High-Index Refractive Element Aberration Correction
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Solution Overview
Problem
High-index optical materials used in microlithographic projection exposure apparatuses suffer from aberrations due to inhomogeneous optical properties, limiting their practical application in achieving high numerical apertures and resolution.
Innovation Solution
A projection objective design with a high-index refractive optical element having refractive indices greater than 1.6 at 193 nm, incorporating multiple optical surfaces with correction mechanisms such as surface deformations and birefringent layers to correct aberrations caused by variations in refractive index, birefringence, absorption, and scattering across the optical element's volume.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-index optical materials are used to increase numerical aperture and resolution, then imaging performance is improved, but aberrations increase due to inhomogeneous optical properties
Solution Approach 1:
The patent applies local quality by introducing spatially varying correction mechanisms (birefringent layers with position-dependent optical properties, localized surface deformations) that specifically address the inhomogeneous regions of high-index optical materials. Different zones of the optical element receive tailored corrections based on their specific aberration characteristics, allowing the system to maintain high numerical aperture while compensating for local optical property variations.
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting optical parameters (birefringence magnitude, orientation, surface curvature) of correction elements to compensate for aberrations in high-index materials. The birefringent layers are designed with spatially varying optical parameters that counteract the inhomogeneous refractive index distribution, enabling the system to achieve both high numerical aperture and acceptable wavefront quality.
2Power
If high-index optical materials are used to achieve numerical aperture greater than 1, then immersion objective performance is improved, but manufacturing complexity increases
Solution Approach 1:
The patent introduces birefringent layers and surface correction elements as intermediary components that mediate between the high-index optical material and the projection light. These intermediaries provide the necessary optical correction without requiring changes to the fundamental high-index material properties, thereby maintaining the numerical aperture >1 capability while adding manageable correction functionality that can be applied through established manufacturing techniques for birefringent materials and surface figuring.
3Ease of manufacture
If conventional optical materials are used, then manufacturing is easier, but numerical aperture is limited to less than 1
Solution Approach 1:
The patent employs composite material structures combining high-index optical materials with birefringent layers and surface correction elements. This composite approach enables the system to achieve numerical aperture greater than 1 by leveraging the high refractive index of the primary optical material while using the birefringent composite layers to correct the inherent aberrations, thus breaking the numerical aperture limit that constrains conventional single-material 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
The design effectively reduces aberrations, enhancing the numerical aperture and resolution of the projection objective, allowing for the use of high-index materials that were previously impractical due to inhomogeneous optical properties.
Implementation Method 1
at least one optical surface includes a birefringent layer which modifies a polarization state of light passing through the high-index refractive optical element
Implementation Method 2
at least one optical surface includes a deformed optical surface which redirects light rays passing through the high-index refractive optical element
Data Source
AI summary
A projection objective of a microlithographic projection exposure apparatus has a high index refractive optical element with an index of refraction greater than 1.6. This element has a volume and a material related optical property which varies over the volume. Variations of this optical property cause an aberration of the objective. In one embodiment at least 4 optical surfaces are provided that are arranged in at least one volume which is optically conjugate with the volume of the refractive optical element. Each optical surface comprises at least one correction means, for example a surface deformation or a birefringent layer with locally varying properties, which at least partially corrects the aberration caused by the variation of the optical property.