Graded Antireflection Coating for Microlithography Lens Heating
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Solution Overview
Problem
Microlithographic projection exposure systems face challenges in maintaining optical performance due to lens heating effects, particularly when non-uniform heating occurs, which can lead to aberrations and refractive index changes in optical elements, especially under off-axis illumination conditions.
Innovation Solution
The use of a projection objective with low absorption antireflection coatings having a graded structure, featuring three or less layers of dielectric material, applied to lens surfaces to reduce reflection and heat generation, with geometrical layer thickness varying to optimize performance across different angles of incidence, thereby minimizing lens heating effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional antireflection coatings are applied to lens surfaces, then reflection is reduced, but absorption increases causing lens heating
Solution Approach 1:
The patent changes the physical parameters of the antireflection coating by using a graded refractive index structure that transitions from high refractive index at the lens surface to low refractive index at the outer surface. This parameter gradient enables the coating to maintain low reflection across wide angle ranges while minimizing absorption and heat generation in the coating layers.
Solution Approach 2:
The patent employs composite coating structures combining multiple dielectric materials with different refractive indices arranged in graded layers. This composite approach creates an optimized optical path that reduces both reflection and absorption simultaneously, resolving the contradiction between reflection reduction and heat generation.
2Manufacturing precision
If off-axis illumination is used to improve resolution, then depth of focus and contrast increase, but non-uniform lens heating occurs causing aberrations
Solution Approach 1:
The patent applies local quality optimization by designing antireflection coatings with spatially varying properties across the lens surface. The graded refractive index structure is tailored to accommodate different angle of incidence conditions across the optical field, providing localized optimization for both on-axis and off-axis rays, thereby maintaining optical performance stability under off-axis illumination.
Solution Approach 2:
The patent introduces dynamic adaptability through optically adaptive elements that can adjust their optical properties in response to varying illumination conditions. This dynamic characteristic allows the system to maintain optimal performance across different operating conditions, including off-axis illumination scenarios that would otherwise cause non-uniform heating and aberrations.
3Device complexity
If standard optical designs optimized for cold condition are used, then design simplicity is maintained, but optical performance deteriorates under lens heating conditions
Solution Approach 1:
The patent implements preliminary action by pre-compensating for lens heating effects during the optical design phase. The antireflection coatings and optical elements are designed with built-in compensation characteristics that counteract the expected thermal distortions before they occur, allowing standard designs to maintain reliability under operating conditions without requiring complex active compensation systems.
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
This approach effectively reduces lens heating and associated aberrations, maintaining optical performance even under non-uniform heating conditions, and extends the applicability of low-absorption antireflection coatings to higher angles of incidence, enhancing the stability of the projection exposure process.
Implementation Method 1
A low absorption antireflection coating having at least one layer of dielectric material is applied to that lens surface
Implementation Method 2
The radiation varied by the mask and the pattern forms projection radiation propagating through the projection objective
Implementation Method 3
A certain fraction of energy of radiation used for the exposure process is absorbed by optical elements within the projection objective, the amount of absorption typically being dependent on the material subject to radiation energy
Implementation Method 4
The heating of lens groups and other optical elements subject to the radiation... will be referred to as 'lens heating' in the following. Lens heating may cause surface deformations of optical elements and/or changes of refractive index of transparent materials directly through increasing the temperature
Data Source
AI summary
Projection objective, projection exposure apparatuses and related systems and components are disclosed.