White Light Interferometry Height Map Phase Signal Processing
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Solution Overview
Problem
Current white light interferometry methods provide only a rough approximation of height locations on a surface, which is not accurate enough for many applications, and are prone to errors due to the use of amplitude signals, whereas phase signals are less error-prone but periodic, leading to inaccuracies in determining zero crossings.
Innovation Solution
The method involves deriving a phase signal from the interference pattern, filtering it to emphasize relevant frequencies, and calculating zero crossings at 0, 2π, and -2π phases to create multiple height maps, which are then combined to minimize jumps and errors, using techniques like Fourier transforms and wavelet transforms to enhance accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the centre of mass of the filtered signal is calculated directly to determine height location, then the method is simple and quick, but the height determination accuracy is insufficient
Solution Approach 1:
The patent applies a preliminary action by raising the magnitude of the inverse Fourier transformed signal to a power of substantially 1.8 before calculating the centre of mass. This preprocessing step sharpens the peak and steepens the slopes of the signal, enabling more accurate height determination while maintaining the simplicity of the centre of mass calculation method.
2Measurement precision
If the magnitude of the inverse Fourier transformed signal is raised to a power of 1.8 to sharpen the peak, then the height determination accuracy is improved, but the computational complexity increases
Solution Approach 1:
The patent changes the parameter of the signal magnitude by raising it to a power of substantially 1.8. This parameter transformation sharpens the peak characteristics of the signal, improving the precision of height determination. The specific power value of 1.8 is optimized to balance accuracy improvement with computational efficiency.
3Measurement precision
If phase signal is used to determine height with greater accuracy, then the measurement precision is improved, but the periodic nature of phase leads to zero crossing inaccuracies
Solution Approach 1:
The patent uses the centre of mass of the processed signal as an intermediary to determine the expected value of the height, which then serves as a reference for accurately identifying the relevant zero crossing of the phase signal. This intermediary approach resolves the ambiguity of periodic phase zero crossings by providing a contextual reference point.
Solution Approach 2:
The patent employs feedback by using the centre of mass calculation result (raised to power 1.8) to identify the expected height value, which then guides the selection of the correct zero crossing from the periodic phase signal. This feedback mechanism ensures that the zero crossing determination is reliable and consistent with the overall signal characteristics.
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 provides a more accurate determination of height locations by reducing errors and jumps in the height maps, resulting in a more precise surface height measurement by leveraging the phase information and correcting for optical and scanning inaccuracies.
Implementation Method 1
white light interferometry with a broadband light source, comprising for each spatial position on the sample the steps of obtaining a interference pattern signal or correlogram during scanning
Implementation Method 2
an optical detector adapted to convert the received light into electrical signals
Data Source
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AI summary
The invention relates to a method and an apparatus for determining the height of a number of spatial positions on the sample, defining a height map of a surface through interferometry with a broadband light source, comprising the following steps for each spatial position: obtaining a correlogram during scanning of the surface plane of the objective and estimating the point of the correlogram where the amplitude of the correlogram is at its maximum, thus determining an approximation of the height of the spatial position on the sample wherein the estimation of the value where the correlogram has its maximum takes place through the steps of subjecting the correlogram to a Fourier transform, subjecting the Fourier transformed signal to a filter, subjecting the filtered signal to an inverse Fourier transform and calculating the location of the centre of mass of this inversed filtered Fourier transformed signal.