HIGH FREQUENCY LOW VOLTAGE PIEZOELECTRIC MEMS DEVICES WITH CLOSE COUPLED ELECTRONICS

Open Access
Author:
Kim, Hyunsoo
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 29, 2010
Committee Members:
  • Thomas Nelson Jackson, Dissertation Advisor
  • Jerzy Ruzyllo, Committee Member
  • Thomas Nelson Jackson, Committee Chair
  • Srinivas A Tadigadapa, Committee Member
  • Susan E Trolier Mckinstry, Committee Member
Keywords:
  • MEMS Devices
  • Ultrasound transducers
  • Quartz crystal oscillators
  • Piezoelectric thin film
  • Piezoelectric micromachined ultrasound transducers
Abstract:
Two different types of piezoelectric MEMS devices, a high frequency quartz resonator and a high frequency ultrasound lead zirconate titanate (PZT) transducer, were developed in this work. The first section of the work aimed at developing oven controlled crystal oscillators (OCXOs) with reduced volume, power, and start-up time. OCXOs provide high frequency accuracy and stability and are widely used for communication and network systems, cellular phones, and GPS navigation systems. In a typical OCXO package, multiple printed circuit boards for the resonator, oscillator circuit, and other temperature sensitive components are packaged in a metal enclosure, which results in a large package volume, large power consumption, and long warm-up time. A crystal-on-chip technique was developed with a high frequency, high-Q MEMS-based quartz crystal resonator as a new approach of OCXO manufacturing. Unlike previous tab mesa–type quartz crystal resonators, for this work a recess was etched in the middle of the quartz crystal substrate so that the resulting resonator would have two different thicknesses: (1) a thinned enclosed active resonator region, and (2). a thick support frame. Controlled wet etching was demonstrated for a 10 µm active layer thickness with a 150-MHz thickness shear mode resonant frequency. The resonator was combined with a control chip in a proof-of-concept miniature OCXO with only 10 mm3 of core volume, power consumption of 1.2 W at start-up, and 300 mW at steady state. The warm-up time to reach the steady state was 190 seconds. The second section of the work aimed to demonstrate a reduced scale device with a lower Q, but a high piezoelectric coefficient material, PZT, for use as an ultrasound transducer. High-frequency ultrasound devices in the >30 MHz range are capable of resolving submicron features; however, they are typically limited by traditional fabrication methods. Moreover, the large-drive voltage level usually requires bulky and expensive cables and complex external electronics. In this work, PZT thin films were used so that the voltages required to excite the transducer were greatly reduced. Therefore, the transducers could be driven directly with CMOS signals. The ultrasound PZT transducer was fabricated by a multilayer dry-etching technique in contrast to the wet etching used for the quartz resonator. The transducer design was based on a partially released unimorph structure used in length- or width-mode vibration. This design used SiO2 and PZT as the passive and active layers, respectively. Because of their high piezoelectric response, sol-gel-deposited PZT films were selected as the transducer material. These films exhibited good ferroelectric properties with remanent polarizations of 25 µC/cm2 and coercive fields of 50 kV/cm. Platinum was used for the top and bottom electrodes to pole and drive the transducer. The PZT and the remaining films in the stack were patterned using ion-beam etching. The transducers were partially released by etching sacrificial silicon in gas-phase xenon difluoride. The transducers have 30 ~ 70 MHz resonance frequencies and low Q (20), and the pitch-catch mode test showed a received signal bandwidth of ~65%. A fabrication method for two-dimensional transducer arrays using a pillar structure was also proposed. A custom-designed CMOS transceiver chip for a high-frequency ultrasound imaging system was designed by Mixed Signal Chip Design Lab, Department of Computer Science and Engineering. The chip size was 10 mm2, and its average power consumption in receive mode was approximately 270 mW with a 3.3V power supply. In comparison with conventional ultrasound imaging systems, the system size and power consumption were remarkably reduced by integrating the MEMS transducers with the CMOS transceiver chip. Moreover, expensive cables and other bulky components are not required because of the close coupling of the CMOS transceiver chip and MEMS transducers and because the proposed system does not necessitate any extra parts. The MEMS transducers were integrated with the CMOS transceiver chip such that the transducers were successfully excited, receiving ultrasound waves during the test. These results serve as a guideline for portable ultrasound-imaging systems with thin-film MEMS transducers.