Characterization and reliability studies towards piezoelectrically actuated RF-MEMS switches

Abstract

This PhD research aims to develop high density piezoelectric RF-MEMS switch arrays to be integrated in an energy-efficient agile RF transceiver with reconfigurable antenna. According to the comparison of performance criteria for various RF-MEMS switches, the galvanic switches are broadband and have a higher isolation than capacitive switches. The piezoelectric switches have better performance in actuation voltage, speed, linearity than electrostatic switches. Therefore, we aim to study towards galvanic RF-MEMS switches using PZT thin film actuators. An accurate characterization of the device under test across a wide range in frequency is important not only for a comprehensive understanding of the device but also for detecting the degradation/breakdown of the device. This thesis studies the calibration and de-embedding methods of pF-level capacitance measurements of the switches, and provides an equivalent circuit presenting the origins of all the parasitics. The low-frequency and quasi-static capacitance-voltage curves yield different results from classical high-frequency and radio-frequency C-V curves around pull-in and pull-out. This phenomenon is explained by a transducer model, which presents the coupling between mechanical and electrical operations of RF-MEMS switches. Then the study goes towards the reliability of metal-insulator-metal (MIM) capacitors with PZT thin film, which are potential piezoelectric actuators of the target device. We find that the ion milling process induced charging should be controlled, or better still eliminated. By comparing two kinds of PZT capacitors, it appears that direct ion bombardments of the PZT surface may cause PZT degradation/damage. The measurement conditions and the stacks of PZT MIM capacitors also influence the reliability. Humidity greatly worsens PZT degradation and breakdown. To measure PZT material quality, it is important to prevent moisture in the measurement. Both reversible and irreversible PZT degradation/breakdown are observed to happen during time-dependent-dielectric-breakdown. At the same composition and layer thickness, the crystal structure of PZT determines the breakdown voltage to a large extent. A higher temperature or a larger voltage leads to a shorter breakdown time.