New Tool Gives Scientists Inside Look At Materials

Now, because of the work of researchers at the Department of Energy's Oak Ridge National Laboratory (ORNL), scientists have a powerful new tool to study interconnects and other materials made up of small disoriented crystal blocks called grains. The new X-ray crystal microscope provides an exciting capability that didn't exist before, said Gene Ice of the Metals and Ceramics Division.

"This is like having a microscope that allows us to see the three-dimensional crystal structure of most materials for the first time," Ice said. "It allows us to watch the evolution of materials on a scale that is too large to worry about every atom and too small to assume that the material is uniform: the so-called mesoscale."

In the case of integrated circuits, researchers can use the X-ray crystal microscope to make measurements of metal interconnects as the chip is processing signals. It can study how the shape, orientation and stresses in the individual interconnect grains influence the tendency to failure.

Previously, researchers could either study isolated single crystals or could study the average properties of many polycrystalline grains. Neither approach gives an entirely accurate picture of what's going on at the scale required to understand the behavior of polycrystalline materials. Existing methods such as electron microscopy provide high resolution for thin, two-dimensional samples, but simply cannot address the need for three-dimensional information on the scale of tens of microns.

The approach developed by Ice, Ben Larson of the Solid State Division and others at ORNL relies on three innovations, the first being a technique that allows for better focusing of X-rays. To accomplish this, researchers have capitalized on emerging thin-film deposition techniques to fabricate special X-ray mirror surfaces. They use these mirrors to focus high-powered X-ray beams from an ultra-high-brilliance, third-generation synchrotron X-ray source such as the Advanced Photon Source at Argonne National Laboratory.

For their second innovation, Ice and colleagues make use of the Laue diffraction technique, a standard crystallographic method for determining crystal orientation. This method has rarely been used where precision measurements are required, but, with today's instrumentation, it can determine the orientation and strain of individual grains to high precision. This provides information critical to studying the interaction of grains.

The third innovation is the development of automatic indexing software that separates the overlapping Laue patterns from the simultaneously illuminated grains of the sample. In addition to identifying the position and orientation of individual grains, this software analyzes X-ray patterns in detail to detect distortions in each grain.

"This is critical because we can now determine what's happening to individual grains instead of taking averages, as we've had to do in the past." Ice said. "It's a far more direct-- and accurate -- approach."

Putting the three steps together, the ORNL scientists use submicron X-ray beams to probe crystal grains within materials. Using an X-ray area detector, they record the X-ray Laue patterns from the grains. They interpret the pattern to determine the number of reflecting grains illuminated and their orientation. With this information and the algorithms developed at ORNL, researchers can determine the strain, stress and orientation in each grain of a polycrystalline material.

Ultimately, this work, which has been published in a number of technical journals, including the Journal of Applied Physics (November 1999), gives scientists a tool to help develop better materials to be used in computers, automobiles, medical equipment and in the generation and transmission of electricity.

For example, John Budai of the Solid State Division is using this technique to understand the forces driving the alignment of high-temperature superconducting grains with RABiTS, a method to manufacture long lengths of ultra-high-performance superconducting wires. In the RABiTS process, the metal substrate is engineered to provide a pre-aligned foundation on which the superconducting grains form. With the X-ray crystal microscope, alignment of the superconducting thin-film grains can be related to the alignment of the engineered substrate. This new information will guide future developments of this award-winning technology.

"We are now working to make this a routinely useable tool, not just a research project," Ice said. "The challenge is to apply it to the broad range of materials performance and failure issues depending critically on individual grains and their evolution on mesoscopic sizes of tenths-to-tens of microns; for instance, grain growth, ceramic sintering, deformation and fracture."

The development of the X-ray crystal microscope by Ice, Larson, Budai, Jin-Seok Chung, Nobumichi Tamura and Jon Tischler builds upon an effort funded initially through seed money and the Laboratory Directed Research and Development programs, both funded by DOE. Collaborating with ORNL on the project have been researchers at Howard University in Washington, D.C., and the University of Illinois.

ORNL is a DOE multi-program research facility managed by Lockheed-Martin Energy Research Corporation.