Variable Size and Shape Distribution of Ferroelectric Nanoislands by Chemical Mechanical Polishing (2006)

Ferroelectric PbTiO3 nanostructures with lateral dimensions typically below 100 nm are grown on platinized silicon substrates by a chemical solution deposition (CSD) technique. Chemical mechanical polishing allows control of the aspect ratio of the grains, since their initial size distribution is thermodynamically driven (see SEM images; above: as grown, below: polished). Piezoresponse force microscopy measurements on a 15-nm-thick PbTiO3 grain indicate that the polishing step has no detrimental effect on ferroelectric properties. Current studies on ferroelectrics predominantly focus on the potential integration of ferroelectric materials into next-generation high-density memory devices such as nonvolatile ferroelectric random-access memories (FeRAMs).1 New fabrication methods have emerged, which allow the production of large-area high-density ferroelectric structure arrays in an effective manner, in which the lateral dimensions of the single ferroelectric cells are reduced down to the nanoscale.2–3 Size effects play an important role in this regime and are therefore intensively studied both theoretically and experimentally.4–8 However, the size of the cell is not the only important issue: Various extrinsic parameters (such as the patterning method and the resulting defects, and the substrate interface and the resulting stress) as well as intrinsic features such as shape dependencies and the aspect ratio of the structures strongly influence their ferroelectric properties.9 Previously, we have presented a method to gain a better insight into the electrical properties of ferroelectric nanostructures by enabling a direct electrical characterization technique.10 Nanoscale lead titanate (PbTiO3) grains were embedded in a low-k dielectric matrix and contacted with collective top electrodes. In order to provide electrical contact to the grain tops the dielectric layer was chemically mechanically polished. In this Communication, we show that it is also feasible to polish PbTiO3 grains without an embedding matrix between the structures. The size distribution of self-assembled grains is usually strongly determined by the growth mechanism. For epitaxial systems, Dawber et al. observed a size distribution in good agreement with Shchukin–Bimberg theory.11 Chemical mechanical polishing (CMP) of self-assembled PbTiO3 grains offers the interesting opportunity to modify these grains in shape and aspect ratio, respectively. The presented technique provides information on the correlation between structure, shape, and ferroelectric properties and might provide feedback for the interpretation of atomic-force-based scanning techniques, such as piezoresponse force microscopy (PFM).12–14 Self-assembled ferroelectric PbTiO3 nanoislands below 100 nm in lateral extension were grown on 1 cm2 (111)-oriented platinized silicon substrates by a 2-butoxyethanol-based chemical solution deposition (CSD) technique. A detailed description of this process is given elsewhere.15, 16 The concentration of the coating solution determines the average grain size and was chosen to result in a maximum grain height of about 50 nm for all samples. A simple reduction of average grain size can be achieved by a higher precursor dilution, as performed by Roelofs et al.17 To vary shape and aspect ratio, an additional processing step has to be introduced. We applied a chemical mechanical polishing step. The as-deposited PbTiO3 grains were polished on a commercial PM4 tabletop polisher using Syton-SF1 as polishing slurry.18 The slurry was diluted with deionized water to a ratio of 1:5. A soft, chemically resistant ChemoMet19 polishing pad was used to polish the grains. All samples were polished for 10 to 30 s each. Grain heights were reduced significantly due to the polishing step and converged. This process was completely abrasive. Removal of entire grains due to the polishing shear forces was not detected. Figure 11 compares SEM images of an unpolished sample with a sample containing extremely thin PbTiO3 grains of only a few nanometers in height, which was polished for 30 s. SEM images: a) an unpolished sample; b) a sample that was polished for 30 s. Only a few nanometers of the original grain height remained after the polishing step. Across the polished samples the material removal rate was not absolutely uniform. The average grain height after polishing differed within a range of about 10 nm. Nevertheless, polishing times that exceeded 30 s resulted in complete abrasion of the grains down to the platinum bottom electrode. Figure 22 displays topography data from atomic force microscopy (AFM) measurements. The area fraction of grains below a certain threshold height is plotted for an unpolished sample and for a sample that was polished for 15 s. Area fraction of PbTiO3 grains below a certain threshold height. For the polished grains the maximum grain height was brought down to about 25 nm, conterminous with a removal of about a half of the initial grain heights. The increased gradient of the polished sample curve indicates the convergence of grain heights. The platinum background of about 20 % can be subtracted, leading to a total area fraction of PbTiO3 grains of 80 %. One has to keep in mind that the radius of the probe tip also contributes to the grain diameters in AFM measurements and therefore the actual PbTiO3 area fraction is lower that that which can be seen in Figure 11. In order to prove whether the polished grains are still ferroelectric, piezoresponse force microscopy measurements were carried out. Figure 33 displays the in-plane piezoresponse signal of a sample that was polished for 15 s and compares it to that of an unpolished sample. For comparability reasons, both measurements were performed with a voltage of 1 V at a frequency of 7 kHz applied to the same probe tip under equal lock-in settings. In-plane piezoresponse signal of an unpolished sample (above) compared to the signal of a sample that was polished for 15 s (below). The grains polished for 15 s show strong and well-defined piezoelectric activity in magnitude and phase. Domain structures are clearly visible. A degradation of piezoelectric activity due to the polishing step is not detectable at this stage. The PbTiO3 grains are not obviously chemically or mechanically deteriorated by the polishing process. To confirm ferroelectric functionality, we reversed the piezoelectric tensor of the polished grains (Figure 44). Here, we switched a selected grain of 15 nm in height bidirectionally by using a dc voltage (−5 V and 5 V respectively) that was applied between a Pt/Ir probe tip and a platinum bottom electrode between two scans. Polarization reversal of the highlighted, uniformly polarized grain is demonstrated in the in-plane phase diagram. Partial switching occurred as the switching voltage was only locally induced to the probe tip. Simultaneous switching in the out-of-plane phase diagram (not shown) indicates 180° switching here. Bidirectional switching of a PbTiO3 grain (highlighted) that was polished for 15 s. The fairly round grain is about 15 nm in height and 100 nm in diameter. The single images depict: a) topographic information, b) in-plane phase information, and c, d) in-plane phase information after applying dc voltages of −5 V and 5 V to the grain, respectively. In conclusion, we have demonstrated the controlled modification of the aspect ratio of ferroelectric nanograins by chemical mechanical polishing. For our configuration the chemical agents do not significantly affect the ferroelectric performance, and fine tuning of the polishing parameters allows manipulation of samples close to the critical thickness of ferroelectricity. With this method, it is now possible to nanomanipulate the shape of individual ferroelectric grains as well as ensembles in a low-k dielectric material. A 2-butoxyethanol-based CSD technique using anhydrous lead(II) acetate and titanium tetrabutoxide for solution synthesis was applied to grow the PbTiO3 nanoislands. Square (111)-oriented platinized silicon wafers of 1 cm in size served as substrates. In order to achieve a maximum grain height of about 50 nm, the 1 M PbTiO3 stock solution was diluted to a volume ratio of 1:5 using filtered 2-butoxyethanol as solvent. The diluted precursor was spin-coated at a revolution speed of 3000 rpm onto the samples under a nitrogen atmosphere. Pyrolysis was carried out for 2 min at 350 °C on a hotplate. In order to crystallize the obtained amorphous film, the samples were annealed at 700 °C for 15 min in a rapid thermal processing (RTP) unit (SHS 100), resulting in separated PbTiO3 nanoislands of typically less than 100 nm in lateral extension. The chemical mechanical polishing step was carried out on a Logitech PM4 precision lapping and polishing machine. The samples were polished for 10 to 30 s each on a ChemoMet chemical-resistant polishing pad. KOH-based Syton-SF1 colloidal silica slurry was used as the polishing suspension. The revolution speed of the polishing plate was adjusted to 30 rpm and the eccentric drive of the polishing head was chosen to cycle 15 times per minute. Following this dedicated polishing step the samples were continuously polished in pure water doubling the polishing time in order to remove any abrasive particles from the samples surface. Subsequently the samples were cleaned in acetone under ultrasonic agitation, rinsed with isopropyl alcohol and deionized water, and finally blown dry with pure nitrogen. SEM images were prepared on a DSM 982 Gemini (Zeiss) column with a maximum resolution of 2.5 nm. PFM measurements were performed on a Jeol 4210 scanning probe microscope. Piezoelectric activity was verified by applying an ac voltage of 1 V at 7 kHz to the probe tip. Switching the grains was accomplished by alternately applying 5 V or −5 V to single selected grains between single PFM scans.