PZT Process Development for pMUT Actuators

The objective of this thesis is the development of a process flow PZT-based of a piezoelectric Micromachined Ultrasound Transducer (pMUT) for large area applications, and subsequently the characterization of the PZT layer and the full device. A brief introduction and a theoretical background is developed in order to have a better understanding of the following experiments. A simple lumped element model is explained and the main process flow is discussed, giving a general idea of the steps. Then, the development of the process is treated, showing some results related to this exotic ferroelectric material, such as the development of a PZT etching process and a Critical Dimension (CD) of 10 um for the patterning of it. Moreover, the development of a stable Bosch Process Deep Reactive Ion Etching (DRIE) is tackled and the improvements will be discussed in details. This is the most critical part, being the main cause of the resonance blue shift and small membrane velocity. Sol-gel and Pulsed Laser Deposition (PLD) PZT are then characterized and compared. Among the PZT Figures of Merit (FoM), the remanent polarization, fatigue, working voltage, breakdown voltage and uniformity are treated. Moreover, an analysis of top electrode material is performed, and a final Ti/Al/Ti top electrode is chosen for the superior remanent polarization (25 uC/cm^2), fatigue (nice signal up to 100 polarizing cycles) and breakdown properties (60 V/um). Afterwards, the pMUT fabricated device is measured, and a resonant frequency of 1.35 MHz is obtained for a 1 mm diameter membrane. In order to verify this resonance, Laser Doppler Velocimetry (LDV), impedance and capacitive measurements are performed.