Compact silicon-based attenuated total reflection (ATR) sensor module for liquid analysis
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-04-12 |
| Journal | Journal of sensors and sensor systems |
| Authors | A. Lambrecht, Carsten Bolwien, Hendrik Fuhr, Gerd Sulz, Annett Isserstedt-Trinke |
| Institutions | University of Freiburg, Fraunhofer Institute for Physical Measurement Techniques |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the development and performance of a compact, silicon-based Attenuated Total Reflection (ATR) sensor module designed for continuous liquid analysis in demanding industrial environments.
- Core Value Proposition: Realization of a compact, cost-effective ATR sensor module using established MEMS packaging techniques, providing robustness and enhanced sensitivity suitable for industrial process analytics.
- Robustness Achievement: The Si ATR element is coated with Nanocrystalline Diamond (NCD), providing resistance against mechanical wear, abrasive components, and aggressive Cleaning-In-Place (CIP) agents (e.g., hot NaOH and HNO3 solutions).
- Sensitivity Enhancement: The NCD coating, combined with an optimized inverse geometry (30° incidence angle), experimentally increased the ATR absorbance change by a factor of 1.8 compared to uncoated Si.
- Performance Metrics: The enhanced sensitivity resulted in a reduction of the Noise Equivalent Concentration (NEC) by a factor of 3 for the coated device compared to the uncoated Si element.
- Target Application Performance: Demonstrated high sensitivity for isocyanate solutions (Basonat in Propylene Carbonate), achieving an estimated NEC of 20 ppm (m/m) with the NCD-coated Si element.
- Module Design: The sensor utilizes a hybrid package incorporating state-of-the-art MEMS thermal emitters and a four-channel thermopile detector array with specific narrowband filters (3.95 ”m to 4.78 ”m).
- Scalability: The hermetically sealed module housing and established processing steps allow for rapid scaling of production volume, similar to standard infrared detectors.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| ATR Crystal Material | Silicon (Si) | N/A | High refractive index material |
| ATR Crystal Dimensions | 37.5 x 10 x 1.5 | mm | Element size |
| NCD Coating Thickness | 500 | nm | Used for coated Si ATR element |
| Wedge Angle (beta) | 40 | ° | Mechanically cut Si crystal geometry |
| Angle of Incidence (alpha) | 30 | ° | At sample face (optimized for NCD enhancement) |
| Absorbance Enhancement Factor | 1.8 | Factor | NCD-coated Si vs. uncoated Si (AC in PC) |
| NEC Reduction Factor | 3 | Factor | NCD-coated Si vs. uncoated Si (AC in PC) |
| Estimated NEC (Basonat in PC, NCD-coated) | 20 | ppm (m/m) | 1 min integration time |
| Measured NEC (AC in PC, NCD-coated) | 0.11 | % (m/m) | 1 min integration time |
| Detector Channels | 4 | N/A | Thermopile array |
| Filter Center Wavelength Range | 3.95 to 4.78 | ”m | Narrowband filters used |
| Sensitivity Gain (Krypton Backfill) | 1.7 | Factor | Hermetic packaging atmosphere optimization |
| Hermetic Sealing Method | Laser welding and soldering | N/A | Ensures long-term stability |
Key Methodologies
Section titled âKey MethodologiesâThe sensor development involved specialized NCD coating, optimized optical design, and robust hybrid packaging techniques.
NCD Film Deposition (HFCVD)
Section titled âNCD Film Deposition (HFCVD)â- Substrate Preparation: Silicon ATR elements (37.5 mm x 10 mm x 1.5 mm) were used as substrates.
- Deposition Method: Hot Filament Chemical Vapor Deposition (HFCVD).
- Process Gases: CH4 and H2 were used as the main reaction gases.
- Temperature Parameters: Substrate temperatures ranged from 750 to 900 °C; filament temperatures ranged from 1900 to 2200 °C.
- Pressure and Rate: Chamber pressures were maintained between 1 and 20 mbar, yielding growth rates of 50 to 400 nm h-1.
- Defect Testing: NCD-coated Si samples were exposed to concentrated KOH (a known Si etchant). Films thicker than 300 nm showed no etch pits after subsequent O2 plasma etching, confirming the absence of pinholes.
Chemical Resistance Testing (CIP Simulation)
Section titled âChemical Resistance Testing (CIP Simulation)âUncoated and NCD-coated Si elements were subjected to aggressive cleaning agents:
- Alkaline Exposure: 5% v/v NaOH solution at 80 °C for 200 h.
- Acid Exposure: 3% v/v HNO3 solution at 40 °C for 200 h.
- Beverage Exposure: Cola at 15 °C for 400 h; Beer at 5 °C for 400 h.
- Result: Coated surfaces were visually unaffected, and FTIR spectra showed no difference before and after treatment, confirming robustness; uncoated Si surfaces were unstable.
Sensor Module Assembly and Optical Design
Section titled âSensor Module Assembly and Optical Designâ- Optical Concept: Inverse geometry using mechanically cut and polished Si crystals with a 40° wedge angle, resulting in a 30° angle of incidence at the sample face.
- Packaging: Hybrid concept where the ATR crystal is soldered directly into the housing lid, which is then laser-welded onto the housing base for a hermetic seal.
- IR Source: An array of four individual MEMS thermal emitters was used to fully illuminate the ATR element.
- Detector System: A line array of four thermopile detectors, each equipped with a specific narrowband pass filter (Table 1).
- Sensitivity Optimization: The hermetic package was backfilled with Krypton (an inert gas with low thermal conductivity) via a copper tube, sealed by cold welding and soldering, achieving a 1.7x sensitivity gain.
Commercial Applications
Section titled âCommercial ApplicationsâThis compact, robust ATR sensor technology is highly relevant for continuous monitoring and quality control in industries requiring chemical resistance and high sensitivity in the mid-infrared (MIR) range.
Process Analytical Technology (PAT)
Section titled âProcess Analytical Technology (PAT)â- Chemical Manufacturing: Continuous, in-line determination of base chemical concentrations, specifically isocyanates (e.g., HDI trimers like Basonat HI100) in polyurethane production, down to 100 ppm levels.
- Pharmaceuticals: Process monitoring, such as tracking crystallization processes (e.g., Paracetamol).
- Oil and Lubricants: Quality control and determination of moisture content in transformer and lubrication oils.
Food and Beverage Industry
Section titled âFood and Beverage Industryâ- Quality Control: Determination of dissolved CO2 concentrations in beverages (facilitated by integration into standard Varivent flanges).
- CIP Environments: Applications requiring sensors that can withstand aggressive cleaning cycles (high temperature, high pH/acid exposure) without degradation of the ATR element surface.
Sensor and Instrumentation
Section titled âSensor and Instrumentationâ- Handheld Instruments: Incorporation into portable devices for fast, effective quality control of delivered raw materials or at-line analysis, replacing bulkier process FTIR spectrometers.
- Scalable Manufacturing: The use of established MEMS packaging techniques allows for high-volume, cost-effective production of robust MIR sensor modules.
- Future Spectral Expansion: The Si platform allows for future integration of Diamond-Like Carbon (DLC) coated Germanium (Ge) ATR elements, which would extend the spectral range into the MIR fingerprint region for enhanced selectivity.
View Original Abstract
Abstract. Infrared attenuated total reflection (ATR) spectroscopy is a common laboratory technique for the analysis of highly absorbing liquids and solids, and a variety of ATR accessories for laboratory FTIR spectrometers are available. However, ATR spectroscopy is rarely found in industrial processes, where compact, robust, and cost-effective sensors for continuous operation are required. Here, narrowband photometers are more appropriate than FTIR instruments. We show the concept and implementation of a compact Si-based ATR module with a four-channel microelectromechanical systems (MEMS) detector. Measurements of liquid mixtures demonstrate the suitability for applications in the chemical industry. Apart from sapphire (for wavelengths below 5 ”m) and diamond (extending to the far-infrared region), most materials for ATR elements do not have either high enough infrared transmission or sufficient mechanical and chemical stability to be exposed to process fluids, abrasive components, or aggressive cleaning agents. However, using diamond coatings on Si improves the stability of the sensor surface. In addition, by proper choice of incidence angle and coating thickness, an enhancement of the ATR absorbance is theoretically expected and demonstrated by first experiments using a compact sensor module with a diamond-coated Si ATR element.