Optical Splitter for Overlay Metrology Phase Error Cancellation
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Solution Overview
Problem
Existing optical systems for alignment, registration, and overlay metrology of integrated circuits are prone to measurement errors due to substrate variations and inherent asymmetries in the imaging system, particularly from diffraction effects and phase modulation of light.
Innovation Solution
An optical system that splits reflected light into multiple portions, directing each portion to separate image sensors and processing them independently before combining the results, effectively canceling out dominant common mode phase errors by averaging the measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single imaging system is used to capture the sample image, then the device complexity is low, but measurement precision deteriorates due to common mode phase errors and optical asymmetries
Solution Approach 1:
The patent divides the imaging system into multiple separate imaging channels (first imaging system and second imaging system), each capturing a portion of the sample image. This segmentation allows independent processing of each channel to cancel common mode phase errors through averaging, thereby improving measurement precision while accepting increased device complexity.
Solution Approach 2:
The patent creates multiple copies of the sample image through separate imaging channels, each with potentially different optical paths and asymmetries. By processing these independent copies and averaging their measurements, the system eliminates common mode errors while maintaining the complexity of having multiple imaging systems.
2Measurement precision
If multiple imaging channels are used to cancel common mode phase errors, then measurement precision improves, but device complexity increases
Solution Approach 1:
The optical system is segmented into multiple independent imaging channels that can be processed separately. Each channel captures image data that can be independently analyzed, allowing the system to cancel common mode phase errors through averaging while managing complexity through modular architecture.
Solution Approach 2:
The system processes images from multiple channels and uses the combined information to correct for optical asymmetries and phase errors. The feedback mechanism involves analyzing the differences between channels and using this information to improve the final measurement accuracy.
3Shape
If diffraction orders are filtered to enhance image contrast, then image quality improves, but measurement precision deteriorates due to phase modulation errors
Solution Approach 1:
Instead of filtering diffraction orders in a single channel, the patent segments the imaging into multiple channels that can independently capture diffraction information. This allows the system to maintain image contrast while eliminating phase modulation errors through independent processing and averaging of multiple channels.
Solution Approach 2:
The patent converts the harmful effect of diffraction-induced phase modulation errors into a beneficial effect by using multiple imaging channels. The errors in individual channels are transformed into cancelable differences between channels, allowing the system to achieve both good image contrast and high measurement precision.
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 significantly improves the accuracy of imaging systems by reducing errors caused by substrate variations and optical imperfections, enhancing the precision of alignment, registration, and overlay metrology.
Implementation Method 1
an optical splitter disposed at the pupil plane, for receiving the reflected light at the pupil plane and splitting the reflected light into at least a first light portion and a second light portion
Implementation Method 2
illumination optics for directing the illumination onto the sample, thereby producing reflected light
Implementation Method 3
first imaging optics for receiving the first light portion and directing the first light portion to a first image sensor
Data Source
AI summary
An optical system including a light source, optics for directing illumination, thereby producing reflected light, optics for receiving the reflected light, a splitter disposed at a pupil plane for receiving the reflected light and splitting it into a first and second portion, first imaging optics for receiving the first portion and directing it to a first sensor to produce a first image portion, the first sensor delivering the first image portion to a processor, second imaging optics for receiving the second portion and directing it to a second sensor to produce a second image portion, the second sensor delivering the second image portion to the processor, and the processor for combining the first image portion and the second image portion into a single image of the sample.