Say Hello to Helium Ion Microscopy

MICROSCOPE MAKERS constantly strive to improve images. Sometimes all it takes is a quick tweak to an existing instrument. Or it can mean decades of effort to develop a fundamentally different concept.

"Our mission was to make a new type of microscope—an alternative to the electron microscope," says John A. Notte, previously with start-up company ALIS Corp. and now a research and development director with microscope manufacturer Carl Zeiss. Instead of using electrons for high-magnification imaging, Notte says, he and his colleagues turned to helium ions.

Helium ions have shorter wavelengths than electrons, so helium ions can form a more tightly focused beam. For a microscope, that means better image resolution.

Last year, a team from Zeiss installed Orion, the first commercially available helium ion microscope (HeIM), at the National Institute of Standards & Technology, in Gaithersburg, Md., as part of a cooperative R&D agreement. This past summer, Zeiss replaced it with the first Orion Plus, a second-generation microscope that includes several design changes suggested by NIST researchers, including improvements to the cooling system for the helium ion source. Several other research facilities around the world have purchased the new microscope.

Helium ions "could be the electrons of the 21st century" for imaging, says David C. Joy, a microscopy expert at the University of Tennessee, Knoxville, and Oak Ridge National Laboratory.

A HeIM operates much like a scanning electron microscope (SEM) but has unique capabilities. The new microscope's helium ions can produce images with subnanometer resolution, which is up to four times better than that of an SEM. The helium-derived images have higher surface contrast and better depth of field, so more of the image is in focus than in SEM-derived images. Most notably, HeIMs can also create images with Rutherford backscattered ions (RBIs), which in this case are high-energy helium ions that rebound off a sample, to give information on chemical composition that a standard SEM cannot.

The semiconductor and nanomanufacturing industries are quite interested in these advances because, for example, a HeIM can clearly image the edge of a microchip or the grooves in a CD, whereas edges "bloom" or appear fuzzy in SEM images. The ability to accurately measure features on the edge of a material or use RBIs to determine the chemical composition of a defect in a semiconductor chip could help improve commercial production processes.

MOST MICROSCOPY EXPERTS agree that HeIMs will not replace SEMs. Rather, the two techniques will complement each other. Scanning electron microscopy "is and will remain the standard imaging tool, as it is a well-established and cost-effective inspection method," says Diederik Maas, a senior scientist who develops microscopes at the Netherlands Organization for Applied Scientific Research (TNO) Nanolab, in Delft, which has a second-generation HeIM. (Three high-resolution SEMs can be purchased for roughly $2 million, the cost of a single HeIM.) But quite a number of applications are challenging or impossible for an SEM, and that is where the HeIM could help and may even create a new niche within microscopy, Maas adds.

Helium ion microscopy is related to field-ion, or field-emission, microscopy, a technology developed in 1955 to look at individual atoms on a cryogenically cooled tungsten tip in an ultra-high vacuum system with small amounts of helium gas in it. In the 1980s, scientists followed up by commercializing a focused-ion-beam microscope with a gallium ion source that could analyze a wider range of samples. However, the problem with using this heavy ion for imaging is that it often sputters away the sample before an image can be captured.

The HeIM is similar to the gallium focused-ion-beam microscope. But in the HeIM, a distinctive pyramid-shaped ion source allows helium ions to form a more tightly focused beam, leading to higher resolution images. In addition, helium ions are lighter than gallium ions and don't cause samples to deteriorate as quickly.

Notte and ALIS colleagues Bill Ward and Nick Economou developed the helium gas ion source, which is the HeIM's distinguishing component. Zeiss acquired ALIS and then developed the HeIM, which is designed to collect images via two modes: secondary-electron mode and RBI mode. In an SEM, electrons are fired from an electron emission source, which uses a combination of heat and electric fields to emit electrons. The electrons collide with the sample and release secondary electrons that eventually reach the detector, which generates signals that are synthesized into the image. In a HeIM, helium ions hit the sample, which releases secondary electrons and RBIs. Different detectors monitor the electrons and RBIs, whose signals lead to separate images.

Helium ion beams have higher mass and much shorter wavelength than electron beams. Helium ions therefore interact much more strongly with materials than do electrons and produce about 100 times more secondary electrons, Joy says. This means that more information goes to the detector, providing highly detailed images.

Secondary-electron images provide useful information about a sample's surface, but researchers studying nanoscale materials are particularly excited about getting information on chemical composition from RBI images.

FEW ANALYTICAL TOOLS have the unique ability to image with RBIs, and those images convey qualitative information about a material's elemental content, says David C. Bell, manager of imaging and analysis at Harvard University's Center for Nanoscale Systems.

For example, Notte and colleagues have shown how tin and lead components of solder may be visually indistinguishable in a secondary-electron image but appear as patches of dark and light, respectively, in an RBI image. Additionally, the number of RBIs generated is proportional to the atomic numbers of elements in the sample, which can help identify the composition of an unknown material or defect on a microchip.

Scientists at Zeiss, NIST, and other research institutions with the new microscopes are working to understand HeIM imaging mechanisms, fine-tune the HeIM's capabilities, and test chemistry-related applications.

John Allgair, a metrology program manager for Sematech, a nonprofit semiconductor industry group, says microelectronic chip makers now rely on automated SEMs to monitor their manufacturing processes. But he notes that there is growing industrial interest in HeIMs because they have better resolution for imaging surfaces and can do much-needed chemical analysis of extraneous small particles and other defects that can form during wafer and microchip fabrication.

One potential drawback of the HeIM is that a beam of helium ions may damage samples more than a beam of electrons would. Therefore, it's important to determine whether HeIM-induced damage can be tolerated in final products, Allgair says.

Nanomaterial researchers say the HeIM is useful for investigating various kinds of materials. Michael T. Postek, chief of NIST's Precision Engineering Division, says scientists in NIST's materials, manufacturing, and chemistry laboratories have used the HeIM to examine the properties of cellulosic nanocrystals and carbon nanotubes. And at Harvard, Bell says, several research groups have used both secondary-electron and RBI modes on the university's first-generation HeIM to examine the chemistry of nanowires and other nanoscale materials.

TNO's Maas describes plans at his institution to use the microscope to inspect nanofabricated structures and as a means of nanofabrication. And groups at the National University of Singapore have used a second-generation HeIM to image dry biological samples with secondary electrons and have used RBIs to image monolayers of graphene, says Daniel S. Pickard, an assistant professor of electrical engineering who works on imaging instruments at the university. He says both kinds of samples have been difficult to image with an SEM because of their extreme fragility.

And with the help of scientists from several institutions, Zeiss's team is making further improvements to the company's HeIM systems. New capabilities available next year will include the option to carry out energy spectroscopy of backscattered helium ions, which will give researchers more information on chemical composition. "The HeIM's performance and capabilities are changing from month-to-month—literally," Notte says.