SiO2 nanodots on Si produced by slow multiply charged ions

Ever since the advent of sources for slow highly charged ions more than twenty years ago the possibility of exploiting their huge amount of potential energy for nano-fabrication, i.e. "writing" an a surface, has captured the imagination of researchers. Such applications have been envisioned for a broad spectrum ranging from information storage via materials processing to biotechnology. One important way to produce nanostructures involves kinetic sputtering with and implantation of fast ions. However, fast ions unavoidably cause some unwanted radiation damage. As opposed to this, potential sputtering (PS), i.e. desorption induced by the potential energy of slow multicharged ions (MCI), holds great promise for a much more gentle nanostructuring tool. It may cause high surface sputter yields even at low !, ion impact energies where kinetic sputtering and thus defect creation in deeper layers is impossible. While the physical mechanisms of PS have already been the subject of extensive investigation, technical applications of slow multicharged ions have so far remained largely unexplored, despite the fact that slow MCI provide unique opportunities desirable for etching, ultra-thin film growth and nanostructure fabrication. Within the here proposed project we intend to utilize beams of slow multicharged ions to produce nanometer sized surface modifications an silicon substrates. This will be achieved by bombarding hydrogen terminated silicon monocrystals in ultra high vacuum with low fluxes of slow MCI. At the MCI impact site (with a nanometer size radius) we expect the hydrogen atoms to be removed by the interaction of the MCI with the surface. By introducing oxygen gas of sufficient partial pressure the now open silicon bonds will react with the OZ molecules, in this way producing ultra shallow silicon oxide nanodots. We intend to study the formation of these nanodots and optimize the conditions by using (non - contact) atomic force - (AFM) and scanning tunneling microscopy (STM), as well as high resolution (10 nm) scanning Auger spectroscopy. Lateron we will investigate whether carbon nanotubes or other multi-molecular structures can be grown an the such produced small silicon oxide nanodots.

Highly charged ions carry a large amount of potential energy which is released upon impact on a surface. It was the main goal of this project to investigate under which circumstances this particular form of energy can give rise to well localized surface modifications. Because of the ever-shrinking length-scale of semiconductor devices, the ability to produce nanometre- sized local alterations on a surface with highly charged ions becomes increasingly interesting as possible semiconductor manufacturing tools. Within the project we first investigated the possibility of using highly charged ions to gain control over silicon surface modifications on a nanometre scale. Using a high precision scanning tunnelling microscope we investigated the local changes to a hydrogen - terminated surface after irradiation with multiply-charged ions and subsequent oxidation. As a second surface we investigated calcium fluoride, which can be used as an insulator in silicon microelectronic devices. In these experiments we applied very highly charged ions (up to Xe48+ ) with a rather low fluence. Surprisingly, the inspection of the irradiated surface with atomic force microscopy revealed the generation of nanometric hillocks protruding from the surface above a well-defined threshold of potential energy. The number of hillocks per unit area was found to be in good agreement with the applied ion fluence (one hillock per single ion impact). By varying the charge state of the incident highly charged ions we have meanwhile achieved control over the size of the nano-hillocks (diameter and height increase with projectile charge state) and discovered the mechanism behind the nanostructure formation on CaF2 . Estimates of the energy density deposited suggest that the threshold is linked to a solid-liquid phase transition ("melting``) on the nanoscale. These investigations will be continued within the joint research activity on "Slow ion induced nanostructuring of surfaces and thin films" of a newly granted EU-network.