Transparent Layer Thickness Measurement Using FFT and Spectral Fitting
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Solution Overview
Problem
Existing methods for measuring the thickness of transparent layers, such as glass plates, are limited to an accuracy of approximately ±1 μm, which is insufficient for precise control in manufacturing processes like wet-chemical etching, especially for high-tech glass products and semiconductor wafers.
Innovation Solution
A method combining Fast Fourier Transformation (FFT) analysis with a Full-Spectral Fitting Procedure (FPO) for Fabry-Perot oscillations, using high-spectral resolution reflectance measurements and advanced calibration techniques to correct for optical and spectral distortions, allowing for nanometer-scale accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If FFT analysis is used to measure thickness from Fabry-Pérot oscillations, then the measurement can be performed efficiently, but the accuracy is limited to approximately ±1 μm due to the nonlinear period of oscillations
Solution Approach 1:
The patent transforms the measurement approach by changing the parameter being analyzed from frequency domain (FFT) to spatial domain (oscillation envelope). Instead of analyzing the periodic Fabry-Pérot oscillations directly, the method extracts the envelope of these oscillations and analyzes its spatial variation, which linearly correlates with thickness and enables nanometer-scale precision
Solution Approach 2:
The patent moves from one-dimensional frequency analysis to two-dimensional analysis by considering both the oscillation frequency and the envelope modulation. This dimensional expansion allows simultaneous extraction of both thickness and material properties while achieving higher precision through the envelope's linear relationship with thickness
2Measurement precision
If standard FFT method with rescaling is applied, then the analysis can be performed with simple algorithms, but the absolute thickness accuracy remains limited to ±1 μm
Solution Approach 1:
The patent performs preliminary extraction of the oscillation envelope from the raw reflectance spectrum before final thickness calculation. This preliminary action separates the envelope modulation from the rapid Fabry-Pérot oscillations, creating a pre-processed signal that contains the thickness information in a linear, easily analyzable form that accelerates subsequent measurement
Solution Approach 2:
The patent extracts the envelope of the Fabry-Pérot oscillations as a separate analytical component. By taking out the envelope modulation from the full oscillating signal, the method isolates the thickness-dependent information from the material-property-dependent oscillations, enabling independent and more precise thickness measurement
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
Achieves an absolute thickness measurement accuracy of ±1 nm, enabling precise control of etching processes and improving uniformity and accuracy in the manufacturing of glass and semiconductor wafers.
Implementation Method 1
spectral reflectance measurement
Implementation Method 2
Fabry-Pérot oscillation, FPO, from the layer in a spectral reflectance measurement
Data Source
AI summary
The present invention relates to a method and an apparatus for measuring the thickness d of a transparent layer (10), and in particular to a method and an apparatus for measuring the thickness d of glass plates during wet-chemical glass thinning in the manufacturing process.The method for measuring the thickness d of a transparent layer (10) includes: determining an approximate thickness dFFT based on a precisely known dispersion n(1) of the material of the layer (10) by performing Fast Fourier Transformation, FFT, analysis on Fabry-Pérot oscillation, FPO, from the layer (10) in a spectral reflectance measurement (900); and performing a FPO full-spectral fitting procedure (910-0) with the approximated thickness dFFT as starting value d0,0 to determine an initial local best fitting thickness dFPO,0. After the initial local best fitting thickness dFPO,0 has been found, the FPO full-spectral fitting procedure (910-i) may be repeated with specific alternative starting values d0,i to determine a global best fitting thickness dFPO.