Optical Blank Absorption Measurement Using Angled Light Rays
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Solution Overview
Problem
Current methods for determining absorption in optical elements used in microlithography are inefficient, particularly in measuring volume absorption, as they are sensitive to surface absorption and require complex polishing and precise alignment, leading to high costs and uncertainties in measurement accuracy.
Innovation Solution
A method involving a heating light ray and a measurement light ray intersecting at an angle of less than 90° within the optical element, using a Shack-Hartmann sensor to measure wavefront deformation, allowing for spatially resolved absorption measurement without sensitive alignment, and enabling the production of optical elements with reduced thermal lens effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a collinear heating and measurement light ray setup is used, then absorption measurement is possible, but surface absorption sensitivity requires very careful polishing and precise alignment
Solution Approach 1:
The patent transitions from a collinear (1D) arrangement to a planar (2D) arrangement where the heating light ray and measurement light ray intersect at an angle less than 90 degrees within the blank. This dimensional change allows the measurement to be performed in a plane rather than along a single line, reducing sensitivity to surface imperfections and alignment errors while maintaining measurement precision.
Solution Approach 2:
The patent changes the geometric parameter of the light ray arrangement by introducing an intersection angle between the heating and measurement light rays. This parameter change from 0 degrees (collinear) to an angle less than 90 degrees modifies the measurement geometry to reduce surface absorption sensitivity and eliminate the need for very careful polishing and alignment.
2Ease of manufacture
If transmission measurement is used to determine absorbance coefficient, then measurement can be performed, but it cannot directly determine absorption and requires correction for reflection losses
Solution Approach 1:
The patent extracts the direct absorption measurement capability from the indirect transmission measurement method. By using a heating light ray that is absorbed and converted to heat, and a separate measurement light ray to detect the thermal effect, the method directly measures absorption without requiring correction for reflection losses, thereby improving measurement accuracy while maintaining simplicity.
Solution Approach 2:
The patent introduces a thermal effect as an intermediary between the heating light ray and the measurement. The absorbed energy converts to heat, which then affects the refractive index or physical dimensions of the blank, and this thermal intermediary is detected by the measurement light ray. This indirect thermal measurement pathway enables direct absorption determination without reflection loss corrections.
3Measurement precision
If separate samples are cut from the blank for measurement, then edge regions with higher absorption can be excluded, but additional production costs and potential destructiveness increase
Solution Approach 1:
The patent makes the measurement method universally applicable to entire blanks rather than requiring separate sample preparation. The angled intersection of heating and measurement light rays allows measurement across the full blank surface, enabling both quality control of individual blanks and statistical evaluation of production batches without cutting samples, thereby reducing production complexity while maintaining measurement precision.
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 precise, cost-effective measurement of absorption in optical elements, reducing the thermal lens effect and allowing for the selection of blanks with low absorption coefficients, thereby minimizing imaging aberrations and dynamic wavefront deformations in microlithography systems.
Implementation Method 1
radiating a heating light ray through the blank for the purpose of heating the blank
Implementation Method 2
The material of the optical elements is heated by the absorbed radiation
Implementation Method 3
the refractive index in the respective material changes in a location-dependent manner, such that a thermal lens effect ('lens heating') is associated with the change in temperature
Implementation Method 4
the refractive index in the respective material changes in a location-dependent manner
Data Source
Figure 1a
Figure 1b
Figure 2a~2b
AI summary
The invention relates to a method for determining the absorption of a blank (2) for producing an optical element (3), comprising: radiating a heating light ray (8) through the blank (2) for the purpose of heating the blank (2), and determining the absorption in the blank (2) by measuring at least one property of a measurement light ray (10) influenced by the heating of the blank (2). In the method, either the heating light ray (8) and the measurement light ray (10) or the heating light ray and a further heating light ray are oriented in such a way that they enter into the blank (2) through a first polished surface (2a) or a second polished surface (2b), situated opposite the first surface, and meet one another exclusively in the interior of the blank (2), preferably in a volume (12) used for the production of the optical element (3). The invention also relates to a corresponding measuring apparatus (1 ), to an optical element (3), and to an optical arrangement therewith.