Chromatically Corrected Projection Objective for Microlithography
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Solution Overview
Problem
Microlithographic projection exposure systems face challenges in achieving low-aberration imaging with broadband ultraviolet radiation sources, particularly due to significant chromatic aberrations such as axial chromatic aberration and spherochromatism, which affect image contrast and resolution, especially at high image-side numerical apertures.
Innovation Solution
The development of a projection objective with a plurality of dioptric optical elements, where each element is made from transparent materials with specific normalized optical dispersion characteristics, ensuring that the sum of the chromatic optical path differences for all rays in an axial ray bundle changes uniformly with wavelength, thereby correcting chromatic aberrations and maintaining high contrast across the image field.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If broadband ultraviolet radiation sources are used for microlithographic exposure, then productivity and versatility are improved, but chromatic aberrations worsen significantly
Solution Approach 1:
The patent applies parameter changes by selecting specific optical materials with controlled dispersion characteristics (normalized optical dispersion Δni) and configuring their arrangement to satisfy specific mathematical relationships. This enables the objective to correct chromatic aberrations across broadband UV radiation, maintaining image resolution while using efficient broadband sources.
Solution Approach 2:
The patent employs composite material strategies by combining multiple optical materials with different dispersion properties in a specific configuration. The use of materials with normalized optical dispersions satisfying the relationship Σ(si·Δni)/Σ(si)≤0.05 creates a composite optical system that corrects chromatic aberrations across the broadband spectrum.
2Manufacturing precision
If high image-side numerical aperture is used, then manufacturing precision and imaging resolution are improved, but chromatic variation of spherical aberration worsens
Solution Approach 1:
The patent controls the chromatic variation of spherical aberration by precisely managing parameters including the normalized optical dispersion of materials, the geometric path lengths through each element, and the arrangement configuration. The mathematical relationship Σ(si·Δni)/Σ(si)≤0.05 ensures that even at high NA, chromatic spherical aberration remains corrected.
3Manufacturing precision
If multiple dioptric optical elements with specific dispersion characteristics are arranged to correct chromatic aberrations, then chromatic correction is improved, but device complexity increases
Solution Approach 1:
The patent applies local quality by assigning specific dispersion characteristics to specific positions in the optical train. Each dioptric optical element is strategically placed and configured with particular normalized optical dispersion values to address local chromatic aberration issues at different stages of image formation.
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 solution significantly reduces chromatic variation of spherical aberration and axial chromatic aberration, allowing for high-contrast imaging of both coarse and fine structures across the entire image field, even with broadband ultraviolet radiation sources, thereby improving the overall imaging performance and resolution.
Implementation Method 1
a plurality of optical elements arranged to image a pattern from an object field in an object surface of the objective to an image field in an image surface region of the objective with electromagnetic radiation
Data Source
AI summary
An objective having a plurality of optical elements arranged to image a pattern from an object field to an image field at an image-side numerical aperture NA>0.8 with electromagnetic radiation from a wavelength band around a wavelength λ includes a number N of dioptric optical elements, each dioptric optical element i made from a transparent material having a normalized optical dispersionΔni=ni(λ0)−ni(λ0+1 pm)for a wavelength variation of 1 pm from a wavelength λ0. The objective satisfies the relation∑i=1NΔni(si-di)λ0NA4≤Afor any ray of an axial ray bundle originating from a field point on an optical axis in the object field, where si is a geometrical path length of a ray in an ith dioptric optical element having axial thickness di and the sum extends on all dioptric optical elements of the objective. Where A=0.2 or below, spherochromatism is sufficiently corrected.