Dense Fluoride Films for DUV Optical Elements
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Solution Overview
Problem
Current fluoride-coated optical elements, such as mirrors, used in below 200 nm laser systems face challenges due to porous and inhomogeneous film structures from thermal evaporation, leading to increased scatter losses and contamination risks, especially in high-repetition rate excimer laser applications where durability and reflectivity are critical.
Innovation Solution
The use of co-deposition techniques with high refractive index lanthanide metal fluorides like LaF3 and GdF3, and low refractive index materials like AlF3 and MgF2, to form dense and homogeneous fluoride films, with specific substrate orientations and layer thicknesses, to enhance film packing density and reduce surface roughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If thermal resistance evaporation is used for fluoride film deposition, then the deposition process is simple and widely accepted, but the resulting fluoride film has low packing density and porous structure
Solution Approach 1:
The patent uses composite fluoride films combining multiple materials (e.g., LaF3/MgF2, GdF3/AlF3) with different refractive indices and deposition characteristics. The high-index fluoride provides dense structure while the low-index fluoride fills voids, creating a composite film with superior packing density and homogeneity compared to single-material thermal evaporation films.
Solution Approach 2:
The patent modifies deposition parameters by using ion beam sputtering instead of thermal evaporation, changing the physical state and energy of deposited atoms. This parameter change enables higher packing density and more homogeneous film structure while maintaining processability through controlled sputtering conditions.
2Ease of manufacture
If thermal resistance evaporation is used for fluoride film deposition, then the process is widely accepted, but the film structure becomes inhomogeneous
Solution Approach 1:
The composite fluoride film structure with alternating high-index and low-index layers creates a more homogeneous overall composition. The low-index material fills in structural voids and irregularities in the high-index layers, resulting in a uniformly dense film structure that reduces scatter losses and improves compositional stability.
Solution Approach 2:
Different regions of the film have different local compositions - high-index fluoride regions provide structural framework while low-index fluoride regions fill voids and smooth interfaces. This local quality variation throughout the film structure achieves overall homogeneity and eliminates the inhomogeneity problems of single-material films.
3Ease of manufacture
If porous fluoride film structure is used, then deposition is easier, but scatter losses increase and contamination risk increases
Solution Approach 1:
The composite film structure eliminates porosity by using low-index fluoride materials to fill the voids and pores that naturally form in thermally evaporated high-index fluoride films. This creates a dense, non-porous composite structure that dramatically reduces scatter losses while maintaining the optical functionality of the original porous film.
4Reliability
If multiple periods of high index and low index layers are used to achieve high reflectivity, then reflectivity improves, but surface/interface roughness and inhomogeneity increase
Solution Approach 1:
Within each bilayer period, the low-index fluoride layer is strategically positioned to smooth the interfaces between high-index layers. This local quality adjustment at each interface prevents roughness accumulation across multiple periods, maintaining low surface roughness even as reflectivity increases with additional layers.
Solution Approach 2:
The composite structure of alternating high-index and low-index fluoride layers creates a self-smoothing effect where the softer low-index material conforms to and smooths the interfaces with the harder high-index layers. This composite approach allows multiple periods to be stacked while maintaining low overall surface roughness and structural homogeneity.
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 co-deposition method results in improved film homogeneity and reduced scatter losses, increasing the durability and reflectivity of optical elements, addressing the limitations of traditional thermal evaporation methods and enhancing performance in high-repetition rate laser systems.
Implementation Method 1
The invention is directed to optical elements that are coated with dense homogeneous fluoride films and to a method of making such coated elements. The coatings materials are a high ('H') refractive index fluoride material and a low ('L') refractive index material that are co-evaporated to form a coating layer
Data Source
AI summary
The invention is directed to optical elements that are coated with dense homogeneous fluoride films and to a method of making such coated elements. The coatings materials are a high ("H") refractive index fluoride material and a low ("L") refractive index material that are co evaporated to form a coating layer of a L-H coating material (a co deposited coating of L and H materials). Lanthanide metal fluorides (for example, neodymium, lanthanum, dysprosium, yttrium and gadolinium, and combinations thereof) are preferred metal fluorides for use as the high refractive index materials with lanthanum fluoride (LaF3)and gadolinium fluoride (GdF3) being particularly preferred. Aluminum fluoride (AlF3) and alkaline earth metal fluorides (fluorides of calcium, magnesium, barium and strontium) are the preferred low refractive index materials, with magnesium fluoride (MgF2) being a preferred alkaline earth metal fluoride.