EUV Projection Objective Graded Coating Transmission
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Solution Overview
Problem
EUV projection objectives face challenges in achieving high overall transmission due to the limited reflectivity of mirrors in the extreme ultraviolet range, which is exacerbated by the need for multiple mirrors with reflective coatings, leading to suboptimal optical parameters like transmission and telecentricity.
Innovation Solution
The use of graded reflective coatings with rotationally-asymmetric or conical section profiles, such as elliptical, parabolic, or hyperbolic thickness profiles, on mirrors with decentered coating axes, enhances the overall transmission by optimizing optical parameters like transmission, telecentricity, and Stokes parameters in the pupil.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple mirrors with reflective coatings are used in EUV projection objectives, then the ability to image EUV patterns is achieved, but the overall transmission is reduced due to limited reflectivity per mirror
Solution Approach 1:
The patent applies local quality by implementing graded reflective coatings where the coating thickness varies continuously across the mirror surface. Different regions of the mirror have different coating thicknesses optimized for their specific incident angle ranges, with thicker coatings at edges for higher incident angles and thinner coatings at centers for lower incident angles, thereby maximizing reflectivity locally across the entire mirror surface
Solution Approach 2:
The patent employs asymmetry by using rotationally-asymmetric coating thickness profiles that are decentered relative to the mirror axis. The coating axis is deliberately offset from the mirror center, creating an asymmetric thickness distribution that optimizes reflectivity for the specific angular distribution of EUV radiation in the projection objective, breaking the conventional symmetric coating approach
2Loss of energy
If conventional rotationally-symmetric graded coatings are used, then some transmission improvement is achieved, but optical parameters like telecentricity and Stokes parameters remain suboptimal
Solution Approach 1:
The patent resolves this contradiction by introducing rotationally-asymmetric (decentered) coating thickness profiles that break the rotational symmetry. This asymmetric design allows simultaneous optimization of multiple optical parameters including telecentricity and Stokes parameters in the pupil, while maintaining improved transmission through the graded thickness distribution
Solution Approach 2:
The patent applies parameter changes by systematically varying the coating thickness as a function of position on the mirror surface according to specific mathematical profiles (elliptical, parabolic, or hyperbolic). These parameter variations in coating thickness are designed to optimize multiple optical parameters simultaneously, transforming the coating from a uniform or simply graded structure to one with spatially-varying properties that control wavefront characteristics
3Loss of energy
If decentered rotationally-symmetric coatings are used, then transmission is improved, but intensity variations and rotationally-asymmetric aberrations increase
Solution Approach 1:
The patent resolves this apparent contradiction by carefully designing the asymmetric coating profile to compensate for intensity variations. The decentered rotationally-asymmetric profile is specifically shaped to balance the increased transmission benefit against the potential for aberrations, creating an optimized compromise that improves transmission while maintaining acceptable intensity uniformity across the beam
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 significantly increases the overall transmission of EUV projection objectives by improving reflectivity and reducing intensity variations, leading to better imaging performance and higher reflectivity across a wider range of incident angles.
Implementation Method 1
a plurality of imaging mirrors provided with reflective coatings are arranged between the object plane and the image plane, and at least one of the mirrors includes a graded reflective coating
Data Source
AI summary
A projection objective, such as for EUV lithography, for imaging a pattern arranged in an object plane into an image plane with the aid of electromagnetic radiation from the extreme ultraviolet range is provided. The projection objective includes a plurality of mirrors provided with reflective coatings and arranged between the object plane and the image plane. At least one of the mirrors includes a graded reflective coating with a rotationally-asymmetric coating thickness profile in the mirror plane on a substrate with a rotationally-asymmetric or rotationally-symmetric surface profile. The projection objective can exhibit increased overall transmission.