Optical System Image Quality via Small-Pupil Diffraction Separation
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Solution Overview
Problem
Existing methods for determining the image quality of optical systems are limited in accuracy and speed, particularly when evaluating the effect of test structures on the phase of light, and require prior knowledge of the test structure's characteristics.
Innovation Solution
A method utilizing a periodic test structure and specific illumination angle distributions within a small pupil area, allowing for the separation of diffraction effects from imaging effects, and enabling the determination of image quality without prior knowledge of the test structure's properties, using a metrology system with a variable aperture to adjust illumination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a periodic test structure is used for determining imaging contribution, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The test structure is segmented into periodic elements with specific geometries (lines, rectangles, or contact holes) arranged in a regular pattern. This segmentation allows the diffraction spectrum to contain discrete, predictable orders that can be selectively illuminated and measured, improving measurement precision while keeping the structure manufacturable
Solution Approach 2:
The method changes the illumination parameters (angle distributions, pupil area selection) to selectively illuminate specific diffraction orders of the periodic test structure. By varying illumination angles and selecting small pupil areas (less than 10% of total pupil area), the method separates diffraction effects from imaging effects, enhancing measurement accuracy without requiring complex test structures
2Measurement precision
If small pupil area illumination is used, then measurement precision is improved, but productivity decreases
Solution Approach 1:
The method performs preliminary calculations of the diffraction spectrum based on the known periodic test structure geometry and illumination parameters. This allows prediction of which diffraction orders will appear and their positions, enabling efficient selection of small pupil areas that capture only the necessary information, thus improving precision without excessive measurement time
Solution Approach 2:
Instead of illuminating the entire pupil area, the method uses partial illumination with small pupil areas (less than 10%, preferably less than 5%) that are sufficient to capture the essential diffraction orders. This partial action reduces the amount of data to be processed and measured, improving both precision and productivity by avoiding redundant measurements
3Measurement precision
If prior knowledge of test structure properties is required, then measurement precision is improved, but ease of operation deteriorates
Solution Approach 1:
The periodic test structure serves itself by providing a diffraction spectrum with predictable, discrete orders based on its geometry. The structure's periodicity automatically generates the necessary spectral information without requiring external calibration or detailed prior knowledge. The method uses the structure's own geometric properties (period, shape) to generate the diffraction pattern that reveals the imaging contribution
Solution Approach 2:
The method is designed to work with any periodic test structure regardless of its specific geometry (lines, rectangles, contact holes). The universal approach uses the common property of periodicity to generate predictable diffraction spectra, eliminating the need for structure-specific calibration procedures and making the method easy to operate across different test structures
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
Improves the accuracy and speed of image quality determination by separating diffraction and imaging effects, allowing for precise characterization of optical systems and enabling adjustments based on the determined image quality.
Implementation Method 1
by using a periodic test structure for determining the imaging contribution of the optical system and/or for qualifying the effect of the test structure on the phase of the light
Implementation Method 2
a projection optic for mapping an object field in the object plane onto an image field in the image plane
Data Source
Figure 1
Figure 2~3
Figure 4~5
AI summary
To determine the image quality of an optical system when illuminated with illumination light (1) within a pupil of the optical system to be measured, and/or to qualify the phase effect of a test structure, a test structure (5) that is periodic in at least one dimension (x) is first positioned in an object plane (4) of the optical system. An initial illumination angle distribution for illuminating the test structure (5) with an initial pupil area, the area of which is less than 10% of the total pupil area, is specified, and the test structure (5) is illuminated at various distance positions (z) relative to the object plane (4). An initial measured aerial image of the test structure (5) is then determined.The process of specifying the illumination distribution, illuminating the structure, and determining the aerial image is then repeated for a further illumination angle distribution. A comparison of the measured aerial images yields an image contribution of the optical system, from which the image quality parameter to be determined and/or a complex-valued diffraction spectrum of the test structure is derived. A metrology system for carrying out the procedure comprises a holder (14) for the test structure (5), illumination optics (9), a presetting device (10) for specifying the illumination angle distributions, the optical system (17) to be examined with regard to its image quality, and a spatially resolved detection device (25) for determining the aerial image. This results in an improved image quality determination method.