Differential Interference Contrast Surface Inspection Using Polarization Equalization
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Solution Overview
Problem
The differential interference detection system is not robust against noise, particularly random noise from changes in reflection direction, contrast changes due to reflectance changes, and interference between detection signals and stray light, leading to errors in height restoration, especially when measuring nanometer-order height displacements, and is limited in measuring height displacements beyond certain wavelength thresholds, resulting in aliasing and large measurement errors.
Innovation Solution
A surface inspection apparatus with a differential interference contrast illumination system that emits light with different phases and polarizations, using two sensors to process interference signals in specific polarization directions to equalize signal intensities, thereby reducing noise influence and enhancing measurement robustness and sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If differential interference contrast measurement is used to achieve high sensitivity in detecting nanometer-order height displacements, then measurement precision is improved, but robustness against noise deteriorates
Solution Approach 1:
The patent changes the polarization parameters of the illumination light to resolve the contradiction. Specifically, it uses s-polarized light for one illumination spot and p-polarized light for another, allowing differential detection that is insensitive to certain noise sources while maintaining high sensitivity for height displacement measurement. This parameter change enables the system to distinguish between signal and noise by exploiting polarization differences.
2Device complexity
If a single wavelength light is used for illumination, then the measurement system is simplified, but the measurement range is limited to ±1/4 of wavelength
Solution Approach 1:
The patent employs periodic modulation of the polarization state of illumination light to extend the measurement range. By periodically changing the polarization angle and detecting the corresponding interference pattern changes, the system can measure height displacements beyond the traditional ±1/4 wavelength limit while maintaining a relatively simple single-wavelength illumination setup.
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
The apparatus achieves highly sensitive and robust surface height measurements, preventing optical noise interference and extending the dynamic range of height measurements beyond previous limitations.
Implementation Method 1
generate light in a first polarization direction and a second polarization direction different from each other from interference light obtained by interfering reflected light of the two illumination spots
Implementation Method 2
a first sensor configured to photoelectrically convert the light in the first interference polarization direction to generate a first signal
Implementation Method 3
a second sensor configured to photoelectrically convert the light in the second polarization direction to generate a second interference signal
Data Source
AI summary
A surface inspection apparatus includes a differential interference contrast detection system. Illumination light is emitted to positions deviated by a predetermined shear amount as two illumination spots having different phases. Light is generated in first and second polarization directions different from each other from interference light obtained by interfering reflected light of the two illumination spots from a surface of a sample. Light in the first and second polarization directions is converted to generate first and second interference signals. A signal processing device is configured to process the first and second interference signals, and the first and second polarization directions of the light generated by the differential interference contrast detection system are set such that an intensity of the first interference signal and an intensity of the second interference signal are the same at an operation point at which there is no phase difference between the two illumination spots.