Overlay Measurement Scatterometer Targets
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Solution Overview
Problem
Current methods for measuring overlay in lithographic processes are not accurate enough, especially at the 32nm half-pitch node and beyond, as they typically measure macro-pitch targets that do not resemble the actual product pitch, leading to a loss of resolution and increased costs with expensive and time-consuming CD SEM measurements.
Innovation Solution
A system using at least three targets with different predetermined overlay biases, where a radiation beam is projected onto each target to detect radiation interactions, and a processor determines the overlay offset by analyzing the measured dimensions and biases to achieve accurate at-resolution overlay measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If macro-pitch targets are used for overlay measurement, then measurement is straightforward, but measurement precision is lost due to resolution loss
Solution Approach 1:
The patent applies local quality by creating targets with different local pitch characteristics. Specifically, it uses a combination of macro-pitch targets (for ease of measurement) and at-resolution targets (for precision). The method measures overlay at multiple pitch scales locally across the substrate, then combines these measurements to achieve both operational ease and measurement precision.
2Measurement precision
If CD SEM measurements are used for at-resolution overlay, then measurement accuracy improves, but manufacturing cost and time increase
Solution Approach 1:
The patent segments the overlay measurement process into multiple pitch components. Instead of using a single expensive CD SEM measurement at at-resolution, it divides the measurement into macro-pitch measurements (fast, low-cost) and at-resolution measurements (slow, high-cost), then combines them through a mathematical model to achieve accurate at-resolution overlay measurement without always requiring the expensive CD SEM.
Solution Approach 2:
The patent introduces an intermediary mathematical model that relates macro-pitch measurements to at-resolution overlay. This model acts as a mediator, translating the easier-to-obtain macro-pitch data into accurate at-resolution overlay values, reducing the need for direct expensive CD SEM measurements.
3Ease of operation
If macro-pitch targets are used, then measurement process is simplified, but the link with real at-resolution overlay is lost
Solution Approach 1:
The patent implements feedback by using measurements from multiple pitch scales (macro-pitch and at-resolution) and feeding them into a unified overlay model. The macro-pitch measurements provide feedback about overall overlay trends, while at-resolution measurements provide feedback about local precision requirements, allowing the system to maintain the link between simplified measurement and actual at-resolution overlay.
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 allows for precise measurement of overlay offset, reducing costs and time while improving accuracy compared to traditional methods, enabling effective overlay control in advanced lithographic processes.
Implementation Method 1
A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured.
Implementation Method 2
A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured.
Data Source
AI summary
A method of measurement of at-resolution overlay offset may be implemented in a scatterometer. At least three targets are provided on a wafer, each target comprising a first marker grating and a second interleaved marker grating and each target having a different overlay bias between its first and second marker. The first and second markers are provided by subsequent lithography steps in a double patterning lithographic process. The targets are measured with a scatterometer and for each target a measured CD of at least one of the markers is determined using reconstruction. The CD of the first marker may be fixed in the reconstruction. The measured CDs and at least one of the overlay biases is used to determine an overlay result corresponding to a minimum measured CD. The overlay result may be determined by fitting a function such as a parabola to the measured CDs and the overlay biases and determining the overlay at the minimum of the fitted function.