Autofocus Error Compensation via Reflectance Pattern Analysis
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Solution Overview
Problem
The variable reflectivity of semiconductor wafers due to circuits and features on their surface affects the accuracy of autofocus measurements, leading to positioning errors in exposure apparatuses used for semiconductor processing.
Innovation Solution
An autofocus system that includes a slit light source assembly, a slit detector assembly, and a control system, which uses reflectance information to compensate for variations in reflectivity, ensuring more accurate measurements and positioning by reconstructing a detector signal insensitive to patterns on the wafer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional autofocus assembly is used to monitor wafer position, then the position measurement function is provided, but measurement precision deteriorates due to variable reflectivity of the wafer surface
Solution Approach 1:
The patent changes the parameter being measured from total reflected light intensity to the spatial distribution pattern of reflected light. By analyzing how the light pattern changes with focus position rather than relying on absolute intensity, the system becomes insensitive to reflectivity variations caused by circuit patterns on the wafer surface.
Solution Approach 2:
The patent uses a reference pattern (the slit light source pattern) that is copied onto the wafer surface and then reflected back. By comparing the reflected copy of this known pattern against the original, the system can determine focus position based on pattern distortion rather than intensity, eliminating the harmful effect of variable reflectivity.
2Measurement precision
If reflectance information is collected and processed to compensate for reflectivity variations, then measurement precision improves, but device complexity increases
Solution Approach 1:
The patent segments the reflected light into multiple discrete detection zones using an array of photodetectors. Each detector monitors a specific spatial region, and the collective pattern of signals from all segments provides focus information. This segmentation approach enables pattern-based measurement without requiring complex computational processing.
Solution Approach 2:
The patent replaces complex mechanical or computational reflectivity compensation systems with a simpler optical-detection approach. By using a slit light source and array detector configuration, the system inherently encodes focus position information in the spatial distribution of reflected light, eliminating the need for separate reflectivity measurement and compensation mechanisms.
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 achieves improved accuracy in positioning the wafer along the optical axis, enabling the manufacture of higher density wafers by correcting for reflectance variations and enhancing the precision of autofocus measurements.
Implementation Method 1
The slit detector assembly detects light reflected off of the first slit area
Data Source
AI summary
An autofocus system (222C) for measuring the position of a work piece (200) along an axis includes a slit light source assembly (236), a slit detector assembly (238), and a control system (224). The slit light source assembly (236) directs a first slit of light (342A) at a first slit area (344A) of the work piece (200). The slit detector assembly (238) detects light reflected off of the first slit area (344A) and generates a first slit signal relating to the amount of light reflected off of the first slit area (344A) at the slit detector assembly (238). The control system (224) uses the first slit signal from the slit detector assembly (238), and first reflectance information of the first slit area (344A) to determine the position of the work piece (200) along the axis. With this design, the autofocus system (222C) can compensate for the changes in reflectivity of the work piece (200). As a result thereof, measurements taken with the autofocus system (222C) are more accurate and the work piece (200) can be positioned with improved accuracy.