Pzt Thin Films for Piezoelectric Mems Mechanical Energy Harvesting

Open Access
Author:
Yeager, Charles B
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 06, 2015
Committee Members:
  • Susan E Trolier Mckinstry, Dissertation Advisor
  • Clive A Randall, Committee Member
  • Qiming Zhang, Committee Member
  • Thomas Nelson Jackson, Special Member
Keywords:
  • ferroelectric domain
  • e31f
  • energy harvesting figure of merit
Abstract:
This thesis describes the optimization of piezoelectric Pb(ZrxTi1-x)O3 (PZT) thin films for energy generation by mechanical energy harvesting, and self-powered micro-electro-mechanical systems (MEMS). For this purpose, optimization of the material was studied, as was the incorporation of piezoelectric films into low frequency mechanical harvesters. A systematic analysis of the energy harvesting figure of merit was made. As a figure of merit (e31,f)2/εr (transverse piezoelectric coefficient squared over relative permittivity) was utilized. PZT films of several tetragonal compositions were grown on CaF2, MgO, SrTiO3, and Si substrates, thereby separating the dependence of composition on domain orientation. To minimize artifacts associated with composition gradients, and to extend the temperature growth window, PZT films were grown by metal organic chemical vapor deposition (MOCVD). Using this method, epitaxial {001} films achieved c-domain textures above 90% on single crystal MgO and CaF2 substrates. This could be tailored via the thermal stresses established by the differences in thermal expansion coefficients of the film and the substrate. The <001> single-domain e31,f for PZT thin films was determined to exceed −12 C/m2 in the tetragonal phase field for x ≥ 0.19, nearly twice the phenomenologically modeled value. The utilization of c-domain PZT films is motivated by a figure of merit above 0.8 C2/m4 for (001) PZT thin films. Increases to the FoM via doping and hot poling were also quantified; a 1% Mn doping reduced εr by 20% without decreasing the piezoelectric coefficient. Hot poling a device for one hour above 120 °C also resulted in a 20% reduction in εr; furthermore, 1% Mn doping reduced εr by another 12% upon hot poling. Two methods for fabricating thin film mechanical energy harvesting devices were investigated. It was found that phosphoric acid solutions could be used to pattern MgO crystals, but this was typically accompanied by damage to the PZT film. An energy harvester was fabricated by etching the MgO substrate down to 10-20 μm under a circular diaphragm device; this structure had a natural frequency of 2.7 kHz and was estimated to provide a maximum RMS power of 8.8 μW/cm2∙g2. Due to the lack of selectivity in the patterning, MgO was not as versatile as silicon substrates, which can be etched rapidly by wet and dry methods. To successfully release a PZT film onto a polymer passive elastic layer, dry (gas) etch methods were preferable. This protected the interfacial bonding between PZT films and Parylene. A 2 cm2 thin film membrane (15 μm Parylene/ 3 μm Cyclotene 4022/ 0.1 μm Pt-Ti/ 1.4 μm PZT (52/48)/ 0.14 μm Pt-Ti/ 1 μm SiO2) was released from a silicon substrate and operated with a 5 Hz natural frequency, the lowest reported for a thin film energy harvester operating in resonant excitation. Though problems existed with buckling of the beam due to tension in the Cyclotene 4022 (a benzocyclobutene, BCB, resin) from curing on a silicon substrate, the cantilevered device was calculated to output up to RMS 0.53 μW/cm2 when swept through an arc >30°. Silicon substrates facilitated scaling in size and quantity of devices compared to MgO substrates, which motivated an investigation into the reduction of 90° domain walls for thin films released from substrate clamping conditions. Circular test structures were designed to produce systematic changes in the clamping condition of {001} PZT thin films. The stiffness of the substrate interface was modified either by using a PZT buffer layer on the substrate or by removing the substrate completely. Films allowed to stress relax upon release, via curling, had reduced domain wall restoring force compared to fully clamped structures, leading to a 72% increase in irreversible domain wall contributions for free-standing 300 μm features. The irreversible dielectric Raleigh coefficient, α, for a 1.64 μm {001} PZT film measured at 20 Hz increased from 40 cm/kV to 71 cm/kV in this way. Griggio et al., Phys. Rev. Let. 108, no. 15 (2012) 157604 reported α of 148 cm/kV at 100 Hz for broken sections of 70 μm diaphragms. To understand the relationship between α reported in those experiments and the results of this thesis, the size dependence of α was investigated by partitioning 300 μm diaphragms into wedges. Both α and the frequency dispersion of α increased as the membrane was sectioned. This was attributed to a decrease in the elastic restoring force for domain walls. Interface (local) stresses were found to have a smaller impact on domain wall mobility, even after the domain structure was annealed above the Curie temperature post release.