damage-free patterning of ferroelectric lead zirconate titanate thin films for microelectromechanical systems via contact printing

Open Access
Author:
Welsh, Aaron Joel
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 26, 2016
Committee Members:
  • Susan E Trolier Mckinstry, Dissertation Advisor
  • Clive A Randall, Committee Member
  • Michael T Lanagan, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • soft lithography
  • thin films
  • dry etching
  • piezoresponse force microscopy
  • electroceramics
  • piezoelectrics
  • ferroelectrics
Abstract:
This thesis describes the utilization and optimization of the soft lithographic technique, microcontact printing, to additively pattern ferroelectric lead zirconate titanate (PZT) thin films for application in microelectromechanical systems (MEMS). For this purpose, the solution wetting, pattern transfer, printing dynamics, stamp/substrate configurations, and processing damages were optimized for incorporation of PZT thin films into a bio-mass sensor application. This patterning technique transfers liquid ceramic precursors onto a device stack in a desired configuration either through pattern definition in the stamp, substrate or both surfaces. It was determined that for ideal transfer of the pattern from the stamp to the substrate surface, wetting between the solution and the printing surface is paramount. To this end, polyurethane-based stamp surfaces were shown to be wet uniformly by polar solutions. Patterned stamp surfaces revealed that printing from raised features onto flat substrates could be accomplished with a minimum feature size of 5 µm. Films patterned by printing as a function of thickness (0.1 to 1 µm) showed analogous functional properties to continuous films that were not patterned. Specifically, 1 µm thick PZT printed features had a relative permittivity of 1050 ± 10 and a loss tangent of 2.0 ± 0.4 % at 10 kHz; remanent polarization was 30 ± 0.4 µC/cm2 and the coercive field was 45 ± 1 kV/cm; and a piezoelectric coefficient e31,f of -7 ± 0.4 C/m2. No pinching in the minor hysteresis loops or splitting of the first order reversal curve (FORC) distributions was observed. Non-uniform distribution of the solution over the printed area becomes more problematic as feature size is decreased. This resulted in solutions printed from 5 µm wide raised features exhibiting a parabolic shape with sidewall angles of ~ 1 degree. As an alternative, printing solutions from recesses in the stamp surface resulted in more uniform solution thickness transfer across the entire feature widths, with increased sidewall angles of ~ 35 degrees. This was at the cost of degrading line edge definition from ~ 200 nm to ~ 500 nm. The loss of line edge definition was mitigated through the combined use of printing from stamp recesses onto raised substrate features. This allowed for printing of PZT features down to 1 µm wide. Solutions could also be transferred onto both fixed and free standing cantilever structures patterned into a substrate surface. Optimization of the stamp removal from the substrate was crucial in increasing sidewall angles of printed PZT films. It was determined that solutions gel once deposited onto the stamp before printing. As a result, printed films could not redistribute easily after transfer had occurred. Through a combination of varying peeling directions and peeling rates, it was possible to deposit thin film PZT on a pre patterned feature ~ 1 µm wide with sidewall angles > 80 degrees. These printing techniques were utilized in printing a 250 nm thick 30/70 PZT onto pre-patterned cantilever structures for use in a bio-functionalized, mass sensing resonating structure in collaboration with a bio-nanoelectromechincal sensing research group from the University of Toulouse, France. The features ranged in lateral size from 30 down to 1 µm. The printed devices exhibited a relative permittivity of 500 ± 10 and a loss tangent of 0.9 ± 0.1 %. The hysteresis loops were well formed, without pinching of the loops, and exhibited remanent polarizations of 24 ± 0.5 µC/cm2, and coercive fields of 110 ± 1 kV/cm. Dry etched features of the same size and thickness displayed a relative permittivity of 445 ± 8 and a loss tangent of 0.9 ± 0.1 %. The hysteresis loops exhibited pinched loops with remanent polarizations of 24 ± 0.7 µC/cm2, and coercive fields of 112 ± 2 kV/cm. Upon cycling, the dry etched films developed a 20 kV/cm imprint with reduced remanent polarizations to 20.5 ± 0.5 µC/cm2. An understanding of the influence of patterning on the material properties is essential to predicting and controlling the behavior of polycrystalline films for MEMS applications. The influence of pinning centers on domain wall motion, particularly near feature sidewalls, in patterned features was explored in reactive ion etched (RIE) and microcontact printed films with the same thickness (i.e. 1 µm) and lateral feature size (i.e. 5 and 10 µm). This was accomplished by measuring global dielectric nonlinearity through Rayleigh and minor hysteresis measurements. For comparative purposes, local quantitative mapping of the piezoelectric nonlinearity was undertaken through the use of band excitation piezo-response force microscopy (BE-PFM). The printed and etched films exhibited differing microstructures which precluded quantitative direct comparisons. However, qualitative trends were identified. The dielectric aging rate of all Rayleigh parameters for the etched films increased with increases in perimeter length. In particular, the aging of the dielectric irreversible/reversible Rayleigh ratio (α/εinit) increased from -7 ± 0.6 %/decade to -11.6 ± 0.7 %/decade (600 to 5 µm in width, respectively). In contrast, the printed films showed very slight aging rates. BE-PFM measurements revealed that defects from the etching process introduced large concentrations of pinning centers near the patterned sidewalls, resulting in reductions in the piezoelectric irreversible/reversible Rayleigh ratio (α/d33,init) as far as 750 nm from the sidewall. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) showed that variations in stoichiometry of crystal quality were not the predominant factor controlling the decreased domain wall mobility near sidewalls. In contrast to the etched films, printed films showed an increase in α/d33,init as the sidewall was approached due to mechanical declamping from the substrate.