CD-SEM Calibration via Reference Mapping Function
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Solution Overview
Problem
Current techniques for determining critical dimensions in scanning electron microscopy, such as CD-SEM, face challenges with arbitrary thresholding and lack of accuracy, especially for objects smaller than 100 nm, due to incomplete characterization and measurement artifacts, leading to unreliable and ambiguous results.
Innovation Solution
A method is developed to calibrate the CD-SEM technique by establishing a mapping function between CD-SEM parameters and those from richer reference techniques like AFM, TEM, or OCD, using a set of coefficients to minimize distance between measured parameters, thereby providing a more accurate and realistic value of critical dimensions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If arbitrary thresholding is used to determine critical dimensions in CD-SEM, then the measurement process is simple and fast, but the measurement precision and reliability deteriorate, especially for objects smaller than 100 nm
Solution Approach 1:
The patent transforms the arbitrary threshold parameter into a physically meaningful parameter by establishing a mapping function that relates the arbitrary threshold value to the actual critical dimension through coefficients determined by reference measurements. This changes the nature of the threshold from a simple intensity cutoff to a calibrated parameter that accounts for the specific measurement conditions and object characteristics.
Solution Approach 2:
The patent introduces a mapping function with coefficients as an intermediary between the arbitrary threshold measurement and the true critical dimension. This intermediary layer, determined through reference measurements against AFM or OCD data, translates the simplified threshold-based measurement into an accurate critical dimension value without requiring complex reference measurements for every sample.
2Measurement precision
If reference techniques like AFM are used to obtain accurate critical dimension measurements, then measurement precision improves, but the measurement time and device complexity increase significantly
Solution Approach 1:
The patent performs reference measurements using accurate techniques like AFM or OCD in advance to determine the mapping function coefficients. Once these coefficients are established through preliminary calibration, they can be applied to subsequent CD-SEM measurements, providing accurate critical dimension values without repeating the time-consuming reference measurement process for each sample.
Solution Approach 2:
The patent creates a mathematical model (mapping function) that copies the relationship between threshold values and critical dimensions observed in reference measurements. This model can then be applied to CD-SEM data, transferring the accuracy benefits of reference techniques to the faster CD-SEM measurements without requiring the physical presence of reference measurement equipment during production measurements.
3Ease of operation
If CD-SEM uses arbitrary thresholding methods, then the device complexity remains low and operation is simple, but the reliability and accuracy of measurements deteriorate
Solution Approach 1:
The patent changes the threshold parameter from an arbitrary value to a calibrated parameter with physical meaning. By determining coefficients through reference measurements and applying them through a mapping function, the threshold becomes a reliable indicator of actual critical dimensions while maintaining the simplicity of the thresholding approach.
Data Source
AI summary
A calibration method for a CD-SEM technique, includes determining a match function converting at least one parameter obtained by modelling a measurement supplied by the CD-SEM technique into a function of at least one parameter representative of a measurement supplied by a characterisation technique different from the CD-SEM technique, the match function being characterised by a plurality of coefficients; performing measurements on a plurality of patterns chosen to cover the desired validity range for the calibration, the measurements being done using both the CD-SEM technique to be calibrated and the reference technique; determining, from the measurements, a set of coefficients of the match function minimising the distance between the functions of the parameters measured using the reference technique and applying the match function to the parameters obtained by modelling measurements supplied by the CD-SEM; using the set of coefficients during the implementation of the calibrated CD-SEM technique.