Raymond Hill

An Introduction to the Helium Ion Microscope.

A new microscope has been developed that uses a beam of helium ions which is focused and scanned across the sample. In principle, and in its applications, it is similar to a traditional scanning electron microscope (SEM). However, the source technology, the sample interaction, and the contrast mechanisms are distinctly different. The helium ion source offers high brightness (4×109 A/cm2sr) and a small energy spread (ΔE/E∼3×10−5), and hence allows the beam to be focused to small probe sizes (as small as 0.25 nm). As the beam interacts with the sample, the beam penetrates relatively deeply before it diverges and hence there is a narrow sample interaction region near the surface. The helium beam generates secondary electrons, scattered helium atoms (ions and neutrals), and other detectable particles from which images can be generated or analysis can be performed.

Scanning Helium Ion Microscopy

Abstract Recent developments in the area of gas field ion sources, coupled with knowledge gained from field ion microscopy, have made the realization of very high brightness ion sources a reality. In particular, an ion source using helium has been produced with a brightness equal to (or even exceeding) that of a cold field emission electron source. The other optical properties of this source (energy spread and angular intensity) are also very favorable, enabling the development of high resolution ion microscopes. One such development is a scanning helium ion microscope (HIM) based on this source technology. In many ways, the HIM has similarities to the scanning electron microscope in terms of construction and use. However the use of helium ions over electrons offers some unique advantages, including smaller focused probe size, beam/sample interactions, and sample charge control. This article describes the principles of operation for the helium ion source. Consideration is then given to a scanning column design, highlighting the small focused probe size that is to be expected. A description of the helium ion beam/sample interaction is presented followed by a review of the signals generated at the sample that can then be used for imaging or analytical purposes. Finally, a series of example applications are presented that highlight the unique capabilities of scanning helium ion microscopy.

Diffraction imaging in a He+ ion beam scanning transmission microscope

The scanning helium ion microscope has been used in transmission mode to investigate both the feasibility of this approach and the utility of the signal content and the image information available. Operating at 40 keV the penetration of the ion beam, and the imaging resolution achieved, in MgO crystals was found to be in good agreement with values predicted by Monte Carlo modeling. The bright-field and annular dark-field signals displayed the anticipated contrasts associated with beam absorption and scattering. In addition, the diffraction of the He ion beam within the sample gave rise to crystallographic contrast effects in the form of thickness fringes and dislocation images. Scanning transmission He ion microscopy thus achieves useful sample penetration and provides nanometer scale resolution, high contrast images of crystalline materials and crystal defects even at modest beam energies.

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.

Scanning Transmission Ion Microscopy and Diffraction Imaging

In this paper we investigate the possibility of applying the “ORION” (ZeissSMT : Peabody, MA) Helium ion scanning microscope (HIM) to imaging in the scanning transmission mode of operation. In particular because the interaction of He+ ions with solids differs in many ways from that for electrons it is necessary to determine the changes in operating conditions, and in image interpretation, that may be required.