Influences of Stress on the Performance of Lead Zirconate Titanate Thin Films
Open Access
- Author:
- Coleman, Kathleen Patricia
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 23, 2020
- Committee Members:
- Susan E Trolier-Mckinstry, Dissertation Advisor/Co-Advisor
Susan E Trolier-Mckinstry, Committee Chair/Co-Chair
Jon-Paul Maria, Committee Member
Clive A Randall, Committee Member
Brian Foley, Outside Member
Raul Bermejo, Special Member
John C Mauro, Program Head/Chair - Keywords:
- piezoelectric
thin film
ferroelectric
PZT
stress
electromechanical - Abstract:
- Lead zirconate titanate (PbZrxTi1-xO3, PZT) thin films are used in piezoelectric microelectromechanical systems (PiezoMEMS) such as sensors, actuators, and energy harvesting devices. The performance of PZT films depends on the piezoelectric response and the structural integrity of the piezoelectric film. Typically, films are driven to higher fields, stresses, and strain levels compared to bulk devices. The relationship between these large input signals, particularly high stresses, and the performance of these films needs to be quantified. This thesis investigates the influences of various stresses on the properties of PZT films and determines their mechanical limits. With small changes in the stress state (~200 MPa), the piezoelectric, dielectric, and ferroelectric properties of PZT films may be tuned. The dielectric and piezoelectric properties of 0.6 µm thick {001} sol-gel Pb0.99⧠0.01(Zr0.52Ti0.48)0.98Nb0.02O3 (PZT) films on Si substrates and thin Ni foils were measured as a function of applied strain and total stress. These films are under different residual stresses arising from thermal expansion mismatch between the film and the substrate. With no additional applied stress, the remanent polarization, Pr was approximately 21 ± 0.2 µC/cm2 and 39.5 ± 2.3 µC/cm2 for PZT films on Si and Ni, respectively. The higher Pr on Ni originates from more “c” domains (out-of-plane polarization) due to the compressive stresses. The link between stress and domain orientation was further explored by applying uniaxial strains. PZT film on 50 µm Ni foil had uniaxial strains of -0.2% to 0.5% applied, while films on Si were only exposed to strains between -0.06 and 0.06%, because of substrate failure. When PZT films on Ni foil were under a 0.5% tensile strain, their Pr decreased by 7-10% and their relative permittivity increased by ~20% relative to zero applied strain. This trend reversed upon compressive strain. In addition, the piezoelectric coefficient, e31,f was -9.0 ± 0.45 µC/cm2 and -7.1 ± 0.35 µC/cm2 on Ni and Si, respectively, and increased in magnitude with applied uniaxial compressive strain. These changes suggest some ferroelastic reorientation. The extent of property changes with stress was also shown to differ for the films on Si and Ni. To explore the relationship between the tunability of properties with residual stress, the PZT films on Ni and Si were electrically characterized from 15 K up to room temperature. At room temperature, the dielectric irreversible Rayleigh parameter, αray, was 15.5 ± 0.1 and 28.4 ± 1.6 cm/kV for PZT on Si and Ni, respectively. The higher αray suggests more irreversible domain wall motion at room temperature, and may be explained by the lower stiffness on the Ni foil reducing the degree of clamping of these films. Below 200 K, αray for the PZT/Si sample exceeds that of the PZT/Ni sample. This is believed to arise from differences in the energy landscape of pinning centers for domain wall motion and was supported by Preisach analysis and the third harmonic phase angle results. The second portion of this thesis focuses on understanding the mechanical limits of PZT thin films. Piezoelectric thin films are vulnerable to fracture, which results in degradation of the structural integrity and device performance. This work explores the fracture process in PiezoMEMS, which is a combination of a crack initiation event in the thin piezoelectric film followed by crack propagation through the subsequent layers. Biaxial bending tests, using the Ball-on-three-Balls (B3B) technique, were performed on stacks containing Pb(Zr0.52Ti0.48)O3 (PZT) thin films on thick (500 µm) Si wafers. First, a series of PZT films of varying thicknesses (i.e. 0.7 µm, 1.3 µm, or 1.8 µm) was tested using the B3B method to determine the relationship between crack initiation stress and film thickness. Crack initiation stress increased when the film thickness decreased. PZT films that were 0.7 µm thick required ~590 MPa to initiate a crack, where 1.8 µm thick films required only 490 MPa to crack. This trend was modeled using a finite fracture mechanics model that necessitates a coupled stress-energy criterion for crack initiation in brittle ceramics. In this model, it was shown that for PZT in this film thickness regime (0.1 to a few microns), the crack initiation stress depended on the thickness. At higher loads, the entire stack would fail. First, a fracture would initiate in the PZT film, enter the LaNiO3 layer, and arrest in the compressive SiO2 layer. With higher loads, the crack could then propagate through the SiO2 and Si layer, failing the entire stack. Weibull analysis shows a significant effect of the thin film thickness on the stack’s strength. The characteristic strength and Weibull modulus were σ0 ~ 1110 MPa and m ~ 28 for the stacks with the 0.7 µm thin PZT film stack, σ 0 ~ 1060 MPa and m ~ 26 for the 1.3 µm film stack, σ 0 ~ 880 MPa and m ~ 10 for the 1.8 µm film stack. This trend in crack propagation was rationalized using linear elastic fracture mechanics indicating the importance of the PZT layers thickness on the initial crack length and the stack’s strength. Since films are typically under electromechanical loading conditions, electromechanical failure was also explored by investigating the relationship between the direction of applied stress and failure pattern. Cracks consistently propagated perpendicular to the maximum tensile stress direction and connected thermal breakdown events, suggesting correlations between electrical and mechanical failure. Additionally, the influence of electric history on the crack initiation stress was also determined using the B3B method. Electrical fields are important for enhancing the properties of piezoelectric thin film, but in this thesis, have been shown to decrease the mechanical load the films can withstand. For 1.6 µm thick PZT films, the crack initiation stress was reduced by ~15% when it was poled or under a DC bias compared to a virgin film. This may be due to a reduction in domain wall motion when poled, or development of local strain from some ferroelastic domain reorientation with an applied field. Overall, residual stress in PZT thin films was shown to influence the properties; at lower applied stress level these properties can be tuned. At higher stresses (around 500 MPa), cracks initiate in the PZT films and the thickness and the electrical history of the PZT layer affects the crack initiation stress. At stress levels greater than 800 MPa, failure was observed in the multilayer stack, as the crack that initiated in the PZT layer would then propagate through the underlying layers. These trends observed in this thesis will allow for further commercialization and improved performance of thin films. Additionally, the methods, models, and calculations used in this study can be expanded to other piezoelectric thin films, brittle ceramic coatings, and multilayers stacks.