Structured Reflective Optical Element with Continuous Plasma Protection
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Solution Overview
Problem
Existing reflective optical elements in EUV lithography are susceptible to etching attacks from hydrogen plasma, leading to reflectivity loss and degradation due to gaps in the reflective coating on structured surfaces with high flank steepness, and existing coating methods are complex for such surfaces.
Innovation Solution
A reflective optical element with a discontinuous reflective coating and a continuous protective layer applied using isotropic methods, which covers the structured surface efficiently, even with flank steepness greater than 60°, and a cap layer to protect the underlying layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a reflective coating is applied to a structured surface with high flank steepness, then the spectral filtering function is achieved, but the reflective coating becomes discontinuous leaving gaps that are susceptible to etching attacks
Solution Approach 1:
The protective function is segmented into two separate layers: a reflective coating that provides spectral filtering (even if discontinuous) and a separate continuous protective layer that prevents etching attacks. This segmentation allows each layer to optimize its specific function without compromising the other.
Solution Approach 2:
A continuous protective layer is introduced as an intermediary between the structured surface and the environment. This protective layer acts as a mediator that prevents direct contact between the etching hydrogen plasma and the exposed surface regions, while allowing the reflective coating to maintain its spectral filtering function.
2Reliability
If a continuous protective layer is applied to cover the structured surface, then protection against etching attacks is improved, but the coating process becomes more complex
Solution Approach 1:
The patent changes the coating parameters by using isotropic coating methods with specific angle ranges (0° to 30° from normal incidence) to achieve continuous coverage. By controlling the coating angle and method, the process achieves the desired protective coverage without excessive complexity.
3Measurement precision
If the flank steepness of the structured surface is increased, then the spectral filtering performance is improved, but the reflective coating coverage becomes discontinuous
Solution Approach 1:
The functional requirements are segmented between the reflective coating (spectral filtering) and the protective layer (continuous coverage). This allows the structured surface to maintain high flank steepness for optimal spectral filtering while the protective layer provides the continuous coverage needed to prevent etching attacks.
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 solution effectively protects the structured surface from etching attacks while maintaining reflectivity by using a thin protective layer applied via isotropic methods, reducing complexity and preventing degradation.
Implementation Method 1
it has been proposed to apply a protective layer by an isotropic coating method, in particular by means of atomic layer deposition (ALD), chemical vapour deposition (CVD) or physical vapour deposition (PVD) with a directed geometry which reduces the anisotropy of the coating method
Implementation Method 2
The mirrors used therein have a substrate to which a reflective coating is applied in order to reflect the EUV radiation. The reflective coating may be configured as a multilayer coating which acts as an interference layer system for the operating wavelength.
Implementation Method 3
The hydrogen ions or free hydrogen radicals cause an etching attack on exposed surfaces of components disposed in the vacuum environment. The etching attack may result in the formation of volatile substances, for example SiH3, SiH4 (silanes), at the surface, which is associated with the removal of the exposed surfaces
Implementation Method 4
On penetration of atomic hydrogen into the material of the (partially) exposed surface, for example of a mirror substrate, stresses may additionally occur, which can lead to layer detachment.
Data Source
AI summary
A reflective optical element (17), in particular for an illumination optical unit of a projection exposure apparatus includes: a structured surface (25a) that preferably forms a grating structure (29), and a reflective coating (36) that is applied to the structured surface (25a). The reflective coating (36) covers the structured surface (25a) discontinuously, and the reflective optical element (17) has at least one protective layer (37) that covers the structured surface (25a) continuously. Also disclosed are an illumination optical unit (4) for a projection exposure apparatus (1) including at least one reflective optical element (17) of this type, to a projection exposure apparatus (1) including an illumination optical unit (4) of this type, and to a method for producing a protective layer (37) on a reflective optical element (17) of this type.