No holes barred?

Experts looking for new insulating films to replace SiO₂ know that inserting minuscule holes in the films lowers the dielectric constant and raises insulating capability. However, the holes can also weaken the insulating material and render it susceptible to damage from high temperatures and etch chemicals. Chipmakers and suppliers know they must find a film that combines the best of both insulation and barrier properties for future IC generations.

A team of materials scientists at NIST may have the answer. The team has been exploring a technique for characterizing so-called nanoporous insulators. Extending a method for studying bulk materials, Ronald Hedden, Barry Bauer, and Hae-Jeong Lee of NIST's polymers division have adapted a neutron-scattering technique called contrast matching to examine holes in samples from International Sematech. Measurements show the size and volume fraction of pores, pore connectivity, and density of the matrix underneath the film. The holes typically measure 5 nm or less.

The three researchers built a flow-through cell (above right) to deliver solvent vapor to thin films. The vapor enables the team to conduct contrast-matching experiments by condensing the vapor in the film pores. The condensation permits neutron scattering in the NG1 8-meter system, shown below with the custom-made small-angle neutron-scattering (SANS) contrast-matching tool, at the NIST Center for Neutron Research. These instruments both provide measurements of film density and supply information on pore connectivity, NIST says. In addition, the technique is able to detect irregularities at the nanometer level in the matrix composition.

The blue schematic shown below depicts the contrast-matching method used with SANS. Starting with the box at lower left, the technique involves filling the pores with liquids that have differing neutron-scattering length density until a match point is found. As the holes fill with solvent and the intensity of neutron scattering drops, the contrast changes in the open pores. At the match point, the only areas of contrast will be closed pores and "heterogeneities" in the wall, the researchers say. In this schematic, none of these features appears, meaning no coherent SANS signal exists.

The NIST researchers say the method has at least one big drawback: It takes three to four days per sample. The team hopes to develop a faster and cheaper alternative, and Hedden says the scattering technique could be combined with other data-gathering methods for characterizing low-k films.

IMAGES COURTESY OF NIST