Catadioptric Projection Objective Antireflection Coatings
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Solution Overview
Problem
Microlithographic projection exposure systems face challenges in achieving high overall transmission while maintaining low uniformity error, particularly due to the influence of reflective coatings on the state of polarization and intensity uniformity across the image field.
Innovation Solution
A catadioptric projection objective is designed with a combination of graded index antireflection layers and multilayer interference antireflection coatings, where the latter is strategically applied to compensate for the negative effects of reflective coatings on uniformity error, optimizing the arrangement of antireflection structures to balance transmission and uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If reflective coatings are applied to mirrors in the catadioptric projection objective, then the reflectivity and light gathering capability are improved, but the uniformity of intensity across the image field deteriorates due to polarization-dependent reflection losses
Solution Approach 1:
The patent applies different antireflection coating strategies to different optical surfaces: graded index layers are applied to most transparent optical elements, while multilayer interference coatings are strategically applied to specific surfaces where they can compensate for polarization-dependent losses from mirror reflections. This local differentiation optimizes both light gathering and intensity uniformity.
Solution Approach 2:
The patent changes the refractive index parameter across the thickness of antireflection layers (graded index from high to low) and uses multilayer interference structures with specific thicknesses and materials to alter the optical path difference, thereby compensating for polarization-dependent reflection losses and improving intensity uniformity across the field.
2Manufacturing precision
If multiple transparent optical elements are used in the projection objective, then the imaging quality and aberration correction are improved, but the overall transmission deteriorates due to cumulative reflection losses at each air-gas interface
Solution Approach 1:
The patent employs graded index antireflection layers where the refractive index continuously changes from high (near substrate) to low (near gas interface), minimizing reflection losses at each air-gas interface of transparent optical elements while maintaining high imaging quality through optimized layer thicknesses and material selection.
Solution Approach 2:
The patent uses composite antireflection structures combining graded index layers with multilayer interference coatings, creating a composite material system that simultaneously reduces reflection losses and maintains optical quality across multiple transparent elements in the projection objective.
3Loss of energy
If graded index antireflection layers are applied to all optical surfaces, then the overall transmission is improved by reducing reflection losses, but the uniformity error increases due to inability to compensate for mirror-induced polarization effects
Solution Approach 1:
The patent applies graded index layers to most optical surfaces to reduce reflection losses, but strategically applies multilayer interference coatings to specific surfaces where they can locally compensate for polarization-dependent losses from mirror reflections, creating a differentiated coating strategy that balances transmission and uniformity.
Solution Approach 2:
The patent introduces multilayer interference coatings as intermediary structures that mediate between the graded index layers and the mirror reflections, compensating for polarization-dependent losses and improving uniformity without significantly compromising the transmission benefits of the graded index structure.
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 achieves high overall transmission (greater than 70%) and low polarized uniformity error (less than 0.5%), effectively addressing the challenges of intensity variation and polarization state changes across the image field.
Implementation Method 1
The antireflection structure includes a graded index layer having a refractive index gradient with a relatively high refractive index on a substrate side of the graded index layer and a relatively low refractive index close to a refractive index of a gas adjacent to a free surface of the graded index layer
Implementation Method 2
at least one transparent optical element having an optical surface provided with a second antireflection structure structured as a multilayer interference antireflection coating
Implementation Method 3
The optical elements include at least one concave mirror having a mirror surface coated with a reflective coating
Data Source
AI summary
The disclosure provides projection objectives which may be used in a microlithographic projection exposure apparatus to expose a radiation-sensitive substrate arranged in the region of an image surface of the projection objective with at least one image of a pattern of a mask arranged in the region of an object surface of the projection objective. The disclosure also provides projection exposure apparatus which include such projection objectives, as well as related components and methods.