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

Advanced single-chip temperature stabilization system for silicon MEMS resonators and gyroscopes

The main objective of this research is to develop temperature and frequency stabilization techniques for silicon MEMS oven-controlled crystal oscillators (MEMS OCXO) with high-frequency stability. The device was built upon an ovenized platform that used a micro-heater to adjust the temperature of the resonator. Structural resistance-based (Rstruc) temperature sensing was used to improve the self-temperature monitoring accuracy of the silicon MEMS resonator. An analog feedback micro-oven control loop and a feedforward digital calibration scheme were developed for a 77MHz MEMS oscillator, which achieved a ±0.3ppm frequency stability from -25°C to 85°C. An AC heating scheme was also developed to enable tighter integration of the resonator, temperature sensor (Rstruc) and heaters. This temperature stabilization technique was also applied to silicon MEMS mode-matched vibratory x/y-axis and z-axis gyroscopes on a single chip. The temperature-induced frequency change, scale factor and output bias variations were all reduced significantly. The complete interface circuit for the single-chip three axes gyroscopes were also developed with an innovative trans-impedance amplifier to reduce the input-referred noise. For the first time, the simultaneous operation of mode-matched vibratory 3-axis MEMS gyroscopes on a single chip was demonstrated.Ph.D

Above-IC RF MEMS devices for communication applications

Wireless communications are showing an explosive growth in emerging consumer and military applications of radiofrequency (RF), microwave, and millimeter-wave circuits and systems. Applications include wireless personal connectivity (Bluetooth), wireless local area networks (WLAN), mobile communication systems (GSM, GPRS, UMTS, CDMA), satellite communications and automotive electronics. Future cell phones and ground communication systems as well as communication satellites will require more and more sophisticated technologies. The increasing demand for size and weight reduction, cost savings, low power consumption, increased frequency and higher functionality and reconfigurability as part of multiband and multistandard operation is necessitating the use of highly integrated RF front-end circuits. Chip scaling has made a major contribution to this goal, but today a situation has been reached where the presence of numerous off-chip passive RF components imposes a critical bottleneck to further integration and miniaturization of wireless transceivers. Microelectromechanical systems (MEMS) technology is a rapidly emerging enabling technology that is intended to replace the discrete passives by their integrated counterparts. In this thesis, an original metal surface micromachining process, which is compatible with CMOS post-processing, for above-IC integration of RF MEMS tunable capacitors and suspended inductors is presented. A detailed study on SF6 inductively coupled plasma (ICP) releasing has been performed in order to ascertain the optimal process parameters. This study has emphasized the fact that temperature plays an important role in this process by limiting silicon dioxide etching. Moreover, the optimized recipe has been found to be independent of the sacrificial layer used (amorphous or polycrystalline silicon) and its thickness. Using this recipe, 15.6 µm/min Si underetch rate with high Si: SiO2 selectivity (> 20000: 1) has been obtained. Single-air-gap and double-air-gap parallel-plate MEMS tunable capacitors have been designed, fabricated and characterized in the pF range, from 1 MHz to 13.5 GHz. It has been shown that an optimized design of the suspended membrane and direct symmetrical current feed at both ports can significantly improve the quality factor and increase the self-resonant frequency, pushing it to 12 GHz and beyond. The maximum capacitance tuning range obtained for a single-air-gap capacitor is 29% for a bias voltage of 20 V. The maximum capacitance tuning range obtained for a double-air-gap capacitor is 207% for a bias voltage of 70 V. The post-processing of X-FAB BiCMOS wafers has been successfully demonstrated to fabricate monolithically integrated VCOs with above-IC MEMS LC tank. Comparing a suspended inductor and the X-FAB inductor with the same design, it has been shown that increasing the thickness of the spiral from 2.3 to 4 µm and having the spiral suspended 3 µm above the passivation layers lead to an improvement factor of 2 for the peak quality factor and a shift of the self-resonant frequency beyond 15 GHz. No significant variation on bipolar and MOS transistors characteristics due to the post-processing has been observed and we conclude that the variation due to post-processing is in the same range as the wafer-to-wafer variation. Based on our metal surface micromachining process, coplanar waveguide (CPW) MEMS shunt capacitive switches and variable true-time delay lines (V-TTDLs) have been designed, fabricated and characterized in the 1 - 20 GHz range. A novel MEMS device architecture: the SG-MOSFET, which combines a solid-state MOS transistor and a metal suspended gate has been proposed as DC current switch. The corresponding fabrication process using polysilicon as a sacrificial layer has been developed to release metal gate suspended over gate oxide by SF6 plasma. Very abrupt current switches have been demonstrated with subthreshold slope better than 10 mV/decade (better than the theoretical solid-state bulk or SOI MOSFET limit of 60 mV/decade) and ultra-low gate leakage (less than 0.001 pA/µm2) due to the air-gap

System and circuit design for a capacitive MEMS gyroscope

In this thesis, issues related to the design and implementation of a micro-electro-mechanicalangular velocity sensor are studied. The work focuses on a system basedon a vibratory microgyroscope which operates in the low-pass mode with a moderateresonance gain and with an open-loop configuration of the secondary (sense) resonator.Both the primary (drive) and the secondary resonators are assumed to have a high qualityfactor. Furthermore, the gyroscope employs electrostatic excitation and capacitivedetection. The thesis is divided into three parts. The first part provides the background informationnecessary for the other two parts. The basic properties of a vibratory microgyroscope,together with the most fundamental non-idealities, are described, a shortintroduction to various manufacturing technologies is given, and a brief review of publishedmicrogyroscopes and of commercial microgyroscopes is provided. The second part concentrates on selected aspects of the system-level design of amicro-electro-mechanical angular velocity sensor. In this part, a detailed analysis isprovided of issues related to different non-idealities in the synchronous demodulation,the dynamics of the primary resonator excitation, the compensation of the mechanicalquadrature signal, and the zero-rate output. The use of ΣΔ modulation to improveaccuracy in both primary resonator excitation and the compensation of the mechanicalquadrature signal is studied. The third part concentrates on the design and implementation of the integratedelectronics required by the angular velocity sensor. The focus is primarily on the designof the sensor readout circuitry, comprising: a continuous-time front-end performingthe capacitance-to-voltage (C/V) conversion, filtering, and signal level normalization;a bandpass ΣΔ analog-to-digital converter, and the required digital signal processing(DSP). The other fundamental circuit blocks, which are a phase-locked loop requiredfor clock generation, a high-voltage digital-to-analog converter for the compensationof the mechanical quadrature signal, the necessary charge pumps for the generationof high voltages, an analog phase shifter, and the digital-to-analog converter used togenerate the primary resonator excitation signals, together with other DSP blocks, areintroduced on a more general level. Additionally, alternative ways to perform the C/Vconversion, such as continuous-time front ends either with or without the upconversionof the capacitive signal, various switched-capacitor front ends, and electromechanicalΣΔ modulation, are studied. In the experimental work done for the thesis, a prototype of a micro-electro-mechanicalangular velocity sensor is implemented and characterized. The analog partsof the system are implemented with a 0.7-µm high-voltage CMOS (ComplimentaryMetal-Oxide-Semiconductor) technology. The DSP part is realized with a field-programmablegate array (FPGA) chip. The ±100°/s gyroscope achieves 0.042°/s/√H̅z̅spot noise and a signal-to-noise ratio of 51.6 dB over the 40 Hz bandwidth, with a100°/s input signal. The implemented system demonstrates the use of ΣΔ modulation in both the primaryresonator excitation and the quadrature compensation. Additionally, it demonstratesphase error compensation performed using DSP. With phase error compensation,the effect of several phase delays in the analog circuitry can be eliminated, andthe additional noise caused by clock jitter can be considerably reduced

RF MEMS reference oscillators platform for wireless communications

A complete platform for RF MEMS reference oscillator is built to replace bulky quartz from mobile devices, thus reducing size and cost. The design targets LTE transceivers. A low phase noise 76.8 MHz reference oscillator is designed using material temperature compensated AlN-on-silicon resonator. The thesis proposes a system combining piezoelectric resonator with low loading CMOS cross coupled series resonance oscillator to reach state-of-the-art LTE phase noise specifications. The designed resonator is a two port fundamental width extensional mode resonator. The resonator characterized by high unloaded quality factor in vacuum is designed with low temperature coefficient of frequency (TCF) using as compensation material which enhances the TCF from - 3000 ppm to 105 ppm across temperature ranges of -40˚C to 85˚C. By using a series resonant CMOS oscillator, phase noise of -123 dBc/Hz at 1 kHz, and -162 dBc/Hz at 1MHz offset is achieved. The oscillator’s integrated RMS jitter is 106 fs (10 kHz–20 MHz), consuming 850 μA, with startup time is 250μs, achieving a Figure-of-merit (FOM) of 216 dB. Electronic frequency compensation is presented to further enhance the frequency stability of the oscillator. Initial frequency offset of 8000 ppm and temperature drift errors are combined and further addressed electronically. A simple digital compensation circuitry generates a compensation word as an input to 21 bit MASH 1 -1-1 sigma delta modulator incorporated in RF LTE fractional N-PLL for frequency compensation. Temperature is sensed using low power BJT band-gap front end circuitry with 12 bit temperature to digital converter characterized by a resolution of 0.075˚C. The smart temperature sensor consumes only 4.6 μA. 700 MHz band LTE signal proved to have the stringent phase noise and frequency resolution specifications among all LTE bands. For this band, the achieved jitter value is 1.29 ps and the output frequency stability is 0.5 ppm over temperature ranges from -40˚C to 85˚C. The system is built on 32nm CMOS technology using 1.8V IO device

Integrated interface electronics for capacitive MEMS inertial sensors

This thesis is composed of 13 publications and an overview of the research topic, which also summarizes the work. The research presented in this thesis concentrates on integrated circuits for the realization of interface electronics for capacitive MEMS (micro-electro-mechanical system) inertial sensors, i.e. accelerometers and gyroscopes. The research focuses on circuit techniques for capacitive detection and actuation and on high-voltage and clock generation within the sensor interface. Characteristics of capacitive accelerometers and gyroscopes and the electronic circuits for accessing the capacitive information in open- and closed-loop configurations are introduced in the thesis. One part of the experimental work, an accelerometer, is realized as a continuous-time closed-loop sensor, and is capable of achieving sub-micro-g resolution. The interface electronics is implemented in a 0.7-µm high-voltage technology. It consists of a force feedback loop, clock generation circuits, and a digitizer. Another part of the experimental work, an analog 2-axis gyroscope, is optimized not only for noise, but predominantly for low power consumption and a small chip area. The implementation includes a pseudo-continuous-time sense readout, analog continuous-time drive loop, phase-locked loop (PLL) for clock generation, and high-voltage circuits for electrostatic excitation and high-voltage detection. The interface is implemented in a 0.35-µm high-voltage technology within an active area of 2.5 mm². The gyroscope achieves a spot noise of 0.015 °/s/√H̅z̅ for the x-axis and 0.041 °/s/√H̅z̅ for the y-axis. Coherent demodulation and discrete-time signal processing are often an important part of the sensors and also typical examples that require clock signals. Thus, clock generation within the sensor interfaces is also reviewed. The related experimental work includes two integrated charge pump PLLs, which are optimized for compact realization but also considered with regard to their noise performance. Finally, this thesis discusses fully integrated high-voltage generation, which allows a higher electrostatic force and signal current in capacitive sensors. Open- and closed-loop Dickson charge pumps and high-voltage amplifiers have been realized fully on-chip, with the focus being on optimizing the chip area and on generating precise spurious free high-voltage signals up to 27 V

Stretching the limits of dynamic range, shielding effectiveness, and multiband frequency response

In this dissertation, an RF MEMS variable capacitor suitable for applications requiring ultrawide capacitive tuning ranges is reported. The device uses an electrostatically tunable liquid dielectric interface to continuously vary the capacitance without the use of any moving parts. As compared to existing MEMS varactors in literature, this device has an extremely simple design that can be implemented using simple fabrication methods that do not necessitate the use of clean room equipment. In addition, this varactor is particularly suited for incorporating a wide range of liquid dielectric materials for specific tuning ratio requirements. Additionally, the shielding effectiveness performance of graphene-doped ABS thin films is investigated. The use of graphene as a replacement for metal fillers in composite EMI shielding materials is quickly becoming a widely-investigated field in the electromagnetic compatibility community. By replacing conventional metal-based shielding methods with graphene-doped polymers, low-weight, field-use temporary shielding enclosures can be implemented that do not suffer from mechanical unreliability and corrosion/oxidation like a traditional metal enclosure. While the performance of composite EMI shielding materials has not yet surpassed metals, the advantages of polymer-based shielding methods could find usage in a variety of applications. Finally, mutliband pre-fractal antennas fabricated via 3D printing are reported. These devices are the first to incorporate the advantages of 3D printing (rapid prototyping, fabrication of complex geometries otherwise unobtainable) with the advantages of self-similar antennas (increased gain and multiband performance) in a single device. The Sierpinski tetrahedron-based antenna design was both computationally modeled and physically realized to illustrate its potential as a solution to enable true multiband communication platforms