PZT piezoelectric films on glass for Gen-X imaging

The proposed adaptive optics system for the Gen-X telescope uses piezoelectric lead zirconate titanate (PZT) films deposited on flexible glass substrates. The low softening transition of the glass substrates imposes several processing challenges that require the development of new approaches to deposit high quality PZT thin films. Synthesis and optimization of chemical solution deposited 1 μm thick films of PbZr_0.52Ti_0.48O ₃ on small area (1 in²) and large area (16 in²) Pt/Ti/glass substrates has been performed. In order to avoid warping of the glass at temperatures typically used to crystallize PZT films (∼700°C), a lower temperature, two-step crystallization process was employed. An ∼80 nm thick seed layer of PbZr_0.30Ti_0.70O₃ was deposited to promote the growth of the perovskite phase. After the deposition of the seed layer, the films were annealed in a rapid thermal annealing (RTA) furnace at 550°C for 3 minutes to nucleate the perovskite phase. This was followed by isothermal annealing at 550°C for 1 hour to complete crystallization. For the subsequent PbZr_0.52Ti_0.48O ₃ layers, the same RTA protocol was performed, with the isothermal crystallization implemented following the deposition of three PbZr _0.52Ti_0.48O₃ spin-coated layers. Over the frequency range of 1 kHz to 100 kHz, films exhibit relative permittivity values near 800 with loss tangents below 0.07. Hysteresis loops show low levels of imprint with coercive fields of 40-50 kV/cm in the forward direction and 50-70 kV/cm in the reverse direction. The remanent polarization varied from 25-35 μC/cm² and e_31,f values were approximately -5.0 C/m². In scaling up the growth procedure to large area films, where warping becomes more pronounced due to the increased size of the substrate, the pyrolysis and crystallization conditions were performed in a box furnace to improve the temperature uniformity. By depositing films on both sides of the glass substrate, the tensile stresses are balanced, providing a sufficiently flat surface to continue PZT deposition. The properties of the large area film are comparable to those obtained on small substrates. While sol-gel processing is a viable approach to the deposition of high quality PZT thin films on glass substrates, preliminary results using RF magnetron sputter deposition demonstrate comparable properties with a significantly simpler process that offers a superior route for large scale production.