Researchers
from the Commerce Department's National Institute of Standards
and Technology (NIST) reported today 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, reported today
at the American Chemical Society's national meeting in Boston,
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.
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. For more information
on NIST, visit www.nist.gov.