EUV Mirror Decoupling Coating for Reflectivity Preservation
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Solution Overview
Problem
In EUV lithography, existing methods for correcting wavefront deviations in mirrors lead to variations in reflectivity, impairing apodization and causing issues with layer materials like Mo and Si during ion beam figuring, resulting in roughening and oxidation, which affect the reflectivity properties of mirrors.
Innovation Solution
A decoupling coating is introduced between the first and second groups of layers in the mirror's reflective coating, using materials with high absorption coefficients for EUV radiation to prevent radiation from reaching the first group, thereby maintaining reflectivity properties during wavefront correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If wavefront correction is performed by processing the reflective coating layers, then the surface form accuracy is improved, but the reflectivity properties deteriorate due to roughening and oxidation
Solution Approach 1:
The reflective coating is divided into two independent groups of layers: a first group for wavefront correction and a second group for maintaining high reflectivity. The correction layers are processed to correct surface form deviations, while the second group of layers remains unprocessed to preserve reflectivity properties.
Solution Approach 2:
A decoupling coating is introduced between the first and second groups of layers to optically decouple them. This intermediate layer prevents radiation that penetrates the second group from reaching the first group, eliminating the etalon effect and allowing independent optimization of both groups.
2Manufacturing precision
If a correction layer with thickness variation is introduced to correct wavefront deviations, then the wavefront accuracy is improved, but the reflectivity spectrum varies due to constructive interference
Solution Approach 1:
The coating structure is segmented into a first group of layers containing the correction layer with thickness variation, and a second group of layers providing stable reflectivity. This segmentation isolates the wavefront correction function from the reflectivity function.
Solution Approach 2:
The decoupling coating acts as an optical intermediary that prevents interaction between the first and second groups of layers. By blocking radiation from reaching the first group, it eliminates the etalon effect that would otherwise cause reflectivity spectrum variations.
3Manufacturing precision
If ion beam figuring is used to correct the surface form, then the manufacturing precision is improved, but the layer materials undergo roughening and oxidation
Solution Approach 1:
The coating is segmented into correction layers and reflectivity layers. Only the correction layers are subjected to ion beam figuring, while the reflectivity layers remain protected from this harsh processing, avoiding roughening and oxidation.
4Manufacturing precision
If laser radiation is used to heat and compact the layers for wavefront correction, then the surface form precision is improved, but the reflectivity is impaired due to shift in reflectivity spectra
Solution Approach 1:
The coating structure separates correction functions from reflectivity functions. Correction layers can be processed by laser radiation without affecting the reflectivity layers, which maintain their original optical properties.
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 decoupling coating effectively shields the first group of layers from EUV radiation, allowing for precise wavefront correction without impairing the mirror's reflectivity, thus maintaining high reflectivity and preventing constructive interference that can lead to unwanted reflectivity maxima.
Implementation Method 1
The decoupling coating has at least one absorption layer designed for absorbing radiation having a used wavelength in the range between 5 nm and 30 nm
Implementation Method 2
a reflective coating comprising a first group and a second group of layers, wherein the second group of layers is arranged between the first group of layers and the substrate
Data Source
Figure 1a~5c
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Figure 9
AI summary
The invention relates to a mirror (1) for EUV lithography, comprising a substrate (2) and a reflective coating (3, 4), wherein the reflective coating comprises a first group (3) of layers (3a, 3b) and a second group (4) of layers (4a, 4b), wherein the first group (3) and second group (4) of layers (3a, 3b; 4a, 4b) are designed in each case for reflecting radiation having a used wavelength in the range of between 5 nm and 30 nm, wherein the first group (3) of layers (3a, 3b) is arranged between the substrate (2) and the second group (4) of layers (4a, 4b), and wherein a decoupling coating (6) is arranged between the first group (3) and second group (4) of layers (3a, 3b, 4a, 4b), said decoupling coating being designed for optically decoupling the second group (4) of layers (4a, 4b) from the first group (3) of layers (3a, 3b) by preventing the radiation having the used wavelength from reaching the first group (3) of layers (3a, 3b). The reflective coating (3, 4) preferably has a correction layer (5) having a layer thickness variation for correcting the surface form of the mirror (1). The invention also relates to a projection optical unit and an optical system for EUV lithography comprising at least one such mirror, a method for correcting the surface form of such a mirror, and methods for correcting the imaging properties of such a projection optical unit.