Variable Wavelength Interferometry Without Path-Difference Scanning
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Solution Overview
Problem
Conventional scanning white-light interferometry systems face limitations in measuring sample properties due to impractical determination of sample reflectance changes with incident angle, requiring large spot sizes and complex path difference scanning, which are slow and prone to vibration-induced errors, leading to reduced signal-to-noise ratio and inaccurate phase measurements.
Innovation Solution
A variable wavelength interferometric system using a tunable light source with narrow band illumination and fixed path difference, combined with polarization states, captures images at the back focal plane to extract structural information, enabling precise measurement of complex reflectance properties without path difference scanning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If scanning white-light interferometry uses broadband light with large range of incident angles to achieve diffraction limited spot size, then measurement resolution is improved, but determination of sample properties becomes impractical due to reflectance changes with incident angle
Solution Approach 1:
The patent changes the illumination parameter from broadband white light to narrow band light with variable peak wavelength. This allows maintaining diffraction limited spot size while using a single incident angle, eliminating the problem of reflectance changes with angle and enabling practical sample property measurements.
Solution Approach 2:
The patent introduces a tunable light source that dynamically adjusts the peak wavelength of narrow band illumination. This dynamic parameter adjustment enables measurement of sample properties at different wavelengths while maintaining the advantages of narrow band illumination, providing both resolution and versatility.
2Loss of information
If conventional interferometry uses path difference scanning to collect interferometric data, then comprehensive sample information is obtained, but measurement speed decreases and vibration-induced errors increase
Solution Approach 1:
Instead of scanning path difference to obtain wavelength information (conventional approach), the patent inverts the approach by scanning wavelength to obtain path difference information. This is achieved by using a tunable narrow band light source and capturing interferometric images at multiple wavelengths, which eliminates the need for mechanical path difference scanning and its associated problems.
Solution Approach 2:
The patent replaces the mechanical path difference scanning system with an optical wavelength scanning system. By using a tunable light source and capturing images at the back focal plane, the system eliminates mechanical movement during measurement, thereby increasing measurement speed and reducing vibration-induced errors.
3Measurement precision
If path difference scanning is used to capture interferometric fringes, then phase information is obtained, but measurement accuracy decreases due to vibration-induced errors and reduced signal-to-noise ratio
Solution Approach 1:
The patent replaces mechanical path difference scanning with optical wavelength scanning. By tuning the light source wavelength and capturing interferometric images at the back focal plane, the system obtains phase information without mechanical movement, thereby eliminating vibration-induced errors and improving measurement stability and signal-to-noise ratio.
Solution Approach 2:
The patent performs wavelength scanning and interferometric image capture before any potential vibration or drift can affect the measurement. By completing the entire measurement sequence using only optical tuning without mechanical movement, the system ensures consistent measurement conditions throughout the process.
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 system improves measurement speed and accuracy by reducing vibration errors and optimizing signal-to-noise ratio, allowing for high-resolution characterization of small features in semiconductor devices with reduced measurement time and enhanced data quality.
Implementation Method 1
uses a wavelength tunable light source to produce a narrow band illumination beam with variable peak wavelength
Implementation Method 2
An interferometric objective directs light to the sample and a reference surface and recombines the reflected light to produce interference
Implementation Method 3
At least one polarizer generates one or more polarization states of the sample illumination and of the reference illumination
Implementation Method 4
At least one camera captures images of the interference illumination at the back focal plane of the interferometric objective
Data Source
AI summary
A variable wavelength interferometer operates in the back focal plane of the objective and produces a narrow band illumination beam with a peak wavelength that is varied over a range of wavelengths. An interferometric objective directs light to the sample and a reference surface and recombines the reflected light to produce interference. At least one polarizer generates one or more polarization states of the sample illumination and of the reference illumination. One or more path length differences between the sample and reference illumination is produced. At least one camera captures images of the interference illumination at a back focal plane of the interferometric objective for each combination of peak wavelength and polarization state. The interferometric data at one or more pixels of the camera are used to extract structural information for the sample.