Interferometer Calibration for Lithography Mirror Errors
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Solution Overview
Problem
Lithographic apparatus displacement measuring systems face limitations in accuracy due to unflatness or misalignment of plane mirrors, which affects the measurement of displacements in multiple directions and rotations, requiring complex calibration methods that are either slow or insufficiently repeatable.
Innovation Solution
A method for calibrating an interferometric displacement measuring system using a movable object with a planar reflector, involving obtaining initial and phase-offset measurements to correct for periodic errors, allowing for more accurate displacement and angular position measurement in a lithographic apparatus.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a plane mirror is used in the interferometric displacement measuring system, then displacement measurement can be performed in multiple directions, but unflatness or misalignment of the mirror causes periodic errors that reduce measurement accuracy
Solution Approach 1:
The calibration process is performed in advance to determine correction values for periodic errors caused by mirror unflatness and misalignment. These correction values are stored and applied during actual displacement measurements to eliminate the accuracy degradation while maintaining multi-directional measurement capability.
Solution Approach 2:
The system uses feedback from calibration measurements to continuously improve displacement measurement accuracy. By comparing measured positions with known reference positions, the system generates correction values that are fed back into the measurement process to compensate for mirror imperfections.
2Measurement precision
If complex calibration methods are used to correct mirror errors, then measurement accuracy can be improved, but the calibration process becomes slow and less repeatable
Solution Approach 1:
The calibration process is divided into discrete measurement steps with specific phase offsets. By segmenting the calibration into multiple measurements at different phases and combining the results, the system achieves high accuracy without requiring excessively complex or time-consuming procedures.
Solution Approach 2:
The calibration method utilizes periodic measurements with different phase offsets to characterize and correct mirror errors. This periodic approach allows the system to efficiently capture the periodic error patterns and generate correction values through a systematic, repeatable process.
3Measurement precision
If the movable object is rotated to obtain angular position measurements, then comprehensive calibration data can be collected, but measurement time increases due to the need for multiple position measurements
Solution Approach 1:
Angular position measurements are performed during the calibration phase before actual production measurements. The system pre-characterizes the relationship between mirror orientation and measurement errors, storing correction values that can be applied during normal operation without requiring repeated angular measurements.
Solution Approach 2:
The calibration process systematically varies the phase offset parameter to obtain multiple measurements at different phases. By changing this parameter and analyzing the results, the system efficiently extracts angular position information and error characteristics without requiring exhaustive measurements at all possible angles.
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 improves the accuracy of displacement measurements by eliminating periodic errors, enabling more precise tracking of mirror shape drift and maintaining high precision in lithographic processes.
Implementation Method 1
The two beams are then brought together and interfere. Where the beams interfere a fringe pattern will be formed.
Implementation Method 2
As the reflector mounted on the measured object moves, the path length of the measurement beam changes and the fringe pattern will shift position (in a heterodyne interferometer, the phase of the intensity signal of the measurement detector changes relative to the phase of the intensity signal of the reference detector)
Data Source
AI summary
An interferometric displacement measuring system operable to measure displacements of a movable object of the lithographic apparatus in a first direction using a measurement beam of radiation and a reflector. The reflector being substantially planar and substantially perpendicular to the first direction. Calibration is obtained using a first set of measurements of the angular position movable object. A phase offset in the measurement beam is affected. A second set of measurements of the angular position of the movable object is obtained. The interferometric displacement measuring system is calibrated based on the first and second sets of measurements.