Piezoelectric Aluminum Nitride Vibrating RF MEMS for Radio Front-End Technology

This dissertation presents analytical and experimental results on a new class of contour-mode aluminum nitride piezoelectric resonators. The realization of contour mode shapes in piezoelectric resonators, different from commercially available FBAR resonators for which the thickness sets the resonant frequency, permits the definition of multiple frequencies on the same silicon substrate in a very economical manner by using one sole lithographic step. Multi-frequency and multi-band single-chip resonant platforms can be fabricated using this new class of resonators. The use of piezoelectric transduction also has a definitive advantage over electrostatically-driven contour-mode resonators by solving the problem of large motional resistance. Piezoelectric body forces intrinsically offer higher electromechanical coupling coefficients than corresponding surface-based electrostatic forces, making possible low values of motional resistance and the direct interface of these devices with 50 Ω systems. Experimental results from different contour-mode structures have demonstrated that rectangular plates and circular rings constitute the most promising topologies for the realization of next-generation multi-frequency resonant platforms. Rectangular plate resonators have shown, for example, quality factor, Q, of 2,100 in air and motional resistance of 125 Ω at a frequency of 85 MHz. At the same time circular ring resonators were fabricated on the same substrate demonstrating Q of 2,900 in air at 473 MHz and a motional resistance of approximately 84 Ω. The highest Q of 4,300 was reported for a ring resonator at 230 MHz in air.

To further prove the commercial viability of such technology, ladder filters were realized out of arrays of contour-mode resonators. Intermediate frequency (IF) filters at 93 and 236 MHz were fabricated using rectangular plates and circular rings, respectively. These filters show very promising performance, being characterized by low insertion losses (4 dB at 93 MHz), large close-in and out-of-band rejection (approximately 40 dB and above 27 dB, respectively, for a 93 MHz filter) and fairly sharp roll-off with a 20 dB shape factor of 2.2. The filters described in this dissertation are about 20X smaller than existing SAW technology, commonly used in the IF bands for cell phones. In addition, with a temperature coefficient of -25 ppm/°C, they have 40 % lower temperature sensitivity than SAW filters. Given the relatively small value of the motional resistance of the individual resonators, these piezoelectric contour-mode filters can be interfaced to 50 Ω systems by on-chip components. Another important feature of these devices is the ability to lithographically define the mass loading mechanism needed to get the few percent frequency shift used in ladder filters. This feature is unique of contour-mode technology and is another economical advantage in the manufacturing process over thickness-defined resonant devices.

In order to solve capacitive feedthrough problems experienced in high frequency resonators, a new class of two-port stacked resonators is introduced. Similarly to macroscale resonant transformers, for the first time, a two-port vertical topology was successfully implemented in piezoelectric aluminum nitride micromachined rectangular plates. These rectangular plate resonators have shown Q of 1,700 with a motional resistance of 175 Ω at 83 MHz and a feedthrough capacitance of only 20 fF. This topology maintains large electromechanical coupling, eliminates spurious responses and especially reduces feedthrough capacitance offering the potential for higher frequencies of operation.

In addition a robust and high-yield microfabrication process for AlN microstructures was developed at UC Berkeley for the first time. The process is low temperature (TMAX < 400 ºC) and uses standard CMOS-based fabrication steps, therefore offering the opportunity for a fully integrated transceiver with CMOS electronics on the same chip.