DESIGN, FABRICATION, TEST, AND EVALUATION OF RF MEMS SERIES SWITCHES USING LEAD ZIRCONATE TITANATE (PZT) THIN FILM ACTUATORS
Open Access
- Author:
- Polcawich, Ronald Gene
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 03, 2007
- Committee Members:
- Susan E Trolier Mckinstry, Committee Chair/Co-Chair
Clive A Randall, Committee Member
Srinivas A Tadigadapa, Committee Member
Thomas R Shrout, Committee Member
Madan Dubey, Committee Member - Keywords:
- PZT
RF MEMS
Switch - Abstract:
- The aim of this thesis was to design and prototype a robust, low voltage RF MEMS switch for use in military phased arrays. The frequencies of interest for this work include very low frequencies down to DC operation with the upper limit extending to at least 40 GHz. This broad frequency requirement requires a robust high frequency design and simulation using microwave transmission lines. With the aid of researchers at the US Army Research Laboratory, co-planar waveguide (CPW) transmission lines were chosen and designed to provide a low loss, 50 ohm impedance transmission line for the switch. CPW designs allow for both series and shunt switch configuration with this work focusing on a series switch. Furthermore, a series switch an ohmic contact was chosen as opposed to capacitive contacts. Piezoelectric actuation is chosen for the switch to enable operating voltages less than 10 volts while still maintaining a restoring force to prevent stiction. To meet these demands, lead zirconate titanate (PZT) thin films have been chosen for the piezoelectric actuator. Mechanical modeling of cantilevers comprised of an elastic layer and a Pt-PZTPt actuator were used to demonstrate feasibility of closing large gaps between switch contacts. Placement of the actuator to minimize perturbations to the RF transmission line is critical for broadband performance. Using fabrication design rules, electro-mechanical modeling, and high frequency design, the actuators were designed to fit with the RF gap between the RF conductor and ground planes of the CPW transmission line. Optimal performance was obtained with the actuators mechanically isolated from a majority of the RF transmission except for a small section that provides the contact pad to enable switch closure. The resulting switch is the first demonstrated first surface micromachined RF MEMS switch operating from DC to 65 GHz. This switch has a median actuation voltage below 5 volts with operation as low as 2 volts. Isolation in the off state is better than -20 dB across the frequency band with values better than -60 dB observed for frequency below 1 GHz. The insertion loss in the on state is less than 1 dB below 40 GHz and remains below 2 dB up to 65 GHz. Switching time was demonstrated at 40 μs and was limited by bouncing between the contacts, with initial contact observed at less than 10 μs. The cycle reliability of this switch has been tested with high contact resistance failures observed in the low 106 cycles range. The reliability appears to be limited by organic contamination between the contacting surfaces, possibly the result of resist residue and/or the development of a frictional polymer on the platinum coated contact surface. Testing of CPW transmission lines pointed to the fact that the elastic layer used to stress engineer the PZT actuators significantly contributes to the transmission line loss and limits the insertion loss of the switch in the on state. The insertion loss was improved to better than 0.5 dB below 40 GHz by modifying the fabrication process to eliminate the silicon nitride layer within the elastic layer from underneath of the CPW. This process modification leaves the elastic layer intact beneath the PZT actuators, ensuring proper stress engineering of the actuators. With one application of this RF MEMS switch targeted for military phased arrays, temperature sensitivity is an extremely important characteristic. The PZT SW5.1 design exhibited excellent off state performance with no change in the isolation characteristics from -25ºC to 100ºC. However, in the on state, the actuation voltage is required to increase to 20 volts to ensure operation at 100ºC. The temperature sensitivity was improved by redesigning the RF contact pad to enable co-linear contact between the RF input and output sides of the transmission. This redesign, PZT SW5.2, successfully enables operation of the PZT RF MEMS switch from -25ºC to 100ºC with an actuation voltage less than 10 volts. The isolation of the newer design remains similar to the earlier design with values better than -20 dB up through 50 GHz. The insertion loss of this design exhibited a slight improvement compared to the earlier design with values less than 0.3 dB below 40 GHz and rising to 0.5 dB up through 50 GHz. The DC and RF power handling of the PZT SW5.2 design was evaluated with the goal of investigating the use of eutectic alloys in the contact dimple for improved RF performance at elevated power levels. The testing at DC highlighted the existence of a contamination layer on the switch contacts, evident by the significant reduction in contact resistance with increasing DC current. Subsequently analysis found these devices have residual resist remaining on the switch contacts possibly from carbonization of the resist during the fabrication process. This residue does not affect the power handling as it breaks down at low currents. For RF power handling, the PZT SW5.2 switch demonstrated an average failure of 2 – 3 WRF. A significant majority of the switch failures occurred along the RF contact pad from electromigration failures, as the contact pad has a contact resistance greater than that of the contact dimples.