Spectroscopic Sensor for Thin Film Thickness Measurement
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Solution Overview
Problem
Existing sensors for measuring thin films face challenges in achieving a balance between spectral range, spectral resolution, and signal-to-noise ratio, which are typically mutually exclusive, making it difficult to accurately measure thin plastic films using a single detector and filter combination.
Innovation Solution
A sensor system comprising a spectrometer and a stack of single channel detectors, where the spectrometer analyzes transmitted or reflected radiation, and the detectors analyze radiation that has passed through the film multiple times, utilizing Lambertian scattering to enhance sensitivity, allowing for simultaneous interferometry and absorption measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single detector and filter combination is used, then signal-to-noise ratio is improved, but spectral range is insufficient
Solution Approach 1:
The sensor system divides the detection function into multiple single-channel detectors, each optimized for specific wavelength ranges and measurement modes (transmission, reflection, multi-pass absorption). This segmentation allows each detector to achieve high signal-to-noise ratio in its designated range while the collective system covers the full spectral range from visible to mid-IR.
2Measurement precision
If a compact spectrometer is used, then spectral range and resolution are improved, but throughput is reduced
Solution Approach 1:
Instead of using a single compact spectrometer, the system segments the spectral analysis into a broadband spectrometer for general spectral range coverage and multiple single-channel detectors for specific high-resolution measurements. This segmentation maintains high throughput by allowing parallel detection paths while achieving high spectral resolution where needed through the dedicated single-channel detectors.
3Measurement precision
If interferometry is used for thin films, then measurement accuracy is improved, but absorption is too weak
Solution Approach 1:
The system segments the measurement approach by using interferometry (spectrometer) for very thin films where absorption is weak, and switching to multi-pass absorption detection for thicker films where absorption is sufficient. This segmentation allows optimal measurement technique selection based on film thickness, maintaining high measurement precision across the entire thickness range.
Solution Approach 2:
The system introduces an intermediary decision mechanism that selects between interferometry and absorption measurement modes based on the film thickness. For thin films, interferometry is used as the primary measurement method, while for thicker films, multi-pass absorption becomes the effective measurement approach, with the system automatically transitioning between methods.
4Measurement precision
If absorption is used for thick films, then measurement capability is improved, but spectral range is limited
Solution Approach 1:
The system segments the detection capability across multiple single-channel detectors covering different wavelength ranges (visible, near-IR, mid-IR). Each detector is optimized for specific absorption measurements in its wavelength range, and the system integrates data from all detectors to achieve comprehensive spectral range coverage while maintaining absorption measurement capability for thick films.
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
Enables robust, continuous on-line measurement of thin film thicknesses down to 1 micron or less, providing superior thickness measurement accuracy through enhanced visibility of interference patterns and multi-wavelength absorption techniques.
Implementation Method 1
a spectrometer that detects (i) transmitted radiation that passes through the layer of material
Implementation Method 2
or (ii) reflected radiation that is reflected from the layer of material
Implementation Method 3
the members (such as plates) that form the measurement cell exhibit near perfect Lambertian scattering which means that the angle at which light leaves the plate is independent of the angle at which the light impinges on the plate
Implementation Method 4
the single channel stack is offset from the source such that the stack detects reflected radiation that has passed multiple times through the film
Implementation Method 5
The spectrometer is either (i) positioned directly opposite the source of radiation so that it detects radiation that passes through the film (that is, only 1 pass through the film) or (ii) disposed on the same side of the film as is the source of radiation so that the spectrometer detects radiation that is specularly reflected from the film
Data Source
AI summary
Continuous on-line thin film measurements employ a sensor having a spectrometer for interferometric measurements and a stack of single channel detectors for adsorption measurements. The stack is separated from the spectrometer, which analyzes radiation that emerges (transmitted pass or reflected from) the film, whereas the stack analyzes radiation that has passed through the film multiple times. The spectrometer is (i) positioned directly opposite the source of radiation so that it detects transmitted radiation or (ii) disposed on the same side of the film as is the source of radiation so that the spectrometer detects radiation that is specularly reflected from the film. The sensor includes a broadband radiation source emitting visible to far infrared light which propagates through a measurement cell defined by reflective surfaces exhibiting Lambertian-type scattering. The sensor is capable of measuring thin plastic films with thicknesses down to 1 micron or less.