A Walking, Scanning Tunneling Microscope Combined with a Focused Ion Beam

A walking, concentric-tube STM has been designed to interface with an F a . The outer tube, which stands upright on 3 spherical feet, supports the inner tube and serves as an inertial stepper, allowing the STM to move laterally relative to a sample. The ion beam can pass down through the center of the inner tube, which operates as a conventional tube scanner with the STM tip at its lower end. The STM walks distances of mm with pm-scale resolution, resolves the pyrolytic graphite lattice, and can image regions of up to 3.5 x 3.5 ym. Introduction The scanning tunneling microscope (STM) and the focused ion beam (FIB) are both versatile instruments. The FIB has found many uses as a tool for micr~fabrication.~ Its small beam diameter enables it to perform micron-scale modifications, and its microscopic capability can be used to locate features of interest. The STM is known primarily for its atomic resolution images of s ~ r f a c e s , ~ but it is also attracting increasing attention as a possible tool for nanofabrication.3 We have designed an STM to interface with a FIB in a system that will permit us to image regions ranging in size from mm2 to nm2, and to induce modifications at the pm and nm levels. STM AND NANOFABRICATION The ultimate goal of nanofabrication is to be able to arrange individual atoms as desired. Few tools can address a region of only nanometers in extent, let alone precipitate a controlled response within that region. Primary among the tools suggested for nanofabrication are lens-focused electron beams, which are routinely formed into subnanometer probes.4 The diameter of the beam is not a true measure of the area that is influenced, however, since energy is dissipated throughout a larger region (a manifestation of this is the "proximity effect" witnessed in electron-beam lithography). It is difficult to conceive how electrons with energies in the keV regime could be used to controllably influence the actions of an atom, or small cluster of atoms. The STM "beam" is focused by proximity; that is, the source is so close to the target that the electrons cannot diverge to any great degree during transit between the two. The most important attribute of proximity focusing is that the electrons can have much lower energies than those in available lens-focused beams of comparable diameter. Low particle energy is of critical concern when it is considered that many of the events that are of interest for nanofabrication (e.g. migration, bond breaking, chemical reactions) have activation energies of less than ten electron volts per atom.5 The STM is the only instrument that can presently provide a sub-nanometer beam of such lowenergy particles. Another method by which an STM might exert influence at the nanometer scale is by direct interatomic force. The atomic force microscope (AFM) has exhibited the ability to convert detected changes in the interaction forces between a sharp tip and a substrate into sub-nanometer resolution images.6 If the region of interaction is assumed to be comparable in size to the spatial resolution, then it should be possible to exert forces on nanometer-scale regions using the sharp tip of an STM. This direct-contact mode may prove to be useful for nanomanipulation, perhaps to push atoms around. The STM may be able to exert human influence upon a single atom? but before it becomes possible to arrange atoms at will, the practical problems involved in moving small particles from point A to point B must first be addressed. We plan to study the use of STM for nanomanipulation-to investigate the influence that the STM might exert over nanometer-scale atom clusters.