Hard X-ray Microscope Opens New Vistas

Alexander E. Braun, Senior Editor -- Semiconductor International, 9/4/2008 7:55:00 AM

The newly operational Hard X-Ray Nanoprobe, at the Center for Nanoscale Materials (CNM) at the U.S. Department of Energy’s (Washington, D.C.) Argonne National Laboratories (Argonne, Ill.), is one of the most powerful in its class on the planet. It now provides unprecedented capabilities for the study of nanoscale materials, because the weak interaction of hard X-rays with matter allows researchers to penetrate into materials, see through process gases, and non-destructively study embedded—subsurface—phenomena.

The applications seem limited only by the operator’s imagination, ranging from researching novel nanoscale materials for more efficient photovoltaic cells and providing more efficient lighting, to enabling advanced computing.

Hard X-ray Nanoprobe at Argonne National Laboratory. One of the world's most powerful X-ray microscopes, it will be used to study novel nanoscale materials and devices. (Source: ANL)

Until now, the very characteristics that made this kind of X-ray microscopy desirable have worked against its application. The manufacture of efficient X-ray optics is extremely difficult, and it limits the degree to which X-rays can be focused. However, Argonne solved the problem by using advanced X-ray optics called Fresnel zone plates. These are similar in appearance to the large Fresnel lenses used to focus light in old lighthouses. Together with a laser-based nanopositioning system, Argonne is now able to focus X-rays down to the smallest spot yet achieved with this type of illumination source.

The microscope combines scanning-probe and full-field transmission imaging to create both 3-D visualizations of complex systems and devices, as well as to perform sensitive quantitative analysis of elemental composition, chemical states, crystallographic phase and strain.

According to CNM Division Director Stephen Streiffer, the concept for the X-ray Nanoprobe dates back almost a decade. “Particularly, as X-ray focusing optics improved, we realized that there was an opportunity to make use of our excellent synchrotron source and — with the help of the X-Ray Division of the Advanced Photon Source (APS) at Argonne — build a revolutionary new instrument that could allow us to focus high-energy X-rays down to the 0.10 nm length scale.” Streiffer added that the resurgence of nanomaterials and nanoscience as a scientific discipline, coupled with X-ray optics development, provided the opportunity to strike while the iron was hot.

The Hard X-Ray Nanoprobe was designed, built, and operated in partnership between the CNM and the APS. The CNM researches the development and characterization of novel nanoscale materials and devices. Argonne’s Advanced Photon Source plays a key role in the hard X-rays used by the Nanoprobe beamline, providing its unprecedented capabilities to characterize nanostructures.

The Nanoprobe uses X-rays with photon energies of 3 to 30 KeV to produce images of, initially, 30 nm resolution. This is expected to improve with the continued development of novel X-ray optics, leading to significantly higher spatial resolution being achieved over the Nanoprobe’s lifetime.

The Hard X-ray Nanoprobe is the highest resolution microscope of its type today. It can produce images with initially 30 nm resolution. A much higher spatial resolution is anticipated. (Source: ANL)

Argonne Nanoprobe Beamline Director Jorg Mase said, “When you consider high-resolution imaging, there is a variety of choices that can be had. For example, atomic force microscopes, which are surface-sensitive; electron microscopes for very thin samples; and then, of course, X-rays, which are very penetrating. It’s this unique window that in the case of semiconductors can enable the engineer to look into structures that are thicker than what an electron microscope can handle, at very high spatial linewidth resolutions. With this new X-ray optics you don’t generate enough heat to damage structures so you can look at complex devices, do their tomography without cutting or sectioning, and with volumetric sensitivity.”

As Streiffer explained, “Semiconductor devices are already at nanoscale dimensions. The technology is at the 45 nm node going down to 32 nm. So what we can do is use the penetrating power of the X-rays to non-destructively look inside such devices and start to measure things like structure, strain, and composition in the embedded structure in a way that one cannot do with electron microscopy.”

Another example is catalytic nanoparticles. “If you think about the catalytic converter in your automobile,” said Streiffer, “it essentially consists of metallic nanoparticles supported on a ceramic membrane, obviously operating in a pretty high-temperature, aggressive environment. The sort of thing that we would like to do with the Nanoprobe is take an advanced catalytic nanoparticle, put it in an environment typical to that in which it operates in the real world, and then do X-ray scattering off it to investigate its structure in that aggressive environment. This is still a few years off, but is one of our goals.”

Yet another area of investigation is looking at the chemical and elemental composition of biological materials. “Although this isn’t our main mission, a cell can really be considered as a collection of nanoscale machines,” Streiffer said. “Using the Nanoprobe we can look at its individual elements and begin to measure elemental compositions that might be related, for instance, to a drug therapy or a disease or some other kind of intervention in a way that you cannot do with E-beam.”

This also opens the possibility to study things like nanocomposites, which are composed of nanoparticles; this would enable the tuning not only of properties such as bandgap, but DNA as well. As Mase put it, “It’s been shown that it’s possible to bring some of those nanocomposites into a cell or a structure such as a mitochondrion. This would make it possible to specifically target some sections of DNA, enabling the nanocomposites to act upon it, and heal diseases.”