Lithographic Stage Encoder Purging Cap for Refractive Index Stability
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Solution Overview
Problem
Conventional lithographic apparatuses face inaccuracies in stage position measurement due to environmental factors such as temperature, humidity, and pressure fluctuations, which affect the refractive index of the medium through which the encoder beam propagates.
Innovation Solution
A stage system with an encoder type position measurement system that includes a purging cap enclosing the optical path and a purging medium supply device to maintain a constant propagation medium, reducing the impact of environmental disturbances on position sensing accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If encoder type position sensors are used to measure stage position, then position measurement capability is provided, but measurement precision deteriorates due to environmental factors affecting the refractive index of the propagation medium
Solution Approach 1:
The patent introduces a purging gas system that supplies an inert or controlled atmosphere (such as nitrogen or filtered air) into the measurement space between the encoder grating and the encoder head. This purging medium displaces the ambient air that causes refractive index variations due to temperature, humidity, and pressure changes. By maintaining a stable, controlled gas environment in the optical path, the system eliminates the harmful effects of environmental fluctuations on the encoder beam propagation, thereby preserving measurement precision without requiring complex compensation algorithms or environmental control of the entire lithographic apparatus.
2Measurement precision
If purging gas is supplied to stabilize the propagation medium, then measurement precision improves, but device complexity increases due to additional purging infrastructure
Solution Approach 1:
The purging system is segmented and localized only to the critical measurement region where the encoder beam propagates between the grating and the encoder head. Rather than purging the entire lithographic apparatus, the system applies purging gas selectively to the small volume required for accurate position measurement. This segmented approach minimizes the complexity of the purging infrastructure while effectively stabilizing the refractive index in the measurement path, thereby achieving improved precision without proportionally increasing device complexity.
Solution Approach 2:
The purging gas acts as an intermediary medium between the encoder components and the ambient environment. Instead of directly controlling temperature and humidity in the entire apparatus or using complex refractive index compensation algorithms, the system introduces a controlled gas atmosphere that mediates the interaction between the encoder beam and environmental disturbances. This intermediary purging medium simplifies the overall system by providing a straightforward physical solution to the refractive index stability problem.
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 solution provides a more accurate stage position measurement by stabilizing the propagation medium, thereby minimizing the effects of temperature, humidity, and pressure fluctuations on the encoder beam's path.
Implementation Method 1
A position measurement as obtained from the encoder type position sensor, may hence exhibit a dependency on a propagation characteristic (e.g. a refractive index) of such medium
Data Source
AI summary
A stage system includes a movable stage, and an encoder for measuring a position of the stage, wherein the encoder includes an emitter for emitting an encoder beam, a grating for interacting with the encoder beam, and a detector for detecting the encoder beam having interacted with the grating, the encoder beam in use propagating along an optical path; a purging cap at least partly enclosing the optical path; and a purging medium supply device for supplying a purging medium into the purging cap.