Multilayer Reflective Optical Element for EUV Microlithography
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Solution Overview
Problem
Microlithographic projection exposure apparatuses face challenges in achieving high reflectivity and uniform radiation throughput, especially at extreme ultraviolet wavelengths, due to the limitations of available light-transmissive refractive materials, leading to the use of mirrors with multilayer systems that struggle with varying reflectivity across different angles of incidence and wavelengths.
Innovation Solution
A reflective optical element with a multilayer system comprising sections with varying degrees of periodicity, where one section has a higher deviation in partial stack thickness and layer thickness ratio, while the other section is more periodic, optimizing reflectivity and allowing for precise control of the lateral layer thickness profile through X-ray diffractometry, achieving a broadband reflectivity curve with pronounced peaks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a periodic multilayer system is used to achieve high reflectivity at specific wavelengths, then the reflectivity at the design wavelength is maximized, but the reflectivity varies greatly with wavelength and angle of incidence, reducing broadband performance
Solution Approach 1:
The multilayer system is divided into multiple sections with different periodicities. The first section has a first period length optimized for certain wavelengths, while the second section has a second period length optimized for other wavelengths. This segmentation allows each section to contribute to reflectivity at different spectral ranges, achieving broadband performance while maintaining high reflectivity at specific wavelengths.
Solution Approach 2:
Different sections of the multilayer system have different local properties - specifically different period lengths and layer thickness ratios. The first section has a first period length and the second section has a second period length, creating local variations in optical properties that enable the system to achieve high reflectivity across multiple wavelength ranges simultaneously.
2Device complexity
If the number of periods in the multilayer system is reduced to simplify the structure, then the device complexity is decreased, but the reflectivity curve width is reduced, limiting the bandwidth
Solution Approach 1:
Instead of using a single long periodic structure, the system segments the multilayer into multiple sections with different period lengths. This allows the bandwidth to be extended by combining the spectral responses of different sections, achieving a wider overall reflectivity curve without requiring an excessive number of periods in any single section.
3Adaptability or versatility
If a stochastic multilayer system is used to achieve uniform reflectivity across angles and wavelengths, then the broadband performance is improved, but the lateral profile control becomes difficult, reducing manufacturing precision
Solution Approach 1:
The patent introduces local quality variations through different section designs. The first and second sections have different period lengths and layer thickness ratios, creating controlled local variations in the multilayer structure. This allows the system to achieve uniform broadband reflectivity through the combination of sections while maintaining sufficient regularity in each section for precise lateral profile control during manufacturing.
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 enhances the reflectivity bandwidth over wavelengths and angles of incidence, providing sufficient and sharp peaks for effective control and optimization of the lateral layer thickness profile, ensuring improved imaging performance in microlithographic projection exposure apparatuses.
Implementation Method 1
mirrors are used as optical components for the imaging process. Such EUV mirrors comprise a substrate and a multilayer system arranged on the substrate for reflecting the electromagnetic radiation impinging on the optically effective surface
Implementation Method 2
For controlling the lateral profile during the production of a reflective optical element, it is possible to use X-ray diffraction, for example, wherein the reflectivity is measured depending on the angle of incidence
Data Source
AI summary
A reflective optical element (50) having a substrate (52) and a multilayer system (51) that has a plurality of partial stacks (53), each with a first layer (54) of a first material and a second layer (55) of a second material. The first material and the second material differ from one another in refractive index at an operating wavelength of the optical element. Each of the partial stacks has a thickness (Di) and a layer thickness ratio (Γi), wherein the layer thickness ratio is the quotient of the thickness of the respective first layer and the partial stack thickness (Di). In a first section of the multilayer system, for at least one of the two variables of partial stack thickness (Di) and layer thickness ratio (Γi), the mean square deviation from the respective mean values therefor is at least 10% less than in a second section of the multilayer system.