Overlay Target Grating Asymmetry for Submicrometer Measurement
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Solution Overview
Problem
As semiconductor feature sizes decrease, overlay alignment tolerances become increasingly difficult to measure accurately, especially at submicrometer scales, due to the limitations of conventional image-based overlay measurement targets and diffracted-based overlay metrology, which struggle with small target sizes and increased layer complexity.
Innovation Solution
An image-based overlay measurement system using shifted overlying grating patterns with intentional offsets, where the proximity effect enhances sensitivity to overlay shifts, allowing for precise measurement of asymmetry to determine overlay errors, and self-calibration techniques are employed using multiple overlay measurement pads with different offsets to optimize sensitivity and accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If conventional image-based overlay measurement targets (20×20 μm or 25×25 μm) are used, then measurement capability is maintained, but target size is too large for shrinking scribe lines (50 μm)
Solution Approach 1:
The patent changes the critical parameters of the overlay target by using gratings with pitch and line width optimized for small dimensions. The top and bottom gratings have specific pitch values (e.g., 100 nm to 1 μm) and line width ratios that create measurable asymmetry even at small target sizes, enabling accurate overlay measurement in shrinking scribe lines without sacrificing measurement precision
2Area of stationary object
If overlay targets are made smaller to fit shrinking scribe lines, then space utilization improves, but proximity effects between lines increase and sensitivity to overlay shifts decreases
Solution Approach 1:
The patent optimizes the grating parameters (pitch, line width, spacing) to balance proximity effects with measurement sensitivity. By carefully selecting the pitch-to-line-width ratio and the spacing between top and bottom gratings, the design maintains sufficient asymmetry signal even when lines are close together, preventing the proximity effect from overwhelming the overlay measurement capability
Solution Approach 2:
The patent deliberately introduces asymmetry in the grating design by offsetting the top grating relative to the bottom grating. This intentional offset creates a measurable asymmetric pattern that enhances the sensitivity to overlay shifts. The asymmetric design ensures that even small overlay errors produce detectable changes in the diffraction pattern, maintaining measurement precision despite the small target size and close line spacing
3Measurement precision
If multiple overlay targets are printed to increase measurement coverage, then overlay variation knowledge improves, but the number of targets increases complexity and space requirements
Solution Approach 1:
The patent merges multiple measurement functions into a single integrated overlay target design. By incorporating multiple grating structures with different orientations and pitch values within one compact target, the design enables measurement of overlay variation in multiple directions simultaneously. This merging approach provides comprehensive overlay variation knowledge while avoiding the complexity and space consumption of printing separate targets for each measurement direction
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 improves measurement accuracy and sensitivity by optimizing the design of overlay targets to be close to the image resolution limit, enabling precise determination of overlay errors even at smaller feature sizes, thus addressing the challenges of shrinking scribe lines and increasing layer complexity.
Implementation Method 1
The overlay target is configured so that a lateral separation between the nearest lines in gratings from different layers is sufficiently close to an image resolution limit of the image based metrology device, that proximity effects improve sensitivity of the measured asymmetry to overlay shift
Data Source
AI summary
An image based overlay measurement is performed using an overlay target that includes shifted overlying gratings. The overlay target is imaged and an asymmetry is measured in the image of the overlaid gratings. The asymmetry is used to determine the overlay error. For each measurement direction, the overlay target may include two or more overlay measurement pads with different offsets between the top and bottom gratings. The measured asymmetries and offsets in the overlay measurement pads may be used to determine the overlay error, e.g., using self-calibration. The pitch and critical dimensions of the overlay target may be optimized to produce a greatest change of symmetry with overlay error for a numerical aperture and wavelength of light used by the image based metrology device.