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High-Q Aluminum Nitride RF MEMS Lamb Wave Resonators and Narrowband Filters

  • Author(s): YEN, TING-TA
  • Advisor(s): Pisano, Albert P.
  • Nguyen, Clark T.-C.
  • et al.

The increasing demands for higher performance, advanced wireless and mobile communication systems have continuously driven device innovations and system improvements. In order to reduce power consumption and integration complexity, radio frequency (RF) microelectromechanical systems (MEMS) resonators and filters have been considered as direct replacements for off-chip passive components. In this dissertation, a new radio architecture for direct channel selection is explored. The primary elements in this new architecture include a multitude of closely-spaced narrowband filters (i.e., a filter bank) and an array of low-loss RF switches. This work addresses a number of issues related to this modern channel-select RF front end and explores the potential of utilizing piezoelectric aluminum nitride (AlN) resonator technology to fulfill these technical challenges.

Characteristic studies of acoustic waves propagating in a piezoelectric thin film suggest the use of high-phase-velocity Lamb wave mode vibration for higher frequency applications. The lowest-order symmetric modes (S0 modes) can be efficiently excited, via the d31 (e31) piezoelectric coefficient, by utilizing interdigital transducer (IDT) electrodes, enabling co-fabrication of devices operating from tens of megahertz up to a few gigahertz on the same chip. An AlN "overhang" fine frequency selection technique is experimentally studied, allowing precise relative frequency control of an array of Lamb wave resonators (LWR) to 0.1%. Experimental results suggest the resonance frequency of Lamb wave resonators can be linearly adjusted by up to 5% with no significant effects on other resonator parameters. The first high temperature testing of AlN Lamb wave resonators above 600°C verifies its potential of being used in a harsh environment sensing telemetry. With a correct AlN/SiO2 thickness ratio, the first-order temperature coefficient of frequency (TCF) of a LWR can be reduced from -25 ppm/K to 3.9 ppm/K. In addition, increasing the input power level from -15 dBm to 10 dBm causes no bifurcation instability or frequency hysteresis on AlN Lamb wave resonators and only 0.05% frequency drift is recorded, showing an excellent power handling capability.

A number of different resonator topologies are studied and demonstrated in this work as possible candidates for the filter bank. Mechanically-coupled filters utilize quarter-wavelength coupling beams to eliminate the mass-loading effect to adjoining resonators, and the bandwidths are determined by the equivalent stiffness of the coupling beam and the resonator itself. Numbers of identical resonators are mechanically-coupled as a filter with center frequency at 710 MHz and 0.4% fractional bandwidth (FBW). Furthermore, by introducing AlN overhang selection technique, an array of electrically self-coupled filters are fabricated with evenly-spaced center frequencies around 735 MHz and 500 kHz bandwidths (0.07% FBW). An array of ladder filters with center frequencies around 440 MHz and 2 MHz bandwidths (0.5% FBW) are also demonstrated, without post-process trimming. These closely and evenly spaced AlN Lamb wave filters demonstrate the potential to realize a purely mechanical, high performance, yet low-power RF front-end system.

To further improve filter performance, capacitive-piezoelectric Lamb wave resonators, featuring sub-micron air gaps between piezoelectric structural layer and electrodes, are demonstrated with the aim of reducing interface energy dissipation. Quality factors of these capacitive-piezo Lamb wave resonators are measured over 5,000 at 940 MHz, posting the highest reported Q for single AlN resonators using d31 (e31) transduction. The Q * f products above 4.7×10^12 exceed those of commercialized FBAR and SAW resonators. Although the motional impedance of these devices inevitably rises to 1 kilo-ohm; when electrodes are separated from the AlN, this value is still much lower than conventional electrostatic resonators and can be easily terminated with on-chip matching networks. While designing the surface micromachining fabrication process dedicated to these capacitive-piezo devices, a thorough AlN etch rate table including commonly encountered cleaning and wet/dry etch steps is established.

Although a large part of this dissertation concerns Lamb wave resonators, the last part of this dissertation focuses on a special corrugated cantilever beam design to improve conversion efficacy of a piezoelectric energy harvester. These vibration-sensitive piezoelectric AlN energy harvesters utilize corrugated cross-section cantilevers to achieve the same energy conversion effectiveness as that in a bimorph beam design, yet using a simple fabrication process similar to that of a unimorph beam. Due to the opposite signs of strains, the generated electric fields above and below the neutral plane have opposite polarities, and the generated energy can be extracted separately without the common cancellation issues encountered in a single piezoelectric beam design. This approach provides superior performance while simultaneously simplifying the fabrication process. A prototype multi-fold device resonating at 853 Hz with output power of 0.17 microwatt under a 1 G acceleration is recorded. Based on superb material properties and the 600°C thermal testing performed on RF resonators, these AlN energy harvesters offer a promising solution to scavenge vibration energies from harsh environments for advanced microsensor systems.

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