Overlay Targets With Moiré Fringes for Precise Lithography Alignment
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Solution Overview
Problem
Conventional overlay targets in semiconductor processing lack the accuracy required for small feature sizes due to large CCD pixel sizes, leading to measurement errors in overlay alignment, which become more significant as ICs scale down.
Innovation Solution
The use of overlay targets with integrated pattern recognition features and moiré fringe patterns that amplify overlay errors by shifting interference pattern peaks relative to a reference point, allowing for more precise measurement through increased gain factors, enabling detection of small overlay errors with improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional overlay targets are used with standard CCD pixels, then the measurement system is simple and easy to operate, but the measurement precision deteriorates due to large pixel sizes relative to overlay errors
Solution Approach 1:
The patent changes the physical parameters of the overlay target by introducing moiré fringe patterns with specific pitch relationships. The interference pattern has a period T that is much larger than the overlay error δ, creating a magnification effect where small physical displacements produce large observable phase shifts in the interference pattern, thereby improving measurement precision without requiring smaller CCD pixels
Solution Approach 2:
The patent introduces an intermediary interference pattern formed by overlapping two grating patterns with different pitches. This intermediary pattern acts as a mediator that converts small overlay errors into large phase shifts observable by the CCD, effectively bridging the gap between the small feature sizes and the relatively large pixel sizes
2Reliability
If overlay errors are not amplified, then the measurement process is straightforward, but the detection capability deteriorates for small overlay errors in scaled ICs
Solution Approach 1:
The patent changes the measurement parameter from direct displacement measurement to phase shift measurement of an interference pattern. The phase shift φ is related to the overlay error δ by φ = (2π/T)δ, where T is the interference pattern period. By designing T to be much larger than δ, the system amplifies the measurement signal, improving both detection reliability and precision for small overlay errors
Solution Approach 2:
The patent transitions from measuring overlay in one dimension (direct spatial displacement) to measuring it through another dimension (phase space of the interference pattern). This dimensional transformation allows small physical displacements to be detected as large phase variations, enhancing detection capability
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 enhances the detection of overlay errors by amplifying them up to ten times or more, reducing reliance on interpolation and quantization, and providing more accurate measurements using a larger fraction of CCD pixels for data extraction.
Implementation Method 1
the third grating overlaps the first grating to provide a first interference pattern and the fourth grating overlaps the second grating to provide a second interference pattern
Implementation Method 2
overlay targets with integrated pattern recognition features and moiré fringe patterns that amplify overlay errors by shifting interference pattern peaks relative to a reference point
Data Source
AI summary
Embodiments disclosed herein include a lithography reticle set and methods of using such reticle sets. In an embodiment, the set comprises a first reticle and a second reticle. In an embodiment, the first reticle comprises a first grating having a first pitch, and a second grating having a second pitch. In an embodiment, the second reticle comprises a third grating having a third pitch, wherein the third pitch is different than the first pitch, and a fourth grating having a fourth pitch, wherein the fourth pitch is different than the first pitch. In an embodiment the third grating overlaps the first grating and the fourth grating overlaps the second grating when two or more edges of the first reticle are aligned with two or more edges of the second reticle. In an embodiment the first reticle or the second reticle further comprises a pattern recognition feature.