Colin A. Sanford

Rapid and precise scanning helium ion microscope milling of solid-state nanopores for biomolecule detection

We report the formation of solid-state nanopores using a scanning helium ion microscope. The fabrication process offers the advantage of high sample throughput along with fine control over nanopore dimensions, producing single pores with diameters below 4 nm. Electronic noise associated with ion transport through the resultant pores is found to be comparable with levels measured on devices made with the established technique of transmission electron microscope milling. We demonstrate the utility of our nanopores for biomolecular analysis by measuring the passage of double-strand DNA.

Understanding imaging modes in the helium ion microscope

Recent investigations are gaining us a better understanding of the nature of the beam-sample interactions in the helium ion microscope and what they mean for the image information provided. In secondary electron (SE) imaging, for example, the surface sensitivity is attributed to the low SE-II fraction. Voltage contrast imaging shows the ability to see both buried structures and to probe the conductance to ground of surface contacts. It is found, however, that the prominence of these two types of contrast varies oppositely with beam energy, yielding information about the nature of the interactions that gives rise to them. Transmission ion imaging can yield information about material density, atomic number, grain structure, and electronic structure. It is possible to capture the top-side SE signal, bright field signal, and dark field signal from a given sample simultaneously. The detection of diffraction contrast is under investigation.

Beam induced deposition of platinum using a helium ion microscope

Helium ion microscopy is now a demonstrated practical technology that possesses the resolution and beam currents necessary to perform nanofabrication tasks, such as circuit edit applications. Due to helium’s electrical properties and sample interaction characteristics relative to gallium, it is likely that the properties and deposition characteristics of beam induced deposited films will be different than those produced using gallium focused ion beam technology. However, there is at this date very little literature discussing the use of helium beams for beam induced chemistry or characterization of the resulting films. In this article, the authors present initial results regarding the deposition of platinum using a helium ion microscope and a gaseous organometallic precursor. Within this work a Carl Zeiss ORION™ helium ion microscope was used along with an OmniGIS unit to deposit platinum while exploring a variety of controllable parameters such as beam current, beam overlap, and size of deposition.

Nanopillar growth by focused helium ion-beam-induced deposition

A 25 keV focused helium ion beam has been used to grow PtC nanopillars on a silicon substrate by beam-induced decomposition of a (CH(3))(3)Pt(C(P)CH(3)) precursor gas. The ion beam diameter was about 1 nm. The observed relatively high growth rates suggest that electronic excitation is the dominant mechanism in helium ion-beam-induced deposition. Pillars grown at low beam currents are narrow and have sharp tips. For a constant dose, the pillar height decreases with increasing current, pointing to depletion of precursor molecules at the beam impact site. Furthermore, the diameter increases rapidly and the total pillar volume decreases slowly with increasing current. Monte Carlo simulations have been performed with realistic values for the fundamental deposition processes. The simulation results are in good agreement with experimental observations. In particular, they reproduce the current dependences of the vertical and lateral growth rates and of the volumetric deposition efficiency. Furthermore, the simulations reveal that the vertical pillar growth is due to type-1 secondary electrons and primary ions, while the lateral outgrowth is due to type-2 secondary electrons and scattered ions.

Analysis and metrology with a focused helium ion beama)

The newly introduced ORION™ helium ion microscope has been used for high resolution imaging and nanofabrication. More recently, an energy sensitive detector has been developed that permits the measurement of the energy spectrum of the backscattered helium ions. The spectra can be analyzed directly or compared with the simulated spectra from hypothetical models of the specimen. The technique can provide information about the elemental composition of the specimen or structural information (for example, layer thickness) of the specimen.

Analysis of subsurface beam spread and its impact on the image resolution of the helium ion microscope

By virtue of its extremely bright gaseous field ion source, the ORION™ helium ion microscope has demonstrated a probe size smaller than 0.3 nm, when operating the microscope in its (typical) high-resolution imaging mode, i.e., surface imaging with secondary electron signal. The authors combined SRIM-models of beam spread in the sample with models for secondary electron signal generation and escape, for a wide range of beam energies and sample materials, in order to calculate the effect of beam spread on image resolution. It was found that the effect on resolution is larger for sample materials with higher atomic numbers, and that the effect is inversely proportional to beam energy. The magnitude of the calculated effect on image resolution ranges from 0.005 to 0.08 nm, which is typically an order of magnitude smaller than the currently experimentally measured image resolution of the helium ion microscope.

Creating nanohole arrays with the helium ion microscope

Helium Ion Microscopy has been established as a powerful imaging technique offering unique contrast and high resolution surface information. More recently, the helium ion beam has been used for nanostructuring applications similar to a gallium focused ion beam. A key difference between helium and gallium induced sputtering is the less intense damage cascade which lends this technique to precise and controlled milling of different materials enabling applications. The helium ion beam has been used for drilling 5nm holes in a 100nm gold foil (20:1 aspect ratio) while the gallium beam sputtered holes of a similar aspect ratio seem to be limited to a 50nm hole size. This paper explores the drilling of nanopores in gold films and other materials and offers an explanation for the observed differences in results between helium and gallium ions.