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X-Ray Tool Aids Laser Design

Photonics Spectra
Jul 1997
Kathleen G. Tatterson

MURRAY HILL, N.J. -- Scientists at Bell Laboratories have developed a tool that uses x-ray technology to measure strain in small volumes of material for laser manufacturing and quality control applications.

According to the company, the x-ray microprobe generates an x-ray spot 2 µm across so users can make measurements of features 1000 times smaller than with conventional optical probes using visible light. "It's the difference between looking at something in your hand and looking at it under a microscope," said Ken Evans-Lutterodt, Bell Labs team scientist.
Researchers at parent company Lucent Technologies are using the device to measure strain and determine the three-dimensional structure of its laser modules used in fiber optic communications. Too much strain can cause defects that "gobble up electrons" and render a laser inoperable, said Eric Isaacs, Bell Labs researcher.
Another advantage is the ability to make penetrating measurements nondestructively. Conventional measurement techniques can monitor only the top of a material's surface. Before the microprobe, Lucent scientists had to destroy a module to make accurate measurements of strain and atomic displacement.
The XMP microprobe uses x-ray light from the National Synchrotron Light source at Brookhaven National Laboratory and the Advanced Photonic Light Source at Argonne National Laboratory. Using a pair of elliptical-shaped x-ray mirrors, one focusing horizontally and one vertically, the device manipulates the bright beams and sends them through an array of mirrors, lenses and pinholes. "It works like a pinhole camera. These new optical elements can now be made small enough to make this possible," Isaacs said.
The x-ray diffraction determines the three-dimensional atomic structure of materials. The microprobe can then map boundaries of different materials. "If you hit chromium, you know it's chromium," he said.
The team is looking at using the device for applications in electromigration, the mass transport of metal by momentum exchange between thermally activated metal ions and conducting electrons. When electromigration occurs in transistors having metal interconnecting films, it can produce voids that cause failure. Semiconductor manufacturers could use the XMP technology to gauge quality control in chip making. Researchers say that the technique could also have implications in biology for intricate cell imaging using "soft x-rays."
The XMP will remain strictly as a research tool for Lucent, and Bell Labs will not be offering it commercially. "It's not a tool we're in the business of selling access to," Evans-Lutterodt said. "We are building a tool to meet (Lucent's) needs."

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