Reliability and Aging in Patterned Pb(Zr0.52Ti0.48)O3 Films

Open Access
Author:
Yang, Jung In
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 31, 2016
Committee Members:
  • Susan E Trolier-Mckinstry, Dissertation Advisor
  • Susan E Trolier-Mckinstry, Committee Chair
  • Clive A Randall, Committee Member
  • Venkatraman Gopalan, Committee Member
  • Srinivas A Tadigadapa, Outside Member
Keywords:
  • Ferroelectric
  • Reliability
  • Aging
  • HALT
  • Conduction mechanism
  • Patterned PZT thin film
Abstract:
This research was targeted at understanding how and why aging and reliability are influenced by lateral feature size in PZT thin films patterned by ion beam milling and reactive ion etching. Lead zirconate titanate, PZT thin films (with Zr/Ti ratios of 52/48) were grown on a PbTiO3 seed layer and patterned into feature sizes of 100 μm, 50 μm and 10 μm using argon ion beam milling. It was found that as the pattern dimensions decreased, the remanent polarization increased, presumably due to the fact that the ferroelectric polarization near the feature perimeter is not as severely clamped to the substrate. It is believed that clamping at the film/substrate interface produces deeper wells that prevent some fraction of the spontaneous polarization from switching at high field. The dielectric nonlinearity in the serpentine patterned PZT thin films was described using the Rayleigh law at modest ac electric fields. Both Rayleigh parameters (εinit and α) increased with decreasing serpentine line width after recovery annealing. It was observed that εinit of 500 mm square patterns was 1510 ± 13; with decreasing serpentine line width, εinit rose from 1520 ±10 for the 100 μm serpentine to 1568 ± 23 for the 10 μm serpentine. The irreversible parameter, α, for the square patterns was 39.4 ± 3.2 cm/kV and it increased to 44.1 ± 3.2 cm/kV as the lateral dimension is reduced. However, etched patterns also aged more rapidly. Based on this, it was concluded that sidewall damage produces shallow wells that lower the Rayleigh constants of aged samples at small fields. These observations are consistent with a model in which shallow wells can be overcome by the large fields used to measure the remanent polarization and the large unipolar electric fields typically used to drive thin film piezoelectric actuators. 600nm thick PZT (52/48) thin films were also patterned into squares and antenna-shapes with feature sizes of 10 μm, 8 μm, 5 μm, and 2 μm using a reactive ion etching process. The effect of feature size on (1) highly accelerated lifetime tests (HALT), (2) the current-voltage (I-V) characteristics, and (3) the property stability as function of time using Rayleigh measurements was then investigated. HALT measurements were made at temperatures ranging from 120 oC to 180 oC and electric fields ranging from 100 kV/cm to 225 kV/cm. The characteristic time to failure t50 increased with decreasing feature size. The activation energy for all feature sizes were 0.82eV ~ 1.1eV; the range of voltage acceleration factors for all feature sizes was 3.4 ~ 5.2. It is hypothesized that the sidewall and the bulk of the film both contribute to the observed feature size dependence of the median time to failure and activation energy in patterned PZT thin film. HfO2 was deposited using ALD to minimize sidewall damage such as dangling bonds on the etched surfaces. After ALD coating, the lifetime of all features sizes increases because the sidewall damage is partially recovered without significant changes in the activation energies. I-V data were measured at temperatures ranging from 50oC to 150oC to investigate possible conduction mechanism. The most reasonable fits were obtained using a Schottky emission model. To investigate the aging mechanisms for patterned PZT films, Rayleigh measurements were conducted as a function of time. Two methods were used to obtain preferred polarization states in the ferroelectric. First, patterned PZT films were exposed to a dc bias (poling); secondly, films were poled at elevated temperature to promote alignment of defect dipole complexes. Both poled and hot poled samples have loops that are shifted negatively along the electric field axis due to development of the internal field. Thermal poling at 150 oC reduced aging rates to ~ 0.6 %/decade for εinit and 1.1 %/decade for α. In contrast, the aging rate of unpoled samples were ~ 1.3 %/decade for εinit and ~1.8 %/decade for α value. Also, all feature sizes showed the same trend; α / εinit decreased rapidly over short time scales (on the order of 1-150 seconds) and then gradually decreased at longer time.