Scanning Interferometry Surface Analysis Model Correlation
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Solution Overview
Problem
Conventional scanning interferometry techniques face challenges in accurately analyzing complex surface structures, particularly in determining surface-height offsets and material parameters, due to limited coherence length and under-resolved features, which affects the precision of surface profiling and characterization in applications like flat-panel display metrology and semiconductor wafer analysis.
Innovation Solution
A method involving the comparison of scanning interferometry signals with model signals, using correlation functions and phase compensation to identify surface-height offsets and determine test object parameters, such as thin film thickness and index of refraction, by calculating merit values that account for similarities and differences between experimental and theoretical signals in the frequency domain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional scanning interferometry is used to measure surface profile, then the measurement process is simple, but the measurement precision is insufficient for complex surface structures
Solution Approach 1:
The patent applies preliminary action by generating model signals that represent expected interferometry patterns for various surface structures before actual measurement. These pre-computed models are then compared with experimental data to identify surface features, eliminating the need for complex real-time analysis during measurement.
Solution Approach 2:
The patent uses model signals as an intermediary between the raw interferometry data and the final surface structure characterization. By introducing these theoretical models that incorporate known optical properties and surface geometry, the system bridges the gap between simple measurement and complex analysis, enabling accurate determination of surface-height offsets and material parameters.
2Measurement precision
If the coherence length of interfering light is limited, then the interferogram shows localized fringes near zero OPD, but features under the optical resolution limit cannot be resolved
Solution Approach 1:
The patent introduces model signals that encode information about sub-resolution features through their effect on the overall interferometry signal shape. By comparing experimental data with these models that account for thin film parameters, discrete structures, and other complex surface features, the system recovers information about features smaller than the optical resolution limit without requiring higher coherence length.
Solution Approach 2:
The patent transitions from direct spatial domain analysis to frequency domain analysis, adding a dimensional perspective that enables resolution of features beyond the classical diffraction limit. By analyzing the spectral content and phase information of the interferometry signal, the system can extract details about surface structures that are not visible in the direct image.
3Measurement precision
If model-based comparison with merit value calculation is used, then the accuracy of parameter determination is improved, but the analysis complexity increases
Solution Approach 1:
The patent applies preliminary action by pre-calculating model signals for a range of possible parameter values before comparison with experimental data. This allows the system to quickly evaluate multiple hypotheses without performing computationally intensive optimizations during the measurement process, reducing analysis time while maintaining accuracy.
Solution Approach 2:
The patent applies local quality by focusing the merit value calculation on specific features of the interferometry signal that are most sensitive to the parameters of interest. Rather than comparing entire signals, the method identifies and compares critical regions, reducing computational load while preserving measurement accuracy.
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
This approach enhances the accuracy and precision of surface structure analysis, enabling the determination of complex surface features and parameters, even those under-resolved by conventional methods, thereby improving metrology in flat-panel displays and semiconductor wafer inspection.
Implementation Method 1
an interferometer combines measurement light reflected from the surface of interest with reference light reflected from a reference surface to produce an interferogram
Implementation Method 2
A limited coherence length of the interfering light can be produced, for example, by using a white-light source, which is referred to as scanning white light interferometry (SWLI)
Data Source
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AI summary
A method includes comparing a scanning interferometry signal obtained for a location of a test object to each of multiple model signals corresponding to different model parameters for modeling the test object, wherein for each model signal the comparing comprises calculating a correlation function between the scanning interferometry signal and the model signal to identify a surface-height offset between the scanning interferometry signal and the model signal and, based on the identified surface-height offset, calculating a height-offset compensated merit value indicative of a similarity between the scanning interferometry signal and the model signal for a common surface height. The method further includes, based on the respective merit values for the different model signals, determining a test object parameter at the location of the test object.