Nanostructured ferroelectric materials are of growing interest in international research because of their impact on the future development of non-volatile ferroelectric random access memory (FeRAM) [1] as well as on the potential utilization of scanning-probe-based ferroelectric mass-storage media.[2] For both memory concepts, the scaling of the ferroelectric properties needs to be understood in order to elucidate possible limitations to memory densities. Furthermore, the question arises as to how ferroelectric nanostructures can be generated with a predefined registration as a link to the process integration of devices. In addition to several theoretical and experimental approaches to calculate a size effect on phase transitions in fine ferroelectric particles, [3±5] various experimental studies have been carried out in order to investigate scaling effects of ferroelectric nanograins on different substrates.[6±10] A major drawback that the commonly used top±down approaches encounter is the difficulty in achieving ferroelectric structures with lateral sizes below 50 nm. In addition, erosive top±down approaches like focused ion beam (FIB) [6] technology or dryetching methods [8] inevitably induce severe lattice damage to the sidewalls of the fabricated capacitors. Hence, it becomes hard to distinguish between real intrinsic ferroelectric properties and the impact of processing artifacts. Alternative non-invasive approaches like nanosphere lithography [10] and electron-beam direct writing (EBDW) [7] have almost no influence on the intrinsic material properties. Still, the minimum structure sizes achieved are in the range of 160 nm for nanosphere lithography and about 100 nm for EBDW, as the gel film used in the latter method is not optimized for obtaining the smallest possible features by electron-beam exposure, in contrast with dedicated electron-beam resists. To overcome these drawbacks, our group introduced a ªbottom±upº method, growing separated PbTiO 3 and Pb(Zr x Ti 1±x )O 3 (PZT) grains on Si/SiO 2 /TiO 2 /Pt(111) substrates by a chemical solution deposition (CSD) technique. The as-deposited thin film broke up into single-crystalline islands after the crystallization process due to a very high precursor dilution, [11,12] which resulted in the microstructural instability of the thin films.[13] Within the scope of these studies, PbTiO 3 grains of sizes below 15 nm were fabricated, emerging at grain boundaries of the underlying platinum electrode. It was found that for grains smaller than approx. 20 nm, no piezoresponse could be observed by piezoresponse force microscopy (PFM) measurements at room temperature, whereas larger grains clearly showed ferroelectric behavior. [14] This vanishing of detectable piezoresponse for small grains is discussed elsewhere.[15]One major drawback still existed within these studies: the crystalline islands merely featured a low short-range ordering. In order to achieve a high registration, a prepatterned field becomes necessary. [16] In this communication, we report on the re...
Ferroelectrics hold promise for high-density non-volatile data storage device use. Their eventual performance will strongly depend on the available displacement current, which primarily scales with the area but might to some extent be enhanced by substrate-induced homogeneous strain while the interface at the same time controls the coercive field. As the lateral dimensions persistently decrease, the only way to keep track of the real figures of merit with realistic electrodes is to use a macroscopic configuration instead of scanning probe approaches with undefined interfaces. We report on a novel approach to integrating arbitrarily patterned, highly registered ferroelectric nanoislands fabricated by a template controlled chemical solution deposition approach into a matrix of a low-k dielectric spin-on glass. These structures with a narrow lateral size distribution below 100 nm are subsequently polished in a chemical-mechanical polishing step to expose their very tops, whose piezoelectrically active area depends on the polishing time. At this stage our findings indicate a full piezoelectric functionality of the locally exposed nanoislands. The structures are ready for macroscopic top electrodes to average the displacement current over hundreds of almost identical structures, to provide a nanoscale scaling behaviour of the individual ferroelectric capacitors.
A bottom‐up fabrication route for PbTiO3 nanograins with dimensions of 30 nm, based on predefined TiO2 nanostructures, is presented on p. 2346 by Karthäuser, Rathgeber, and co‐workers. The patterning of the TiO2 seeds was performed by means of a self‐organized micellar monofilm used as an etching template. The dimensions of the micelle template are transferred across all process steps to the final PbTiO3 nanograins. A bottom‐up fabrication route for PbTiO3 nanograins grown on predefined TiO2 nanostructures used as seeds is presented. The structuring of the TiO2 seeds is performed using a self‐organized template constructed from a gold‐loaded micellar monofilm. With this fabrication process, TiO2 seeds and PbTiO3 grains with diameters of 12 and 30 nm, respectively, are prepared without the need for electron‐beam lithography. The dimensions of the structure imposed by the micellar template are transferred through all the processing steps to the final PbTiO3 grains. Furthermore, it is shown that the intermicelle distance and the degree of order in the dried monofilm is mainly determined by the preparation conditions, such as the pulling velocity in the dipping process and the strength of the surface–micelle interaction, and not necessarily by the architectural properties (block length and ratio) of the diblock copolymers that build the micelles. The intermicelle spacing in the dried film is much smaller than the micelle dimensions in solution, and approaches the dimensions of a fully collapsed micelle when the dipping process is performed slowly enough.
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