Single-Material Grating Waveplates for Pupil Polarization Filtering
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Solution Overview
Problem
Conventional waveplates struggle to achieve precise control of phase retardation and optical-axis orientation for efficient suppression of surface scattering in wafer inspection, particularly at short wavelengths, due to limitations in manufacturing and material transparency.
Innovation Solution
Implementing single-material gratings with form-birefringence on reflective substrates to convert elliptically polarized scattered light into uniformly oriented linear polarization using spatially varying phase retardation, combined with a linear polarizer to filter out surface scattering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional birefringent waveplates are used, then uniform phase retardation is achieved, but precise control of phase retardation and optical-axis orientation for surface scattering suppression is difficult
Solution Approach 1:
The patent applies local quality by making the phase retardation spatially varying across the waveplate aperture rather than uniform. The phase retardation is designed to vary according to a specific function (e.g., linear or quadratic) to match the polarization characteristics of surface scattering at different angles, enabling precise control of the optical transformation for scattering suppression.
Solution Approach 2:
The patent segments the waveplate into multiple regions with different phase retardation values. This is achieved by dividing the aperture into zones where each zone has a specific phase retardation designed to linearize the elliptical polarization of scattered light at that particular angle, thereby achieving precise control through spatial segmentation.
2Manufacturing precision
If multiple quartz segments are used to control optical-axis orientation, then polarization transformation is achieved, but wavefront quality and integration into one piece becomes difficult
Solution Approach 1:
The patent uses composite materials by combining multiple quartz segments with different optical axis orientations and phase retardations into a single integrated waveplate. The segments are precisely joined to maintain wavefront quality while achieving the desired spatial variation in optical properties for effective surface scattering suppression.
Solution Approach 2:
The patent changes the optical parameters (phase retardation and optical-axis orientation) continuously or in steps across the waveplate aperture. This is achieved by varying the thickness or material properties of different segments to create a gradient in optical parameters, enabling precise control of polarization transformation without requiring complex integration of many small segments.
3Manufacturing precision
If free-form polishing is used to control wavefront error, then surface profile precision is improved, but manufacturing complexity increases
Solution Approach 1:
The patent changes the surface profile parameters systematically using free-form polishing to achieve the desired phase retardation distribution. The polishing process is controlled to create specific thickness variations across the waveplate that correspond to the required phase retardation function, enabling precise wavefront control through parameter optimization.
4Manufacturing precision
If thin-film coating process is used for form-birefringent waveplates, then spatially varying phase retardation is achieved, but transparency at very short wavelengths is limited
Solution Approach 1:
The patent replaces the thin-film coating approach with a alternative material or structure that achieves form-birefringence without relying on multiple transparent layers. This may involve using a single-material grating structure or reflective substrate approach that maintains transparency at very short wavelengths while still providing the required spatially varying phase retardation.
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
Effectively suppresses surface scattering noise in optical inspection systems, enhancing defect detection sensitivity by converting elliptical to linear polarization across the collection pupil, thereby improving signal-to-noise ratio.
Implementation Method 1
Implementing single-material gratings with form-birefringence on reflective substrates to convert elliptically polarized scattered light into uniformly oriented linear polarization using spatially varying phase retardation
Implementation Method 2
combined with a linear polarizer to filter out surface scattering
Data Source
AI summary
A method includes illuminating a target and collecting light scattered from the illuminated target. The collected light scattered from the illuminated target has an elliptical polarization that varies spatially across a collection pupil. The method also includes converting the polarization of the collected light from the elliptical polarization that varies spatially across the collection pupil to a linear polarization that is uniformly oriented across the collection pupil, using one or more single-material gratings. The one or more single-material gratings have phase retardation that varies spatially across the collection pupil in accordance with the elliptical polarization. The method further includes filtering out the light having the linear polarization that is uniformly oriented across the collection pupil, using a linear polarizer.