Projection Mirror Motion to Suppress Lithography Cyclic Errors
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Solution Overview
Problem
Cyclic errors in position measurement systems of lithographic apparatuses, such as interferometers and encoder systems, cause inaccuracies due to thermal drift, affecting the accuracy of mirror positioning and impacting the performance of the lithographic process.
Innovation Solution
A method and system that involves continuously moving position-controlled mirrors during the projection phase to shift the main frequency of cyclic errors above the control loop bandwidth, using feedback and feedforward controllers to adapt mirror movements and reduce alignment errors, and employing notch filters to filter out cyclic error frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If cyclic errors are calibrated and compensated in software, then measurement accuracy is improved, but the compensation becomes inaccurate over time due to thermal drift causing cyclic errors to change
Solution Approach 1:
The patent applies dynamics by continuously moving the mirror during projection phases rather than keeping it static. This continuous movement ensures that cyclic errors manifest as high-frequency signals that can be filtered by the control loop, maintaining measurement accuracy without requiring stable long-term calibration. The system adapts dynamically to thermal drift by continuously operating in a regime where cyclic errors are suppressed through motion.
Solution Approach 2:
The patent changes the operational parameter of mirror movement from static or intermittent to continuous during projection phases. This parameter change transforms the frequency characteristics of cyclic errors, pushing them above the control loop bandwidth where they can be effectively rejected. This allows the system to maintain measurement precision without relying on stable calibration over time.
2Measurement precision
If mirrors are kept stationary during measurements, then position measurement can be performed, but cyclic errors cause position dependent offsets that hamper optical substrate table position measurements
Solution Approach 1:
The patent resolves this contradiction by making the mirror dynamic rather than static. During projection phases, the mirror continuously moves, which transforms cyclic errors into high-frequency components that the control loop can filter. This dynamic operation eliminates position-dependent offsets while still allowing accurate position measurements to be performed through the control system.
Solution Approach 2:
The patent implements continuous mirror movement during projection phases rather than intermittent measurement-only movement. This continuous useful action ensures that cyclic errors are consistently suppressed throughout the projection process, maintaining measurement accuracy for substrate table positioning without requiring the mirror to be completely stationary.
3Manufacturing precision
If mirrors are adjusted after optical measurements, then positioning corrections can be made, but cyclic errors cause the mirror not to reach the correct position
Solution Approach 1:
The patent applies dynamics by continuously moving the mirror during projection phases, which transforms cyclic errors into high-frequency signals that the control loop can reject. This ensures that both optical measurements and subsequent positioning adjustments are performed under conditions where cyclic errors are suppressed, guaranteeing that the mirror reaches the correct position without position-dependent offsets.
4Productivity
If mirrors scan during exposures, then lithographic process continues, but dynamic disturbances occur due to cyclic errors impacting performance
Solution Approach 1:
The patent embraces dynamics by continuously moving the mirror during projection phases including exposures. This continuous movement transforms cyclic errors into high-frequency components that the control loop naturally filters, allowing the mirror to scan during exposures without introducing dynamic disturbances. The system maintains position measurement accuracy even during active scanning because the cyclic errors are pushed above the control bandwidth.
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
Effectively reduces the negative impact of cyclic errors on mirror positioning accuracy, enhancing the optical performance and alignment precision of the lithographic process.
Implementation Method 1
The principle of an interferometer is based on interference of two beams originating from the same source where one beam travels between transmitter, target, e.g. the mirror, back to a receiver and the other beam travels over a fixed distance.
Data Source
AI summary
A method of reducing cyclic error effects in a lithographic process having a projection phase and an idle phase, the method including controlling in a first control loop a first position of a first module, the first module being a position controlled mirror of a projection system, the first control loop having a first bandwidth and including a first position measurement system having a first cyclic error, wherein controlling the first position includes continuously moving the first module at least during the projection phase, such that a first main frequency of the first cyclic error will be above the first bandwidth of the first control loop.