Overlay Target Modeling Backscattered Electrons in Stacked Gratings
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Solution Overview
Problem
Current semiconductor metrology methods face challenges in accurately measuring overlay between multiple layers due to signal contamination and the inability to distinguish between secondary and backscattered electrons, leading to increased measurement time and inaccuracy, especially in stacked layer structures.
Innovation Solution
A grating-over-grating structure with a calibration and overlay scan location is used, along with an electron beam-based output acquisition subsystem, to form an acquisition image and solve for the overlay by applying design-based filters and transfer functions to improve signal-to-noise ratio and account for scattering, optimizing parameters using symmetry and asymmetry analysis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If spatially separated targets are used for electron beam overlay measurement, then signal distinction between layers is improved, but manufacturing cost increases due to large open areas and measurement time increases due to beam placement distortion
Solution Approach 1:
The target structure is segmented into distinct grating regions (first grating and second grating) that are spatially separated in the design but overlap when projected, allowing signal separation through pattern recognition rather than physical isolation. This enables faster measurement without sacrificing signal distinction.
Solution Approach 2:
The solution transitions from spatial separation in the physical domain to separation in the signal processing domain by using differential signal analysis. The overlay measurement is extracted from the difference between signals from the first and second gratings, effectively separating layer signals without requiring physical separation.
2Measurement precision
If spatially separated targets are used for electron beam overlay measurement, then signal distinction between layers is improved, but manufacturing cost increases due to large open areas
Solution Approach 1:
The target is segmented into compact grating regions that can be densely packed on the wafer, eliminating the need for large open areas. The gratings are designed with specific geometries that enable signal differentiation through their structural differences rather than spatial separation.
Solution Approach 2:
The invention changes the distinguishing parameter from spatial position to structural configuration. By varying the grating parameters (such as line width, spacing, or orientation) between the first and second gratings, the signals can be differentiated without requiring large physical separation, thus reducing manufacturing cost.
3Measurement precision
If full modeling of electron beam stack interaction is performed, then measurement accuracy is improved, but measurement time increases significantly
Solution Approach 1:
Instead of performing complete electron beam stack interaction modeling, the invention uses a simplified differential signal approach that captures the essential overlay information. By measuring the difference between signals from the two gratings, the method obtains sufficient accuracy without the computational burden of full modeling, enabling faster in-line production control.
Solution Approach 2:
The invention creates a simplified mathematical model that copies the essential behavior of the complex electron beam interaction. The differential signal relationship serves as a simplified representation that captures overlay information without requiring detailed knowledge of the complete physical interaction process.
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 precise measurement of overlay with reduced measurement time and improved accuracy, overcoming signal contamination and target inaccuracy, and is compatible with microelectronics fabrication without the need for spatial separation of signals.
Implementation Method 1
there may be an inability to distinguish whether a secondary electron signal corresponds to secondary electrons 4 from the top layer 2, which may be denoted as SE, or secondary electrons 5 from the buried layer 3
Implementation Method 2
there may be an inability to determine whether the backscattered electron (BSE) signal 6 or 7 corresponds to the top layer 2 or a buried layer 3
Data Source
AI summary
An overlay target includes a grating-over-grating structure with a bottom grating structure disposed on a specimen and a top grating structure disposed on the bottom grating structure. The overlay target further includes a calibration scan location including the bottom grating structure but not the top grating structure and an overlay scan location including the top grating structure and the bottom grating structure.