Thin-film PZT on polymer and glass substrates: breaking the rules
Open Access
- Author:
- Liu, Tianning
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 30, 2020
- Committee Members:
- Tom Jackson, Dissertation Advisor/Co-Advisor
Tom Jackson, Committee Chair/Co-Chair
Susan E Trolier-Mckinstry, Committee Chair/Co-Chair
Mehdi Kiani, Committee Member
Christopher D. Rahn, Outside Member
Kultegin Aydin, Program Head/Chair
Susan E Trolier-Mckinstry, Dissertation Advisor/Co-Advisor - Keywords:
- flexible devices
flexible electronics
piezoelectrics
ultrasonic transducers
PZT
PZT thin films
microfabrication
anisotropic conductive film - Abstract:
- This dissertation investigates the material properties, device performances, and processing techniques for lead zirconate titanate (PZT) thin-films on flexible polymer and curved glass substrates. For the study of PZT films on polymeric substrates, a release process has been developed to transfer high temperature crystallized PZT films from Si to thin layers of solution-cast polyimide through dissolution of an underlying sacrificial etch layer. This process not only enabled the assessment of domain responses in fully released piezoelectric films as a result of declamping, but also made useful contributions to the development of flexible piezoelectric devices. For a 1 µm thick blanket PZT film transferred to a 5 µm thick polyimide layer, superior electrical properties resulted relative to the same films on Si. Polarization-electric field hysteresis measurements showed a ~45% increase in the remanent polarization (from 17.5 µC/cm2 to 26 µC/cm2). High-temperature poling either induced more ferroelectric and ferroelastic realignment or significantly increased the density of domain walls in released films, which resulted in a reduction in relative permittivity of 17% compared to only 3% for clamped films. These measurement results confirmed the correlation between the reduced substrate clamping and improved domain wall mobility in released PZT films on polymeric substrates. Evaluation of piezoelectric responses in the released state was attempted through measurements of d33 using a commercial double-beam laser interferometer. Windows were etched in the polyimide layer so that the laser beams could directly probe the top and bottom electrodes of the released PZT structure. However, large apparent d33 values were measured due to a critical error associated with excess bending in the PZT membrane. Though correct d33 data in the released state were not successfully obtained, the experiments identified a critical artifact in the system, which has been overlooked by reports in the literature. The above progress in the development of released PZT films enabled the design and fabrication of flexible ultrasonic transducers using 1 µm thick PZT on 5 µm thick polyimide substrates. The transducers were patterned single elements of PZT bar-resonators operating in width extension mode with center frequencies in the 8-80 MHz range. Pitch-catch was demonstrated with two 100 µm × 1000 µm neighboring elements respectively transmitting and receiving against an aluminum reflector at 1.5 cm distance. The element in receive detected a 0.2 mV pre-amplified signal for a driving frequency of 9.5 MHz. Characterization with a hydrophone evaluated a sound pressure output of these devices at 33 kPa and a -6dB bandwidth at 32%. PZT film development processing on unusual substrates was expanded through integrating thin-film PZT actuators on curved glass mirror segments for deterministic figure correction using the converse piezoelectric effect. Crystallization conditions, photolithography procedures and anisotropic conductive film (ACF) bonding were reinvented to accommodate the use of the unconventional substrate. Two generations of X-ray mirror prototypes are reported: a cylindrical optic (HFDFC3) with 112 actuator cells and a pseudo-conical piece (C1S04) with 288 actuator cells routed through an insulation layer using two-level metallization. Absolute figure correction could not be achieved for HFDFC3 due to processing-related impact being outside of the actuator dynamic range. However, it provided empirical influence function data that, through finite element analysis simulations, proved the feasibility of figure correction by the integrated PZT actuators. C1S04 resulted in a 15% low yield upon fabrication. A rework process was developed to remove the top-level metallization and the insulation layer to assess the failure mechanism. Most of the shorted PZT actuator cells were repaired by a novel defect-burnout approach that used a combination of pogo pins, current-voltage characteristic measurements, and laser-cutting to vaporize the conductive filaments. The final yield was improved to 99%.