PhD defence Jiahui Wang

Jiahui Wang is a PhD student in the MESA+ research group Semiconductor Components. Her supervisor is Jurriaan Schmitz. 

characterization and reliability studies towards piezoelectrically actuated rf-mems switches 

This PhD research is part of the EPAMO project aiming 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. So, we aim to study towards galvanic RF-MEMS switches using PZT thin film actuators. This thesis presents characterization and reliability studies towards piezoelectrically actuated RF-MEMS switches.

The reliability problem, especially the reliability of the PZT thin film actuators, is the main obstacle of the development of the target device. The main failure modes of RF-MEMS switches include stiction, contact degradation, dielectric charging/breakdown, creep, RF power induced failure, and PZT actuator degradation/breakdown. The dielectric breakdown mechanism, leakage current, charging and TDDB mechanism are introduced in chapter 2 and provide a background knowledge of the reliability studies on PZT thin films in chapter 6 and 7.

An accurate characterization of the switches under test across 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. Chapter 3 focuses on the calibration and de-embedding of pF-level capacitance measurements, and provides an equivalent circuit presenting the origins of all the parasitics. The small-signal capacitance of the switches under test is measured across 11 orders of magnitude in frequency from 10 mHz to 1 GHz. The capacitance-voltage (C-V) curves measured by five different methods are compared. The low-frequency and quasi-static C-V curves yield different results from classical high-frequency and radio-frequency C-V curves. The low-frequency and quasi-static capacitance is up to 200% and 25% higher than the high-frequency and radio-frequency capacitance around the pull-in and pull-out voltages. This phenomenon is explained by a transducer model expressed in chapter 4. A transducer model presents the coupling between mechanical and electrical operations of RF-MEMS switches. In the up-state, a one-dimensional spring-mass model quantitatively reproduces the C-V behaviour. In the down-state, the difference between low frequency and high-frequency capacitances is attributed to squeeze-film effects in the residual gas volume between the top electrode and the dielectric film. This assumption is supported by measurements at reduced pressure.

The measurement of spring constant is important to characterize the RF-MEMS switches; it may be a suitable parameter for in-line process control if a fast and reliable wafer-level electrical measurement is developed. The spring constant of four RF-MEMS capacitive switch designs extracted using vibrometer, pull-in voltage, and low-field C-V measurements, are compared in chapter 5. The pull-in voltage method is considered a suitable way for in-line process control.

In chapter 6 and 7, 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. The influence of plasma etching process on PZT properties and reliability are studied in chapter 6. 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 reliability is significantly worsened when positive voltage is applied on the top electrode, whereas not much change in reliability is observed by applying negative voltage to the top electrode.

Chapter 7 studies the influence of the measurement conditions and the stacks of PZT MIM capacitors on 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 TDDB. 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.