Overlay Error Metrology Using Offset Targets and Image Models
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Solution Overview
Problem
Existing methods for overlay error measurement in semiconductor manufacturing are inaccurate due to reliance on asymmetry indicators, which are affected by line profile asymmetry and beam illumination asymmetry, and require large pitch targets that do not conform to design rules, leading to errors in estimating overlay errors for actual device structures.
Innovation Solution
The use of two overlay targets with programmed offset distances, collecting scatterometry data at different azimuth angles to produce symmetric signals, and applying image-based measurement models trained on raw image data to calculate overlay errors, reducing sensitivity to structural asymmetries and improving measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If asymmetry-based scatterometry overlay measurement is used, then overlay error can be characterized, but measurement accuracy deteriorates due to coupling with line profile asymmetry and beam illumination asymmetry
Solution Approach 1:
The patent extracts and removes the harmful asymmetry components from the measurement signal. By separating the overlay-induced asymmetry from the structure-induced asymmetry through differential measurements and signal processing, the method isolates only the relevant overlay information while eliminating the coupling effects that degrade measurement accuracy.
Solution Approach 2:
The patent intentionally introduces asymmetry in the form of programmed overlay offsets in the target structures. By creating known asymmetric configurations with controlled overlay shifts, the method enables accurate overlay measurement through differential comparison, transforming the harmful asymmetry coupling into a useful measurement mechanism.
2Measurement precision
If large pitch targets are used for scatterometry overlay measurement, then sufficient signal can be generated, but the targets do not conform to design rules and errors occur in estimating overlay errors for actual device structures
Solution Approach 1:
The patent applies local quality by creating target structures with specific local characteristics (programmed overlay offsets) that are embedded within the actual device structures. This allows the targets to maintain the same pitch and design rule conformity as the actual devices while providing the necessary signal for measurement through localized asymmetric features.
Solution Approach 2:
The patent merges the metrology target function with the actual device structures. Instead of using separate large-pitch targets, the method combines overlay measurement capabilities directly into the device-patterned structures themselves, allowing simultaneous fulfillment of design rule conformity and sufficient signal generation for accurate overlay measurement.
3Measurement precision
If multiple specialized target structures are used for overlay measurement, then comprehensive overlay characterization can be achieved, but device complexity increases and measurement reliability deteriorates due to errors in representing actual device structures
Solution Approach 1:
The patent applies universality by designing target structures that serve multiple functions simultaneously. The same structures provide both the actual device pattern and the overlay measurement reference, eliminating the need for separate specialized targets. The programmed overlay offsets enable these structures to function as both device features and metrology targets, reducing overall system complexity while maintaining measurement reliability.
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 allows for precise measurement of overlay errors using fewer targets, reduces errors associated with traditional methods, and improves measurement sensitivity by considering the entire image, thereby enhancing the accuracy and reliability of overlay error determination.
Implementation Method 1
scatterometry based overlay measurements... based primarily on differential measurements of optical signals corresponding to diffraction from pairs of targets
Data Source
AI summary
Methods and systems for measuring overlay error between structures formed on a substrate by successive lithographic processes are presented herein. Two overlay targets, each having programmed offsets in opposite directions are employed to perform an overlay measurement. Overlay error is measured based on zero order scatterometry signals and scatterometry data is collected from each target at two different azimuth angles. In addition, methods and systems for creating an image-based measurement model based on measured, image-based training data are presented. The trained, image-based measurement model is then used to calculate values of one or more parameters of interest directly from measured image data collected from other wafers. The methods and systems for image based measurement described herein are applicable to both metrology and inspection applications.