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Nanoscale Toughness Measurements

Moore’s law is simply stated: the number of transistors on an integrated-circuit logic device doubles roughly every two years. This pace is accomplished through a combination of greater transistor density and larger devices. The greater transistor density causes problems in the interconnect structure—the array of miniature wires (and insulating materials) that distribute power and clock signals to the transistors. Wires in the interconnect structure begin to interfere with each other through a phenomenon called “cross-talk” when they become too close, and cross-talk causes devices to slow. Incorporation of low-dielectric constant (low-k) insulators into the interconnect structure of advanced integrated circuits will eliminate cross-talk but such incorporation remains a major technological challenge facing the microelectronics industry.

Innovative materials engineering has developed many varieties of low-k thin films. However, the fact is that the significant reductions in dielectric constant necessitated the incorporation of porosity, transforming the dielectrics from completely solid to foam-like. Porosity degrades mechanical properties, and consequently failure of low-k materials by brittle fracture remains a device manufacturing and reliability hurdle. Improvement of the fracture toughness (a measure of the resistance to fracture) of low-k dielectrics is a difficult challenge according to the current International Technology Roadmap for Semiconductors (ITRS), and yet there exists no reliable method to measure toughness of these nanoscale thin films. Without a technique to measure fracture toughness of such films, new materials must be incorporated and “tried out” in test devices; an expensive and time-consuming device development proposition.

NIST Nanomechanical Properties Group researchers have developed a nanoindentation-based technique to measure the fracture toughness of low-k dielectrics. Nanoindentation is routinely used to measure the elastic modulus and hardness of thin films and materials in small volumes, but the measurement of fracture properties is much less well developed. Cracks as small as 300 nanometers were formed when low-k films were indented with the apex of a diamond “cube-corner” probe The surface lengths of these cracks were then measured by scanning-electron microscopy. The crack lengths vary in an odd and complex way with regard to indentation forces, film thickness, film stress, and the elastic properties of the low-k material and silicon substrate. These variables were all incorporated into a contact and fracture mechanics model that allows for the extraction of a materials property, fracture toughness, as well as a performance-based metric, the critical film thickness for spontaneous fracture.

The measurement technique and model were published in a two-part series: “Indentation fracture of low-dielectric constant films, Part I. Experiments and observations,” D.J. Morris and R.F. Cook, J. Mater. Res. 23 (2008) 2429-2442.“Indentation fracture of low-dielectric constant films, Part II. Indentation fracture mechanics model ,” D.J. Morris and R.F. Cook, J. Mater. Res. 23 (2008) 2443-2457

CONTACT: Robert Cook (x3207)