|Ph.D Student||Etin Aleksej|
|Subject||Chemical Solution Deposition of Nano-Structured PZT Films|
|Department||Department of Chemical Engineering||Supervisors||Professor Gideon Grader|
|Dr. Gennady Shter|
|Full Thesis text|
PbZrxTi1-xO3 (PZT) films are important due to their piezoelectric, pyroelectric and ferroelectric properties that can be used in Micro-Electro-Mechanical-Systems (MEMS) and non-volatile memory (FeRAM). Chemical Solution Deposition (CSD), i.e. spin-coating of a liquid precursor, emerges as the most promising method for industrial production of high quality PZT films since it allows relatively easy stoichiometry control along with high throughput. Although the CSD method has a great potential, it has not yet found wide applications in the industry. A fundamental understanding of the processes employed in the CSD is essential for its effective implementation, and was a goal of this research.
We have found that the main current drawbacks of the CSD-based films are the non-uniformity of composition and coating thickness on the nano- and macro-scale, respectively. The nano-scale effects include either Pb-deficient (Pb/(Ti+Zr)<1) or Pb-rich (Pb/(Ti+Zr)>1) layer on the surface of films, while the depth profile exhibits gradient of Zr/Ti stoichiometry throughout the film thickness. The deviations from ideal Pb stoichiometry occur due to Pb loss during processing that can be circumvented by carefully controlled Pb excess in the precursor. At the same time the Zr/Ti depth distribution depends on the thermal decomposition of Zr and Ti precursors and uniformity was obtained when their decomposition was simultaneous. The PZT films with uniform stoichiometry on the nano-scale showed excellent ferroelectric properties with remnant polarization of ~40 mC/cm2, coercive field of ~50 kV/cm and fatigue only after 107 cycles, among the best results reported for PZT films on Pt electrodes.
While the nano-scale stoichiometry defines the films ferroelectric properties, the macro-defects are intolerable since they decrease the yield of devices and process reliability. Study of the surface tension and composition evolution during spin-coating showed that evaporation of the volatile by-products causes significant increase of the surface tension. The increase of the surface tension is responsible for the formation of macro-defects and was avoided by the use of the low surface tension and high boiling point co-solvents such as octanol.
The last part of the work shows novel evidence of gelation in the intermediate, “green” films. The images inside the cracks of these films reveal for the first time that the crack sides are connected by necks ~100 nm long and ~10 nm thick as consistent with a gel structure.
Finally the developed technology for deposition of high quality uniform PZT films on large substrates was transferred to RAFAEL. Piezoelectric films prepared according to the developed technology were used in micro-gyro device.