Modelization, simulation and design of micro-electro-mechanicazed systems (mems) preconcentrators for gas sensing

Hoy en día, la normativa sobre gases que son tóxicos a muy bajas concentraciones se está haciendo muy estricta. Por esta razón, los dispositivos como los preconcentradores que permiten la detección de estos gases a bajas concentraciones se están investigando cada vez más.En esta tesis se hace un estudio sobre preconcentradores planos fabricados en silicio, con la idea de poder integrarlos con el sistema sensor. Estos preconcentradores se han simulado para comprender mejor su funcionamiento. Se ha hecho un estudio para conseguir una óptima homogenización de temperatura en el área calentada. Se han fabricado nuevos diseños con calefactores optimizados. También se han fabricado membranas grandes para poder depositar más material adsorbente y así incrementar la capacidad de concentración. Los nuevos diseños han sido caracterizados para validar las simulaciones y poder crear un modelo que sirva para probar nuevas ideas de diseño evitando los largos y costosos procesos de fabricación.Nowadays the allowed limits of volatiles that are toxic at very low concentrations are becoming restrictive. For this reason, preconcentrators which let detection at low levels are becoming more important and its study is increasing.In this thesis, our aim is to design a planar preconcentrator in silicon technology, in order to be able to design in the same substrate the preconcentrator and the sensor system. Simulations have been developed to study its behaviour. A good homogenisation temperature is needed in order to obtain big concentrations in a narrow desorption peak. We will develop new designs which improve homogenisation temperature. Also, a large area is needed in order to have more adsorbent material which ensures more concentration. New design have been fabricated and characterized. Experimental results validate our simulations and let us to develop future designs avoiding time and cost fabrication

Microelectromechanical Systems for Wireless Radio Front-ends and Integrated Frequency References.

Microelectromechanical systems (MEMS) have great potential in realizing chip-scale integrated devices for energy-efficient analog spectrum processing. This thesis presents the development of a new class of MEMS resonators and filters integrated with CMOS readout circuits for RF front-ends and integrated timing applications. Circuit-level innovations coupled with new device designs allowed for realizing integrated systems with improved performance compared to standalone devices reported in the literature. The thesis is comprised of two major parts. The first part of the thesis is focused on developing integrated MEMS timing devices. Fused silica is explored as a new structural material for fabricating high-Q vibrating micromechanical resonators. A piezoelectric-on-silica MEMS resonator is demonstrated with a high Q of more than 20,000 and good electromechanical coupling. A low phase noise CMOS reference oscillator is implemented using the MEMS resonator as a mechanical frequency reference. Temperature-stable operation of the MEMS oscillator is realized by ovenizing the platform using an integrated heater. In an alternative scheme, the intrinsic temperature sensitivity of MEMS resonators is utilized for temperature sensing, and active compensation for MEMS oscillators is realized by oven-control using a phase-locked loop (PLL). CMOS circuits are implemented for realizing the PLL-based low-power oven-control system. The active compensation technique realizes a MEMS oscillator with an overall frequency drift within +/- 4 ppm across -40 to 70 °C, without the need for calibration. The CMOS PLL circuits for oven-control is demonstrated with near-zero phase noise invasion on the MEMS oscillators. The properties of PLL-based compensation for realizing ultra-stable MEMS frequency references are studied. In the second part of the thesis, RF MEMS devices, including tunable capacitors, high-Q inductors, and ohmic switches, are fabricated using a surface micromachined integrated passive device (IPD) process. Using this process, an integrated ultra-wideband (UWB) filter has been demonstrated, showing low loss and a small form factor. To further address the issue of narrow in-band interferences in UWB communication, a tunable MEMS bandstop filter is integrated with the bandpass filter with more than an octave frequency tuning range. The bandstop filter can be optionally switched off by employing MEMS ohmic switches co-integrated on the same chip.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/109069/1/zzwu_1.pd