Zoned Order Sorting Filter Underlayers for Dispersion Reduction
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Solution Overview
Problem
Zoned diffraction order sorting filters in optical metrology systems experience anomalies in measured diffraction signals due to unwanted dispersion at zone joints, caused by varying optical lengths and sharp changes in the thin-film stack filters, leading to image spreading and shifting, which affects the accuracy of wavelength measurements.
Innovation Solution
The introduction of underlayers with adjusted thicknesses to equalize the total optical lengths or ensure phase matching across zone joints in the thin-film stack filters, minimizing internal reflections and dispersion, and the use of separate calibration curves to correct for nonlinear wavelength-to-pixel location calibration at these joints.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If zoned order sorting filters are used to prevent detection of unwanted higher diffraction orders, then the spectrometer can measure over a wide wavelength range, but unwanted dispersion and image spreading occur at zone joints causing measurement anomalies
Solution Approach 1:
An underlayer is introduced as an intermediary component between the substrate and the thin-film stack filter in each zone. This underlayer has a specific refractive index (intermediate between substrate and filter) and thickness designed to equalize optical path lengths at zone joints, thereby reducing unwanted dispersion and image spreading while maintaining the filter's wavelength range coverage capability
Solution Approach 2:
The thickness of the underlayer is specifically designed and optimized to change the optical path length parameter. By adjusting the underlayer thickness, the optical path lengths in different zones are equalized at the zone joints, which minimizes dispersion and improves measurement precision across the entire wavelength range
2Reliability
If thin-film stack filters are used in each zone to block specific wavelengths, then diffraction order sorting is achieved, but sharp changes in filter characteristics at zone joints cause image spreading and shifting
Solution Approach 1:
The underlayer is applied locally in each zone with specific thickness and refractive index properties tailored to that zone's requirements. This local optimization ensures that at each zone joint, the optical path lengths are equalized, minimizing image spreading and shifting while maintaining the zone-specific wavelength blocking characteristics
Solution Approach 2:
The underlayer is designed to equalize the optical path lengths across zone joints, creating an equipotential optical environment. This ensures that light waves emerge from adjacent zones with matched phases and optical path lengths, eliminating abrupt changes in image position and reducing dispersion at the joints
3Adaptability or versatility
If the optical path length varies across different zones, then the filter can be designed with different passband characteristics, but this causes dispersion at zone joints and anomalies in measured signals
Solution Approach 1:
The underlayer thickness is specifically optimized to compensate for variations in optical path length across zones. By adjusting this parameter, the system maintains customized passband characteristics for each zone while equalizing the total optical path length at zone joints, thereby eliminating dispersion-induced signal anomalies
Solution Approach 2:
The underlayer utilizes the inherent refractive index difference between the substrate and filter materials by introducing an intermediate layer. This converts the potentially harmful effect of refractive index mismatches into a beneficial optical path length equalization mechanism, reducing dispersion while maintaining zone-specific filter characteristics
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 reduces unwanted dispersion and anomalies at zone joints, enhancing the accuracy and reliability of diffraction signal measurements by ensuring consistent optical paths and phase alignment across zones, thereby improving the overall precision of wavelength calibration and measurement in optical metrology systems.
Implementation Method 1
the thicknesses of the underlayers of each pair of adjacent zones are adjusted such that the total optical lengths traversed by light of the wavelength falling at each zone joint are substantially equal in the two zones adjacent the joint
Implementation Method 2
an order sorting filter (OSF) is provided where underlayers are deposited underneath the thin-film stack filters of all OSF zones
Implementation Method 3
A spectrometer typically employs a blazed diffraction grating to disperse an optical signal onto an array detector
Data Source
AI summary
A zoned order sorting filter for a spectrometer in a semiconductor metrology system is disclosed with reduced light dispersion at the zone joints. The order sorting filter comprises optically-transparent layers deposited underneath, or on top of thin-film filter stacks of the order sorting filter zones, wherein the thicknesses of the optically-transparent layers are adjusted such that the total optical lengths traversed by light at a zone joint are substantially equal in zones adjacent the zone joint. A method for wavelength to detector array pixel location calibration of spectrometers is also disclosed, capable of accurately representing the highly localized nonlinearities of the calibration curve in the vicinity of zone joints of an order sorting filter.