Metrology Target Design for Pattern Placement Error Reduction
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Solution Overview
Problem
Existing metrology targets face challenges with pattern placement error (PPE) due to non-agreeing structural scales between devices and metrology targets, leading to printability issues and errors in overlay measurements.
Innovation Solution
Incorporating sub-resolved assist features with the same periodicity as the target structures and applying a learning procedure to derive PPE corrections, which are used to adjust measurements and reduce discrepancies caused by scanner aberrations and other asymmetric effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If standard metrology targets with large pitches are used for overlay measurements, then measurement coverage is improved, but pattern placement error increases and printability deteriorates
Solution Approach 1:
The target structure is divided into multiple segments with different pitch characteristics. The target includes both large-pitch periodic structures for measurement coverage and device-like structures with smaller pitches for reduced PPE, allowing simultaneous achievement of measurement coverage and precision
Solution Approach 2:
Different regions of the metrology target are assigned different structural qualities. Some regions use large-pitch periodic structures optimized for measurement coverage while other regions use device-like structures with smaller pitches optimized for low PPE, allowing each region to serve its specific function
2Manufacturing precision
If device-like target structures with small pitches are used, then pattern placement error is reduced, but optical contrast and printability become challenging
Solution Approach 1:
The target structures incorporate asymmetric design elements that enhance optical contrast while maintaining small pitch dimensions. The asymmetric features create stronger diffraction signals and improve measurability without increasing the pitch size, thereby maintaining low PPE
Solution Approach 2:
The target design transitions from two-dimensional planar structures to three-dimensional structures with varying heights and depths. This dimensional change enhances optical contrast through increased light scattering and diffraction effects, allowing small-pitch structures to maintain both low PPE and high measurability
3Measurement precision
If asymmetric aberrations are present in scanner systems, then measurement accuracy deteriorates, but correcting all aberrations increases system complexity
Solution Approach 1:
The asymmetric aberration effects are extracted and isolated into separate correction terms that can be independently calculated and applied. By separating the aberration correction into distinct components, the system can address specific asymmetric errors without requiring complete redesign of the entire optical system
Solution Approach 2:
The correction approach changes the parameters of the measurement model by introducing aberration-specific correction terms. Rather than physically modifying the optical system, the method adjusts measurement parameters and calculation models to compensate for asymmetric aberrations, reducing hardware complexity
Data Source
AI summary
Metrology methods and targets are provided for reducing or eliminating a difference between a device pattern position and a target pattern position while maintaining target printability, process compatibility and optical contrast—in both imaging and scatterometry metrology. Pattern placement discrepancies may be reduced by using sub-resolved assist features in the mask design which have a same periodicity (fine pitch) as the periodic structure and/or by calibrating the measurement results using PPE (pattern placement error) correction factors derived by applying learning procedures to specific calibration terms, in measurements and/or simulations. Metrology targets are disclosed with multiple periodic structures at the same layer (in addition to regular target structures), e.g., in one or two layers, which are used to calibrate and remove PPE, especially when related to asymmetric effects such as scanner aberrations, off-axis illumination and other error sources.