Lithographic Metrology Optical System Polarization Angle Optimization
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Solution Overview
Problem
Metrology systems in lithographic apparatuses face challenges in achieving reproducible measurements due to variations in optical components, particularly in the ratio of intensities of polarizations caused by manufacturing errors in reflective elements, leading to inaccuracies in measuring substrate height and alignment.
Innovation Solution
The implementation of an optical system that directs a beam of radiation with perpendicular polarizations through a diffractive element and reflective elements at carefully selected angles of incidence to reduce variations in the intensity ratio, thereby minimizing the impact of optical component variations and improving measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional optical systems are used in metrology apparatuses, then the system is simple and easy to manufacture, but the measurement precision deteriorates due to variations in optical components
Solution Approach 1:
The patent applies parameter changes by carefully selecting and optimizing the angles of incidence for the beam on the first and second reflective elements. By adjusting these angular parameters, the system reduces the sensitivity to manufacturing variations in the reflective elements, thereby improving measurement precision while maintaining a relatively simple optical configuration.
Solution Approach 2:
The patent introduces asymmetry in the optical path by using two reflective elements at different angles of incidence rather than symmetric arrangements. This asymmetric configuration helps to minimize the impact of manufacturing errors in individual reflective elements on the overall measurement accuracy.
2Reliability
If reflective elements with standard manufacturing tolerances are used, then the device is easy to manufacture, but the reliability of measurements deteriorates due to variations in intensity ratio of polarizations
Solution Approach 1:
The patent changes the operational parameters (angles of incidence) to reduce sensitivity to manufacturing variations. By optimizing these parameters, the system achieves reliable measurements even when reflective elements are manufactured within standard tolerances, without requiring tighter manufacturing specifications.
Solution Approach 2:
The patent converts the potentially harmful effect of manufacturing variations in reflective elements into a benefit by designing an optical configuration where these variations have minimal impact on the measurement. The specific angular arrangement ensures that intensity ratio variations due to manufacturing errors are reduced, making the system more reliable.
3Stability of the object's composition
If the optical system is highly sensitive to optical component variations, then the measurement resolution can be high, but the stability of measurements deteriorates due to machine-to-machine variations
Solution Approach 1:
The patent optimizes the angular parameters of the optical system to reduce sensitivity to manufacturing precision variations. By selecting specific angles of incidence for the reflective elements, the system achieves stable measurements across different machines even when manufacturing precision varies within normal tolerances.
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 approach enhances the accuracy and reliability of metrology systems by reducing machine-to-machine variations and sensitivity to optical component errors, leading to more consistent and precise measurements of substrate height and alignment.
Implementation Method 1
directing a beam of radiation toward a diffractive element
Implementation Method 2
the first reflective element reflects the beam toward a second reflective element at a second angle of incidence
Implementation Method 3
The beam has a first polarization and a second polarization that is perpendicular to the first polarization
Data Source
AI summary
A method to reduce sensitivity of a level sensor, arranged to measure a height of a substrate, to variations of a property of an optical component in the level sensor includes directing a beam of radiation toward a diffractive element and directing the beam, via an optical system, to a first reflective element at a first angle of incidence. The beam has a first polarization and a second polarization that is perpendicular to the first polarization. The first reflective element reflects the beam toward a second reflective element at a second angle of incidence causing the beam to impinge on the substrate. The first and second angles of incidence are selected to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements.