No description available.

This thesis reports on the design and fabrication of micro-electro-mechanical (MEM) resonators on silicon that are piezoelectrically-transduced for operation in the very high frequency (VHF) range. These devices have a block-type or beam-type design, and are designed to resonate in their in-plane and out-of-plane bulk extensional modes. Two piezoelectric materials were taken into consideration, zinc-oxide (ZnO) and lead-zirconate-titanate (PZT). The resonators are fabricated on silicon-on-insulator (SOI) wafers and the metal/piezo/metal stack of layers forming the device is built and patterned on the device layer silicon via photolithography techniques, RF sputtering (for the piezo-layer) and electron-beam evaporation (for the metal layers). The designing aspect involved ANSYS simulations of the mode-shapes and estimation of frequencies, and these have correlated well with experimental results. Devices with RF sputtered ZnO were successfully fabricated and tested to give high quality factors at reasonably high frequencies. A gold ground plane was implemented to reduce the feed-through level and increase the signal-to-noise ratio. Extensive characterization of PZT was also done as a replacement for ZnO, as the former material has a much higher piezoelectric coefficient (~20X that of ZnO) and can therefore extend the operation of these MEM resonators into the UHF range. Although the basic design of the device remains the same, incorporation of PZT complicates the process flow considerably with respect to the chemistry now involved with the patterning of different layers. The frequency response for ZnO-based resonators as well as all the characterization data for PZT has been reported.

A low-temperature (<300°C) method to fabricate electrostatically actuated microelectromechanical (MEM) clamped-clamped beam resonators has been developed. It utilizes an amorphous silicon carbide (SiC) structural layer and a thin polyimide spacer. The resonator beam is constructed by DC sputtering a tri-layer composite of low-stress SiC and aluminum over the thin polyimide sacrificial layer, and is then released using a microwave O 2 plasma etch. Deposition parameters have been optimized to yield low-stress films (<50MPa), in order to minimize the chance of stress-induced buckling or fracture in both the SiC and aluminum. Characterization of the deposited SiC was performed using several different techniques including scanning electron microscopy, EDX and XRD. / Several different clamped-clamped beam resonator designs were successfully fabricated and tested using a custom built vacuum system, with measured frequencies ranging from 5MHz to 25MHz. A novel thermal tuning method is also demonstrated, using integrated heaters directly on the resonant structure to exploit the temperature dependence of the Young's modulus and thermally induced stresses.

No description available.

The most accurate method for performing analog signal processing in MOS (metal-oxide-semiconductor) integrated circuits is through the use of switched-capacitor circuits. A switched-capacitor circuit operates as a discrete-time signal processor. These circuits have been used in a variety of applications, such as filters, gain stages, voltage-controlled oscillators, and modulators. A switched-capacitor circuit contains operational amplifiers (opamps), capacitators, switches, and a clock generator. Capacitors are used to define the state variables of a system. They store charges for a defined time interval, and determine the state variables as voltage differences. Switches are used to direct the flow of charges and to enable the charging and discharging of capacitors. Nonoverlapping clock signals control the switches and allow charge transfer between the capacitors. Opamps are used in order to perform high-accuracy charge transfer from one capacitor to another. The goal of this research is to design and explore future low-voltage switched-capacitor circuits, which are crucial for portable devices. Low-voltage operation is needed for two reasons: making reliable and accurate systems compatible with the submicron CMOS technology and reducing power consumption of the digital circuits. To this end, three different switched-capacitor integrators are proposed, which function with very low supply voltages. One of these configurations is used to design a lowpass ����� modulator for digital-audio applications. This modulator is fabricated and tested demonstrating 80 dB dynamic range with a 1-V supply voltage. The second part of this research is to show that these low-voltage circuits are suitable for modern wireless communication applications, where the clock and signal frequencies are very high. This part of the research has focused on bandpass analog-to-digital converters. Bandpass analog-to-digital converters are among the key components in wireless communication systems. They are used to digitize the received analog signal at an intermediate center frequency. Such converters are used for digital FM or AM radio applications and for portable communication devices, such as cellular phones. The main block, in these converters, is the resonator, which is tuned to a particular center frequency. A resonator must be designed such that it has a sharp peak at a specific center frequency. However, because of circuit imperfections, the resonant peak gain and/or the center frequency are degraded in existing architectures. Two novel switched-capacitor resonators were invented during the second part of this research. These resonators demonstrate superior performance as compared to previous architectures. A fourth-order low-voltage bandpass ����� modulator, using one of these resonators, has been designed. / Graduation date: 2002