High performance readout circuits and devices for Lorentz force resonant CMOS-MEMS magnetic sensors

In the last decades, sensing capabilities of martphones have greatly improved since the early mobile phones of the 90’s. Moreover, wearables and the automotive industry require increasing electronics and sensing sophistication. In such echnological advance, Micro Electro Mechanical Systems (MEMS) have played an important role as accelerometers and gyroscopes were the first sensors based on MEMS technology massively introduced in the market. In contrast, it still does not exist a commercial MEMS-based compass, even though Lorentz force MEMS magnetometers were first proposed in the late 90’s. Currently, Lorentz force MEMS magnetometers have been under the spotlight as they can offer an integrated solution to nowadays sensing power. As a consequence, great advances have been achieved, but various bottlenecks limit the introduction of Lorentz force MEMS compasses in the market. First, current MEMS magnetometers require high current consumption and high biasing voltages to achieve good sensitivities. Moreover, even though devices with excellent performance and sophistication are found in the literature, there is still a lack of research on the readout electronic circuits, specially in the digital signal processing, and closed loop control. Second, most research outcomes rely on custom MEMS fabrication rocesses to manufacture the devices. This is the same approach followed in current commercial MEMS, but it requires different fabrication processes for the electronics and the MEMS. As a consequence, manufacturing cost is high and sensor performance is affected by the MEMS-electronics interface parasitics. This dissertation presents potential solutions to these issues in order to pave the road to the commercialization of Lorentz force MEMS compasses. First, a complete closed loop, digitally controlled readout system is proposed. The readout circuitry, implemented with off-the-shelf commercial components, and the digital control, on an FPGA, are proposed as a proof of concept of the feasibility, and potential benefits, of such architecture. The proposed system has a measured noise of 550 nT / vHz while the MEMS is biased with 300 µA rms and V = 1 V . Second, various CMOS-MEMS magnetometers have been designed using the BEOL part of the TSMC and SMIC 180 nm standard CMOS processes, and wet and vapor etched. The devices measurement and characterisation is used to analyse the benefits and drawbacks of each design as well as releasing process. Doing so, a high volume manufacturing viability can be performed. Yield values as high as 86% have been obtained for one device manufactured in a SMIC 180 nm full wafer run, having a sensitivity of 2.82 fA/µT · mA and quality factor Q = 7.29 at ambient pressure. While a device manufactured in TSMC 180 nm has Q = 634.5 and a sensitivity of 20.26 fA/µT ·mA at 1 mbar and V = 1 V. Finally, an integrated circuit has been designed that contains all the critical blocks to perform the MEMS signal readout. The MEMS and the electronics have been manufactured using the same die area and standard TSMC 180 nm process in order to reduce parasitics and improve noise and current consumption. Simulations show that a resolution of 8.23 µT /mA for V = 1 V and BW = 10 Hz can be achieved with the designed device.En les últimes dècades, tenint en compte els primers telèfons mòbils dels anys 90, les capacitats de sensat dels telèfons intel·ligents han millorat notablement. A més, la indústria automobilística i de wearables necessiten cada cop més sofisticació en el sensat. Els Micro Electro Mechanical Systems (MEMS) han tingut un paper molt important en aquest avenç tecnològic, ja que acceleròmetres i giroscopis varen ser els primers sensors basats en la tecnologia MEMS en ser introduïts massivament al mercat. En canvi, encara no existeix en la indústria una brúixola electrònica basada en la tecnologia MEMS, tot i que els magnetòmetres MEMS varen ser proposats per primera vegada a finals dels anys 90. Actualment, els magnetòmetres MEMS basats en la força de Lorentz són el centre d'atenció donat que poden oferir una solució integrada a les capacitats de sensat actuals. Com a conseqüència, s'han aconseguit grans avenços encara que existeixen diversos colls d'ampolla que encara limiten la introducció al mercat de brúixoles electròniques MEMS basades en la força de Lorentz. Per una banda, els agnetòmetres MEMS actuals necessiten un consum de corrent i un voltatge de polarització elevats per aconseguir una bona sensibilitat. A més, tot i que a la literatura hi podem trobar dispositius amb rendiments i sofisticació excel·lents, encara existeix una manca de recerca en el circuit de condicionament, especialment de processat digital i control del llaç. Per altra banda, moltes publicacions depenen de processos de fabricació de MEMS fets a mida per fabricar els dispositius. Aquesta és la mateixa aproximació que s'utilitza actualment en la indústria dels MEMS, però té l'inconvenient que requereix processos de fabricació diferents pels MEMS i l’electrònica. Per tant, el cost de fabricació és alt i el rendiment del sensor queda afectat pels paràsits en la interfície entre els MEMS i l'electrònica. Aquesta tesi presenta solucions potencials a aquests problemes amb l'objectiu d'aplanar el camí a la comercialització de brúixoles electròniques MEMS basades en la força de Lorentz. En primer lloc, es proposa un circuit de condicionament complet en llaç tancat controlat digitalment. Aquest s'ha implementat amb components comercials, mentre que el control digital del llaç s'ha implementat en una FPGA, tot com una prova de concepte de la viabilitat i beneficis potencials que representa l'arquitectura proposada. El sistema presenta un soroll de 550 nT / vHz quan el MEMS està polaritzat amb 300 µArms i V = 1 V . En segon lloc, s'han dissenyat varis magnetòmetres CMOS-MEMS utilitzant la part BEOL dels processos CMOS estàndard de TSMC i SMIC 180 nm, que després s'han alliberat amb líquid i gas. La mesura i caracterització dels dispositius s’ha utilitzat per analitzar els beneficis i inconvenients de cada disseny i procés d’alliberament. D'aquesta manera, s'ha pogut realitzar un anàlisi de la viabilitat de la seva fabricació en massa. S'han obtingut valors de yield de fins al 86% per un dispositiu fabricat amb SMIC 180 nm en una oblia completa, amb una sensibilitat de 2.82 fA/µT · mA i un factor de qualitat Q = 7.29 a pressió ambient. Per altra banda, el dispositiu fabricat amb TSMC 180 nm presenta una Q = 634.5 i una sensibilitat de 20.26 fA/µT · mA a 1 mbar amb V = 1 V. Finalment, s'ha dissenyat un circuit integrat que conté tots els blocs per a realitzar el condicionament de senyal del MEMS. El MEMS i l'electrònica s'han fabricat en el mateix dau amb el procés estàndard de TSMC 180 nm per tal de reduir paràsits i millorar el soroll i el consum de corrent. Les simulacions mostren una resolució de 8.23 µT /mA amb V = 1 V i BW = 10 Hz pel dispositiu dissenyat

Design, fabrication, characterization and reliability study of CMOS-MEMS Lorentz-Force magnetometers

Tesi en modalitat de compendi de publicacionsToday, the most common form of mass-production semiconductor device fabrication is Complementary Metal-Oxide Semiconductor (CMOS) technology. The dedicated Integrated Circuit (IC) interfaces of commercial sensors are manufactured using this technology. The sensing elements are generally implemented using Micro-Electro-Mechanical-Systems (MEMS), which need to be manufactured using specialized micro-machining processes. Finally, the CMOS circuitry and the MEMS should ideally be combined in a single package. For some applications, integration of CMOS electronics and MEMS devices on a single chip (CMOS-MEMS) has the potential of reducing fabrication costs, size, parasitics and power consumption, compared to other integration approaches. Remarkably, a CMOS-MEMS device may be built with the back-end-of-line (BEOL) layers of the CMOS process. But, despite its advantages, this particular approach has proven to be very challenging given the current lack of commercial products in the market. The main objective of this Thesis is to prove that a high-performance MEMS, sealed and packaged in a standard package, may be accurately modeled and manufactured using the BEOL layers of a CMOS process in a reliable way. To attain this, the first highly reliable novel CMOS-MEMS Lorentz Force Magnetometer (LFM) was successfully designed, modeled, manufactured, characterized and subjected to several reliability tests, obtaining a comparable or superior performance to the typical solid-state magnetometers used in current smartphones. A novel technique to avoid magnetic offsets, the main drawback of LFMs, was presented and its performance confirmed experimentally. Initially, the issues encountered in the manufacturing process of MEMS using the BEOL layers of the CMOS process were discouraging. Vapor HF release of MEMS structures using the BEOL of CMOS wafers resulted in undesirable damaging effects that may lead to the conclusion that this manufacturing approach is not feasible. However, design techniques and workarounds for dealing with the observed issues were devised, tested and implemented in the design of the LFM presented in this Thesis, showing a clear path to successfully fabricate different MEMS devices using the BEOL.Hoy en día, la forma más común de producción en masa es una tecnología llamada Complementary Metal-Oxide Semiconductor (CMOS). La interfaz de los circuitos integrados (IC) de sensores comerciales se fabrica usando, precisamente, esta tecnología. Actualmente es común que los sensores se implementen usando Sistemas Micro-Electro-Mecánicos (MEMS), que necesitan ser fabricados usando procesos especiales de micro-mecanizado. En un último paso, la circuitería CMOS y el MEMS se combinan en un único elemento, llamado package. En algunas aplicaciones, la integración de la electrónica CMOS y los dispositivos MEMS en un único chip (CMOS-MEMS) alberga el potencial de reducir los costes de fabricación, el tamaño, los parásitos y el consumo, al compararla con otras formas de integración. Resulta notable que un dispositivo CMOS-MEMS pueda ser construido con las capas del back-end-of-line (BEOL) de un proceso CMOS. Pero, a pesar de sus ventajas, este enfoque ha demostrado ser un gran desafío como demuestra la falta de productos comerciales en el mercado. El objetivo principal de esta Tesis es probar que un MEMS de altas prestaciones, sellado y empaquetado en un encapsulado estándar, puede ser correctamente modelado y fabricado de una manera fiable usando las capas del BEOL de un proceso CMOS. Para probar esto mismo, el primer magnetómetro CMOS-MEMS de fuerza de Lorentz (LFM) fue exitosamente diseñado, modelado, fabricado, caracterizado y sometido a varias pruebas de fiabilidad, obteniendo un rendimiento comparable o superior al de los típicos magnetómetros de estado sólido, los cuales son usados en móviles actuales. Cabe destacar que en esta Tesis se presenta una novedosa técnica con la que se evitan offsets magnéticos, el mayor inconveniente de los magnetómetros de fuerza Lorentz. Su efectividad fue confirmada experimentalmente. En los inicios, los problemas asociados al proceso de fabricación de MEMS usando las capas BEOL de obleas CMOS resultaba desalentador. Liberar estructuras MEMS hechas con obleas CMOS con vapor de HF producía efectos no deseados que bien podrían llevar a la conclusión de que este enfoque de fabricación no es viable. Sin embargo, se idearon y probaron técnicas de diseño especiales y soluciones ad-hoc para contrarrestar estos efectos no deseados. Se implementaron en el diseño del magnetómetro de Lorentz presentado en esta Tesis, arrojando excelentes resultados, lo cual despeja el camino hacia la fabricación de diferentes dispositivos MEMS usando las capas BEOL.Postprint (published version

A high precision accelerometer-based sensor unit for the acquisition of ultra low distortion seismic signals

Over 800,000 people worldwide lost their lives to earthquakes in the last decade and on average 171 people die every day due to earthquake related damage to structures and buildings. Precisely understanding the effects ground motion has on manmade structures is crucial to making them earthquake resistant. This can only be achieved by the precise measurement, recording, and analysis of ground displacement trends during a seismic event. Although there is a vast amount of recorded seismological data available, current technology and processing methods fail to represent accurate ground displacement over time as the considerable technological challenges have yet to be overcome. Raw seismic data has so far been primarily acquired with instruments utilising geophone or accelerometer based sensors. These instruments produce prominent time domain displacement errors due to the various system and sensor inaccuracies, and due to non-linear response. Since accelerometers provide acceleration over time data: whilst geophones are velocimeters, and therefore provide velocity over time data; in order to derive true ground displacement over time, a double, or single numerical integration is required respectively. During this essential numerical integration processes of data from such sensors, even small in magnitude errors accumulate to yield rather large displacement trend offsets over a typical event recording period of 60 to 120 seconds. In addition, the numerical integration process itself poses considerable challenges due to the theoretically infinite number of samples and the accurate determination of initial conditions required for an exact mathematical result to be obtained. The latter, is currently performed by averaging an up to 60 second pre-event data trend stored on the instrument. Most post-integration data from current instruments appears to contain low frequency drifts amongst other noise artefacts, and generally requires baseline correction algorithms in an attempt to correct for these effects. Such corrections, although helpful, only aid to minimise the perceived effects of an assumed and collective source of error, and hence are largely unable to tackle the individual error contribution of each element within the system. Since individual element contribution is of a dynamic nature, the validity of these algorithms is limited by the accuracy of the initial assumptions made about a specific set of data. Faced with such a multivariable and uncertain dynamic behaviour, where even mathematical system modelling is of inadequate long term accuracy, a solution that aims to directly minimise these errors at source, rather than attempt to correct them postacquisition, is of immense importance when it comes to the recording, analysis, and understanding of earthquakes. This thesis describes the design, implementation, and evaluation of a High Precision Active Gyro Stabilised (HPAGS) sensor unit of exceptional performance for the provision of highly accurate ground displacement data. Experimental results demonstrated that the device described herein, was able to diminish the inherent non-linear and environment-dependant effects of current sensors, and thus was able to provide highly improved time domain displacement data

A CMOS Multi-Sensor System for 3D Orientation Determination

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