Catadioptric Projection Objective with Low-Absorption Prisms
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Solution Overview
Problem
Conventional catadioptric projection objectives face challenges in maintaining high throughput and stability across a broad spectral range, particularly with mercury g-, h-, and i-lines, and suffer from lens heating issues that introduce aberrations and reduce long-term performance.
Innovation Solution
A catadioptric projection objective design incorporating a concave mirror, a positive lens group, and prisms made from low-absorption materials, optimized for unit magnification with minimal absorption coefficient, ensuring low lens heating and improved color correction using two different optical materials with significant Abbe number difference, facilitating high throughput and prolonged stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional catadioptric projection objectives are used to image a broad spectral range including mercury g-, h- and i-lines, then high throughput is achieved, but lens heating introduces aberrations and reduces long-term performance
Solution Approach 1:
The patent changes the material parameters of the optical elements by selecting specific glass types with optimized absorption coefficients and dispersion characteristics. The positive lens group uses glass with absorption coefficient k < 0.001 cm⁻¹ in the design wavelength band, and the prisms use glass with specific dispersion properties (Abbe number νHg ≥ 20), fundamentally altering the thermal and optical parameters to eliminate lens heating while maintaining broad spectral performance
Solution Approach 2:
The patent employs a composite optical system combining different glass materials with complementary properties: the positive lens group uses low-absorption glass (k < 0.001 cm⁻¹) while the prisms use higher-dispersion glass (Abbe number νHg ≥ 20). This composite material approach allows simultaneous optimization for minimal heating and effective chromatic aberration correction across the broad spectral range
2Productivity
If a broad design wavelength band including mercury g-, h- and i-lines is used, then high throughput is enabled, but chromatic aberration becomes difficult to correct
Solution Approach 1:
The patent applies local quality correction by assigning different dispersion characteristics to different parts of the optical system. The positive lens group uses glass with low dispersion (high Abbe number νHg ≥ 20) to minimize chromatic effects, while the prisms use glass with higher dispersion to provide necessary chromatic correction. This localized differentiation of material properties enables effective aberration correction across the broad spectral range
Solution Approach 2:
The patent uses composite materials with complementary dispersion properties: the positive lens group employs glass with Abbe number νHg ≥ 20 (low dispersion) and the prisms employ glass with higher dispersion. This composite approach creates a balanced chromatic aberration correction system that maintains optical performance across the broad wavelength band from 365 nm to 436 nm
3Ease of manufacture
If optical elements with high absorption coefficient are used, then manufacturing is simpler, but lens heating increases and introduces aberrations
Solution Approach 1:
The patent fundamentally changes the absorption parameter of the optical materials by selecting glass with absorption coefficient k < 0.001 cm⁻¹ in the design wavelength band. This extreme parameter optimization reduces lens heating to negligible levels while maintaining manufacturing feasibility through standard optical glass fabrication processes
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 design achieves high throughput and prolonged stability with reduced aberrations, enabling efficient exposure across the mercury g-, h-, and i-lines, while minimizing lens heating effects and maintaining optical performance over varying conditions.
Implementation Method 1
The positive lens group has positive refractive power... Each optical element of the positive lens group and the first and second prisms is made from a transparent material having Abbe number νHg
Implementation Method 2
The projection objective includes, along its optical axis, a concave mirror... The concave mirror is arranged adjacent to the positive lens group
Implementation Method 3
at least the thickest optical element of the positive lens group and the first and second prisms are made from optical material having an absorption coefficient k that is less than 0.001 cm−1 in the design wavelength band
Data Source
AI summary
A catadioptric projection objective has, along an optical axis, a concave mirror; a positive lens group with positive refractive power arranged adjacent to the concave mirror and spaced apart therefrom; a first prism and a second prism each having respective first fiat surfaces and second flat surfaces, wherein the second flat surfaces are arranged adjacent to the positive lens group and on opposite sides of the optical axis, and wherein the first flat surfaces are arranged adjacent to an object plane and an image plane, respectively, of the projection objective. The projection objective has unit magnification and at least one focus at a wavelength in a design wavelength band that includes mercury g-, h- and i-lines. At least the thickest optical element of the positive lens group and the first and second prism are made from optical material having an absorption coefficient k<0.001 cm−1 in the design wavelength band.