Overlay Focus Selection for Multi-Peak Wafer Measurement
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Solution Overview
Problem
Conventional overlay measurement methods fail to accurately determine the optimal focus when multiple layers with high-contrast focuses are stacked, leading to unclear images and potential alignment errors in semiconductor wafer manufacturing.
Innovation Solution
An overlay measurement device and method that includes a light source, lens assembly, detector, and controller to obtain depth-specific focus images, extract candidate focuses based on maximum contrast indexes, and select an optimal focus using statistical parameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional focus determination methods are used on multi-layer wafers, then the measurement process is simple, but the image clarity and measurement precision deteriorate due to multiple high-contrast focuses
Solution Approach 1:
The patent segments the focus determination process into distinct stages: acquiring multiple focus images at different depths, extracting candidate focuses from each image, and selecting the optimal focus through statistical analysis. This segmentation allows the system to handle multi-peak focus scenarios by processing each layer's focus information separately and then integrating the results.
Solution Approach 2:
The patent introduces a depth dimension by acquiring focus images at multiple depth positions (first depth, second depth, etc.). This transforms the traditional single-plane focus measurement into a multi-depth measurement system, enabling the detection and differentiation of multiple high-contrast focuses from different layers.
2Quantity of substance
If multiple layers are stacked on the wafer, then the circuit density increases, but the focus measurement becomes ambiguous with multiple high-contrast points
Solution Approach 1:
The system uses feedback mechanisms by calculating contrast indexes for multiple candidate focuses and applying statistical analysis to determine the optimal focus. The contrast index calculation provides feedback on image quality at each depth, and the statistical analysis (standard deviation calculation) provides feedback to identify the most reliable focus point among multiple candidates.
Solution Approach 2:
The patent changes the measurement parameter from single-plane contrast to multi-depth contrast by acquiring images at different depths and calculating contrast indexes for each. This parameter change enables the system to distinguish between multiple high-contrast focuses by analyzing their respective depth positions and contrast characteristics.
3Measurement precision
If conventional overlay measurement is performed without focus optimization, then the measurement speed is fast, but the overlay measurement precision deteriorates
Solution Approach 1:
The patent performs preliminary focus optimization before the actual overlay measurement by acquiring multiple focus images and determining the optimal focus in advance. This preliminary action ensures that subsequent overlay measurements are performed at the correct focus, preventing rework and improving overall measurement efficiency despite the initial time investment.
Solution Approach 2:
The system performs self-service by automatically acquiring multiple depth-specific images, calculating contrast indexes, and determining the optimal focus without requiring manual intervention. This automated self-service approach minimizes the time penalty of focus optimization while ensuring accurate focus determination for multi-layer wafers.
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
Enhances measurement accuracy by identifying the optimal focus for multi-peak wafer layers, reducing worker dependency and ensuring consistent results through automated optimization.
Implementation Method 1
an objective lens configured to condense the illumination on a measurement position of at least one point in the overlay measurement target
Implementation Method 2
a detector configured to acquire a focus image at the measurement position based on a beam reflected on the measurement position
Data Source
AI summary
An overlay measurement device includes: a light source configured to direct an illumination to an overlay measurement target in which a first overlay key formed in a first layer and a second overlay key formed in a second layer stacked on an upper portion of the first layer are positioned, a lens assembly having an objective lens and a lens focus actuator, a detector configured to obtain a focus image, and a controller configured to control the lens assembly to acquire depth-specific focus images of the overlay measurement target, extract a plurality of candidate focuses from the acquired depth-specific focus images, and select an optimal focus by applying predetermined parameter to the plurality of candidate focuses.