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A WALKING, SCANNING TUNNELING
MICROSCOPE COMBINED WITH A FOCUSED ION BEAM
G. Shedd, Prue Russell
To cite this version:
G. Shedd, Prue Russell. A WALKING, SCANNING TUNNELING MICROSCOPE COMBINED WITH A FOCUSED ION BEAM. Journal de Physique Colloques, 1989, 50 (C8), pp.C8-497-C8-500.
�10.1051/jphyscol:1989885�. �jpa-00229983�
COLLOQUE DE PHYSIQUE
Colloque C8, Supplement au noll, Tome 50, novembre 1989
A WALKING, SCANNING TUNNELING MICROSCOPE COMBINED WITH A FOCUSED ION BEAM
G.M. SHEDD and P.E. RUSSELL
Precision Engineering Center and Department of Materials Science North Carolina State University, Box 7918, Raleigh, NC 27695, U.S.A.
Abstract: 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 sub- nanometer 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 low- energy 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.
Equipment
FOCUSED ION BEAM
One of the motivations for interfacing STM with a FIB is that the FIB'S liquid metal ion source (LMIS) can be used as a source of atom clusters and droplets of liquid metal. As the extraction field is increased beyond the level required for normal ion emission, a larger and larger proportion of the beam current will be due to clusters of atoms, Support from ONR (Contract N00014-86-K-0681) and NSF (Grant DMR-86578 13) is gratefully acknowledged.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989885
until the field finally induces the Taylor cone to become unstable, and emission increasingly takes the form of large clusters and charged droplets of the liquid metal.8 The electrostatic Ienses and scanners of the FIB column can be used to focus and deflect clusters and droplets, permitting control of their dismbution on a substrate.
The FIB system has been described elsewhere.9 Briefly, it consists of a liquid metal ion source mounted to an electrostatic optical column that accelerates ions to energies of up to 25 keV. The FIB is mounted to a UHV chamber that houses an X-Y-Z specimen stage. A computer is used to control scanning and blanking of the ion beam, and to acquire secondary electron signals for imaging.
Figure 1. The walking double-tube STM. The inner tube scans during imaging; the outer tube is used for offsets and walking. By bending the tube slowly, then straightening quickly, the STM walks toward the sample wedge by stick-slip motion. The FIB has a clear view of the tip through the tube and tip holder.
SCANNING TUNNELING MICROSCOPE
The STM illustrated in Figure 1 was designed to be used in combination with a focused beam (ion or electron).
It is a variation on several tube-in-tube designs that have appeared in the literature.lO-l2 Two piezoelectric tubes of different diameters are attached concentrically to a rigid flange. The inner tube operates as a conventional STM tube scanner, scanning the tip parallel to the sample surface (X-Y plane), while moving the tip perpendicular to the surface (Z) to maintain a constant tip / sample separation. The tip-holder is a removable, plug-shaped piece of insulator that fits inside the inner STM tube where it rests on a washer-shaped disc that covers the bottom of the tube. A hole drilled through the tip holder (at a 15' angle to the central axis) contains a metal sleeve into which the shank of an STM tip can be placed. The STM tip protrudes beyond the end of the scanning tube, through the hole in the center of the disc. Another hole bored through the tip holder, parallel to the tube axis, provides an opening through which a FIB, or SEM can view the sample and tip.
The entire double-tube assembly is supported on three spherical feet mounted to the bottom rim of the outer tube, which is used to apply X, Y, and Z offsets. If the outer tube is bent slowly to one side, then straightened almost instantaneously, the feet slip relative to their contact points. This stick-slip sequence produces a sub-micron step which, when repeated, can produce a macroscopic "walking" motion in the X and Y directions.
By placing the sample on a wedge-shaped sample holder beneath the tip, trans>.erse STM motion toward the uphill part of the wedge reduces the tip-to-sample separation distance. Motion across the wedge's slope (in the X direction) will not change the Z height macroscopically, so coarse X motion is possible without moving the tip out of the Z piezo offset range. The ability of the STM to "walk" simplifies the coarse tip-to-sample approach, and increases the area on the sample accessible for imaging without requiring complicated mechanical design.
An STM of this design has been built using 12.5-mm-long piezoelectric tubes of 6.25 mm and 12.5 mm diameters. The feet are 3-mm-diameter Teflon balls fastened to the outer rube with cyanoacrylate; a silicon wafer serves as the smooth supporting surface. Electrical connections are made to the STM with 25 pm gold wires suspended from above.
STM walking and imaging are accomplished through computer control of a commercially-available STM electronics unit.13 The sawtooth waveforms applied to the outer tube during walking are generated by digital-to- analog convertors @AC's) controlled by the computer. During iinaging, the inner tube can be scanned by the same DAC's, or by an automated scan generator included with the control electronics. A feature that has recently been added to our STM imaging systems is the ability to perform continuous-scan, real-time imaging. The incorporation of this imaging capability will greatly facilitate the task of finding clusters on a substrate.
Results and Discussion
STM PERFORMANCE
To test the walking motion, pieces of gold-coated silicon were mounted to the outer tube to serve as target electrodes for two capacitance gauges which measured STM displacements in the X and Y directions. Using the capacitance gauges for guidance, the STM walked from one comer of a 62.5 pm square through the center to the opposite comer, back through the center to the original position, and then completely around the square's perimeter.
After a retroreflector was attached to the top of the STM, a laser interferometer was used to measure longer- range motion along the Y axis (X motion was again monitored with a capacitance gauge). The X position reading was used as feedback by the walker control software to minimize the STM's net X motion. Thus configured, the STM could walk 2 mm in the positive Y direction, and then back to the original position without straying more than a few pm from a straight line. During excursions of 1 mm, the X position reading could be maintained constant to within f 1 pm. Step size varied from tens of nm to tenths of pm, depending on the magnitude of the voltage step (50-200 V) and on the frictional properties of the walking surface.
- L . m - I I - , - I I ,
-100 100 300 500 700 900 1100 Y displacement (pm)
Figure 2. Displacevent of the STM during a 1 mm excursion along the Y axis. Points are separated by 50 Y steps (200 V). X deviation is less than f 1 pm as measured with a capacitance gauge.
It should be mentioned that the walking motion of the STM is not completely predictable. An example of this can be seen in Figure 2, which is a graph of X-Y displacement data collected during a 1 mm translation of the STM.
Data points were acquired after each 50 steps taken in the Y direction. During the first 350 pm of the translation the average Y displacement per step was .24 pm, but after that point the response changes abruptly to .50 pm per step.
The frictional forces between the feet and surface can vary with position, and with the direction of motion, causing anisotropy and nonlinearity in the response. Therefore, it is important that some form of positioning information be available for precise motion to be possible. When the STM is interfaced to an FIB or SEM this information can be provided by images of the tip.
To confirm the STM's imaging capability, the atomic lattice of graphite, and thin gold films on Si, were imaged.
A 1.6-pm-period optical disk was used to measure the maximum field of view of the STM scanner. When a scan signal of 260 V was applied to a single X electrode on the inner tube, the STM tip traversed a distance of ~ 3 . 5 pm.
The STM design thus has the ability to cover a fairly large range of imaging applications, from the atomic scale up to the pm scale relevant for imaging features micromachined by the FIB. Since this test was performed, the STM electronics have been modified to provide complementary bipolar scan voltages which should effectively double the scan range.
CLUSTERS
The FIB has been used to deposit Ga clusters and droplets onto a graphite substrate. Droplets ranging in diameter from tens of nm to several pm were imaged with field-emission SEM and STM. The deposition technique will be developed further using feedback provided by STM images. The parameters of primary concern are the particle size and deposition density as functions of extracted ion current. Substrate damage (and its relation to dose and beam energy) will be characterized to see if it is problematic.
PLANS
A second walking STM has been built, using shorter tubes (12.5 mm) to increase the stability and resonance characteristics of the smcture. Based on experience with the first model, it is expected that the complementary bipolar scan voltages should scan the shorter tube over an image area of more than 3 x 3 pm in size.
Before initiating nanomanipulation studies a few modifications to the standard STM configuration are planned.
A digitally-triggerable, capacitive-discharge circuit will be attached to the tip to allow short-duration voltage pulses to be applied during nanomanipulation experiments. This will enable the study of field-induced and current-induced effects. It may also be necessary to implement a non-standard tip scanning motion to avoid displacing clusters unintentionally during STM imaging. Several authors have noted a smearing effect when imaging small clusters, as if the clusters were being moved by the tip.14.15 h o u r laboratory, structural changes have been observed during imaging of Ga droplets. A technique recently suggested to avoid such problems when imaging biological samples may used.16 It involves lifting the tip during X-Y point-to-point translations, followed by vertical motion to establish the Z height at each position.
Conclusion
The STM design described in this paper has exhibited the ability to image over a wide range of magnifications, from atomic resolution to a 3.5 x 3.5 pm image field. It has walked millimeters along a straight (* pm) line, and has traced a relatively complex square path of several hundred pm in length. This precise motion capability opens up lwger portions of the sample surface for STM exploration without the need for complex translation mechanisms.
The STM's configuration also facilitates applications that combine focused beams and STM.
The STM will be used to study atom cluster nanomanipulation. This paper has briefly discussed STM's application to nanofabrication, and has tried to make the point that STM is uniquely suited as a tool for influencing atomic-scale events.
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