Template-Controlled Integration and Characterization of Bottom-Up Grown Ferroelectric Nanoislands (2009)

The tremendous progress in thin-film processing has opened up an exciting new field of applications for ferroelectric materials in information and semiconductor technology today. Exemplarily, the development of the Ferroelectric Random Access Memory (FeRAM) was enabled, which uses the spontaneous polarization of ferroelectric thin-films to store digital information non-volatile. Beyond, ferroelectric films are likewise employed due to their outstanding dielectric, piezoelectric and pyroelectric properties. The prospective success of any of these technologies will first and foremost depend on their potential to fulfill the markets requirement for ever growing integration densities. The scalability of ferroelectric materials however is still under discussion and therefore subject to various research projects. Size-effects in real systems are expected to be often rather extrinsically determined by technological issues like processing induced damage and boundary conditions than intrinsically. The general question arises, how ferroelectric nanostructures can be processed with Gbit integration densities for future devices without deteriorating their physical properties. In addition, electrical characterization of these structures is hampered as the electric charge to detect scales with lateral cell dimension. A surface charge of a 100 nm2 capacitor that features a 30 µC/cm2 polarization is compensated by merely 180 electrons for example. Hence, ferroelectricity of capacitors below 200 x 200 nm2 in lateral dimensions was so far only shown qualitatively and indirectly via their piezoelectric properties. In the framework of this thesis, bottom-up grown ferroelectric lead titanate (PTO) nanoislands are studied in terms of their potential for integration and electrical characterization. The production method applied is based on Chemical Solution Deposition (CSD) which is especially suited as it features a non-destructive, parallel processing of smallest ferroelectric nanostructures. A new method, based on pre-defined artificial titanium oxide nucleation sites, is introduced to enable growth of the ferroelectric crystals with a precise arrangement and a uniform size distribution. Using this technique, the ferroelectric nanostructure arrays achieve integration densities up to 6.4~Gbit/cm2 – for the first time obtained in the field of well-ordered functional ferroelectric nanostructures. Regarding electrical characterization of the nanoislands, an integration method based on flowable oxide layers and Chemical Mechanical Polishing (CMP) is presented. It is shown, that using the polishing step electrical contact can be provided to completely insulated, ferroelectric nanoislands of lowest heights (< 20 nm). A significant deterioration of the ferroelectric properties due to this processing method is not detected which is supported by direct electrical characterization of the embedded nanoislands. Electrical hysteresis measurements are facilitated by connecting the islands in parallel. For the first time displacement current peaks are recorded on ferroelectric nanostructures featuring typical lateral dimensions below 10000 nm2.