CONCEPTION, TECHNOLOGIE ET PACKAGING DE CELLULES À VAPEUR DE CÉSIUM POUR LES HORLOGES ATOMIQUES DE TYPE MEMS

Atomic clocks are nowadays among the most accurate time and frequency standards, and are used, e.g.,for international time distribution service or in global navigation satellite systems. During the last severalyears, based on reliable and well-stabilized microelectromechanical systems (MEMS) technology and onthe availability of lasers on chip (single-mode vertical-cavity surface-emitting laser; VCSEL),considerable work has been performed by different groups around the world to develop miniaturizedversion of atomic clocks, called MEMS atomic clocks (MAC) or chip scale atomic clocks (CSAC).The goal of this thesis was to design and develop the technology of Cs vapor cells along with thermalanalysis for the thermal management of fully packaged Cs vapor cell for MEMS atomic clock. This workhas been carried-out in the framework of European project “MEMS atomic clock for timing, frequencycontrol and communication” (MAC-TFC). Two different architectures of Cs vapor cell have beenconsidered. The first one operates on the transmission of light through the cell and thus the Cs-vapor, andis called transmissive cell or T-cell. Such T-cell is made of silicon based deep-cavities sandwichedbetween two borosilicate glass wafers. For their fabrication, deep reactive ion etching (DRIE) process hasbeen optimized in order to produce smooth enough side walls of silicon cavities. In addition, specificanodic bonding process has been developed to fill the cavities with buffer gas at the required pressure.Second version of Cs vapor cell is based on the reflection of laser light inside the KOH etched siliconcavity sealed by one borosilicate glass wafer and is called reflective cell or R-cell. R-cells, as an advantageover the T-cells, allow e.g. a longer interaction of light/atom inside the Cs cavity, whereas location ofoptical source and detection elements on the same side of cell leads to better clock compactness. For theirfabrication, wet KOH etching, employed to realize the cavities inside the silicon with near mirror like(111) planes, has been studied and optimized. Further, diffraction gratings for routing of circularlypolarized light have been designed, fabricated and integrated on top of the Cs vapor R-cell. In bothversions of Cs vapor cells, our goal was to simplify the related clock assembly by doing maximumintegration and alignment at the wafer level, thanks to refractive and diffractive micro-optical componentswhile thermal analysis has been also performed for the thermal management of fully packaged Cs vaporcell (transmissive one) based on the Low temperature co-fired ceramics (LTTC) packaging.Les horloges atomiques sont de nos jours parmi les normes de temps et de fréquences les plus précises etsont utilisées, par exemple, pour les services de distributions travaillant à l'heure internationale ou pour lessystèmes de navigation globaux par satellite. Au cours des dernières années, un travail considérable a étéaccompli par différents groupes à travers le monde pour développer une version miniaturisée des horlogesatomiques, basée sur la technologie des systèmes microélectromécaniques qui est fiable, bien stabilisée etsur la disponibilité de lasers sur puces (diode laser monomode à cavité verticale émettant par la surface ouVCSEL : Vertical-Cavity surface emitting Laser). Ce type d'horloge atomique est appelé horloge atomiqueMEMS ou horloge atomique sur puce (CSAC : Chip Scale Atomic Clocks).L'objectif de cette thèse était de concevoir et développer la technologie des cellules à vapeur de Césium(Cs) ainsi qu’une analyse thermique pour sa gestion thermique lorsqu’elle est complètement packagéepour les horloges atomiques MEMS. Ce travail a été réalisé dans le cadre du projet européen "MEMSatomic clock for timing, frequency control and communication" (MAC-TFC). Deux conceptionsdifférentes de la cellule à vapeur de Cs ont été considérées. La première est basée sur la transmission de lalumière à travers la cellule et donc au travers de la vapeur de Cs et est appelée cellule transmissive ou T-cell. Ces T-cells sont réalisées à base de cavités profondes générées dans du silicium et prises en sandwichentre deux wafers de verre borosilicaté. Pour leur fabrication, le processus de gravure profonde par ionsréactifs (DRIE-Deep reactive ion etching) a été optimisé afin de produire des cavités dans le silicium dontles parois soient suffisamment lisses. De plus, le procédé de soudure anodique a été développé pourremplir les cavités avec du gaz tampon à la pression requise. La deuxième version de la cellule à vapeurde Cs est basée sur la réflection de la lumière du laser à l'intérieur des cavités gravées dans le silicium parKOH et scellées par un wafer de verre borosilicaté. Cette cellule est appelée cellule réfléchissante ou R-cell. Les R-cells permettent, par rapport aux T-cells, une interaction lumière/atome plus longue dans lescavités contenant du Cs, tandis que la localisation de la source optique et des éléments de détection dumême côté de la cellule permet la réalisation d’une horloge plus compacte. Pour leur fabrication, lagravure humide par KOH, employée pour générer les cavités à l'intérieur du silicium avec des parois dontla surface est proche de celle d’un miroir (111), a été étudiée et optimisée. De plus, les réseaux dediffraction pour le guidage de la lumière polarisée circulairement ont été conçus, fabriqués et intégrés surla partie supérieure de la R-cell à vapeur de Cs. Pour les deux versions des cellules à vapeur de Cs, notreobjectif était de simplifier l'assemblage relatif à l'horloge en faisant un maximum d'intégration et d'alignement à l'échelle du wafer, grâce à des composants micro-optiques réfractifs et diffractifs. Une analyse thermique a aussi été effectuée pour la gestion thermique de la cellule à vapeur de Cs complètement packagée (T-cell) à base de céramiques cofrittée à basse température (LTCC : Lowtemperature Co-fired ceramics)

Characterization of Cs vapor cell coated with octadecyltrichlorosilane using coherent population trapping spectroscopy

We report the realization and characterization using coherent population trapping (CPT) spectroscopy of an octadecyltrichlorosilane (OTS)-coated centimeter-scale Cs vapor cell. The dual-structure of the resonance lineshape, with presence of a narrow structure line at the top of a Doppler-broadened structure, is clearly observed. The linewidth of the narrow resonance is compared to the linewidth of an evacuated Cs cell and of a buffer gas Cs cell of similar size. The Cs-OTS adsorption energy is measured to be (0.42 $\pm$ 0.03) eV, leading to a clock frequency shift rate of $2.7\times10^{-9}/$K in fractional unit. A hyperfine population lifetime, $T_1$, and a microwave coherence lifetime, $T_2$, of 1.6 and 0.5 ms are reported, corresponding to about 37 and 12 useful bounces, respectively. Atomic-motion induced Ramsey narrowing of dark resonances is observed in Cs-OTS cells by reducing the optical beam diameter. Ramsey CPT fringes are detected using a pulsed CPT interrogation scheme. Potential applications of the Cs-OTS cell to the development of a vapor cell atomic clock are discussed.Comment: 33 pages, 13 figure

Laser light routing in an elongated micromachined vapor cell with diffraction gratings for atomic clock applications

International audienceThis paper reports on an original architecture of microfabricated alkali vapor cell designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane. Gratings have been specifically designed to diffract circularly polarized light in the first order, the latter having an angle of diffraction matching the (111) sidewalls orientation. Then, the length of the cavity where light interacts with alkali atoms can be extended. We demonstrate that a longer cell allows to reduce the beam diameter, while preserving the clock performances. As the cavity depth and the beam diameter are reduced, collimation can be performed in a tighter space. This solution relaxes the constraints on the device packaging and is suitable for wafer-level assembly. Several cells have been fabricated and characterized in a clock setup using coherent population trapping spectroscopy. The measured signals exhibit null power linewidths down to 2.23 kHz and high transmission contrasts up to 17%. A high contrast-to-linewidth ratio is found at a linewidth of 4.17 kHz and a contrast of 5.2% in a 7-mm-long cell despite a beam diameter reduced to 600 μm

Design and Thermal Modeling of a Microregenerator

International audienceThis paper reports design and thermal modeling of a micro-scale regenerative heat exchanger (microregenerator) dedicated to a free piston Stirling generator for waste heat recovery at low temperatures. The selected geometry for the microregenerator is a microchannel integrated with silicon photo-etched micropillars, the porosity is around 80% and the shape factor of the etched microstructures is around 0.35. The gas flow is set to be incompressible and viscous (note that no viscous heating was involved in the computation).Two-dimensional heat transfer in the porous medium with staggered pattern configuration under unidirectional steady state flow conditions was numerically investigated using a CFD tool (Fluent 15.0). The investigated working fluids are respectively air, helium (He) and hydrogen (H2). The obtained numerical results confirm that for a better thermal heat diffusion in the fluid path, helium is found to be the most interesting candidate

The design, fabrication and characterization of fluidic membranes for micro-engines with the aim of frequency lowering

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Deep Wet-Etched Silicon Cavities for Micro-Optical Sensors: Influence of Masking on {111} Sidewalls Surface Quality

International audienceIn this paper, we investigate the influence of different masking parameters onto the surface quality of the {111} sidewalls in order to generate specifically deep cavities by wet-anisotropic-etching of bulk silicon, for optical sensors using cavity sidewalls as reflectors. Mask alignment with crystal planes prior to wet-etching is found to be essential in order to avoid the appearance of additional planes during long etching. Mask deposition processes have been also evaluated. Among the different employed mask materials, Cr/Au gives the best results. It is then shown that cavities as deep as 1 mm with low roughness sidewalls can be simply produced with potassium hydroxide solution with periodic piranha cleanin

Single-step deep reactive ion etching of ultra-deep Silicon cavities with smooth sidewalls

International audiencetA process based on deep reactive ion etching (DRIE) has been developed and optimized for the fabricationof millimeter deep silicon cavities with smooth sidewalls. The process combines two approaches whichinvolve an optimized etching process based on the classical Bosch process (Alcatel A601E equipment)followed by the use of an aqueous etchant solution of potassium hydroxide (KOH) to smooth the surfaceand remove the fluorocarbon contaminants remaining after the DRIE process. As DRIE highly dependson the opening size of the patterned etch mask, different opening sizes have been tested to completelyetch through a 1.4 mm thick silicon wafer. Additionally, the effect of different etch-stop materials ontothe sidewalls quality has also been characterized. Sidewall quality of etched-through cavities was char-acterized by scanning electron microscopy (SEM) and contact surface profilometry. This single-step DRIEetching followed by short exposure to KOH solution permits to smooth sidewalls and achieve a surfaceroughness as low as 50 nm, which is the roughness typically obtained with the Bosch process on standarddepths

Microfabrication of hybrid fluid membrane for microengines

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Conception et modélisation thermique d’un microrégénérateur

International audienceRésumé - Cet article présente la conception et la modélisation thermique d’un microrégénérateur en vue de l’intégrer dans une micromachine Stirling à membrane hybride. Le microrégénérateur conçu est formé par des microplots gravés en silicium logés entre deux wafers de verre. Les microplots sont disposés en quinconce le long du microcanal. La modélisation des échanges thermiques entre fluide et parois chauffées pour différents gaz sous un régime laminaire pour un fluide visqueux incompressible a montré que la diffusion thermique est le mode de transfert prépondérant

Design, fabrication and CFD modeling of a Stirling engine microregenerator

International audienceThis paper reports the design, fabrication and CFD modeling of a microregenerator in order to be integrated in a multiphase piezoelectric smart membrane Stirling engine. The application aims to recover waste heat at low temperatures and to convert it into electricity via the piezoelectric element. The suggested geometry for the microregenerator is a microchannel integrated with staggered DRIE etched silicon micropillars, the whole is encapsulated between two glass wafers. The porosity ranges from 0.8 to 0.9. A 2D numerical study of the microregenerator thermofluidic performances was investigated. Three gases (air, helium andhydrogen) were investigated. The gas flow is set to be incompressible and viscous under laminar unidirectional steady flow conditions for low Reynolds number (<10). According to the obtained results, helium has shown the highest pressure drop since it’s the most viscous gas. The pressure drop encountered with helium gas is more than twice the one registered with hydrogen. Nevertheless it was the fastest gas to heat up. Trade-off between heat transfer and pressure drop needs to be reached