Aspheric Surface Measurement via Interferometric Scanning
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Solution Overview
Problem
Current methods for measuring aspherical surfaces and wavefronts in optical systems face limitations in precision, accuracy, and efficiency, particularly in high-accuracy applications like EUV lithography, where existing techniques are prone to measurement errors, long lead times, and increased costs due to the need for complex apparatus calibration and additional fabrication steps.
Innovation Solution
An interferometric scanning method that generates a reference wavefront from a known origin, aligns a test optic to intersect with the aspheric surface at points of common tangency, and uses interferograms to determine phase information, allowing for the calculation of axial and vertical coordinates of the aspheric surface, reducing sensitivity to vibrations, temperature changes, and air turbulence, and enabling measurement of large departures and surface slopes with high spatial resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If null-lenses or computer generated holograms are used to compensate for aspherical wavefronts, then measurement precision is improved, but device complexity increases and additional fabrication steps are required
Solution Approach 1:
The patent extracts and eliminates the complex compensation elements (null-lenses, computer generated holograms) from the measurement system. Instead of using these additional components to compensate for aspherical wavefronts, the invention directly measures aspherical surfaces using a spherical reference wavefront, removing the source of complexity while maintaining measurement capability.
Solution Approach 2:
The patent inverts the conventional approach by not trying to make the reference wavefront match the aspherical test surface, but rather using a simple spherical reference wavefront to measure the aspherical surface directly. This inversion eliminates the need for complex compensation elements and their calibration.
2Ease of operation
If indirect test methods are applied by measuring surface of each lens element in a null system, then measurement can be performed, but measurement precision deteriorates due to accumulation of measurement errors
Solution Approach 1:
The patent removes the multi-step indirect measurement process that requires measuring each lens element separately and then calculating the combined wavefront. Instead, it performs a direct single-step measurement of the complete optical system, eliminating the accumulation of measurement errors inherent in multi-step indirect methods.
3Measurement precision
If refractive index, lens thickness and air-spacing are measured carefully in null systems, then measurement accuracy is improved, but loss of time increases due to additional measurement steps
Solution Approach 1:
The patent merges the measurement of multiple parameters (refractive index, lens thickness, air-spacing) into a single direct interferometric measurement of the complete optical system. Instead of performing separate measurements of each parameter and then calculating the wavefront, the invention directly measures the wavefront in a single step, eliminating the time-consuming multi-step measurement process.
4Measurement precision
If the wavefront of the reference wavefront is made aspherically to fit the wavefront to be measured, then measurement precision is improved, but device complexity increases and calibration becomes more difficult
Solution Approach 1:
The patent inverts the conventional approach by not making the reference wavefront aspherical to match the test surface, but rather using a simple spherical reference wavefront to measure the aspherical surface directly. This inversion maintains measurement precision while eliminating the complexity of creating and calibrating aspherical reference wavefronts.
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 provides high-accuracy, absolute measurements of aspherical surfaces and wavefronts, reducing measurement errors and lead times, and increasing productivity by simplifying the measurement process and reducing costs, while maintaining precision across various measurement conditions.
Implementation Method 1
interferometric scanning methods and apparatus for measuring aspheric surfaces and wavefronts
Data Source
AI summary
Interferometric scanning method(s) and apparatus for measuring test optics having aspherical surfaces including those with large departures from spherical. A reference wavefront is generated from a known origin along a scanning axis. A test optic is aligned on the scanning axis and selectively moved along it relative to the known origin so that the reference wavefront intersects the test optic at the apex of the aspherical surface and at one or more radial positions where the reference wavefront and the aspheric surface intersect at points of common tangency (“zones”) to generate interferograms containing phase information about the differences in optical path length between the center of the test optic and the one or more radial positions. The interferograms are imaged onto a detector to provide an electronic signal carrying the phase information. The axial distance, ν, by which the test optic is moved with respect to the origin is interferometrically measured, and the detector pixel height corresponding to where the reference wavefront and test surface slopes match for each scan position is determined. The angles, α, of the actual normal to the surface of points Q at each “zone” are determined against the scan or z-axis. Using the angles, α, the coordinates z and h of the aspheric surface are determined at common points of tangency and at their vicinity with αmin≦α≦αmax, where αmin and αmax correspond to detector pixels heights where the fringe density in the interferogram is still low. The results can be reported as a departure from the design or in absolute terms.