UHV-AFM Studies of Nanodefect Formation by Slow Multicharged Ions at Insulator Surfaces

Formation of nanodefects on mono-crystalline insulator surfaces due to impact of slow multicharged ions will be studied by means of AFM (atomic force scanning microscopy) in UHV (ultra-high vacuum). In project phase 1, in- situ measurements are planned with a multicharged ion source facility (5 GHz ECRIS) at TU Wien. Targets to be irradiated (e.g., LiF, CaF2, MgF2, SiO2, Al2O3, MgO, various semiconductors) will be prepared from commercial research grade monocrystals in an UHV target chamber which is electrically insulated for primary ion deceleration down to impact energies of a few eV. The UHV-AFM to be purchased is the currently only commercially available instrument operating under genuine UHV conditions (OMICRON UHV AFM/STM) and in non-contact mode. It will be attached to the UHV chamber in order to permit transfer of target crystals for AFM inspection before and after the MCI irradiation. For satisfactory AFM performance the AFM jar is evacuated by a 1000 ltr/s ion pump during the AFM measurements. The collision chamber features a CF 35 side flange through which probes can be transferred into a portable UHV station via a bakeable UHV lock, which is of importance both for project phase 2 (see below) and to permit AFM service for other research groups. With proper AFM operation, in project phase 2 external target irradiation will be carried out. Particular target crystals (including semiconductors as Si and GaAs) will be irradiated at a suitable external highly charged ion beam facility as, e.g., the Lawrence Livermore National Laboratory EBIT. To this purpose targets crystals will first be AFM-inspected in the UHV apparatus at TU Wien and then transferred via the UHV lock into a portable UHV chamber. After the external irradiation the respective target crystals will be re-inserted into the UHV chamber and AFM-based searches for PSI-related surface damages will be made, focusing in first place on dependences of PSI related surface defects on the MCI charge and -impact energy. In project phase 3, PSI studies will be carried out with insulator surfaces covered by regular patterns of metal clusters of variable size ("self-organized" metal cluster formation). One candidate target material could be NaCl covered by Na clusters produced by in-situ electron irradiation of NaCl (or other alkalihalides), another one Al2O3 covered by Ag clusters as known from recent research on novel surface catalysts. After having learned how to produce such complex target surfaces in situ in a hopefully simple and reproducible way, they will be irradiated by slow MCI and the resulting surface damage will again be studied by means of AFM. The main goal of this last project phase is to achieve sufficient clarity on whether coverage of insulator surfaces by small metal clusters would be a possible way for producing regularly- structured PSI induced surface-nanodefects on insulator surfaces relevant for microelectronics- and nanotechnology related applications.

Surface inspection techniques (STM/S-Scanning Tunneling Microscopy /-Spectroscopy, contact -and non-contact mode AFM-Atomic Force Microscopy) have been used for studying with atomic resolution nanoscopic damages on surfaces of monocrystalline insulators and highly-oriented pyrolytic graphite (HOPG) caused by the impact of single slow multiply charged ions (MCI induced "potential sputtering-PS"). AFM on single-crystalline insulator surfaces in ultra-high vacuum reached only atomic resolution for well prepared and cleaned targets samples, and when flooding them with slow electrons for avoiding surface charge-up from the MCI impacts. Target samples were transferred in an ultra-high vacuum vault between the AFM, other surface-analytical instruments and the ion bombardment chamber. Slow ions with given kinetic energy but in different charge states produced nanostructures of clearly different size, which for the first time unanimously demonstrated PS on particular insulator surfaces. Systematic investigations will now be made for the most promising materials like SiO2 and Al2 O3 . We will also apply AFM for detection of large molecules deposited on substrates during quantum-optical diffraction (M. Arndt and A. Zeilinger, University of Vienna), and develop with P. Scheier (University of Innsbruck) "gray-scale nanowriting" (STM with tunneling current control) on Si nanoparticle-covered HOPG surfaces. MCI induced SiO2 nanodot growth on hydrogen terminated silicon surfaces is the subject of the new FWF project No. 16178 PHY (F. Aumayr), and our acquired expertise is called in the planned EU-FP6 infrastructure network "Surface-, thin film- and interface nanostructuring by slow MCI beams".