Diffraction Overlay Metrology Using Composite Gratings
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Solution Overview
Problem
Diffraction-based overlay metrology in lithographic processes faces challenges in accuracy due to sensitivity to vibrations and optical aberrations, and the limitations of illumination beam cross-section size, which affects the measurement of overlay errors near critical structures and the size of gratings required.
Innovation Solution
A method and system that utilize a composite grating with first and second gratings of identical pitch, illuminated by a common beam with an annular cross-section at oblique angles to measure the intensity difference of first and negative first order diffracted beams, allowing for more precise overlay error determination while avoiding interference from surrounding regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If image based overlay error measurement is used, then the position of mask pattern relative to substrate pattern can be determined, but the measurement becomes sensitive to vibrations and optical aberrations, reducing accuracy
Solution Approach 1:
The patent replaces image-based optical measurement with diffraction-based measurement. Instead of capturing and analyzing images of the overlay structures, the system uses diffraction patterns generated by illuminating the composite grating at oblique angles. This substitution of measurement mechanism eliminates sensitivity to vibrations and optical aberrations that plague image-based methods.
Solution Approach 2:
The patent changes the measurement parameter from image intensity distribution to diffraction order intensity ratios. By measuring the intensity ratio between positive and negative first-order diffracted beams rather than analyzing images, the system achieves vibration-insensitive overlay error determination. This parameter transformation fundamentally resolves the sensitivity issue.
2Measurement precision
If a common illumination beam with annular cross-section is used to illuminate the composite grating, then interference from surrounding regions is avoided, but the beam cross-section size is limited affecting measurement capability
Solution Approach 1:
The patent transitions from measuring in the spatial domain to measuring in the angular/frequency domain through diffraction. By illuminating the composite grating at oblique angles and measuring diffraction orders, the system achieves spatial selectivity without being constrained by beam cross-section area. The diffraction geometry provides inherent spatial filtering that eliminates interference from surrounding regions while maintaining measurement capability.
Solution Approach 2:
The patent uses asymmetric illumination at oblique angles to selectively enhance diffraction from the composite grating structure. The oblique incidence creates asymmetric diffraction patterns where the intensity ratio between positive and negative first-order beams directly reflects the overlay error. This asymmetric illumination strategy enables precise measurement while maintaining a compact beam footprint.
3Measurement precision
If smaller grating sizes are used to enable measurements closer to critical structures, then measurement proximity to critical features is improved, but the diffraction signal strength decreases
Solution Approach 1:
The patent employs periodic illumination at multiple oblique angles to sequentially measure diffraction patterns from the composite grating. By systematically varying the illumination angle and measuring the intensity ratio of diffracted beams at each angle, the system accumulates measurement data that compensates for the reduced signal strength from smaller gratings. This periodic measurement approach extracts maximum information from weak diffraction signals.
Solution Approach 2:
The patent uses the measured intensity ratio of diffracted beams as feedback to determine overlay error. The system continuously monitors the diffraction signal and uses the intensity ratio relationship (which is directly proportional to overlay error) to provide real-time measurement feedback. This feedback mechanism enables accurate measurement even with reduced signal strength from smaller grating structures.
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 accuracy and precision of overlay error measurement by reducing the impact of vibrations and optical aberrations, enabling measurements closer to critical structures with smaller grating sizes, thus improving the accuracy and reliability of lithographic processes.
Implementation Method 1
measuring a first intensity of a first order diffracted beam from the first composite grating; and providing a second illumination beam for illuminating at least the first composite grating
Data Source
AI summary
Systems, methods, and apparatus are provided for determining overlay of a pattern on a substrate with a mask pattern defined in a resist layer on top of the pattern on the substrate. A first grating is provided under a second grating, each having substantially identical pitch to the other, together forming a composite grating. A first illumination beam is provided under an angle of incidence along a first horizontal direction. The intensity of a diffracted beam from the composite grating is measured. A second illumination beam is provided under the angle of incidence along a second horizontal direction. The second horizontal direction is opposite to the first horizontal direction. The intensity of the diffracted beam from the composite grating is measured. The difference between the diffracted beam from the first illumination beam and the diffracted beam from the second illumination beam, linearly scaled, results in the overlay error.