Lithographic Scatterometry Calibration via Differential Target Segmentation
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Solution Overview
Problem
Existing lithographic inspection methods using principal component analysis are sensitive to variations in the underlying structure, leading to significant errors in measuring parameters like focus, dose, and contrast, as the calibration relationship derived from one layer or process cannot be reused in different layers or processes.
Innovation Solution
A method involving a reference pattern with first and second parts of different sensitivities to variations in a parameter, where differential measurement results are obtained and decomposed into basis functions and coefficients to establish a calibration function, allowing for the determination of parameter values in target patterns.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If principal component analysis is used to measure lithographic parameters from scatterometry data, then measurement can be achieved without computationally-intensive reconstruction, but the measurement is highly sensitive to variations in underlying structure leading to significant errors
Solution Approach 1:
The target pattern is divided into two distinct parts with different sensitivities to the parameter being measured. By segmenting the measurement into two separate responses and combining them through subtraction, the method isolates the parameter of interest while canceling out structure variations, thus maintaining measurement speed while improving accuracy.
Solution Approach 2:
The patent introduces an intermediary measurement approach where instead of directly measuring the parameter from a single scatterometry signal, it uses the difference between two scatterometry measurements from different parts of the target pattern. This intermediary differential measurement serves as a mediator that eliminates sensitivity to underlying structure variations.
2Measurement precision
If calibration is performed for each layer and process to ensure accuracy, then measurement precision is maintained, but the time and resources required for recalibration increase significantly
Solution Approach 1:
The patent creates a universal calibration approach where a single calibration relationship can be applied across multiple layers and processes. The differential measurement method produces calibration data that is insensitive to underlying structure variations, making the calibration universally applicable and eliminating the need for layer-specific recalibration.
Solution Approach 2:
The patent performs calibration once using the differential measurement method, which establishes a robust calibration relationship that accounts for structure variations. This preliminary calibration action is sufficient for all subsequent measurements across different layers and processes, eliminating the need for repeated recalibration activities.
3Device complexity
If a single target pattern is used for scatterometry measurement, then the measurement process is simple, but the measurement is sensitive to variations in underlying structure
Solution Approach 1:
The target pattern is divided into two parts with different sensitivities to the parameter being measured. This segmentation allows the measurement system to capture two different responses that, when combined through subtraction, isolate the parameter of interest while canceling out structure variations, thus improving accuracy without significantly increasing complexity.
Solution Approach 2:
The patent merges two scatterometry measurements from different parts of the target pattern through a subtraction operation. This combining of measurements creates a differential signal that eliminates sensitivity to underlying structure variations, achieving improved measurement precision while maintaining relatively simple measurement system complexity.
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 reduces the sensitivity to variations in the underlying structure, enabling more accurate and reliable measurement of parameters like focus and dose without the need for recalibration in each layer or process.
Implementation Method 1
One form of 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
measuring the radiation reflected or scattered from the target pattern
Data Source
AI summary
In a scatterometric method differential targets with different sensitivities to parameters of interest are printed in a calibration matrix and difference spectra obtained. principal component analysis is applied to the difference spectra to obtain a calibration function that is less sensitive to variations in the underlying structure than a calibration function obtained from spectra obtained from a single target.