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Near-IR spectroscopy Near-infrared (NIR) spectroscopy has served to quantify and qualify materials for many applications. Using NIR, a device can derive a spectral signature from a particular material by shining a broadband light on a sample and measuring its spectral response. The spectral response provides a reference for performing identification and/or quantification analysis using predeveloped analysis models. NIR differentiates itself from other approaches by generating accurate data immediately on multiple chemical and physical parameters at the same time. No preparation of different sample types is required. While wet chemical and other conventional analysis methods can demand a delay until certain transformations occur in the sample, with NIR this need is eliminated. NIR light can even pass through plastic and glass to provide analysis of materials inside a package or container. While NIR spectroscopy can provide significant advantages including quick results and some portability for on-site use, technical and market barriers have severely limited the development of versatile tools for expanded commercial use, not to mention consumer use. In the case of a consumer device, the obvious prerequisite is miniaturization that can enable integration into an easy-to-carry mobile device form factor. Price point and scalability are also a challenge: Recent prices have hovered between $10,000 and $20,000. Much more integration is required to make manufacturing scalable to hundreds of thousands of units. Given the need for robust, highaccuracy optics and circuitry, there is no choice but to move toward semiconductorstyle manufacturing. In essence, spectrometers must be as cheap as chips. Engineering teams have tried a variety of techniques to achieve better integration and smaller form factors for a NIR device, including micromirror arrays, linear variable filters, tunable Fabry-Perot filters and other approaches. These advances have helped move spectrometers toward a module size for some portability, but even more versatility is required for nonexpert use, including an increase in spectral range for a single sensor. The question a b Measurements conducted on Cerelac infant cereal show the spectral signature for plain Cerelac and also the presence of plain milk and powdered milk in the mixture (a). Measurements of the ability to detect different kinds of sugars based on spectral signatures (b). is: What approach can get the industry to a chip-scale size with the necessary capabilities? MEMS: A key to the consumer market MEMS (microelectromechanical systems) holds great promise for the development of an FT-NIR (Fourier transform near-infrared) device that meets the major requirements for the broader market. For devices with moving parts such as mirrors, MEMS has an established track record and offers a major advantage over other approaches: miniaturization to chip scale, use of semiconductor style wafers, and etching techniques for high-volume batch-style production, ultimately providing pricing at the component level. The ability to develop moving structures at microscale that can be produced in high volumes and at wafer-scale cost has opened the door to new untapped markets, a recent example being the motion-sensing accelerometers found in smartphones, automobiles and other consumer equipment. Si-Ware Systems has developed a variety of MEMS capabilities through its combined work in MEMS, ASIC development and photonics, and through the creation of a proprietary technology called SiMOST (Silicon integrated Micro Optical Systems Technology) that enables the Si-Ware Systems October 2017 Photonics Spectra 49
■ MEMS-Based Spectroscopy replication of semiconductor capabilities for the photonics industry. The technology pulls from a library of well-characterized and validated optical and mechanical components to design and fabricate optical benches — on a single silicon chip. The team used this technology to develop a fully monolithic Michelson interferometer with moving mirrors. The Michelson interferometer, the core of any FT-NIR spectrometer, is an optical interferometer. A beamsplitter splits the incident beam into two paths: One of the beams is reflected by a moving mirror and the other is used as a reference when reflected by a fixed mirror. The moving mirror controls the optical path, or simply the delay, of the first beam and thus the two reflected beams interfere, producing a pattern that corresponds to the spectral content of the input light. The latter is captured by the single photodetector, generating an “interferogram.” The spectrum of the input light is directly generated by applying a Fourier transform over the interferogram. The three-dimensional SiMOST spectrometer design is printed onto masks in the same way that other MEMS devices are produced. These masks pattern the design onto silicon wafers by photolithography. The patterns are subsequently etched in layers, using batch processes. The chips are then diced and packaged, enabling unprecedented economies of scale that significantly lower costs. The scanning electron microscope photo of the miniaturized version of the Michelson interferometer shows all the optical components (fixed mirror, moving mirror and beamsplitter) as well as the mechanical components (a MEMS comb drive micro-actuator) integrated onto the single chip. The components are aligned using a single photolithography process. They are fabricated with a single deep reactive ion-etching (DRIE) process. The dedicated ASIC chip complements the functionality of the interferometer, resulting in the creation of a full spectrometer. It achieves this by generating the From Quality Control to Health Monitoring To capitalize on the potential of this technology to allow for a small, low-cost and scalable NIR spectral sensor, a chipsized spectral sensor module was developed. The photodetector, the MEMS chip and the ASIC chips have been housed under one roof in a single 18 × 18-mm package. The smaller footprint enables the creation of new usage models and applications for spectroscopy. Portable spectrometry, in-line process monitoring, wireless spectrometry networks under an IoT umbrella, and integration into mobile consumer devices are just a few examples. Each of these usage models can include qualitative and/or quantitative analysis of materials in different sectors including medical, industrial, food and beverage, forensics, and law enforcement applications. includes a wide steel tube to plunge into the soil to secure a representative sample. When the button is pushed, the scanner activates, obtains the soil signatures and relays them via mobile phone to a cloudbased database for a determination on the amount of nitrogen, phosphorus, potassium Portable soil analysis As an example of a new usage model made possible by the smaller FT-NIR form factor, a company called SoilCares, in the Netherlands, has incorporated the FT- NIR-based spectrometer into a ruggedized portable instrument for field use. The tool Exploded view of the spectral sensor in a chip-scale package. Si-Ware Systems www.photonics.com
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