Defect Detection in Titanium-Doped Silica Glass Using Oblique Light
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Solution Overview
Problem
Current methods for detecting defects in titanium-doped silica glass substrates for EUV lithography are inadequate, particularly for large, thick blanks with brownish coloration, as they struggle with light absorption and sample size limitations, leading to inaccurate detection of internal defects and their depth positions.
Innovation Solution
A method involving face grinding and determining defect positions using a light beam incident at a predetermined angle, with scattered light detected vertically above the surface, allowing for precise removal of the surface layer to eliminate defects, and a system utilizing a light source and detection element to determine defect positions in the surface layer of Ti-doped silica glass blanks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If optical measurement methods with vertical light incidence are used to detect defects in Ti-doped silica glass, then defect position can be determined in transparent glasses, but the method fails for colored glasses with high light absorption and for thick samples where light intensity decreases considerably
Solution Approach 1:
The patent changes the measurement geometry from vertical incidence to oblique incidence, introducing a new dimensional aspect to the measurement. By directing light at an angle α between 5° and 75° relative to the surface normal, the method enables defect detection in thick, light-absorbing materials where vertical measurement would fail due to excessive attenuation.
Solution Approach 2:
The patent modifies the measurement parameters by changing the angle of light incidence from the conventional vertical direction to an oblique angle within a specific range (5°-75°). This parameter change allows the light to penetrate and interact with defects in thick, colored glass samples while still enabling position determination through the relationship between incidence angle and scattered light detection position.
2Area of stationary object
If the sample size is increased to accommodate large mirror substrate blanks, then EUV lithography requirements are met, but the light intensity decreases considerably with path length making defects invisible
Solution Approach 1:
By measuring at an oblique angle rather than vertically, the effective optical path through the material is extended in a controlled manner, allowing light to interact with defects throughout the thickness of large samples while maintaining sufficient intensity for detection.
Solution Approach 2:
The patent replaces the conventional vertical transmission measurement approach with an oblique incidence scattered light measurement system. This substitution enables defect detection in large-area, thick samples by detecting scattered light at an angle rather than relying on direct transmission, thereby maintaining measurement capability despite increased sample dimensions.
3Stability of the object's composition
If Ti-doped silica glass is used to achieve zero thermal expansion for EUV lithography, then mask and mirror substrate requirements are met, but the glass develops a brownish coloration that increases light absorption
Solution Approach 1:
The patent changes the measurement parameters (light incidence angle, detection geometry) to accommodate the optical properties of Ti-doped glass. By measuring at oblique angles and detecting scattered light rather than transmitted light, the method achieves defect detection capability in materials with high absorption coefficients caused by titanium doping.
Solution Approach 2:
The patent substitutes the conventional transmission-based defect detection method with a scattered light detection method at oblique incidence. This substitution enables measurement in Ti-doped silica glass where the titanium content provides the required thermal stability but creates excessive light absorption for conventional measurement techniques.
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 enables simple and accurate detection of defects within the surface layer of Ti-doped silica glass blanks, optimizing the production of mirror substrate blanks by ensuring no bubbles or inclusions reach the final mirror surface, thus enhancing the imaging quality for EUV lithography.
Implementation Method 1
b2) the light is scattered on a defect in the blank
Data Source
AI summary
A method for producing a mirror substrate blank made from titanium-doped silica glass for EUV lithography, having a thickness of at least 40 millimeters, includes steps of face grinding the surface of the blank and identifying data on defects in a surface layer of the blank. Light penetrates the blank at a predetermined angle of incidence α of less than 90° at a location on the flat surface of the blank. The light scatters on a defect in the blank, and the scattered light is detected at a distance x from the penetration location on the surface of the blank by a light detection element arranged perpendicularly thereabove. The method further includes steps of determining the position of the defect in the surface layer based on the obtained data, and partial or complete removal of the surface layer in consideration of the position determination and forming the mirror substrate blank.