MICROMACHINED SWITCHES AND CANTILEVER ACTUATORS BASED ON PIEZOELECTRIC LEAD ZIRCONATE TITANATE (PZT)
Open Access
- Author:
- Gross, Steven Joseph
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 26, 2004
- Committee Members:
- Thomas Nelson Jackson, Committee Chair/Co-Chair
Srinivas A Tadigadapa, Committee Chair/Co-Chair
Susan E Trolier Mckinstry, Committee Member
Kenji Uchino, Committee Member
Christopher Rahn, Committee Member - Keywords:
- MEMS
PZT
microfabrication
etching
cantilevers
xenon difluoride
ion beam etching
piezoelectric
actuators
unimorph
bimorph - Abstract:
- In this work, the first piezoelectric surface-micromachined switches were developed and demonstrated. To date, MEMS switches based on electrostatic actuation have exhibited excellent RF performance including low loss, high isolation, and low power consumption. Many systems stand to benefit from these switches, and other RF MEMS devices. A significant drawback to MEMS switches so far has been the high actuation voltages, typically greater than 50V. This work has empirically confirmed that exploiting piezoelectric actuation offers a considerable advantage over electrostatics. A switching time of 2 ms was measured with only 30 volts applied. This is one of the fastest reported switching speeds for MEMS device, as well as a comparatively low actuation voltage. The piezoelectric switches in this study were designed based on unimorph cantilevered bender actuators. Electrical switching was accomplished by a metal contact on the end of the actuator that makes and breaks a connection with a gold transmission line. A low-stress silicon nitride was used as the structural layer. Sol-gel deposited lead zirconate titanate (PZT) films were selected as the actuation material due to their comparatively high piezoelectric response. These films exhibited good ferroelectric properties with remanent polarizations of 20-25 mC/cm2 and coercive fields of 50 kV/cm. Gold interdigitated electrodes were used to pole and drive the microactuators and switches. The PZT, and remaining films in the stack, were patterned using ion-beam etching. Hard chromium and titanium masks, used in conjunction with oxygen, were developed for ion milling, and found to have an etch-rate selectivity over PZT of roughly 3 to 1. The devices were released by etching sacrificial silicon in gas-phase xenon difluoride. Released cantilevers had a propensity for upward curvature due to unbalanced residual stresses in the films. A systematic approach, based on varying the PZT thickness, was devised to produce flat cantilevers, and ultimately, functioning switches. An investigation into the stress mechanisms was undertaken to gain insight into the stress mechanisms with the aim of improving yield. The study revealed that the stress of the sol-gel deposited zirconia was not stable when exposed to the atmosphere. Presumably, this was a result of hydration due to the porous nature of the film. Reactively sputtered titania films were explored as an alternative barrier layer. PZT films deposited on 100 nm of titania exhibited good ferroelectric properties, comparable to those deposited on zirconia. Unimorph actuators using titania were fabricated, and the field-induced deflection was measured. Due to the symmetric nature of the bimorph, and its predicted superior performance, this structure was explored as an alternative to the unimorph. It was found that the curvature of these structures could be controlled post-release by increasing the annealing temperature. Based on these initial results, bimorph actuators look very promising for MEMS switches. The deflection of a zirconia buffered actuator, 280mm in length, was measured to be 10 mm at 100 volts (160 kV/cm). A 280 mm long titania actuator exhibited a 15 mm at 200 volts (333 kV/cm). A switch with a tapered geometry was used to measure the transmitted RF power as function of frequency, and was found to have an isolation of more than 60 dB up to 10 MHz.