Résonateur opto-mécanique pour la détection photoacoustique

OPTO-MECHANICAL MICRO-RESONATOR FOR PHOTO-ACOUSTIC DETECTION

• Auteur / Autrice : Julien Charensol
• Direction : Aurore Vicet
• Type : Projet de thèse
• Discipline(s) : Électronique
• Date : Inscription en doctorat le 30/09/2021
• Etablissement(s) : Université de Montpellier (2022-....)
• Ecole(s) doctorale(s) : École doctorale Information, Structures, Systèmes (Montpellier ; 2015-....)
• Partenaire(s) de recherche : Laboratoire : IES - Institut d'Electronique et des Systèmes
• Partenaire(s) de recherche : Equipe de recherche : Composants à Nanostructures quantiques pour le Moyen InfraRouge (NANOMIR)

Mots clés

Mots clés libres

Laser

Mesure

Photoacoustique

Mems

Laser

Mems

Sensing

Photoacoustics

Résumé

Nous avons développé ces dernières années des résonateurs mécaniques en silicium spécifiquement dessinés pour la spectroscopie photo-acoustique de gaz. Les derniers designs de résonateurs mécaniques que nous avons réalisés nous ont permis d'obtenir lors de la détection de méthane à l'aide d'un laser à puits quantiques interbande émettant à 2.3 µm des limites de détection de 15ppmv pour une seconde d'intégration et de 5.5ppmv pour 15 secondes d'intégration. Afin d'obtenir ce niveau de performance, nous avons travaillé sur un design de résonateur couplé permettant de séparer la zone chargée de la collecte de l'énergie photo-acoustique de la partie en charge de la transduction électrique par effet capacitif. Ces résonateurs mécaniques entièrement développés à l'IES constituent une approche originale de la spectroscopie photo-acoustique, protégée par un brevet de l'université de Montpellier. Les performances de ces résonateurs posent les bases du développement de capteurs de gaz extrêmement compacts sur substrat silicium, matériau choisi pour son haut potentiel d'intégration. Le sujet de thèse que nous proposons porte sur l'étude de l'intégration du laser utilisé pour la spectroscopie photo-acoustique au plus près du résonateur mécanique. Les lasers seront développés et fabriqués à l'IES. L'IES possède tous les savoir-faire et moyens matériesl (épitaxie, salle blanche) permettant de réaliser des sources laser performantes et monomodes émettant entre 2.3 et 20 µm. Les applications visées par ce type de capteur peuvent être extrêmement larges, aucun gaz cible n'est privilègié, car en choisissant la source laser la mieux adaptée tout type de gaz pourra être détecté. Il sera également possible d'utiliser simultanément plusieurs sources laser afin d'obtenir des capteurs conservant une excellente sélectivité en milieu réel où un grand nombre de gaz peuvent être présents. À titre d'exemple nous pouvons citer l'aide au diagnostic médical avec l'analyse de l'air exhalé. Cette étude permettra de finalement franchir une nouvelle étape importante dans la miniaturisation des capteurs de gaz photo-acoustique. Le laboratoire IES offre les conditions idéales à la réalisation de cette thèse puisqu'il possède en son sein les compétences et savoir-faire dans tous les domaines concernés : conception et fabrication des des lasers à semi-conducteur, etude et réalisation des résonateurs Si et spectroscopie des gaz.

Applications are invited for an PhD student position at University of Montpellier (France), in the NANOMIR group. The group is located at the Electronics and Systems Institute (IES), on the University premises. IES (Electronics and Systems Institute) is a laboratory specialized in reliable electronic systems and components for observation and measurement applications. Thanks to its experience and know-how in the development of systems and components used in sectors such as electronics, microelectronics, photonics, energy, heat and acoustics, IES is able to provide technological and scientific solutions for the observation, measurement and analysis of the physical phenomena surrounding us and thus participate in resolving some of the societal issues encountered at the beginning of the 21st century (energy, water, food, health, urban planning, safety, evolution climate, pollution, etc.). Gas sensors find more and more various applications such as human health, with breath analysis and diagnostic, agri-food monitoring, or air quality qualification. To be efficient, they must exhibit three important qualities: sensitivity, selectivity and stability. A considerable amount of commercial gas sensors are available based on different techniques: electro-chemical sensors, mass sensors and optical sensors. Despite this great diversity, only sensors based on optical absorption like TDLS (tunable diode laser spectroscopy), can perfectly discriminate the spectral signature of species among others, thus give a perfect selectivity, combined with a very high sensitivity. However, optical detection sensitivity is strongly related to the used photodetector performances and spectral range. Most optical sensors are large, because the sensitivity is proportional to the optical path and reducing their size is a current research topic. In photo-acoustic spectroscopy, the optical detection is replaced by an acoustic detection. A laser, emitting at a wavelength corresponding to the absorption line of a targeted gas, is focused into a gas chamber and the measurement is performed by detecting the acoustic pressure generated by the local warming caused by optical absorption. In 2002, Rice University proposed to use a piezoelectric mechanical resonator (a commercial quartz tuning fork QTF) instead of a microphone to amplify the response of the photo-acoustic effect. This new technique was called QEPAS (Quartz-Enhanced PhotoAcoustic Spectroscopy). In this case, the laser is focused between the prongs of a QTF. The photoacoustic effect generates a pressure on the arms of the QTF. When the laser is modulated at the resonant frequency of the tuning fork, its active piezo-mechanical mode is excited, and the response is amplified by the mechanical quality factor of the system. This method avoids using expensive mid-infrared optical detectors with restricted spectral ranges while providing sensitivities down to a few particles per billion in volume, equivalent to those of the best photoacoustic systems. The photoacoustic spectroscopy is the best method to achieve both excellent selectivity and sensitivity while being very compact. QEPAS is potentially the best method to achieve selective, sensitive, compact and stable sensors. However, the main limitation of QEPAS systems comes from the use of QTF. These components are cheap and easy to use, but they are not specifically designed for this application. The goal of this thesis is to participate to the ANR NOMADE project and the “prématuration” project COMPAS which aim to create a new generation of integrated sensors based on mechanical micro-resonators for high-selectivity and sensitivity gas photo-acoustic detection (sub-ppm concentrations), in a very compact, robust, portable system. Based on photoacoustic detection, they will offer the possibility to address a wide variety of gas, by simply replacing the exciting laser source. Never has a micro-mechanical resonator been entirely thought and designed for photoacoustic detection. These projects aim at studying all physical concepts to optimize the micro-resonator design and improve the sensor sensitivity. Capacitive MEMS transducers offer advantages compared to other pressure sensors, by providing a potential excellent sensitivity with easy measurement. The micro-resonators will be developed in the silicon material technology, in order to benefit from its maturity. More specifically, SOI (Silicon On Insulator) based fabrication technique is chosen to fabricate the micro-resonators. The technological process is then simpler and opens the way to optimized geometries that should improve the sensibility of the sensor. The sensitivity improvement will allow placing the exciting laser next to the micro-resonator, eliminating the need for any optical element used to focalize the laser light. Without optical elements, we will be able to propose very compact and robust gas sensors. The sensor composed of the laser source and micro-mechanical resonator should be 0.5 cm long and 0.25 cm large. It will be the smallest photoacoustic gas sensors ever realized with such sensitivity and selectivity. These compact and robust detectors will allow the measurement and analysis of atmospheric air on a large variety of sites. The task of the PhD student will be the fabrication of MEMS (samples preparation, lithography, wet and dry etching), their characterization using microscopy techniques (scanning electron microscopy, optical microscopy), profilometry, laser Doppler vibrometry, photoacoustic, impedance meter. Also, the student will need to study physical aspect of the resonators. For this study COMSOL Multiphysics Modeling Software will be used as well as other language programming such as Python