Terahertz Coating Thickness Measurement via Optical Calibration
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Solution Overview
Problem
The challenge of unambiguously determining the thickness and refractive index of thin coating layers using terahertz radiation is complicated by limited information content in the reflected signal, especially in 'wet on wet' applications where layer intermixing occurs, leading to difficulties in direct analytical solutions.
Innovation Solution
A method involving calibration with single and multi-layer samples, using constrained optimization routines with known optical properties and thickness bounds, to minimize the difference between simulated and measured terahertz signals, allowing for accurate thickness calculation despite variations in layer properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If numerical optimization techniques are used to determine coating thickness and refractive index, then measurement precision is improved, but processing time increases due to extensive solution space exploration
Solution Approach 1:
The patent applies preliminary action by performing a calibration phase before actual measurements. During calibration, the system pre-determines the optical properties (refractive index, absorption coefficient) of coating materials using reference samples with known thicknesses. This preliminary characterization creates a library of optical parameters that can be directly applied to subsequent measurements, eliminating the need to perform full optimization for each new measurement and thus reducing processing time while maintaining accuracy.
Solution Approach 2:
The patent changes parameters by transitioning from a full optimization approach to a simplified calculation approach. Once optical properties are known from calibration, the system uses these fixed parameters to calculate coating thickness directly from measured terahertz signal characteristics, rather than performing extensive numerical optimization. This parameter fixation significantly reduces computation time while preserving measurement precision.
2Object-affected harmful factors
If terahertz radiation is used to measure thin coating layers, then non-invasive measurement is achieved, but measurement precision deteriorates due to limited information content in the reflected signal
Solution Approach 1:
The patent uses an intermediary approach by introducing calibration reference samples with known coating thicknesses and optical properties. These intermediaries bridge the gap between the terahertz measurement system and the unknown samples. By measuring the terahertz reflection from known reference samples, the system determines the optical properties of the coating materials, which then serve as intermediaries to enable accurate thickness calculation for unknown samples without requiring invasive measurements.
Solution Approach 2:
The patent addresses the limited information problem by changing the approach from direct measurement to indirect calculation. Instead of relying solely on the limited information in the reflected signal from unknown samples, the system uses terahertz measurements on reference samples to determine optical parameters, then applies these parameters to calculate thickness from the same limited signal information. This parameter transformation converts an underdetermined problem into a solvable one.
3Productivity
If wet on wet coating application is used, then productivity is improved, but manufacturing precision deteriorates due to layer intermixing and interface roughness
Solution Approach 1:
The patent addresses the precision problem in wet-on-wet coating by changing the measurement and analysis parameters. Instead of assuming sharp, well-defined layer interfaces, the system uses terahertz time-domain spectroscopy to measure the actual optical response, which captures the effects of intermixing and roughness. By analyzing the full time-domain waveform and fitting it to models that account for interface imperfections, the system can accurately determine individual layer thicknesses even when layers are intermixed, thus maintaining manufacturing precision despite the productive wet-on-wet application method.
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 processing time and enhances accuracy in determining coating thickness, accounting for interlayer variations and environmental conditions, while simplifying the solution space exploration.
Implementation Method 1
the terahertz time-domain waveform reflected from such a stack of thin films can be calculated rapidly using a matrix formalism of Fresnel equations
Implementation Method 2
as described by the refractive index of each material (coatings and substrate)
Data Source
AI summary
A method for determining the thickness of a plurality of coating layers. The method comprises the steps of performing a calibration analysis on calibration data to determine initial values and search limits of optical parameters of the plurality of coating layers, irradiating the plurality of layers with a pulse of THz radiation in the range from 0.01 THz to 10 THz, detecting the reflected radiation to produce a sample response derived from the reflected radiation, producing a synthesized waveform using the optical parameters and predetermined initial thicknesses of the layers, varying the thicknesses and the optical parameters within the search limits to minimize the error measured between the sample response and the synthesized waveform, and outputting the thicknesses of the layers.