Depolariser Thickness Profile for Microlithography Polarisation Uniformity
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Solution Overview
Problem
Microlithographic projection exposure systems face challenges in eliminating local variations in residual polarization distribution in the pupil plane, leading to unwanted effects such as moiré patterns and uneven polarization, which can impact the quality of microstructured components like integrated circuits and LCDs.
Innovation Solution
Incorporating a depolariser with a microlens array, where the depolariser and microlens array are arranged upstream of the pupil plane, and the depolariser produces a periodic succession of orthogonal polarisation states matching the periodicity of the microlens array, effectively reducing residual polarization to a maximum of 5% or even achieving unpolarised light by ensuring that light beams passing through microlenses at corresponding positions bear orthogonal polarisation states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Hanle depolariser is used to depolarise light from a laser source, then the light is effectively depolarised, but local variations in residual polarisation distribution occur in the pupil plane due to interaction with the microlens array periodicity
Solution Approach 1:
The patent changes the physical parameters of the depolariser by introducing a laterally inhomogeneous thickness profile. This creates a position-dependent retardation that compensates for the periodic polarisation effects introduced by the microlens array, thereby eliminating local variations in residual polarisation while maintaining overall depolarisation effectiveness.
Solution Approach 2:
The depolariser is designed with spatially varying properties - the thickness and/or refractive index differ at different lateral positions. This local variation in the depolariser's characteristics allows it to counteract the position-dependent polarisation effects caused by the microlens array periodicity, achieving uniform polarisation distribution across the pupil plane.
2Productivity
If a microlens array with periodicity is used to produce angular distribution, then light mixing is achieved, but interaction with the depolariser creates residual polarisation distribution with local variations
Solution Approach 1:
By modifying the thickness profile of the depolariser to be laterally inhomogeneous, the patent creates a position-dependent optical path difference that compensates for the periodic structure of the microlens array. This ensures that light beams from different microlenses experience different retardation, preventing the formation of periodic residual polarisation patterns and achieving uniform polarisation distribution.
3Reliability
If the depolariser is positioned upstream of the microlens array, then effective depolarisation is achieved, but periodic superpositioning effects create moiré patterns in the pupil plane
Solution Approach 1:
The laterally inhomogeneous thickness profile of the depolariser introduces a continuous variation in retardation across the beam profile. This prevents the periodic alignment of polarisation states that would otherwise occur due to the microlens array periodicity, thereby eliminating moiré patterns while preserving the depolariser's effectiveness.
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 reduces or eliminates local variations in residual polarization, resulting in a more uniform and unpolarised light distribution in the pupil plane, enhancing the quality of microlithographic components by preventing periodic superpositioning effects and ensuring consistent illumination.
Implementation Method 1
a depolariser which in conjunction with a light mixing system disposed downstream in the light propagation direction at least partially causes effective depolarisation of polarised light impinging on the depolariser
Implementation Method 2
a microlens array which is arranged upstream of the light mixing system in the light propagation direction and in which a plurality of microlenses are arranged with a periodicity
Implementation Method 3
a first wedge plate of birefringent material transparent to light of the working wavelength
Data Source
AI summary
The disclosure relates to an illumination system of a microlithographic projection exposure apparatus. The illumination system can include a depolarizer which in conjunction with a light mixing system disposed downstream in the light propagation direction at least partially causes effective depolarization of polarized light impinging on the depolarizer. The illumination system can also include a microlens array which is arranged upstream of the light mixing system in the light propagation direction. The microlens array can include a plurality of microlenses arranged with a periodicity. The depolarizer can be configured so that a contribution afforded by interaction of the depolarizer with the periodicity of the microlens array to a residual polarization distribution occurring in a pupil plane arranged downstream of the microlens array in the light propagation direction has a maximum degree of polarization of not more than 5%.