NIST team reports method to characterize new insulating materials for microelectronics

August 26, 2002

Researchers from the Commerce Department's National Institute of Standards and Technology (NIST) reported on Aug. 20, 2002, at the American Chemical Society meeting in Boston, that they have developed methods for characterizing key structural features of porous films being eyed as insulators for the ultrathin metal wires that will connect millions of devices on future microprocessors and increase processor speed. The advance will help semiconductor manufacturers and their materials suppliers home in on the most promising "nanoporous film" candidates for shielding miles of interconnecting wire on next-generation microprocessors.

Microprocessors, or central processing units, are both the "brains" and "engines" of computers and other microelectronic devices.

To increase processor speed, semiconductor manufacturers seek to reduce chip sizes. However, size reduction introduces problems with electrical interference between circuit elements ("cross-talk"), motivating development of better insulating materials. Current insulating materials such as silicon dioxide and fluorinated silicate glass (FSG) are approaching their limits as devices are squeezed ever closer on a chip.

To prepare better insulating films, many materials suppliers are developing films interspersed with very small holes that measure about 5 nanometers (billionths of a meter) in diameter or less. Introducing nanometer-sized air bubbles lowers a material's dielectric constant, or "k" value -- a measure of insulating performance. Air, the ideal insulator, has a dielectric constant of 1. Silicon dioxide and FSG, in contrast, have values of about 4.2 and 3.7, respectively.

For several years, the NIST team has been performing a variety of measurements to help the semiconductor industry characterize potential nanoporous insulators. Working with the NIST Center for Neutron Research, materials scientists Ronald Hedden, Barry Bauer, and Hae-Jeong Lee of the NIST Polymers Division developed a neutron scattering technique for surveying minuscule holes in film samples supplied by International SEMATECH, the consortium of chipmakers. Combined with information gathered with other methods, neutron scattering measurements reveal the size and volume fraction of pores, the connectivity among pores and the density of the underlying matrix.

The "Swiss cheese" approach to developing new insulators presents some formidable challenges. To be sure, riddling a material with tiny holes (or bubbles) lowers its "k" value, but changes other important properties as well. Also to be taken into account, for example, are a candidate material's strength and hardness, how well it adheres to different substrates, and whether it can withstand high temperatures and etch chemicals during processing.

"An ideal replacement for silicon dioxide would provide the desired level of insulation without compromising barrier properties," explains Bauer. "The more fully we can characterize the pore structure and properties of these nanoporous materials, the more straightforward the search becomes."

Bauer, Hedden and Lee led an effort to extend contrast matching -- a neutron-based technique for studying bulk materials -- to nanoporous thin films. They succeeded by pumping solvent vapor mixtures into a special flow-through chamber containing the films. The vapor condenses into the pores, permitting neutron scattering measurements that probe the film density while yielding valuable information about pore connectivity. The new neutron technique also can detect nanometer-sized inhomogeneities in the composition of the matrix.

Though the results are promising, the contrast-matching procedure is time-consuming, taking three to four days per sample. One goal of the team is to develop a simpler alternative that industry could use to character films faster and more cheaply. Hedden says neutron scattering measurements could be used as a benchmark to evaluate data gathered with other approaches.
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As a non-regulatory agency of the U.S. Department of Commerce's Technology Administration, NIST develops and promotes measurements, standards and technology to enhance productivity, facilitate trade and improve the quality of life.

National Institute of Standards and Technology (NIST)

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