THE INFLUENCE OF CLAMPING AND RESIDUAL STRESS ON SCALING EFFECTS IN Pb(Zr0.3Ti0.7)O3 THIN FILMS FOR PIEZOMEMS APPLICATIONS
Open Access
- Author:
- Denis, Lyndsey
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 06, 2019
- Committee Members:
- Susan E Trolier-Mckinstry, Dissertation Advisor/Co-Advisor
Susan E Trolier-Mckinstry, Committee Chair/Co-Chair
Roman Engel-Herbert, Committee Member
Clive A Randall, Committee Member
Michael T Lanagan, Outside Member
Jacob L. Jones, Special Member - Keywords:
- Rayleigh Analysis
Dielectric Response
Domain Wall Motion
X-ray Diffraction
Residual Stress
PiezoMEMS Devices - Abstract:
- Ferroelectric thin films such as lead zirconate titanate (PZT) have high dielectric and piezoelectric properties which can be utilized in actuators, sensors, transducers, and energy harvesters in microelectromechanical systems (MEMS). A small film thickness enables low voltage operation of such devices; however, property degradation limits the extent that the film thickness can be reduced while maintaining performance. This dissertation describes the impact that different electrical and mechanical boundary conditions have on the dielectric properties of PZT thin films for a variety of thicknesses (ranging from 0.27 to 1.11 µm). Specifically, variations in elastic layer thickness, substrate clamping, residual stress and domain state were investigated. A novel approach to quantitatively deconstruct the relative permittivity into three contributions (intrinsic, reversible extrinsic and irreversible extrinsic) was developed using a combination of X-ray diffraction and Rayleigh analysis. This work aims to determine which factors are associated with scaling effects in tetragonal {001} textured Pb0.99(Zr0.3Ti0.7)0.98Nb0.02O3 (PZT 30/70) thin films. It is generally accepted that scaling effects play a key role in the suppression of ferroelectric responses in thin films (< 1 μm), though the type and extent of this contribution is still debated. Scaling effects refer to the size-induced degradation of properties at length scales exceeding those associated with the ferroelectric stability limit. For a blanket PZT 30/70 film clamped to a Si substrate, the thickness dependence of the irreversible and reversible Rayleigh coefficients was investigated using Rayleigh analysis. The irreversible Rayleigh coefficient was found to be thickness dependent. By partially releasing the films from the substrate, the suppression of extrinsic contributions to the relative permittivity was alleviated. A greater increase in the irreversible Rayleigh coefficient was observed for thicker films (1.11 μm) compared to thinner films (0.56 μm – 0.27 μm). Therefore, substrate clamping contributes to scaling effects. After the films were partially declamped from the substrate, the irreversible contributions increased up to 23% in Nb-doped films and became more frequency-dependent (by up to 29%). Defects in a low-dielectric, Mn-doped seed layer also suppressed the extrinsic as well as the intrinsic contributions to the relative permittivity and contributed to the observed thickness dependence in the irreversible and reversible Rayleigh coefficients. The influence of the seed layer on dielectric properties was accounted for using a capacitor in series model. The suppressed frequency dependence in the clamped films was attributed to the pinning of irreversible domain walls active at lower frequencies. Both the seed layer and substrate clamping contributed to the pinning of irreversible domain walls. The thickness dependence of intrinsic and extrinsic contributions to the dielectric properties was elucidated from a combination of X-ray diffraction and Rayleigh analysis. In situ synchrotron X-ray diffraction was used to understand the influence of residual stress and substrate clamping on the domain state, ferroelastic domain reorientation and electric-field-induced strain in PZT 30/70 thin films. A thickness-dependent in-plane tensile stress developed in clamped, blanket PZT films during processing which dictates the domain structure even after poling. Defects and thermal stresses contribute to the greater in-plane tensile stress in the thinner films, resulting in this thickness dependence. However, after the films were partially declamped from the substrate and annealed, the residual stress was alleviated. As a result, the thickness dependence of the volume fraction of c-domains largely disappeared, and the out-of-plane d-spacings for both a and c-domains increased in the thinnest film. By poling the films, irreversible changes in the domain state and domain structure were induced as a result of 90° domain reorientation, domain coarsening, and lattice strain. Upon poling, thicker films experienced a greater coarsening of c-domains which resulted in reduced domain wall densities and a larger change in the reversible Rayleigh coefficient. The volume fraction of c-domains was used to calculate the intrinsic relative permittivity; the reversible Rayleigh coefficient was then used to separate the intrinsic and the reversible extrinsic contributions. The reversible extrinsic response contributed to more than 70% of the overall relative permittivity and was thickness dependent even after poling and upon release. Some PiezoMEMS devices, such as cantilevers and fixed-fixed beams, operate in a partially released state. For these devices, the thickness of the passive layer is known to tune the rigidity, deflection and resonance frequency for tailored device performance. However, as the rigidity of the device increases, at some point the ferroelastic response of the active layer will be suppressed. Therefore, the influence of passive layer thickness on the performance of the active layer is of interest. The deflection of cantilevers and fixed-fixed beams with a tetragonal {001} PZT 30/70 active layer and a SiO2 elastic layer in an IrO2/PZT/PbTiO3/Pt/TiO2/SiO2 device stack was characterized experimentally and by finite element modeling. Differences in the magnitude of the tip displacement were attributed to the variances in rigidity of the device stack associated with different SiO2 thicknesses. It was found that bending of the cantilevers was dictated by the competing integrated stresses of the IrO2 top electrode and the SiO2 elastic layer. Upon release, the PZT films showed increased reversible Rayleigh coefficients but decreased irreversible Rayleigh coefficients, regardless of SiO2 thickness (2.035 and 0.76 µm), for both cantilever and fixed-fixed beam geometries. These data suggest that upon release, at least some of the domain walls transition from irreversible to reversible motion. For cantilevers and fixed-fixed beams with the thinner SiO2 layers, the electroded PZT region deflected downward, placing PZT under further in-plane tension. This was correlated with a further decrease in the irreversible and reversible domain wall motion contributions to the relative permittivity.