Local Origin of Macroscopic Properties and Patterning in PbZr(1-x)Ti(x)O(3) Films

Open Access
Author:
Bintachitt, Patamas
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 28, 2009
Committee Members:
  • Prof Susan Trolier Mc Kinstry, Dissertation Advisor
  • Susan E Trolier Mckinstry, Committee Chair
  • Thomas R Shrout, Committee Member
  • Christopher Muhlstein, Committee Member
  • Thomas Nelson Jackson, Committee Member
Keywords:
  • Ferroelectrics
  • Piezoelectric
  • Thin Films
  • Piezoelectric Force Microscopy (PFM)
  • MEMS
  • Lead Zirconate Titanate PZT
Abstract:
The lead zirconate titanate (PZT) films that are most widely utilized in microelectromechanical systems (MEMS) are polycrystalline, with random orientation of the grains. The resulting transverse piezoelectric coefficient, e31,f, is approximately -6 to -7 C/m2 for PbZr0.52Ti0.48O3. This thesis describes the preparation of high quality PZT films, the nonlinearity in their dielectric and piezoelectric responses, as well as their patterning by reactive ion etching. Two types of PZT films were prepared: PZT films with mixed {001} and {111} orientation and those with strong levels of {001} orientation. Dense polycrystalline PZT films on (111)Pt/Ti/SiO2/(001)Si substrates (with thicknesses from 0.28 to 4.40 µm) were obtained by optimizing the chemical solution deposition process. Higher piezoelectric coefficients (about -12 C/m2) can be obtained if significant {001} orientation is achieved. Thus, one goal of the research was to obtain {001} oriented PZT films on Pt-coated Si substrates. In this work, PbTiO3 buffer layers were chosen due to good lattice matching with PZT films, and the strong propensity for development of {001} orientation. The pyrolysis, crystallization steps, and lead excess addition of PbTiO3 buffer layers deposition were investigated. Using a thin PbTiO3 buffer layer and controlled pyrolysis conditions allowed {100} oriented PZT films to be prepared. The PbTiO3 buffer layer can be used over a full wafer to provide orientation. Higher piezoelectric coefficients, e31,f of -14 and -10 C/m2 were achieved for {001} PZT thin films of 1.0 µm and 0.24 µm thickness, respectively using appropriate poling conditions. The local and global domain wall contributions were studied by piezoelectric nonlinearity and dielectric nonlinearity in both {001}-textured PZT films and PZT films with mixed {001} and {111} orientation. Band Excitation Piezoelectric Force Microscopy (BE-PFM) was shown to enable quantitative measurements of the local piezoelectric nonlinearity. It was found that films over the thickness range probed showed Rayleigh-like behavior. 4 μm thick films were nearly uniform in their Rayleigh coefficient, suggesting that any heterogeneities in the response developed at lateral length scales below the resolution of the PFM measurement. In contrast, thinner films showed significantly more patchiness in their response, so that fluctuations in behavior developed at a lateral length scale on the order of 0.6 to 2.5 micron. These variations did not appear to be correlated directly with the surface topology. The ability to detect lateral inhomogeneity in the nonlinearity of thinner films is likely to be a function of the volume probed in the measurement. Finally, it is hypothesized that the same population of domain wall contributes to the local and global nonlinearity. Nanoindentation measurements were conducted in an attempt to distinguish 180° and non-180° domain wall motion in these films. Non-180° domain walls can be moved by both electrical and mechanical fields. In contrast, 180° domain walls can be moved by electric fields, but not by uniform stresses. In PZT films with mixed {111} and {001} orientation, some ferroelastic wall motion took place during loading, at stress levels on the order of GPa. The reduced elastic modulus is much higher on the unloading curve. Thus, on unloading, it is believed that there is no contribution from mechanical softening associated with ferroelasticity. The global polarization switching in polycrystalline, {001}-textured films on Si, and {001}-films on SrRuO3 / SrTiO3 was studied through first order reversal curves (FORC) in order to assess the Preisach distribution governing the switching behavior. Then, the local polarization switching in polycrystalline PZT films was studied by using Switching Spectroscopy Piezoresponse Force Microscopy (SS-PFM). Acquisition of multiple hysteresis loops allows polarization switching parameters including nucleation biases, coercive biases, and the amount of switchable response to be mapped in real space. SS-PFM was studied on both bare PZT film surfaces and in capacitor structures (metal/ PZT film/ metal). The capacitor structure shows the evolution of correlated switching of 102 – 103 grain clusters with well-defined imprint and nucleation biases. A transition from a regime where the domain wall motion is over a short range to the formation of clusters to complete switching is observed. The switchable polarization as a function of bias window allows the voltage dependence and spatial distribution of regions with reversible and irreversible wall motions to be mapped. The final chapter of experimental work describes the patterning of PZT films for MEMS. The ability to dry etch large depths of ferroelectric materials such as lead zirconate titanate is important in both microelectromechanical systems and in high frequency medical ultrasound transducers. Dense Pb(Zr0.52Ti0.48)O3 films (≥1 μm) were used to study the etching characteristics. The films were etched using an Inductively Coupled Plasma Reactive Ion Etcher (ICP-RIE) with a mixture of Ar and SF6 gas. The variation of the etch rate with gas flow rate, source power, substrate holder power, and operation pressure and the uniformity of etching were investigated. The maximum etch rate achieved was 0.31 µm/min. The electrical properties of PZT films directly exposed to the plasma after etching (εr = 1300, tanδ = 3.29%, Pr = 29.8 µC/cm2, Ec = 39 kV/cm) are similar to the electrical properties of PZT before etching (εr = 1400, tanδ = 4.35%, Pr = 27.1 µC/cm2, Ec = 32 kV/cm). Small changes in the minor hysteresis loops were found, suggesting that there may be a small change in the internal field induced by the patterning process. In addition, sidewall angles of > 80° have been achieved using a non-contact method (laser writer) for the lithography step.