If aperture signal is adjusted to reduce noise correlation length, then measurement precision is improved, but device complexity and operation complexity increase
Solution Approach 1:
The patent implements feedback by measuring the noise correlation length and using this measurement to adjust the aperture signal. The system creates a closed-loop control where the noise characteristics are continuously monitored and the aperture parameters are automatically adjusted to optimize the noise correlation length. This feedback mechanism simplifies operation by automating what would otherwise require complex manual adjustments.
Solution Approach 2:
The patent replaces complex mechanical adjustments of the aperture system with automated electronic control based on noise correlation length measurements. Instead of requiring manual manipulation of aperture components, the system uses electronic feedback control to adjust aperture parameters, substituting mechanical complexity with automated measurement and control algorithms.
3Measurement precision
If multiple frames are measured and evaluated, then measurement precision is improved through noise cancellation, but loss of time increases
Solution Approach 1:
The patent applies preliminary action by measuring and characterizing the noise properties from multiple frames before final CD measurement. The system collects noise data in advance, determines statistical properties including noise correlation length, and uses this pre-characterized noise information to improve subsequent measurements. This preliminary noise characterization reduces the need for extensive repeated measurements, thereby reducing overall time loss.
Solution Approach 2:
The patent uses copying by measuring multiple frames to create redundant copies of the sample image under identical conditions. These copied frames are then processed to cancel out random noise through statistical analysis. By creating and analyzing multiple copies, the system extracts the true signal while eliminating noise, improving precision without requiring additional measurement time beyond the initial set of N frames.
Data Source
AI summary
A method of improving an SEM image includes (a) measuring a first SEM image, (b) determining a noise correlation length with respect to the first SEM image, (c) based on the noise correlation length being greater than 0, adjusting an aperture signal of an SEM equipment, and repeating (a), (b) and (c) until the noise correlation length is substantially 0.