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 |
Technical Documentation & Analysis: Diamond-Coated Si ATR Sensor Module
Section titled âTechnical Documentation & Analysis: Diamond-Coated Si ATR Sensor ModuleâExecutive Summary
Section titled âExecutive SummaryâThis research successfully demonstrates the implementation of a compact, robust silicon-based Attenuated Total Reflection (ATR) sensor module, leveraging Nanocrystalline Diamond (NCD) coatings to meet stringent industrial process requirements.
- Enhanced Robustness: NCD coatings (500 nm thickness) applied via HFCVD provided exceptional chemical and mechanical stability, successfully resisting aggressive Cleaning-In-Place (CIP) agents (e.g., 5% NaOH at 80 °C for 200h) where uncoated Si failed.
- Performance Amplification: Contrary to typical expectations, the NCD coating resulted in a significant absorbance enhancement factor of 1.8 compared to uncoated Si elements, boosting the sensorâs sensitivity.
- High Sensitivity Achieved: The sensor module demonstrated a Noise Equivalent Concentration (NEC) sensitivity of 20 ppm (m/m) for isocyanate solutions (Basonat in Propylene Carbonate), suitable for demanding process analytics.
- Optimal Geometry: The final design utilized mechanically cut and polished Si crystals with a 40° wedge angle, yielding an angle of incidence of 30° at the sample face, which is critical for achieving the observed absorbance enhancement.
- Scalable Manufacturing: The hybrid packaging concept, utilizing established MEMS thermal emitters and thermopile detectors, enables efficient, reproducible, and cost-effective manufacturing suitable for high-volume industrial deployment.
- 6CCVD Value Proposition: This work validates the use of high-quality CVD diamond films for creating rugged, high-performance ATR elements, a core capability offered by 6CCVD for advanced sensor applications.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the research paper detailing the sensor design and performance metrics.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| ATR Element Substrate | Silicon (Si) | N/A | Material chosen for established packaging technologies |
| ATR Element Dimensions | 37.5 x 10 x 1.5 | mm | Custom size requiring precision cutting |
| NCD Coating Thickness | 500 | nm | Applied via HFCVD for robustness and enhancement |
| Wedge Angle ($\beta$) | 40 | ° | Mechanically cut angle for optimal coupling |
| Angle of Incidence ($\alpha$) | 30 | ° | Angle at the sample face, resulting in absorbance enhancement |
| Absorbance Enhancement Factor | 1.8 | N/A | NCD-coated Si vs. uncoated Si (30% AC in PC) |
| Noise Equivalent Concentration (NEC) | 20 | ppm (m/m) | Estimated sensitivity for Basonat in PC (1 min integration) |
| Sensitivity Gain (Krypton) | 1.7 | N/A | Factor achieved by inert gas backfilling of housing |
| Chemical Resistance Test (NaOH) | 5% v/v, 80 °C, 200h | N/A | NCD coating showed no degradation |
| Chemical Resistance Test (HNO3) | 3% v/v, 40 °C, 200h | N/A | NCD coating showed no degradation |
| Spectral Range (MIR) | 3 < $\lambda$ < 20 | ”m | Mid-Infrared range for ATR spectroscopy |
Key Methodologies
Section titled âKey MethodologiesâThe compact ATR sensor module relied on established MEMS packaging techniques and specialized Hot Filament Chemical Vapor Deposition (HFCVD) for diamond film growth.
- Substrate Preparation: Silicon (Si) ATR elements were mechanically cut and polished to achieve the required 40° wedge angle ($\beta$).
- Diamond Deposition (HFCVD): Nanocrystalline Diamond (NCD) films were grown using the HFCVD process.
- Reaction Gases: Methane (CH4) and Hydrogen (H2).
- Substrate Temperature: 750 to 900 °C.
- Filament Temperature: 1900 to 2200 °C.
- Chamber Pressure: 1 to 20 mbar.
- Growth Rate: 50 to 400 nm h-1.
- Target Thickness: 500 nm NCD film.
- Robustness Testing: Coated Si elements were exposed to concentrated KOH, followed by O2 plasma etching to inspect for pinholes via optical and SEM microscopy.
- Chemical Resistance Validation: NCD-coated elements were subjected to prolonged exposure (200h) to aggressive industrial cleaning agents (5% NaOH, 3% HNO3) and beverages (cola, beer) to confirm suitability for CIP processes.
- Sensor Assembly: The ATR crystal was soldered directly into a hermetically sealed housing lid, which was then laser-welded onto the base, incorporating MEMS thermal emitters and a four-channel thermopile detector array.
- Sensitivity Optimization: The package was backfilled with Krypton (an inert gas with low thermal conductivity) to increase thermopile sensitivity by a factor of 1.7.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & Capabilitiesâ6CCVD is uniquely positioned to supply the advanced CVD diamond materials and precision engineering services required to replicate, scale, and extend the performance demonstrated in this compact ATR sensor research.
Applicable Materials
Section titled âApplicable MaterialsâThe research highlights the need for robust, high-transmission materials capable of withstanding harsh chemical environments while maintaining high optical quality.
- Optical Grade Single Crystal Diamond (SCD): For applications requiring the highest infrared transmission and lowest scattering losses, 6CCVD offers optical grade SCD plates up to 500 ”m thick. SCD provides superior intrinsic hardness and chemical inertness compared to NCD films on Si, offering the ultimate ATR element stability.
- High-Purity Polycrystalline Diamond (PCD): For cost-effective, large-area ATR elements, 6CCVD supplies high-purity PCD wafers up to 125 mm in diameter. Our PCD can be tailored for high infrared transmission across the MIR range (3 ”m < $\lambda$ < 20 ”m), matching the spectral requirements of this sensor.
- Custom Diamond Films (NCD/UCD): While the paper used NCD coatings on Si, 6CCVD can supply Ultra-Nanocrystalline Diamond (UNCD) or Nanocrystalline Diamond (NCD) films directly deposited onto customer-supplied substrates (Si, Ge, etc.) or provide free-standing diamond plates for superior bulk performance.
Customization Potential
Section titled âCustomization PotentialâThe success of this ATR sensor relies heavily on precise geometry (40° wedge) and high surface quality. 6CCVDâs in-house engineering capabilities ensure these specifications are met.
| Requirement from Paper | 6CCVD Custom Solution | Benefit to Customer |
|---|---|---|
| Custom Dimensions (37.5 x 10 x 1.5 mm) | Precision laser cutting and dicing of SCD/PCD plates up to 125 mm. | Enables exact replication of the required ATR element size and geometry. |
| Wedge Angle ($\beta = 40^\circ$) | High-precision mechanical shaping and polishing services. | Guarantees the critical 30° angle of incidence ($\alpha$) necessary for absorbance enhancement. |
| Surface Quality (Ra < 1 nm) | State-of-the-art polishing services: Ra < 1 nm (SCD) and Ra < 5 nm (PCD). | Minimizes reflection losses and maximizes the efficiency of the evanescent wave coupling. |
| Hybrid Integration (Metallization) | Internal metalization capability (Au, Pt, Pd, Ti, W, Cu). | Allows for direct integration of diamond ATR elements into hermetically sealed MEMS packages via soldering or bonding, as described in the paper. |
Engineering Support
Section titled âEngineering Supportâ6CCVDâs in-house PhD team specializes in CVD diamond optics and electrochemistry. We offer comprehensive support for projects involving Process Analytical Technology (PAT) and ATR Spectroscopy.
- Material Selection: We assist engineers in selecting the optimal diamond grade (SCD vs. PCD) and thickness (0.1 ”m to 500 ”m) to balance cost, optical transmission, and mechanical robustness for specific chemical environments (e.g., strong acids, bases, or abrasive slurries).
- Optical Design Consultation: Our experts can advise on optimizing ATR element geometry and surface preparation to maximize the absorbance enhancement factor for similar isocyanate or liquid analysis projects.
- Global Supply Chain: We offer reliable global shipping (DDU default, DDP available) to ensure timely delivery of custom diamond components worldwide, supporting rapid prototyping and scaling.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
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.