EUV Mask Inspection Test Structure for Wave-Front Aberration Metrology
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Solution Overview
Problem
Current EUV reticle inspection systems face challenges in accurately measuring wave-front aberration due to the limitations of existing diagnostic masks, which are optimized for high peak reflectivity rather than angular bandwidth, leading to difficulties in achieving balanced background suppression, feature resolution, and image quality, especially with thicker absorber materials that interfere with aberration metrology.
Innovation Solution
A test structure and method for measuring wave-front aberration in EUV inspection systems, featuring a substrate with low reflectivity and a multilayer stack with alternating layers of different refractive indices, optimized to have fewer than 15 pairs, and a capping layer to prevent oxidation, designed to provide high contrast and optimized reflectivity, allowing for precise measurement of wave-front aberration across the pupil of the inspection system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a multilayer stack with many alternating pairs is used to increase peak reflectivity, then the reflectivity is improved, but the angular bandwidth is reduced and wave-front aberration measurement precision deteriorates
Solution Approach 1:
The patent changes the physical parameters of the multilayer stack by reducing the number of alternating layer pairs from typical high-reflectivity counts (40-50 pairs) to a reduced count (5-15 pairs). This parameter change trades some peak reflectivity for increased angular bandwidth, enabling the diagnostic mask to provide sufficient light intensity across the full numerical aperture for accurate wave-front aberration measurement.
Solution Approach 2:
The patent applies local quality by creating a specialized multilayer stack with non-uniform properties - using fewer layer pairs in specific regions where angular bandwidth is critical for aberration measurement, while maintaining adequate reflectivity through optimized layer thicknesses and materials. This allows different regions of the optical system to receive appropriately tailored light intensity characteristics.
2Object-generated harmful factors
If a thick absorber layer is used in the diagnostic mask, then the background suppression is improved, but the feature resolution and image quality deteriorate due to interference with aberration metrology
Solution Approach 1:
The patent changes the thickness parameter of the absorber layer from thick (which provides strong background suppression) to thin (which allows sufficient light transmission for accurate wave-front measurement). This parameter optimization finds the balance point where the absorber layer is thin enough to permit adequate light intensity for aberration metrology while still providing sufficient contrast for background suppression.
3Illumination intensity
If a multilayer stack optimized for high peak reflectivity is used, then the reflectivity is improved, but the angular bandwidth is reduced leading to insufficient light intensity across the full numerical aperture
Solution Approach 1:
The patent optimizes the multilayer stack parameters by reducing the number of alternating layer pairs and adjusting layer thicknesses to broaden the angular bandwidth. This ensures sufficient light intensity is delivered across the full numerical aperture of the inspection system, enabling accurate wave-front aberration measurement across the entire pupil, rather than only at narrow angles.
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 proposed solution enables accurate measurement of wave-front aberration with improved image quality and contrast, supporting fine-resolution lithographic patterning and increased angular bandwidth, effectively addressing the limitations of existing diagnostic masks by using a thin multilayer reflector design with reduced bilayer counts and optimized thickness, resulting in a cleaner image and enhanced signal-to-noise ratio.
Implementation Method 1
a multilayer (ML) stack portion formed on the substrate and comprising a plurality of alternating pairs of layers having different refractive indexes so as to reflect EUV light
Implementation Method 2
a multilayer (ML) stack portion formed on the substrate and comprising a plurality of alternating pairs of layers having different refractive indexes so as to reflect EUV light
Implementation Method 3
each alternating pair has a thickness that is optimized so that EUV light diffracted from the test structure substantially fills an entrance pupil area of the inspection system
Data Source
AI summary
Disclosed is test structure for measuring wave-front aberration of an extreme ultraviolet (EUV) inspection system. The test structure includes a substrate formed from a material having substantially no reflectivity for EUV light and a multilayer (ML) stack portion, such as a pillar, formed on the substrate and comprising a plurality of alternating pairs of layers having different refractive indexes so as to reflect EUV light. The pairs have a count equal to or less than 15.