The newly developed ORION helium ion microscope provides high magnification images that exceed the capabilities of scanning electron microscopes.

The ORION is the first commercially available helium ion microscope and is available through Carl Zeiss SMT. Images: Carl Zeiss SMT

Since the 1940s, the traditional scanning electron microscope (SEM) has evolved into a valuable imaging and analysis tool due to its long depth of focus, high magnification, and easy to interpret images. The 1986 Nobel Prize in Physics was awarded (in part) to Ernst Ruska for his original work on the design of the SEM which is now ubiquitous in the science laboratory. But advancements in SEM performance have stagnated due to two undesired effects. First, an electron’s low mass means that it behaves partly like a wave, so it diffracts and interferes with beam focusing. Second, the electron beam scatters broadly as it strikes the sample, producing information from regions other than the region being probed. Due to these effects, a typical sample will produce images with a resolution from 2 to 5 nm on a SEM.

The newly developed ORION helium ion microscope, from Carl Zeiss SMT, Peabody, Mass., addresses the present limitations of the SEM. The helium ion is about 8,000 times more massive than an electron, so its deBroglie wavelength is less than 100 fm—small enough that the diffraction effects are negligible. Consequently, the focused probe size of the helium beam can be as small as 0.25 nm. On the other hand, the mass of the helium is much lighter than traditional ion beams (such as gallium or argon) so that there is negligible sputtering of the sample. In fact, the helium beam stays relatively collimated as it penetrates into the sample, so the information produced is comparable to the focused probe size. For all of these reasons, the helium ion beam is able to produce images which reveal surface details at the sub-nanometer scale.

As the helium ion beam strikes the sample, a variety of particles is produced. The varying yield of these particles—and their ability to be collected—is the basis for creating the strong contrast in the helium ion microscope images. One of the available particles is the secondary electron, several of which are ejected for each ion that hits the surface. Since the production rate depends on the topography of the sample, the images provide valuable highlights and shadows that can easily convey the 3-D shape of the feature. The above images show the strong topographic contrast and high resolution available in the helium ion microscope. Since the beam penetration depends upon the crystallographic orientation of the sample, the images can also show the different crystallographic domains which constitute a sample.

In addition to the secondary electrons, the incident helium ions can actually be scattered out of the sample. Since the scattering probability depends on the atomic number of the scattering nuclei, images based upon the scattered helium atoms provide elemental contrast.

In summary, a new form of microscopy using helium ions is now commercially available in the ORION helium ion microscope. The images offer resolution and contrast not available through traditional focused ion beams or SEMs.

—John Notte, Director of R&D, Carl Zeiss SMT Inc.
—Nicholas Economou, CEO ,Carl Zeiss SMT Inc.
—Bill Ward, CTO,Carl Zeiss SMT Inc.