Released Piezoelectric Thin Films for Piezoelectric Micromachined Ultrasound Transducers (PMUT)
Open Access
- Author:
- Tipsawat, Pannawit
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 12, 2024
- Committee Members:
- John Mauro, Program Head/Chair
Susan Trolier-McKinstry, Chair & Dissertation Advisor
Tom Jackson, Major Field Member
Sri-Rajasekhar Kothapalli, Outside Unit & Field Member
Andrea Arguelles, Major Field Member - Keywords:
- Piezoelectric thin films
PMUT
MEMS
Effective longitudinal piezoelectric coefficient
PZT
Neurostimulation
Released piezoelectric thin films
Acoustic
Ultrasound - Abstract:
- This thesis focuses on the exploration of released PZT thin films, from the fundamentals that govern the magnitude of the piezoelectric response to applications. Quantitative measurement of the effective longitudinal piezoelectric coefficient of partially released PZT thin films was conducted to provide a better understanding of declamping from the substrate. Partially released structures were employed in the development and optimization of piezoelectric micromachined ultrasound transducers (PMUT) phased arrays tailored for neuromodulation applications. Further steps in developing piezoelectric devices for neuromodulation applications involved investigating the PZT PMUT with a fully released structure to offer feasibility in flexible and conformable applications such as implantable ultrasound stimulation. The investigation of partially released PZT thin films was realized using the double beam laser interferometry technique to suppress the effect of substrate bending; such bending commonly inflates the values inferred from single beam laser interferometry. The partially released structures with 2 mol% Nb-doped Pb(Zr0.52Ti0.48)O3 thin film were fabricated using a two-step backside etching process with ZnO serving as both a sacrificial layer and the silicon deep reactive ion etch stop layer. This fabrication approach allowed the released boundary to be well-defined and provided a laser path for backside probing. Significant improvement in the effective longitudinal piezoelectric coefficient (d*33,f) was observed in the partially released structure, with the released structures exhibiting a 3-fold increase in d*33,f compared to clamped samples, reaching values of 420 ± 8 pm/V in the 75% released structure. This enhancement is attributed to the change in stress level, the reduction in mechanical constraints, and improved domain wall mobility in the released structures. The results confirm that substrate declamping can substantially elevate the piezoelectric performance of thin films, bringing them closer to that of bulk ceramics. Partially released PZT-based PMUT phased arrays on a silicon-on-insulator substrate were designed and fabricated. Utilizing a 1.5 µm thick 2 mol% Nb-doped Pb(Zr0.52Ti0.48)O thin film, a 32-element PMUT phased array was optimized for neuromodulation. The array was designed using k-Wave simulations to achieve a focal distance (F) of 20 mm and steering angles (θ_s) ranging from -60° to 60°. The rigid PMUT phased array was tested in a water tank and driven with 14.6 V unipolar pulses using appropriate time delays for beamforming and steering. The maximum peak-to-peak acoustic pressure from the phased array with beamforming was found to be 0.44 MPa at 1.4 MHz, with axial and lateral resolutions of 9.2 and 1 mm, respectively. The achievable acoustic intensity (I_SPPA = 1.29 W/cm2) achieved at low driving voltages underscores the potential of rigid PMUT arrays for low-intensity focused ultrasound stimulation. Fully flexible PMUT arrays were also explored to enable conformality to curved and complex surface structures such as the skull or brain membrane, aiming for potential implantable applications. The transition to flexible PMUTs involved fabricating these devices on polyimide substrates using a transfer and release method with a ZnO sacrificial layer. A critical aspect of this design was the incorporation of an electroplated Ni metal rigid support layer to optimize the operational resonance frequency, ensuring compatibility with the pitch required for effective beamforming. A rectangular prototype PMUT was demonstrated using a capacitor stack of Pt/PZT/Pt with a 1 µm thick 2 mol% Nb-doped Pb(Zr0.52Ti0.48)O3 thin film, deposited on a silicon substrate and patterned via plasma etching with an electroplated Ni hard mask. However, the design was prone to the high stress levels; the asymmetric structure led to stress concentrations and resulted in damaged devices after release. Challenges encountered included cracking and electrode delamination. To address the stress-related issues, the introduction of a compressive layer, such as SiO2, is proposed to counterbalance the stress levels. Additionally, adjusting the electrode coverage from 60% to 100% should reduce stress concentration at the edges of the electrodes, thereby mitigating the risk of cracking and peeling. This thesis represents significant findings on the piezoelectric response of released PZT structures and advancements in the design, fabrication, and application of PMUTs. The findings provide insights into both rigid and flexible device configurations, highlighting the potential for these technologies in developing next-generation implantable ultrasound stimulation devices. The detailed exploration of material properties, device architectures, and fabrication techniques contributes to a deeper understanding of the critical factors influencing PMUT performance.