New X-Ray Microprobe From Bell Labs

Tom Topalian
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NEW X-RAY MICROPROBE FROM BELL LABS HELPS LUCENT DESIGN BETTER LASER MODULES

FOR RELEASE TUESDAY, MAY 6, 1997

MURRAY HILL, N.J. -- Lucent Technologies is designing better laser modules thanks to a new tool developed by Bell Labs scientists -- an X-ray microprobe (XMP) that measures strain in smaller volumes of material and detects trace elements better than any other non-destructive deep probe in the world.

The instrument produces an X-ray spot only two microns across -- one two-
hundredth the diameter of the period at the end of this sentence. That’s 1,000 times smaller than spots produced by conventional X-ray probes.

The XMP makes it possible for scientists to study micron-size features in a range of structures, from naturally patterned biological systems to lithographically patterned devices such as electronic circuits and solid-state lasers. They’re interested in strain, the displacement of atoms from their naturally occurring positions, for example, because the right amount of strain is essential to efficient laser operation but too much strain can cause defects that make a laser inoperable.

"Bell Labs inventions like this one substantially reduce trial-and-error experimentation, which accelerates the ‘time to market’ for new Lucent products," said John Pilitsis, vice president, Optoelectronics Products, Lucent Technologies Microelectronics Group.

The XMP was used to generate a "map" of the micro-structural properties of the 266 EM-ILM (electro-absorptive modulated isolated laser module) made by Lucent’s Microelectronics Group. The module, used in fiber-optic communications systems, integrates a laser and modulator and can transmit 2.5 billion bits of information per second over 600 kilometers of fiber.

Lucent engineers used the map to adjust their device design, varying the chemical composition, thickness, and strain of the atomic layers of the laser to optimize the structure of the laser.

Bell Labs researchers developed the scanning X-ray microprobe at the National Synchrotron Light Source at Brookhaven National Laboratories in Upton, N.Y. Synchrotrons are large machines, some as much as a kilometer in circumference, that accelerate electrons in a circular orbit to emit X-rays and other electromagnetic radiation.

An XMP takes these very bright X-rays and sends them through high-quality X-ray optics, such as mirrors, lenses and pinholes. It uses X-ray diffraction, a technique in use for more than 80 years to determine the three-dimensional atomic structure of materials. The wavelength of X-rays is ten thousand times smaller than that of visible light, so X-rays can make measurements of smaller features than can be made with optical probes, which use visible light to discern features as small as one-
thousandth the thickness of a human hair.

The Bell Labs XMP consists primarily of a pair of X-ray mirrors, one focusing horizontally and the other vertically. Within its two-micron spot size, the XMP can accurately and non-destructively measure the average distance between atoms in a solid.

"The X-ray microprobe will revolutionize characterization of materials in a broad range of disciplines, including device engineering, materials science, chemistry, biology and environmental sciences," said Pierre Wiltzius, head of the Bell Labs Condensed Matter Physics Research department.

"There are many other problems of interest to Lucent Technologies that can take advantage of the X-ray microprobe," added researcher Ken Evans-Lutterodt. "A key problem for silicon-based microelectronics is long-term device reliability. Thermal and electromigration stresses result in the development of voids in the aluminum wires that provide electrical interconnection between sub-micron devices on silicon chips. These voids lead to device failure. The XMP allows characterization of the stresses and the development of voids."

The Bell Labs research team is now refining the XMP to provide spot sizes one-fifth as large as those produced by their current XMP.

"An important aspect of this work is that it demonstrates how a long-term research project, like the XMP development, can have a very tangible impact on the short-term needs of a high-tech company like Lucent Technologies," said Eric Isaacs.

X-ray microprobes are being developed by teams of scientists at other synchrotron sources, such as the Advanced Photon Source in Argonne, Ill., and the European Synchrotron Radiation Facility in Grenoble, France. The Bell Labs XMP team includes Evans-Lutterodt, and Isaacs, of the Physical Research Lab, Matthew Marcus and William Lehnert, of the Silicon Electronics Research lab, and former Bell Labs researcher Alastair MacDowell, of the Advanced Light Source in Berkeley, Calif.

Isaacs and Phil Platzman, of the Bell Labs Physical Research Lab, presented information on the development and current uses of the Bell Labs XMP and X-ray synchrotron sources at this year’s American Physical Society annual meeting.

Lucent Technologies designs, builds and delivers a wide range of public and private networks, communications systems and software, consumer and business telephone systems and microelectronics components. Bell Labs is the research and development arm of the company.

Lucent’s Microelectronics Group designs and manufactures integrated circuits, optoelectronic components, and power systems for the computer and communications industries. Lucent’s Optoelectronics Group is the number-one worldwide supplier of optoelectronic components for telecommunications and cable TV applications.

More information about Lucent Technologies, headquartered at Murray Hill, N.J., is available at http://www.lucent.com.

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