EUV Mask Buried Defect Detection via Multi-Angle Reflectance
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Solution Overview
Problem
Current inspection tools are unable to effectively detect and characterize buried defects in multilayer lithography masks, as they can only provide surface information and do not account for defect propagation through the multilayer stack, leading to defects being undetectable and difficult to repair.
Innovation Solution
A reflective mask inspection system that uses short wavelength radiation to analyze spatially resolved reflectance characteristics from different angles, allowing for the characterization of buried defects by capturing and comparing reflectance images at various rotation and tilt angles, enabling the detection of defects that may not cause surface changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If atomic force microscope (AFM) is used to measure defects at the mask surface, then surface defect information can be obtained, but no information regarding the bulk of the defect or its propagation through the ML stack can be discerned
Solution Approach 1:
The patent transitions from surface-only measurement (2D/3D surface topology) to multi-angle reflectance measurement that probes the bulk multilayer stack. By measuring reflectance at multiple angles and wavelengths, the system accesses information from different depths and layers, effectively adding a dimensional aspect to defect characterization that reveals bulk properties invisible to surface-only techniques.
Solution Approach 2:
The patent introduces reflectance measurements at multiple angles and wavelengths as an intermediary method to indirectly probe bulk defect properties. Rather than directly imaging the bulk (which AFM cannot do), the system uses optical reflectance as a mediator that interacts with the multilayer stack and carries information about bulk defects back to the detector, enabling indirect characterization of buried defects.
2Difficulty of detecting and measuring
If current inspection tools are used to detect defects, then surface defects can be identified, but buried defects in the multilayer stack remain undetectable
Solution Approach 1:
The patent moves from single-angle surface inspection to multi-angle reflectance measurement. By varying the angle of incidence and measuring reflectance at multiple angles, the system probes different depths within the multilayer stack, enabling detection of buried defects that do not manifest at the surface but affect the optical properties of underlying layers.
Solution Approach 2:
The patent changes multiple parameters simultaneously: measurement angle, wavelength, and polarization state. By sweeping through different angles of incidence and wavelengths, the system creates varying interaction conditions with the multilayer stack, causing buried defects to manifest as anomalies in the reflectance data that can be detected and characterized even though they remain invisible at the surface.
3Measurement precision
If AFM surface measurement and simulation is used to characterize defects, then some defect shape information can be obtained, but defect propagation through the ML stack cannot be analyzed
Solution Approach 1:
The patent replaces single-point surface topology measurement with distributed multi-angle reflectance measurement across the multilayer stack. This dimensional change in measurement approach enables observation of how defects propagate through different layers, as each angle and wavelength probes different depths and provides information about defect morphology at various positions within the stack.
Solution Approach 2:
The patent implements a feedback loop where multi-angle reflectance measurements are taken, compared against reference data from known good regions, and used to iteratively refine defect characterization. The system uses the measured reflectance anomalies to infer defect properties and propagation, continuously improving the characterization accuracy by comparing expected versus actual reflectance patterns at multiple angles and wavelengths.
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 method allows for the accurate characterization and detection of buried defects, improving defect detection capabilities beyond surface-level analysis and enabling more precise repair strategies.
Implementation Method 1
A reflective mask inspection system that uses short wavelength radiation to analyze spatially resolved reflectance characteristics
Implementation Method 2
EUV masks, for example, usually have 40 bilayers of molybdenum and silicon, which have been successively coated onto a substrate. In general, a Bragg reflection of around 70% at 13.5 nanometer (nm) wavelength is targeted.
Data Source
AI summary
A reflective mask inspection system comprises a short wavelength radiation source for irradiating a reflective mask. A detector system detects the short wavelength radiation reflected from the reflective mask and a controller compares reflectance images of the reflective mask from the detector to characterize the mask. The system analyzes the spatially resolved reflectance characteristics of the substrate from different angles with respect to normal to the substrate and/or at different angles of rotation of the substrate. This information can be used to then analyze the mask for buried defects and then characterize those defects. This technique improves over current systems that rely on atomic force microscopes, which can only provide surface information.