EUV Projection-Optical Systems OoB Light Management
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Solution Overview
Problem
Current projection-optical systems for EUV lithography face issues with out-of-band (OoB) light, which causes thermal aberrations, background exposure noise, and wafer heating, leading to degraded optical performance and alignment precision due to the limitations of conventional filters and reflectors.
Innovation Solution
The use of a dual-reflector configuration where one reflector has a lower reflectance for OoB light and another with higher reflectance for the same wavelength, strategically positioned to absorb OoB light before it reaches the wafer, reducing thermal deformation and optical property deterioration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a membrane-type filter is used to block OoB light, then OoB light transmission is reduced, but the filter is fragile, difficult to manufacture in large diameters, and exhibits low EUV light transmission
Solution Approach 1:
The patent changes the physical state and optical parameters by using a multilayer-film reflector instead of a membrane filter. The multilayer film comprises alternating layers of high-refractive-index material (e.g., Mo, Ru, Rh) and low-refractive-index material (e.g., Si, SiO2, MgF2), creating constructive and destructive interference patterns that selectively reflect OoB light while transmitting EUV light. This parameter change in material composition and layer structure resolves the contradiction between OoB blocking and filter reliability.
Solution Approach 2:
The patent employs composite multilayer-film structures combining different materials with complementary optical properties. The alternating layers of high and low refractive index materials create a composite structure that achieves superior optical filtering performance. This composite approach eliminates the fragility and manufacturing difficulties of single-material membrane filters while maintaining effective OoB light blocking.
2Object-affected harmful factors
If a multilayer-film reflector with antireflective layer is used as OoB filter, then OoB light reflection is suppressed, but EUV light is also absorbed and intensity must be increased, causing more thermal deformation
Solution Approach 1:
The patent applies local quality by designing the multilayer-film reflector with specific layer thicknesses and material compositions optimized for different wavelength ranges. The antireflective layer is engineered with precise thickness control to be anti reflective only for OoB wavelengths while maintaining high reflectance for EUV wavelengths. This localized optical property differentiation allows selective wavelength management, suppressing OoB reflection without absorbing EUV light, thereby avoiding the thermal deformation problem.
3Manufacturing precision
If conventional photolithography with wavelengths of approximately 190 nm or greater is used, then current technology is available, but resolving power of 50 nm or less is not achievable
Solution Approach 1:
The patent replaces conventional refractive optical elements with reflective optical elements (mirrors and multilayer-film reflectors) suitable for EUV wavelengths. Since materials have refractive indices very close to unity at EUV wavelengths, conventional lenses and prisms cannot function. The invention substitutes refraction-based optics with reflection-based optics, enabling EUV lithography systems to achieve the required resolving power for sub-50 nm manufacturing precision.
4Illumination intensity
If EUV light source radiates various wavelengths including OoB wavelengths, then complete spectrum is produced, but thermal aberrations, background exposure noise, and wafer heating occur
Solution Approach 1:
The patent extracts and removes OoB wavelengths from the EUV light spectrum using the multilayer-film reflector as a wavelength-selective filter. The reflector is designed to reflect OoB light away from the optical path while allowing EUV light to pass through to the wafer. This extraction of harmful wavelengths from the complete spectrum eliminates thermal aberrations, background exposure noise, and wafer heating while preserving the beneficial EUV illumination for high-resolution lithography.
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 configuration significantly decreases OoB radiation on the wafer, minimizing thermal aberrations and maintaining optical system performance by effectively managing radiant energy absorption and reducing reflector deformation.
Implementation Method 1
the first reflector absorbs sufficient light of the predetermined wavelength, which reduces the influence of the light of predetermined wavelength on the reflector that is positioned closer to the second surface than the first reflector
Implementation Method 2
grazing-incidence mirrors (providing total reflection due to their index of refraction being slightly less than 1) and multilayer-film reflectors (which combine and superpose multiple phases of light weakly reflected at layer interfaces) are used
Data Source
AI summary
Projection-optical systems are disclosed that reduce OoB radiation doses on the wafer while reducing deterioration of optical properties of the systems. An exemplary system includes a first reflector having a reflectance for light of a second predetermined wavelength, different from light of a first predetermined wavelength, that is less than a predetermined reflectance. The system also includes a second reflector having a reflectance for light of the second wavelength which is greater than the predetermined reflectance. When the reflectors in the system are classified as reflectors having a high percentage of overlap for the reflecting regions corresponding to two different points on the wafer, and reflectors having a low percentage of overlap for the reflecting regions, then, among the reflectors having a lower percentage of overlap for the reflecting regions, the most upstream reflector in the light path of the system is the second reflector.