Through-Focus Overlay Metrology for Semiconductor Wafer Alignment
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Solution Overview
Problem
Image-based overlay metrology in semiconductor manufacturing faces challenges in achieving high accuracy and throughput due to variations in image quality caused by depth-dependent factors such as focal depth and aberrations, requiring trade-offs between capturing multiple images for accuracy and single images for throughput.
Innovation Solution
A metrology system and method that adjust the object plane of a detector to capture reference images at multiple focal planes, allowing for single-image overlay determination at a selected measurement plane that corresponds to the reference overlay within a tolerance, thereby optimizing measurement accuracy and eliminating the need for external calibration tools.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple images are captured at different focal planes to improve measurement accuracy, then overlay measurement precision improves, but measurement throughput decreases
Solution Approach 1:
The system performs preliminary through-focus imaging to capture images at multiple focal planes before the actual overlay measurement. This preliminary action characterizes the depth-dependent image quality variations and identifies the optimal focal plane for measurement, enabling subsequent single-image measurements to be performed at the predetermined optimal plane without requiring multiple images during production measurement.
Solution Approach 2:
The system dynamically adjusts the focal plane position based on depth-dependent image quality metrics. By characterizing the relationship between focal depth and image quality (sharpness, contrast, aberrations), the system can adaptively select or adjust the measurement focal plane to optimize both accuracy and throughput for different measurement conditions.
2Productivity
If a single image is used to capture features on multiple layers for higher throughput, then measurement throughput improves, but measurement accuracy deteriorates due to image quality variations
Solution Approach 1:
The system performs preliminary through-focus imaging to identify the optimal focal plane that provides the best image quality for the specific overlay target and measurement conditions. This preliminary characterization enables subsequent single-image measurements to be performed at the predetermined optimal plane, achieving both high throughput and high accuracy without requiring multiple images.
Solution Approach 2:
The system changes the focal plane parameter to optimize image quality for single-image overlay measurement. By adjusting the focal depth to match the depth of the overlay target features, the system maximizes image sharpness and contrast, thereby improving measurement accuracy while maintaining the high throughput benefit of single-image measurement.
3Productivity
If the object plane is fixed at a single focal plane, then measurement throughput is maximized, but measurement accuracy deteriorates due to depth-dependent aberrations and image quality variations
Solution Approach 1:
The system performs preliminary through-focus imaging to characterize depth-dependent image quality variations and identify the optimal focal plane for the specific measurement conditions. This preliminary action enables the system to set the object plane at the optimal position before measurement, ensuring both high throughput and high accuracy.
Solution Approach 2:
The system makes the object plane position dynamic rather than fixed, allowing adjustment to different focal depths based on the specific measurement requirements and depth-dependent image quality characteristics. This dynamic adjustment enables optimization of both throughput and precision for different measurement scenarios.
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 enables high-accuracy, high-throughput overlay measurements by selecting an optimal measurement plane that balances image quality metrics, reducing systematic errors and facilitating self-referenced overlay measurements without external tools, while monitoring process variations across samples.
Implementation Method 1
an object plane of the detector with respect to the sample is adjustable
Implementation Method 2
image quality associated with imaged features on each sample layer, which may vary based on factors such as a depth of field or location of the plane (e.g., focal plane)
Data Source
AI summary
A metrology system includes a controller coupled to a detector to image a sample based on the light captured by an objective lens, where an object plane of the detector with respect to the sample is adjustable. The controller may direct the detector to generate reference images of an overlay target on the sample at multiple object planes including at least a first reference image at a first sample layer and a second reference image at a second sample layer. The controller may further determine a reference overlay between the first layer and the second layer at the overlay target based on the first reference image and the second reference image. The controller may further select a measurement object plane for single-image overlay determination that corresponds to the reference overlay within a selected tolerance. The controller may further determine overlay for additional overlay targets at the measurement plane.