EUV Projection Optical System with Distant Entrance Pupil
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Solution Overview
Problem
Microlithography projection optical systems face challenges in achieving uniform imaging across the field due to field-dependent shading effects and aberrations, particularly when using EUV wavelengths, which affect the resolution and quality of microstructured components.
Innovation Solution
A microlithography projection optical system with a telecentric design at the object plane, incorporating freeform reflective elements and a specific arrangement of optical elements that ensures chief rays are substantially parallel and at an angle of 3° or more to the normal, allowing for the use of phase shifting masks and reducing field-dependent shading effects, while maintaining high resolution and low aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the entrance pupil is located close to the object plane, then the optical system is more compact, but field-dependent shading effects increase and imaging uniformity deteriorates
Solution Approach 1:
The patent moves the entrance pupil from a position close to the object plane to a position far away (more than 2.8 m), effectively using spatial dimensionality to resolve the contradiction. This distant pupil position enables uniform chief ray angles across the field while maintaining a compact optical system through careful arrangement of reflective elements.
Solution Approach 2:
The patent employs asymmetric optical paths with chief rays at angles of 3° or more to the normal, rather than symmetric on-axis configurations. This asymmetric design allows the entrance pupil to be positioned far from the object plane while maintaining compact overall system dimensions through non-traditional ray trajectories.
2Device complexity
If chief rays are at small angles to the normal, then the optical system is simpler, but phase shifting mask functionality is limited and imaging uniformity deteriorates
Solution Approach 1:
The patent changes the critical parameter of chief ray angle from small angles to 3° or more relative to the normal. This parameter change enables phase shifting mask functionality and improves imaging uniformity across the field, while the telecentric design at the image plane maintains relative system simplicity.
3Manufacturing precision
If the entrance pupil is located far from the object plane, then imaging uniformity improves, but the optical system becomes more complex and larger
Solution Approach 1:
The patent replaces traditional dioptric (refractive) elements with catoptric (reflective) elements to achieve the distant entrance pupil configuration. This substitution allows the entrance pupil to be positioned more than 2.8 m from the object plane while managing system complexity through reflective optics that can fold the optical path.
4Ease of manufacture
If conventional optical designs are used, then the system is easier to manufacture, but EUV wavelength imaging suffers from field-dependent shading and resolution limitations
Solution Approach 1:
The patent employs composite optical systems combining multiple reflective elements with different functions (imaging mirrors, beam steering mirrors, field flattening elements). This composite approach addresses EUV-specific challenges like field-dependent shading and resolution limitations while maintaining manufacturability through modular element design.
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 system achieves improved uniformity and resolution across the field, enabling the production of high-quality microstructured components with reduced aberrations and distortion, particularly suitable for EUV lithography.
Implementation Method 1
A microlithography projection optical system that includes a plurality of optical elements arranged to image radiation from an object field in an object plane to an image field in an image plane
Data Source
AI summary
A microlithography projection optical system is disclosed. The system can include a plurality of optical elements arranged to image radiation having a wavelength λ from an object field in an object plane to an image field in an image plane. The plurality of optical elements can have an entrance pupil located more than 2.8 m from the object plane. A path of radiation through the optical system can be characterized by chief rays having an angle of 3° or more with respect to the normal to the object plane. This can allow the use of face shifting masks as objects to be imaged, in particular for EUV wavelengths.