DUV Mirror Reflectance Coating Surface Roughness Reduction
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Solution Overview
Problem
Deep ultraviolet (DUV) mirrors require high reflectance coatings, but surface roughness in these coatings leads to light scattering, reducing reflectivity below the necessary 99% due to variations in the film stack and substrate interfaces.
Innovation Solution
A method involving a cap layer of silicon dioxide with a sacrificial layer, etched using oxygen radical plasma to reduce surface roughness, repeated multiple times to achieve a final surface roughness at least 10% smaller than initial, ensuring high reflectance across DUV wavelengths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a multilayer dielectric coating is formed to achieve high theoretical reflectance, then the reflectance can reach 100%, but surface roughness in the fabricated film stack causes light scattering and reduces reflectivity below 99%
Solution Approach 1:
A cap layer is deposited on the uppermost dielectric layer before final assembly, serving as a protective barrier that prevents surface roughness degradation during subsequent handling and assembly processes. This preliminary protective action ensures the surface remains smooth throughout manufacturing.
Solution Approach 2:
The cap layer acts as an intermediary between the delicate multilayer dielectric structure and the external environment, isolating the precision-fabricated layers from sources of surface degradation while allowing the optical function to proceed uninterrupted.
2Productivity
If standard deposition processes are used to form the reflectance coating, then the manufacturing process is simple and fast, but the surface roughness of the uppermost layer is insufficient
Solution Approach 1:
The cap layer is deposited in advance using optimized parameters that prioritize surface smoothness over deposition speed. This preliminary step with controlled, slower deposition ensures the uppermost surface achieves the required precision while the overall process remains efficient.
Solution Approach 2:
Deposition parameters are adjusted specifically for the cap layer formation, using lower deposition rates and optimized conditions to achieve superior surface roughness characteristics compared to standard rapid deposition processes used for bulk layers.
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 method significantly improves reflectance to above 99% by reducing surface roughness, enhancing the optical performance of DUV mirrors by minimizing scattering losses.
Implementation Method 1
etching the sacrificial layer down to the cap layer without substantially reducing the thickness of the cap layer
Implementation Method 2
plasma etching with a plasma that contains oxygen radicals
Data Source
AI summary
A method of reducing surface roughness of DUV reflectance coatings for a DUV mirror to improve the reflectance of the DUV mirror includes: forming the reflectance coating on a substrate, the reflectance coating including a film stack comprising multiple dielectric layers, including an uppermost layer. The method also includes adding to the uppermost layer a cap layer comprising SiO2 and having an upper surface with an initial RMS amount of surface roughness. The method further includes adding a sacrificial layer to the upper surface of the cap layer, wherein the sacrificial layer comprises SiO2. The method also includes etching the sacrificial layer down to the cap layer so that the upper surface of the cap layer has a final RMS amount of surface roughness that is less than the initial amount of surface roughness.