Whole Field Thin Film Stress Evaluation via Interferometry
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Solution Overview
Problem
Current methods for evaluating thin film stress, such as mechanical, interference, and X-ray diffraction, are inadequate for precise whole field stress evaluation, particularly in amorphous films, and fail to predict crack or peel-off locations effectively.
Innovation Solution
A method using a commercial interferometer to measure wavefront phases before and after thin film deposition, with image subtraction and fitting with Zernike polynomials to determine phase differences, allowing for whole field stress distribution analysis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional stress evaluation methods (mechanical, interference, X-ray diffraction) are used, then stress measurement can be performed, but only average stress can be obtained and whole field stress distribution cannot be evaluated
Solution Approach 1:
The patent segments the wavefront phase information into multiple Zernike polynomial terms, where each term corresponds to specific stress distribution characteristics. By fitting the measured wavefront phase with Zernike polynomials and analyzing specific coefficients, the method extracts localized stress information from the overall wavefront, enabling whole field stress distribution evaluation while maintaining measurement precision.
Solution Approach 2:
The patent transitions from measuring only average curvature (scalar value) to measuring wavefront phase distribution (spatial field). By using interferometric wavefront measurement and Zernike polynomial decomposition, the method adds spatial dimension information to stress evaluation, allowing visualization and analysis of stress distribution across the entire film field rather than just an average value.
2Ease of operation
If average curvature measurement is used for stress evaluation, then measurement process is simple, but crack or peel-off location prediction is not possible
Solution Approach 1:
The patent applies local quality analysis by examining specific Zernike polynomial coefficients that correspond to localized stress features. Instead of treating the film as a uniform structure with average stress, the method identifies local stress concentrations through wavefront phase analysis, enabling prediction of specific locations where cracks or peel-off are likely to occur while maintaining operational simplicity.
3Measurement precision
If higher precision curvature radius measurement is adopted, then average stress evaluation accuracy improves, but whole field stress distribution and local stress concentration still cannot be determined
Solution Approach 1:
The patent performs preliminary wavefront phase measurement and Zernike polynomial fitting before stress calculation. By pre-processing the interferometric data to extract phase information and decompose it into Zernike coefficients, the method prepares the information in a form that simultaneously provides both precise average stress values and localized stress distribution characteristics, preventing loss of local stress concentration information.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables precise evaluation of thin film stress distribution, predicting crack or peel-off locations and improving the accuracy of stress measurement beyond average stress evaluation, facilitating better understanding and prevention of film damage in optical systems.
Implementation Method 1
measuring associated wavefront of the substrate by an interferometer with a laser having a wavelength of λ, so as to obtain a first wavefront phase; measuring associated wavefront for the specimen, so as to obtain a second wavefront phase
Data Source
AI summary
A novel method for whole field thin film stress evaluation is provided. Through the provided method, the whole filed thin film stress distribution for an optical thin film would be developed with a commercial interferometer, so that a whole field evaluation for the crack or peel-off of thin film is hence achievable.