Transmissive Diffraction Grating for EUV Aberration Measurement
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Solution Overview
Problem
Existing transmissive diffraction gratings for EUV radiation in lithographic systems suffer from wavefront distortion and interference effects due to significant differences in refractive indices between the absorbing layer and the surrounding medium, leading to inaccurate aberration measurements in phase-stepping measurement systems, especially at high numerical aperture projections.
Innovation Solution
A transmissive diffraction grating with an absorbing layer made of aluminum nitride (AlN) or aluminum, having a refractive index close to 1 for EUV radiation, reduces wavefront distortion by minimizing the difference in refractive indices with the surrounding medium, and is designed with a thickness that balances absorption and mechanical properties, potentially eliminating the need for a support layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a traditional absorbing layer (chromium, nickel, or cobalt) with high extinction coefficient is used, then sufficient absorption of EUV radiation is achieved, but significant wavefront distortion and interference effects occur due to large refractive index differences with the surrounding medium
Solution Approach 1:
The patent changes the refractive index parameter of the absorbing layer material to be close to that of the surrounding medium (air or vacuum). By selecting aluminum or aluminum nitride with refractive indices near 1 for EUV radiation, the patent minimizes wavefront distortion and interference effects while maintaining sufficient absorption through optimized layer thickness
Solution Approach 2:
The patent employs composite material structures, specifically aluminum nitride (AlN) or aluminum (Al) absorbing layers, which combine the properties of high EUV absorption with refractive indices matching the surrounding medium. This composite approach resolves the contradiction between absorption efficiency and optical distortion
2Loss of energy
If the absorbing layer thickness is increased to improve absorption, then more EUV radiation is absorbed, but wavefront distortion and three-dimensional effects increase
Solution Approach 1:
The patent optimizes the thickness parameter of the absorbing layer to achieve a balance between absorption and wavefront distortion. By controlling the thickness to specific ranges (e.g., 5-50 nm for aluminum, 10-100 nm for aluminum nitride), the patent maintains sufficient absorption while minimizing optical path differences and three-dimensional effects
Solution Approach 2:
The patent applies local quality optimization by tailoring the absorbing layer thickness and material properties specifically for the EUV wavelength range, creating a localized solution that addresses both absorption and wavefront distortion requirements in the critical measurement region
3Strength
If a support layer is added to provide mechanical stability, then the mechanical properties of the diffraction grating are improved, but the complexity of the device structure increases and additional optical interfaces are introduced
Solution Approach 1:
The patent extracts and eliminates the support layer from the traditional two-layer structure, demonstrating that a single thin absorbing layer made of aluminum or aluminum nitride can provide both the required mechanical stability and optical performance. This simplification removes additional optical interfaces that would cause further wavefront distortion
Solution Approach 2:
The absorbing layer material itself (aluminum or aluminum nitride) provides both the absorption function and the mechanical support function, making the structure self-sufficient. The material's inherent mechanical properties enable the thin layer to maintain structural integrity without requiring an additional support layer
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 design enhances the accuracy of aberration maps by reducing wavefront errors and three-dimensional effects, improving the optical performance and mechanical stability of the diffraction grating while maintaining sufficient absorption of EUV radiation.
Implementation Method 1
an absorbing layer provided with a two-dimensional array of through-apertures, wherein the absorbing layer is formed from a material which has a refractive index for the radiation having the first wavelength in the range 0.96 to 1.04
Implementation Method 2
a two-dimensional diffraction grating (i.e. a diffraction grating causing a two-dimensional diffraction pattern) for a shearing phase-stepping interferometric measurement system
Data Source
AI summary
A transmissive diffraction grating for a phase-stepping measurement system for determining an aberration map for a projection system comprises an absorbing layer. The diffraction grating is for use with radiation having a first wavelength (for example (EUV radiation). The absorbing layer is provided with a two-dimensional array of through-apertures. The absorbing layer is formed from a material which has a refractive index for the radiation having the first wavelength in the range 0.% to 1.04.