Paul P. Donohue

Sputtered lead scandium tantalate thin films: crystallization behaviour during post-deposition annealing

Lead scandium titanate (PST) thin films were deposited by RF dual magnetron sputtering and then annealed either by vacuum furnace or combined rapid thermal annealing (RTA) and furnace anneal. The film structure was investigated by x-ray diffraction, scanning electron microscopy and transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy techniques. Lead loss was more severe using furnace annealing than the combined RTA and furnace anneal. The annealed films were characterized by the presence of voids and exhibited relaxor ferroelectric characteristics. PST perovskite crystal grains were found to co-exist with pyrochlore matrix in the furnace-only annealed films, whilst in RTA annealed films no apparent pyrochlore morphology was observed in the TEM image. Lead was found to diffuse through the bottom electrode Pt layer during the annealing. Films treated by combining RTA and furnace annealing have shown pyroelectric coefficients under field of up to 500 µC m-2 K-1, a dielectric loss of below 0.007 and a merit figure for thermal detection of 2.5×10-5 Pa-1/2.

Pulse-Extended Excimer Laser Crystallisation of Pb(Zr,Ti)O 3 Thin Films for Integration on Low Thermal Budget Substrates.

Excimer laser annealing has been used to convert low temperature (non-ferroelectric) deposited lead zirconate titanate (PZT) to the perovskite phase without significantly heating underlying layers. A pulse-extension technique has been used to lengthen the laser pulse duration from 25 ns to 374 ns, lowering the surface temperature and improving the heat distribution in the PZT, as compared to the non-extended case, but still not significantly heating the substrate. Initial experiments are reported which have shown the technique to be capable of crystallising over half a 500 nm thick PZT film to perovskite although a melting effect limited the converted thickness. The thickness crystallised is however of the order of that used in FeRAM devices and modelled temperature profiles suggest that the technique provides a tractable solution for high temperature processing of ferroelectric thin films of thickness 200-300 nm on low thermal budget substrates.

On the phase transformation kinetics in lead scandium tantalate thin films

The phase transformations from amorphous to pyrochlore to perovskite in lead scandium tantalate (PST) thin films during a rapid thermal annealing (RTA) process have been studied. Volume fractions for pyrochlore and perovskite were obtained from their respective x-ray diffraction intensities. Two models assuming the starting phase being either pure amorphous or pyrochlore were analysed in detail. Equations have been derived and numerical calculations have been used to simulate the volume fractions for each phase as functions of annealing time. Transformation parameters k and n were obtained by comparing the simulated to the experimental volume fractions using a least-squares curve fitting technique. Transmission electron microscopy, SEM and energy dispersive x-ray spectroscopy were employed to study the lead loss and other factors affecting phase transformations. It is concluded that the starting materials in the PST films deposited at 300°C were mixture of amorphous and pyrochlore and the phase transformations upon RTA are diffusion limited.

CVD of oxide materials for thermal imaging – the role of precursor chemistry

Thermal detectors exploiting ferroelectric lead zirconate titanate (PZT) or the metal insulator phase change material vanadium oxide are used widely to detect infra red radiation. Higher performance systems need to exploit improved materials as the detector element. Such materials are the ferroelectric perovskite, lead scandium tantalate (PST) and the metal insulator phase change material lanthanum barium manganite (LBMO). This paper presents work on the selection of precursors for liquid injection CVD of PZT and the high performance detector materials PST and LBMO. Control of the chemistry for the CVD process is an important step in the realisation of the large area, high quality deposition. In order to achieve this it is necessary to tailor the precursor properties so that vaporisation is efficient and decomposition leads to the desired material composition. It demonstrates how through careful precursor selection it is possible to match precursors in order to deposit high quality materials. It is shown that CVD offers a competitive route for the large area uncooled infrared devices.

Kinetics of Phase Transformations in Lead Scandium Tantalate Thin Films

The phase transformations from amorphous to pyrochlore to perovskite in lead scandium tantalate (PST) thin films during rapid thermal annealing process (RTA) have been studied. Volume fractions for pyrochlore and perovskite were obtained from their respective X-ray diffraction intensities. Two models assuming the starting phase being pure amorphous or pyrochlore were analysed in detail. Equations have been derived and numerical calculation used to simulate the volume fractions for each phase as functions of annealing time. Transformation parameters k and n were obtained by comparing experiment with simulation using a least-squares curve fitting technique. > *Originally presented at 10th European meeting on Ferroelectricity, Cambridge, UK, August 3–8, 2003.

A Model of Phase Transition Kinetics in Lead Scandium Tantalate Thin Films

The phase transformations from amorphous to pyrochlore to perovskite in lead scandium tantalate (PST) thin films during rapid thermal annealing process (RTA) have been studied by X-ray diffraction (XRD) techniques. The growth and decay of the pyrochlore phase, the growth of the perosvkite PST were analysed using the Avrami model. Integral equations have been derived and numerical calculation been used to simulate the volume fractions for pyrochlore and perovskite as functions of annealing time. Transformation parameters k and n were obtained by comparing the simulated to the experimental intensities using a least-squares curve fitting technique. Activation energies for perovskite formation were calculated. The results indicate that the phase transitions are diffusion-limited. TEM, SEM, and EDS were employed to study the lead loss and other factors affecting phase transitions.