P. E. Russell

The scanning tunneling microscope as a tool for nanofabrication

The scanning tunneling microscope (STM) has proven to be an unsurpassed instrument for studying surfaces with sub-nanometer resolution in all three spatial dimensions. The same circumstances that permit the STM to image with atomic resolution also allow it to function as a highly localized probe capable of exerting human influence upon regions of only nanometers in extent. Demonstrations of the STMs ability to modify matter at the nanometer scale give evidence of its promise as a tool for nanofabrication applications. In addition to presenting some recent results of STM modifications of Au and graphite substrates, this paper introduces a system for classifying the various mechanisms by which an STM may induce modifications, and classifies published and new experimental observations within this framework.

Tip–sample forces in scanning probe microscopy in air and vacuum

Forces between a probe and sample have been measured in air and vacuum using a rocking beam force balance sensor capable of simultaneously obtaining both force and tunneling topographs. Force versus distance curve measurements demonstrate the effects of capillary forces in air but not in vacuum. Simultaneous scanning tunneling microscope (STM) imaging and force measurement in air show spatial force variations consistent with repulsive force tunneling. Our results support observations of large repulsive forces in STM occurring from surface contaminants. These results show how forces can affect image formation in both STM and scanning force microscopy.

Scanning probe tip geometry optimized for metrology by focused ion beam ion milling

We have developed a novel technique for producing tips for scanning probe microscopy. The shape of the tip is optimized for applications where high aspect ratio surfacetopography is the norm, as with integrated circuit structures. The technique involves focused ion beam(FIB)milling of a tip which was previously shaped to a nominal geometry by electrochemical etching. The ion milling pattern is annular, and the ion beam is collinear with the axis of the tip. The process allows control of the ion milling dose using 20 keV Ga+ with submicron control of the annular patterns. The result is a narrow, tapered structure approximately 20 μm in length which ends in a point with a radius of curvature between 30 and 50 nm when grains dominate the sputter process, and radii of about 3–4 nm when there is no evidence of grain structure effects. This microstructure is 3 μm in diameter at the base and it protrudes from a portion of the shank of the macrostructure where the diameter is about 15 μm. We have sufficient control over the sputter process to yield the final tip length, taper, and radius. Cone angles between 12° and 15° over the first two microns from the apex can be achieved routinely, by the correct choice of annulus and ion dose. Sputter simulations predict the correct shape of the final tip profile, and show the effect of varying the ion beam focus, dose, and inner and outer annulus radii. Tips with the desired geometry have been produced in polycrystalline tungsten, iridium, and platinum‐iridium. Significant improvements in scanning tip microscopy(STM) images have been consistently observed with these FIB milled tips.

Scanning probe tips formed by focused ion beams

Probe tips for scanning tunneling microscopy have been sharpened using focused ion beam milling. Reproducible tips were formed on polycrystalline W and Pt‐Ir shanks, but this technique is not limited to these materials. The tips were found to have cone angles of 12±3° and radii of curvature as sharp as 4 nm. Focused ion beam machining allows precise control of the final shape of the tips which is important in metrology measurements of various nanostructure devices.

H2O enhanced focused ion beam micromachining

The use of H2O vapor as a chemical adjunct for focused ion beam micromachining has been investigated. The presence of H2O vapor during micromachining with a 25 keV Ga+ beam increases the removal rate of carbon‐containing materials such as polyimide, PMMA, and other resists by a factor of 20 (relative to physical sputtering), and that of diamond by a factor of 10. In addition, H2O causes a decrease in the removal rate of some other materials (e.g., Si and Al) by as much as a factor of 10, effectively increasing the selectivity of polymers over these other materials by as much as a factor of 200. The dependence of the removal rate on H2O pressure at the sample, pixel dwell time, pixel size, pattern frame time (corresponding to pattern size), and current density has been investigated utilizing PMMA. PMMA removal rates were calculated by measuring the depth of rectangular pits micromachined into PMMA films under the various experimental conditions. In addition to investigating the effect of H2O on material re...

Chemically and geometrically enhanced focused ion beam micromachining

Improvements in focused ion beam (FIB) material removal rates utilizing geometric and chemical enhancement were investigated. Geometrical optimization of FIB micromachining of Permalloy and diamond was investigated to determine the magnitude of material removal rate gains that could be attained by increasing the angle of the ion beam with respect to the sample surface normal. The combination of geometrical optimization with chemical enhancement (C2Cl4 for Permalloy and H2O and XeF2 for diamond) was then investigated to determine whether additional gains in material removal rate could be attained. FIB sharpening of a diamond nanoindenter tip is also presented as a practical example of diamond micromachining with H2O as the removal rate enhancing species.Improvements in focused ion beam (FIB) material removal rates utilizing geometric and chemical enhancement were investigated. Geometrical optimization of FIB micromachining of Permalloy and diamond was investigated to determine the magnitude of material removal rate gains that could be attained by increasing the angle of the ion beam with respect to the sample surface normal. The combination of geometrical optimization with chemical enhancement (C2Cl4 for Permalloy and H2O and XeF2 for diamond) was then investigated to determine whether additional gains in material removal rate could be attained. FIB sharpening of a diamond nanoindenter tip is also presented as a practical example of diamond micromachining with H2O as the removal rate enhancing species.

A scanning tunneling microscope with a capacitance‐based position monitor

A serious impediment to precision metrology with scanning probe microscopes is the unreproducible, nonlinear behavior of the piezo ceramic actuators. We have developed a simple solution to the problem by measuring the position of the scanning head with capacitors. The circuit monitoring the gap between the plates has a linear response with slope 12 μm/V and noise of 0.1 nm/(Hz)1/2. The linearity of the system is verified by comparing the capacitor output with a scan of a grating that is periodic in two dimensions. We analyze the errors associated with the technique and show how to reduce them to acceptable levels.

Platinum/iridium tips with controlled geometry for scanning tunneling microscopy

An electrochemical etching procedure using a saturated CaCl2/H2O/concentrated HCl (60%/36%/4% by volume) solution has been developed to fabricate platinum/iridium tips with controlled geometry for scanning tunneling microscopy (STM). These tips, which have a high aspect ratio (5°–10° cone half angle) and a small radius of curvature (∼500 A), are particularly useful for the imaging and metrology of precision‐engineered surfaces. Initial attempts to use the tips showed them to be unreliable for STM imaging. Auger electron spectroscopy indicated the presence of a carbon contamination layer. By eliminating CO2 from the etching environment, the contamination was reduced and the reliability of the resulting tips improved dramatically. The imaging versatility of these tips is demonstrated for sputter‐deposited gold on silicon, for a gold‐coated polymethylmethacrylate lithographic test pattern, and for highly oriented pyrolytic graphite.

Focused ion beam fabrication of metallic nanostructures on end faces of optical fibers for chemical sensing applications

Focused ion beam (FIB) fabrication of fiber optic sensors, mainly chemical sensors, which are based on plasmonics-active nanostructures formed on the cleaved tips of optical fibers, is reported. The nanostructures fabricated included nanoholes in optically thick metallic films as well as metallic nanopillars and nanorods. The sensing mechanism is based on detecting shifts in surface plasmon resonances (SPRs) associated with nanoholes in metallic films and localized SPRs of metallic nanopillars and nanorods, when the refractive index of the medium surrounding the nanostructures is changed. These sensors can be employed for the detection of chemical agents in air as well as liquid media surrounding the sensors. FIB milling was employed to fabricate ordered arrays of nanoholes in optically thick (100–240nm) metallic films deposited on cleaved end faces of multimode, four-mode, and single-mode optical fibers. Separately, metallic nanorods and nanopillars were formed by first depositing a metallic (gold or sil...

Microfabrication of AFM tips using focused ion and electron beam techniques

Abstract Production of reproducible AFM tips with narrow and straight tip shanks is important for metrology and imaging of various submicron and high topography structures. Possibilities of micromachining commercial AFM pyramidal tips with focused ion beam (FIB) techniques have been investigated to improve the sharpness of the pyramidal tips. Also, the fabrication of electron-beam-induced microtips grown on the top of the pyramidal tips has been developed using a combination of focused ion and electron beam techniques. Microtips were reproducibly grown with tip shanks of 1.0 μm length and 0.1 μm diameter. The tip geometry was found to degrade only slightly after extensive AFM imaging, indicating minimal wear on the microtip during AFM scanning. The radius of curvature remained as sharp as 25 nm after tens of hours of AFM imaging in the contact mode. The microtips were found to be considerably stronger and more stable when they were grown on pyramids which had been heavily implanted with Ga ions.