Overlay Target Design for Metrology Stability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional scatterometers face measurement instability when dealing with thick stacks due to significant path length differences between radiation diffracted by lower and upper gratings, and struggle with targets that have grating pitches smaller than required for non-zero diffraction orders, leading to oscillations and measurement inaccuracies.
Innovation Solution
Designing an overlay target with overlapping periodic structures where the second periodic structure has a pitch smaller than or equal to the wavelength of the measurement radiation, ensuring no propagative non-zero diffraction in reflection, and allowing indirect diffraction in transmission to minimize path length differences and stabilize measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If conventional scatterometry is used with large gratings (40 μm by 40 μm), then mathematical reconstruction is simplified by treating the grating as infinite, but the target size cannot be reduced for positioning among product features
Solution Approach 1:
The patent changes the diffraction order parameter by blocking the zeroth order and detecting only higher orders (dark field scatterometry). This allows smaller target sizes while maintaining measurement capability, as the higher diffraction orders provide sufficient signal for overlay measurement without requiring the target to be larger than the illumination spot
Solution Approach 2:
The patent utilizes intensity asymmetry between +1st and -1st diffraction orders to measure overlay. By comparing the intensities of these opposite diffraction orders, the system can determine overlay error even with small targets, converting the asymmetry caused by misalignment into a measurable signal
2Area of moving object
If dark field scatterometry is used to measure small targets (smaller than illumination spot), then overlay measurement on small targets is enabled, but measurement instability occurs in thick stacks due to significant path length differences between radiation diffracted by lower and upper gratings
Solution Approach 1:
The patent extracts and eliminates the problematic path length difference effect by using dark field detection that blocks the zeroth order. By removing the specular reflection component and detecting only higher diffraction orders, the system eliminates the oscillations caused by path length differences in thick stacks, achieving stable measurements
Solution Approach 2:
The patent introduces an intermediary blocking element (aperture or spatial filter) that blocks the zeroth diffraction order. This intermediary component prevents the problematic direct reflection path from reaching the detector, thereby eliminating the oscillations and stabilizing the measurement signal for thick stack targets
3Adaptability or versatility
If the second periodic structure has pitch smaller than required for propagating non-zero diffraction orders, then targets with small pitch can be measured, but oscillations occur due to evanescent diffraction modes
Solution Approach 1:
The patent substitutes the mechanical/direct detection approach with an optical field-based approach using dark field scatterometry. By blocking the zeroth order and detecting higher orders in the far field, the system can measure small pitch targets where evanescent modes would otherwise cause oscillations, as the optical detection method naturally filters out the problematic near-field effects
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 stabilizes overlay measurements by eliminating propagative diffraction in reflection and maintaining diffraction in transmission, reducing measurement instability and improving accuracy for thick stacks and small pitch targets.
Implementation Method 1
measuring an overlay target twice under certain conditions, while either rotating the overlay target or changing the illumination mode or imaging mode to obtain separately the −1st and the +1st diffraction order intensities
Implementation Method 2
the zeroth order of diffraction (corresponding to a specular reflection) is blocked, and only higher orders processed
Data Source
AI summary
Disclosed is a method of measuring a target, associated substrate comprising a target and computer program. The target comprises overlapping first and second periodic structures. The method comprising illuminating the target with measurement radiation and detecting the resultant scattered radiation. The pitch of the second periodic structure is such, relative to a wavelength of the measurement radiation and its angle of incidence on the target, that there is no propagative non-zeroth diffraction at the second periodic structure resultant from said measurement radiation being initially incident on said second periodic structure. There may be propagative non-zeroth diffraction at the second periodic structure which comprises further diffraction of one or more non-zero diffraction orders resultant from diffraction by the first periodic structure. Alternatively, the detected scattered radiation may comprise non-zero diffraction orders obtained from diffraction at said the periodic structure which have been disturbed in the near field by the second periodic structure.