Wafer Alignment via Form Birefringence Targets
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Solution Overview
Problem
Current lithographic alignment systems face challenges in accurately measuring the location of alignment marks due to variations in the alignment apparatus and substrate materials, leading to 'on-process' accuracy errors and limited robustness, particularly in differentiating phase offsets and diffraction orders.
Innovation Solution
The system directs an illumination beam with a specific polarization state to form a diffracted beam from an alignment target with sub-wavelength diffraction gratings, splitting it into polarization sub-beams to measure the polarization state, allowing for precise determination of the alignment target's location, which is insensitive to process and structure variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional alignment systems are used to measure alignment mark locations, then the measurement process is simple, but the measurement precision deteriorates due to on-process accuracy errors and sensitivity to apparatus and substrate variations
Solution Approach 1:
The invention changes the measurement parameter from intensity-based detection to polarization state detection. By measuring the polarization state of diffracted light rather than intensity, the system achieves immunity to many sources of error including illumination intensity variations and substrate reflectivity differences, thereby improving both measurement precision and reliability
Solution Approach 2:
The invention replaces conventional intensity-based optical detection with polarization-based detection. This substitution allows the system to extract alignment information from the polarization state of diffracted light, which is insensitive to many environmental and process variations that affect intensity measurements
2Measurement precision
If sub-wavelength diffraction gratings are used as alignment targets, then the form birefringence effect is enhanced for better polarization differentiation, but the manufacturing precision requirements increase
Solution Approach 1:
The invention uses sub-wavelength pitch gratings to induce strong form birefringence effects. By making the grating pitch smaller than the wavelength of incident light, the system creates artificial anisotropic optical properties that produce measurable polarization changes, enabling precise alignment measurements while using standard semiconductor fabrication techniques
3Measurement precision
If polarization-based alignment measurement is implemented, then the accuracy improves by differentiating phase offsets, but the device complexity increases due to polarization optics requirements
Solution Approach 1:
The invention introduces a polarizer as an intermediary element in the optical path. This polarizer converts the polarization state information of the diffracted light into intensity variations that can be measured by conventional detectors, thereby enabling accurate alignment measurement without requiring complex polarization-sensitive detection systems
Solution Approach 2:
The invention exploits polarization state changes as the intermediary carrier of alignment information. By measuring how the polarization state of light changes after interacting with the alignment target, the system can differentiate between various diffraction orders and phase offsets with high accuracy
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 robustness of alignment by effectively measuring the location of alignment targets despite variations in the alignment apparatus and substrate, improving the precision of overlay error measurements.
Implementation Method 1
alignment target comprising a sub-wavelength diffraction grating to induce a form birefringence
Implementation Method 2
directing an illumination beam with a first polarization state to form a diffracted beam with a second polarization state from an alignment target
Data Source
AI summary
An alignment method includes directing an illumination beam with a first polarization state to form a diffracted beam with a second polarization state from an alignment target, and passing the diffracted beam through a polarization analyzer. The alignment method further includes measuring a polarization state of the diffracted beam and determining a location of the alignment target from the measured polarization state relative to its initial polarization state. The alignment target includes a plurality of diffraction gratings with a single pitch and two or more duty cycles, wherein the pitch is smaller than a wavelength of the illumination beam, and the location of the alignment target corresponds to the duty cycle of the diffraction grating.