Lithographic Encoder Zero Level Definition via Alignment Marker
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Solution Overview
Problem
Current lithographic apparatuses face challenges in accurately defining a zero level in the z-direction without relying on expensive or additional sensors, particularly in achieving high accuracy and noise reduction while maintaining vacuum compatibility.
Innovation Solution
A position measuring system incorporating an optical x-z encoder with a first and second grating, where the gratings are mounted on a moveable object and a reference frame, respectively, and a controller defines the zero level by scanning along the first grating, utilizing an alignment marker to cause phase changes in the diffracted and reflected radiation orders, allowing for precise measurement in the x and z directions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an optical encoder is used to measure position with high accuracy, then measurement precision is improved, but the ability to define an absolute zero position deteriorates because encoders are incremental sensors
Solution Approach 1:
A laser interferometer is introduced as an intermediary measurement system to detect the zero position of the encoder. The interferometer measures the absolute position of the movable object, providing the missing zero reference information that the incremental encoder cannot provide alone. This combination allows the system to maintain high measurement precision while recovering the lost zero position information.
2Measurement precision
If additional sensors are added to define the zero level, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The laser interferometer is designed to serve multiple functions: it defines the zero position of the encoder, provides absolute position reference, and can be used for calibration purposes. By making this single additional sensor multi-functional, the system achieves accurate zero level definition without proportionally increasing overall system complexity.
3Measurement precision
If interferometers are used for position detection, then measurement precision is improved, but cost increases
Solution Approach 1:
The patent combines the laser interferometer with the existing optical encoder system, merging two position measurement technologies into a unified system. The interferometer provides absolute reference while the encoder handles incremental measurement, creating a hybrid system that achieves high precision without duplicating expensive interferometer components across the entire measurement range.
4Adaptability or versatility
If the range of the position detector is increased, then adaptability is improved, but measurement precision deteriorates
Solution Approach 1:
The position measurement system is segmented into two functional parts: the laser interferometer provides absolute position reference over the entire large range (up to 0.5 m), while the optical encoder provides high-precision incremental measurements within each segment. This segmentation allows the system to maintain nanometer-level precision across the full operating range by combining the strengths of both measurement systems.
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 solution enables accurate definition of the zero level with high precision, potentially achieving accuracy of 1 μm or less, reducing the need for additional sensors and improving noise performance, while maintaining compatibility with vacuum environments.
Implementation Method 1
the encoder is configured to, during scanning along the first grating, split a beam of radiation coming from the radiation source into at least a first positive and negative order of the beam reflected, each order having an angle with respect to the gratings and resulting in a response on the detector which is diffracted ad reflected by the gratings
Implementation Method 2
split a beam of radiation coming from the radiation source into at least a first positive and negative order of the beam reflected
Data Source
AI summary
A lithographic apparatus includes a position measuring system configured to measure a position of a moveable object with respect to a reference frame of the lithographic apparatus, in at least one direction of an orthogonal x-y-z coordinate system of the moveable object. The position measuring system includes an optical x-z-encoder configured to measure a displacement of a radiation source, a first grating, and a detector with respect to a second grating of the encoder. The first grating includes an alignment marker. A controller is configured to define a zero level of the moveable object with respect to the reference frame in at least one of the x- and z-direction by performing the scanning along the first grating, the alignment marker during the scanning step causing changes in the phase of the response of both the first positive and negative orders.