David C. Ferranti

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.

Helium Ion Microscopy (HIM) for the imaging of biological samples at sub-nanometer resolution

Scanning Electron Microscopy (SEM) has long been the standard in imaging the sub-micrometer surface ultrastructure of both hard and soft materials. In the case of biological samples, it has provided great insights into their physical architecture. However, three of the fundamental challenges in the SEM imaging of soft materials are that of limited imaging resolution at high magnification, charging caused by the insulating properties of most biological samples and the loss of subtle surface features by heavy metal coating. These challenges have recently been overcome with the development of the Helium Ion Microscope (HIM), which boasts advances in charge reduction, minimized sample damage, high surface contrast without the need for metal coating, increased depth of field, and 5 angstrom imaging resolution. We demonstrate the advantages of HIM for imaging biological surfaces as well as compare and contrast the effects of sample preparation techniques and their consequences on sub-nanometer ultrastructure.

Neon Ion Beam Lithography (NIBL)

Existing techniques for electron- and ion-beam lithography, routinely employed for nanoscale device fabrication and mask/mold prototyping, do not simultaneously achieve efficient (low fluence) exposure and high resolution. We report lithography using neon ions with fluence <1 ion/nm(2), ∼1000× more efficient than using 30 keV electrons, and resolution down to 7 nm half-pitch. This combination of resolution and exposure efficiency is expected to impact a wide array of fields that are dependent on beam-based lithography.

Fabrication and initial characterization of ultrahigh aspect ratio vias in gold using the helium ion microscope

Toward the end goal of creating transducers with nanometer scale sensing features, the helium ion microscope (HIM) has been employed to create and characterize high aspect ratio features in gold films. The HIM has a spot size less than 1 nm, uses a chemically inert noble gas (He), which does not deposit/implant any species that may contaminate the material being patterned, and is able to rapidly generate arrays of vias in Au. Hence, the HIM is an ideal tool to generate these ultrahigh aspect ratio features. The authors characterize the vias, also using HIM, by measurements of feature size, lateral milling resolution, sidewall angle, and fabrication speed. Two novel methods were employed to enable the characterization due to the very small size of the features. A significant reduction in via width is achieved, as compared with traditional focused ion beam milling.

Chemically enhanced FIB repair of opaque defects on molybdenum silicide photomasks

The characteristics of an ideally repaired opaque defect on a molybdenum silicide (MoSiaObNc) photomask are: (1) the total removal of the MoSiaObNc defect, leaving no residual MoSiaObNc; (2) a smooth, level quartz surface (no over-etch) after the MoSiaObNc is removed; (3) minimal riverbedding of the quartz at the perimeter of the MoSiaObNc defect; and (4) maximum light transmission (%T) at the i-line (365 nm) and DUV (248 nm) lithographic wavelengths. Achieving these ideal repair characteristics is becoming increasingly difficult as the patterned features become smaller, as the lithographic wavelength becomes shorter and as phase shifting mechanisms are implemented. A chemical process has been developed to enhance the FIB (focused ion beam) etching of MoSiaObNc defects. Using this chemical process, a FIB protocol has been developed which enhances the removal of a MoSiaObNc defect while inhibiting the removal of quartz. AFM (atomic force microscopy) indicates that (1) MoSiaObNc is totally removed, (2) the quartz remains smooth and level (no over-etch), and (3) the riverbends are, at this time, 10 - 45 nm; our target is 1 - 15 nm. The MoSiaObNc etch process reduces optical staining due to implanted gallium

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.

Resolution Limits of Secondary Electron Dopant Contrast in Helium Ion and Scanning Electron Microscopy

As the miniaturization of semiconductor devices continues, characterization of dopant distribution within the structures becomes increasingly challenging. One potential solution is the use of the secondary electron signal produced in scanning electron (SEMs) or helium ion microscopes (HeIMs) to image the changes in electrical potential caused by the dopant atoms. In this article, the contrast mechanisms and resolution limits of secondary electron dopant contrast are explored. It is shown that the resolution of the technique is dependent on the extent of electrical potential present at a junction and that the resolution of dopant contrast can be improved in the HeIM after an in-situ plasma cleaning routine, which causes an oxide to form on the surface altering the contrast mechanism from electrical potential to material contrast.

Advanced Nanofabrication using Helium, Neon and Gallium Ion Beams in the Carl Zeiss Orion NanoFab Microscope

Direct write focused ion beam (FIB) machining represents the fastest and most flexible method to fabricate nano-scaled devices for prototyping and research applications. The use of a FIB combined with SEM allows for immediate inspection and refinement steps in the patterning process to assure the desired fidelity. FIB technology has thus found wide-spread use in fields such as photonics, nanofluidics, TEM sample preparation, integrated circuit modification, MEMS and more. Conventional gallium FIBs use a liquid metal ion source (LMIS) with many notable drawbacks such as a lower limit for feature sizes that can be achieved and undesirable Ga implantation. Ion microscopy with helium and neon beams created from a gas field ion source (GFIS) demonstrates great flexibility for many nanofabrication applications. The beam-sample interaction dynamics of helium and neon beams at moderate voltages extends direct write fabrication and inspection further into the sub-10 nm regime. The Carl Zeiss Orion NanoFab is a commercially available ion beam microscope offering three different species of ions to extend beyond the limits of gallium with the addition of helium and neon beams.

Structure and properties of polymer core-shell systems: Helium ion microscopy and electrical conductivity studies

Peculiarities of the structural organization and electrical properties of two core-shell polymer systems under different fabrication protocols have been studied with a combination of helium ion microscopy (HIM) and current-voltage characterization. The systems under study included a submicrometer core of a ferroelectric polymer polyvinylidene fluoride and a shell of intrinsically conducting or semiconducting polymer polyaniline (PANI) or poly(3-methylthiophene) (P3MT). Application of HIM allowed identification of the polymer components, visualization of the electrically conductive percolation network of PANI or P3MT, and its variation due to thermal annealing and/or interaction with the environment. HIM is proved to be a powerful tool for characterization of not only the morphology but also of the charge distribution and conductivity properties on the nanoscale. The specific contrast formation in HIM imaging is due to differences in local electrical conductivity of the components. The authors have demonst...

Advancements in focused ion beam repair of MoSiON phase-shifting masks

As advanced photolithography moves the printable feature size from 0.25 micrometer to 0.18 micrometer various mask types are being used to improve resolution. One example is the attenuated phase shift MoSiON mask. This in turn requires the development of new mask repair techniques that provide acceptable levels of transmission and minimize phase error. In this study we present the results of opaque defect repairs on MoSiON DUV masks, utilizing a new focused ion beam (FIB) process. Opaque defects were repaired by scanning the defect area with a gallium ion beam in the presence of an etchant gas. Dose enhancement on the order of 20x was achieved, relative non-gas enhanced sputtering on the MoSiON absorber material to a non gas enhanced gas enhanced sputtering, resulting in repaired regions with excellent transmission properties, and minimal quartz damage (riverbed). The optimization of the FIB repair process is discussed and the results of post repair characterization, utilizing AIMS and AFM are presented.