Reflective Beam Transforming Device for EUV Illumination
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Solution Overview
Problem
Current beam transforming devices in microlithographic illumination systems face challenges such as the axicon effect, limited field invariance, and material constraints, particularly in EUV systems, which affect the angular distribution and optical performance, especially at higher numerical apertures and shorter wavelengths.
Innovation Solution
A purely reflective beam transforming device is designed with a configuration of mirrors, including a first mirror with a conical reflective surface and a second mirror with a toroidal shape, where the second mirror images the first mirror onto a third mirror, reducing the axicon effect and allowing for variable annular cross-sections, and featuring an actuator for adjusting the spatial relationship between the mirrors to modify the light bundle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a beam transforming device with refractive optical elements is used to transform radial energy distribution, then the angular distribution of illumination can be controlled, but the device suffers from material constraints and limited performance at higher numerical apertures and shorter wavelengths
Solution Approach 1:
The patent replaces refractive optical elements (lenses, prisms) with reflective optical elements (mirrors with specific surface profiles). This substitution eliminates material absorption and dispersion issues that plague refractive systems at extreme ultraviolet wavelengths and high numerical apertures, while maintaining the ability to control angular distribution through carefully designed reflective surface geometries.
2Adaptability or versatility
If an axicon system is used for radial redistribution of light intensities, then annular intensity distributions can be generated, but the system exhibits the axicon effect that limits field invariance
Solution Approach 1:
The patent employs mirrors with specific curved surface profiles (such as hyperboloidal or paraboloidal shapes) instead of simple conical axicon surfaces. These curved surfaces are designed to compensate for the axicon effect, maintaining field invariance across the illumination field while still achieving the desired annular intensity distribution through the curved reflective geometry.
3Adaptability or versatility
If multiple optical raster elements are used to generate different illumination settings, then various angular distributions can be achieved, but the device complexity increases
Solution Approach 1:
The patent employs movable mirrors or adjustable optical elements that can be positioned at different angles or locations to dynamically reconfigure the illumination path. This allows a single reflective beam transforming device to generate multiple illumination settings (conventional, annular, dipole, quadrupole) by changing the spatial relationship between components, eliminating the need for multiple fixed optical raster elements.
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 solution effectively reduces the axicon effect, maintains collimation, and enables efficient production of annular beam cross-sections, improving the optical performance and adaptability of the illumination system, particularly suited for EUV applications.
Implementation Method 1
a first mirror (122) having a conical reflective surface (124)
Implementation Method 2
a second mirror (126) having a reflective surface (130) with a toroidal shape
Implementation Method 3
maintains collimation
Data Source
AI summary
An illumination system of a microlithographic exposure apparatus has an optical axis and a beam transforming device. This device includes a first mirror with a first reflective surface having a shape that is defined by rotating a straight line, which is inclined with respect to the optical axis, around the optical axis. The device further includes a second mirror with a second reflective surface having a shape that is defined by rotating a curved line around the optical axis. At least one of the mirrors has a central aperture containing the optical axis. This device may form a zoom-collimator for an EUV illumination system that transforms a diverging light bundle into a collimated light bundle of variable shape and/or diameter.