DEVELOPMENT OF PIEZORESISTIVE SILICON NANOWIRES (SINWS) AND THE APPLICATIONS IN NEMS TECHNOLOGY.

Ph.DDOCTOR OF PHILOSOPH

Pressure Sensors for Pressure Garment

Pressure garments, also called compression garments, are used for the treatment of hypertrophic scars, venous insufficiency, lymphoedema and other illnesses. The beneficial influence of pressure on the treatment of hypertrophic scars has not been scientifically proven. For other illnesses such as venous diseases, there exists an international disagreement on the optimal level of pressure to be applied in order to obtain beneficial results. The reason for these is the difficulties in measuring the interface pressure between the pressure garment and the skin. This work focused in the enhancement and study of the structure of an inductively coupled pressure sensor for pressure garment application. A three-dimensional model of the sensor using finite element analysis (FEA) was constructed. The model served to study the behavior of the sensor with different membrane dimensions and to study the influence of the mechanical properties of the materials in the sensor results. Three different membrane designs were developed and constructed. The sensor prototypes were tested and they behaved similarly to the constructed model. A bossed membrane made of transparency film was found to have a linear behavior and a measurement range from 0 to 65 mmHg. These results lead to future reduction in size and enhancement of the behavior of the inductively coupled pressure senso

Integration of electronic and optical techniques in the design and fabrication of pressure sensors

Since the introduction of micro-electro-mechanical systems fabrication methods, piezoresistive pressure sensors have become the more popular pressure transducers. They dominate pressure sensor commercialization due to their high performance, stability and repeatability. However, increasing demand for harsh environment sensing devices has made sensors based on Fabry-Perot interferometry the more promising optical pressure sensors due to their high degree of sensitivity, small size, high temperature performance, versatility, and improved immunity to environmental noise and interference. The work presented in this dissertation comprises the design, fabrication, and testing of sensors that fuse these two pressure sensing technologies into one integrated unit. A key innovation is introduction of a silicon diaphragm with a center rigid body (or boss), denoted as an embossed diaphragm, that acts as the sensing element for both the electronic and optical parts of the sensor. Physical principles of piezoresistivity and Fabry-Perot interferometry were applied in designing an integrated sensor and in determining analytic models for the respective electronic and optical outputs. Several test pressure sensors were produced and their performance was evaluated by collecting response and noise data. Diaphragm deflection under applied pressure was detected electronically using the principle of piezoresistivity and optically using Fabry-Perot interferometry. The electronic part of the sensor contained four p-type silicon piezoresistors that were set into the diaphragm. They were connected in a Wheatstone bridge configuration for detecting strain-dependent changes in resistance induced by diaphragm deflection. In the optical part of the sensor, an optical cavity was formed between the embossed surface of the diaphragm and the end face of a single mode optical fiber. An infrared laser operating at 1.55 was used for optical excitation. Deflection of the diaphragm, which causes the length of the optical cavity to change, was detected by Fabry-Perot interference in the reflected light. Data collected on several sensors fabricated for this dissertation were shown to validate the theoretical models. In particular, the principle of operation of a Fabry-Perot interferometer as a mechanism for pressure sensing was demonstrated. The physical characteristics and behavior of the embossed diaphragm facilitated the integration of the electronic and optical approaches because the embossed diaphragm remained flat under diaphragm deflection. Consequently, it made the electronic sensor respond more linearly to applied pressure. Further, it eliminated a fundamental deficiency of previous applications of Fabry-Perot methods, which suffered from non-parallelism between the two cavity surfaces (diaphragm and fiber), owing to diaphragm curvature after pressure was applied. It also permitted the sensor to be less sensitive to lateral misalignment during the fabrication process and considerably reduced back pressure, which otherwise reduced the sensitivity of the sensor. As an integrated sensor, it offered two independent outputs in one sensor and therefore the capability for measurements of: (a) static and dynamic pressures simultaneously, and (b) two different physical quantities such as temperature and pressure

Smart Platform for Low-Cost MEMS Sensors – Pressure, Flow and Thermal Conductivity

In a technological world that is trending towards smart and autonomous engineering, the collection of quality data is of unrivalled importance. This has led to a huge market demand for the development of low-cost, small and accurate sensors and thus has resulted in significant research into sensors, with the aim of advancing the price/performance ratio in commercial solutions. Micro Electro Mechanical Systems (MEMS) have recently offered an attractive solution to miniaturise and drastically improve the performance of sensors. In this thesis, MEMS technology is exploited to create a multi-sensor technology platform that is used to fabricate several sensing technologies. Piezo-resistive and piezo-electronic pressure sensors are designed, fabricated and tested. Different doping profiles, stress-engineered structures and electronic devices for pressure transduction are investigated, with focus on their sensitivity and non-linearity. A ring is fabricated in the metal layer around the circumference of the membrane that alleviates the effects of over/under etching. This is achieved by creating a new rigid edge of the membrane in the metal layer, which has tighter fabrication tolerances. A piezo-MOSFET is developed and shown to have greater sensitivity than similar state-of-the-art devices. Flow sensors based on a heated tungsten wire are designed, fabricated, tested and substantiated with numerical modelling. Calorimetric and anemometric driving modes are optimised with regards to device structure. Thermodiodes are also used as the temperature transduction devices and are compared to the traditional resistor method and showed to be preferable when further miniaturising the sensor. Thermal conductivity gas sensors based on a heated tungsten resistor are designed, tested and substantiated with numerical modelling. Holes through the membrane are used to improve the sensitivity to measuring carbon dioxide by 270%. Asymmetric holes are utilised to prove a novel method of measuring thermal conductivity in a calorimetric method. Designs improving this new concept are outlined and substantiated with analytical and numerical models. Linear statistical methods and artificial neural networks are used to differentiate flow rate and gas concentration using three on-membrane resistors. With membrane holes, the discrimination between gases in the presence of flow is improved. Neural networks provide a viable solution and show an increase in the accuracy of both flow rate and gas concentration. The main objective of the work in this thesis was to develop low-cost, low-power, small devices capable of high-volume production and monolithic integration using a single smart technology platform for fabrication. The smart technology platform was used to create pressure sensors, flow sensors and thermal conductivity gas sensors. Within each sensing technology, proof-of-concepts and optimisations have been carried out in order to maximise performance whilst using the low-cost, high-volume fabrication process, ultimately helping towards smart and autonomous engineering solutions driven by data

Remotely interrogated MEMS pressure sensor

This thesis considers the design and implementation of passive wireless microwave readable pressure sensors on a single chip. Two novel-all passive devices are considered for wireless pressure operation. The first device consists of a tuned circuit operating at 10 GHz fabricated on SiO2 membrane, supported on a silicon wafer. A pressure difference across the membrane causes it to deflect so that a passive resonant circuit detunes. The circuit is remotely interrogated to read off the sensor data. The chip area is 20 mm2 and the membrane area is 2mm2 with thickness of 4 µm. Two on chip passive resonant circuits were investigated: a meandered dipole and a zigzag antenna. Both have a physical length of 4.25 mm. the sensors show a shift in their resonant frequency in response to changing pressure of 10.28-10.27 GHz for the meandered dipole, and 9.61-9.58 GHz for the zigzag antenna. The sensitivities of the meandered dipole and zigzag sensors are 12.5 kHz and 16 kHz mbar, respectively. The second device is a pressure sensor on CMOS chip. The sensing element is capacitor array covering an area of 2 mm2 on a membrane. This sensor is coupled with a dipole antenna operating at 8.77 GHz. The post processing of the CMOS chip is carried out only in three steps, and the sensor on its own shows a sensitivity of 0.47fF/mbar and wireless sensitivity of 27 kHz/mbar. The MIM capacitors on membrane can be used to detune the resonant frequency of an antenna

Conception, fabrication et caractérisation d'un microphone MEMS

Electret microphones dedicated to consumer electronics and medical applications (hearing aids) have reached the miniaturization limits. Since the release of the first microphone based on Silicon micromachining, electret microphones are constantly replaced by MEMS microphones. MEMS (Micro-Electro-Mechanical Systems) microphones use Silicon that provides exceptional mechanical characteristics along with good electric properties and mature fabrication technology. Regardless of the transduction principle (capacitive, piezoresistive, piezoelectric, optical), all of the MEMS microphones reported in the state of the art literature are based on a membrane deflecting out of the plane of the base wafer. Most of the reported microphones and all of the commercially available MEMS use capacitive transduction. Downscaling of capacitive microphones is problematic, since the sensitivity depends on capacitance value. Moreover capacitive sensors suffer of high sensitivity to parasitic capacitance and nonlinearity. The drawbacks of capacitive detection may be overcome with use of piezoresistive properties of Silicon nanowires. Unlike the classical piezoresistors integrated into silicon membrane, suspended nanowires do not suffer of leakage current. Further improvement of piezoresistive detection is possible since the longitudinal piezoresistive coefficient rises inversely proportional to nanowire section. This thesis presents the considerations of novel MEMS microphone architecture that uses microbeams which deflect in the plane of the base wafer. Signal transduction is achieved by piezoresistive nanogauges integrated in the microsystem and attached to the microbeams. Acoustic pressure fluctuations lead to the deflection of the microbeams which produces a stress concentration in the nanogauges. Accurate simulations of the discussed transducer couple acoustic, mechanical and electric behavior of the system. Due to micrometric dimensions of the MEMS acoustic system, thermal and viscous dissipative effects have to be taken into account. To reliably predict the sensor behavior two acoustic models are prepared: the complete Finite Element Model based on the full set of linearized Navier-Stokes equations and the approximative model based on the Lumped Elements (Equivalent Cirtuit Representation). Both models are complementary in the design process to finally retrieve the frequency response and the noise budget of the sensor. The work is completed by the description of the technological process and the challenges related to the prototype microfabrication. Then the approach to the MEMS microphone characterization in pressure-field and free-field is presented.Les microphones à électret dédiés à l'électronique grand public et les applications médicales (les audioprothèses) ont atteint les limites de la miniaturisation. Depuis la sortie du premier microphone basé sur une technologie microsystème sur silicium (MEMS: Micro-Electro-Mechanical Systems), les microphones à électret sont progressivement remplacés par les microphones MEMS. Les MEMS utilisent le silicium car il offre des caractéristiques mécaniques exceptionnelles avec de bonnes propriétés électriques et la technologie de fabrication est maintenant bien maîtrisée. La plupart des microphones MEMS qui sont décrits dans la littérature sont constitués d’une membrane qui vibre en dehors du plan du capteur, et utilisent la transduction capacitive. La miniaturisation de tels microphones est limitée car leur sensibilité est liée à la valeur de la capacité qui dépend de la taille de la membrane. En outre, les capteurs capacitifs sont très sensibles aux capacités parasites et aux non-linéarités. Cette thèse présente une nouvelle architecture de microphone MEMS qui utilise des micro-poutres qui vibrent dans le plan capteur. La transduction du signal est réalisée par des nanojauges piézorésistives intégrées dans le microsystème et attachées aux micro-poutres. Ce système de détection original ne présente pas les inconvénients de la détection capacitive et à la différence des piézorésistors classiques intégrés dans la membrane de silicium, les nanofils suspendus permettent d’éliminer les courants de fuite. De plus, l'amélioration de la détection est possible puisque le coefficient piézo-résistif longitudinal est inversement proportionnel à la section du nanofil. Les fluctuations de pression acoustique entraînent les déviations des micro-poutres qui produisent une concentration de contraintes dans les nanogauges. Le comportement du capteur, que l’on cherche à modéliser, est lié à des phénomènes mécaniques, acoustiques et électriques qui sont couplés. En raison des dimensions micrométriques du MEMS, les effets des dissipations thermique et visqueuse doivent être pris en compte dans le comportement acoustique. Pour prédire de façon fiable le comportement du capteur, deux modèles vibroacoustiques sont utilisés: un modèle éléments finis basé sur l'ensemble des équations de Navier-Stokes linéarisées et un modèle approché basé sur un schéma à constantes localisées (représentation par circuit électrique équivalent). Les deux modèles sont complémentaires dans le processus de conception pour déterminer la réponse en fréquence et le taux de bruit du capteur. Le travail est complété par la description des processus technologiques et les défis liés à la fabrication du prototype. Puis deux approches pour la caractérisation fonctionnelle du microphone MEMS sont présentées, la première en tube d’impédance, la seconde en champ libre

Development of dynamic pressure sensor for high temperature applications

Pressure measurement under high temperature environments is required in many engineering applications and it poses many practical problems. Pressure patterns are highly desirable for health monitoring for improved performance and accurate prediction of remaining life of systems used in various applications. Data acquisition in harsh environments has always been a major challenge to the available technology. Sensing becomes more intricate in case if it has to operate under extreme conditions of temperature. Propulsion system applications represent one such area that requires a sensor that is absolutely accurate and has utmost sensitivity coupled with the ability to withstand high temperature. The need for such sensors is driven by the dependence of the performance of propulsion system on pressure pattern encountered along the gas path. Associated with that, high resolution, small size, low time dependent drift and stable range of measurement will complete the performance of such Microsystems Sensors using the current technology are capable of reliable measurement for a limited time at an extremely high cost and are bulky thereby preventing online monitoring. Improvement in the durability of the sensors requires new technology and will definitely open new areas of research. A number of technologies have been lately investigated, these technologies targeting specific applications and they are limited by the maximum operating temperature. The objective of this research is to develop a dynamic pressure measurement system that would be capable of operating at high temperatures with the technology of the device based on Silicon Carbon Nitride (SiCN). The principle of operation is based on the drag effect. Silicon carbon-nitride (SiCN) is a material that has been little explored. The service temperature of SiCN is in the range of 1400°C. The structure is produced from a liquid polymer precursor that could be originally formed into any shape. The proposed micro sensor can measure dynamic pressure and detects flow which is very important to know as the flow continuity is critical in many applications. Furthermore pressure measurement can be used as a base for many aspects. For example the proposed micro sensor could be designed and packaged to be fitted in the gas turbine engine. The correlation of the acquired data from the sensors may provide valuable timely information on imminent instability in the gas flow, detect leakage, improve efficiency et

EUROSENSORS XVII : book of abstracts

Fundação Calouste Gulbenkien (FCG).Fundação para a Ciência e a Tecnologia (FCT)