Fringe-Projection Autofocus Error Correction Without Reference Mirror
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Solution Overview
Problem
Conventional optical autofocus systems face significant measurement errors due to air turbulence, which are difficult to quantify and control, especially in unsealed environments, leading to inaccuracies in target surface profiling.
Innovation Solution
A lithographic autofocus system employing a fringe-projection system that projects an irradiance pattern with specific opto-geometrical parameters to reduce profiling errors caused by air turbulence, using data-processing circuitry to calculate and subtract phase data associated with optical path length variations, without relying on a reference mirror.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional optical autofocus systems operate in unsealed environments, then ease of operation is improved, but measurement precision deteriorates due to air turbulence
Solution Approach 1:
The patent converts the harmful effect of air turbulence into a measurable signal by using the same turbulent air as the reference medium. The system measures the optical path difference between the measurement beam (interacting with target surface) and reference beam (interacting with reference surface) through the turbulent air, thereby converting the environmental disturbance into useful reference data for error compensation.
Solution Approach 2:
The patent introduces an intermediary reference surface (flat surface) that interacts with the reference beam in the same turbulent environment. This intermediary provides a stable reference point that allows the system to separate and quantify the air turbulence effects from the actual target surface profile, enabling correction of the measurement errors.
2Measurement precision
If a reference mirror is used in fringe-projection autofocus system, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent implements self-service by using the measurement beam's own reflected light from the target surface as the reference beam. The system splits the reflected light into measurement and reference paths, allowing the system to self-reference without requiring external reference mirrors or additional light sources, thereby reducing device complexity while maintaining measurement precision.
Solution Approach 2:
The patent merges the reference beam path with the measurement beam path by using the same optical components and light source. The reference and measurement beams are combined in the interferometer, eliminating the need for separate reference mirror assemblies and reducing overall system complexity while maintaining the ability to correct air turbulence effects.
3Measurement precision
If multiple wavelengths are used for phase measurement, then measurement precision is improved, but use of energy increases
Solution Approach 1:
The patent applies partial action by using a limited number of discrete wavelengths (e.g., two or three specific wavelengths) rather than a continuous broad spectrum. This selective wavelength approach provides sufficient precision for phase measurement while minimizing energy consumption compared to using all possible wavelengths, achieving an optimal balance between measurement accuracy and energy efficiency.
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 system effectively reduces errors induced by air turbulence, enhancing the accuracy and precision of target surface profiling by directly measuring phase data from the interference fringes at multiple wavelengths, thereby improving the operational performance of the autofocus system.
Implementation Method 1
an optical projection system containing a diffraction grating to define interference fringes on a target surface by projecting at least two optical beams formed by said diffraction grating from light incident thereon
Implementation Method 2
an optical projection system structured to project an irradiance pattern representing interference fringes onto a target surface
Data Source
AI summary
Fringe-projection autofocus system devoid of a reference mirror. Contributions to error in determination of a target surface profile caused by air non-uniformities are measured based on multiple measurements of the target surface performed at different wavelengths, and/or angles of incidence, and/or grating pitches and subtracted from the measured profile, rendering the system substantially insensitive to presence of air turbulence. Same optical beams forming a fringe irradiance pattern on target surface are used for measurement of the surface profile and reduction of measurement error by the amount attributed to air turbulence.