M. A. Todd

A study of reactively sputtered lead scandium tantalate films

Abstract Sputtered thin films of perovskite-structured ferroelectric Pb(Sc1/2, Ta11/2)O3, (PST), produced using near stoichiometric ratios of the constituent metals require a post-deposition anneal temperature of 900[ddot]C to promote crystallization of the films in the perovskite phase. In this paper, we report that modification of the metal targets, resulting in the inclusion of a large excess of Pb in the deposited films, forms perovskite structured PST at a much lower substrate temperature of 600[ddot]C. The results of the permittivity and dielectric measurements and transmission electron microscope studies carried out on a PST film deposited on MgO-sapphire substrate are presented in this paper.

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.

Sputtered lead scandium tantalate thin films: a microstructural study

Lead scandium tantalate (PST) thin films have been deposited on a platinized silicon substrate with and without a buffer layer of MgO at the temperature of 525°C. It was found that PST films on the substrate without a buffer layer were strongly (1 1 1) oriented perovskite, whilst films on the substrate with a buffer layer showed the presence of second-phase pyrochlore, and the films were (1 1 1) and (1 1 0) oriented. These structural differences were believed to result from the structural differences between the platinum layers immediately below the respective PST layers. The “lines” which divide PST into “network” of islands were found to be no more than wider grain boundaries, rather than “cracks” as believed previously. Micro-beam diffraction and energy dispersive X-ray analysis showed that grain boundaries were tantalum-rich and lead-deficient compared to perovskite grain centres. Electrical properties, such as relative permittivity and dielectric loss, for the films were also measured.

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.